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Solar PondsSolar Ponds | Low Temperature Solar Thermal in Australia | Further Information | ReferencesLow temperature solar thermal technologies, especially those that do not generate electricity, rely on the scientific principles behind the Greenhouse Effect to generate heat. Electromagnetic radiation from the sun, including visible and infrared wavelengths, penetrates into the collector that is absorbed by the surfaces inside the collector. Once the radiation is absorbed by the surfaces within the collector, the temperature rises.
Solar PondsWhen 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 preheated water for generating steam could utilise solar pond technology. Solar ponds use the natural properties of salt water to collect and store heat energy. Two main types of solar pond exist: salt gradient and membrane ponds.
Salt Gradient PondsThe 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 absorption of solar radiation and to prevent groundwater contamination (see Figure 1). 
Figure 1 Schematic diagram of a solar pond
(Adapted from Twidell and Weir, 1998) Sunlight is absorbed by the salty water and lining, heating the lower layers of the pond. The concentrations of the salt layers are designed so that even when the bottom layer is heated to near boiling (93oC), it is still denser than the second layer, thus inhibiting the convection of heat throughout the pond. Heat is extracted from this layer by pumping the salt solution from the bottom layer through an external heat exchanger. Alternatively, a heat transfer fluid may be used to pump through a heat exchanger placed on the bottom of the pond.
Membrane PondsMembrane ponds behave in a similar fashion to salt gradient ponds, except a thin transparent membrane is used to separate each layer of the pond. Heat is removed from the pond using the same technology as salt gradient ponds.
Solar Pond TechnologyIsrael 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. Several demonstration projects are operational in the United States of America, including a salt gradient pond used for recreational buildings and pool heating. There is an example of a 5MW system at Beit Ha'Arava in Israel (see Figure 2).  Figure 2 The El Paso Salinity Gradient Solar Pond
(courtesy of the University of Texas, El Paso).
Low Temperature Solar Thermal Technologies in AustraliaFor some of the research in solar thermal technologies in Australia click here
Further InformationRISE Resources - Information regarding available renewable energy resources.RISE Technologies - An extensive collection of information regarding renewable energy technologies.RISE Applications & System Design - Renewable energy application information and system designs.RISE System Displays - Case studies and information on installed renewable energy systems & performance data.The Energy Resources Institute – Solar Thermal The Solarserver – Solar Thermal Technologies in the United States Australian National University – Solar Thermal Energy Research Wikipedia – Solar Energy Renewable Energy Commercialisation in Australia – Solar Thermal
ReferencesTwidell, J.T. and Weir, A.D., 1986, Renewable Energy Resources. Spon Press, London. El Paso Salinity Gradient Solar Pond, 1996. Webpage. http://www.solarpond.utep.edu/ (Accessed 23 February 2007). |
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