Impact of Stack Temperature on Solid Oxide Electrolysis System Operation for Lunar Production of Hydrogen and Oxygen Propellants

Abstract

Solid oxide electrolysis (SOEC) systems have the potential to produce hydrogen and oxygen propellant from lunar ice found in the permanently shadowed cream on the moon. Because SOEC systems electrolyze steam instead of liquid water as in more conventional alkaline and proton-exchange membrane electrolysis systems, they require lower stack voltages for thermal-neutral operation (about 1.29 V/cell vs. 1.48 V/cell for liquid-fed cells) and thus can have lower stack specific energy as defined by stack power requirements per kg of hydrogen produced. However, at a system level, power requirements in a remote lunar setting for vaporizing the steam and flowing it through the stack increase system-level power demands such that the advantages of the SOEC system are reduced due to the balance of plant loads contributing as much as 20 to 25% of the system energy at 800 deg. C. Our team at Mines collaborated with our partners at OxEon Energy to demonstrate an SOEC system with a stack operating temperature of 800 deg. C that demonstrated an overall system-level specific energy just below 50 kWh electric per kg of hydrogen. That study and a supporting model validation study provided a basis for showing the viability for high-temperature SOEC to be scaled-up for lunar production of hydrogen and oxygen propellant for propulsion. The current study builds on that previous work by exploring the impact of SOEC stack temperature on both the stack and balance of plant power requirements. This study explores the trade-offs between reduced balance of plant parasitics and increased stack mass with lower SOEC operating temperatures and explores the advantages of higher conducting mixed-ionic-electronic conducting electrolytes with electron blocking layers to increase current densities at lower temperatures.This study was performed using stack and balance of plant models that have been calibrated in the lab-scale demonstration of an SOEC system with an electrically driven steam generator, a scroll-compressor for driving the steam flow, and cathode and anode exhaust recuperators for heat recovery to the stack inlet steam flow. The system also includes H2 drying from the SOEC cathode exhaust by heat exchange with the incoming water stream. The current study looks updates the previous model by replacing the compressor with a higher pressure steam generator and an expansion valve that allows the operation of the system without the parasitic loads of a compressor. In the current study, three different electrolytes traditional yttrium-stabilized zirconia (YSZ), higher conductivity scandium-stabilized zirconia (ScSZ), and mixed ionic-electronic conducting gadolinium-doped ceria (GDC) with a YSZ blocking layer. The GDC bilayer electrolyte cells have the highest conductivity and allow for reasonable hydrogen production at operating temperatures below 700 deg. C. The results show that the lower operating temperatures enabled by the GDC bilayer electrolyte and by the ScSZ electrolytes not only reduce the system mass with smaller heat exchangers and thermal insulation, but also lower requirements for steam generation by enabling more heat recovery in the steam generator. The impact of stack operating temperatures on the feasibility of scaling up a lunar-deployed SOEC system to produce more than 1 kg/h are presented to further emphasize the feasibility of SOEC systems as part of a lunar propellant production facility.