3.3.6 Passive Cooling and Heating of Rooms

‘Passive design’ is the design that does not require mechanical heating or cooling. Homes that are passively designed take advantage of natural energy flows to maintain thermal comfort.
Passive design in your home:

• Significantly improves comfort.
• Reduces or eliminates heating and cooling bills.
• Reduces greenhouse gas emissions from heating, cooling, mechanical ventilation and lighting.

'Building envelope’ is a term used to describe the roof, walls, windows, floors and internal walls of a home. The envelope controls heat gain in summer and heat loss in winter. Its performance in modifying or altering climatic extremes is greatly improved by passive design. Well-designed envelopes maximize cooling air movement and exclude sun in summer. In winter, they trap and store heat from the sun and minimize heat loss to the external environment. Buildings, as they are designed and used today, contribute to serious environmental problems because of excessive consumption of energy and other natural resources. Demands of energy use in buildings and environmental damage arise because of energy-intensive solutions sought to meet the requirements of heating, cooling, ventilation and lighting.

However, buildings can be designed to meet the occupant’s need for thermal and visual comfort at reduced levels of energy and resources consumption. Energy consumption in new constructions can be controlled by way of adopting an integrated approach to building design.
Primary steps in this approach are as below:

• Incorporate solar passive techniques in a building design to minimize load on conventional systems (heating, cooling, ventilation, and lighting)
• Design energy-efficient lighting and HVAC (heating, ventilation, and air-conditioning) systems.
• Use renewable energy systems (solar photovoltaic systems/solar water heating systems) to meet a part of building load.
• Use low energy materials and methods of construction and reduce transportation energy.

Climate and architecture

India is divided into six climatic zones based on different climatic conditions. Knowledge of climate at a given location can help in the design of solar passive buildings that eliminate the adverse effects of climate, yet simultaneously take advantage of effects that are beneficial. For instance, in a place like Mumbai (Indian coastal mega city), a building can be designed in such a way that appropriate shading prevents solar radiation and adequate ventilation reduces humidity. In a place like Shimla (Indian hill station), where the climate is cold and cloudy, a building can be designed to make maximum use of sunlight, and thereby keep its interiors as warm as possible.
The various climatic factors that affect the solar passive design are listed below:

• Wind velocity
• Ambient temperature
• Relative humidity
• Solar radiation

3.3.6.1 Solar Passive Techniques

Various concepts and techniques are used to design energy-efficient buildings. Some of these are as described below:

Direct heat gain
The direct heat gain technique is generally used in cold climates. The basic principle is that sunlight is admitted into the living spaces directly through openings or glazed windows to heat walls, floors, and inside air. The glazed windows are generally located facing south to receive maximum sunlight during winter. They are usually double-glazed with insulated curtains to reduce heat loss during the night. During the day the heat is stored in walls and floors.

Thermal storage walls
In this approach, a thermal storage wall is placed between the living space and the glazing. It prevents solar radiation to enter the living space. Radiation is absorbed by the storage wall, and then transferred into the living space. Thermal storage walls include brick, cement and clay walls, water walls, transwalls.

Evaporative cooling
Evaporative cooling is a passive cooling technique generally employed in hot and dry climates. It works on the principle that when warm air is used to evaporate water the air itself becomes cool.

Passive desiccant cooling
Passive desiccant cooling method is effective in a warm and humid climate. Natural cooling of the human body through sweating does not occur in highly humid conditions. To decrease the humidity level of the surroundings, desiccant salts or mechanical de-humidifiers are used.

Induced ventilation
Passive cooling by induced ventilation can be most effective in hot and humid climates as well as in hot and dry climates. This method involves the heating of air in a restricted area through solar radiation; thus, creating a temperature difference and causing air movements or drafts. The drafts cause hot air to rise and escape from the interior causing effecting cooling.

Earth berming
Earth-berming technique is used for both passive cooling and heating of buildings. It is based on the fact that the earth acts like a massive heat sink. Thus, underground or partially sunk buildings remain cool in summer and warm in winter.


