Experimental study on thermal behavior of new mixed medium phase change material for improving productivity on salt gradient solar pond

Five identical cylindrical salinity-gradient solar ponds (SGSPs) with internal heat exchanger were constructed and operated. In order to optimize the performance of the SGSPs, mixtures of NaCl and Na(2)SO(4)were used. Radiation flux, temperature of the zones, ambient temperature, inlet, and outlet temperature of the internal heat exchangers were measured. It was shown a controlled amount of Na(2)SO(4)improves the thermal and salinity stability of the pond in the normal operation and heat extraction and lowers the pond temperature drop ratio to the water outlet temperature drop during the heat extraction, which means an improvement in the energy storage capacity. The pond with higher percentage of Na(2)SO(4)requires less time to stabilize. Higher percentage Na(2)SO(4)reduces the density gradient between the upper and lower convective zones of the pond and leads to rapid destruction of the upper layer. Maximum ability of heat extraction corresponds to the pond with 0.75% Na2SO4. In addition, to prevent the algae growth at higher percentage Na2SO4, spraying of HCl on the pond surface was used.