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structural reinforcement and sometimes sealing to ensure water tightness. Underground caverns must be adapted or excavated to house turbines, pumps and associated equipment. In addition, a closed-loop reservoir system must be established, often combining underground volumes with surface features such as quarries or lakes. “Sealing and waterproofing are critical,” Olrog


notes. You also need to consider how water moves through the system over repeated cycles, and how that interacts with the surrounding geology. Surface integration varies significantly between


sites. In some cases, a disused quarry can be repurposed as a reservoir, minimising additional excavation. In others, natural water bodies may serve as reservoir, simplifying hydraulics but introducing new environmental and permitting considerations. Site selection is therefore critical. Depth is a primary driver, as it directly determines hydraulic head and energy capacity. Geological stability is equally critical, both for structural integrity and long-term operational reliability.


Existing infrastructure, including shafts and access tunnels, can significantly reduce capital expenditure, while proximity to grid connections is essential for commercial viability. While the global potential is considerable, Olrog emphasises that only a subset of sites will meet the combined technical and commercial criteria required for development. “There are many candidates globally,” he says, “but probably a subset are investment grade.” “We use a structured multi parameter screening and elimination framework to identify and prioritise suitable underground sites,” Olrog adds.


Project configuration and


cost dynamics Mine Storage’s approach is inherently site-specific, with each project configured according to local conditions rather than standardised design templates.


Power capacity is typically determined by grid connection constraints, while energy storage capacity is a function of shaft depth and system volume. The Norberg project in central Sweden provides a clear example. Built around a decommissioned iron-ore mine, it is designed to deliver 24MW of power and 138MWh of storage capacity, with a rock quarry serving as the upper reservoir and the existing shaft providing the hydraulic conduit. The configuration reflects both the physical characteristics of the site and the available grid connection capacity. Similarly, the Vånga project utilises an abandoned


stone quarry in combination with a natural lake, creating a system with 30MW of peak power and 70MWh of storage. In this case, the use of a natural lower reservoir reduces the need for extensive civil works but increases surface visibility and alters the permitting landscape. “The power rating is typically constrained by the grid connection,” Olrog explains. “The energy capacity, on the other hand, is determined by the available head and the volume of the system. So each project is optimised individually.” From a cost perspective, the most significant advantage of mine-based systems lies in the reuse of existing infrastructure. Avoiding large-scale dam construction not only reduces capital expenditure but also shortens development timelines and lowers environmental impact.


“Dam construction is a major cost driver in conventional projects,” Olrog says. “If you can eliminate that and leverage existing excavation, you have a clear economic benefit.”


However, this does not imply that mine-based


systems are low-cost by default. Underground works, including shaft rehabilitation, sealing and equipment installation, remain capital-intensive. Electromechanical systems must also be designed to operate reliably under the specific conditions of each site. In terms of operational characteristics, mine-based storage is best suited to mid-duration


www.waterpowermagazine.com | June 2026 | 11


Mine Storage is planning an energy storage facility in Norberg, Sweden


Fredrik Olrog, CEO, Mine Storage


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