What Is a Solar Pond? Benefits and Drawbacks

A solar pond may be the simplest, most economical, and most sustainable way to store solar energy. It may also be the most counter-intuitive: You don't need to have a degree in physics to know that heat rises, but in a solar pond, heat energy is stored at the bottom of the pond and insulated by cold water above it.

How Solar Ponds Work

While surprising, the physics of solar ponds are actually quite simple: The bottom of a pond is lined with salts, as much as a few meters deep, which are then heated naturally by the sun. Because the salts are heavier than water, they remain at the bottom of the pond, while the cooler top layer of water acts as an insulator of the heat below. As long as the upper layer of water remains clear and free of salt so that sunlight can penetrate to the bottom of the pond, temperatures at the bottom can reach up to near-boiling.

Depending on the size and depth of the solar pond, large amounts of heat can be stored. The deeper the pond, the longer duration of the heat storage is, though it takes longer for the storage area to reach the desired temperature. A wider, shallower pond heats more quickly, due to more exposure to solar radiation as well as higher temperatures—but it cannot store that high heat for as long of a duration. The ideal size may depend on the ultimate use case for the solar pond.

Saltwater basins such as the Great Salt Lake or the Dead Sea can have portions of their area converted into solar ponds. The Salton Sea in southern California, currently in development as a brine extraction for lithium for electric vehicles, has also been studied by NASA and others as a potential site for supplying thermal energy for electricity generation.

The Environmental Benefits of Solar Ponds

One of the main benefits of solar ponds is how little energy and materials are needed to construct and maintain them. Excavation is the most energy-intensive part of the installation process. Depending on the compactability of the underlying soil, a solar pond may need to be lined with clay or other non-porous material before adding salt. The only other materials are common table salt (NaCl) or a briny solution to fill the bottom of the pond, and freshwater.

Freshwater is periodically needed to flush salts from the upper layer and replenish water losses from evaporation. Likewise, salt or brine needs to be added to the bottom layer to account for natural losses as pond waters mix. Otherwise, the system is self-maintaining.

Solar ponds can act as year-round energy storage and are not subject to the same kinds of seasonal variability of hydroelectric storage (dams), another form of long-term storage. Heat-storing ponds are also available for a wide variety of uses, such as industrial heating, chemical production, agricultural uses, desalination, and electricity production.

Given the low cost and simplicity of solar ponds, they can be constructed close to the point where their energy is needed. Whether used for heat or electricity, this advantage reduces the requirement to transport or transmit energy or its sources long distances via pipelines, shipping, and trucking, or transmission wires. Once installed, the low-maintenance costs of solar ponds make them nearly emissions-free, and the embodied carbon in the materials can also be near zero.

Limitations and Drawbacks

Solar ponds are generally used to directly provide heat to buildings and for industrial purposes, as the efficiency of converting the stored heat to electricity is very low (2%) and is generally not economically viable. To generate electricity from a solar pond, a Rankine engine cycle is often used because the turbine it uses to produce electricity is driven by a fluid with a lower boiling point than water; the heat from a solar pond is insufficient to generate steam from plain water.

Rather than clay, durable plastic, polyethylene, or other non-renewable and potentially toxic may be needed to line the bottom of the pond. The amount of freshwater needed for constructing and maintaining the pond might be prohibitive in arid climates or where freshwater is scarce, while the opposite may also be true; an area with a high water table may prevent excavation deep enough to create a solar pond. Adequate sunlight might not be available in some regions, especially at higher latitudes where solar insolation is weaker, and regular heavy rains and monsoons can penetrate deep into a solar pond and disrupt the stability of its separate layers.