Multi-timescale flexibility for low-carbon Nordic energy systems: comparative roles of batteries, district heating, and hydropower

Abstract

The Nordic power system is undergoing a rapid transition toward high shares of variable renewable energy and widespread electrification of heating, transport, and industry. While extensive hydropower resources provide a strong foundation for system flexibility, declining synchronous inertia and pronounced winter demand peaks introduce new challenges for both operational stability and long-term adequacy. Consequently, ensuring reliable operation increasingly depends on the coordinated deployment of complementary flexibility resources across multiple temporal horizons rather than expansion of any single technology. This paper presents a systematic review of literature published between 2019 and 2026 and develops a review-based multi-timescale classification that links grid service requirements with the operational capabilities of battery energy storage systems (BESS), district heating with thermal energy storage (TES), and hydropower reservoirs. The synthesis shows that BESS primarily supports fast frequency response and short-term congestion management, TES provides demand shifting and peak reduction over hourly-to-daily timescales, and hydropower delivers long-duration balancing and seasonal adequacy. The review further identifies that current electricity market structures tend to prioritize short-duration stability services while undervaluing longer-duration flexibility, potentially creating a mismatch between investment incentives and evolving system requirements. By integrating evidence across multiple operational timescales, the proposed classification provides a unified framework for evaluating complementary flexibility resources and offers practical insights for system planning, market design, and the reliable operation of renewable-dominated low-carbon power systems.