Optimization and performance analysis of a near-zero emission SOFC hybrid system based on a supercritical CO2 cycle using solar energy

Renewable energy represented by solar energy has the characteristics of intermittency and fluctuation. To ensure that the grid can operate safely and stably after the large-scale grid connection of renewable energy power, a new hybrid system consisting of a solid oxide fuel cell (SOFC), a solar power tower plant, and a supercritical CO2 (S-CO2) Brayton cycle is proposed in this study from the perspective of energy, exergy, and economy. By utilizing the cold source loss of the S-CO2 cycle to provide part of the energy required for the alkaline electrolyzer, it can efficiently produce and store hydrogen, achieving zero greenhouse gas emissions while increasing efficiency. Meanwhile, the high-temperature exhaust gas from SOFC is recovered by the S-CO2 cycle. The system considers two operation modes, switching two types of S-CO2 cycle configurations to efficiently utilize solar energy according to the variation of solar radiation energy. The thermal efficiency of the proposed system is improved by 16.61% and 6.88% compared with the other two typical S-CO2 cycle power generation systems, and the net power output reaches 8.0 MW. A parameter study shows that increasing the split ratio is beneficial to improve the thermodynamic performance of the system. However, at a split ratio less than 0.76, it is beneficial to the total cost rate of the system. The results of multi-objective optimization show that the exergy efficiency of the combined system and the total system cost rate reach 56.86%, 513.10$/h and 56.45%, 481.59$/h for the system with an additional simple S-CO2 cycle and the system with an additional recompression S-CO2 cycle, respectively, and the optimum point is selected by the TOPSIS and LINMAP methods.