Salt gradient solar pond as a thermal energy storage system: A review from current gaps to future prospects

A Salt Gradient Solar Pond (SGSP) is an artificial pond or natural lake, able to collect and store the incident solar energy, characterizing by a specific vertical gradient of salt concentration. SGSPs have been studied for many years for its long-term thermal storage capacity, simplicity and low cost. This review presents an overview of the studies on the topic, presenting the achievements obtained and highlighting, for the first time on the SGSP reviews, the limiting factors where improvements are needed. In this sense, attenuation of solar radiation approaches, different numerical models developed and aspects affecting the SGSP performance are dealt with. The choice of the geometrical shape and the thickness of the zones is the first step in the design phase. The salt, in function of type and concentration, affects the stability of the pond. The heat extraction modality is crucial for the overall efficiency and a wide description of the position of the heat extraction and the cross-section shape of the tubes is provided. Then, over the last years, different enhancing methods have been proposed, such as floating rings, gels, porous and phase change materials, to maximize the performance and to reduce the heat losses with the environment. At the end, the key points on which further research should focus are discussed.