SOLAR RADIATION

HOW MUCH SOLAR ENERGY STRIKES THE EARTH?
The sun generates an enormous amount of energy - approximately 1.1 x 10 E20 kilowatt-hours every second. (A kilowatt-hour is the amount of energy needed to power a 100 watt light bulb for ten hours.) The earth's outer atmosphere intercepts about one two-billionth of the energy generated by the sun, or about 1500 quadrillion (1.5 x 10 E18 ) kilowatt-hours per year. Because of reflection, scattering, and absorption by gases and aerosols in the atmosphere, however, only 47% of this, or approximately 700 quadrillion (7 x 10 E17 ) kilowatt-hours, reaches the surface of the earth.

The amount of incident energy per unit area and day depends on a number of factors, e.g.:
latitude
local climate
season of the year
inclination of the collecting surface in the direction of the sun.

TIME AND SITE
The solar energy varies because of the relative motion of the sun. This variations depend  on the time of day and the season. In general, more solar radiation is present during midday than during either the early morning or late afternoon. At midday, the sun is positioned high in the sky and the path of the sun's rays through the earth's atmosphere is shortened. Consequently, less solar radiation is scattered or absorbed, and more solar radiation reaches the earth's surface.

The amounts of solar energy arriving at the earth's surface vary over the year, from an average of less than 0,8 kWh/m2 per day during winter in the North of Europe to more than 4 kWh/m2 per day during summer in this region. The difference is decreasing for the regions closer to the equator.
The availability of solar energy varies with geographical location of site and is the highest in regions closest to the equator. Thus the average annual global radiation impinging on a horizontal surface which amounts to approx. 1000 kWh/m2 in Central Europe, Central Asia, and Canada reach approx. 1700 kWh/m2 in the Mediterranian and to approx. 2200 kWh/m2 in most equatorial regions in African, Oriental, and Australian desert areas. In general, seasonal and geographical differences in irradiation are considerable (see the table bellow) and must be taken into account for all solar energy applications.

Variations of solar irradiation (tilt angle South 30Deg.) in Europe  and Caribbean region in kWh/m2.day.

CLOUDS
The amount of solar radiation reaching the earth's surface varies greatly because of changing atmospheric conditions and the changing position of the sun, both during the day and throughout the year. Clouds are the predominant atmospheric condition that determines the amount of solar radiation that reaches the earth. Consequently, regions of the nation with cloudy climates receive less solar radiation than the cloud-free desert climates. For any given location, the solar radiation reaching the earth's surface decreases with increasing cloud cover. Local geographical features, such as mountains, oceans, and large lakes, influence the formation of clouds; therefore, the amount of solar radiation received for these areas may be different from that received by adjacent land areas. For example, mountains may receive less solar radiation than adjacent foothills and plains located a short distance away. Winds blowing against mountains force some of the air to rise, and clouds form from the moisture in the air as it cools. Coastlines may also receive a different amount of solar radiation than areas further inland.
The solar energy which is available during the day varies and depends strongly on the local sky conditions. At noon in clear sky conditions, the global solar irradiation can in e.g. Central Europe reach 1000 W/m2 on a horizontal surface (under very favourable conditions, even higher levels can occur) whilst in very cloudy weather, it may fall to less than 100 W/m2 even at midday.

POLLUTION
Both man-made and naturally occurring events can limit the amount of solar radiation at the earth's surface. Urban air pollution, smoke from forest fires, and airborne ash resulting from volcanic activity reduce the solar resource by increasing the scattering and absorption of solar radiation. This has a larger impact on radiation coming in a direct line from the sun (direct radiation) than on the total (global) solar radiation. On a day with severely polluted air (smog alert), the direct solar radiation can be reduced by 40%, whereas the global solar radiation is reduced by 15% to 25%. A large volcanic eruption may decrease, over a large portion of the earth, the direct solar radiation by 20% and the global solar radiation by nearly 10% for 6 months to 2 years. As the volcanic ash falls out of the atmosphere, the effect is diminished, but complete removal of the ash may take several years.

POTENTIALS
Solar radiation provides us at zero cost with 10,000 times more energy than is actually used worldwide. All people of the world buy, trade, and sell a little less than 85 trillion (8.5 x 1013 ) kilowatt-hours of energy per year. But that's just the commercial market. Because we have no way to keep track of it, we are not sure how much non-commercial energy people consume: how much wood and manure people may gather and burn, for example; or how much water individuals, small groups, or businesses may use to provide mechanical or electrical energy. Some think that such non-commercial energy may constitute as much as a fifth of all energy consumed. But even if this were the case, the total energy consumed by the people of the world would still be only about one seven-thousandth of the solar energy striking the earth's surface per year.

In some developed countries like in the United States people consume roughly 25 trillion (2.5 x 10E13 ) kilowatt-hours per year. This translates to more than 260 kilowatt-hours per person per day - this is the equivalent of running more than one hundred 100 watt bulbs all day, every day. U.S. citizen consumes 33 times as much energy as the average person from India, 13 times as much as the average Chinese, two and a half times as much as the average Japanese, and twice as much as the average Sweden.

Even in such heavy energy consuming countries like USA solar energy falling on the land mass  can many times surplus the energy consumed there. If only 1% of land would be set aside and covered by solar systems (such as solar cells or solar thermal troughs) that were only 10% efficient, the sunshine falling on these systems could supply this nation with all the energy it needed. The same is true for all other developed countries. In a certain sense, it is impractical - besides being extremely expensive, it is not possible to  cover such large areas with solar systems. The damage to ecosystems might be dramatic. But the principle remains. It is possible to cover the same total area in a dispersed manner - on buildings, on houses, along roadsides, on dedicated plots of land, etc. In another sense, it is practical. In many countries already more than 1% of land is dedicated to the mining, drilling, converting, generating, and transporting of energy. And the great majority of this energy is not renewable on a human scale and is far more harmful to the environment than solar systems would prove to be.

SOLAR ENERGY UTILISATION
In most places of the world much more solar energy hits a home's roof and walls as is used by its occupants over a year's time. Harnessing this sun's light and heat is a clean, simple, and natural way to provide all forms of energy we need. It can be absorbed in solar collectors to provide hot water or space heating in households and commercial buildings. It can be concentrated by parabolic mirrors to provide heat at up to several thousands degrees Celsius. This heat can be used either for heating purposes or to generate electricity. There exist also another way to produce power from the sun - through  photovoltaics. Photovoltaic cells are devices which convert solar radiation directly into electricity.

Solar radiation can be converted into useful energy using active systems and passive solar design. Active systems are generally those that are very visible like solar collectors or photovoltaic cells. Passive systems are defined as those where the heat moves by natural means due to house design which entails the arrangement of basic building materials to maximize the sun's energy.

Solar energy can be converted to useful energy also indirectly, through other energy forms like biomass, wind or hydro power. Solar energy drives the earth´s weather. A large fraction of the incident radiation is absorbed by the oceans and the seas, which are warmed than evaporate and give the power to the rains which feed hydro power plants. Winds which are harnessed by wind turbines are getting its power due to uneven heating of the air. Another category of solar-derived renewable energy sources is biomass. Green plants absorb sunlight and convert it through photosynthesis into organic matter which can be used to produce heat and electricity as well. Thus wind, hydro power and biomass are all indirect forms of solar energy.

PASSIVE SOLAR ENERGY USE

Passive solar design, or climate responsive buildings use existing technologies and materials to heat, cool and light buildings. They integrate traditional building elements like insulation, south-facing glass, and massive floors with the climate to achieve sustainable results. These living spaces can be built for no extra cost while increasing affordability through lower energy payments. In many countries they also keep investment in the local building industry rather than transferring them to short term energy imports. Passive solar buildings are better for the environment while contributing to an energy independent, sustainable energy future.
Passive solar system uses the building structure as a collector, storage and transfer mechanical equipment. This definition fits most of the more simple systems where heat is stored in the basic structure: walls, ceiling or floor. There are also systems that have heat storage as a permanent element within the building structure, such as bins of rocks, or water-filled drums or bottles. These are also classified as passive solar energy systems. Passive solar homes are ideal places in which to live. They provide beautiful connections to the outdoors, give plenty of natural light, and save energy throughout the year.

HISTORY
Building design has historically borrowed its inspiration from the local environment and available building materials. More recently, humankind has designed itself out of nature, taking a path of dominance and control which led to one style of building for nearly any situation. In 100 A.D., Pliny the Younger, a historical writer, built a summer home in Northern Italy featuring thin sheets of mica windows on one room. The room got hotter than the others and saved on short supplies of wood. The famous Roman bath houses in the first to fourth centuries A.D. had large south facing windows to let in the sun's warmth. By the sixth century, sunrooms on houses and public buildings were so common that the Justinian Code initiated “sun rights” to ensure individual access to the sun. Conservatories were very popular in the 1800's creating spaces for guests to walk through warm greenhouses with lush foliage.

Passive solar buildings in the United States were in such demand by 1947, as a result of scarce energy during the prolonged World War 2, that Libbey-Owens-Ford Glass Company published a book entitled Your Solar House, which profiled forty-nine of the nations greatest solar architects.

In the mid-1950's, architect Frank Bridgers designed the world's first commercial office building using solar water heating and passive design. This solar system has been continuously operating since that time and the Bridgers-Paxton Building is now in the National Historic Register as the world's first solar heated office building.
Low oil prices following World War 2 helped keep attention away from solar designs and efficiency. Beginning in the mid-1990's, market pressures are driving a movement to redesign our building systems to more in line with nature.

Passive Solar Space Heating

Site
Landscaping and Trees
According to the U.S. Department of Energy report, “Landscaping for Energy Efficiency” (DOE/GO-10095-046), careful landscaping can save up to 25% of a household's energy consumption for heating and cooling. Trees are very effective means of shading in the summer months as well as providing breaks for the cool winter winds. In addition to contributing shade, landscape features combined with a lawn or other ground cover can reduce air temperatures as much as 5 degrees Celsius in the surrounding area when water evaporates from vegetation and cools the surrounding air. Trees are wonderful for natural shading and cooling, but they must be located appropriately so as to provide shade in summer and not block the winter sun. Even deciduous trees that lose their leaves during cold weather block some winter sunlight - a few bare trees can block over 50 percent of the available solar energy.

Windows

Glass permits sun radiation of wavelengths 0.4 to 2.5 microns to pass through it. As this radiant energy collides with opaque objects on the other side of the glass, it's wavelength increases to 11 microns. Glass acts as an opaque barrier to light of this wavelength thereby trapping the sun's energy. The amount of light penetrating a glass is dependent on the angle of incidence. The optimum angle of incidence is 90o. When sunlight strikes the glass at 30o or less, the most radiation is reflected.

Understanding the Solar Spectrum and Heat Transfer
To make good choices on glazing, it is needed to understand a bit about light and heat. The sunlight that strikes the Earth is comprised of a variety of wavelengths and different glazing will selectively transmit, absorb, and reflect the various components of the solar spectrum. Likewise, reducing glare (via reflection or tinting) is helpful in the workplace by allowing the transmission of visible, or natural, light it is possible to save energy for artificial light. But perhaps the greatest effect on human comfort levels is determined by infrared heat transfer. By specifying the right type of glass, it is possible to trap the infrared heat for warmth, or reflect the infrared heat to prevent warming.

There are three ways that heat moves through a glazing material. The first is conduction. Conductive heat is transferred through the glazing by direct contact. Heat can be felt by touching the glazing material. The second form of heat transfer is radiation; electromagnetic waves carry heat through a glazing. This produces the feeling of heat radiating from the surface of the glazing. The third method of heat transfer is convection. Convection transfers heat by motion, in this case, air flow. The natural flow of warm air toward colder air allows heat to be lost or gained.
The R-value of a glazing - its insulating capabilities or resistance to the flow of heat - is determined by the degree of conduction, radiation, and convection through the glazing material. However, air infiltration will also determine the overall R-value of a glazing system. The amount of heat that travels around a glazing is as important as the heat transfer through a glazing. Air can leak in or out of a building around the glazing via the framing. The quality, workmanship, and the installation of the entire glazing system, including the framing, affects air infiltration.
Advances in glass technology have perhaps been the single largest contributor to building efficiency since the 1970s and they play an important roll in solar design. Some window advances include:
Double and triple pane windows with much higher insulating values.
Low emissivity or Low-E glass employing a coating which lets heat in but not out.
Argon (and other) gas filled windows that increase insulating values above windows with just air.
Phase-change technologies that can switch from opaque to translucent when a voltage is applied to them.

Basic Glass Types
Glazing materials include glass, acrylics, fibreglass, and other materials. Although different glazing materials have very specific applications, the use of glass has proven the most diverse. The various types of glass allow the passive solar designer to fine-tune a structure to meet client needs. The single pane is the simplest of glass types, and the building block for higher performance glass. Single panes have a high solar transmission, but have poor insulation - the R-value is about 1,0. Single pane glass can be effective when used as storm windows, in warm climate construction (unless air conditioning is being used), for certain solar collectors, and in seasonal greenhouses. Structures using single pane glass will typically experience large temperature swings, drafts, increased condensation, and provide a minimal buffer from the outdoors.

Perhaps the most common glass product used today is the double pane unit. Double pane glass is just that: two panes manufactured into one unit. Isolated glass (thermopane) incorporate a spacer bar (filled with a moisture absorbing material called a desiccant) between the panes and are typically sealed with silicone. The spacer creates a dead air space between the panes. This air space increases the resistance to heat transfer; the R-value for double pane is about 1,8-2,1. Huge air spaces will not drastically increase R-value. In fact, a large air space can actually encourage convective heat transfer within the unit and produce a heat loss. A rule of thumb for air space is between 1 and 2 centimetres. It is also possible to go as large as 10-12 centimetres without creating convective flow, but at that point you are dealing with a very large and awkward unit. The demand for greater energy efficiency in building and retrofitting homes has made insulated glass units the standard. With good solar transmission and fair insulation, such unit is a large improvement over the single pane. Windows, doors, skylights, sunrooms, and many other areas utilize double pane glass.

HIGH PERFORMANCE GLASS
High performance or enhanced glass offers even better R-value and solar energy control. By further improving the insulating capability of glass, it is possible dramatically increase  also design options. What were once insulated walls may become sunrooms. Solid roofs and ceilings become windows to the sky. Dark rooms can “wake up” to natural light, solar heat gain, and wonderful views. For a relatively small increase in cost it is possible to improve efficiency, provide better moisture and UV protection, and gain design flexibility. A variety of high performance glass is now available.

What are the advantages of this glass? Low emissivity (Low-E) glass is succeeding double pane glass in energy efficient buildings. Emissivity is the measure of infrared (heat) transfer through a material. The higher the emissivity, the more heat is radiated through the material. Conversely, the lower the emissivity, the more heat is reflected by the material. Low-E coatings will reflect, or re-radiate, the infrared heat back into a room, making the space warmer. This translates into R-values from 2.6 to 3.2. In warmer climates it is possible to reverse the unit and re-radiate infrared heat back to the outside, keeping the space cooler. Low-E glass improves the R-value, UV protection, and moisture control.Gas-filled windows increase R-value. Properly done, gas-filling will increase the overall R-value of a glass unit by about 1,0. The air within an insulated glass unit is displaced with an inert, harmless gas with better insulation properties. Typical gases used are Krypton and Argon.

Window curtains
In addition to decorative functions, curtains can be used to reduce the heat losses that occur during the cold months as well as the heat gains during the warmer months. The plywood box over the curtain top prevents warm ceiling air from moving between the glass and curtain. The curtain should drop at least 30 cm below the window for it to be effective. The optimum condition would be for it to drop to the floor.

Thermal mass
Solar radiation hitting walls, windows, roofs and other surfaces is adsorbed by the building and is stored in thermal mass. This stored heat is then radiated to the interior of the building. Thermal mass in a solar heating system performs the same function as batteries in a solar electric system (see chapter on photovoltaics). Both store solar energy, when available, for later use.

Thermal mass can be incorporated into a passive solar room in many ways, from tile-covered floors to water-filled drums. Thermal mass materials, which include slab floors, masonry walls, and other heavy building materials, absorb and store heat. They are a key element in passive solar homes. Homes with substantial south-facing glass areas and no thermal storage mass do not perform well.

It is important to know that dark surfaces reflect less, therefore, absorb more heat. In case of a dark tiled floor, the floor will be able to absorb heat all day and radiate heat into the room at night. The rate of heat flow is based on the temperature difference between heat source and the object to which the heat flows. As described above heat flows in three ways - conduction (heat transfer through solid materials), convection (heat transfer through the movement of liquids or gasses), and radiation. All surfaces of a building lose heat via these three modes. Good solar design works to minimize heat loss and maximize efficient heat distribution. The need for thermal mass (heat-storage materials) inside a building is very climate-dependent. Heavy buildings of high thermal mass are consistently more comfortable during hot weather in hot-arid and cool-temperate climates, while in hot-humid climates there is little benefit. In cool-temperature climates the thermal mass acts as a cold-weather heat store thus improving overall comfort and reducing the need for auxiliary heating, except on overcast or very cold days. In intermittently heated buildings, however, it tends to increase the heat needed to maintain the chosen conditions.

Providing adequate thermal mass is usually the greatest challenge to the passive solar designer. The amount of mass needed is determined by the area of south-facing glazing and the location of the mass. In order to ensure an effective design it is important to follow these guidelines:
Locate the thermal mass in direct sunlight. Thermal mass installed where the sun can reach it directly is more effective than indirect mass placed where the sun's rays do not penetrate. Houses that rely on indirect storage need three to four times more thermal mass than those using direct storage.
Distribute the thermal mass. Passive solar homes work better if the thermal mass is relatively thin and spread over a wide area. The surface area of the thermal mass should be at least 3 times, and preferably 6 times, greater than the area of the south windows. Slab floors that are 8 to 10 centimetres thick are more cost effective and work better than floors 16 to 20 inches thick.
Do not cover the thermal mass. Carpeting virtually eliminates savings from the passive solar elements. Masonry walls can have drywall finishes, but should not be covered by large wall hangings or lightweight panelling. The drywall should be attached directly to the mass wall, not to covers fastened to the wall that create an undesirable insulating airspace between the drywall and the mass.
Select an appropriate mass colour. For best performance, finish mass floors with a dark colour. A medium colour can store 70 percent as much solar heat as a dark colour, and may be appropriate in some designs. A matte finish for the floor reduces reflected sunlight, thus increasing the amount of heat captured by the mass and having the additional advantage of reducing glare. The colour of interior mass walls does not significantly affect passive solar performance.
Insulate the thermal mass surfaces. There are several techniques for insulating slab floors and masonry exterior walls. These measures should introduced to achieve the  energy savings. Unfortunately, problems in some case can arise like with termite infestations in foam insulation for perimeter slabs. This can complicate the issue of whether and how to insulate slab-on-grade floors.
Make thermal mass multipurpose. For maximum cost effectiveness, thermal mass elements should serve other purposes as well. Masonry thermal storage walls are one example of a passive solar design that is often cost prohibitive because the mass wall is only needed as thermal mass. On the other hand, tile-covered slab floors store heat, serve as structural elements, and provide a finished floor surface. Masonry interior walls provide structural support, divide rooms, and store heat.

