Winter heating is energy intensive, but it is possible to save up warmth over summer and release it over winter.

Several seasonal heat storage systems are already in use.

In Marstal, Denmark, for example, a large, covered reservoir is heated with solar thermal collectors in summer. In winter, the reservoir produces enough warmth to cover half of the heating requirements of around 2000 residents.

More and more variants of such heat storage systems are being built, and not just in Europe. What technologies are available and how do they work?

Reservoirs as heat stores: practical for district heating

In the case of underground heat storage tanks, large pits are dug and sealed with foil. About 70,000 cubic meters of water are then poured into an area of around one hectare.

This water is heated up to 90 degrees Celsius (194 Fahrenheit) using solar thermal systems, waste heat from factories or trash incineration plants. An insulated lid ensures that just 10% of the stored heat is lost throughout the course of the year, and in winter, the warm water flows from the large storage tank into the district heating network to warm homes. 

To date, there are six large geothermal storage tanks in Denmark, one in Tibet and three are under construction in Germany and Poland. Interest in storage technology is growing.

Another option is to repurpose disused underground coal mines. 

In Germany's Ruhr region, a heat storage system is already being tested in an old mine shaft.

Advantages and disadvantages: Large underground reservoirs store excess heat for the winter at low cost and lend themselves to feeding district heating networks. While construction costs are relatively low, they do require enough space for the basins.

Energy efficiency: High. Only around 10% of the heat is lost per year. 

Where can they be used? Everywhere.

Large heat tank: Heat storage in buildings

Large water tanks in buildings can also store heat for months. For this purpose, steel tanks of up to 260 cubic meters are installed in the middle of new buildings. They can be several storeys high and are very well insulated.

Because steel tanks are more expensive and the buildings in which they are contained have to be specially constructed, investment costs are around 10 times higher than for geothermal storage tanks. A storage tank with 50 cubic meters costs around €170,000 ($184,000), including a large solar system.

This building complex (pictured) in Frankfurt has several tanks holding a total of 50 cubic meters of water. A solar thermal system on the roof heats the tanks to 80 degrees in summer. In conjunction with a heat pump, this provides winter warmth for 56 apartments.

Advantages and disadvantages: Heat storage tanks in buildings are practical, well-insulated and save on additional pipes. The buildings can largely be heated with solar power, which keeps running costs very low. However, the additional construction outlay may deter building owners.

Energy efficiency: Very high: The heat storage tank is in the middle of the house and hardly any heat is lost.

Where can it be used: In new buildings worldwide.

Geothermal energy storage: Good for heat pumps

Heat can also be stored in the ground to supply existing buildings. To do this, water pipes are laid in the ground and the soil is insulated on the sides and at the top.

In summer, the ground is heated with hot water from a solar thermal system. In winter, a heat pump uses this stored geothermal energy to supply warmth to homes.

This Berlin residential complex from the 1920s (photo) is heated with such a geothermal storage tank. 

Advantages and disadvantages: Retrofitting is possible. However, space is needed next to the building to lay pipes in the ground. Around 40 square meters are needed for a detached house, but more is required for larger buildings.

Energy efficiency: High. Heat pumps work very efficiently with this geothermal energy, even on very cold winter days. 

Where can it be used? Wherever there is enough space next to the building.

Seasonal storage with hydrogen: good for municipalities 

Heat can also be stored using hydrogen. The technology already exists in some buildings. 

In summer, excess solar power is used to produce hydrogen in an electrolysis process. This is stored in pressurized cylinders. In winter, the hydrogen is converted back into electricity and heat using fuel cells to supply buildings.

In the future, hydrogen could account for almost 9% of Germany's district heating network, according to long-term scenarios calculated on behalf of the German government. The hydrogen would be stored primarily in large underground salt caverns, which are currently used to store natural gas. 

Advantages and disadvantages: For energy systems and municipal utilities, hydrogen can be a good addition when combined with other storage systems to cover electricity and heating requirements in a climate-neutral way all year round. However, such systems are very expensive for individual houses; the installation in this building near Bonn (photo) cost around 550,000 euros. 

Energy efficiency: Poor. The conversion of electricity to hydrogen and its recovery to electricity in winter generates around 40% waste heat. This waste heat should be used promptly to reduce energy losses. Other seasonal storage systems are usually more efficient.

Where can they be used? Everywhere.

This article was originally published in German and edited by Anke Rasper.