Solar Evaporation Ponds, also known as Solar Ponds, are innovative systems that capture and store solar thermal energy. In use for more than a hundred years, these ponds leverage some basic physical principles to heat water at the bottom of the pond, using salt to change the density of the water and prevent the convection of heat to the surface.
Heat from the sun warms the bottom of the pond, but as salt is added to the water, the bottom layers become more dense , and sink rather than rise, turning the normal process of convection on its head, and trapping heat at the bottom, covered by the less dense and less salty layers, which provide an insulating cover. This process works so well that the heated layers at the bottom of the pond are hot enough to drive turbines to create electricity.
The solar pond uses similar physical principles to the Salt Evaporation Pond, but to a different purpose. Here, the goal is not to evaporate the water and retrieve the salt, but to salt the water and retrieve the heat. And while electricity storage is still a difficult challenge for us, storing heat is a lot easier – some ponds can retain heat without sunlight for a month or more.
The steps of the solar evaporation pond are concisely described in this way:
- The Salinity Gradient. A pond of salt water begins to create a gradient of salinity, as less salty and less dense water rises while saltier and denser water sinks, with most dense at bottom and least dense at the surface.
- Solar Absorption. Sunlight penetrates the water and is absorbed at the bottom of the pond, while less saline water takes some of the heat passing through to become even more diffuse and rise.
- Heat Trapping. The high-salinity bottom layer becomes heated but cannot rise because of its greater density, preventing convection and heat loss.
- Insulation. The top layer acts as an insulating blanket, further reducing heat loss.
- Heat Extraction. The stored heat can be extracted for various applications, such as electricity generation or industrial processes.
In terms of electricity generation, the solar pond can use about one-fifth of the land area required by solar panels to produce the same amount of electricity. And although the generative efficiency of photovoltaic systems is greater than the solar pond, the life span of the pond is greater. As with any pond, a suitable pond liner is important to prevent seepage and for ease of maintenance.
In Israel, the Beit HaArava pond was the largest solar pond for electricity generation, covering 210,000 square meters and producing 5 Megawatts of power. In India, the Bhuj, Gujarat pond supplied 80,000 liters of hot water daily to a dairy plant. And in El Paso, Texas, a solar pond comprising four-fifths of an acre powers 20% of a food corporation’s operations.
Solar ponds can be integrated into existing renewable energy systems, especially with their ability to store heat when other energy generation sources such as sun and wind are unavailable. This brings stability to a naturally intermittent power system. Recent advances in solar pond technology include new methods for extracting heat that enhance their overall performance and energy output. And research is ongoing to improve the salt-gradient structure of solar ponds, as well as developing new salts and additives that can enhance the performance and stability of solar ponds.