When developing a thermal storage system or simply comparing materials it is useful to look at the storage capacity of the proposed building materials which is referred to as the volumetric heat capacity (J/m3. Deg. Celsius) or more commonly the specific heat and the rate at which the material can take up and store heat. Some examples of common storage materials are given in the following table:

Early solar designers used water (stored in large containers) as the heat storage medium. Although water is cheap, the containers and the space they take are not. Some solar designers turned to rock storage bins as reservoirs for thermal mass. It took three times as much rock to store the same amount of heat as an equivalent volume of water and the moist warm environment of the bins became breeding grounds for odor producing fungi and bacteria. The high cost and the foul odors started to give solar design a bad name. Both water and rock heat storage require complicated control systems, pumps, and blowers. Heat storage is not common in today‘s solar energy utilisation. Main reason for this is that all of these systems rely on electricity, require maintenance, and are subject to periodic breakdown.

Thermal insulation

Cooling
In many parts of the world a passive solar building needs cooling as much as heating. One of the best, time proven methods of cooling is thermal coupling with the earth's constant temperature. Dropping the ground floor at least one meter into the earth provides a more even exterior temperature which aids cooling as well as heating. Adequate structural engineering, drainage, and damp proofing are essential in below ground areas. Thermal isolation is the best and most economical way to temper the building's environment. Using the earth's thermal mass keeps the house at a reasonable temperature, and so does good insulation. Shades located outside and inside the windows, ventilation and reflective films on the windows are also very important in order to control temperature inside the building.

External Shades and Shutters
Exterior window shading treatments are effective cooling measures because they block both direct and indirect sunlight outside of the home. Solar shade screens are an excellent exterior shading product with a thick weave that blocks up to 70 percent of all incoming sunlight. The screens absorb sunlight so they should be used on the exterior of the windows. From outside, they look slightly darker than regular screening, but from the inside many people do not detect a difference. Most products also serve as insect screening. They should be removed in winter to allow full sunlight through the windows. A more expensive alternative to the fibreglass product is a thin, metal screen that blocks sunlight, but still allows a view from inside to outside. Hinged decorative exterior shutters which close over the windows are also excellent shading options. However, they obscure the view, block daylight completely, may be expensive and may be difficult for many households to operate on a daily basis.

Interior Shades and Shutters
Shutters and shades located inside the house include curtains, roll-down shades, and Venetian blinds. Interior shutters and shades are generally the least effective shading measures because they try to block sunlight that has already entered the room. However, if passive solar windows do not have exterior shading, interior measures are needed. The most effective interior treatments are solid shades with a reflective surface facing outside. In fact, simple white roller blinds keep the house cooler than more expensive louvered blinds, which do not provide a solid surface and allow trapped heat to migrate between the blinds into the house.

Reflective Films and Tints
Reflective film, which adheres to glass and is found often in commercial buildings, can block up to 85% of incoming sunlight. The film blocks sunlight all year, so it is inappropriate on south windows in passive solar homes. However, it may be practical for unshaded east and west windows. These films are not recommended for windows that experience partial shading because they absorb sunlight and heat the glass unevenly. The uneven heating of windows may break the glass or ruin the seal between double-glazed units.

Ventilation
Ventilation is the changing of air in buildings to control oxygen, heat and contaminants. Ventilation may occur in few forms. Building orientation, form, plan and user actions also alter air flow paths. Natural ventilation consumes no energy and has few if any running costs, but depends on weather conditions and can be difficult to control. Mechanical and air-conditioned ventilation are energy-driven alternatives to natural ventilation, normally dictated by building type, site and function. They can be particularly efficient as supplements to natural ventilation. Mechanical ventilation uses fans and ducts to supply and extract air in localised areas such as a kitchen. Air conditioning both treats and supplies air. It is particularly useful to cool air below ambient temperatures.

SOLAR ARCHITECTURE & ACTIVE SYSTEMS
It is important to design the house with the aim to incorporate active solar systems (see below) like collectors or photovoltaic modules as  well. The building should orient these appliances due south. Tilt of the solar collectors should be in Europe and North America more than 50° (from horizontal) to maximize winter heat collection. Solar collectors should be thermally locked with the roof. Non-tracking photovoltaics receive the most yearly insolation (exposure to the sun's rays) when tilted at an angle, from horizontal, equal to the building's latitude. Design of the building's roof should be done to such angles and southern orientation as integral aspects of the building. Hot water collectors and photovoltaic panels should be located as close as possible to their main areas of use. It is important to concentrate these areas of use. For example, putting the bathrooms and kitchen close together economizes on their installation and minimizes energy loss. All appliances should be selected with efficiency as the prime criterion.

SUMMARY

SITE
It is important to become familiar with the energy flows of house surroundings. The nature and relationship of the lay of the land, water courses, vegetation, soil types, wind directions, and exposure to the sun should be investigated. A site suitable for solar design should balance and complement these elements. It must have unobstructed exposure to the sun from 9 am to 3 pm during the heating season.

HEATING
In Northern hemisphere orientation due south of the main solar insolating spaces, i.e. greenhouse, and/or main daytime activity areas is important. Glass should be open to the sun patterns during the winter. By facing of the windows to the south, and virtually none to the north maximaze solar gain. Multiple pane glass in all windows is recommended.

THERMAL MASS
Thermal mass including masonry floors, walls and water storage is important to absorb ambient heat during the day and release it at night. Insulation of the building further  minimize heat loss through windows, walls and roof.

NATURAL HEAT FLOW
It is useful to design the house with the natural heat flow in mind. Hot air rises, so placing  some activity areas on a second floor to draw heat up from a lower collector area and across other areas can save a lot of energy. Buffer areas of the building (unheated rooms, or partially heated spaces such as utility rooms, vestibules and storage areas) should be oriented due to the north to lessen the impact of the winter's cold. Using a vestibule on doors to the exterior can lead to energy savings. Vestibules cut heat loss and provide a buffer zone between the exterior and the interior.

SOLAR COLLECTORS

Using energy from the sun to heat water is one of the oldest uses of solar energy. Solar collectors are the heart of most solar energy systems. The collector absorbs the sun's light energy and changes it into heat energy. This energy is than transferred to a fluid or air which are used to warm buildings, heat water, generate electricity, dry crops or cook food. Solar collectors can be used for nearly any process that requires heat.
Domestic hot water is the second-highest energy cost in the typical household in Europe or North America. In fact, for some homes it can be the highest energy expenditure. Solar water heating can reduce domestic water heating costs by as much as 70%. Designed to pre-heat the domestic water that is supplied to conventional water collector, it can result in remarkable savings. It's easy to install and almost maintenance free.
Today, solar water heating systems are being used for single family houses, apartment buildings, schools,  car washes, hospitals, restaurants, agricultural farms and different industries. This is a diverse list of private, commercial and industrial buildings, but they all have one thing in common -  they all use  hot water. Owners of these buildings have found that solar water heating systems are cost-effective in meeting their hot water needs all over the world.

HISTORY
Solar water heating was used long before fossil fuels dominated our energy system. The principles of solar heat have been known for thousands of years. A black surface gets hot in the sun, while a lighter coloured surface remains cooler, with white being the coolest. This principle is used by solar water collectors which are one of the best known applications for the direct use of the sun's energy. They were developed some two hundred years ago and the first known flat plate collector was made by Swiss scientist Horace de Saussure in 1767, later used by Sir John Herschel to cook food during his South Africa expedition in the 1830's.
Solar technology advanced to roughly it's present design in 1908 when William J. Bailey of  the Carnegie Steel Company (USA), invented a collector with an insulated box and copper coils. This collector was very similar to the thermosyphon system (described bellow).  Bailey sold 4000 units by the end of World War I and a Florida businessperson who bought the patent rights sold nearly 60 000 units by 1941. In the U.S. the rationing of copper during World War II sent the solar water heating market into a sharp decline.
Little interest was shown in such devices until the world-wide oil crisis of 1973. This crisis promoted new interest in alternative energy sources. As a result, solar energy has, received increased attention and many countries are taking a keen interest in new developments. The efficiency of solar heating systems and collectors has improved from the early 1970s. The efficiencies can be attributed to the use of low-iron, tempered glass for glazing (low-iron glass allows the transmission of more solar energy than conventional glass), improved insulation, and the development of durable selective coatings.

SOLAR COLLECTOR MARKET
Solar domestic hot-water systems are technically mature and available practically all over the world. The market for flat-type collectors has been reported as substantial in Israel, China, Cyprus, Japan, Australia, Austria, Germany, Greece Turkey and USA. Sales in Europe are mainly for domestic water heating, which may also include space heating and heating swimming pools. World production of solar collectors in 1995 was 1,3 million m2 where market in Europe and Mediterranean countries is reported to be about 40% of the world market. Total amount of installed solar collectors exceeded 30 million m2 and the development of sales was very rapid since 1980. Since 1989 there is steady increase with around 20 % per year.
Among countries in Europe, Greece has become the leader in production of solar systems and exports 40% of all collectors produced and comprises 30% of the market in Germany.  The industry‘s goal for the year 2005 represents 1,3 million systems and 5 million m2 of collectors.  A project on Crete will need 20,000 collectors over two years.  The Greek market installs 70,000 solar systems a year, reducing CO2 emissions by 1,5 million tonnes.
Sales in the EU in 1996 were reported to be over 0,7 million m2 of glazed collectors and about 0,15 million m2 of unglazed collectors (Renewable energy world, Sept. 1998). All the indications are that this trend will continue at a rapid pace since measures are being taken all over the EU for the promotion of solar systems.

Installed solar collector area in the world (Source: IEA SHC programme: Solar Thermal Collector Market in IEA Member Countries, December 2002)

Installed solar collector area per head of population was 0,5 m2 in Cyprus in 2002 the largest in Europe and followed by Greece and Austria. Collector area per head of population increased in Austria up to 0,2 m2 in 2002 and amounted total area of 1,5 million m2.  Austria is first in sales per capita followed by Greece  but both countries still fall behind the world leaders Israel and Cyprus. Analysis of statistical figures like collector area per head of population shows that favourable climatic conditions have less influence than socio-economic boundary conditions. The success in Cyprus is explained not only by the absence of any other local source of energy but also by countries regulation. Strong legislation promoting solar energy utilisation is in force also in Israel. Israel and Cyprus have imposed statutory requirements for solar heating systems in all new buildings. These requirements were introduced in stages: thus in Israel initially all new apartment buildings of up to eight storeys were required to have a community solar water heating system with appropriate storage tanks. This was later extended to all new dwellings in the country. Finally in 1983 new regulations required hotels, hospitals and schools to install solar water heating equipment. These regulations were coupled with financial incentives. A similar attempt has also been made in Cyprus and it was recently estimated that 90 % of individual dwellings and 15 % of apartments in Cyprus are now equipped with solar water heaters.

POTENTIALS
In Europe the total rapidly exploitable potential for solar collectors production is estimated to be 360 million m2 , representing a market volume of 50 billion USD at an annual average growth rate of 23%. In 2005 the area occupied by glazed solar collector installations in the EU was expected to rise to 28 million m2. Moreover, unglazed solar collectors for heating swimming pools  are expected to reach 20 million m2.

SOLAR COLLECTORS TYPES
Typical solar collectors collect the sun's energy usually with rooftop arrays of piping and net metal sheets, painted black to absorb as much radiation as possible. They are encased in glass or plastic and angled towards south to catch maximum sunshine. The collectors act as miniature greenhouses, trapping heat under their glass plates. Because solar radiation is so diffuse, the collectors must have a large area.

Solar collectors can be made in various sizes and constructions depending on requirements. They give enough hot water for washing, showers and cooking. They can be used also as pre-heaters for existing water heaters. Today there are several collectors on the market. They can be divided into several categories. One of them is division according temperature they produce:
Low-temperature collectors provide low grade heat, less than 50 degrees Celsius, through either metallic or non-metallic absorbers for applications such as swimming pool heating and low-grade water.
Medium-temperature collectors provide medium to high-grade heat (greater than 50 degrees Celsius, usually 60 to 80 degrees), either through glazed flat-plate collectors using air or liquid as the heat transfer medium or through concentrator collectors that concentrate the heat to levels greater than “one sun.” These include evacuated tube collectors, and are most commonly used for residential hot water heating.
High-temperature collectors are parabolic dish or trough collectors primarily used by independent power producers to generate electricity for the electric grid.

Batch Solar Water Collectors

Flat-Plate Collectors
Flat-plate collectors are the most common collectors for residential water heating and space-heating installations. A typical flat-plate collector is an insulated metal box with a glass or plastic cover called the glazing and a dark-coloured absorber plate. The glazing can be transparent or translucent. Translucent (transmitting light only) low-iron glass is a common glazing material for flat-plate collectors because low-iron glass transmits a high percentage of the total available solar energy. The glazing allows the light to strike the absorber plate but reduces the amount of heat that can escape. The sides and bottom of the collector are usually insulated, further minimising heat loss.
The absorber plate is usually black because dark colours absorb more solar energy than light colours. Sunlight passes through the glazing and strikes the absorber plate, which heats up, changing solar radiation into heat energy. The heat is transferred to the air or liquid passing through the flow tubes. Because most black paints still reflect approximately 10% of the incident radiation some absorber plates are covered with “selective coatings,” which retain the absorbed sunlight better and are more durable than ordinary black paint. The selective coating used in the collector consists of a very precise thin layer of an amorphous semiconductor plated on to a metal substratum. Selective coatings has both high absorptivity in the visible region and low emissivity in the long-wave infrared region.
Absorber plates are often made of metal usually copper or aluminium because they are both good heat conductors. Copper is more expensive, but is a better conductor and is less prone to corrosion than aluminium. An absorber plate must have high thermal conductivity, to transfer the collected energy to the water with minimum temperature loss. Flat-plate collectors fall into two basic categories: liquid and air. And both types can be either glazed or unglazed.

Liquid Collectors
In a liquid collector, solar energy heats a liquid as it flows through tubes in the absorber plate. For this type of collector, the flow tubes are attached to the absorber plate so the heat absorbed by the absorber plate is readily conducted to the liquid.
The flow tubes can be routed in parallel, using inlet and outlet headers, or in a serpentine pattern. A serpentine pattern eliminates the possibility of header leaks and ensures uniform flow. A serpentine pattern can pose some problems for systems that must drain for freeze protection because the curved flow passages will not drain completely.
The simplest liquid systems use potable household water, which is heated as it passes directly through the collector and then flows to the house to be used for bathing, laundry, etc. This design is known as an “open-loop” (or “direct”) system. In areas where freezing temperatures are common, however, liquid collectors must either drain the water when the temperature drops or use an antifreeze type of heat-transfer fluid.
In systems with heat-transfer fluids, the transfer fluid absorbs heat from the collector and then passes through a heat exchanger. The heat exchanger, which generally is in the water storage tank inside the house, transfers heat to the water. Such designs are called “closed-loop” (or “indirect”) systems.
Glazed liquid collectors are used for heating household water and sometimes for space heating. Unglazed liquid collectors are commonly used to heat water for swimming pools. Because these collectors need not withstand high temperatures, they can use less expensive materials such as plastic or rubber. They also do not require freeze-proofing because swimming pools are generally used only in warm weather.

Air Collectors
Air collectors have the advantage of eliminating the freezing and boiling problems associated with liquid systems. Although leaks are harder to detect and plug in an air system, they are also less troublesome than leaks in a liquid system. Air systems can often use less expensive materials, such as plastic glazing, because their operating temperatures are usually lower than those of liquid collectors.
Air collectors are simple, flat-plate collectors used primarily for space heating and drying crops. The absorber plates in air collectors can be metal sheets, layers of screen, or non-metallic materials. The air flows through the absorber by natural convection or when forced by a fan. Because air conducts heat much less readily than liquid does, less heat is transferred between the air and the absorber than in a liquid collector. In some solar air-heating systems, fans on the absorber are used to increase air turbulence and improve heat transfer. The disadvantage of this strategy is that it can also increase the amount of power needed for fans and, thus, increase the costs of operating the system. In colder climates, the air is routed between the absorber plate and the back insulation to reduce heat loss through the glazing. However, if the air will not be heated more than 17°C above the outdoor temperature, the air can flow on both sides of the absorber plate without sacrificing efficiency.
The best features of air collector systems are simplicity and reliability. The collectors are relatively simple devices. A well-made blower can be expected to have a 10 to 20 year life span if properly maintained, and the controls are extremely reliable. Since air will not freeze, no heat exchanger is required.
However, the use of solar air heating collectors is still limited to supply hot air for space heating and for drying of agricultural products mainly in developing countries. The major limitations for the wide adoption of solar air heaters are the high cost for commercially produced solar air heaters, the large collector area required due to the low density and the low specific heat capacity of the air compared to liquid heat transfer fluids, the extended air duct system required, the high power requirement for forcing the air through the collector, and the difficulty of heat storage. In countries with comparatively low insolation and extended periods of adverse weather, supplementary heat is required which increases investment costs to a level which limits its competitiveness to conventional heating systems. Promising ways to reduce the collector cost are the integration of the collector into the walls or roofs of buildings and the development of collectors which can be constructed using prefabricated components.

Solar wall.

Heating with the solar wall .

