Two-dimensional simulation and critical efficiency analysis of high-temperature steam electrolysis system for hydrogen production

High-temperature steam electrolysis (HTSE) is a promising method for highly efficient large-scale hydrogen production. The HTSE process not only reduces the amount of thermodynamic electrical energy requirement but also decreases the polarization losses, which improves the overall efficiency of hydrogen production. In this paper, a two-dimensional simulation method of the efficiency of the HTSE system integrated with high-temperature gas-cooled nuclear reactor (HTGR), which changes two parameters simultaneously in a reasonable range while keeping one parameter constant, was presented. Compared with one-dimensional analysis method, the effects of electrical efficiency (ηel), electrolysis efficiency (ηes,), and thermal efficiency (ηth) on overall efficiency (ηoverall) were investigated more objectively and accurately. Moreover, the critical concepts of ηes and ηth were put forward originally, which were very important to determine the optimum electrolysis voltages and operation temperatures in the actual HTSE processes. The calculated critical value of ηes was ΔG(T)/ΔH(T) and the actual ηes should be higher than the theoretically calculated one in order to maintain the high hydrogen production efficiency of HTSE system. Also, it was very interesting to find that the critical ηes was the theoretical maximum efficiency in SOFC mode. Furthermore, the critical value of ηth was equal to the value of ηel, which means the overall efficiency decreases with the ηes increasing if the ηth in the actual HTSE process is less than the critical value of ηth. Therefore, it is very important to control the ηth higher than the critical value in the actual HTSE process to get high overall system efficiency.