Solar pond

 

A solar pond is a pool of saltwater which acts as a large-scale solar thermal energy collector with integral heat storage for supplying thermal energy. A solar pond can be used for various applications, such as process heating, desalination, refrigeration, drying and solar power generation.

Figure 2.10 – Solar Pond

Solar pond collects and stores solar thermal energy. The saltwater naturally forms a vertical salinity gradient also known as a "halocline", in which low-salinity water floats on top of high-salinity water. The layers of salt solutions increase in concentration (and therefore density) with depth. Below a certain depth, the solution has a uniformly high salt concentration.

A solar pond has three distinctive zones.

· The top layer is the surface zone that has a low salt content and is at atmospheric temperature. It is also called the upper convective zone (UCZ)

· The bottom layer has a very high salt content and is at a high temperature, 70°C-90°C. This is the zone that collects and stores solar energy in the form of heat and it is called the lower convective zone (LCZ).

· There is an intermediate insulating zone with a salt gradient. It establishes a density gradient that prevents heat exchange by natural convection, and hence it is called the nonconvective zone (NCZ). In this zone, salt content increases with depth, creating salinity.

When solar energy is absorbed in the water, its temperature increases, causing thermal expansion and reduced density. If the water were fresh, the low-density warm water would float to the surface, causing a convection current.

The temperature gradient alone causes a density gradient that decreases with depth. However the salinity gradient forms a density gradient that increases with depth, and this counteracts the temperature gradient, thus preventing heat in the lower layers from moving upwards by convection and leaving the pond. This means that the temperature at the bottom of the pond will rise to over 90 °C while the temperature at the top of the pond is usually around 30 °C.

The heat trapped in the salty bottom layer can be used for many different purposes, such as the heating of buildings or industrial hot water or to drive an organic Rankine cycle turbine or Stirling engine for generating electricity.