HOW IT WORKS ?
Solar air heaters can be classified based on the mode of air circulation. In the bare plate collector, which is the most simple solar air heater, the air passes through the collector underneath the absorber. This kind of solar air heater is only suitable for temperature rise between 3 - 5 deg. Celsius due to the high convection and radiation losses at the surface. The top losses can be reduced significantly by covering the absorber with a transparent material of low transitivity for infrared radiation. The air flow occurs in this kind of solar air heater either underneath the absorber or between absorber and transparent cover. Due to the transparent cover, the incident radiation on the absorber is reduced slightly, but due to the reduction of the convective heat losses, temperature rise between 20 and 50 degrees Celsius can be achieved depending on insolation and air flow rate. A further reduction of the heat losses can be achieved if the air is made to pass above and underneath the absorber since this doubles the heat transfer area. The heat losses due to radiation will be reduced by this process due to lower absorber temperature. However, there is simultaneous reduction in the absorptivity of the absorber due to dust deposit if air flow is above or on both sides of the absorber.
Some solar air collectors eliminate the cost of the glazing, the metal box, and the insulation. Such a collector is made of black, perforated metal. The best heat transfer can be achieved by using porous material as absorber.  The sun heats the metal, and a fan pulls air through the holes in the metal, which heats the air. For residential installations, these collectors are available in different sizes. Typical collector  2,4-meter by 0,8-meter panels are capable of heating 0,002 m3 per second of outside air. On a sunny winter day, the panel can produce temperatures up to 28°C higher than the outdoor air temperature. Transpired air collectors not only heat air, but also improve indoor air quality by directly preheating fresh outdoor air. These collectors have achieved very high efficiencies - more than 70% in some commercial applications. Plus, because the collectors require no glazing or insulation, they are inexpensive to manufacture.

Evacuated-Tube Collectors
Conventional simple flat-plate solar collectors were developed for use in sunny and warm climates. Their benefits are greatly reduced when conditions become unfavourable during cold, cloudy and windy days. Furthermore, weathering influences such as condensation and moisture will cause early deterioration of internal materials resulting in reduced performance and system failure. These shortcomings are reduced in evacuated-tube  collectors.
Evacuated-tube collectors heat water in residential applications that require higher temperatures. In an evacuated-tube collector, sunlight enters through the outer glass tube, strikes the absorber tube, and changes to heat. The heat is transferred to the liquid flowing through the absorber tube. The collector consists of rows of parallel transparent glass tubes, each of which contains an absorber tube (in place of the absorber plate in a flat-plate collector) covered with a selective coating. The heated liquid circulates through heat exchanger and gives off its heat to water that is stored in a solar storage tank.
Evacuated tube collectors are modular tubes which can be added or removed as hot-water needs change. When evacuated tubes are manufactured, air is evacuated from the space between the two tubes, forming a vacuum. Conductive and convective heat losses are eliminated because there is no air to conduct heat or to circulate and cause convective losses. There can still be some radiant heat loss (heat energy will move through space from a warmer to a cooler surface, even across a vacuum). However, this loss is small and of little importance compared with the amount of heat transferred to the liquid in the absorber tube. The vacuum in the glass tube, being the best possible insulation for a solar collector, suppresses heat losses and also protects the absorber plate and the “heat-pipe” from external adverse conditions. This results in exceptional performance far superior to any other type of solar collector.

Evacuated-tube collectors are available in a number of designs. Some use a third glass tube inside the absorber tube or other configurations of heat-transfer fins and fluid tubes. One commercially available evacuated-tube collector stores 19 litres of water in each tube, eliminating the need for a separate solar storage tank. Reflectors placed behind the evacuated tubes can help to focus additional sunlight on the collector.
Due to the atmospheric pressure and the technical problems related to the sealing of the collector casing, the construction of an evacuated flat-plate collector is extremely difficult. To overcome the enormous atmospheric pressure, many internal supports for the transparent cover pane must be introduced. However, the problems of an effective high vacuum system with reasonable production costs remain so far unsolved. It is more feasible to apply and adapt the mature technology related to the lamp industries with proven mass production. Building a tubular evacuated solar collector and the maintenance of its high vacuum, similar to light bulbs and TV tubes, is practical. The ideal vacuum insulation of the tubular evacuated solar collector, obtained by means of a suitable exhausting process, has to be maintained during the life of the device to reduce the thermal losses through the internal gaseous atmosphere (convection losses).
In high temperature region these collectors are more efficient than flat-plate collectors for a couple of reasons. First, they perform well in both direct and diffuse solar radiation. This characteristic, combined with the fact that the vacuum minimizes heat losses to the outdoors, makes these collectors particularly useful in areas with cold, cloudy winters. Second, because of the circular shape of the evacuated tube, sunlight is perpendicular to the absorber for most of the day. For comparison, in a flat-plate collector that is in a fixed position, the sun is only perpendicular to the collector at noon. Evacuated-tube collectors achieve both higher temperatures and higher efficiencies than flat-plate collectors, but they are also more expensive.

Concentrating Collectors
Concentrating collectors use mirrored surfaces to concentrate the sun's energy on an absorber called a receiver. They also achieve higher temperatures than flat-plate collectors, however concentrators can only focus direct solar radiation, with the result being that their performance is poor on hazy or cloudy days. The mirrored surface focuses sunlight collected over a large area onto a smaller absorber area to achieve high temperatures. Some designs concentrate solar energy onto a focal point, while others concentrate the sun's rays along a thin line called the focal line. The receiver is located at the focal point or along the focal line. A heat-transfer fluid flows through the receiver and absorbs heat. Concentrators are most practical in areas of high insolation, such as those close to the equator and in the desert areas.
Concentrators perform best when pointed directly at the sun. To do this, these systems use tracking mechanisms to move the collectors during the day to keep them focused on the sun. Single-axis trackers move east to west; dual-axis trackers move east and west and north and south (to follow the sun throughout the year). Concentrators are used mostly in commercial applications because they are expensive and because the trackers need frequent maintenance. Some residential solar energy systems use parabolic-trough concentrating systems. These installations can provide hot water, space heating, and water purification. Most residential systems use single-axis trackers, which are less expensive and simpler than dual-axis trackers. For more information about concentrating collectors see chapter Solar Thermal Power Production.

SOLAR COOKERS AND STILLS
There exists also some other inexpensive, “low-tech” solar collectors with specific functions like solar box cookers (used for cooking) and solar stills producing inexpensive distilled water from virtually any water source.
Solar box cookers (see chapter on Solar cooking) are inexpensive to buy and easy to build and use. They consist of a roomy, insulated box lined with reflective material, covered with glazing, and fitted with an external reflector. Black cooking pots serve as absorbers, heating up more quickly than aluminium or stainless steel cookware. Box cookers can also be used to kill bacteria in water if the temperature can reach the boiling point.
Solar stills (see chapter on Solar water distillation) provide inexpensive distilled water from even salty or badly contaminated water. They work on the principle that water in an open container will evaporate. A solar still uses solar energy to speed up the evaporation process. The stills consist of an insulated, dark-coloured container covered with glazing that is tilted so the condensing fresh water can trickle into a collection trough. A small solar still, which is about the size of kitchen stove, can produce up to ten litres of distilled water on a sunny day.

Technology Examples
Solar energy has a variety of practical and cost-effective applications in today's homes and buildings. The main applications of solar collectors are as follows :
hot water preparation in households, commercial buildings and industry,
water heating in swimming pools,
space heating in buildings,
drying crops and houses,
space cooling and refrigeration,
water distillation,
solar cooking.

The technologies for all applications are considered to be mature and for the first two, under the appropriate conditions, economically viable. Separate chapter is devoted to concentrating collectors which are cost effectively used for power production especially in regions with high insolation (see chapter on Solar Thermal Power).

Solar Thermal Residential Water Heating

Today, several million homes and businesses use solar water heating systems. These systems are providing consumers a cost-effective and reliable choice for hot water. Taking a shower with solar-heated water, or heating a house with solar-heated air or water, is a natural and simple method for both conserving energy and saving fossil fuels. When a solar heating system has been designed and installed correctly, it can be aesthetically appealing and also add to the value of the home. On new construction, they can be worked into the building design to be almost invisible, while on existing construction it can be a real challenge to make them fit in.
A solar water collector is saving an owner money but it also help protect the environment. Emissions of one to two tons of carbon dioxide are saved by a single conventional water collector every year. Other pollutants, such as sulphur dioxides, carbon monoxide and nitrous oxides are also displaced when a homeowner decides to tap into a solar energy.

Hot water production is the most widely distributed utilisation of direct solar heating. An installation consists of one or more collectors in which a fluid is heated by the sun, plus a hot-water tank where the water is heated by the hot liquid. Even in the areas of low insolation like in Northern Europe a solar heating system can provide 50-70% of the hot water demand.  It is not possible to obtain more, unless there is a seasonal storage (see chapter below). In Southern Europe a solar collector is able to cover 70-90% of the hot-water consumption. Heating water with the sun is very practical and cost effective. While photovoltaics (see chapter on photovoltaics) range from 10-15% efficiency, thermal water panels range from 50-90% efficiency. In combination with a wood stove coil/loop, virtually year round domestic hot water can be obtained without the use of fossil fuels.

HOW IS A SOLAR WATER COLLECTOR COMPETITIVE WITH CONVENTIONAL HEATERS ?
Costs of complete solar water heating systems differs considerably from country to country (in Europe and the USA e.g. between 2000 - 4000 USD). They also depend on hot water requirements and the climate conditions in the area. This is usually a higher initial investment than required for an electric or gas heater but when adding all of the costs involved with heating water in home, the life-cycle cost of a solar water heating system is usually lower than traditional heating system. It must be noted that simple pay-back time for investment into solar heating system depends on prices of  fossil fuels substituted by solar energy. In EU countries pay-back times are generally less than 10 years. The expected life span of the solar heating system is 20-30 years.
Important feature of solar installation is energy pay-back time - time needed to produce as much energy by solar system as it was needed to produce this system. In Northern Europe with less solar radiation than in other parts of the world a solar heating system for hot-water preparation has an energy pay back period of 3-4 years.

HOW MUCH ENERGY CAN WE GET ?
The amount of energy we can get from solar heating system depends on available insolation and efficiency of the solar system. Insolation differs widely in the world and is crucial for solar system. The amount of solar radiation available in some regions of the world is given in chapter Solar Radiation. The efficiency of solar system depends on efficiency of solar collector and losses in the hot water circulation system. As the later depends on various specific parameters we will focus only on solar collector efficiency. Efficiency is defined as the ration between the amount of energy produced and solar energy falling down on collector. Efficiencies are different for different collector types and depends on solar intensity, thermal and optical losses - higher losses means lower efficiencies. Thermal losses are minimal if the temperature of water used for application is the same as ambient air temperature. Thus simple absorber without glazing used for pool heating achieve the highest efficiencies up to 90%. But when these collectors are used for warm domestic hot water preparation (water temperature 40 degrees Celsius higher than ambient air temperature) their efficiencies are usually lower than 20%.  In this case the best results are achieved by flat-plate collectors (with selective coatings) and evacuated tube collectors  which are best suited for this application. When higher water temperatures are needed (e.g. for space heating) evacuated -tube collectors are the best but also the most expensive.

Solar collector efficiencies for insolation typical for Central Europe at noon
during summer day - 800 W/m2. Efficiency at temperature difference (*)

 * Difference between ambient temperature and temperature of water inside solar collector.
** Values are related to lower insolation during early spring (400 W/m2).

Low efficiency of evacuated tube collector in low temperature region is caused by high optical losses on curved surface of the glass.
Bearing in mind that there are huge differences between prices of collectors it is obvious that the crucial criteria for collector type selection is purpose of its utilisation. A comparison of different collector types and their economy features are given in the table below.

Typical characteristics of different types of solar collectors according German ministry of economy  are following.

Guidelines on Solar Water Heating System Sizing
A solar water heating system can be used as the sole source for hot water or may include a back-up conventional system to meet heavy or unusual hot water requirements throughout the year. Systems are usually sized according to the number of rooms, people and household water needs. There are several different configurations of solar water heating systems. In general, however, there are two main types: active systems which have pumps and  controls to deliver solar heat to the storage tank, and passive systems like thermosiphons which utilise natural circulation of hot water.
When designing a solar water heating system, it is important to decide first how much hot water will be used per average day. If the amount of hot water is known, the size of system (collectors, storage tank) have to be calculated. Here are some general remarks on what should be taken into consideration when designing solar heating system.

Solar Collector
The main part of the solar heating system are the solar collectors. Most frequently used are flat-plate collectors consisting of an absorber where the solar radiation is transferred to heat in the solar collector fluid, insulation along the edge and under the absorber a case that holds everything together, and allows the necessary ventilation and a glass or plastic cover.
When glass is used as cover, it is important that the iron content is low or zero, so at least 95% of the solar radiation pass through the glass. In practice no more than single layer of glass is used. If a plastic cover is used, it is important that the plastic can stand up to the UV-rays from the sun. It has been found that polycarbonate plates are very satisfactory.
The absorber can be made of a plate with tubes where the collector fluid flows. Usually the absorber is made of copper or stainless steel. Experience have shown, that best absorber tubes are those made from copper. Ordinary steel tubes cause big problems with corrosion. It is essential that the absorber can stand up to the UV-light from the sun, and the stagnation temperature (dry-boiling temperature), which is 100-140 deg.C for solar collectors without selective coating, and 150-200 deg.C with selective coating.
Construction of a flat plate collector requires soldering and brazing of tubes and physically bonding the tubes to sheet. The more physical contact between the sheet and the tubes, the more heat transfer to the fluid moving through the tubes. The absorber is often covered by a selective black coating, which absorbs the sun rays, but holds back the heat radiation. The problem with normal black paint is that it will outgas, or boil off the metal under the extreme heat. Also, under normal cases, black paint will radiate heat, rather than absorb it for transfer to the fluid.
Many choices for the framework of solar collectors are reasonably available. Wood, plastic, steel or aluminium have all been used with varying degrees of success, but nothing is as good as aluminium. Aluminium weathers the elements with very low maintenance, and has colour choices baked on, so there is no need to paint the exterior of solar panel. Over the years, plastics have proven to be a poor choice for the major parts of a solar panel. For the exterior, plastic has a nasty habit of degrading from the sun's ultraviolet rays. Plastic discolours and eventually becomes brittle and cracks. Plastic also has a high coefficient of expansion. This means it expands and contracts so much that making the joints weather tight is difficult. Using steel for framework means also some problems. One is that the panels need painting regularly and two, they react chemically with the copper interior.
Solar collectors are usually mounted directly on top of the roof, or at a frame placed on a flat roof or the ground. Solar collectors can also be integrated in the roofing. In some cases  problems with sealing between the solar collector and the rest of the roof can arise.
The size of solar collectors depends on the daily hot water requirements. In general one person may require approx. up to 50 litres of hot water at approx. 55° to 60° degrees Celsius per day (for domestic bathing only, without laundry). It has been shown that in average 1-1,5 m2 solar collector area is needed per 50 litres daily consumption of hot water. Selection of size would also depend on availability of standard products. Prizes vary with the collector size and with the installation charges. Installation is simplest when the system is incorporated in the initial planning of the construction of a new house. This allows the architect to incorporate the collectors into the plan, both esthetically and economically.

SOLAR COLLECTOR ORIENTATION
The orientation of solar collectors (which way they face and how they are tilted) optimizes their collection ability. The earth's atmosphere absorbs and reflects a significant portion of solar radiation. Thus, the most energy that can be gathered on any given sunny day is at solar noon, when the direct beam radiation is least affected by the atmosphere. Solar noon is true south in the northern hemisphere. Although orienting the collectors to true south will normally maximize performance, a variation within 20° east or west is acceptable without additional collector surface area.
A solar collector that traces the sun, will usually receive about 20% more solar radiation than a south facing optimum placed collector. This additional output do not compensate the costs related to a construction, which has to trace the sun. Usually it will be cheaper to install a 20% larger solar collector.
Local weather patterns (i.e., morning haze or prevailing afternoon cloudiness) should also be considered in collector orientation. If local weather is not a factor and collectors cannot be faced true south, orienting them to the west is generally preferable due to higher afternoon temperatures (collectors have less heat loss with higher outside temperatures).
Since elevation of the sun varies throughout the year depending on local latitude, collectors should be tilted towards the sun depending upon application. In general, seasonal differences in irradiation are considerable and must be taken into account for all solar energy applications. Tilting the collecting surface some 30...50 degrees to the South in the Northern Hemisphere or to the North in the Southern Hemisphere yields somewhat better wintertime results for the region in question, but also some losses in summer. Space heating systems are tilted more to the position of the winter sun. In the tropics, a nearly horizontal receiving surface is generally most advantageous because of the sun's high altitude. The most desired angle of inclination to mount the solar collector is the local latitude. Positive difference between latitude and roof angle results better system performance in winter. Lower solar collector mounting angle than the local latitude will result in greater system performance in summer. Variations of solar collector tilt angle for architectural reasons can be compensated with additional collector size.

Storage Tank
The storage tank shall store the solar heat. This is done by storing hot water until it is needed. There are several different sizes of tanks available. All tanks must have connections for cold water inlet and hot water outlet as well as two connections for circulation pipes. Hot water storage tanks can easily be fitted to a stand. The most efficient is a vertical tank with good temperature stratification, so the cold inlet water aren't mixed with the warmer water at the top of the tank. A horizontal tank reduces the output by 10-20%.
The heat from the solar collectors is delivered to the water in a heat exchanger. As heat exchanger is mostly used a coil in the bottom of the tank, or a cap around the tank with collector fluid. In low-flow and self-circulating systems a cap are always used. In low-flow systems the solar collector fluid flows slowly down through the cap of the storage tank, which gives a stratification of collector fluid in the cap corresponding to the stratification in the tank. This gives more ideal heat transfer, and thereby a higher efficiency than in traditional systems.
All hot water storage tanks must be well insulated to keep the water hot during the night. Heat loss depends on many factors (ambient temperature, wind, season, etc.) and will be approximately 0,5 to 1 degree Celsius per hour during the night. The insulation of the tank must be so good, that hot water from a sunny day still is hot two days later. Especially the top must be well insulated, and without thermal bridges. Experience shows that a minimum thickness of insulation of 100 mm should be maintained.
It must be ensured that piping from the storage tank do not lead to self-circulation, which can drain the tank for hot water during periods without hot water consumption. If there is a flow tube pipe for the hot water, this must not be connected to the cold water; but has to enter at the upper part of the tank. Usually the outlet of the storage tank is equipped with a scalding protection, so the water delivered for use never gets warmer than e.g. 60 deg.C, regardless of the temperature in the tank.
The solar water collector storage tank should have a size of 80 litres of hot water storage volume per person with a hot water consumption of 50 litres per day. These are the average values. If the home have a dishwasher, washing machine, several children taking daily showers or baths during the day, so all of this water usage must be figured into the total water needs.

