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Solar hot water
Solar hot water describes heated water achieved through the usage of solar energy.

Technique
In order to heat water using solar energy, a collector is placed on, or forms the roof of a building, or on a wall facing the sun. In some cases, the collector may be free-standing. The working fluid is either pumped or driven by convection through it. Active control or simple physics ensures it only moves when a net gain in heat will occur.

The collector could be made of a simple glass topped box with copper pipes in it, or a set of metal tubes surrounded by an evacuated (near vacuum) glass cylinder. A parabolic mirror can also be added to concentrate the sun's light on the tube.

A simple water heating system would pump cold water out to a collector to be heated, the heated water flows back to a collection tank. This type of collector can provide enough hot water for a family, for very little or no monthly cost.

Heat is stored in a hot water tank. The volume of this tank will be larger with solar heating systems in order to allow for bad weather, and because the optimum final temperature for the absorber is lower than a typical immersion or combustion heater.

The working fluid for the absorber may be the hot water from the tank, but more usually at least in pumped systems will be a separate loop of fluid containing anti-freeze and a corrosion inhibitor which delivers heat to the tank through a heat exchanger - a coil of copper tubing within the tank.

If a central heating system is also present and heats water, then either the solar heat will be concentrated in a pre-heating tank that feeds into the tank heated by the central heating, or the solar heat exchanger will be lower in the tank than the hotter one.

The water from the collector can reach very high temperatures in good sunshine, or if the pump fails. Designs should allow for relief of pressure.