Investigation and optimization of the performance of a solar assisted power generation system working with CO2

In this study, the performance of the solar energy-based power generation system working with CO2 was investigated, and multi-objective optimization was carried out. For this aim, the dynamic mathematical modeling of the evacuated U-tube solar collector was performed at first using computer software. The results of the modeling were compared with an experimental research in the literature, and the deviation between the theoretical and experimental study was found to be 6.3%. Afterward, the performance analysis of the solar collector was carried out using the meteorological data of Isparta. From the analysis, it was determined that the fluid temperature could reach around 250 ºC. The thermal energy absorbed by solar collectors was assumed to be used for driving the CO2-based power cycle. In this regard, the performances of different CO2-based transcritical Rankine and supercritical Brayton cycles were examined to determine the best cycle suitable for the solar thermal process. According to the comparative analyses, the best performance was obtained for the supercritical Brayton cycle with intercooling and reheating for low and mid-temperature applications. In the last part, thermodynamic, exergo-economic and exergo-environmental analyses of the determined cycle have been conducted under steady state and dynamic conditions followed by optimization. According to the results, the annual solar energy falls on the solar collector was determined to be 17 696.8 kWh, the absorbed heat energy by the working fluid circulating in the collectors was calculated as 4 800 kWh, while the average energy efficiency of the collector was 27.12%. Considering the analyses, the annual power generation by the supercritical cycle was found to be 547.03 kWh, while the average energy and exergy efficiencies of the solar energy-based system were calculated as 3.09% and 3.31%, respectively. For the transient conditions, the optimum number of collectors was found to be 20 according to the results of optimization. In addition, the exit temperature of the fluid from the collector was estimated as 165 ºC, the net power produced was 0.311 kW, the total capital cost of the cycle was 31 529 $, and the sustainability index of the system was 1.042 with respect to the optimization results.