Solar Collector Circuit
The solar collector circuit connects the solar collector to the storage tank. The components of the circuit are:
a pump that ensures circulation (not needed in self-circulating systems). The pump is usually controlled by a difference thermostat, so it starts running, when the solar collector is a bit warmer than the storage tank. If the storage tank has a heat exchanger coil at the bottom, a more simple control system can be used; e.g. a light sensor, or a timer that starts the pump during day time.
pipelines connecting hot water storage tank and collectors. Layout of pipelines should secure to be of shortest possible distance. Pipes should not be exposed to the weather if possible. Best is to keep them inside the house where possible. It is important to have several separate pipes from the collector to the taps to reduce heat losses (smaller pipes) and to give a fast supply of hot water to the user, with a maximum delay of about 10 to 20 seconds. Pipelines must be produced of a non-corroding material. Systems with open expansion are most risky to get corrosion problems.
a one-way valve which prevents that the solar collector fluid runs backwards at night, and empties the storage tank for heat (not necessary in all kinds of installations).
an expansion tank; either an open container at the top of the installation, or a pressurised expansion tank that contains minimum 5% of the solar collector fluid.
overpressure protection (only in connection with pressurized expansion tank); must be a type that manage to let out the solar collector fluid, if the system is boiling. There must always be an accumulation tank to the fluid in case of boiling. This is normally a safety valve and a non-return valve (check), or a non-return valve and a vent pipe which will  release over-pressure due to the increase of volume by heating.
air outlets, automatic or simply screws; must be used at all height points in the system, as air pockets always will appear.
filling valve.
dirt filter for the pump, to remove dirt, e.g. from the installation (can be spared in some installations).
manometers and thermometers according to need.
the solar collector fluid must be able to stand frost, and must not be toxic.

Usually is used an approved liquid, consisting of water with 40% propylene glycol (can stand  minus 20 deg.C), and a substance that can be seen and tasted, if solar collector fluid leak to the tap water. Oil can also be used as collector fluid, but it is difficult to make a collector circuit with oil tight.

MAINTENANCE
The simplicity of solar water heating systems means that maintenance is minimal. Required maintenance will depend on type of system. Experience shows that once or twice a year it must be controlled, that there are enough fluid and pressure on the system. Once a year it should be checked that the solar collector fluid hasn't become acid. Acid indicator paper can be used. Acid fluid should be changed. In case the system is boiling, it is simply needed to fill new fluid on the system; as the old fluid may be damaged by the boiling.
An important consideration when designing a system is the freeze-protection requirements. Some storage tanks must be softened, and the anti-corrosion zinc block shall be changed after approximately  10 years, it prolongs the life span significantly.

GUIDELINES FOR SOLAR COLLECTOR SYSTEM SIZING
For a typical solar water collectors (heating from 8 to 45 deg.C) with selective absorbers, the following hand rules can be used:
in average 50 litres of hot water per person and day is needed.
1-1,5 m2 solar collector area is needed per 50 litres daily consumption of hot water.
the storage tank shall be 40-70 litres per m2 solar collector or 80 litres per head.
the heat exchanger in the storage tank shall be able to transfer 40-60 W/deg.C per m2 solar collector at 50 deg.C.

If these guidelines are followed, a typical solar water collector installed in Northern Europe will cover 60-70% of the annual hot water consumption, and be able to produce 350-500 kWh/m2 per year. For larger buildings (e.g. hotels, hospitals, apartment blocks), the collector areas and storage volumes required per head are smaller, but good dimensioning needs detailed analysis of demand and local climate conditions. The experience shows that solar systems for hot water preparation should be designed to be as simple as possible and not oversized.

Example
For a family with 4 persons which uses 200 litre of hot water each day solar collector with 6 m2 area are needed.  During the year they can produce up to 3000 kWh of clean energy. When solar collectors substitute the oil boiler than net saving can achieve at least 300 litres of oil annually.

THERMOSIPHON
Thermosiphons are solar water heating systems with natural circulation (i.e. by convection) which can be used in non-freezing areas. These systems are not the highest in overall efficiency but they do offer many advantages to the home builder. They are simple to make and most of these devices operate without the assistance of an electric pump. This thermosiphon circulation occurs because of the variation of water density with its temperature. With the heating of the water in the collector (usually flat-plate), the warm water rises, and since it is connected in a riser pipe to the hot water storage tank and a down-comer pipe again to the collector, it is replaced by the cooler, heavier cold water from the bottom of the hot water storage tank. It is therefore necessary to place the collectors below the hot water storage tank and to insulate both connecting circulation pipes.
Thermosiphon systems have serious problems with their collectors freezing and bursting, even in areas with only one or two mild freezes a year. It only takes one frozen night to ruin an unprotected collector. Some systems are designed to avoid freeze damage by using 10 centimetres or larger copper tubing in a double glazed, insulated enclosure. Quite simply, the volume of water in system is too large to freeze and burst in a mild freeze. This type of installations is popular in sub-tropical and tropical areas.

The complete thermosiphon circulation system may be divided into three separate sections:
The flat plate collector (absorber).
The circulation piping.
The hot water storage tank (boiler).

Usually solar collector is located on a lower story, porch, or shed roof so that the top of the panel is at least 50 centimetres  below the bottom of the storage tank. Tank location is usually in a second story, an attic, sometimes a cupola - somewhere that ensures an 50 cm vertical height difference between panel and the tank.

Solar Pool Heating
Solar pool heating system is a wise investment. In the USA the Department of Energy has identified swimming pools as a huge consumer of energy across the country, and has recognized pool heating as one of the most cost-effective means of reducing energy consumption. Solar pool heating systems are being used in virtually every area of the United States or Europe. Over 200 000 pools are heated by solar in the United States alone. The oldest systems have been in use for more than 25 years, and are cost-effective, highly reliable and require minimal maintenance. Important fact is that they function well and are cost-effective for the swimming season even in northern climates. Systems can also be designed for indoor pools as well as for larger municipal and commercial pools.

Despite the fact that price of installation varies on the size of the pool and other site-specific installation conditions if solar systems are installed in order to reduce or eliminate fuel or electricity consumption, they generally pay for themselves in energy savings in many countries in two to four years. Moreover solar pool heating can extend the swimming season by several weeks without additional cost.
Most homes can accommodate a solar pool heating system. These systems can be as simple as water running through a black hose. For outside pools, the only thing which is needed is the absorber portion of the solar collector. Inside pools need standard solar collectors to provide winter heating.
Although solar collectors are often installed on a roof, they can be installed wherever they can be exposed to the sun for a good portion of the day. The type of roof or roofing material is not important. The appropriate area of solar collectors required for a given swimming pool is directly related to the area of the pool itself. The proper ratio of pool area to solar collector area will vary according to such factors as location, the orientation of the solar collectors, the amount of shading on the pool or solar collectors, and the desired swimming season. In general, however, the area of solar collectors required is usually 50% to 100% of the pool surface area.

HOW DO SOLAR POOL HEATING SYSTEMS WORK?

PLACING THE SYSTEMS
Systems can quite easily be placed out of sight in a remote places, for example upon a suitable roof; however some basic design rules should be observed. The chosen site should be level or slightly sloping (less than 30 deg. to horizontal) with the return manifolds higher than the infeed manifolds and all hoses rising steadily from one to the other to ensure all air is expelled during operation.
Both a non-return valve and a vacuum release valve should be fitted to systems placed at more than 1 meter above pool level to prevent the reverse flow of water into the pool and the flattening of hoses when the collector drains at the end of each operating cycle. All connections into the pool filtration circuit must be made after the filter unit and, if applicable, before any existing conventional heater to avoid pressurising the solar system.

OPERATION AND MAINTENANCE
The simplicity of solar pool heating systems means that operation and maintenance requirements are minimal. In fact, in most cases no additional maintenance beyond normal filter cleaning and winter close-up is necessary. The system should be drained in the winter months; however, in some cases even this may not be necessary because the system drains itself. In addition, solar pool heating equipment is so reliable that many solar pool collector manufacturers provide warranty coverage for their products which far exceeds that of automobiles and household appliances.

SOLAR SPACE HEATING
So far only systems for warm water preparation have been described. An active solar heating plant can provide hot water, and additional heating via the central heating system at the same time. To get a reasonable output, the central heating temperature must be as low as possible (preferably around 50 deg.C), and there must be a storage for the space heating. A smart solution is to combine the solar heating installation with under-floor heating, where the floor function as heat storage.
Solar heating installations for space heating usually give less profit than hot-water installations, both according economy and energy, as heating is seldom needed during summer. But if heat is needed during summer (like in some mountain areas), then space heating installations is a good idea. In central Europe, some 20% of the total heat load of a traditional house, and close to 50% low energy house, could be supplied by an advanced active solar heating system employing water storage only. The remaining heat need to be drawn from auxiliary energy systems. To increase the solar fraction, would in practice require larger storage capacities.
For single houses, systems with well-insulated water tanks between 5-30 m³ have been constructed especially in Switzerland (so-called Jenni system) but the costs are too high and the storage is often unpractical. The solar fraction of a Jenni-system is >50% and may reach even 100%.
If all of the load in the above example were supplied by an up-to-date active solar heating system, a 25 m² collector area and 85 m³ storage water tank with 100 cm insulation around would be needed. Improving the energy storage capacity of the storage unit, would dramatically improve the practical possibilities for storage.
Although individual solar space heating is technically feasible, it is likely that it would be far more cost effective to invest in insulation to cut space heating demands.

SEASONAL STORAGE
If a far larger collector together with a much larger storage tank were fitted, solar energy should be able to supply energy for several houses. Basic problem with solar energy is related to the fact that most of the energy is needed during the winter when solar insolation is the lowest and on the other side much of summer potential output can not be used because the demand is mostly not there. So capital investment into larger collectors with larger gains would be wasted.
Despite this fact there are several installations using summer heat produced by solar collectors and saved through to the winter. These installations are using large storage tanks (seasonal storage). Problem is that the volume of hot water storage needed to supply a house is almost the same size as the house itself. In addition, the tank would need to be better insulated. A normal domestic hot water cylinder would require insulation of 4 metres thick to retain most of its heat from summer to winter. It therefore pays to make  storage volume really  enormous. This reduces the ratio of surface area to volume.
Large solar heating plants for district heating are now in use, e.g. in Denmark, Sweden, Switzerland, France or USA. Solar modules are mostly installed directly at the ground in larger fields. Without a storage such solar heating installation would cover approximately 5% of the annual heat demand, as the plant never must produce more than the minimum heat consumption, including loss in the district heating system (by 20% transmission loss). If there is a day-to-night storage, then the solar heating installation can cover 10-12% of the heat demand including transmission loss, and with a seasonal storage up to 100%. There is also a possibility to combine district heating with individual solar water collectors. Then the district heating system can be closed during summer, when the sun provides hot water, and there is no need for space heating.

PRESENT SOLAR STORAGE SYSTEMS
Large-size seasonal storage systems for communities have been demonstrated in several countries but are still too expensive. The size of a central storage system may range from a few thousand m³ up to a few 100 000 m³. The largest storage project in Europe is in Oulu, Finland where a large rock cavern heat storage of 200 000 m³ will be connected to a combined heat and power plant burning biomass. This district heating plant was built under the EU-Thermie programme.
Another successful project with seasonal storage of hot water has been constructed in Lyckebo, Sweden. This project is using a rock cavern filled with water (volume of 105 000 m3) and flat plate solar collectors with area of 28 800 m2 which supply 100% energy (8500 MWh/a) for space and water heating of 550 dwellings. All houses are connected to communal district heating system. The temperature of supply water is 70 degrees Celsius and the temperature of return water is 55 degrees.
The pay-back times of such installations are very long. The important lesson from space heating systems has been that it is essential to invest in energy conservation and passive solar design first and then to use solar energy to help  supply the remaining reduced load.

COMBINING SOLAR WITH OTHER RENEWABLE SOURCES
Combining renewable energy sources such as solar heat with solar storage in form of biomass may be a good solution. Or, if the remaining load of a low energy house is very low, some liquid or gaseous biofuels with advanced burners together with solar heating may be used.
Solar heating together with solid biomass boilers may provide interesting synergy and also solution to the seasonal storage of solar energy. Using biomass in the summer may be non-optimal, as the boiler efficiencies at partial loads are low and also relative piping losses may be high - in smaller systems using wood in the summer may even be uncomfortable. Solar heating may well provide 100% of the summertime loads in such cases. In the winter, when the solar yield is negligible, the biomass options provides almost all of the heat needed.
Experiences notably from central Europe with solar heating and biomass together are positive. Some 20-30% of the total load is typically provided by solar heating and the main load, i.e. 70-80% of the total load, by biomass. Combined solar heat and biomass may be used for both single-family houses and for district heating. For central European conditions, around 10 m³ of biomass (e.g. wood) would be enough for a single-family house with solar heating system replacing well up to 3 m³ per year in a household.

Solar Thermal Commercial Water Heating
Many businesses use solar water heating to preheat the water before using another method to heat it to boiling or for steam. Being less dependent on fluctuating fuel prices is another factor that makes solar system a wise investment. In many cases installation of solar water heating will derive an immediate and significant savings in energy costs. Depending on the volume of hot water needed and the local climate a business can realize savings of 40 - 80% on electric or fuel bills. For example the 24-story Kook Jae office building in Seoul, South Korea meets over 85% of its daily hot water needs with a solar hot water heating system. The system has been in operation since 1984 and is so efficient that it has exceeded it's design specifications and even provides 10 to 20 percent of the annual space heating requirement.

Solar heating at Kook Jae building.

There are several different configurations of solar water heating systems. In general, however, the amount of hot water that a commercial business demands requires an active system. Active systems typically consist of solar collectors on a south-facing roof (in Northern hemisphere), and a storage tank near the existing water collector. When sufficient heat is present in the solar panel, a “controller” turns on a pump which begins circulating fluid, either water or antifreeze, through the solar panel. The fluid picks up the heat from the collector and transfers the heat to the potable water supply which is stored in a tank until needed. If the solar-heated water is not at the desired temperature, a back-up energy source can be used to bring the water temperature up to the desired level. The type and size of a system is calculated by determining ‘ water-heating load similar to the way described in chapter on solar collector sizing for households (see above). Similarly required maintenance for commercial systems will depend on the type and size of system, but the simplicity of solar water heating systems means that maintenance is minimal.
While for many businesses the biggest advantage of a solar water collector is the resulting savings in utility bills, value must be placed on the substantial environmental benefit. Air pollutants, such as sulphur dioxides, carbon monoxide and nitrous oxides are also displaced when a business owner decides to tap into a cleaner source of energy - the sun.

Industrial Process Heat
Industry requires heat in a variety of temperature ranges, depending on the process at hand. Many of these processes can be served by collectors ranging from the flat-plate variety, which are restricted to temperatures below 100 degrees C, to concentrating collectors which can produce temperatures of several hundred degrees.

SOLAR COOLING
The world demand of energy for air-conditioning and cooling is still increasing. This is not only due to an increasing wish for comfort in highly industrialized countries but also follows the necessity of e.g. food storage and medical applications in hot climates especially third world countries.
Today there are mainly three techniques available for active cooling. First of all the compression machine driven by electricity which is today the standard cooling device in Europe. On the other hand there is the absorption cooling machine using heat as driving force. Both compression and absorption machines are able to provide air conditioning, i.e. chilled water at about 5°C, and refrigeration, i.e. temperatures below 0°C. There is a third possibility which is desiccant and evaporative cooling used for air conditioning. All systems can be driven by solar energy and in addition have the advantage of using absolute harmless working fluids like simple water, solutions of certain salts in water or ammonia. Possible applications of this technology are not only air-conditioning but also refrigeration (food storage etc.).
The vast use of present compression cooling machines is also responsible for an increasing peak demand of electrical power in summer which reaches already the capacity limit in some southern countries. Because most of the electrical power stems from fossil fired power plants this also increases the production of CO2 which is no longer acceptable. A more innovative approach is to use solar energy from thermal collectors as driving force for air-conditioning systems. This idea is very promising in the sense that to some extent the demanded cooling power is correlated with the incident solar radiation intensity which also delivers the driving force.
In principle compression cooling machine can be driven by solar energy i.e. by electricity from photovoltaic panels but we will restrict to sorption cooling machines using heat from a thermal solar collector due to the advantage of using environmental harmless refrigerants and the higher market penetration of thermal solar collectors. A higher market penetration is also found for absorption cooling machines compared to desiccant cooling systems. Moreover absorption machines can also be used as retrofit in standard air conditioning systems using chilled water.  Solar collectors are used for vaporization heat in absorption machine.
In Kuwait, where air conditioning is essential for summer cooling in residential, commercial and public buildings, the use of solar for air conditioning has received serious attention during the seventies and eighties. Development has primarily focused on modifying conventional steam-fired cooling systems for use with solar-heated water at temperatures below 100°C. Some attention has also been paid to using photovoltaic systems to generate the electricity needed to operate a conventional vapour compression air conditioning unit.

SOLAR DRYING
A solar collector that heats air, can be used as a cheap heat source for drying crops like corn, fruit or vegetable. Since solar air collectors can efficiently increase the ambient air temperature by 5 to 10 degrees Celsius (some sophisticated devices by even more), it can also be used effectively for air conditioning in warehouses.
The use of simple and low cost solar air collectors for heating the drying air of crop dryers offers a promising alternative to reduce the tremendous post harvest losses in developing countries. The lack of adequate storage and preservation facilities in the developing countries result in considerable food losses. Although reliable estimate of the magnitude of the post harvest losses in these countries is not possible, some references indicates estimates of about 50 to 60%. To avoid such losses, growers usually sell of their produce immediately after harvest at low prices. Reduction in these losses through the processing of fresh products into dried products would be of great significant to growers and consumers alike. In several developing countries, open air sun drying is the widely practiced method of food preservation. This involves spreading the fresh material on the ground, on rocks, along the roadside, or on the roofs. The advantage of this method lies in its simplicity and cheapness. However, the quality of the final product is low due to long drying time, contamination by dirt and dust, infestation by insects and degradation by overheating. Furthermore, drying to a low moisture content is difficult resulting in spoilage during subsequent storage. The introduction of solar dryers is an appropriate technology that can help to improve the quality of the dried products and to reduce the wastage.
Various types of small scale solar dryers were developed for drying small amounts of agricultural products in developing countries. In the natural convection dryers, the solar air heater is either incorporated into the dryer, or the air heater is connected to a cabinet or chamber dryer. The solar air-collector may consist of a black mat covered by a plastic plate. The air is drawn through the mat, where it is heated, and thereafter blown through the crops. These dryers can be used both in arid and humid regions for drying fruits, vegetables and spices. Due to their enlarged capacity they are mainly used on larger farms or by cooperatives for producing high quality products. Integrating the solar air heater into the south oriented roof of the barn is common system used in industrialized countries for drying hay.
Solar dryers are usually classified according to the mode of air flow into natural convection and forced convection dryers. Natural convection dryers do not require a fan to pump the air through the dryer. The low air flow rate and the long drying time, however, result in low capacity and product quality. Thus, this system is restricted to the processing of small quantities agricultural surplus for family consumption. Where large quantities of fresh produce are to be processed for the commercial market, forced convection dryers should be used. One fundamental disadvantage of forced convection dryers lies in their requirement of electrical power to run the fan. Since the rural or remote areas of many developing countries are not connected to the national electric grid, the use of these dryers is limited to electrified urban areas. Even in the urban locales with grid-connected electricity, the service is unreliable. In view of the prevailing economic difficulties in most of these countries, this situation is not expected to change in the foreseenable future. The application of photovoltaic to generate the electricity required by the fan could boost the dissemination of solar dryers in the developing countries.
In developed countries the solar air heater usually consists of a black absorber foil, a transparent plastic foil where the air is forced by a fan between the space. To enlarge the collector area, the roof is extended southward to the ground and the whole roof is used as collector. The solar greenhouse dryer is used for drying medicinal and aromatic plants on large farms. By using a photovoltaic driven blower, it can be secured that only when the sun shines, air is blown in. Such installations are commonly used in summer cottages in Denmark and Sweden, where they keep the houses dry most of the year.
While solar drying has many advantages over sun drying, lack of control over the weather is the main problem with both methods. In many regions weather is not suitable for sun or solar drying because there are few consecutive days of high temperatures and low humidity. It is likely that the food will sour or mold before drying is completed.

