The role of a cavern thermal energy storage as part of an evolving district heating system

Vantaa Energy is planning to implement the world’s largest cavern thermal energy storage in the city of Vantaa. The amount of heat contained in the facility, 90 GWh, would meet the heating need of a medium-sized Finnish town and it also corresponds to about five per cent of the annual heat consumption of Vantaa. Among the largest heat storage facilities throughout the world until now have been the Vojens (200,000 m3) and Marstal (75,000 m3) storage facilities connected to solar heat systems. They can store about 11 GWh or 4 GWh heat, depending on the temperature levels. The systems are based on the utilisation of solar heat and therefore the size of the storage facility in relation to the district heat load is easily tens of percentage points. Built in a rock cavern, the estimated capacity of the Mustikkamaa heat storage facility in Helsinki, which was used as oil storage in its time, is 11.6 GWh. The corresponding share of the district heat load of the entire city of Helsinki is less than 0.2 per cent.

The greatest innovation in Vantaa storage is the higher temperature level enabled by the hydrostatic pressure of the storage facility located deep in the bedrock. As a result, the capacity will be almost double that of a normal heat storage facility of a similar size.

However, what is more important than technological innovations and scale is the role of the storage facility in the evolvement of district heating in Vantaa towards a carbon-neutral system.
New low-carbon solutions are often more expensive than traditional solutions in terms of their investment costs and they must achieve a high utilisation rate in order to be cost-effective. However, the district heating system can only accommodate production corresponding to heat consumption at that moment and there is not always room for all available heat production. The storage solution creates this space and can increase the utilisation rate of new heat supply by just that much that they are techno-economically profitable and therefore are also implemented. There are substantial seasonal variations in heating: consumption during the summer is only about one-tenth of the heat needed in the coldest times of the year.

Even in the current situation, the cavern thermal energy storage improves the utilisation of heat produced by the Vantaa waste-to-energy plant by storing it for the peak of the heating season. The same goes with the utilisation of waste heat in general: data centres, shops and cooling systems can be included more efficiently as part of the production structure of district heat. This way, it will be possible to eliminate the particularly expensive peak production based on natural gas and, in addition to costs, the overall emissions of district heat will also fall. Investments in peak heat production in the form of a pellet or natural gas boiler are economically unviable unlike investment in a heat storage facility.

Therefore, a cavern thermal energy storage brings direct benefits and it also enables adding new, low-carbon heat supply to the system. When the solutions are developed, the emission reduction methods will also further increase the cost effectiveness of the system.

Thanks to the district heating system, i.e. a large connected heat load, the cavern thermal energy storage can also be used as energy storage by regulating co-generation or by utilising production based on heat pumps. In terms of their investment costs, heat storage facilities are significantly cheaper than, for example, battery storage facilities. The investment cost of a heat storage facility is around €1/kWh, while that of a battery storage facility is approximately €100/kWh at best.

The district heating system of the future is flexible with regard to its consumption and production structure. A large-scale heat storage facility supports flexible production and that way improves the possibilities of sector integration between heating and electrical systems. This feature in the system will have a monetary value for district heating companies as variable renewable electricity generation increases.

This electricity generation based on wind and solar power is growing at a tremendous pace throughout the world. In Europe, there is also a lot of discussion about replacing property- or dwelling-specific gas and electric boilers with district heating solutions. As district heat develops in Europe, interest towards heat storage solutions will definitely grow along with the need to increase flexibility in the energy system. The heating and cooling sector in general offers many interesting possibilities for both flexibility and the reduction of emissions.

Dr. Miika Rämä
Research Team Leader
VTT