Thermal Integration of Solid Oxide Electrolysis Cell Systems with External Heat Sources to Maximize Heat Recuperation and Exergetic Efficiency
Authors: Choi Saeyoung , Hong Jongsup 
Published: September 2023

Green hydrogen is expected to play a significant role as an energy carrier in the future energy system. As the issue of climate change due to greenhouse gas emissions has recently emerged, efforts are being made to change from a carbon-based energy society to a hydrogen-based energy society worldwide. Among several hydrogen production technologies, green hydrogen refers to hydrogen that does not generate carbon dioxide at all in production processes such as water electrolysis by using renewable energy. The hydrogen production through water electrolysis is more mature than other green hydrogen production technologies and is the most popular green hydrogen production method because it is easy to mass-produce with a modular structure. In particular, a solid oxide electrolysis cell (SOEC) system, which has the highest efficiency among electrolysis systems, is drawing significant attention.
The high efficiency of SOEC systems originates from heat utilization for water electrolysis, which makes it critical to have optimal thermal integration. The SOEC systems operating at the high temperature above 700°C need a significant amount of thermal energy for steam generation and inlet gas heating. To deal with these thermal energy requirements, the internal heat recuperation and the external heat source integration with system components must be implemented in an effective manner. The thermal integration is the most important part of the SOEC system design. However, the majority of previous studies have concentrated on the performance of unit-cells and stacks, ignoring thermal energy consumption of the systems and heat integration.
The purpose of this study is to explain the basis of thermal integration and to investigate the effects of internal and external thermal integration on the whole system performance. The basic concept of the thermal integration of SOEC system is presented while considering internal heat recuperation and coupling with external heat sources. By using temperature-heat transfer analysis, the procedure to design the optimal thermal integration of the SOEC system is described. Based on the proposed system, a parametric study with respect to stack operating conditions (i.e., stack inlet temperature, air utilization, steam conversion, hydrogen fractions) is conducted in order to elucidate their effect on the system level performance. Then, the effects of external heat sources such as steam injection, indirect heat transfer with hot fluids and heat supply by an electrical heater are examined, with pinch analysis and exergetic performance evaluation. The results show that high temperature external heat is not always a prerequisite for the SOEC system. The systematic comparison with several heat integration methods provides a clear guideline to maximize the exergetic efficiency when designing and operating SOEC systems. Through the analysis, this study provides understanding the internal and external characteristics of the thermal integration.