Utilizing supercritical carbon dioxide/propane mixture for efficient heat extraction from salinity gradient solar ponds

Efficient heat extraction from salinity gradient solar ponds remains a critical challenge for their practical application and widespread adoption. This study addresses this challenge by investigating the use of supercritical carbon dioxide (sCO2)/propane binary mixtures as high-performance heat transfer fluids in the internal heat exchanger of solar ponds, which represents a novel approach in this field. A comprehensive three-dimensional numerical analysis is conducted, and the system is optimized using response surface methodology with a Box-Behnken design and Analysis of Variance to systematically assess the influence of design and operational parameters on key performance indicators, including heat extraction rate, outlet fluid temperature, and pumping power. The results reveal that introducing propane modifies the flow dynamics of sCO2 by altering the balance between centrifugal and buoyancy forces, which reduces the Richardson number and significantly affects the thermal and hydraulic behavior. Notably, higher mass fluxes amplify thermal variations, while increased propane content stabilizes them. Pumping power initially decreases with propane addition up to a 60 % mass fraction, followed by a slight increase. Optimization demonstrates that mass flux predominantly governs heat extraction, whereas propane mass fraction is more influential in increasing the outlet temperature. The best-performing configurations achieve a 110.6 % increase in heat extraction rate and a 4.8 % increase in outlet temperature compared to the central point, highlighting the potential of sCO2/propane mixtures for efficient solar pond thermal management.