Solar hot water (SHW) systems harness thermal energy from sunlight in order to generate heat for domestic hot water. Similar to solar photovoltaic systems, the loss of solar insolation during the winter significantly affects production; thus, a backup water heating source will be needed to continue providing hot water during the winter. Solar thermal systems are considered renewable technologies because they use heat captured directly from the sun.
SHW systems use a collector to capture solar thermal energy, designed as flat plates or evacuated tubes. A heat exchange liquid is circulated with a pump to capture heat from the collector when the collector temperature exceeds the hot water storage tank. A heat exchanger is then used to transfer heat from the heat exchange liquid to heat the hot water tank. In climates where temperatures drop below freezing, freeze protection is required to prevent the risk of freeze damage.