Thermal Analysis of a Solar Pond Power Plant Operated With a Direct Contact Boiler

A solar pond power plant operated with a direct contact boiler was thermally analyzed. A binary cycle system of concentrated brine, and an organic working fluid were considered. Brine temperature of 80°C, condensation at 30°C, a 75 percent efficient turbine, and 70 percent efficient pumps were specified for the analysis. The current study involved six working fluids: butane, pentane, hexane, and refrigerants R113, R114, R12. Each of these fluids exhibited a maximum efficiency of the system at characteristic operating conditions of the boiler. The system efficiency increased as the boiler pressure approached that of the pond. Net electrical outputs of 7–8.4 percent of the heat inputs were obtained for the low pressure fluids, such as pentane, hexane, and R113. Gravity flow of brine to boiler made the system efficiency approach the thermal efficiency of the power cycle. A single component vapor to turbine was considered, as the generated organic vapor appeared to contain less than 1.5 percent of water vapor for the fluids examined. Solute losses in brine by direct contact in boiler were estimated for pentane, as 120 kg per year per km² of pond, or 60 kg/MW_e•year, based on 2 MW_e/km² net electrical output. Such losses are practically negligible; however, selection of the working fluid has to be determined on an economic basis.