Thermodynamic and Economic Analysis of State-of-the-Art Reversible SOFC-SOEC Systems Using Stable Rare-Earth Nickelate Oxygen Electrodes
Authors: Colella Whitney 
Published: July 2021

Gaia Energy Research Institute (Gaia) will present a preliminary thermodynamic and techno-economic analysis of a state-of-the-art (SOA) reversible solid oxide electrolyzer (SOE) using stable rare-earth nickelate oxygen electrodes. Part of the significance of this work is that it quantifies the potential future cost of producing extremely pure hydrogen and oxygen using SOA reversible SOEs in dollars per kilogram of hydrogen ($/kg H2) and oxygen ($/kg O2). This work also analyzes the impact of thermal storage on increasing reversible SOE round-trip efficiency, and the value of electrochemically-compressing the H2 and O2.

Primary cost drivers of solid oxide fuel cells (SOFC) systems tend to include the power density of the SOFC stack. Primary cost drivers of SOE systems tend to include (1) system capital costs and (2) the electricity consumed per unit of hydrogen / oxygen produced. In SOEs, electricity consumption can be displaced by heat consumption. In a combined reversible SOE system, heat produced by the SOFC can be stored and reused to provide heat to the SOE. The extent of this heat reuse is estimated to be a primary cost driver, because for every unit of electricity input displaced by reused heat as an input, the SOE produces more hydrogen and oxygen per unit of electricity input. Heat reuse by the reversible SOE can increase the hydrogen and oxygen output per unit of electricity by up to ~30%. Efficient design of this heat reuse subsystem is crucial to the reversible SOE achieving round-trip efficiencies of >70%, a necessity for viability in the long-duration energy storage market. The work examines the primary cost drivers for the reversible SOE system, including the heat storage and reuse subsystem. This research effort considers valid heat storage media including, but not limited to, (1) the ceramic materials of the reversible SOE stack, (2) steam exhausted from the SOE stack (in the same stream as the hydrogen but with the hydrogen potentially electrochemically separated at high temperature via proton-conducting ceramics), and (3) phase change materials, such as molten salts.

The reversible SOE stack being evaluated is a unique, all-ceramic stack, and therefore, the materials of the stack have the potential to maintain greater stability as a thermal storage media compared with stacks composed of multiple material types each with potentially quite different thermal expansion coefficients. A primary failure mechanism of high temperature electrochemical systems is a mismatch in thermal expansion coefficients of electrochemical stack materials. For example, historically, the FuelCell Energy Inc. stationary, commercial Molten Carbonate Fuel Cell (MCFC) system has been limited to a turn down ratio of only 12% of maximum rated power over the course of one (1) hour, primarily due to mismatches in the thermal expansion coefficients of its MCFC stack materials.

The all-ceramic reversible SOE stack evaluated here is also unique in that it has the capacity to supply hydrogen and oxygen at pressure. This analysis also quantifies the value of this electrochemically-compressed hydrogen and oxygen, compared with hydrogen and oxygen produced by mechanical compression. Based on techno-economic analyses of electrochemical and mechanical compression, electrochemical compression has the most compelling cost advantage over mechanical compression in the lower pressure range (i.e., for example, 1 to 100 bar). Electrochemical compression appears to have the highest theoretical efficiency advantage over mechanical compression across a larger pressure range going up to very high pressures (1 bar to 900 bar). When practical, measured, in-use efficiencies are considered, electrochemical compression appears to have a substantial potential advantage over mechanical compression in part because mechanical compression systems are often not designed to maintain high efficiencies at lower throughputs and are limited in their turn-down ratio (in some cases, their turn-down ratio is limited to only 25% of maximum flow rate.) Electrochemical compression with an all-ceramic stack is estimated to be less limited in response time and less limited in turn-down ratio due to more uniform thermal expansion coefficients, compared with some mechanical compressor systems built today.

Model results indicate that the reversible SOE is expected to produce hydrogen at less than $2/kg H2, which is the DOE Fuel Cell Technologies Office (FCTO) H2 production cost target. Model results further indicate that, between near-term and far-term cases, the estimated energy storage costs are expected to decline due to decreases in (1) the reversible SOE system capital costs (mainly at the stack), (2) net system-wide energy consumption, (3) the extent of waste heat dissipated by the system, and (4) operations and maintenance costs. Energy storage costs are also analyzed using single-variable sensitivity studies and Tornado charts.