Estimation of the thickness of the lower convective layer of solar ponds

In solar ponds, the lower convective layer plays a dual role. It provides a means for the extraction of energy from the pond. It acts also as an in-built seasonal energy store. The ground beneath the pond acts as an additional energy store. To enhance the ground energy storage, a method was earlier proposed by the authors employing trapezoidal-shaped trenches at the bottom of the pond. A rigorous method is presented for the determination of the thickness of the lower convective layer in the case of a flatbottomed pond and the trench depth for a pond with trapezoidal trenches. The energy storage to be provided depends on the magnitude and the pattern of energy extraction. For a constant extraction rate, the required thickness of the lower convective layer or the trench depth increases with an increase in the rate. For a sinusoidal extraction pattern, the thickness or depth increases with an increase in the phase lag of the extraction pattern from the insolation and exhibits a minimum for the amplitude of the extraction pattern for phase lags less than about 95 days. The results also indicate that, for a given heat load (total energy extracted in a year), there exists an area for which the cost of the pond and the associated system for energy utilization is a minimum.