(434d) Optimal Operation of Solid Oxide Electrolysis Cell Considering Long Term Physical and Chemical Degradation and System Performance

With an increased emphasis on flexible and modular energy systems, there is significant interest in the adoption of solid oxide cell (SOC) technologies for stationary power and hydrogen production. The solid oxide electrolysis cell (SOEC) in particular has been proposed as an efficient method to convert intermittent renewable energy sources into hydrogen for energy storage. To maximize efficiency, high temperature operation of SOCs is desired. However, high temperature electrolysis operation can lead to significant degradation shortening the life of SOCs (Sohal et al., 2012). Degradation of SOCs can occur due to chemical changes such as the change in microstructural composition in the SOC electrodes that is particularly exaggerated under electrolysis operation. Degradation and resulting failure can occur due to physical damage as well due to thermo-mechanical stresses (Clague et al., 2012) that develop in the cell due to temperature gradient in the cell and temperature variation under dynamic operation.

While complete elimination of degradation is difficult, if not impossible, (Kamkeng & Wang, 2022), operational strategies can be developed for slowing down degradation. These include, for example, constant voltage operation over extended periods of time (Naeini et al., 2021). However, this strategy will lead to gradual decrease in current density and therefore, decrease in the H₂ production rate. Other recommendations include operation at low current densities and lower temperatures to minimize degradation effects (Parhizkar & Roshandel, 2017). However, this strategy will lead to lower H₂ production rates and system efficiency thus leading to higher specific cost for H₂ generation. Due to conflicting effects in SOC operation between efficiency, degradation and capacity utilization, it is desired to optimize the cell operational profile over its lifetime accounting for the operating costs, capital costs, prices for electricity and H₂. Dynamic optimization with due consideration of degradation effects is desired for obtaining the optimal operational profile.

In this work we couple degradation models with a first principles non-isothermal model (Bhattacharyya et al., 2009) of a SOC. Models for the microstructural degradation includes various degradation mechanisms in the oxygen electrode, fuel electrode and electrolyte. The model accounts for the effect of deteriorated microstructures on thermal gradients, activation polarization and cell efficiency. For modeling physical degradation, cell components were modeled as a flat multilayer plate generating results for bending axis and curvature as a function of the operating conditions. The models provide information of spatio-temporal variation of stress, failure probability, and microstructural degradation as a function of operating conditions.

An optimization algorithm is developed for maximizing the economics of the SOC under a "quasi-steady state" assumption. Variations in the demand and price profiles are considered for optimizing the operational profile. Comparisons with existing recommendations in the literature for reducing chemical degradation show considerable differences in the operational profile. Results are presented comparing overall degradation rates, thermal gradients, system efficiencies, and economics considering operation over tens of thousands of hours.

References

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