In recognition of the need to shift from natural gas to renewable energy, Dronninglund District Heating set a goal back in 2005 o have solar heat cover 50 percent of its annual energy production. Today, Dronninglund has a solar district heating solution consisting of 2,982 solar panels covering 37,573 m3.

In the summer, the solar thermal plant produces more energy than the city needs. To save energy for a cloudy day, the surplus heat is stored in a pit thermal storage tank that holds a total of 60,000 m3 of water. In autumn, the stored heat is gradually released to meet the city’s district heating needs. To further utilise the storage, the system has an integrated heat pump to cool the storage by using the heat before the next summer. The heat pump was initially an absorption heat pump supplied with heat from a biooil boiler located at a heating centre nearby. It is now replaced by a compressor heat pump located at the pit thermal storage, where it uses both ambient air and the stored heat as an energy source.

The combination of a large seasonal storage tank and a heat pump enables 40-50 percent of the annual heat demand to be met by solar thermal energy. It has also reduced CO2 emissions by two tonnes per household per year.