Energy storage is vital for the energy transition, enabling reliable power grids based on intermittent renewables. Reversible solid oxide cell (rSOC) technology is promising for seasonal energy storage. The novel finding from this work is that optimised air recirculation for rSOC in endothermic electrolyser mode leads to efficiency being nearly independent of current density. Thereby the operating region of highest efficiency is expanded from the thermoneutral point to the entire endothermic region, leading to highly efficient part-load operation. Air recirculation increases fuel cell mode efficiency too, particularly at higher loads. This widens the efficient operating window in both modes. These findings emerge from a thermodynamic study of an rSOC-based energy storage system with ammonia as fuel. A process design is developed and optimised for efficiency, supported with detailed exergy analysis. First, ammonia synthesis subsystem integrated with the rSOC system in electrolyser mode is optimised. Second, rSOC outlet air recirculation is optimised for high system efficiency. Finally, rSOC operating points are optimised for highest round-trip efficiency. We find the least exergy destruction for the ammonia synthesis subsystem at 170 bar synthesis pressure and 30 °C condensation temperature (without needing refrigeration). The overall system achieves round-trip efficiencies up to 60.3%.

In this work, dynamic modelling of a system based on a reversible solid oxide cell (rSOC) is developed so that it can be integrated with the grid for power balancing. The focus of this work is on the dynamic operation of a system, which is investigated using representative profiles of wind electricity production. In addition, the effect and challenges of dynamic operation on the system and stack itself are studied. Detailed operation strategies are defined during the switching process from one operational mode to another and are implemented on the dynamic process model. Simulation results show that when the rSOC system is operated in solid oxide electrolysis (SOE) and solid oxide fuel cell (SOFC) modes alternatively, energy balancing can be implemented on a continuous basis. In this process, the results show that the rSOC system operates in a safe operating range and does not deviate from the pre-defined limits. This is due to the accurate strategies developed for the switching process. It is also observed from the simulation results that the switching time is significantly influenced by the initial power of the first and the final power of the later operational mode. The proposed model of rSOC was validated using experimental data, and good agreement with experimental data was demonstrated.

Hydrogen is yet to be widely accepted as a fuel for everyday operation due to stringent safety regulations involved around it. In the meanwhile, methanol could be a potential fuel of the future. In this work, an extensive thermodynamic investigation on an energy storage system with a reversible solid oxide stack at its core is presented. The current investigated system can operate either as an electrolyzer or as a fuel cell. It uses steam for electrolysis (charging mode) and methanol for fuel cell operation (discharging mode). A process model of the entire system is formulated by using Aspen Plus™. Energy and exergy efficiency have been reported for both modes of operation, along with maximum roundtrip efficiency that can be achieved for the entire system operation. Results indicate that during electrolysis mode, a maximum energy and exergy efficiency of 67.94% and 72.30% can be achieved and for fuel cell mode operation, the numbers are 74.14% and 62.61% respectively. The maximum reported value of RT efficiency is 64.32% which is quite high considering the infancy of reversible solid oxide technology and the fact that methanol is used as the fuel.

The power to gas process concept is promising for the next generation of grid electricity storage and stabilization technologies. The electricity-driven fuel production can be chosen to be the efficient energy carrier for excessive grid power. Here, a reversible solid oxide cells system integrated with methane synthesis (ReSOC-MS) is proposed for the grid stabilization application at MW class. Besides H2, CH4 can be inclusively synthesized at grid surplus conditions as a transportation friendly energy carrier. A control strategy is proposed for this combined system, based on the grid state and H2 tank state of the system for the normal SOFC mode and SOEC mode operating. Simulation results of these two modes operating demonstrate that the ReSOC-MS can achieve an 85.34% power to gas efficiency at SOEC mode and 46.95% gas to power efficiency at SOFC mode.

The power to gas concept is promising for the next generation of electrochemical energy storage and grid stabilization technologies. The fuel produced from electricity-driven fuel production can be an efficient energy carrier for excessive grid power. Here, a reversible solid oxide cell(s) system integrated with methane synthesis (ReSOC-MS) is proposed for the grid stabilization application at Mega Watts class. CH ₄ can be synthesized at grid surplus conditions and can be a transportation friendly energy carrier. A control strategy is proposed for this combined system, based on the grid state and H ₂ tank state of the system for the normal solid oxide fuel cell (SOFC) mode and solid oxide electrolysis cell (SOEC) mode. Simulation results of these two operational modes demonstrate that the ReSOC-MS can achieve 85.34% power to gas efficiency in SOEC mode and 46.95% gas to power efficiency in SOFC mode. Dynamic simulations of stepping grid state for 5000 s operation show that the power to gas efficiency can be higher than 70%, thereby successfully demonstrating the capability of grid-balancing and methane production.

This paper presents a mini-review in the field of energy storage using reversible solid oxide cells (rSOCs) for development of energy storage systems for the future. Such energy storage systems fall under the category of power-to-X-to power systems where excess electrical energy produced through renewables is stored in the form of chemicals and the same chemicals are used for conversion back to power. The main competitors of energy storage systems based on rSOC are pumped hydro storage, compressed air storage and batteries and it is envisioned that with better heat integration techniques, the round trip efficiency of rSOC systems can be improved to reach the target value of 80% as specified in the joint EASE-EERA report for European energy storage technology.