The simplest type of solar pond is very shallow, about 5 to 10 cm deep, with a radiation absorbing (e.g., black plastic) bottom. A bed of insulating material under the pond minimizes loss of heat to the ground. A curved cover, made of transparent fibre glass, over the pond permits entry of solar radiation but reduces losses by radiation and convection. In shallow solar pond, the water soon acquires a fairly uniform temperature. However, experience shows that the water in such a pond usually heats up only a few degrees, because of the natural convection currents which are set into motion as soon as heat is absorbed at the bottom. In a deeper pond also temperature variations generally exist. Loss of heat from the surface, especially at night, then results in circulation of water by convection.
Most people know that fluids such as water and air rise when heated. The salinity gradient stops this process when large quantities of salt are dissolved in the hot bottom layer of the body of water, making it too dense to rise to the surface and cool.
Generally, there are three main layers. The top layer is cold and has relatively little salt content. The bottom layer is hot up to 100°C (212°F) and is very salty. Separating these two layers is the important gradient zone. Here salt content increases with depth. Water in the gradient cannot rise because the water above it has less salt content and is therefore lighter. The water below it has a higher salt content and is heavier. Thus, the stable gradient zone suppresses convection and acts as a transparent insulator, permitting sunlight to be trapped in the hot bottom layer from which useful heat may be withdrawn or stored for later use.