We are blessed with Solar Energy in abundance at no cost. The solar radiation incident on the surface of the earth can be conveniently utilized for the benefit of human society. One of the popular devices that utilize the solar energy is solar hot water system (SHWS). Solar water heating system is a device that uses solar energy to heat water for domestic, commercial, and industrial needs.
Working of a solar water heater
The system is generally installed on the roof or open ground, with the collectors are glazed on the sun facing side to allow solar radiation to come and connected to a continuous water supply. A black absorbing surface (absorber) inside the flat plate collectors absorbs solar radiation and transfers the energy to water flowing through it. Heated water is collected in the tank which is insulated to prevent heat loss. Circulation of water from the tank through the collectors and back to the tank continues automatically due to density difference between hot and cold water (thermosyphon effect).Water flows through the tubes, absorbs solar heat and becomes hot. The heated water is stored in a tank for further use. The water stored in the tank remains hot overnight as the storage tank is insulated and heat losses are small.
Parts of the Solar Water Heater System
A solar water heater system consists of a solar collector, a storage tank kept at a height behind the collector, and pipelines. The collector usually comprises copper tubes welded to copper sheets (both coated with a highly absorbing black coating) with a toughened glass sheet on top and insulating material at the back. The entire assembly is placed in a flat box.
Most solar water heaters require a well-insulated storage tank. Solar storage tanks have an additional outlet and inlet connected to and from the collector. In two-tank systems, the solar water heater preheats water before it enters the conventional water heater. In one-tank systems, the back-up heater is combined with the solar storage in one tank.
Three types of solar collectors are used for residential applications:
Flat-plate collector: It is the heart of a solar water heating system. It consists of an absorber plate which is coated on its sun facing surface with an absorbent coating, also called selective coating. The absorber consists of a grid of metallic tubes and sheets. Water flows through the tubes. Sheet absorbs the solar radiation falling on it and transfers it to water. The absorber plate is placed in a top open box to protect it from weather. The space between back and sides of the absorber and the box is filled with insulation to reduce heat losses. The front of the box is covered with a high transmittance glass plate. Flat plate collectors are the most common type used in the domestic solar water heating systems in India as they are relatively cheaper when compared to others.
Glazed flat-plate: collectors are insulated, weatherproofed boxes that contain a dark absorber plate under one or more glass or plastic (polymer) covers. Unglazed flat-plate collectors typically used for solar pool heating — have a dark absorber plate, made of metal or polymer, without a cover or enclosure.
Integral collector-storage systems: Also known as ICS or batch systems, they feature one or more black tanks or tubes in an insulated, glazed box. Cold water first passes through the solar collector, which preheats the water. The water then continues on to the conventional backup water heater, providing a reliable source of hot water. They should be installed only in mild-freeze climates because the outdoor pipes could freeze in severe, cold weather.
Evacuated-tube solar collectors: Evacuated Tube Collector is made of parallel rows of transparent borosilicate glass tubes evacuated for providing insulation. Each tube contains a glass outer tube and metal absorber tube attached to a fin. The fin’s coating helps absorption of solar radiation and transfers the heat to the water which flows through the inner tube.
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