Flexibility needed: challenges for future energy storage systems [Guest editorial]

The articles in this special section focus on the technologies that will support future energy storage systems. In modern power systems, we are transitioning to an increasing penetration of massive low-cost wind and solar generation, which will require indispensable system flexibility for balancing requirements to maintain system performance. The existing actors have limited technical capabilities to provide the needed flexibility, and new alternatives are required. The flexibility providers are diverse and being assessed extensively, with a clear sense of urgency.