Solar ponds are a type of passive solar energy technology whereby pools of saltwater are used to collect and store solar thermal energy — making use of the natural formation of a salinity gradient in such bodies of water, whereby heat isn’t easily transferred between a high-salinity layer at the bottom of the body of water in question and a low-salinity layer on top.
As a result of this naturally occurring vertical salinity gradient — known as a halocline — heat stored in the lower portions of such a “solar pond” isn’t easily lost, and can thus be used at will to move turbine and thus to generate electricity.
While quite a different way of making use of passive solar energy design principles than say passive solar houses, solar updraft towers, or solar thermal power plants, solar ponds are fairly economically attractive — due to the low costs of the materials in question, and the low-tech nature of the systems.
Despite that being the case, the technology hasn’t been widely deployed in recent times — with only a few large-scale deployments being known in the western world. Below I’ll provide a bit more in the way of details.
Solar Ponds — Utilizing Vertical Salinity Gradients To Store Solar Thermal Energy In Water
To reiterate the above, the basic idea of solar ponds is to utilize the stratification and lack of mixing between different layers in a body of saltwater to store heat in the lower layers which don’t easily mix with the higher ones.
In other words, one captures heat in the bottom layers (when sunlight hits the preferably dark bottom of the pond) and then stores it there naturally due to the lack of convection in highly saline bodies of water (between layers, that is, not within layers where convection does occur readily).
To explain that yet another way — as depth in a saline body of water increases, so does salinity; once a certain concentration and depth are reached, a saturated-brine of a near-constant salt concentration is formed. In this brine, convection will occur easily but not with other less saline layers, only internally. As a result of this, heat loss is minimized.
It’s possible using such a system for lower brine layers in a solar pond to stay near a constant ~70–90° Celsius while the higher lower-salinity layers remain around ~20–30° Celsius — depending upon specific design.
As a result, those wanting to generate electricity using this high-temperature water can easily and leisurely do so due to the lack of heat loss in the solar pond in question.
As noted earlier in the article, the primary advantage of such a system is its low-tech nature — all that one needs to do is a dig a pond and fill it with a highly saline water, and then install a system of taking heat and/or water from the lower brine layers. Other advantages include 24/7 (or nearly so) operation in relatively warm or temperate climates, and easy maintenance (all that really needs to be done is the addition of freshwater on top occasionally).
Disadvantages largely revolve around the large amount of space involved and the low conversion efficiencies (solar energy to electricity conversion efficiencies, that is).
Solar Pond Examples
There aren’t many examples of commercial-scale solar ponds out there — really just the Beit HaArava solar pond (formerly) in Israel; the TERI/GDDC project in Bhuj (India); and Bruce Foods Corporation’s facility in El Paso (Texas).
All 3 projects in question have reportedly functioned quite well but have not been replicated elsewhere.
The Beit HaArava solar pond — totaling an area of 210,000 m² -– reportedly featured a peak output of 5 MW and was operated until 1988. The Gujarat Dairy Development Corporation project in India reportedly supplied around 22,000,000 kWh of thermal energy a year to the dairy plant — before the regional dairy collapsed following the Bhuj earthquake and GDDC faced financial problems. And the Bruce Foods’ solar pond — totaling 0.8-acre (3,200 m2) — reportedly provides around 20% of the energy to the plant in El Paso.
Image via Solvay
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