SOLAR COOKING
Successful solar cookers were first reported in Europe and India as early as the 18th century. Solar cookers and ovens, absorb solar energy and convert it to heat, which is  captured inside an enclosed area. This absorbed heat is used for cooking or baking various kinds of food. In solar cookers temperatures as high as 200 degrees Celsius can be achieved.
Solar cookers come in may shapes and sizes. For example there are: box ovens, concentrating-type or reflector cookers, solar steam cookers etc. This list could go on forever. Designs vary, but all cookers trap heat in some form of insulated compartment. In most of these designs the sun actually strikes the food.

BOX-TYPE SOLAR COOKERS
Box-type solar cookers consist of a well-insulated box with a black interior, into which black pots containing food are placed. The cover of the box usually comprises a two-pane “window” that lets solar radiation enter the box but keeps the heat from escaping. This in addition to a lid with a mirror on the inside that can be adjusted to intensify the incident radiation when it is open and improve the box's insulation when it is closed.

The main advantages of box-type solar cookers are:
They make use of both direct and diffuse solar radiation.
Several vessels can be heated at once.
They are light and portable.
They are easy to handle and operate.
They needn't track the sun.
The moderate temperatures make stirring unnecessary.
The food can be kept warm until evening.
The boxes are easy to make and repair using locally available materials.
They are relatively inexpensive (compared to other types of solar cookers).

There are some disadvantages too:
Cooking must be limited to the daylight hours.
The moderate temperatures make for long cooking times.
The glass cover causes considerable heat losses.
Such cookers cannot be used for frying or grilling.

Thanks to their simple construction, relatively low cost, uncomplicated handling and easy operation, solar cooking boxes are the most widely used type of solar cooker. There are all sorts of box-type solar cookers: mass-produced, hand-crafted, do-it-yourself types etc. with shapes resembling a suitcase or a wide, low box, and stationary types made of clay, with a horizontal lid for tropical and subtropical areas or an inclined lid for more temperate regions. Standard models with aperture areas of about 0,25 m2 are the rule for a family of five, and larger versions measuring 1 m2 and more are available on the market.

GUIDELINES FOR CONSTRUCTION
Since the heat absorbed by the inner box needs to be conducted to the area beneath the cooking pots, the best choice of material is aluminium, because it is a very good heat conductor. Additionally, aluminium is good for reasons of corrosion prevention, i.e. iron sheet boxes, even galvanized ones, could not stand up indefinitely to the hot, humid conditions that are created inside during the cooking process. Sheet copper is prohibitively expensive.
No metal parts should placed to the outside around the top rim of the inner box: thermal bridges must be avoided. The insulation may consist of glass, rock wool or some natural material like residue from the processing of peanuts, coconuts, rice, corn, etc. Whatever kind of material is used, it must be kept dry.
The cover could consist of one or two panes of glass with a layer of air between them. The pane-to-pane clearance usually amounts to 10...20 mm. Recent experiments have shown that a honeycomb structure of transparent material that divides the inner space into small vertical compartments can substantially reduce the cooker's heat losses, thus increasing its efficiency accordingly. The inside cover pane is exposed to substantial amounts of thermal stress, for which reason tempered (safety) glass is frequently used; otherwise, both panes may consist of normal window glass with a thickness of about 3 mm.

The outer cover, or lid, of the solar cooking box always serves as a reflector to amplify the incident radiation. The reflecting surface may consist of an ordinary glass mirror (heavy, expensive, fragile, but easily obtainable anywhere), plastic sheet with a reflecting coating (Mylar, Tedlar, etc.; cheap, but not very durable and hard to find), or a metal mirror (unbreakable). In an emergency, even foil from empty cigarette packs will do the job.
The outer box of the solar cooker may be made of wood, glass-reinforced plastic (GRP) or metal. GRP is light, inexpensive and fairly weather-resistant, but not necessarily stable enough for continuous use. Wood is more stable, but also heavier and less weather-resistant. A metal case aluminium with wooden bracing offers the best finish and is adequately stable with regard to mechanical impact and the effects of weather. An aluminium-clad wooden box is the most stable of all, but it is expensive and time-consuming to make, in addition to being heavy.
The capacity of a normal box-type solar cooker with a 0.25 m2 area of incidence (aperture) amounts about 4 kg ready-to-eat food, or enough to feed a family of five.

The inside of a solar cooking box can reach a peak temperature of over 150 deg.C on a sunny day in the tropics; that amounts to a thermal head of 120 deg.C, referred to the ambient temperature. Since the water content of food does not heat up beyond 100 deg. C, a loaded solar cooker will always show an accordingly lower inside temperature. The temperature inside of the solar cooker drops off sharply when the vessels are placed inside it. Also important is the fact that the temperature remains well below 100 deg.C for the greater part of the cooking time. Nevertheless the boiling temperature of 100 deg. C is not necessary for most vegetables and cereals.
The average achievable cooking times in box-type solar cookers amount to somewhere between 1 and 3 hours for good insolation and a reasonable fill volume. Thin-walled aluminium vessels yield much shorter cooking times than stainless steel pots.

The time taken for cooking is also influenced by the following factors:
The cooking time is shortened by strong insolation and viceversa
High ambient temperatures shorten the cooking time, and viceversa
Small volumes (shallow fill) in the pot make for shorter cooking times, and vice versa.

REFLECTOR COOKERS
The most elementary kind of reflector cooker is one that consists of (more or less) parabolic reflectors and a holder for the cooking pot situated at the cooker's focal spot. If the cooker is properly aligned with the sun, the solar energy bounces off of the reflectors such that it all meets at the focal spot, thus heating the pot. The reflector can be a rigid axial paraboloid, made for example from sheet metal or from a reflecting foil. The reflecting surface is usually made of treated aluminium or a mirror-finish metal or plastic sheet, but it may also consist of numerous little flat mirrors cemented onto the inside of the paraboloid. Depending on the desired focal length, the reflector may have the shape of a deep bowl that completely “swallows” the pot (short focal length, pot shielded from the wind) or that of a shallow plate with the cooking pot mounted in the focal point a certain distance above or in front of it.
All reflector cookers exploit only direct insolation and must track the sun at all times. The tracking requirement makes them somewhat complicated to handle, depending on the nature and stability of the stand and adjusting mechanism.

The advantages of reflector cookers include:

The ability to achieve high temperatures and accordingly short cooking times.
Relatively inexpensive versions are possible.
Some of them can also be used for baking.

The above mentioned merits stand in contrast to the following drawbacks, some of which are quite serious:
Depending on its focal length, the cooker must be realigned with the sun every 15 minutes or so.
Only direct insolation is exploited, i.e. diffuse radiation goes unused.
Even scattered clouds can cause high heat losses.
The handling and operation of such cookers is not easy; it requires practice, a good grasp of the working principle.
The reflected radiation is blinding, and there is danger of injury by burning when manipulating the pot in the cooker's focal spot.
Cooking is restricted to the daylight hours.
The cook must stand out in the hot sun (single exception: fixed-focus cookers).
The efficiency is heavily dependent on the momentary wind conditions.
Any food cooked around noon or in the afternoon gets cold by evening.

Particularly the cooker's complicated handling, in combination with  the fact that the cook has to stand out in the sun, is a major impediment with regard to the acceptance of reflector cookers. But in China, where the food demands high cooking power and temperature, eccentric axis reflector cookers have been disseminated and accepted in a large number.

THERMAL OUTPUT
The thermal output of a solar cooker is determined by the insolation level, the cooker's effective collecting area (usually between 0.25 m2 and 2 m2), and its thermal efficiency (usually between 20% and 50%). Table below compares some typical area, efficiency and cooking-power values for a box-type solar cooker and a reflector.

Standard values for area, efficiency and power output of reflector cookers and cooking boxes

As a rule, reflector cookers have a much larger collecting area than do cooking boxes. Consequently, they are able to generate a much higher power output, meaning that they can boil more water, cook more food, or process comparable amounts in less time. On the other hand, their thermal efficiency is lower, because the cooking pot is completely exposed to the cooling effects of the surrounding atmosphere.
In many tropical and subtropical countries, one can count on clear skies and normal daily insolation patterns for most of the year. At about midday, when the global radiation reaches up to 1000 W/m2 , the thermal output levels (50 to 350 W, depending on the type and size of the cooker) may be regarded as quite realistic. The insolation is naturally lower during the morning and afternoon hours and cannot be fully compensated for by solar tracking.
By way of comparison: burning 1 kg of dry wood in one hour yields approximately 5000 W times the thermal efficiency of the cooking facility (15% for a three-stone hearth and 25-30% for an improved cookstove used in developing countries). The thermal power actually reaching the cooking pot therefore amounts to between 750 and 1500 W.
Insolation drops off sharply under cloud and during the rainy season. The lack of direct radiation leaves reflector cookers without the slightest chance, and cooking boxes can do little more than keep prepared food warm. The weak point of solar cooking is that no matter what kind of device is used: on cloudy and rainy days (up to between 2 and 4 months per year in most Third World countries) cooking has to be done according to conventional methods, e.g. over a wood/dung fire or on a gas/kerosene-fuelled cooker.

SOLAR RADIATION
The first and foremost prerequisite for success in a solar cooker application is adequate insolation, with only infrequent interruptions during the day and/or the year. The duration and intensity of solar radiation must suffice to allow the use of a solar cooker for prolonged, worthwhile regular periods. While cooking with solar energy is possible in Central Europe on a sunny summer day, a minimum irradiation of 1500 kWh/m2 per year (corresponding to a mean daily insolation of 4 kWh/m2 per day) should be available for any solar cooker. But these annual data can sometimes be misleading. The essential condition for solar cooking is a reliable “summer weather”, i.e. essentially predictable sequences of regular cloudless days.
Supply of solar energy varies substantially from country to country, even within the Third World's tropical belt. Thus, local data must be referred to - and they are not always available. Some examples: In India solar radiation in most regions is good to very good for purposes of solar energy exploitation. The yearly averages of daily annual global radiation range from 5 to 7 kWh/m2 per day, depending on the region. In most places, the insolation reaches its minimum during the monsoon season and is nearly as weak again during the months of December and January.
In Kenya's climate and insolation potential are favourable to the use of solar cookers. Kenya is close to the equator and therefore has a purely tropical climate. In Nairobi, the daily irradiation alternates between 3.5 kWh/m2 per day in July and 6.5 kWh/m2 per day  in February, but it remains practically uniform (6.0...6.5 kWh/m2 per day) in other regions of Kenya like Lodwar. Solar irradiation in Nairobi is adequate for cooking with solar energy nine months a year (excluding June through August). On the other hand, conventional cooking facilities must be relied on for cloudy or hazy days. In the Lodwar area, though, solar cookers can be used year-round.

SOLAR COOKERS FOR DEVELOPING COUNTRIES
The purpose of solar cookers, of course, is to save energy in the face of a double energy crisis: the poor people's energy crisis is the increasing scarcity of firewood, and the nation's energy crisis is the growing pressure on its balance of payments. Solar cooker should be judged with that in mind.
Compared to other nations, developing countries consume very little energy. For example, India's 1982 per capita energy consumption rate, at 7325 GJ, was one of the world's lowest. But the country's energy consumption rate is increasing nearly twice as fast as its gross national product. The same is true for the  most developing countries.
The poor majority of the people in developing countries cover most of their energy requirement in a non-commercial way, using traditional, locally available sources of energy and their own physical labour. They simply cannot afford to buy any appreciable amounts of commercial energy.
The logical consequence is a relative shortage of fuel for use by the poor, whose living conditions deteriorate even more as a result. Solar cookers could at least try to compensate.
If the “poor” majority of the Third World's people is the target group, then solar cookers must be first and foremost to the benefit of the rural population.

COOKING-ENERGY QUANTITIES
The daily fuel requirement varies according to the kind of food being cooked and the number of warm meals. In the typical developing country, each native burns one ton of firewood each year.
In India, the average family needs somewhere between 3 and 7 kg of wood per day; in the cooler regions, the daily firewood demand varies between just under 20 kg in the winter and 14 kg in the summer. In the southern part of Mali, the average 15-member (!) family burns about 15 kg of wood each day. A survey conducted in an Afghan refugee camp in Pakistan showed a daily firewood demand of up to 10 kg per family and day. More than half of the wood used in the average household goes for baking, and the remainder is used for cooking. Additional wood is needed for heating in the wintertime, of course.
Despite the fact that above examples indicate that the required amounts of cooking energy are extremely variable much cooking energy can be saved by using solar cookers.
The prime function of solar cookers is to help reduce firewood consumption, since most cooking fires are still fuelled with firewood. The trouble is, firewood is usually quite inexpensive in comparison with kerosene, bottled gas or electricity (based on relative energy content).Increasing, uncontrolled felling of wood for people's own use and for selling are a main cause of deforestation, desertification, erosion, receding groundwater levels and it has long-term adverse effects on the ecological balance. Pakistan's meager forest heritage and rampant deforestation in Kenya show that such fears are well-founded. If denudation of the Sudan's forests continues at the present rate, they will be gone by the year 2005.
For most solar cookers, little data is available on the actual cost of production. Since most of those solar cookers are prototypes that do not yet display the technical maturity needed for series production, pertinent information is of low indicative value. Due to the chronic shortage of foreign currency in the Third World, preference should be given to cookers that can be made locally using indigenous materials.
The problem is that practically any amount of money paid for solar cooker, however small, would still be too expensive for most rural households as long as firewood can be gathered for free and the farmers earn very little money.
On the whole, solar cookers could, at best contribute little toward a national energy policy. But they could make a very substantial contribution toward improving the living conditions of the poor and helping them overcome their own energy crisis.

SOLAR WATER DISTILLATION
Many people throughout the world do not have access to clean water. Of the 2.4 billion people in developing countries, less than 500 million have access to safe drinking water, let alone distilled water. The answer to these problems is a solar still. A solar still is a simple device that can convert saline, brackish, or polluted water into distilled water. The principles of solar distillation have been around for centuries. In the fourth century B.C., Aristotle suggested a method of evaporating sea water to produce potable water. However, the first solar still was not produced until 1874, when J. Harding and C. Wilson built a still in Chile to provide fresh water to a nitrate mining community. This 4700 m2 still produced 24000 litres of water per day. Currently there are large still installations in Australia, Greece, Spain and Tunisia, and on Petit St. Vincent Island in the Caribbean. Smaller stills are commonly used in other countries.
Practically any seacoast and many desert areas can be made inhabitable by using sunshine to pump and purify water. Solar energy does the pumping (see chapter on  photovoltaics), purification, and controls seawater feed to the stills.

SOLAR STILL BASICS
The most common still in use is the single basin solar still. The still consists of an air tight basin that holds the polluted or salt water, covered by a sloped sheet of glass or plastic. The bottom of the basin is black to help absorb the solar radiation. The cover allows the radiation to enter the still and evaporate the water. The water then condenses on the under side of the cover (which is cooled by the outside air), and runs down the sloped cover into a trough or tube. The tube is also inclined so that the collected water flows out of the still.
The process is exactly Mother Nature's method of getting fresh water into the clouds from oceans, lakes, swamps, etc. All the water we have ever consumed has already been solar distilled a several thousand times around the hydrologic cycle.

SOLAR STILL PERFORMANCE
Operation of the still requires no routine maintenance and has no routine operating costs. The rated production of the still is an estimated annual average and is not exact, as the amount of sunshine can vary widely. Stills produce more in hot climates than in cold ones, more at low latitudes than high, and more in summer than in winter. At the 23° North latitude of the central Bahamas, the estimated average production of the installation was 12 times higher in June than in mid-winter. In higher latitudes, addition of a mirror to the rear of each still increases winter production. Some stills also functions in freezing climates. In general solar still can produce 1 litre of distilled of water a day per square meter of still. On very sunny days over one litre of water can be gained. The still is usually filled once daily, at night or in the morning.

STILL COSTS
The cost of a solar distillation system will vary widely, due to size and site-specific circumstances. The stills are usually inexpensive to build. Some small models designed in the USA cost 25 USD with glass or 18 USD with plastic (the amount of water produced is smaller). If the stills are used for one year, they will produce water at approximately 10 cents per litre.

WATER QUALITY
The distilled water produced is of very high quality, normally better than that sold in bottles as distilled water. It routinely tests lower than one part per million total dissolved solids. It is also aerated, as it condenses in the presence of air inside the still. The water may taste a little strange at first because distilled water does not have any of the minerals which most people are accustomed to drinking. Tests have shown that the stills eliminated all bacteria, and that the incidence of pesticides, fertilizers and solvents is reduced by 75–99,5%. This is of great importance for many countries where cholera and other water borne diseases are killing people daily.

DESIGNING SOLAR STILL
There are a few things to keep in mind when designing the solar still:
The tank can be made of cement, adobe, plastic, tile, or any other water resistant material.
If plastic is used to line the bottom of the still or for the condensate trough, make sure the tank never remains dry. This could melt the plastic.
Insulation should be used if possible. Even a small amount will greatly increase the efficiency of the still.
The container holding the distilled water should be protected from solar radiation to avoid re-evaporation.

