HIGH-EFFICIENT reversible solid oxide fuel cell coupled with waste steam for distributed electrical energy storage system

Recently, the penetration of renewable energy into the power sector has dramatically increased; thus, electrical energy storage (EES) systems with long duration time, high capacity, and high energy density are essential. Reversible solid oxide fuel cell (RSOFC) systems have become a promising candidate for this important role. In this study, a new RSOFC system coupled with waste steam considering the compatibility between charge and discharge modes is proposed. A lumped model of RSOFC stack was integrated into EBSILON^®Professional commercial software for system analysis. Parametric studies were conducted to examine the effects of fuel composition, waste steam temperature, and steam conversion ratio on three system round-trip efficiency types: reference system round-trip efficiency (η_RT1), electrical round-trip efficiency (η_RT2), and exergy round-trip efficiency (η_RT3). Base case calculation shows that the system round-trip efficiencies η_RT1, η_RT2, and η_RT3 are 37.9%, 53.8% and 49.6%, respectively. In the parametric analysis, as hydrogen volume concentration in the hydrogen-steam mixture in the SOEC mode was increased from 10% to 60%, η_RT1 and η_RT2 increased, reaching a maximum value of 54.2% and 38.7% at 30% and 40% of H₂ concentration, respectively. The exergy round-trip efficiency had the same trend as η_RT2 and reaches maximum value of 50.1%. The waste steam temperature had a small effect on all round-trip efficiency types. The increase in steam conversion ratio relatively improved η_RT1 and η_RT3, but negligibly influences η_RT2. The proposed system provided a solution for upgrading the EES system performance via integrating a RSOFC system with low exergy waste steam. The efficiencies η_RT2 and η_RT3 were much higher than η_RT1 because of the use of waste steam. The exergy round-trip efficiency with consideration of waste steam exergy was a good indicator of system performance.