Seasonal heat storage, Vantaa, Finland

In cooperation with Vantaa Energy and AFRY Finland, we are building the world's biggest seasonal heat energy storage underground using the alliance model.

The development phase of the project started in 2021. During this phase, the parties to the alliance jointly plan the project and its implementation to match Vantaa Energy’s goals and determine its target cost and incentive scheme.

After the development phase, Vantaa Energy will decide separately on the implementation of the investment and on the transition to the implementation phase of the project.

In the implementation phase, the alliance will build the world’s biggest seasonal heat energy storage underground. It is a seasonal thermal energy storage facility of around one million cubic metres, which stores renewable energy from sources such as solar, wind and waste heat during the summer. The energy is stored during periods of low heating demand and the stored heat is utilised during the winter period when heating demand is high. Renewable energy from sources such as solar, wind and waste heat is stored in the seasonal heat storage in water with a temperature of 140-150 degrees Celsius. With a capacity of 90 gigawatt-hours, the seasonal heat storage facility can meet the annual heating needs of an average-sized Finnish city.

The seasonal heat storage is the most important part of the Fossil-free 2026 project with which Vantaa Energy aims to completely phase out fossil fuels by 2026. This supports Vantaa’s goal of carbon neutrality by 2030.

“Vantaa Energy's seasonal heat storage is an innovative project for a better living environment, and it supports YIT’s strategic objective to reduce CO2 emissions through our operations. We are excited that we and AFRY were chosen to implement this project. The alliance model offers the best prerequisites for achieving the goals set by the client, and we are happy that our cooperation can continue even more closely in the joint alliance organisation,” says Pasi Tolppanen, Head of Infrastructure segment at YIT.

The strength of the project is the alliance

In an alliance, the parties to the project are responsible for the project’s planning and construction and they share the risks and benefits. An alliance is an open and cooperative project model in which the parties operate in a shared project organisation.

In addition to a good team spirit, alliances are characterised by innovations which help to achieve better quality and safety as well as influence the environment and cooperation, for example.

“YIT and AFRY have strong expertise in alliances. We can now harness their expertise for this great project of Vantaa Energy that supports climate goals. In the development phase, we plan together the best way to implement a seasonal heat storage that meets the client's objectives, taking into account the environment and residents of the area,” says Project Manager Markku Okkonen from YIT.

In the preparation phase, the alliance partners are working together to find solutions for implementing the best possible seasonal heat storage from the point of view of nearby residents, stakeholders and climate goals.

Rock tunneling requires special skills and a lot of background work

10 questions and answers regarding the Vantaa seasonal heat storage

Alliance Model


Project name
Seasonal heat storage
Vantaa Energy
Project status
Business segment