SOLAR THERMAL POWER PRODUCTION
In addition to using the warmth of the sun directly, it is possible (in areas with high level of solar radiation) to use the heat to make steam to drive a turbine and produce electricity. If undertaken on a large scale, solar thermal electricity is very cost-competitive. The first commercial applications of this technology appeared in the early 1980's, and the industry grew very rapidly. Today, utilities in the U.S. have installed more than 400 megawatts of solar thermal generating capacity, providing electricity to 350,000 people and displace the equivalent of 2,3 million barrels of oil annually. Nine plants in California's Mojave Desert are generating 354 MWe of solar electric capacity, and have accumulated 100 plant-years of commercial operating experience.  The technology is maturing to the point where officials say it can compete directly with conventional power technologies in many regions of USA. A number of opportunities for solar thermal projects may open soon in other regions of the world. India, Egypt, Morocco, and Mexico have active programs that will receive grants from the Global Environment Facility, and independent power producers are designing power projects in Greece, Spain, and the US.
According to the way how the heat is produced solar thermal power plants can be divided between solar concentrators (mirrors) and solar ponds.

SOLAR CONCENTRATORS
Solar thermal electric power plants generate heat by using lenses and reflectors to concentrate the sun's energy. Because the heat can be stored, these plants can generate power when it is needed, day or night, rain or shine.
Large mirrors - of the point focusing type or the line focusing variety - can concentrate solar beams to such an extent that water can be converted to steam with enough power to drive a generating turbine. Enormous fields of such mirrors have been constructed by Luz Corp. in the Californian desert, for the production of 354 MW of electric power. Such systems can convert solar to electric power with an efficiency of about 15%.
All solar thermal technologies except solar ponds achieve high temperatures by utilizing solar concentrators to reflect sunlight from a large area to a smaller receiver area. A typical system consists of the concentrator, receiver, heat transfer, storage system and a delivery system.
The sun's heat can be collected in a variety of different ways. Today‘s technology includes solar parabolic troughs, solar parabolic dish and power towers. Because these technologies involve a thermal intermediary, they can be readily hybridized with fossil fuel and in some cases adapted to utilize thermal storage. The primary advantage of hybridization and thermal storage is that the technologies can provide dispatchable power (dispatchability means that power production can be shifted to the period when it is needed) and operate during periods when solar energy is not available. Hybridization and thermal storage can enhance the economic value of the electricity produced and reduce its average cost.

Solar Parabolic Troughs

Nine trough systems, built in the mid to late 1980's by Luz International set up electricity-generating plants in the southern California desert with a total installed capacity of 354 MW, making parabolic troughs the largest solar thermal electric generating producers to date. These plants supply electricity to the Southern California Edison utility grid. In 1984 Luz International installed Solar Electric Generating System I (SEGS I) in Daggett, California. It has an electricity capacity of 13,8 MW. Oil is heated in the receiver tubes to 343°C to produce steam for electricity generation. SEGS I contains six hours of thermal storage, and uses natural gas-fueled super heaters to supplement the solar energy when solar energy is not available. Luz also constructed additional plants, SEGS II through VII, with 30 MW capacity each. In 1990, Luz completed construction of SEGS VIII and IX in Harper Lake, each with 80 MW capacity. As a result of numerous regulatory and policy obstacles, Luz International and four subsidiaries filed for bankruptcy on November 25, 1991. Three companies now operate and maintain SEGS I - IX under the same contract that Luz International had negotiated with Southern California Edison. Plans to construct SEGS X, XI, and XII were canceled, eliminating 240 MW of additional planned capacity.

Solar Parabolic Dish/engine 

One such application was the STEP project in the state of Georgia (USA). The Solar Total Energy Project (STEP) was a large solar parabolic dish system that operated between 1982 and 1989 in Shenandoah, Georgia. It consisted of 114 dishes, each 7 meters in diameter. The system furnished high-pressure steam for electricity generation, medium-pressure steam for knitwear pressing, and low-pressure steam to run the air conditioning system for a nearby knitwear factory. In October 1989, Georgia Power shut down the facility due to the failure of its main turbine, and lack of funds for necessary plant repairs.

A cooperative venture between Sandia National Lab and Cummins Power Generation is  recently attempting to commercialize 7.5 kilowatt (kW) dish/engine systems. The systems are out of the component stage and into the validation stage. When they accumulate sufficient running time, they will be ready for the marketplace. Cummins hopes to sell 10,000 units a year by 2004. Other companies are also entering into parabolic dish/Stirling technology. Stirling Technology, Stirling Thermal Motors, and Detroit Diesel have teamed up with Science Applications International Corporation in a $36 million joint venture with the Department of Energy, to develop a 25 kW membrane dish/Stirling system.
The National Renewable Energy Laboratory (NREL) and the Cummins Engine Company are testing two new receivers for dish/engine solar thermal power systems: the pool-boiler receiver and the heat-pipe receiver. The pool-boiler receiver operates like a double boiler on a stove. It boils a liquid metal and transfers the heat energy to an engine on top. The heat-pipe receiver also uses a liquid metal, but instead of pooling the liquid, it uses a wick to transfer the molten liquid to a dome receiver.

Solar Central Receivers or Power Towers

The first power tower “Solar One” built near Barstow in Southern California, successfully demonstrated this technology for electricity generation. This facility operated in the mid-1980's, used a water/steam system to generate 10 MW of power. In 1992, a consortium of U.S. utilities decided to retrofit Solar One to demonstrate a molten-salt receiver and thermal storage system. The addition of this thermal storage capability makes power towers unique among solar technologies by promising dispatchable power at load factors of up to 65%. In this system, molten-salt is pumped from a “cold” tank at 288 deg.C and cycled through the receiver where it is heated to 565 deg.C and returned to a “hot” tank. The hot salt can then be used to generate electricity when needed. Current designs allow storage ranging from 3 to 13 hours.

Technology Comparison

Characteristics of solar thermal electric power systems (as of 1993).

(p) = predicted; (d) = demonstrated;

Comparison of Major Solar Thermal Technologies.

Solar Thermal Power Cost and Development Issues
The cost of electricity from solar thermal power systems depends on a multitude of factors. These factors include capital and operating and maintenance cost, and system performance. However, it is important to note that the technology cost and the eventual cost of electricity generated is significantly influenced by factors “external” to the technology itself. As an example, for troughs and power towers, small stand-alone projects will be very expensive. In order to reduce the technology costs to compete with current fossil technologies, it will be necessary to scale-up projects to larger plant sizes and to develop solar power parks where multiple projects are built at the same site in a time phased succession. In addition, since these technologies in essence replace conventional fuel with capital equipment, the cost of capital and taxation issues related to capital intensive technologies will have a strong effect on their competitiveness.

COST VERSUS VALUE
Through the use of thermal storage and hybridization, solar thermal electric technologies can provide a firm and dispatchable source of power. Firm implies that the power source has a high reliability and will be able to produce power when the utility needs it. As a result, firm dispatchable power is of value to a utility because it offsets the utility's need to build and operate new power plants. Dispatchability implies that power production can be shifted to the period when it is needed.  This means that even though a solar thermal plant might cost more, it can have a higher value.

BENEFITS
Solar thermal power plants create two and one-half times as many skilled, high paying jobs as do conventional power plants that use fossil fuels.
California Energy Commission study shows that even with existing tax credits, a solar thermal electric plant pays about 1,7 times more in federal, state, and local taxes than an equivalent natural gas combined cycle plant. If the plants paid the same level of taxes, their cost of electricity would be roughly the same.

POTENTIAL
Utilizing only 1% of the earth's deserts to produce clean solar thermal electric energy would provide more electricity than is currently being produced on the entire planet by fossil fuels.

FUTURE
Over 700 megawatts of solar thermal electric systems should be deployed by the year 2003 in the U.S. and internationally. The market for these systems should exceed 5,000 megawatts by 2010, enough to serve the residential needs of 7 million people which will save the energy equivalent of 46 million barrels of oil per year.

SUMMARY
Solar thermal power technologies based on concentrating technologies are in different stages of development. Trough technology is commercially available today, with 354 MW currently operating in the Mojave Desert in California. Power towers are in the demonstration phase, with the 10 MW Solar Two pilot plant located in Barstow (USA), currently undergoing testing and power production. Dish/engine technology has been demonstrated. Several system designs are under engineering development, a 25 kW prototype unit is on display in Golden (USA),  and five to eight second-generation systems have been scheduled for field validation in 1998. Solar thermal power technologies have distinct features that make them attractive energy options in the expanding renewable energy market world-wide.
Solar thermal electricity generating systems have come a long way over the past few decades. Increased research and development of solar thermal technology will make these systems more cost competitive with fossil fuels, increase their reliability, and become a serious alternative for meeting or supplying increased electricity demand.

Solar Ponds
Neither focusing mirrors nor solar cells can generate electricity at night. For this purpose the daytime solar energy must be stored in storage tanks, a process which occurs naturally in a solar pond.
Salt-gradient solar ponds have a high concentration of salt near the bottom, a non-convecting salt gradient middle layer (with salt concentration increasing with depth), and a surface convecting layer with low salt concentration. Sunlight strikes the pond surface and is trapped in the bottom layer because of its high salt concentration. The highly saline water, heated by the solar energy absorbed in the pond floor, can not rise owing to its great density. It simply sits at the pond bottom heating up until it almost boils (while the surface layers of water stay relatively cool)! This hot brine can then be used as a day or night heat source from which a special organic-fluid turbine can generate electricity. The middle gradient layer in solar pond acts as an insulator, preventing convection and heat loss to the surface. Temperature differences between the bottom an surface layers are sufficient to drive a generator. A transfer fluid piped through the bottom layer carries heat away for direct end-use application. The heat may also be part of a closed-loop Rankine cycle system that turns a turbine to generate electricity.

This type of power station has been tested at Beit Ha'Arava (Israel) near the Dead Sea. Israel leads the world in salt-gradient solar pond technology. Ormat Systems Inc. has installed several systems in the Dead Sea. The largest is a 5 MW electric system. This 20 hectare pond converts sunlight to electricity at an efficiency of about 1%. It consists of a pond of water with very high salinity in its lower depths. Although the solar pond operated successfully for several years, in 1989 it was shut down for economical reasons. The largest solar pond in the USA is a 0,3 hectare pond in El Paso, Texas, which has operated reliably since its start in 1986. The pond runs a 70 kW (electric) organic Rankine-cycle turbine generator, and a 20 000 litres per day desalting unit, while also providing process heat to an adjacent food processing company. The pond has reached and sustained temperatures higher than 90 deg. C in its heat-storage zone, generated more than 100 kW of electric power during peak output , and produced more than 350 000 litres of potable water in a 24 hour period. During five year operation, it has produced more than 50 000 kWh of electricity. A man-made, salt-gradient solar pond was  built in Miamisburg, (Ohio, USA) and it heats a municipal swimming pool and a recreational building.

PHOTOVOLTAICS
Photovoltaics (PV) is the term derived from Greek word for light - photos- and the name for unit of electromotive force - volt. Photovoltaics means direct generation of electricity from light. Recently this process is utilised by means of solar cells. The “solar cells”, made from semiconductor materials such as silicon, produce electric currents when exposed to sunlight. By manufacturing modules which contain dozens of such solar cells and connecting the modules large power stations can be built. The largest photovoltaic power station that has yet been constructed is the 5 MW system at Carrisa Plain, California. The efficiency of photovoltaic power stations is presently about 10% but individual solar cells have been fabricated with efficiencies exceeding 20%.

HISTORY OF PHOTOVOLTAICS
The history of photovoltaics dates back to 1839 and major developments evolved as follows:
In 1839 Edmund Becquerel, a French physicist observed the photovoltaic effect.
In 1883 Selenium PV cells were built by Charles Edgar Fritts, a New York electrician. Cells converted light in the visible spectrum into electricity and were 1% to 2% efficient. (light sensors for cameras are still made from selenium today).
In the early 1950's the Czochralski meter was developed for producing highly purecrystalline silicon.
In 1954 Bell Telephone Laboratories produced a silicon PV cell with a 4% efficiency and later achieved 11% efficiency.
In 1958 the US Vanguard space satellite used a small (less than one watt) array to power its radio. The space program has played an important role in the development of PV's ever since.
During the 1973-74 oil price shock several countries launched photovoltaic utilization programmes, resulting in the installation and testing of over 3,100 PV systems in USA alone, many of which are in operation today.

PV MARKET

Progress is visible in many parts of the world. The Japanese government is investing $250 million a year to increase manufacturing capacity from 40 MW (1997) to 190 MW (2000) and national programs are being launched in Europe, driven by energy independence and environment.  These programs,
combined with environmental pressures such as climate change, can accelerate growth of the PV industry.  Shell Solar has built the world's largest PV manufacturing facility in Germany, with current annual production of 10 MW and future growth to 25 MW. The cost was 50 million Mark.

PV UTILISATION
For a range of applications solar cells are technically feasible and economically viable alternative to fossil fuels. A solar cell can directly convert the sun's irradiation to electricity and this process  requires no moving parts. This results in a relatively long service life of solar generators. PV systems have been the best choice for many jobs since the first commercial PV cells were developed. For example, PV cells have been the exclusive power source for satellites orbiting the earth since the 1960s. PV systems have been used for remote stand-alone systems throughout the world since the 1970s. In the 1980s, commercial and consumer product manufacturers began incorporating PV into everything from watches and calculators to music boxes. And in the 1990s, many utilities are finding PV to be the best choice for thousands of small power needs.

PV systems are now generating electricity to pump water, light up the night, activate switches, charge batteries, supply the electric utility grid, and more. PV systems produce power in all types of weather.  On partly cloudy days they can produce up to 80% of their potential energy delivery; on hazy/humid days, about 50%; and on extremely overcast days, they still produce up to 30%.
PV cells are no longer just available in panels. Different companies are incorporating PV into light-weight, flexible and durable roofing shingles, as well as inverted curtain walls for building facades. These new products make the economics of photovoltaics more attractive by incorporating the PV cells into building materials. In remote areas or locations, PV is the most cost-effective, reliable and durable energy solution available. For grid-connected systems PV can provide, in some regions, a cost competitive energy solution. In all regions, both remote and grid connected, PV provides clean energy without the polluting effects of conventional power sources.
Solar powered water pumping systems are effective and economical for virtually any water pumping need. Electric utilities in the USA found that it is more economical to use PV powered water pumps than to maintain distribution lines to remote pumps. Several utilities are offering photovoltaic water pumping systems as customer service options.

Other agriculture solutions include electric fence charging and lighting. In greenhouse or hydroponics operations, solar can provide the power for water circulation, fans, lights and climate control equipment.
 

PV is successfully utilised also in village electrification. Today two billion people in the world are without electricity. A large portion live in the developing world, where 75% of the population lives without electricity. There is rarely a utility grid in these remote, rural or suburban villages. Experience shows that PV delivers cost-effective electricity for basic services, such as:
light
water pumping
communications
health facilities
businesses

People not served by a power grid often rely on fossil fuels like kerosene and diesel. There are a number of problems associated with the use of fossil fuels.
Imported fossil fuels drain foreign currency.
Transporting is difficult because of infrastructure.
Maintenance of fossil fuel generators is difficult because of lack of spare parts.
Generators pollute the environment by loud noises and exhaust.

Electric lights powered by PV are more effective than kerosene lights in developing countries, and installing a PV system is usually less expensive than extending the power lines. Moreover, many developing countries are located in areas with high insolation levels, providing them with a free abundant source of energy year round. Using photovoltaics to generate electricity from sunlight is simple and has proven reliable in tens of thousands of applications world-wide.

During the next decades, a large part of the world's population will be introduced to electricity produced by PV systems. These PV systems will make the traditional requirements of building large, expensive power plants and distribution systems unnecessary. As the costs of PV continue to decline and as PV technology continues to improve, several potentially huge markets for PV will open up. For example, building materials that incorporate PV cells will be designed right into homes, helping to ventilate and light the buildings. Consumer products ranging from battery-powered hand tools to automobiles will take advantage of electricity - producing components containing PV materials. Meanwhile, electric utilities will find more and more ways to use PV to supply the needs of their customers.

The EU wants to double the share of renewables by 2010, and key actions include one million PV systems (500,000 roof and the export of 500,000 village systems) with total installed capacity of 1 GW. BP Amoco (one of the world's leading marketers of petroleum products) will incorporate solar energy into 200 of its new service stations in Britain, Australia, Germany, Austria, Switzerland, the Netherlands, Japan, Portugal and Spain, France and the US. The $50 million program will involve 400 panels, generating 3.5 MW and saving 3,500 tonnes of CO2 emissions every year.  The project will make BP Amoco one of the world's largest users of solar power, as well as one of the largest manufacturers of cells and modules. The solar panels will generate more power than consumed for lighting and pump power, and will be grid-connected to allow excess electricity to be exported during the day and the shortfall imported at night. The world market for photovoltaics will reach 1,000 MW by 2010 and 5 million MW by 2050, according to the president of BP Solar.

TECHNOLOGY
Solar electric systems are simple to operate and have no moving parts; however, PV cells employ sophisticated semiconductor devices, many of which are similar to those developed in the integrated circuit industry. PV cells operate on the physical principle that electric current will flow between two semiconductors with different electrical properties when they are put in contact with each other and exposed to light. A collection of these PV cells constitutes a PV panel, or module. PV modules, because of their electrical properties, produce direct rather than alternating current (AC). Direct current (DC) is electric current that flows in a single direction. Many simple devices, such as those that run on batteries, use direct current. Alternating current, in contrast, is electric current that reverses its direction at regular intervals. This is the type of electricity provided by utilities and required to run most modern appliances and electronic devices. In the simplest systems, DC produced by PV modules is used directly. In applications where AC is necessary, an inverter can be added to the system to convert the DC to AC.

PV CELLS
Today's solar cell production is almost exclusively based on silicon. About 80% of all modules are fabricated using crystalline silicon cells (multicrystalline and single crystalline) and about 20% are based on amorphous silicon thin film cells. The crystalline cells  are the more common, generally blue-coloured frosty looking ones. Amorphous means noncrystalline, and these look smooth and change color depending on the way you hold them. Monocrystalline silicon  has the best efficiency - about 14% of the  sunlight can be utilized - but it is more  expensive than multicrystalline silicon, which  typically has 11% efficiency.  Amorphous  silicon is widely used in small appliances such  as watches and calculators, but its efficiency  and long-term stability are significantly lower;  consequently, it is rarely used in power  applications.
On a laboratory and/or a pilot production scale there are, however, several alternative thin film solar cells under development which may penetrate the market in the future. The most advanced of the presently investigated thin film systems are:
amorphous silicon (a-Si: H) cells,
cadmium telluride/cadmium sulfide (CTS) cells,
copper indium diselenide or copper indium/gallium diselenide (CIS or CIGS) cells, crystalline silicon thin film (c-Si film) cells and
nanocrystalline dye sensitised electrochemical (nc-dye) cells.

