Ecological flow energy storage: an effective approach for grid flexibility

The growing integration of intermittent renewables, especially solar PV, challenges grid stability and limits the operational flexibility of conventional hydropower. Beyond hydrological variability, hydropower is constrained by mandatory ecological flow releases, which restrict its ability to reduce generation during high solar output or ramp up during peak demand due to water scarcity. This paper introduces Ecological Flow Energy Storage (EFES), a cost-effective, environmentally sound solution to enhance grid flexibility. EFES involves building small reservoirs downstream of existing hydropower plants to temporarily store ecological flows. This decouples ecological requirements from electricity generation, enabling hydropower plants to halt generation during solar-rich periods without breaching environmental mandates. Stored water can later be released through upstream turbines during low solar availability or peak demand, effectively turning a regulatory constraint into a strategic energy storage asset. EFES supports better integration of renewables, optimizes hydropower dispatch, and enables economic arbitrage, all while preserving downstream ecological services. In Brazil, the estimated EFES potential is 14.3 GW, with a storage cost that can be ten times lower than battery storage. This paper explores the technical feasibility, economic viability, and environmental implications of EFES, highlighting its promise as a scalable, low-cost energy storage solution for modern power systems. • A proposal for increasing the flexibility of hydropower for both ecological and energy services • Downstream micro-reservoirs can decouple hydropower generation from ecological flow. • Enabling hydropeaking to reduce solar curtailment • In Brazil, Ecological Flow Energy Storage can shave ∼14.3 GW peak. • A scalable, low-cost path to integrate VRE via hydropower flexibility