Transient Analysis of Lower and Ground Zone Energy Extraction in Solar Ponds with Temperature Drop across Exchangers

Abstract

Solar ponds involve energy extraction generally from their lower convective zone. Sometimes, energy is extracted from ground below them too, in order to enhance the output. A transient model is presented that studies such a pond system. The novelty lies in the fact that temperature drop across exchanger surfaces in the two zones are included into the relevant equations, which has not been considered in earlier transient models. A transient analysis for a one-year period under the climatic conditions of Copiapo, Chile has been carried. The finite difference method has been used to solve the relevant equations via a MATLAB code. The model is successfully validated with a simpler version available in the literature. It has been shown that not considering this effect leads to a significant overestimation of the performance. A maximum error of about 13% is seen in the value of outlet temperature of the extracted water stream. Further, it is observed that exchanger pipes in the ground should be of as little cross section as possible. It is also seen that places where year average ambient temperature ground isotherm is deeper prove more productive for such pond systems. Finally, it is observed that mass flux corresponding to ground energy extraction should be low from outlet temperature point of view but should be higher from outgoing energy perspective.