The intermittent nature of renewable energy sources makes it difficult for electric utility companies to effectively implement these technologies in the power grid due to mismatches between supply and demand. Solid oxide cells are electrochemical devices that are receiving a lot attention as an effective power grid balancing technology, given their ability to operate as Solid Oxide Fuel Cells (SOFC) for electric power generation, and as Solid Oxide Electrolysis Cells (SOEC) for chemical fuel production. Solid Oxide Reversible Cells (SORC) are capable of working alternately in these two modes, thus can be used to store electricity in the form of fuel when energy supply in the grid exceeds the energy demand and can release this energy when demand exceeds the supply from generation systems.

The purpose of this project was to design an energy storage system that uses solid oxide reversible cells and syngas as fuel, consuming it to produce electricity during operation as a fuel cell and producing it back to store electric power through the co-electrolysis of water and CO2 when working as an electrolysis cell. The design and steady-state simulation of the system was performed using the process simulation software Aspen Plus, where a base configuration of the plant was constructed and improved using as main criteria the roundtrip efficiency and exergy efficiency achieved. This approach allowed to locate the main sources of energy and exergy losses, therefore strategies could be implemented to reduce them, finally achieving a more advanced and efficient system. Overall, it was possible to attain improvements in system roundtrip efficiency from 29% to 44%; in SOFC exergy efficiency from 44% to 64%; and in SOEC exergy efficiency from 66% to 68%.

A lot of scientific research has been focusing on energy storage systems recently and there are numerous reasons for that. First of all, they can nullify the intermittent nature of renewable energy technologies, by storing excess energy in times of heightened solar irradiance and wind levels and utilizing it when electricity demand is surging. A combination of energy storage systems along with renewable energy technologies can eliminate CO2 emissions in the future. It can also lead to state development, by liberating countries from the dependency on costly fossil fuel imports. Finally, population growth will result in increased energy peaks and energy storage systems can be seen as the means for achieving those enhanced power requirements.
In the current thesis, an extensive thermodynamic investigation of an efficient energy storage system based on steam electrolysis is presented. The core of the system is a reversible solid oxide cell stack. It can operate either as electrolysis (charging mode) or as a fuel cell (discharging mode). Apart from the core, around the stack, various balance of plant components can be placed for the synthesis of a plethora of fuels. In this case study, methanol is synthesized.
At first, process design of a model capable of converting electrical energy to methanol and vice versa is formulated in process simulation software Aspen Plus®. Extensive energy and exergy analysis have been conducted on the system for the identification of process conditions which maximize energy and exergy efficiency of each mode of operation. Furthermore, extensive exergy flow diagrams have been drawn in order to pinpoint the components which mostly contribute to the total exergy losses. Finally, roundtrip efficiency optimization has also been performed and the respective process conditions have been reported. For the calculation of the hot and cold utility of the system, the pinch technology has been employed.
Results indicate that during electrolysis mode energy and exergy efficiencies of 68.74% and 77.67% respectively, can be achieved when thermoneutral operation is applied. The same results for fuel cell mode operation are 60.22% and 56.78% respectively. Exergy and energy efficiency during fuel cell mode are still limited due to the intense refrigeration system employed for CO2 condensation. For maximization of roundtrip efficiency, a thermal energy storage system was additionally employed in the process design which stores heat energy from fuel cell mode in order to satisfy the thermal requirements during the endothermic electrolytic operation. The maximum reported value of roundtrip efficiency is 56.72% while in scientific literature a maximum value of 54.3% has been cited, showing a clear improvement.