Key Performance Indicators for the monitoring of large-scale battery storage systems

Sammanfattning: In the context of the fight against climate change, the electricity sector is experiencing a complete renewal. Power grids are undergoing a transformation from centralized and unidirectional systems to multilevel and more integrated networks with, among others, the insertion of intermittent Renewable Energy Sources (RES) on the production side and with the emergence of new consumer behaviors on the demand side. In this context, Battery Energy Storage Systems (BESS) are gaining momentum. Their excellent technical performances combined with a falling price make these storage solutions applicable to multiple scales and applications, ranging from the electrification of rural areas to the reinforcement of modern power grids. Large scale BESSs are complex systems, for which the electrochemical cells are only the elementary building blocks. Such storage systems consist of a hierarchical assembly of these cells, a complex control structure, a precise thermal management and a reversible power conversion apparatus, cooperating to ensure a smooth and safe operation. To deal with this complexity, BESS owners and operators need synthetic indicators to quickly assess the operation of their storage systems. In this work, this question of the monitoring of large scale BESSs is addressed with a selection, implementation and discussion of Key Performance Indicators (KPI). After a presentation of the multiple components constituting a BESS, a review of the main KPIs found in the literature is proposed. This preliminary phase concluded with the definition of four main categories covering the multiple aspects of the operation of a BESS: operation, performance, ageing and safety. Where needed, a choice was made to choose the estimation techniques offering the best tradeoff between accuracy, ease of implementation and computational load. Then, the overall implementation strategy used to take advantage of the large amount of data available was presented. The results were obtained for actual large-scale Li-Ion BESS projects, covering multiple applications and chemistries. Based on these illustrative results, the robustness and the accuracy of the indicators was discussed. More importantly, a special attention was paid to the methodology, meaning and interdependencies of these KPIs to enable battery owners to better understand their system.