In addition to above concepts, there are many other solar passive techniques such as wind towers, earth air tunnels, curved roofs and air vents, which can be incorporated according to the requirements of the buildings.

Advantages of solar passive buildings

With the incorporation of solar passive concepts into a building a large quantity of energy can be saved. Furthermore, these concepts help provide comfortable living conditions to the inhabitants in an eco-friendly manner. However, they cannot totally eliminate the use of conventional energy for modern facilities such as air-conditioning.

Cost and payback period

The cost of a building may increase by about 5-15% because of incorporation of solar passive concepts. However, the investment may be recovered within a period of 1-7 years due to energy savings.

3.3.6.2 Passive Solar Heating

Passive solar heating is one strategy of ‘solar design’. When combined properly, this strategy can contribute to heating, cooling, and day lighting of any building. Passive solar heating in particular uses building components to collect, store and distribute solar heat gains to reduce the demand for space heating. It does not use mechanical equipments because the heat flow is by natural means (radiation, convection & conductance) and thermal storage is in the structure itself.

It is best to incorporate passive solar heating into a building during the initial design. Window design, especially glazing choices, is a critical factor for determining the effectiveness of passive solar heating. Passive solar systems do not have a high initial cost or long-term payback period, both of which are common with many active solar heating systems. In hot climates, large south-facing windows are used, as these have the most exposure to the sun in all seasons. Although passive solar heating systems do not require mechanical equipments for operation, yet fans or blowers should be used to assist the natural flow of thermal energy. Thus, the passive systems assisted by mechanical devices are referred to as ‘hybrid’ heating systems.

Architectural design of the building usually consists of: buildings with rectangular floor plans, elongated on an east-west axis; a glazed south-facing wall; a thermal storage media exposed to the solar radiation which penetrates the south-facing glazing; overhangs or other shading devices which sufficiently shade the south-facing glazing from the summer sun; and windows on the east and west walls, and preferably none on the north walls.

The following are general recommendations that should be followed in the design of passive solar heated buildings:

Five Elements of Passive Solar Home Design

Following five elements constitute a complete passive solar home design. Each performs a separate function, but all five must work together for the design to be successful.

Aperture (Collector): It is the large glass (window) area through which sunlight enters the building. Typically, the aperture(s) should face within 30° of true south and should not be shaded by other buildings or trees from 9 a.m. to 3 p.m. each day during the winter season.

Absorber: It is the hard, dark surface of the storage element. This surface, which could be that of a masonry wall, floor, partition (phase change material), or water container, sits in the direct path of sunlight. Sunlight hits the surface that is absorbed as heat.

Thermal Mass: The materials that retain or store the heat produced by sunlight are ‘thermal mass’. Difference between the absorber and thermal mass, although they often form the same wall or floor, is that the absorber is an exposed surface whereas thermal mass is the material below or behind that surface.

Distribution: Distribution is the method by which solar heat circulates from the collection and storage points to different areas of the house. A strictly passive design will use three natural heat transfer modes — conduction, convection, and radiation — exclusively. In some applications, however, fans, ducts and blowers may help with the distribution of heat through the house.

Control Roof: Overhangs can be used to shade the aperture area during summer months. Other elements that control under- and/or overheating include electron sensing devices e.g. differential thermostat that signals a fan to turn on, operable vents and dampers that allow or restrict heat flow, low-emissive blinds, and awnings.

Advantages

Disadvantages

• In areas where experienced solar architects and builders are not available, construction costs can run higher than for conventional homes, and mistakes can be made in the choice of building materials especially window glass. Passive solar homes are often built using glass that, unfortunately, rejects solar energy. Such a mistake can be costly. Choosing glass for passive solar designs isn't easy. The right glass choice depends on which side of the building (east, west, north or south) the glass is installed and the climate.
• In addition, room and furniture layouts need to be planned carefully to avoid glare on equipment such as computers and televisions.
• During the summer or in consistently warm climates, day lighting could actually increase energy use in a building by adding to its air-conditioning load.