PV cells are “sandwiches” of silicon, the second most abundant material in the world. Ninety-nine percent of today's solar cells are made of silicon (Si), and other solar cells are governed by basically the same physics as Si solar cells. One layer of silicon is treated with a substance to create an excess of electrons. This becomes the negative or “N” layer. The other layer is treated to create a deficiency of electrons, and becomes the positive or “P” layer. Assembled together with conductors, the arrangement becomes a light-sensitive NP junction semiconductor. It's called a semiconductor, because, unlike a wire, the unit conducts in only one direction; from negative to positive. When exposed to sunlight (or other intense light source), the voltage is about 0,5 Volts DC, and the potential current flow (amps) is proportional to the light energy (photons). In any PV, the voltage is nearly constant, and the current is proportional to the size of the PV and the intensity of the light.
Photovoltaic cells are made from hyper pure silicon that is precisely doped with other materials. The hyper pure silicon substrates used to make PV cells are very expensive. After all, the same amount of hyper pure silicon used in a single 50 Watt PV module could have been made into enough integrated circuits for about two thousand computers. The remainder of the materials used by PV cells are aluminum, glass, and plastic - all inexpensive and easily recyclable materials.

PV production facility.

SOLAR MODULES
Solar modules are an array of solar cells  which are interconnected and encapsulated  behind a glass cover. The stronger the light falling down on the cells and the larger the cell surface, the more  electricity is generated and the higher the  current. Modules are rated in peak watts (Wp). A watt is the unit used to express the  power of a generator or the demand of a consumer. One peak watt is a specification  which indicates the amount of power generated  under rated conditions, i.e. when solar  irradiance of 1 kW/m2 is incident on the cell at a temperature of 25 deg. C. This level of intensity is achieved when weather conditions are good and  the sun is at its zenith. No more than a cell of  10 x 10 cm is necessary to generate a peak watt.  Larger modules, 1 m x 40 cm in size, have an  output of about 40-50 Wp. Most of the time, however, the irradiation is  below 1 kW/m2. Furthermore, in sunlight the  module will warm up beyond the rated temperature. Both effects will reduce the module's performance. For typical conditions an average output  of about 6 Wh per day and 2000 Wh per year  per peak watt can be expected. To have the idea of how  much that is, 5 Wh is the energy consumed by a 50 W lamp in 6 minutes (50W x 0,1h = 5Wh) or by a small radio in one hour (5W x 1h =  5Wh).

Although some differences still exist in product quality, most international companies produce fairly reliable units which can be expected  to work for 20 years. Meanwhile, suppliers  guarantee the specified power output for a period of up to 10 years. The most decisive criterion for the  comparison of different modules is the price per peak watt. In other words, it is possible to get  more power for the money with a 120 Wp  module which costs USD 569 (4,74 USD/Wp) than with a  “cheap” 90 Wp module that costs USD 489 (5,43 USD/Wp). The rated efficiency of a system is a less important consideration.

PV ADVANTAGES
High Reliability
PV cells were originally developed for use in space, where repair is extremely expensive, if not impossible.  PV still powers nearly every satellite circling the earth because it operates reliably for long periods of time with virtually no maintenance.

Low Operating Costs
PV cells use the energy from sunlight to produce electricity - the fuel is free.  With no moving parts, the cells require low-maintenance. Cost-effective PV systems are ideal for supplying power to communication stations on mountain tops, navigational buoys at sea, or homes far from utility power lines.

Non-polluting
Because they burn no fuel and have no moving parts, PV systems are clean and silent. This is especially important where the main alternatives for obtaining power and light are from diesel generators and kerosene lanterns.

Modular
A PV system can be constructed to any size.  Furthermore, the owner of a PV system can enlarge or move it if his or her energy needs change.  For instance, homeowners can add modules every few years as their energy usage and financial resources grow.  Ranchers can use mobile trailer-mounted pumping systems to water cattle as they are rotated between fields.

Low Construction Costs
PV systems are usually placed close to where the electricity is used, meaning much shorter wire runs than if power is brought in from the utility grid. In addition, using PV eliminates the need for a step-down transformer from the utility line. Fewer wires mean lower costs and shorter construction time.

HOW MUCH DOES PV-GENERATED ELECTRICITY COST?
There is no simple answer.  Many small PV systems designed to power a few fluorescent lights and a small TV in remote hoses are much cheaper than the next best alternatives running a new power line, replacing and disposing of primary batteries (those batteries that are used once and then disposed of, such as flashlight batteries), or using an engine generator. The cost of electricity from larger systems, those able, for example, to power a modern home, is evaluated according to the cost per kilowatt hour (kWh). The cost depends on the initial cost, interest on the loan (for paying the initial cost), the cost of system maintenance, the expected lifetime of the system, and how much electricity it produces.  Using typical borrowing costs and equipment life, the cost of PV-generated energy in USA in  1998 ranged from $0,20 to $0,50/kWh.

HOW MUCH SPACE DOES PV TAKE?

Simple PV Systems
The sunlight that creates the need for water pumping and ventilation can be harnessed using the most basic PV systems to meet those same needs. Photovoltaic modules produce the most electricity on clear, sunny days. Simple PV systems use the DC electricity as soon as it is generated to run water pumps or fans. These basic PV systems have several advantages for the special jobs they do. The energy is produced where and when it is needed, so complex wiring, storage, and control systems are unnecessary. Small systems, under 500 W, weigh less than 70 kilograms making them easy to transport and install. Most installations take only a few hours. And, although pumps and fans require regular maintenance, the PV modules require only an occasional inspection and cleaning.

Solar Water Pumping

 Advantages of using PV-powered pumps include:
low maintenance
ease of installation
reliability
scalability

Solar power differs fundamentally from conventional electric or engine-powered systems, so solar pumps often depart from the conventional. PV arrays produce DC power, rather than the AC from conventional sources. And, the power available varies with the sun's intensity. Since it costs less to store water (in tanks) than energy (in batteries) solar pumps tend to be low in power, pumping slowly through the duration of the solar day.
Simple, efficient systems are the key to economical solar pumping. Special, low-power DC pumps are used without batteries or AC conversion. Modern DC motors work well at varying voltage and speed. The better DC motors require maintenance (brush replacement) only after periods of 5 years or more. Most solar pumps used for small scale application (homes, small irrigation, livestock) are “positive displacement” pumps which seal water in cavities and force it upward. This differs from faster, conventional centrifugal type pumps (including jet and submersible pumps) which spin and “blow” the water up.

Positive displacement pumps include piston, diaphragm, rotary vane, and pump jacks. They work best for low volumes, particularly where variable running speeds occur. Centrifugal, jet and turbine pumps are used for higher volume systems. Electronic matching devices known as Power Trackers and Linear Current Boosters allow solar pumps to start and run under low-light conditions. This permits direct use of the sun's power without bothersome storage batteries. Solar trackers may be used to aim the panels at the sun from morning to sunset, extending the useable period of sunlight. Storage tanks usually hold a 3-10 day supply of water, to meet demands during cloudy periods. Solar pumps use surprisingly little power. They utilize high efficiency design and the long duration of the solar day, rather than power and speed, to lift the volume of water required.
In areas where photovoltaic pumps have entered into competition with diesel-driven pumps, their comparatively high initial cost is offset by the achieved savings on fuel and reduced maintenance expenditures. Studies concerning the economic efficiency of photovoltaic pumping systems confirm that they are often able to yield cost advantages over diesel-driven pumps, depending on the country-specific situation.

PV SYSTEMS WITH BATTERIES
The most simple solutions have certain drawbacks - the most obvious one  being that in case of PV powered pump or fan could only be  used during the daytime, when the sun is  shining. To compensate for these limitations, a battery is added to the system. The battery is charged by the solar generator, stores the energy and makes it available at the times and  in the amounts needed. In the most remote and hostile environments, PV-generated electrical energy stored in batteries can power a wide variety of equipment. Storing electrical energy makes PV systems a reliable source of electric power day and night, rain or shine. PV systems with battery storage are being used all over the world to power lights, sensors, recording equipment, switches, appliances, telephones, televisions, and even power tools.

A solar module generates a direct current (DC), generally at a voltage of 12  V. Many appliances, such as lights, TV's,  refrigerators, fans, tools etc., are now available for 12V DC operation. Nevertheless the majority of common electrical household appliances are  designed to operate on 110 V or 220 V alternating current (AC). PV systems with batteries can be designed to power DC or AC equipment. People who want to run conventional AC equipment add a power conditioning device called an inverter between the batteries and the load. Although a small amount of energy is lost in converting DC to AC, an inverter makes PV-generated electricity behave like utility power to operate everyday AC appliances, lights, or computers.
PV systems with batteries operate by connecting the PV modules to a battery, and the battery, in turn, to the load. During daylight hours, the PV modules charge the battery. The battery supplies power to the load whenever needed. A simple electrical device called a charge controller keeps the batteries charged properly and helps prolong their life by protecting them from overcharging or from being completely drained. Batteries make PV systems useful in more situations, but also require some maintenance. The batteries used in PV systems are often similar to car batteries, but are built somewhat differently to allow more of their stored energy to be used each day. They are said to be deep cycling. Batteries designed for PV projects pose the same risks and demand the same caution in handling and storage as automotive batteries. The fluid in unsealed batteries should be checked periodically, and batteries should be protected from extremely cold weather.
A solar generating system with batteries supplies electricity when it is needed. How much electricity can be used after sunset or on cloudy days is determined by the output of the PV modules and the nature of the battery bank. Including more modules and batteries increases system cost, so energy usage must be carefully studied to determine optimum system size. A well-designed system balances cost and convenience to meet the user's needs, and can be expanded if those needs change.

DESIGNING PV HOME SYSTEM WITH BATTERIES

CALCULATION OF ENERGY DEMAND
In case of designing PV powered home system the first step to make is to create a list of all electrical appliances in the household. Check the power input required for the operation of these appliances and put this on the list.
As an example average data on power consumption for some devices are in the table below, but it is important to bear in mind that these are only rough estimations. To calculate power consumption (E) of the system with inverter (using AC devices) it is needed to make correction (multiply average consumption by C to calculate the total power demand Ptot).
 

To operate other electrical appliances such as refrigerators, irons, big fans, cooking plates, etc., you would need a bigger and more expensive system. Since such a system is not standardized but will be tailored specifically to your needs, calculation have to be done by an expert.

Second step is to estimate the amount of time per day that the specific appliances are in operation. This maybe as much as 10 hours for a lamp in the living room, but perhaps only 10 minutes for one in the store. Add these data to your list in a second column in table bellow. Finally, you should make a third column where you list the daily energy requirement. Calculate this figure by multiplying the power by the operating period, e.g. 27 W x 4 h = 108 Wh. When you have added up all the figures in this column, you will have your overall energy demand (E).

The next step consists of estimation of the amount of solar insolation which can be expected at home site. In most cases, these figures can be obtained from local PV suppliers or at a local weather station. Important figure is the annual average solar insolation as well as the average in the month with the worst climatic conditions (some general data can be found in chapter on Solar radiation).

Using the first figure, PV system can be adjusted to the average insolation per year, which means there are some months with more energy than required or calculated and some months with less. If you use the second (low case) figure, you will always have at least enough energy to meet your requirements, except in unusually bad weather periods. However, the PV module will have to be larger and it will also be more costly.
Now you can calculate the rated power of the PV module. Use your energy demand figure (in Wh/d), multiply it by 1,7 to allow for energy losses in the system and divide it by the solar insolation figure (in Wh/d), e.g. 280 (Wh/d) x 1,7/ 5 (kWh/d) = 96,2 W. Unfortunately, PV modules  are only available with a few power ratings. Using a 50 W module, for example, you can build generators of 50 W, 100 W, 150 W, etc.. With a power demand of 95 W, a two-module system would be the best match. Choose the number of modules whose total power rating corresponds approximately to the value you have calculated. If the two figures differ significantly, you have to undersize or oversize the generator. In the first case, the PV system will not be able to meet overall energy demand. Decide whether this partial supply option would be acceptable to you. In the second case, you will have surplus energy.
Designing the battery size depends on energy demand and the number of PV modules. For above mentioned example battery capacity of 60 Ah per module as a minimum should be used and 100 Ah as an optimum. Such a battery can store 1200 Wh at 12 V. This capacity can cover 4 days of energy needs for above mentioned example with daily energy consumption of 280 Wh.

SYSTEM DC VOLTAGE
In the past, almost all systems used 12 V DC as their base voltage. This was because the systems were small and extensively employed 12 V DC appliances powered directly from the battery. Now, with the arrival of efficient and reliable inverters, 12 Volt use has declined and 24 V DC is becoming the favored battery voltage. At this moment, the system's DC voltage should be determined by how much power the system cycles daily. Systems producing and consuming less than 2,000 Watt-hours daily are best served by 12 Volts. Systems cycling over 2,000 and less than 6,000 Watt-hours daily should use 24 V DC as a base voltage. Systems cycling over 6,000 Watt-hours daily should use 48 Volts.
System voltage is a very important factor effecting the choice of inverter, controls, battery chargers, and system wiring. Once these components are bought, they usually cannot be changed. While some hardware, like PV modules, can be reconnected from 12 to higher voltages, other hardware like inverters, controls, and wiring is specified for a particular voltage and must operate there.

BATTERY
A battery stores the energy delivered by the  solar generator and provides power for various  appliances. As a component of an SHS a battery has to fulfil three tasks:
It covers peak loads which the PV modules cannot meet on its own (buffer).
It provides energy during the night  (short-term storage).
It compensates for periods of bad weather or  of unusually high energy demand  (medium-term storage).
Automotive batteries, which are available all over the world at reasonable prices, are the  most commonly employed type of battery.  However, they are designed to deliver high  currents over short periods. They cannot  withstand the continuos cycles of charging and  discharging that are typical for solar systems. The industry has developed batteries,  sometimes called solar batteries, which meet  these conditions. Their main feature is low  sensitivity to cyclic operation.
Unfortunately,  there are only a few developing countries in  which such batteries are produced, and  imported batteries may be very expensive owing to transport costs and customs  duties. In such cases, a heavy-duty truck  battery may be an appropriate, easily  accessible alternative, even if it has to be  replaced more often.
In the case of large PV systems, the capacity of one  battery may not be sufficient. If so, more than  one battery, can be  switched in parallel, i.e. all poles marked + and  all marked - are connected to each other. Thick  copper wires, preferably less than 30 cm long,  should be used for the connection. During charging, batteries produce gases  which are potentially explosive. Thus, you should avoid using an open fire nearby.  However, gassing is relatively low, especially if  a charge regulator is used; the risk is thus no  greater than that normally involved in the use  of automotive batteries in cars. Nevertheless,  the batteries need to be well ventilated.  Therefore you should not cover them up or put  them in boxes.

The capacity of a battery is indicated in  ampere-hours (Ah). A 100 Ah, 12 V battery,  for instance, can store 1,200 Wh (12 V x 100  Ah). However, the capacity will vary,  depending on the duration of the charging or  discharging process. In other words, a battery  will deliver more energy during a 100 h  discharging period than during a 10 h period.  The charging period is indicated by an index to  the capacity c, e.g. C100 for 100 hours. Note  that suppliers may use different reference  periods.
When storing energy in batteries, a certain  amount of energy is lost in the process.  Automotive batteries have efficiencies of about  75%, while solar batteries may perform slightly  better. Some of the battery capacity is lost in each charging-discharging cycle and eventually drops  to a level at which the battery has to be  replaced. Solar batteries have a longer lifetime  than heavy-duty automotive batteries, which  last about 2 or 3 years.

SIZING THE PV SYSTEM'S BATTERY
It is important to size the PV systems battery with a minimum of four days of storage. Consider the system that consumes 2,480 watt-hours daily. If we divide this figure by system voltage of 12 V DC, we arrive at a daily consumption of 206 Ampere-hours from the battery. So four days of storage would be 4 days X 206 Ampere-hours per day or 826 Ampere-hours. If the battery is a lead-acid type, then we should add 20% to this amount to ensure that the battery is never fully discharged. This brings our ideal lead-acid battery up to a capacity of 991 Ampere-hours. If the battery is nickel-cadmium or nickel-iron, then this extra 20% capacity is not required because alkaline batteries don't mind being fully discharged on a regular basis.

THE CHARGE REGULATOR
A battery can only be expected to last several  years if a good charge regulator is employed. It  protects the battery against overcharging and deep-discharging, both of which are harmful to  the battery. If a battery is fully charged, the regulator  reduces the current delivered by the solar generator to a level which equalizes the natural losses. On the other hand, the regulator  interrupts the amount of energy supplied to the  load appliances when the battery has  discharged to a critical level. Thus, in most  cases a sudden interruption in supply is not a system failure, but rather an effect of this  safeguard mechanism.
Charge regulators are electronic components and, as such, may be affected by malfunctions  and improper handling of the systems.  Improved designs are equipped with safeguards  to prevent damages to the regulator and other  components. These include safeguards against short  circuit and battery reverse polarity (mixing up  of the batteries' +/- poles) as well as a blocking  diode to prevent overnight battery discharge. Many models indicate certain states of  operation and malfunctions by means of LEDs (light emitting diodes = small lamps). A few  even indicate the state of charge. Nevertheless the state of charge is  difficult to determine and can only be roughly estimated.

THE INVERTER
The inverter converts low voltage DC power (stored in the battery and produced by the PVs) into standard alternating current, house power (120 or 240 V AC, 50 or 60 Hz). Inverters come in sizes from 250 watts (about 300 USD) to over 8,000 watts (about 6,000 USD). The electric power produced by modern sine wave inverters is far purer than the power delivered to your wall sockets by your local electric utility. There are also “modified sine wave” inverters that are less expensive yet still up to most household tasks. This type of inverter may create a buzz in some electronic equipment and telephones which can be a minor problem. The better sine wave inverters have made great improvements in performance and price in recent years. Inverters can also provide a “utility buffer” between your system and the utility grid, allowing you to sell your excess power generated back to the utility for distribution by their grid.

CABLES
A simple means of avoiding unnecessary  losses is to use appropriate cables and to attach them properly to the devices. Cables should  always be as short as possible. The ones  connecting the different appliances should have  a cross-sectional area of at least 1.6 mm2. To  ensure that the voltage loss does not exceed 3%, the cable between the PV generator and the battery should have a cross-section of 0.35 mm2  (12 V- system) or 0.17 mm2 (24 V-system) per  metre and module. Thus, a 10 m cable for 2 modules would require at least 10 x 2 x0,35  mm2 = 7 mm2. Since cables with a cross-section  exceeding 10 mm2 are difficult to handle and even difficult to get, higher losses have to be accepted in some cases.  If a part of this cable is exposed to the open air, it should be designed so that will withstand all weather conditions. Tolerance to ultraviolet rays may be an important feature.

