Revisiting Gradient Layer Heat Extraction in Solar Ponds Through a Realistic Approach

Abstract

In this paper, the concept of heat extraction from the gradient zone (GZ) in solar ponds has been analyzed in a more realistic manner to overcome the drawbacks of previously conducted studies. For this purpose, a net heat transfer coefficient has been invoked to investigate the heat transfer occurring from the GZ to the exchanger installed in this zone, in addition to the storage zone (SZ). Analytical solutions for temperature profiles in the GZ and the corresponding exchanger have been obtained which are further used to investigate various aspects of the thermal performance of the pond. The consideration of realistic heat transfer across the GZ exchanger reveals that the ideal thickness of GZ yielding maximum power output is always under-predicted by the idealized assumption of the literature. Unlike intuitive perception, the conventional assumption of an infinite heat transfer coefficient does not affect the pond stability because, for all practical purposes, the critical salt diffusion rate predicted by it is always larger than the actual critical value required for ensuring stable pond operation. However, as expected, the rate of exergy destruction caused by the pond’s operation is found to be underestimated by the idealized assumption. This study provides a useful analytical tool to make more realistic predictions on various performance parameters of solar ponds utilizing the heat stored in their GZ.