Process integration and exergy-based assessment of high-temperature solid oxide electrolysis configurations

Solid oxide electrolysis (SOEL) is considered an efficient option for largely emission-free hydrogen production and, thus, for supporting the decarbonization of the process industry. The thermodynamic advantages of high-temperature operation can be utilized particularly when heat integration from subsequent processes is realized. As the produced hydrogen is usually required at a higher pressure level, the operating pressure of the electrolysis is a relevant design parameter.

The study compares pressurized and near-atmospheric designs of 126 MW SOEL systems with and without the integration of process heat from a downstream ammonia synthesis and the inefficiencies that occur in the processes. Furthermore, process improvements by sweep-air utilization are investigated. Pinch analysis is applied to determine the potential of internal heat recovery and the minimum external heating and cooling demand. It is shown that pressurized SOEL operation does not necessarily decrease the overall power consumption for compression due to the high power requirement of the sweep-air compressor. The exergetic efficiencies of the standalone SOEL processes achieve similar values of $\varepsilon =81\%$. Results further show that integrating the heat of reaction from ammonia synthesis can replace almost the entire electrically supplied thermal energy, thereby improving the overall exergetic efficiency by up to 3.5 percentage points. However, the exergetic efficiency strongly depends on the applied air ratio. The highest exergetic efficiency of 86 % can be achieved by employing sweep-air utilization with an expander. The results demonstrate that integrating downstream process heat and applying sweep-air utilization can significantly enhance overall efficiency and thus reduce external energy requirements.