Energy, exergetic and economic analysis and multi-objective optimization of atmospheric and pressurized SOFC based trigeneration systems

Abstract

In this study, the novel trigeneration systems based on the atmospheric and pressurized SOFC are proposed and investigated. The proposed systems include the SOFC based top cycles and an innovative bottom cycle which combines a transcritical CO2 cycle, a transcritical organic Rankine cycle, and a LNG cold energy utilization system. The mathematical model is developed, and the energetic, exergetic and economic analysis is conducted accordingly. Then, the effects of key parameters, such as current density, operating temperature, steam-to-carbon ratio (STCR) on system performance are evaluated. Under design conditions, the results show that the electrical efficiencies, the overall system efficiencies, the exergy efficiencies, and the total cost rate of the integrated systems can reach 57.14%, 59.56%, 55.12%, 16.73 $/h for the atmospheric system and 68.1%, 68.8%, 65.7%, 19.84 $/h for the pressurized system, respectively. Parameter study shows that the thermodynamic performance of pressurized system is significantly better than that of the atmospheric system, but economic cost is much higher. SOFC inlet temperature has a positive effect on the pressurized system performance, while this positive effect is found when the inlet temperature is less than 510 °C for atmospheric system. Besides, the multi-objective optimization by using genetic algorithm (NSGA-II) for the novel trigeneration systems has been performed. And the technique for order preference by similarity to an ideal solution (TOPSIS) based on entropy weight is performed to choose the final optimum design point of the combined systems. The multi-objective optimization results demonstrate that the exergy efficiency and the total cost rate achieve the values of 56.1%, 16.82 $/h and 66.83%, 12.02 $/h for atmospheric and pressurized system, respectively, at the optimal balance point selected by the TOPSIS method.