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Possibly, depending on the city’s size and heat demand. Vantaa Energy says its planned Varanto facility in Vantaa, Finland, could supply enough stored heat to warm a medium-sized Finnish city or town for up to a year. That is a company comparison, not a universal conversion: 90 GWh is a measure of thermal energy, and actual coverage would depend on demand and delivery conditions.
What is Varanto?
Varanto is a planned seasonal thermal energy storage facility designed to feed heat into Vantaa’s district-heating network. Instead of storing electricity in batteries, it will store heat in pressurized hot water held in underground rock caverns. Vantaa Energy currently estimates the facility could be operational by 2030. Vantaa Energy’s project page describes its planned design and schedule.
How can 90 GWh heat a city?
GWh measures energy, while MW measures the rate at which energy can be delivered. Varanto’s stated 90 GWh is its planned thermal storage capacity; its planned discharge capacity is 200 MW. Those figures describe different things: the first is the amount of heat stored, and the second is the rate at which the system can release heat. They do not mean the facility produces 90 GWh of electricity.
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Vantaa Energy’s comparison is that the full store could heat a medium-sized Finnish city or town for as long as a year. The phrase “medium-sized” matters: cities differ in population, building stock, weather, district-heating coverage, and demand. The comparison should be understood as an illustration of scale, not a guarantee that any particular city could be heated for exactly 12 months.
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What is the underground system designed to do?
The project owner describes three rock caverns, each about 40 metres wide, 20 metres high, and 300 metres long. Their combined volume is about 1.1 million cubic metres, including process facilities; the lowest cavern floor is planned to be 100 metres below ground. The caverns will contain hot water under pressure, allowing temperatures of up to 140°C without the water boiling. Heat would then be delivered to buildings through the district-heating network. Vantaa Energy provides the cavern and temperature specifications.
Where does the heat come from?
Storage shifts heat availability over time; it does not determine how that heat was produced. Project descriptions identify potential inputs including industrial waste heat, heat from waste-to-energy processes, and electric boilers using renewable electricity. Vantaa Energy’s project includes two 60 MW electric boilers. The listed sources describe the intended system, not a promise that every source will always be used or that stored heat is automatically renewable.
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AFRY’s project announcement outlines potential heat sources and its EPCM role. VTT describes Varanto as part of a hybrid heating system combining thermal storage, electric heating, and waste heat.
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What is the project’s cost and current timeline?
Vantaa Energy’s current estimate puts the project cost at about €300 million, and the company says it received a €19 million investment grant from Finland’s Ministry of Economic Affairs and Employment. These are project estimates and grant information, not a final audited outturn. A 2025 Tampereen Energia presentation gives an approximately €220 million figure including electric boilers; the sources do not explain the difference in scope, so the figures should not be treated as directly reconciled.
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The latest schedule on the project owner’s page says the facility could be operational by 2030. Earlier expectations were more ambitious: AFRY described heat distribution potentially starting at the end of 2028, and VTT also referred to completion in 2028. Those earlier dates do not replace the owner’s current estimate.
What is known—and not yet proven—about performance?
Varanto is a project under implementation, so its stated capacity and output remain planned rather than demonstrated operating results. The International Energy Agency’s 2024 report describes the project as designed to store 90 GWh in 1.1 million cubic metres of water under pressure at 140°C. It supports the design description, not verified annual heat delivery from an operating facility. The IEA’s 2024 report places the project in the broader energy-innovation context.
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Engineering simulations are also not equivalent to years of operational data. Rock Mechanics Consulting Finland says its 2023 work simulated excavation and an assumed 50 years of operation, examining cavern layout, thermal cycling, temperature, rock-mass movement, and fractures. That describes design analysis, not evidence that the facility has already operated for 50 years. The consultancy summarizes its Varanto analysis.
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Vantaa Energy characterizes Varanto as the world’s largest cavern seasonal thermal energy store by its stated size and capacity. The IEA’s 2024 report supports that the project was designed as first of its scale, but that does not establish the superlative as a permanent global ranking as other projects are built. The owner’s page presents its characterization; it should be read as a project-company claim rather than a continuously verified comparison.
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