Process Simulation and Comparison of Various High Temperature Steam Electrolysis Plants

Kurzfassung

In scope of this diploma thesis a process simulation model was built to observe the effects of different operating methods on hydrogen production by means of high temperature electrolysis (HTE) of steam. The targets of this work were to analyze solar heat integration and probable SOEC (solid oxide electrolyzer cell) operation. In addition, a simple SOEC model was generated based on the Nernst equation and experimentally ascertained data, and was then implemented in the Aspen Plus® simulation program. Statements about the SOEC working conditions are possible in the ohmic resistance area of the cell specific operating voltage vs. current density curve (U-i-curve). In order to determine the behavior of the SOEC when combined with a solar energy supply, its qualities were examined under varying basic conditions in a parameter study. From this, the optimum SOEC working conditions were determined and the default was defined for the solar hydrogen production plant. A steady hydrogen production of 1000 Nm3H2/h is required to fulfill the assumption that the production plant substitutes a standard sized German filling station. Different plant types and plant operating procedures were simulated and examined, and the most promising cases were laid out and compared as complete systems with a solar field, solar tower, receiver and storage. The internal heat exchange process was intensively examined to optimize the plant efficiency. The different cases are compared based on the production efficiency and cost. The results indicate that the HTE is more advantageous than conventional electrolysis because it takes solar HT (high-temperature) heat as a part of the total required steam electrolysis energy. Approximately one sixth of the required electrolysis energy is provided by solar heat integration, while assuming possible high SOEC operating temperatures of more than 800 °C. The internal resistance of the cell decreases with higher temperatures and, as a result, the efficiency of the steam electrolysis increases. Based on the high costs of HT heat storage and their current industrial area of application, a process variation that integrates middle temperature storage is a reasonable alternative. In this variation, the SOEC is driven in two operating methods throughout the day. In periods of high solar radiation, solar heat can be coupled directly into the SOEC. During the remaining time, the absent heat is produced by ohmic heat in the SOEC, and therefore a higher electricity demand is required. The average thermal to hydrogen efficiency of this performance class with two different operating methods throughout the day is 24.1 %, when using the LHV of hydrogen.