Performance evaluation of a direct-biogas solid oxide fuel cell-micro gas turbine (SOFC-MGT) hybrid combined heat and power (CHP) system

Abstract

The combination of a direct-biogas solid oxide fuel cell (SOFC) with a micro gas turbine (MGT) system offers great potential as a green decentralized combined heat and power (CHP) system. To evaluate the potential use of biogas as the main source of energy for a direct-biogas SOFC-MGT hybrid CHP system, a sensitivity analysis was conducted under diverse operating conditions to investigate the influence of key operating parameters of the hybrid CHP system with the consideration of operational constraints. The key parameters in this study were SOFC reforming agent, SOFC fuel utilization factor (Uf), turbine inlet temperature (TIT), and compression ratio. The influence of variation in operating parameters on plant performance was evaluated for the overall system and SOFC efficiencies as well as the heat-to-power ratio (TER), the power ratio of MGT to SOFC (PMGT/PSOFC), and the size of the SOFC stack. As a reforming agent for direct-biogas SOFC, steam is more preferable than a traditional air–steam mixture in terms of material limitations and SOFC efficiencies; however, an air–steam mixture with a small amount of air boosts the useful heat output and electricity generated by an MGT without significantly affecting overall system efficiency. The increase in Uf improves the electrical power output produced by the SOFC stack, but also requires more fuel to be fed to the burner, resulting in an increase in useful heat energy. Increasing the compression ratio improves the system electrical efficiency but lowers useful heat generation; nevertheless, increasing TIT decreases the system electrical efficiency but improves the efficiency of the CHP system. To achieve the optimum operating conditions of the hybrid CHP system, the operating parameters should be determined based on the desired energy outcomes.