Finnish construction and infrastructure company, YIT Corporation, has secured a €100 million, three-years, excavation contract to build the cavern system for Vantaan Energia’s seasonal heat/thermal storage facility, Varanto, which is set to become the world’s largest. YIT will begin excavation work for the Varanto seasonal heat storage project in Vantaa city’s Kuusikonmaki area in Finland, in December 2025.
The Varanto Thermal Energy Storage Project in Finland
Varanto seasonal thermal energy storage facility is being developed by Vantaan Energia in Vantaa, a city just outside Helsinki. It has a capacity of about 90 GWh and cavern volumes exceeding 1.1 million cubic meters, making it the largest underground seasonal heat storage system in the world. The heat storage system will store hot water at up to 140 degrees Celsius in bedrock caverns deep below ground and release it for use in district heating when needed.
Factsheet for Varanto Seasonal Thermal Energy Storage Project
Location: Kuusikonmaki in Vantaa, Finland
Developer/Owner: Vantaan Energia
Excavation Contractor: YIT Corporation in a €100 million contract.
Storage Capacity: 90 GWh of heat. This is equivalent to annual heat demand of a medium-sized Finnish city.
Cavern Volume: 1.1 million m3. This is from three caverns approx. 20 m wide, 300 m long, and 40 m high.
Operating Temperature: Up to 140 degrees Celsius, thanks to groundwater pressure at depth.
Cost: Over €200 million
Additional Technology Use: Two 60 MW electric boilers to support district heating when electricity prices are low.
Completion Date: 2028
Project Timeline
January 2021: AFRY and YIT selected by Vantaan Energia for development of the thermal storage project.
September 2021: Planning under way. Development phase also begins.
July 2021: Construction process expected to begin in 2022 for the 90 GWh storage project.
June 2025: Competitive tendering process launched for excavation contract covering over a million cubic meters of rock. Start of construction expected in late 2025.
November 2025: YIT selected as excavation contractor under a €100 million contract, with work to begin December 2025. Contract duration is about 3 years.
2028: Completion of Varanto heat storage project and start of operations expected.
Varanto Heat Storage Project and Finland’s Energy Transition
Varanto represents a flagship project in Finland’s transition toward fossil-free district heating and thermal-energy systems. It’s also an eye to Vantaan Energia’s goal of phasing out fossil fuels by 2026 and achieving carbon neutrality by 2030.
Like battery energy storage systems, Varanto also addresses the mismatch that often arises between renewable energy generation and demand. Finland’s southern town of Pornainen is also home to the world’s largest sand battery. The 100 MWh commercial battery project was commissioned in June 2025.
Outlook on Varanto: The World’s Largest Seasonal Heat Storage Project
Once commissioned, Varanto thermal energy storage will provide Vantaa’s district heating network with flexibility in allowing the system to switch between modes of electric boilers, waste heat, and heat storage, as per the cheapest, cleanest, and more reliable input at any moment.
YIT’s excavation start in December 2025 is also a key milestone for the project. The next probable focus will be on rock engineering, liner and insulation works, heat integration systems and capped off with commissioning of the storage.
