3.3.2 Solar Water Heater

Water heating is one of the most common applications of solar energy for domestic and industrial applications. Like solar dryers the water-heating systems are also available in natural convection and forced convection designs. Natural convection water heating system is also known as thermo- siphon water heating system that consists of a flat plate solar collector, insulated water storage tank and necessary insulated pipe fittings. Solar water heaters collect sun energy. They usually have rooftop solar collectors with arrays of piping and net of metal sheets.

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Solar water heaters collect the sun’s energy usually with rooftop solar collectors with arrays of piping and net metal sheets, painted black to absorb as much radiation as possible, and a hot water storage tank to enable hot water use at night. Usually the collectors are encased in glass or plastic and angled towards south to catch maximum sunshine. They are painted black to absorb as much radiation as possible. The collectors act as miniature greenhouses trapping heat under their glass plates.
In areas where air freezes the solar collectors must be filled with anti-freeze or emptied when frost risk occurs. In warmer areas, the water for use can be circulated directly through the collector. If the storage tank is above the collector, the hot water can circulate from the collector to the storage having natural convection’ no pump is required.

The size of solar collectors needed vary greatly with location and the acceptance of less hot water some days. In a location with a minimum insolation of 4 kWh/day in monthly average, such as most of South Asia, choose 3 m2 pr. 100 ltr of daily hot water use. This will give sufficient hot water except for a few days a year. The hot water tank should be large enough to store hot water from day till night, often is used the same size as the daily hot water use; but if most hoit water is used during the day, it can be smaller. The tank must be insulated well enough to keep the temperature during the night, with for instance maximal 5’C temperature loss.
Efficiency of solar water heaters to convert sun energy into heat energy is 25-50%. The efficiency actually depends on the system and how much the water temperature is higher than the surrounding temperature. Daily solar energy inflow varies between 4 and 8 kWh/m2 as monthly average in typical South Asian locations. It gives heat production of 1-4 kWh/m2 per day depending on type, usage, etc.

The system need to be installed by people with plumbing skills. A short training (a few days) on special requirements for solar water heaters is an advantage, in particular to avoid design flaws where the hot water is cooled by unwanted circulation or un-insulated pipes or tank. The pipes and system must be good enough to withstand corrosion of the water used, and the pipe-combination that can cause corrosion must be avoided (such as iron and cupper in the self-circulating system). The collectors must be placed in free sunshine, faced south or horizontally. The inlets and outlets of the tank should be placed so cold water is not mixed with hot water outlet. Cold water should be let into the tank in the bottom, preferably in a horizontal inlet.
A regular inspection by skilled people is recommended, for instance every 2. year. In systems with back-up heating, such as electric water heating, it is recommended to turn off the back-up heating from time to time to test the output of the solar heating. Lifetime solar hot waters are often 20 years if well maintained and if corrosion problems are avoided.
Solar hot water is used for washing, showers and cleaning, as well as for industrial appliances. An important application is in health facilities. In off-grid installations a ‘high hot water tank’ can be used as storage tank, filled with hot water once a day in the morning. High-temperature collectors with mirrors to concentrate the sunlight (parabolic dish or trough shaped) can produce steam that can be used in industries or in special solar power plants.

Life of solar water heaters is often 15-20 years if well maintained and if corrosion problems are avoided. Estimated cost is approximately Rs. 18000 for a 100 litres capacity of Solar Water Heater (SWH). For higher capacity, it ranges from Rs. 110 to Rs. 150 per installed litre.

Applications:

Residential Buildings – Domestic Systems: The domestic solar hot water systems substitute or supplement electrical geysers in bathrooms and kitchens used for bathing and washing purposes.

Institutional/Commercial Systems: Commercial uses of solar hot water systems are in hotels, hostels, hospitals, holiday resorts and pilgrim centers. Most of the institutional systems are large-scale hot water plants ranging from 1,000 to 10,000 liters storage capacity. The systems are ‘forced circulation’ ones fitted with electrical pumps to aid the circulation.

Industrial Systems: Boilers supply low-pressure steam ranging from 110°C to 150°C. Source of energy in the conventional boilers is furnace oil or coal. As energy conservation measure, solar hot water systems can be used for pre-heating of boiler feed water to meet the energy requirement partly. Flat plate collectors can provide hot water at about 80°C on sunny days at locations having favorable climatic conditions.