As South-East Europe transitions into a new operational phase for its power systems, the integration of long-duration energy storage (LDES) is becoming increasingly essential. The region is witnessing a rapid increase in renewable energy capacity, particularly in wind and solar, which surpassed 34 GW by the end of 2025. This growth has been driven mainly by Serbia, Romania, Greece, Croatia, and Bulgaria, with Bosnia and Herzegovina, North Macedonia, and Montenegro also beginning to deploy utility-scale renewable projects. By 2030, variable renewable energy sources are expected to account for over 45% of annual electricity generation, a significant increase from less than 25% just ten years prior.
This shift towards renewables exposes vulnerabilities in the existing power systems of the region. Currently, flexibility resources are largely reliant on conventional hydropower and thermal generation. While hydropower plays a crucial role in countries like Serbia and Bosnia and Herzegovina, its seasonal variability poses challenges. Additionally, aging thermal fleets face rising fuel costs and regulatory pressures. Short-duration batteries can provide intraday balancing but fall short during extended renewable shortfalls. LDES technology emerges as a solution to bridge this gap.
Defined as systems capable of delivering electricity for 8 to 72 hours or longer, LDES can effectively manage prolonged low-wind periods in winter and multi-day solar deficits due to weather patterns. Projections indicate that by 2030, South-East Europe will likely experience between 5–10 multi-day low-renewable events per year, where renewable output dips below 20% of installed capacity for over 48 hours. In the absence of LDES, these deficits are typically managed through lignite or gas imports, which carry both economic and political risks.
The introduction of LDES fundamentally alters the economics of dispatch within the region. A single 1 GW / 24 GWh LDES unit can replace approximately 1.3–1.6 GW of open-cycle gas turbines based on loss-of-load probability assessments relevant to South-East European load profiles. Deploying between 10–15 GWh of LDES per country could enable Western Balkan nations to address critical multi-day deficits without resorting to fossil fuel dispatch, potentially reducing peak-period imports by 20–35%.
The stability of the grid is another area where LDES can make a substantial impact. Compared to their Western European counterparts, South-East European transmission systems operate with lower inertia margins, especially during periods of high renewable output when thermal units are offline. Frequency deviations and voltage fluctuations already impose significant costs on transmission operators through redispatching and reserve activation. Studies suggest that each gigawatt of LDES could cut annual balancing energy costs by €35–55 million by mitigating prolonged imbalance periods.
The economic rationale for integrating renewables is further solidified by current curtailment rates for utility-scale solar in parts of South-East Europe, which can reach 6–10%. This is primarily due to midday oversupply and limited export capacity. Long-duration storage could absorb excess generation over longer periods rather than just daily peaks, potentially reducing curtailment by more than 60%, thereby enhancing effective solar capacity factors.
The cross-border implications of LDES deployment are also significant. With South-East Europe being only partially market-coupled, interconnector utilization often faces constraints during peak demand events. Implementing LDES across jurisdictions can alleviate pressure on shared corridors by reducing peak exports and imports during critical times. Regional simulations indicate that coordinated deployment of between 40–50 GWh of LDES could decrease cross-border emergency flows by 25–30%.
The industrial sector stands to benefit from these developments as well. Energy-intensive industries in Serbia, Romania, and Bulgaria are increasingly vulnerable to fluctuating power prices and carbon compliance costs. LDES allows these industries to shift demand towards stored low-cost renewable energy during deficit periods. For large industrial consumers utilizing between 200–400 GWh per year, access to long-duration storage could lead to annual electricity cost reductions of between 8–12%.
Adequacy considerations further underscore the necessity for LDES in the region. Many Western Balkan power systems currently operate with reserve margins below 15%, providing little room for outages or underperformance from renewables. A strategic deployment of between 5–7 GW of LDES by 2035 could facilitate the retirement or reduced operation of inefficient thermal plants while significantly lowering annual emissions by an estimated 6–9 million tonnes of CO₂.
The main obstacles facing LDES adoption are structural rather than technical. Currently, long-duration storage often goes unrecognized in national adequacy assessments and grid planning processes across South-East Europe. Fragmented revenue streams and inadequate permitting frameworks treat storage as ancillary rather than essential infrastructure, leading private capital to view it as a high-risk investment despite its evident system value.
Cumulatively addressing the flexibility gap in South-East Europe will necessitate investments ranging from €18–25 billion in long-duration storage by 2040. This investment parallels the cost associated with maintaining aging thermal fleets but promises markedly improved outcomes: reduced emissions, enhanced renewable utilization, and greater security of supply.
The evolution towards long-duration energy storage represents a pivotal moment for South-East Europe’s power landscape. It transforms renewable capacity into a controllable resource while redefining grid stability as a service rather than a limitation. As renewable penetration increases alongside diminishing legacy flexibility resources, the region’s capacity to implement long-duration storage at scale will be crucial in determining the sustainability and manageability of its energy transition.
