Active indirect solar water heating system
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Basically, solar water heaters can be either active or passive. Active systems use electric pumps, controls, and other such equipment to circulate water or heat-transfer fluids through the collectors. They also require electric power to activate pumps and/or controls.
Passive solar water heating systems require no moving parts and rely on thermodynamics rather than electric power to circulate the household water or a heat-transfer fluid through the system. Passive solar water heaters are the simplest solar water heating systems and they are also called bread box or batch heaters. These systems are most common in regions that do not experience extensive periods of below freezing temperatures.
Active solar water heaters can be further characterized as direct (open loop) or indirect (closed loop) systems. These terms explain the way the water is heated. With direct heating system household water from the storage tank is circulated through the solar collector, heated by the sun and returned to the storage tank. Direct active systems use pumps to circulate household water through the collectors. Direct solar thermal systems work best in warmer climates where the system is less prone to freezing.
Indirect systems use a heat-transfer fluid (usually a glycol-water antifreeze mixture) to collect heat and a heat exchanger to transfer the heat to household water. Indirect active systems also use pumps to circulate heat-transfer fluid through the collectors. Heat exchangers transfer the heat from the fluid to the household water stored in the tanks. Indirect systems are popular in areas subject to extended freezing temperatures because they offer good freeze protection.
Direct systems are more efficient that indirect ones but they require more maintenance to keep the pipes clear of mineral deposits.