Thermal Management of a Reversible Solid Oxide System for Long-Term Renewable Energy Storage

The ambitious plan of California to provide 50% of its electricity from zero-carbon sources by 2025, 60% by 2030, and finally 100% by 2045 results in a substantially expanded use of intermittent renewable energy sources like solar and wind power. One of the challenges in implementation of this plan is to design energy storage systems that are able to harmonize the intrinsic fluctuations of these sources and meet the mismatch between the time and location of electricity generation and consumption. In this study, a self-sustaining and efficient energy storage system has been designed for integrating various sources of intermittent renewable energy into buildings with different electricity demand patterns. The case study investigates the challenges of coupling a wind farm in Palm Springs with a large-scale industrial building using a novel solid oxide fuel cell (SOFC) and solid oxide electrolysis cell (SOEC) system for renewable hydrogen energy storage.

The system is sized for an 18 MW wind farm which is complemented by an integrated 10 MW SOEC (nominal power rated at the thermoneutral point) and 5.5 MW SOFC system connected to a commercial building with the maximum electric demand of 5.4 MW. A detailed quasi-2D model of an anode supported SOFC and a cathode supported SOEC with co-flow configuration of air and fuel flows was developed in Simulink/Matlab to analyze the thermochemical and electrochemical behavior for transient conditions. The system was designed with integrated balance of plant (BoP) including air blowers, steam generators, hydrogen burner and heat exchangers that were also physically modeled in Simulink/Matlab for dynamic simulations. The main challenge of the system is its thermal management without relying on external heat sources, i.e., providing the required heat for preheating inlet air and fuel streams to both SOEC and SOFC and keeping them warm even for long-duration stand-by conditions. Despite the efficiency advantage of solid oxide cells, their elevated operating temperature (973 K -1273 K) can create thermal stresses due to repeated cycling (heat-up, start-up and shut-down) over long-term transient operation. In order to prevent the cell degradation in continual dynamic conditions, a set of control strategies was designed to minimize the temporal and spatial gradients of temperature within the positive electrode-electrolyte-negative electrode (PEN) assembly. The implemented control strategy confirms that the PEN temperature gradient of both SOFC and SOEC are in their acceptable range of 10 K/cm and the average PEN temperature remains well controlled near the design-point operating temperatures of 1073 K and 1023 K, respectively, under all operating conditions. The controllers used for this purpose actuate the temperature and flowrate of inlet air to both SOFC and SOEC to meet the thermal criteria.

The suggested system configuration is designed to take advantage of the exothermic nature of electrochemical reactions in SOFC and the afterburner to provide the thermal demand for SOEC when it is in endothermic or standby mode. SOFC and SOEC are thermally integrated through an optimized network of heat exchangers such that heat is carried from fuel cell side to electrolyzer side via the afterburner off-gas. The system was simulated for a 2-week time window in January to convert the measured electric power dynamics from the wind farm to H₂ using SOEC while the SOFC follows the electric load of the building demand and provides the thermal demand of system. Simulation results showed that under sever conditions, i.e., extreme fluctuations and low levels of wind generation, which results in highly dynamic and endothermic operation SOEC, SOFC is required to increase its power generation beyond the building demand (especially on the weekends) in to keep the energy system thermally stable. This reduced the efficiency of SOFC to 55% and resulted in H₂ deficiency of 10 kg. Hence, solar PV was incorporated into the system to convert otherwise curtailed electricity into renewable H₂ and boost the reliability and efficiency of system under undesired conditions.