Exergy analysis and parameters optimization for zero-carbon/low-carbon fuels on SOFC-engine hybrid system

In order to investigate the exergy effects of NH3, H2, and natural gas on the solid oxide fuel cell (SOFC) and engine hybrid system, a SOFC-HCCI hybrid system model incorporating exhaust gas energy recycling technology is designed and established. In this study, the effects of fuel type, fuel flow rate, equivalence ratio, and steam-fuel ratio on the system exergy efficiency are analyzed. Besides, the exergy loss under different fuels is discussed. Eventually, operating parameters are optimized using the particle swarm optimization algorithm. Results indicate that the system exergy efficiency improves with the fuel flow rate increasing, first raises and then declines with the equivalence ratio increasing, and decreases with the steam-fuel ratio increasing. When NH3, H2, and natural gas are used as fuels, the highest exergy efficiency is 56.3 %, 60.2 %, and 56.4 %, respectively. The system exergy efficiency when H2 is used as fuel is higher than that of NH3 and natural gas at all operating conditions. The exergy destruction analysis results show that the total exergy destruction in the heat exchanger accounts for the largest proportion of 20.2 % when using NH3. The total absolute exergy destruction of the system is the largest when natural gas is used as fuel. Exergy losses of the system mainly occur in the SOFC module and the engine module, and relative exergy losses are both more than 30 %. Optimization results indicate that the system has a high exergy efficiency when the equivalence ratio is near 1.