(Invited) Battery Durability and Reliability Under Electric Utility Grid Operations

Abstract

Battery Energy Storage Systems (BESSs) show promise to help renewable energy sources integration onto the grid. These systems are expected to last for a decade or more, but the actual battery degradation under different real-world conditions is still largely unknown.In this work we analyzed different types of usage for batteries similar to deployed 1MW and 2MW BESS in the Hawaiian archipelago. These batteries are used for a mix of different applications such as voltage smoothing, curtailment, reserve and frequency regulation. With different usages come different battery degradation. A new protocol encompassing all those applications was designed to be able to test the cells and assess their durability in the field. We investigated the impact of frequency regulation duty cycles with different SOC ranges, depth of discharge and temperatures. We also investigated cells with varying duty cycles and temperatures to decipher whether the degradation can be predicted from a laboratory experiment.Based on the BESS representative usage profiles, we designed cycle-aging and calendar-aging experiments to test the degradation in a controlled fashion. The frequency regulation duty cycle was extracted from a 3 years usage analysis of a previous generation lithium titanate BESS in operation in Hawai'i since 2012. Laboratory testing was used to investigate the degradation in detail to improve remaining useful life predictions. The degradation mechanisms, and their path dependency, were analyzed using incremental capacity analysis and mechanistic modeling. It was found that despite the fact that most of the cells did less than 0.5% capacity after more than 400 days of testing, they actually degraded significantly. Cell temperature had the strongest impact on the loss of lithium inventory whereas duty cycle had a big impact on the loss of positive electrode material. Interestingly, the impact of SOC in the calendar aging experiments, was found to be counterintuitive as batteries lost capacity faster at low SOCs than at high. Based on our results we established a 20 year forecast of the BESS degradation under different usages.