Solar water heating systems use the energy of the sun's rays to heat water before it enters a standard gas or electric water heater. By reducing the amount of power needed to run the standard water heater, a solar water heating system can reduce the average family's hot water bill by 50 percent. (See References 1)
Evacuated Tube Collectors
The most efficient type of solar water heater uses evacuated tube collectors, which are metal or glass tubes surrounded by a vacuum and contained inside larger glass tubes. Water or a freeze-proof heat transfer liquid flows through the interior tube, where it is heated by the sun. Because a vacuum surrounds the interior tube, it is extremely well insulated and can heat water even in cold temperatures or on overcast days. (See References 2)
Flat Plate Collectors
Flat plate collectors are a simpler version of the concept behind evacuated tube collectors, but instead of vacuum-insulated glass tubes, a flat plate collector uses plain copper tubes. The plate on which the parallel tubes are mounted is designed to absorb the sun's heat, and the entire assembly is held inside a well-insulated box with a glass top. Most hold 40 gallons of water and can provide for one person's hot water needs. (See References 2)
Batch collectors or integrated collector-storage systems heat water inside a large tank or box and store it there until needed. These systems are less desirable than flat plate or evacuated tube collectors, because they are prone to wide temperature variations. On hot, sunny days, the water may get so hot that it must be cooled before use. In cold climates, the water may freeze in the inlet pipe if there is no demand to move it through the system. (See References 2)
Moving the Hot Water
Solar heating systems either heat water directly or heat a transfer fluid, which then heats water indirectly inside a storage tank through a heat exchanger. Indirect systems are advantageous in cold climates, because the transfer fluid can be freeze-proof. Fluid can either be pumped through the solar water heater and into the house with an electric pump, a process known as an active or forced-circulation system, or by using the power of convection, which is a passive system. (See References 2)
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