Numerical investigation of the nanofluid effects on the heat extraction process of solar ponds in the transient step

In this paper, a modification of previous thermal modeling methods of solar ponds has been employed to simulate the heat extraction process by nanofluids in the transient step. A hypothetical rectangular solar pond with a cross section of 10 × 10 m2, and 2.7 m depth has been considered as the case study. The city of Tehran has been assumed to be the location of the pond. Using the synoptic data of the location, the heat storage process is modeled in August 2016. The model showed that after 24 days, the temperature of the lower convective zone (LCZ) reaches to 98.66 °C with 40.5093 GJ of stored thermal energy. At this point, the heat extraction process is modeled for a 48 h period after which this pond cannot provide enough thermal energy anymore. The thermal performance of heat removal for water and six different water-based nanofluids are investigated and compared theoretically. The nanofluids include Ag/water, Cu/water, CuO/water, Al2O3/water, SWCNT/water, and MWCNT/water. Also, different volume fractions between 0.1% and 5% are modeled. The amounts of the extracted heat for 15 different concentrations of the selected nanoparticle types are calculated. Using the results, the threshold concentration has been determined for each type. Moreover, the heat extraction rate, the percentage of the extracted heat, and the mean outlet temperature are investigated at the threshold concentrations. For all of these parameters, the SWCNT/water nanofluid has determined to have the best performance at its threshold concentration of 0.1%v/v. For the amount of the extracted heat, the model showed the minimum amount of 5.8141 GJ for water, and the maximum amount of 10.2892 GJ for the SWCNT/water nanofluid at its threshold concentrations.