TRACKERS
PV modules work best when their cells are perpendicular to the Sun's incoming rays. Adjustment of static mounted PV modules can result in from 10% (in winter) to 40% (in summer) more power output yearly. Tracking means mounting the array on a movable platform which follows the sun's daily motion. A tracker is a special PV mounting rack that follows the path of the sun. In general the extra energy captured by following the sun must be weighed against the costs of installing and maintaining the tracking system.
Trackers cost money just like PV modules. In many countries it is not cost effective to track less than eight modules (e.g. in the USA). Under eight modules, we will get more power output for money if we spend the money on more panels rather than a tracker. At eight panels in the system, the tracker starts to pay off. There are exceptions to this rule, for example array direct water pumps. If PVs are directly driving a water pump, without a battery in the system, then it is cost effective to track two or more PV modules. This has to do with technical details like the peak voltage required to drive the pumps electric motor.

THE LAMPS
Due to their excellent efficiency and long  lifetime, energy saving lamps should always be used in PV operated systems. Fluorescent tubes or the new compact fluorescent lamps (CFL) are suitable in many cases, 18 W CFL lamp is able to substitute traditional 100 W incadescent light bulb. CFL lamps require electronic ballasts to be operated with a DC system. The quality of such ballasts varies considerably and  sometimes proves to be very poor. Low-quality ballasts will result in high costs for continuous replacement of worn-out tubes. It is important for ballasts to have a good efficiency, a high number starting cycles, reliable ignition at low temperatures and low voltages (10.5 V), and protection against short-circuit, open circuit, reverse polarity and radio interference. Despite the fact that most CFL lamps on the market are working only with AC current there are few companies offering also DC powered lamps.

LIFETIME AND PRICING OF COMPONENTS
A very important consideration in the economic analysis is the lifetime of a PV system. Lifetimes of the various components of a PV power supply have been estimated, based on experiences gained over the past few years.
The lifetime of PV panels is estimated at 20 years. Proper encapsulation and the use of low-iron tempered glass ensure a lifetime which may go well beyond.
Galvanized iron frames and anchors are part of most PV systems. Properly galvanized material should last as long as the panels although some
maintenance may be required.
Batteries. Depending on the character of the charge/discharge cycles, the average lifetime of the so-called “Solar Batteries”, has been 4 years.
Battery chargers are assumed to last at least 10 years.
Inverters are assumed to last for 10 years.

Rough guidelines for pricing of the several components:
Inverters - USD 0.50/W
Frames (galvanized) - USD 0.30/Wp
Control Devices - USD 0.50/Wp
Cables - USD 0.70/m
Local stationary batteries - USD 100/kWh capacity
PV modules - USD 5 /Wp.

PV WITH GENERATORS
Working together, PV and other electric generators can meet more varied demands for electricity, conveniently and for a lower cost than either can meet alone. When power must always be available or when larger amounts of electricity than a PV system alone can supply are occasionally needed, an electric generator can work effectively with a PV system to supply the load. During the daytime, the PV modules quietly supply daytime energy needs and charge batteries. If the batteries run low, the engine-generator runs at full power its most constant fuel-efficient mode of operation until they are charged. And in some systems the generator makes up the difference when electrical demand exceeds the combined output of the PV modules and the batteries. Systems using several types of electrical generation combine the advantages of each. Engine-generators can produce electricity any time. Thus, they provide an excellent backup for the PV modules (which produce power only during daylight hours) when power is needed at night or on cloudy days. On the other hand, PV operates quietly and inexpensively, and does not pollute. Using PV and generators together can also reduce the initial cost of the system. If no other form of generation is available, the PV array and the battery storage must be large enough to supply night time electrical needs.
However, having an engine-generator as backup means fewer PV modules and batteries are necessary to supply power whenever it is needed. Including generators makes designing PV systems more complex, but they are still easy to operate. In fact, modern electronic controllers allow such systems to operate automatically. Controllers can be set to automatically switch generators or to supply AC or DC loads or some of each. In addition to engine generators, electricity from wind generators, small hydro plants, and any other source of electrical energy can be added to make a larger hybrid power system.

GRID-CONNECTED PV

An approved, utility-grade inverter converts the DC power from PV modules into AC power that exactly matches the voltage and frequency of the electricity flowing in the utility line, and also meets the utility safety and power quality requirements. Safety switches in the inverter automatically disconnect the PV system from the line if utility power fails. This safety disconnect protects utility repair personnel from being shocked by electricity flowing from the PV array into what they would expect to be a dead utility line.  In some countries utilities are establishing rate structures that may make PV grid-connected systems more economical. (At today‘s prices, when the cost of installing a utility-connected PV system is divided by the amount of electricity it will produce over 30 years, PV- generated electricity is almost everywhere more expensive than power supplied by the utility.) For example, some utilities charge higher prices at certain times of the day. In some parts of the USA, the highest charges for electricity under this time-of-day pricing structure are now nearly equal to the cost of energy from PV. The better the match between the electrical output of the PV modules and the time of highest prices, the more effective the system will be in reducing utility bills.
Grid connected systems are growing especially in USA and Europe. One such a project was commissioned in California. Twelve homes in a major housing development in Compton (southern California) are using integrated solar roof tiles to provide household electricity from sunlight.  Central Park Estates, an affordable single-family housing development, uses solar roof tiles as an integral and aesthetically pleasing part of the homes. The solar roofs are connected to the local power grid, and meters will ‘spin backwards' when the PV cells produce excess power.

UTILITY-SCALE PV
Electric, gas, and water utilities have been using small PV systems economically for several years. Most of these systems are less than 1 kW and use batteries for energy storage. These systems are performing many jobs for utilities, from powering aircraft warning beacons on transmission towers to monitoring air quality of fluid flows. They have demonstrated the reliability and durability of PV for utility applications and are paving the way for larger systems to be added in the future.
Utilities are exploring PV to expand generation capacity and meet increasing environmental and safety concerns. Large-scale photovoltaic power plants, consisting of many PV arrays installed together, can prove useful to utilities. Utilities can build PV plants much more quickly than they can build conventional power plants because the arrays themselves are easy to install and connect together electrically. Utilities can locate PV plants where they are most needed in the grid because siting PV arrays is much easier than siting a conventional power plant. And unlike conventional power plants, PV plants can be expanded incrementally as demand increases. Finally, PV power plants consume no fuel and produce no air or water pollution while they silently generate electricity. Unfortunately, PV generation plants have several characteristics that have slowed their use by utilities. Under current utility accounting, PV-generated electricity still costs considerably more than electricity generated by conventional plants. Furthermore, photovoltaic systems produce power only during daylight hours and their output varies with the weather.
Utility planners must therefore treat a PV power plant differently than a conventional plant in order to integrate PV generation into the rest of their power generation, transmission, and distribution systems. On the other hand, utilities are becoming more involved with PV. For example in USA utilities are exploring connecting PV systems to the utility grid in locations where they have a higher value. For example, adding PV generation near where the electricity is used avoids the energy losses resulting from sending current long distances through the power lines. Thus, the PV system is worth more to the utility when it is located near the customer. PV systems could also be installed at locations in the utility distribution system that are servicing areas whose populations are growing rapidly. Placed in these locations, the PV systems could eliminate the need for the utility to increase the size of the power lines and servicing area. Installing PV systems near other utility distribution equipment such as substations can also relieve overloading of the equipment in the substation.
Photovoltaics are unlike any other energy source that has ever been available to utilities. PV generation requires a large initial expense, but the fuel costs are zero. Coal- or gas- fired plants cost less to build initially (relative to their output) but require continued fuel expense. Fuel expenses fluctuate and are difficult to predict due to the uncertainty of future environmental regulations. Fossil fuel prices will rise over time, while the overall cost of PVs (and all renewable energy resources) is expected to continue to drop, especially as their environmental advantages are valued.

PV ELECTRICITY COST

The table below shows calculated electricity cost produced by PV system in US cents per kWh as function of the investment cost and the efficiency. The row headings on the left show the total cost, per peak kW (kWp), of a photovoltaic installation. The column headings across the top refer to the annual energy output in kilowatt-hours expected from each installed peak kilowatt. This varies by geographic region (see the figure for Europe). It also depends on the path of the sun relative to the panel and the horizon.

Source: Wikipedia

Insolation in Europe can be seen from the figure bellow.

Key to the PV development in the future are the costs of investment. And they tend to be falling steadily due to the decreasing costs of PV modules. Especially PV modules made from thin-films already reached the production price level bellow 1000 USD/kWp (2009) which is making them attractive option in several regions of the world even without governmental subsidies. Several experts predict that many PV producing companies will reach the cost of 1 USD/W before 2012. This target will be a huge competition factor for consumers and businesses because then PV panels will be able to generate power cheaper than other fossil fuel sources in numerous regions of the world. With price of PV modules less than 1 USD per Watt, solar electricity could be produce for less than 0,1 USD/kWh (see the figure above) and that is highly competitive. The average price of electricity in Europe and U.S is higher than this and due to the fossil fuel scarcity is expecting to rise in the future.

Guideline for Estimation of  Solar Potentials, Barriers and Effects

Solar heating
This section is mainly covering active solar heating, where the solar energy is transferred to heat in solar collectors and from there transported by a fluid to its final use. Another important use of solar heat is passive solar heating, where buildings are designed to capture the maximum of the solar energy coming through windows and upon walls to be used for space-heating.

Energy Content
The yearly incoming solar energy varies from 900-1000 kWh/m2 North of the Baltic Sea to e.g. 1077 kWh/m2 in Central Europe (Hradec Kralove in Bohemia) and up to 1600 kWh/m2 in Mediterranean and Black Sea areas on a horizontal surface. On a south sloping surface, the incoming solar energy is about 20% higher.

Resource Estimation
The incoming solar energy on most buildings exceed the energy consumption of the building, e.g. a 5 storey apartment house in Hradec Kralove receives 1077 kWh/m2, while each storey consumes about 150 kWh/m2 for heating and 25-50 kWh/m2 for light and cooking, adding up to 875 - 1000 kWh/m2 for the 5 storeys together (all measured per. m2 horizontal surface).
While the incoming solar energy is sufficient over the year, the practical usable resource is limited by the fluctuations of the solar energy and the storage capacity. Reasonable good estimates of usable solar heat can be made as a fraction of the different heat demands.

For house-integrated systems, the limitations are normally that solar heating can only cover 60-80% of the hot water demand and 25 - 50% of space heating. The variations are depending on location and systems used. In Northern Europe the limitations are respectively 70% and 30% for hot water and space heating coverage.

For central solar heating systems for district heating, analyses and experience show that these systems can cover 5% of consumption without storage, 10% with 12 hour storage and about 80% with seasonal storage. These figures are based on district heating systems which have 20% average energy losses and mainly deliver to dwellings. The energy delivered from solar heating systems without storage is by far the cheapest solution.

For industries that uses heat below 100oC, solar heating can cover about 30% if they have a steady consumption of heat. For drying processes solar energy can cover up to 100% depending on season, temperature, and limitations to drying period.
Solar heating to swimming pools can cover most of the heat demand for indoor pools and up to 100% for outdoor pools used during summer.

To evaluate the potential for solar heating is, thus, most a question of assessing the demand for low-temperature heat.

Barriers
Most applications for solar heating are well developed, and the technical barrier is more lack of local availability of a certain technology than lack of the technology as such. Thus the main barriers, beside economy, are:
lack of information of available technologies and their optimal design and integration in heating systems.
lack of local skills for production and installation.

In some occasions lack of access to solar energy can be a barrier. For active solar heating it is almost always possible to find a place for the solar collectors with enough sunshine. For passive solar energy, where the solar energy is typically coming through normal windows, neighbouring buildings or high trees can give a severe reduction of the solar energy gain.

Effect on economy, environment and employment
Economy
The economy of using solar energy ranges from almost no costs, when simple passive solar energy designs are integrated into building design and land-use planning to very high costs for solar heating systems with seasonal storage. For solar heating systems, some typical prices are for installed systems:

Notes:
The application for single family hot water, Northern is a typical system for hot water as used in Nordic countries and Germany with anti-freeze agent, high insolation, and closed circuit. The single family Southern Europe is a single family system as used in Greece. Prices in Central & Eastern Europe can be considerably lower. Self-built systems are also considerably cheaper.
The annual production is given for Northern European conditions, except for the Southern European single family system, where production is given for Southern European conditions.
The savings are net savings, in most applications in Northern Europe, the solar heat replaces an oil or gas boiler that has a very low efficiency (often 30-50%) during summer. The total savings can then be 2-3 times larger than the net savings.

Environment
The heat produced in a solar heater replaces energy produced in more polluting ways, which is the main environmental effect. The energy produced to produce a solar heater is equivalent to 1-4 years of production of the solar heater.
Usually the solar collectors are mounted on top of a roof, in which case there is no local impact of the environment.

Effects of employment
The majority of the employment is in the production and installation of solar heaters. Based on Danish experience, the employment is estimated to 17 man-years to produce and install 1,000 m2 of solar heaters for families. These 1,000 m2 replaces 800 MWh of primary energy (net energy production 400 MWh). With 30 years lifetime of the solar heaters, the constant employment of producing solar heaters to replace 1 TWh will be 700 persons.

Country Estimates
In principle all heat demand can be covered by solar energy with seasonal storage. There is therefore no absolute limit to this resource, only economical limitations. In Denmark it is estimated that without seasonal storage, solar energy can cover 13% of the heat demand, including commercial and industrial use. In more sunny places, this fraction is naturally larger.

Photovoltaics Electricity
Photovoltaic (PV) cells produce direct current electricity with output varying directly with the level of solar radiation. PV cells are integrated in modules which are the basic elements of PV systems. PV modules can be designed to operate at almost any voltage, up to several hundred Volt, by connecting cells and modules in series. For applications requiring alternating current, inverters must be used.

PV cell efficiency is calculated as the percentage difference between the irradiated power (Watt) per area unit (m2), and the power supplied as electric energy from the photovoltaic cell. There is a distinction between theoretical efficiency, laboratory efficiency, and practical efficiency. It is important to know the difference between these terms, and it is of course only the practical efficiency which is of interest to users of photovoltaics.

Practical efficiency of mass produced PV cells:
single crystalline silicon : 16 - 17%
polycrystalline silicon : 14 - 15%
amorphous silicon  : 8 -  9%

PV systems are usually divided in:
1. Stand-alone systems that rely on PV power only. Beside the PV modules they include charge controllers and batteries.
2. Hybrid systems that consists of a combination of PV cells and a complementary means of electricity generation such as wind, diesel or gas. Often smaller batteries and chargers/controllers are also used in these systems.
3. Grid connected systems, which work as small power stations feeding power into the grid.

Tips and Applications
When designing a photovoltaic installation a number of things must be taken into consideration, if an optimum solution is wanted. At first it must be clarified, how much energy is demanded from the photovoltaic installation. After that the total daily consumption in Ampere hours (Ah) must be estimated. From the total daily and weekly consumption the total energy storage capacity can be calculated. It must be considered how many days without sun, the installation shall be capable of functioning. At the end it can be calculated, how many photovoltaic modules are required to produce sufficient energy. The photovoltaic application can also be combined with other energy sources. A combination of small wind generators and photovoltaics is an obvious possibility. The energy can be stored in good lead batteries (solar batteries, traction-batteries) or in nickel/cadmium batteries.

Resource estimation
The solar energy which is available during the day varies because of the relative motion of the sun, and depends strongly on the local sky conditions. At noon in clear sky conditions, the solar irradiation can reach 1000 W/m2 while, in very cloudy weather, it may fall to less than 100 W/m2 even at midday. The availability of solar energy varies both with tilt angle and the orientation of surface, decreasing as the surface is moved away from South.

Commercial cells are sold with rated output power (Watt peak power, Wp). This corresponds to their maximum output in standard test conditions, when the solar irradiation is near to its maximum at 1000 W/m2, and the cell temperature is 25oC. In practice, PV modules seldom work at these conditions. Rough estimate of the output (P) from PV systems can be made according to the equation:

P (kWh/day) = Pp (kW) * I (kWh/m2 per day) * PR

where:
Pp  is rated output power in kW, which is equivalent to efficiency * area in m2
I  is solar irradiation on the surface, in kWh/m2 per day
PR is Performance Ratio determined by the system.

Daily mean solar irradiation (I) in Europe in kWh/m2 per day (sloping south, tilt angle from horizon 30 deg.):

Typical Performance Ratios:
0.8 for grid connected systems
0.5 - 0.7 for hybrid systems
0.2 - 0.3 for stand alone systems for all year use
 

Typical System Performance
Stand alone systems have low yields because they operate with an almost constant load throughout the year and their PV modules must be sized to provide enough energy in winter even though they will be oversized during summer.
Typical professional systems in Europe have annual average yields of 200 - 550 kWp.

Hybrid systems have higher performance ratio, because they can be sized to meet the required load in the summer and can be backed up by other systems like wind or diesel in the winter and in bad weather.
Typical annual average yield is 500 - 1250 kWh/kWp depending on the losses caused by the charge controller and the battery.

Grid connected systems have the highest Performance Ratio because all of the energy which they produce can either be used locally or exported to the grid.
Typical annual yield is 800 - 1400 kWh/kWp.

Barriers
Despite a sharp decline in costs, PV cells currently cost 5 US$/Wp (4 ECU/Wp). Electricity generation costs is currently 0.5 - 1 ECU/kWh, which is higher than from other renewable energy sources. In the future, the costs of PV are expected to fall with increasing utilization. Despite its high costs, PV electricity can be cheaper than other sources in remote areas without electric grid and where production of electricity by other means like diesel is difficult or environmentally unacceptable (mountain areas).

Effects on economy, environment and employment
When the only cost-effective applications of PV systems in Europe are remote areas without electric grid, it will have a positive economical effect only for those areas.

There are no environmental effects of using PV systems. Environmental problems can occur in the production of the cells, and in the production and (improper) disposal of the batteries.

The use of PV is not expected to have any measurable employment effect in Europe for the time being.

Hand Rule
In a typical photovoltaic system based on  crystalline Silicon with 12% efficiency each kWp of installed power capacity can produce 1150 kWh of electricity per year for grid connected systems and 300 kWh/year for stand alone systems in Central Europe.

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