Improvement of salt gradient solar ponds' performance using nanoparticles inside the storage layer

The solar pond is one of the usual tools used for solar energy harvesting in the world, although its low efficiency is still a big challenge. In this study, to enhance the thermal efficiency of the laboratory-scale salinity gradient solar pond, three different nanoparticles, i.e., SiO₂, Fe₃O_4, and ZnO, were dispersed into a salt-water solution as the based liquid. Nanoparticles' mass concentrations used in the based liquid were 0.012%, 0.036%, and 0.06%. The simulated sunlight thermal energy was supplied by a 500 W halogen lamp. The prepared nanofluids were used as lower layer of the pond. Approximately 3 days after filling the pond, the stable salinity gradient was formed and after almost 2 days of exposure to the simulated sunlight, the temperature of the pond layers reached thermal equilibrium. The results showed that lower layer temperature increased continuously with nanoparticles' concentration, for all nanofluids. Based on the measured temperature, the pond thermal performance was calculated, showing that all mentioned nanoparticles could enhance the thermal efficiency of the SGSP. The maximum lower layer temperature (~ 47 °C) and thermal efficiency enhancement ratio (35.13%) were obtained for the 0.06% ZnO nanofluid. This nanofluid showed the minimum light scattering (particle size parameter = 0.248) and transmittance (near zero) over the UV–Vis wavelength, leading to the increased thermal efficiency of the pond, as compared to the SiO₂ and Fe₃O₄ nanofluids. In addition, using nanoparticles, the time required to reach equilibrium conditions in the pond was decreased, with the maximum of about 9 h.