Reversible SOFC/SOEC System Development and Demonstration

Abstract

The OxEon Energy team continues its 30+ year solid oxide fuel cell (SOFC) development history with the design, fabrication, and installation of two reversible solid oxide electrolysis (SOEC)/SOFC demonstration modules (rSOC), at Idaho National Laboratory (INL) and a private, stand-alone microgrid, scheduled for installation and commissioning in early 2023. OxEon’s SOEC/SOFC technology builds on the success of the SOEC stack installed on NASA’s Mars Perseverance Rover that has produced high-purity O2 by electrolyzing Mars atmosphere CO2 nine times to date. OxEon Energy’s technology space integrates cross-sector coupling to produce hydrogen or syngas from SOEC, electricity via SOFC, and transportation fuels from syngas through Fischer-Tropsch synthesis. A low energy plasma reformer provides an alternative approach of producing syngas from low value hydrocarbons. OxEon’s four complementary technologies enable a flexible approach to leveling fluctuating energy from renewables and converting it to accessible, storable, and higher value fuels and chemicals. The reversible SOEC/SOFC systems described in this work demonstrate the opportunity to generate and store H2 fuel as a method to stabilize and capture excess production from renewable or nuclear energy sources. The two demonstration units described in this work integrate OxEon’s reversible SOEC/SOFC stacks with an effective and reliable balance of plant (BOP) system. The high temperature electrolysis (HTE) systems produce hydrogen through electrolysis using solid oxide cell (SOC) technology derived from OxEon’s heritage stack technology and the advancements made during the development of stacks for NASA’s Mars2020 mission. The two demonstration units described in this work use the same modular system design based on 4-stack quad assemblies. The INL system consists of three 4-stack quad assemblies to meet the 30 kW SOEC/ 10 kW SOFC target. OxEon also designed the manifold and plenum assembly to interface with INL’s existing 50 kW test stand and scaled the hot section unit (HSU) to enclose the system. Pressure drop across the system is minimized by supplying even flow to each of the three stack quads, and allows for air delivery in SOFC mode with a blower rather than an air compressor. INL system installation and testing is scheduled for early 2023. A previous 10 kW SOEC system demonstration at INL exceeded project objectives with 14.5 kW system power output, with uniform performance measured from each of 4 stacks. OxEon is scheduled to deliver a 20 kW SOEC/ 10 kW SOFC system to the private microgrid at Stone Edge Farm in early 2023. The system is comprised of 2 quad modules and BOP that will connect with onsite hydrogen storage and renewable energy generation plant. The system will generate hydrogen in SOEC mode using renewable energy supplied by the farm’s solar array. Hydrogen produced in SOEC mode will be compressed and stored by a system designed by HyET Hydrogen B.V. During times of low renewable power generation, the SOFC system will use stored hydrogen to generate power. The Stone Edge Farm system includes two heat exchangers (one for air, one for fuel) that raise the gas feeds to within 50 ⁰C of operating conditions, and minimize pre-heating required for operation. Pre-heating is accomplished with heaters in the HSU enclosure. The feed path is routed to use a portion of the exotherm generated in SOFC mode. The air heat exchanger is oversized for SOEC mode but sized to accommodate the excess flow required for cooling in SOFC mode. The fuel heat exchanger is sized appropriately to deliver H2 in SOFC operation and steam in SOEC operation. Both systems apply mechanical compression to the stacks outside of the HSU enclosure. This design produces greater force than if the springs are enclosed in the hot zone and reduces the insulation envelope size. The end load is applied through a loading rod, an upper load plate, layers of insulation, and an additional outer load plate, placing the springs outside the insulation package that surrounds the hot region where the stacks are located. Low thermal conductivity ceramic rods minimize heat loss through the load transmission path. The materials set used in the rSOC systems uses a scandia-stabilized zirconia electrolyte-supported cell design with nickel-cermet fuel electrode and perovskite air electrodes. Green electrolyte is tape cast, cut, and fired to produce a dense electrolyte of about 250 microns thickness. Electrode inks are applied via screen printing, then fired to form porous electrode layers. Recent advancements in the air side electrode barrier layer, air electrode layers, and fuel electrode catalyst have improved stack performance and stability. Figure 1