The battery is a 190,000 m³ tank filled with hot water. The battery is charged by heating the water in the tank using district heat. When the battery is fully charged, the temperature of the water is over 90 degrees Celsius.

The thermal battery is used to heat the water circulating in the district heating pipelines when district heat is used extensively, i.e. during frost periods. Conversely, when less district heat is consumed than what is produced, the district heat water is used to charge the battery.

The maximum storage capacity of the battery is 8 GWh, which is an impressive number – it equals the annual heat consumption of 400–500 detached houses!

Reducing oil consumption

The district heat battery is not only economical but also an environmentally friendly way of producing district heat.

“As the produced district heat can be discharged from the battery when necessary, the use of oil in the production of district heat during peak periods of consumption is significantly reduced,” says Tommi Kantola, Production Director at Oulun Energia.

The Laanila biopower plant and the Toppila power plant, whose primary fuel is wood, are the two main district heat production units. When their power is insufficient during heat consumption spikes, oil heating plants around the city are started.

As the underground thermal storage enables the optimisation of district heat production, the use of low-emission main production units can be maximised and the use of fossil fuel reduced.

“Thanks to the combined effect of the underground battery and another smaller battery in Toppila, oil use can be estimated to be around half of what it would be without the batteries. The underground battery accounts for approximately half of this,” says Kantola.

Oulun Energia was a pioneer

The underground tank, which was used by the factories in the Laanila area, is a huge fuel tank mined in the bedrock. When it was no longer needed for fuel use, it was converted into a district heat battery in the late 1990s. For a long time, it was Europe’s largest thermal storage mined in bedrock. Nowadays, various other energy companies are building similar thermal storages, many of which were modelled after the Oulu template.

“We have had it for so long that it feels commonplace. At one time, it was probably built due to financial reasons, that is, to optimise production, but perhaps even then they realised that this was also important from the environmental perspective,” Kantola says.

Underground battery

• Between its upper and lower edges, the battery extends from 20 to 40 metres below ground
• The cavern is approximately 20 metres high
• Capacity 190,000 m³
• Storage capacity 8 GWh
• Charging and discharging power 40 MW
• Water temperature above 90 degrees Celsius at its hottest

This article was originally published in Oulun Energia’s customer magazine 3/2021. Text by Kati Jurkko.