Development of an Efficient rSOC Based Renewable Energy Storage System

Abstract

The works presented here aim at developing an innovative renewable energies storage solution, so-called “Smart Energy Hub” (SEH), based on reversible Solid Oxide Cell (rSOC) technology. This systemis able to operate either in electrolysis (SOEC) or in fuel cell (SOFC) mode in order to store excess electricity to produce hydrogen, or when energy needs exceed local production, to produce electricity (and heat) again, from hydrogen or any other hydrocarbon fuel locally available. To be more efficient, the rSOC system is completed with an electrochemical storage solution allowing fast response to the electrical energy needs.In the frame of the REFLEX project, a SEH, made of 3 modules of 4 rSOC stacks, able to produce 16 Nm3/h in SOEC mode and up to 15 kWe in SOFC mode will be installed for an in-field demonstration in a technological park. Here the system will provide electricity and heat to the headquarters and maximize the auto-consumption of local renewable energies mixture. The rSOC system will be coupled to a Battery Energy Storage System (BESS), based on 8 lithium ion battery modules serially connected, able to store 50 kWh.Innovative work was done on each component to maximize, since this first try, the overall efficiency of the system prepared for field operation.Both cells and stacks have been optimised to increase performance (+20%) in both SOFC and SOEC modes, and their behaviour in rSOC operation have been characterized at different conditions. To increase the power density forward, enlarged cells have also been developed. The cell active area was near doubled passing from 100 cm² for reference cell to 196 cm². Concurrently, stack sectional dimension increased of about 40% only. This extension was validated up to stack level by performance testing of the enlarged stack. Voltage evolution, as current density increases, is very comparable for both stacks (reference and with enlarged cells). Voltage scattering is, also, quite similar. Steam starvation effect appears at the same level of steam conversion slightly higher than 80%, which shows that even for enlarged cells appropriate fluidic distribution was obtained.Large power modulations have been validated at stack level in three modes: SOEC, SOFC in H2 and SOFC in CH4, with ranges 58-100%, 23-100% and 13-100% respectively.To increase overall system efficiency by minimizing the energy losses in the balance of plant (BoP) components, specific reversible power electronics have been developed and an efficiency as high as 96% has been obtained, associated with a wide operation window (see figure). High performance heat exchangers and a steam generator able to generate stable steam flows have also been selected.Modelling activities have been performed to support the system design and to find the operating conditions in which the system to be built should/could be operated to ensure achieving maximum efficiency, while still operating the system in safe and lifetime-optimal conditions. The benefit of a recycling loop has been evaluated. Furthermore, techno-economic simulations have been carried out to evaluate market driven requirements on system sizing and costs to be competitive with other storage solutions.The article provides the results associated to these activities. Figure 1