The top layer has a low salt content because the salt has leached out. The insulating middle layer with a salt gradient creating a density gradient sufficient to prevent heat exchange by natural convection.  The bottom layer has a higher salt content from upper layers leaching.  Because the pond's bottom is black it has high optical absorption so all the incoming sunlight will go into heating the bottom layer.  Normally as the bottom layer is heating it would rise through normal convection.  However the middle layer creates a density gradient separating the cooler surface water from the warm bottom layer.

In a desert environment, the top layer is usually around 70°F because of exposure to much cooler ambient evening air temperatures.  The bottom layer will rise to as high as 200°F.  This temperature delta is enough to run a closed-loop Rankine Cycle Turbine or a Stirling Cycle Engine. (See Solar Thermal in Primer)   The hot salt water is pumped through a heat exchanger wrapped around a tank filled with a Freon type substance with a low phase change temperature. As this fluid runs from the cooler temperature surface water to the much warmer temperature bottom layer, the sun warmed water instantly flashes the fluid into a pressurized vapor. After the vapor spins a turbine as it expands, just like an old steam engine, it is then forced into another tank where it's condensed back to the liquid stage again by cooler surface water.  This closed loop cycle continues generating electricity even after the sun goes down.  More Solar Thermal applications are explained in greater detail in the  Primer chapter on Solar Thermal Energy.

Solar thermal gradient ponds are in operation in many countries including the US, Europe, India, Israel and Australia; around 60 solar heat gradient ponds worldwide.  The largest solar pond in the US is the El Paso Solar Pond, a research, development, and demonstration project operated by the University of Texas at El Paso.  In 1985, the El Paso Solar Pond became the first in the world to deliver industrial process heat to a commercial manufacturer; in 1986 it became the first solar pond electric power generating facility in the United States; and in 1987 became the nation's first experimental solar pond powered water desalting facility.  The pond ran a 70kW(electric) organic Rankine-cycle turbine generator, and a 20,000 liters per day desalinating unit, while also providing process heat to an adjacent food processing company.

Salt Ponds or Solar Thermal Gradient Ponds have been around for quite some time.  An Israeli Company (Ormat) built an experimental 5 MW solar thermal gradient pond that operated from 1983- 1990 near the Dead Sea. This Solar Thermal Gradient Pond is a solar thermal energy technology that captures solar thermal energy and stores it in a very dense, saline layer of water at the bottom of specially designed artificial pond.  A black colored absorbing material is used as a liner in this shallow pool to maximize the absorption of solar radiation.
The salt water pond is several meters deep and as the salt leaches out of the surface water it stratifies into three zones; the upper convective layer, the non-convective layer and the lower convective layer.

Salt gradient pond at the University of Texas/El Paso

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