Effect of fuel-electrode off-gas recirculation in ReSOC system coupled with waste steam for electrical energy storage system

摘要

Reversible solid oxide fuel cell (ReSOC) is a promising candidate for electrical energy storage system with high efficiency in both charge and discharge modes. There are several ways to improve ReSOC system efficiency such as utilizing external heat, using heat storage, operating at high pressure and fuel recirculation. Fuel recirculation has been shown to increase system efficiency by increasing reactant utilization in system and heat recovery performance. However, if steam is produced by waste heat, the electrical and exergy round-trip efficiencies become important. There is a limited understanding of the compatibility of ReSOC system with various fuel recirculation scenarios. Therefore, this study investigated the effect of fuel recirculation on all round-trip efficiencies of ReSOC system coupled with waste steam. Fuel recirculation by blower and by ejector was also compared. Three groups of fuel-recirculation scenarios were investigated. The scenarios represent different temperatures of recirculated fuel off-gas, which are (1) low temperature, (2) intermediate temperature and (3) high temperature. The study also took into account the performance change of components in charge and discharge modes. The EBSILON~RProfessional software was used for system analysis. The base case results revealed that all fuel-recirculation scenarios increased round-trip efficiencies by 4 - 8 % point compared to reference system. In which, all round-trip efficiencies increased with hydrogen concentration while the reference system showed a decreasing trend at higher hydrogen concentration. Using blower for fuel recirculation resulted in slightly higher round-trip efficiencies than using ejector. The scenarios 1 had round-trip efficiencies nearly unchanged with waste steam temperature. Nevertheless, in the scenario 2 and 3, the electrical round-trip efficiency relatively increases with waste steam temperature. At very high waste steam temperature (700°C), using blower for fuel recirculation at intermediate temperature can remarkably reduce system efficiencies.