Solar Ponds

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Solar Ponds Unlike ocean waters where warmer water is at the surface, solar ponds are shallow lakes of salty water where higher temperatures are in the bottom layers and colder temperatures are near the surface. As solar radiation penetrates shallow waters, it is absorbed at the bottom, raising its temperature. If the water is fresh, the buoyancy causes mixing of the water and temperature will soon become uniform throughout. If water contains some salt, it becomes heavier than fresh water and sinks to the bottom, retaining the heat and temperature gradient (Figure 10-14). Temperatures as high as 95°C can be reached at the bottom layers. The temperature difference between the bottom and the surface layers can be used to design and construct large-scale power production and desalination plants. Heat is extracted from hot salty fluid at the bottom of the pond and passed through heat exchangers to heat a working fluid, causing it to evaporate. The vapor is used to drive a turbine, similarly to conventional steam power plants. Fresh water is produced as the byproduct of these processes. To operate the plant continuously, the salt concentration gradient must be maintained. As a result of convection, there is always some diffusion of salt from the bottom to top layers. To maintain stability, salt must be added at the bottom and removed at the top. Larger ponds and calmer wind conditions are preferable. Larger ponds have a larger surface to perimeter ratio, and convective effects are less important. To prevent wind from disturbing surfaces and to reduce salt mixing, smaller ponds install 17 Avery, W. H., and Dugger, R. D., “Hydrogen Generation by OTEC Electrolysis and Economical Energy Transfer to World Markets via Ammonia and Methanol,” NREL Business/Technology Books: ISBN 0899343317, 1997. 18 McNichols, J. L., et al., “Thermoclines: A Solar Thermal Energy Resource for Enhanced Hydroelectric Power Production”, Science, 203, pp 167-168, January 1979. Figure 10-14 Solar pond 231 Chapter 10 - Solar Energy suppression rings that cover pond surface. Additionally, many ponds contain a large amount of salt making them naturally suitable as solar ponds. The largest solar ponds are in Israel, where the hot, dry climate is ideal for their operations (Figure 10-15). Solar ponds have found numerous applications in drying, desalination, process heat, refrigeration, and power generation. Their cost is considerably lower than that of flat plate and photovoltaic solar collectors. Besides the initial cost of construction, the only cost associated with solar ponds is that of maintenance. This includes preventing growth of algae in the upper convection layer and maintaining the salt gradient.


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