Solar hot water systems are one type of solar thermal energy system since they are simple and designed only to heat water (as opposed to drive a steam turbine to produce electricity). As a result, they can be extremely cost effective, paying for themselves in under five years, and are typically cheaper than photovoltaic solar arrays.
See also: the solar energy diagram
You can build a solar hot water system with simple tools from the local hardware store. All you need is a surface that can absorb heat and transfer it to a closed loop of piping that contains water. However, the system must circulate water effectively, and it must be heat- and freeze-proof.
There are two primary types of solar hot water systems: passive (or “close-coupled”) systems position the water tank next to, or above, the solar collectors. Water moves through the system via thermosiphon flow, which is a fancy way of saying that hot water goes up and cold water goes down without a pump. Pump-circulated systems simply add a mechanical pump to the mix.
Like photovoltaics, solar hot water systems need steady insolation to function properly, and they may not receive this for all of the year. In the case where there isn’t enough sunlight in winter, hot water systems can be augmented by natural gas or electric heat.
Solar hot water systems are used extensively in Israel, Greece, Turkey, China, Australia, Japan, Austria, and China.