Degradation of lithium-ion batteries that are simultaneously servicing energy arbitrage and frequency regulation markets

Abstract

Lithium-ion batteries are currently deployed around the world to provide both energy intensive and power intensive electricity grid services. Because of this, there is interest in simultaneously servicing multiple markets to maximize the use of both the energy and power operating envelope of a given battery. To evaluate the long-term economic potential of such strategies, many researchers apply degradation curves based on a simple cycling regimen rather than a complex grid services profile. Building upon previous work that examined single service battery degradation, this study evaluates the degradation patterns of simultaneous services by superimposing energy arbitrage and frequency regulation signals. This new methodology was applied to two common lithium-ion chemistries (NMC and LFP) and the results were compared to single service degradation results obtained from the same cell types. Frequency regulation alone caused the least amount of capacity degradation. The simultaneous services increased the degradation rate as the energy arbitrage proportion increased, with the highest degradation rate caused by energy arbitrage alone. Degradation rate is highest throughout the first 1000 cycles and becomes constant after that (up to the 2500 cycles tested), suggesting that as batteries are cycled, they are less sensitive to the severity of the service profile. From this a degradation factor is derived to inform developers on the impacts of different service profiles. Energy efficiency only marginally declined throughout the cycling period. This information will improve the fidelity of economic models by increasing degradation modeling accuracy for simultaneous services.