We know that water and air become lighter and rise above, when heated. When water is heated by solar radiation, hot water from the bottom of the pond rises and reaches the surface. It loses the heat gained, to the atmosphere. The result is that pond water remains at a temperature nearer to that of the atmosphere. This loss of heat is prevented by dissolving salts in the bottom of the pond. It makes water denser and hence, it cannot rise to the surface. Thus, solar energy remains trapped in the pond.
India was the first Asian country to have established a solar pond in Bhuj, in Gujarat. It is designed to supply about 220 lakh kwh of thermal energy annually.