Reversible Solid Oxide Cells: A Study on System Simulation

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Introduction In recent years, renewable energy such as solar and wind power has been attracting attention as clean energy. However, such power generation exhibits significant fluctuations in output depending on weather conditions and time of day. During times when renewable power generation exceeds electricity demand, it would be beneficial to utilize such excess renewable electricity to produce hydrogen, to be stored it in storage tanks. In contract, if the supply of electricity is insufficient to meet the electricity demand, the fuel cell operation can provide electricity.Reversible solid oxide cells (r-SOC) can be a core device for reversible energy systems because they can reversibly switch between fuel cell operation and steam electrolysis using the same cell. When incorporating r-SOC into a system, the heat management and storage methods have to considered in the system design1 2. However, it is not easy to evaluate the performance of prototype systems with various combinations of BOPs (balance-of-plants), such as evaporators and heat exchangers. The objective of this study is therefore to propose a concept of such a reversible energy system which enables power generation, energy storage, and their control using reversible cells, through system simulation. Experimental In this study, we used chemical engineering simulation software (Aspen Plus) to calculate the chemical reaction processes and heat balance based on thermodynamic and chemical equilibria. Figure 1 (a) shows a simplified diagram of a r-SOC system using a reversible solid oxide cell. This r-SOC system was assumed to use a common storage tank for both SOEC and SOFC modes, in which H2 and H2O were stored. It was also assumed that the direction of fuel flow is the same in both modes and that the two modes can be switched by switching the direction of the current only. This system was built on Aspen Plus and simulated. In this study, we calculated the efficiency of a 1kW-class SOEC system for hydrogen production, assuming the hydrogen produced in the SOEC mode is consumed in the SOFC mode within a continuous operation period of 24 h (a fixed period of time).Simulations were also performed assuming different storage methods. Hydrogen has a low volumetric energy density, so its density must be increased when it is stored. Here, hydrogen storage alloys and high-pressure hydrogen tanks were considered as typical examples.As a typical property of hydrogen storage alloys, the reaction is exothermic during hydrogen absorption and endothermic during hydrogen desorption3 4. We constructed a system that utilizes this feature.Simulation models for the high-pressure hydrogen tanks were created assuming 35MPa and 70MPa tanks. Compression was assumed to be isentropic. Results and Discussion Figure 1 (b) shows that the efficiency in the SOFC mode with different current densities and corresponding operation times, assuming that hydrogen produced in the SOEC mode is fully consumed in the SOFC mode as described above. The black symbols/line shows the efficiency when using a hydrogen storage alloy, and the blue and red symbols/line show the efficiency when using a 35 MPa and 70 MPa high-pressure tanks, respectively. When hydrogen storage alloys were used, the efficiency was almost equal to that of reversible systems. It was also found that compactness can be achieved by the storage method without pressurization. When using a high-pressure hydrogen tank, the entire system could be downsized, but the efficiency decreased due to energy loss in pressurization. The use of hydrogen storage alloys is expected to realize a reversible energy system capable of highly-efficient power generation, hydrogen production, and reversible operation.