Solar power in Denmark contributes to a goal to use 100% renewable energy by 2050. The goal of 200 MW of photovoltaics by 2020 was reached eight years early, in 2012, and 36 MW is being installed each month. A total of 3,400 MW is expected to be installed by 2030. Many solar-thermal district heating plants exist and are planned in Denmark.
A large solar-thermal district heating plant on the island of Aeroe provides a third of Marstal's energy, and is being expanded. The plant uses seasonal thermal energy storage (STES) in the form of a large lined pit that is filled with gravel and water, and insulated on top. This enables solar heat collected primarily in summer to be used year-round. Similarly ice has historically been stored for summer use in many countries.
In Braedstrup, the community's solar district heating system stores heat in a borehole STES (BTES) facility that uses 19,000 cubic metres of underground strata as a heat battery. It can hold 500 mwh of heat at a temperature of 65 oC. Two water tanks provide additional heat storage. When extracting heat, a 1.5 MW heat pump boosts the temperature to 80 oC, for circulation in the district heating loop. The present system is the first expansion of an original smaller system, and now provides 20% of the community's heat on an annual basis, from a solar collector area of 10,600 square metres. A second expansion is planned, to provide 50% of the heat demand from a total solar collector area of 50,000 square metres and using an enlarged BTES store. The remainder of the demand is provided by electric and gas-fired boilers.
The Braedstrup system is designed to integrate with the national electric grid. The heat pump and electric boiler are used when there is surplus wind power available on the grid, contributing to the stability of the system and maximal use of the wind power. The natural gas boiler is used when this renewable electricity is not available.