A novel electrical energy storage system based on a reversible solid oxide fuel cell coupled with metal hydrides and waste steam

Reversible solid oxide fuel cells (RSOFCs), with high energy densities, long operating times, and intermediate power ratings, have become promising devices for renewable energy storage. A metal hydride (MH) tank is a prospective thermochemical heat and hydrogen storage unit. External heat source such as waste steam is a well-known efficiency booster for high temperature electrolysis system. Here, we propose a novel RSOFC system coupled with MH and waste steam. The MH materials of MgH2-5 at.% V and LaNi5 were used for high-temperature MH (HTMH) case and low-temperature MH (LTMH) case calculations, respectively. We found that, In HTMH case, the H-2 compression power was low, but the MH tank produced steam only during the last 29% of the total absorption time. When the MH tank produced steam, the SOEC mode efficiency increased by 19.3% points. In the SOFC mode, the MH tank stored 76% of heat released from stack, and the system efficiency was lower than stack efficiency by 6% points. The system round trip efficiencies of HTMH system and LTMH system were 45.6% and 48.1%, respectively. For a specific HTMH material, there is an optimal current density in the SOEC mode where heat from MH tank can be used completely and the external heat source is minimal. By choosing appropriate operating strategy or MH material, the high temperature MH can result in a system round-trip efficiency comparable to that of a low temperature MH combined with an external heat utilization system.