Optimization of an Electrolysis System for Production of Rocket Fuel from Lunar Ice

NASA, other governments' space agencies, and some commercial entities are pursuing a long-term goal to develop process systems to produce liquefied H ₂ and O ₂ propellants from available lunar ice deposits. H ₂ /O ₂ propellant made from ice in lunar permanently-shadowed regions (PSRs) will enable missions in cislunar space and secondary cislunar launches to further destinations. System balance-of-plant optimization to minimize mass, energy requirements, and life cycle costs can facilitate designs for reliable long-term operation in the extreme environments of PSRs. OxEon Energy and Colorado School of Mines are designing, testing, and developing an integrated steam electrolysis system for production of propellant in a lunar environment. Using OxEon's high-temperature, solid-oxide electrolysis (SOXE) technology, the system simultaneously produces separated streams of H ₂ and O ₂ and compresses the O ₂ electrochemically to facilitate smaller heat recuperators and passive O ₂ liquification in the PSR, thereby eliminating the need for high-maintenance, mechanical compression. In this study, Mines and OxEon have focused optimization on a lab-scale demonstration system by developing integrated component models in MATLAB utilizing the Cantera thermochemical toolbox to simulate performance of the electrolyzer stack, steam compressor, and heat exchangers as a function of operating conditions using 0-D and 1-D flow path models. The integrated system model has been applied in an optimization algorithm to minimize system specific work input, specific dry mass, and/or estimated cost per mass unit of fuel production. Optimization for a lab-scale demonstration system, which included impacts of downstream cryogenic cooling processes for LH2 and LO2 production, determined that operating conditions with high-steam generation pressures (~1 bar), low steam compression ratios, and high SOXE stack H ₂ O utilizations (> 90 ^% ) minimize system specific work and cost per mass unit of H2 produced. Results from these optimization studies have informed a system design for fabrication and testing in a Mines cryo-vacuum chamber. Further system analysis and optimization will determine how a flight-worthy system fits within NASA plans for full-scale liquid propellant production in a lunar PSR, taking icy regolith extraction and cryogenic storage into account.