Past and present of grid energy storage – A comparative assessment of existing technologies

This paper analyzes stationary energy storage technologies for power grids. The first part of the paper presents stationary energy storage technologies, with special regard to those tailored for prolonged operation, distinguishing between those dedicated to electric grid services and those that are not, but which nevertheless constitute an important field of energy storage. As regards grid-oriented technologies, a distinction is made between unidirectional and bidirectional systems, since only the latter can fully complement intermittent, non-programmable, and non-dispatchable renewable energy sources, in the context of complete grid decarbonization. For each major category, the historical development is outlined to highlight the milestones and challenges associated with its maturation, providing insights relevant to the future development of less mature technologies. The operational limits and current state of the art for each technology are also examined. The second part of the paper analyzes the performance of a selected group of technologies providing bidirectional grid services that have reached technological maturity, demonstrated by deployment at industrial scale or commissioning of significant pilot plants (TRL ≥ 7) so that their evaluation and comparison can be carried out based on operational experience rather than on extrapolations from small-scale laboratory experiments or numerical simulations. These technologies are pumped hydro, compressed air, liquid air, flywheel, lithium-ion batteries, sodium‑sulfur batteries, and vanadium flow batteries. A set of eight technical and five economic key performance indicators (KPIs) including three original evaluation metrics, is introduced, to enable meaningful and consistent assessment and comparison across technologies with fundamentally different operating principles and performance characteristics. The results are synthesized into tables and comparative diagrams, which are discussed in detail.