The expansion of renewable energy with its volatile feed-in character places higher demands on the power grid of the future. Large-scale storage systems (LSS) are a promising option for supporting the electricity grid and have been gaining importance in the last years, both on the market for frequency containment reserve (FCR) and in research. The majority of publications investigating the interaction between storage and FCR are based on simulations rather than on field measurements. This paper presents the analyses of multi-year, high-resolution field measurements of the hybrid 6MW / 7.5MWh battery storage „M5Bat“ (two types of lead-acid and three types of lithium-ion batteries) to address this issue. The influence of FCR operation on the operation, cycles, efficiency and degradation of the hybrid LSS and the individual battery technologies is investigated via a statistical evaluation of the historical operating data between 2017 and 2021. The results can be used to investigate and model the influence of FCR operation on the LSS and its different system components and battery technologies.
The data-based analysis of the LSS and the individual battery technologies reveals a high availability of the LSS of over 96.5%. Furthermore, the FCR operation results in an average SOC of the LSS of 50.5%. A capacity test after four years of operation exposes that the lead-acid batteries have experienced a loss of energy capacity of up to 36%, whereas the lithium batteries have only experienced a loss of up to 5%.
The participation in the reserve market exposes the storage system to a high number of cycles with a low average cycle depth of 1.69%. The number of annual equivalent full cycles (EFC) of the LSS is small with an average of 102 and results in an average remaining number of EFC of the battery units after four years of 83.2%. The high number of remaining cycles compared to the SOH of the battery units demonstrates the dominance of calendar ageing compared to cyclic ageing.
The round-trip efficiency (RTE) of the LSS in operation is 72.8% (with self-consumption: 66.2%). In this context, the lithium-ion batteries have a higher average RTE of 97.4% than the lead-acid batteries with 85%. A low average utilisation rate of the transformer and the inverter in FCR operation of less than 16 % leads to higher losses and a lower efficiency. Through an optimised storage design and the value stacking of additional market products on top of FCR, the efficiency can be increased to over 80%.