A solar water heater consists of a solar collector that absorbs solar radiation and generates usable thermal energy. The thermal energy or heat is then transferred to a fluid medium, such as water, another heat transfer fluid, or air, which flows through the collector. The thermal energy can also be stored for night time use or other times when solar radiation is not available. A major application of solar water heating systems is to provide hot water to individual homes. Water can either be heated directly in the collector (direct systems) or indirectly by a heat transfer fluid that is heated in the collector and passes through a heat exchanger to transfer its heat to water. An auxiliary circulating system, such as a pump, is not necessary for the thermosyphon and collector storage systems. They are called passive systems. The three remaining systems are called active systems and a pump or fan is employed to circulate the fluid. Heat pipe systems are also explored for the use in solar water heaters. A heat pipe is a self contained system that transfers heat by evaporation of the working fluid and condensation of vapor. Since there is no moving parts involved the system is more reliable.