When large amounts of low temperature heat is required other forms of solar heating, such as flat plate collectors are too expensive. For example, many industrial and agricultural applications requiring either low temperature heat or steam could utilise solar pond technology. Solar ponds use the natural properties of salt water to collect and store heat energy. The most common example of a solar pond is the salt gradient pond which consists of three differing concentrations of a salt solution, usually sodium or magnesium chloride. Dark coloured absorbing material is often used to line the pool to enhance the asborption of solar radiation and to prevent groundwater contamination.
Sunlight is absorbed by the salty water and lining, heating the lower layers of the pond.
Heat is extracted from this layer by pumping the salt solution from the bottom layer thorugh an external heat exchanger. Alternatively, a heat transfer fluid may be used pumped through a heat exchanger placed on the bottom of the pond. Membrane Ponds Membrane ponds behave in a similiar fashion to salt gradient ponds, except a thin transparent membrane is used to separate each of the layers of the pond. Heat is removed from the pond using the same technology as salt gradient ponds. Solar Pond Technology Israel leads the way in the development of solar pond technology, although several developing nations are also investigating solar ponds for providing low temperature heat requirements for industrial processes.