3.3.6.3 Trombé Wall

Trombé Wall is a passive solar heating system. Trombé wall is a sun-facing wall built from material that can act as a thermal mass (such as adobe, stone, concrete or water tank), combined with an air space, insulated glazing and vents to form a large solar thermal collector. By attaching a translucent cover (fibre-glass board or glass) on the vault, the sun heating effect is created. The absorbing vault face should be painted black in order to absorb as much heat as possible.


During the day, sunlight shines through the glazing and heats the surface of the thermal mass.
At night, heat escapes from the thermal mass, primarily to the outside.
Because of the insulating glazing the average temperature of thermal mass can significantly be higher than average outdoor temperature. If the glazing insulates well enough and outdoor temperatures are not too low, the average temperature of thermal mass will be significantly higher than room temperature, and heat will flow into the house interior. Indirect gain is that the Trombé wall stores heat during the day. Excess heat is vented to the interior space. At night, Trombé wall vents are closed and the storage wall radiates heat into the interior space.

Common Modifications to the Trombé wall:

• Exhaust vent near the top is opened to vent during the summer. Such venting makes the Trombé wall pump in the fresh air during the day even if there is no breeze.
• Windows in the Trombé wall though lower the efficiency, but they may be fitted for natural lighting or aesthetic reasons. If the outer glazing has high ultraviolet transmittance and the window in Trombé wall is of normal glass, this uses ultraviolet light efficiently for heating purpose while protecting people from its harmful effects.
• Electric blowers controlled by thermostats are used to improve air and heat flow.
• Fixed or movable shades, which can reduce nighttime heat losses, might be fitted in the wall.
• It may be trellises to shade the solar collector during summer months.
• Insulating cover can be used at night on the glazing surface.
• Tubes, pipes or water tanks make part of a solar hot water system, and fish tanks as thermal mass.
• Selective surface can be increased for more absorption of solar radiation by the thermal mass.

The specific Lak’a Uta Trombé wall, built in Bolivia, is mounted to the roof after plastering. It is simple frame of prefabricated concrete elements (or small adobes). For glazing (translucent cover) flat fiber plastic boards (calamina plástica) are used. Black paint or black colored earth-mud-plaster is used to make absorbing vault face.

Benefits of Trombé Wall

• Low or zero energy consumption for heating
• Non-toxic; and low cost

Limitations of Trombé Wall

Trombé wall are an effective alternative to heating from stoves or heaters. However the design is neither simple nor easily comprehensible. A number of pre-conditions must be considered, especially, the design application, thermal conditions (well insulated, accumulation of heat, etc.) and maintenance. Before opting Trombé as low cost housing in cold climate, it is recommended to do a thorough preliminary study and appropriate detailed design. A number of web pages can help the specific design process. See the Laka Uta Trombé manual.

3.3.6.4 Passive Solar Cooling

Reducing Internal Heat Gain

Reducing External Heat Gain

Purge Heat

Insulation
Insulation is an essential component of passive design. It improves building envelope performance by minimizing heat loss and heat gain through walls, roof and floors.

Thermal mass
Externally insulated, dense materials like concrete, bricks and other masonry are used in passive design to absorb, store and re-release thermal energy. This moderates internal temperatures by averaging day/night (diurnal) extremes, therefore, increasing comfort and reducing energy costs.

Glazing
Windows and glazing are a very important component of passive design because heat loss and gain in a well insulated home occurs mostly through the windows.

Shading
Shading of glass is a critical consideration in passive design. Unprotected glass is the single greatest source of heat gain in a well insulated home. Shading requirements vary according to climate and house orientation. In climates where winter heating is required, shading devices should exclude summer sun but allow full winter sun to penetrate. This is most simply achieved on north facing walls. East and west facing windows require different shading solutions to north facing windows. In climates where no heating is required, shading of the whole home and outdoor spaces will improve comfort and save energy.

Skylights
Well-positioned and high quality skylights can improve the energy performance of home and bring welcome natural light to otherwise dark areas.