Salinity gradient energy: Solar ponds

Solar ponds generate so-called salinity gradient energy, by using the difference in salt concentration in layers of stratified salt ponds. It's heat that is harvested in solar ponds, in contrast with the type of salinity gradient energy that is called blue energy (see for example this article in New Scientist about the pilot plant at the Dutch Afsluitdijk or this quick comparison of RED versus PRO, the two main concepts.

Several countries, notably Israel and Australia, operate solar ponds. Solar ponds harvest heat added to water by the sun. Salt features prominently in solar ponds as well, but here it is a construction material.

Solar ponds are ponds with at least three separate layers of clear water with different salt concentrations. Salt provides the factor that stabilizes the layers: Density.

Normally, when sunlight heats the bottom of a pond and the lower layers, this water becomes lighter and starts to rise. This starts up convection, which attempts to achieve a stable density distribution. Water with a high salt concentration is much denser - heavier - and remains much denser when it heats up.

Natural water bodies display these phenomena too. The denser water is at the bottom; the lighter water is at the top and the middle layer has a stable gradual salinity increase (or decrease, upward), which is called the halocline. Ocean water does have possibilities for vertical mixing, however.

The Black Sea, to name an example, is on the other hand a stratified basin where water of a higher salinity flows over a threshold (a sill) and is kept behind that threshold. A large number of rivers, including the Danube, flow into the Black Sea, where the lighter freshwater flows over the heavier saltier water. There is almost no verticam mixing.

Solar ponds resemble the Black Sea in their rigid stratification. The top and bottom layers of a solar pond can have convection, but the middle layer is non-convective and acts as a transparent insulator. The top layer has to be topped up regularly to make up for evaporation. The bottom of the pond - often provided with a dark layer (high optical absorption) - heats up and the bottom layer stores the heat.

This hot brine can then be passed along or through heat exchangers. Temperatures of 65 to 80 degrees C can easily be achieved; flow must be high enough to avoid boiling.

India had the Bhuj pond. It was completed in 1993 and provided hot water to a dairy plant until 2000. It supplied 80.000 liters of hot water daily and covered 6000 m2. Financial troubles at the company that took over the pond operation, followed by an earthquake that put the dairy plant out of action put a stop to this solar pond.

The US, Australia and Israel are examples of other countries that have solar ponds. The University of Texas has a solar pond in El Paso and the company Enersalt is currently commercializing solar ponds in Australia.

• Low efficiency;
• Requires large areas of land;
• Biofouling in upper layer;
• Brine leakages to be avoided; freon used in a closed circuit;
• Water has to be kept clean (transparent).

• Clean;
• Easy.

Some solar ponds use membranes to help separate the brine layers, but most do not.

Disclaimer: Angelina Souren and SmarterScience cannot be held liable for any decisions made on the basis of information given on this web page.

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