World’s largest cavern thermal energy storage to warm city year-round

Interesting Engineering carries an article about the caverns which will store renewable energy in the form of heat that can be supplied to the existing district heating system, Caliber.Az reprints the article.

Vaanta Energy, a company owned by the cities of Helsinki and Vaanta in Finland, has ambitious plans to establish the world’s largest cavern-based thermal energy storage system.

Consisting of three underground caverns, the facility is proposed to have an energy storage capacity of 90 gigawatt hours (GWh) and meet the heating needs of a medium-sized city for up to a year. 

With renewable energy facilities becoming the focus of future energy investments, wind and solar infrastructure is being built worldwide. However, the issues of intermittency of this energy solution present another problem before us, which is energy storage. 

Battery-based energy storage, as offered by companies like Tesla, can only serve small-scale facilities. However, industrial-scale storage options are needed. Interesting Engineering has previously reported how companies use sand as an energy storage option, and now Vaanta wants to demonstrate how a cavern can store thermal energy for long periods. 

District heating in Finland

Finland is the largest producer of district heating in the European Union. In 2023, the country produced 37.3 terawatt-hours of heating. Over half of this heating was generated using renewable energy; another 14 percent was recovered from waste heat.

In Vaanta, most homes are connected to the municipality-run underground heating system, a 373-mile (600 km) long network. Hot water is pumped through the system, which is connected to heat exchangers installed in buildings. After heat transfer, the cooler water is returned to the production plant to be reheated. 

A waste-to-energy plant and a wood-fuelled power plant currently supply energy to the district heating system, but Vaanta Energy plans to introduce renewable energy to the mix as early as 2028. 

Varanto – Cavern thermal energy

The Varanto project design comprises three caverns, each measuring over 900 feet (300 m) long, 131 feet (40 m) high, and 65 feet (20 m) wide. Together, the three caverns will have a total volume of 38.85 million cubic feet (1.1 million cubic metres), making it the largest such facility in the world. 

The structures will be built at a depth of 330 feet (100 m) underground. When renewable energy is produced in excess, two 60 MW water boilers will fill the caverns with hot water. The facility will maintain a high pressure, which will allow water to be heated to 284 degrees Fahrenheit (140 degrees Celsius) without the water boiling over or being converted into steam. 

A smart control system will also allow waste industrial heat to be used to heat the water when available. “Our heat-producing system will work like a hybrid car: alternating between electricity and other forms of production, depending on what is most advantageous and efficient at the time,” explained Jukka Toivonen, CEO of Vaanta Energy in the press release. 

The facility’s total thermal energy capacity is 90 GWh, the equivalent of energy stored in 1.2 million electric vehicles. When fully charged, it can meet the heating needs for the whole year, a press release said. 

Details of the system’s heating cost have not been revealed yet. However, the project, whose construction is expected to start this summer, will cost an estimated 200 million euros ($217 million).