Solar pond

A solar pond is a special pool filled with saltwater that can collect and store energy from the sun. This works because the saltwater naturally forms layers, with less salty water on top and very salty water at the bottom. These layers are called a vertical salinity gradient, or a "halocline". Because the water at the bottom is very salty, it stays warm without mixing with the cooler water above, allowing the pond to store heat for long periods. Solar ponds are useful for heating buildings and other purposes that need a steady supply of warm water.

Description

When the sun shines on the bottom of a shallow pool, it warms the water there. Normally, warm water rises, but solar ponds stop this by using salty water. Salt is added so that the water at the bottom becomes very salty. Because salty water is heavier, it stays at the bottom and doesn’t mix much with the lighter water above. This means the bottom water can get very hot — up to 90 °C — while the top stays around 30 °C. The hot, salty water can then be used to make electricity or provide heat.

convection saturated turbine

Advantages and disadvantages

Solar ponds are especially useful in rural areas of developing countries. They can cover a large space and only need a clay or plastic liner to get started.

One downside is that salt crystals build up and need to be removed, which can be both a cost and a chore. Solar ponds don’t need a special collector, and because they hold so much heat, they can make power even at night. However, they don’t get very hot, so they only change about 2% of sunlight into energy. They also lose water to evaporation, so fresh water is needed to keep the salt levels just right.

Efficiency

Solar ponds collect heat energy, reaching temperatures between 70 and 80 °C, while the surrounding air is about 20 °C. Because of how heat works, the maximum amount of useful energy a solar pond can produce is around 17%. This is lower than power plants that reach much higher temperatures, but solar ponds can be built on a large scale and may cost less overall.

Development

Scientists are working on new ways to improve solar ponds. One idea is to use a special thin material called a membrane. This membrane sits between the layers of water and lets the layers stay separate without mixing the salt. It helps keep the pond working better.

Examples

The largest solar pond used to make electricity was at Beit HaArava in Israel. It operated until 1988 and could make 5 MW of electricity.

India was the first Asian country to build a solar pond, in Bhuj, Gujarat. It started in 1987 and finished in 1993. This solar pond could give out 80,000 liters of hot water every day. It could also give about 22,000,000 kWh of thermal energy each year. Sadly, problems happened later that stopped it from working.

In the U.S., a smaller solar pond in El Paso, Texas, powers part of Bruce Foods Corporation. It was the first solar pond of its kind in the country.