Battery Energy Storage Systems for frequency regulation: Simplified aging evaluation

The increasing exploitation of Renewable Energy Sources (RES) is progressively displacing large conventional power plants, thus reducing system operating reserves and stability margins. Therefore new resources for ancillary service provision are needed. Very fast and flexible response capabilities make Battery Energy Storage Systems (BESS) good candidates to this purpose. However, the related cycling operation may cause early performance degradation due to battery aging. Here, attention is focused on primary and secondary frequency regulation by a BESS, in a stand-alone configuration or supporting a RES-based plant. The BESS response to the measured frequency error and to the secondary control signal in mainland Italy is simulated, in terms of power and energy exchanges and State of Charge (SoC) dynamics. A simplified method is proposed to evaluate battery aging due to such operation: starting from typical characteristic curves Maximum number of cycles to end of life versus Depth of Discharge (DoD), a weighted average DoD, accounting for the actual SoC transient, is computed; the battery expected life is the ratio of the maximum number of cycles and the actual number of partial cycles done, both evaluated at the average DoD. This lifetime is finally compared to the BESS investment pay-back period.