Solar water heaters are part of a solar domestic hot water system that makes use of solar collectors to capture energy from the sun and storage tanks to collect and store the heated water. Depending on the geographical location and propensity for freezing to occur, different designs of these systems are used. So-called direct circulation systems pump the water to be heated through the solar collector and back into the storage system for use. These systems are employed in warmer climates where there is little to no risk of freezing temperatures.
Indirect circulation systems do not pump the water to be heated to the solar collector, but instead, circulate a heat transfer fluid that has a low freezing point. The fluid absorbs the sun’s energy and then is pumped through a heat exchanger where that energy is transferred to the water being heated.
Solar water heaters have the advantage of collecting energy that is free and are relatively simple to operate, but almost always require some form of back-up system to supplement the available supply of hot water during cloudy conditions or for higher than normal demand for hot water.