Benchmarking and Simulation of Solid-Oxide Steam Electrolysis for Propellant Production in a Vacuum Chamber-Simulated Lunar Environment

Abstract

NASA and other space agencies have prioritized the development of in-situ resource utilization (ISRU) technology for producing critical materials such as propellants in space. Electrolysis to split water found in lunar, asteroid, and Martian environments provides a pathway for liquid H2 and O2 production, but the challenges of deploying such technology in extraterrestrial environments put an extreme emphasis on minimizing specific energy (measured in kWhelec/kgH2) and system specific mass (system kg/(kgH2/h)). High-temperature, solid oxide electrolysis cell (SOEC) systems have the potential to achieve lower specific energy than conventional liquid-phase alkaline and polymer electrolyte membrane (PEM) electrolysis systems, but this requires efficient thermal integration and minimization of balance of plant loads in order to take advantage of the lower electrolysis voltages for steam (vs. liquid H2O). The high operating temperatures of SOECs, however, require significant system mass for steam generators, heat exchangers for recuperation, and insulation of the electrolysis stack, particularly for space applications where ambient temperatures may be extremely low.This NASA-sponsored collaboration between OxEon Energy and Colorado School of Mines explored the feasibility of a lunar-based SOEC system, which was modeled and demonstrated at the lab-scale by operating a breadboard system, consisting of a SOEC stack and associated balance of plant BOP) in a cryogenic vacuum chamber in order to simulate a lunar environment. The combined »2.5-kWelec SOEC stack and BOP achieved a specific energy < 50 kWhelec/kgH2 at an H2 production rate of > 0.075 kgH2/h operating in the cryo-vacuum chamber.This study has benchmarked system-level models of the lab-scale SOEC stack based on doped-zirconia electrolytes fabricated at OxEon and designed to operate near thermoneutral voltage (1.28 V/cell) at 800°C. The stack BOP includes a steam generator, an oil-free scroll compressor, cathode and anode exhaust recuperators, and a H2 dryer, which uses cold incoming liquid water feeds to cool the cathode exhaust and condense most of the excess H2O in the H2 product stream out. Operation of this demonstration system provides a basis for designing scaled-up lunar fuel production systems of 40 kgH2/h. The success of the lab-scale tests in terms of thermal integration and the benchmarked models suggest a pathway to such a full-scale SOEC system that can achieve < 46 kWhelec/kgH2 with much lower specific mass.Both the modeling and the lab-scale demonstration tests revealed that system specific energy was most sensitive to stack steam utilization, and that achieving < 50 kWhelec/kgH2 required stack steam utilization > 75%. In addition, effective thermal integration that limited the electrical heating primarily to steam generation is critical to minimizing the SOEC-system BOP loads. The stack was shown to run reliably at steam utilizations > 90% with a single pass feed. Efforts will be presented on how to simulate such high utilization at larger scales for full lunar deployment. Additional testing of the lab-scale system in larger cryo-vacuum chambers at NASA during 2023 will allow for further confirmation of thermal management strategies to ensure reliable SOEC operation for lunar production of H2/O2 propellants.