Optimizing Power in Solid Oxide Electrolysis Systems for Lunar H2 and O2 Production

Abstract

Producing H2 and O2 propellants from icy regolith in the permanently shadowed regions (PSR) of the moon requires development of integrated robust electrolysis systems that operate in cryogenic environments. High-temperature solid-oxide electrolysis (SOXE) stack technology for water splitting with an optimized balance-of-plant to recuperate heat can provide high-purity H2 (after cooling and drying) and O2 (after cooling) for use as rocket propellant. SOXE steam electrolysis can require below 45 kWhelec kgH2 -1, less than alkaline and PEM liquid H2O electrolyzers. However, to achieve these energy advantages, SOXE systems require efficient upstream steam generation and compression, heat recuperation, and water recovery from the H2 product. In this study, optimization of a 3.5 kWelec lab-scale system reveals that the ability of OxEon’s stack technology to operate at steam-utilizations e H2O > 95% can provide a system that meets the target of 45 kWhelec kgH2 -1.The SOXE stack utilizes stabilized zirconia electrolytes similar to the MOXIE (Mars Oxygen In-Situ Resource Utilization Experiment) system and operates at a steady 800ºC. This high operating temperature requires efficient thermal integration and balance-of-plant components to minimize system energy and mass for operation in the PSRs ‘cryo-vac’ conditions, where temperatures can reach as low as 40 K. Figure 1a illustrates the process diagram showing the steam generator, compressor, cathode and anode heat recuperators for heat recovery, and a H2 cooler/dryer for exhaust H2O recovery. Passive radiative coolers for lunar operation prepare the O2 and H2 product streams for liquification. To optimize the system operation, a MATLAB-based process model calculates the performance of the integrated component system model and is called by the MATLAB optimization toolbox to minimize specific power w sp under constraints that maintain reasonable system mass. Optimization variables in this study included SOXE stack H2O utilization e H2O; SOXE stack anode electrochemical compression DP a; steam generator vapor outlet temperature T vap; and compressor pressure ratio P rat. The SOXE cell operated at the thermoneutral voltage (V tn » 1.28 V/cell @ 800℃). BOP power demands included the compressor, SOXE steam preheater, insulation heaters, and supplemental electric heating for the steam generator. Optimization for the 3.5 kWelecSOXE system indicates that the biggest benefit for w sp comes from high eH2O, which reduces freezing required to recover water in the H2 passive cooler. e H2O of 95% with lower T vap around 85℃ and low P rat (around 2.00) provides a path for achieving the target w sp < 45 kWhelec kgH2 -1 . Figure 1