In Vantaa, Finland's fourth-largest city adjacent to the capital Helsinki, construction is underway for a groundbreaking seasonal thermal energy storage facility. Upon completion, this facility, known as Varanto, will stand as the world's largest of its kind by all measures. It operates by storing heat in subterranean caverns, enabling the heating of buildings via the district heating network whenever necessary.

In Finland and neighboring Nordic countries, heat consumption experiences significant seasonal fluctuations. During summer, heat demand dwindles to about one-tenth of the peak load seen in frigid winter months. The innovation of storing inexpensive and environmentally friendly waste heat from sources like data centers, cooling processes, and waste-to-energy assets in underground caverns marks a revolutionary step in the energy transition. Leveraging stored heat aligns ideally with the widespread district heating networks prevalent in Finland and other Nordic regions, to which most properties are seamlessly connected.

The seasonal thermal energy storage facility in Vantaa will be nestled within the city's bedrock, housing three caverns spanning approximately 20 meters in width, 300 meters in length, and 40 meters in height. Situated 100 meters below ground level, these caverns will brim with hot water, attaining temperatures nearing 140 degrees Celsius without boiling or evaporating. These caverns are colossal, with a total volume reaching 1,100,000 cubic meters, including ancillary facilities.

Vantaa Energy CEO Jukka Toivonen underscores the significance of Varanto in the global energy transition, stating: "The world is undergoing a huge energy transition. Wind and solar power have become vital technologies in the transition from fossil fuels to clean energy. The biggest challenge of the energy transition so far has been the inability to store these intermittent forms of energy for later use. Unfortunately, small-scale storage solutions, such as batteries or accumulators, are not sufficient; large, industrial-scale storage solutions are needed. Varanto is an excellent example of this, and we are happy to set an example for the rest of the world."

Cost and period

With a total thermal capacity of 90 gigawatt-hours when fully charged, Varanto could effectively heat a medium-sized Finnish city for an entire year. This energy quantity equates to roughly 1,3 million electric car batteries. The projected cost for this endeavor stands at two hundred million euros.

The estimated cost of the project amounts to approximately 200 million euros, with an additional 19-million-euro investment grant already secured from Finland's Ministry of Economic Affairs and Employment. Construction of the storage facility's entrance is slated to commence in the summer of 2024, with expectations for the seasonal thermal energy storage facility to become operational by 2028.

The most popular form of heating for buildings and homes

In Finland, district heating is the primary method for heating buildings, facilitated by an extensive underground network connecting most properties. Vantaa alone boasts over 600 kilometers of district heating networks, serving around 90% of residents.

Finland leads globally in district heating production and utilization, with the highest per capita production among Nordic countries. Thermal energy generated in local power plants is used to heat residential, commercial, and industrial buildings. In 2023, Finland produced 37,3 terawatt-hours of district heat, with 53% from renewable sources and 14% from waste heat.

In district heating systems, thermal energy from production plants is distributed as hot water through a closed, two-pipe network. Hot water flows to buildings, releases heat, then returns to the production plant for reheating. Heat is transferred to buildings via heat exchangers, ensuring the district heating water itself does not circulate within building networks.

District heating offers environmental sustainability due to its local production and compatibility with various energy sources. It can seamlessly integrate new production methods and technologies, making it highly adaptable.