50 times 48 metres deep

The biggest seasonal borehole thermal energy storage system in Europe is being built in Braedstrup, Denmark. The aim is to store the energy collected in a gigantic solar thermal energy system in an array of borehole heat exchangers over the winter, thereby significantly increasing the solar component of the local district heating network.
Denmark is considered the El Dorado of district heating. Well over 90% of the people living in Copenhagen are connected to the local district heating network, and a central heat supply is run-of-the-mill in many smaller municipalities. The proportion of private households using district heating in the whole of Denmark is an impressive 50%. Less well known is the fact that Denmark is also home to some of the largest solar thermal energy installations in Europe, which also supply district heat to entire municipalities.
About 1,200 houses as well as public buildings and industrial establishments are currently connected to the district heating network in Braedstrup. The hot water demand in summer is completely covered by an existing, 8,000 m² solar energy plant. In terms of the total annual energy demand for heating and hot water, this however only corresponds to a solar contribution of about 10%.
The problem is well known and basically the same as for photovoltaic systems: The sun is the largest source of energy available to mankind – but the energy output of the sun simply isn't constant over the course of a year. While an excess of thermal energy and sunlight readily is available in summer, the figures are on the low side in winter. To counteract this imbalance, research to develop efficient long-term storage systems has been conducted for many years. Such systems should store the excess energy and make it accessible for use in winter.
How the storage system works
The already existing solar field in Braedstrup will be extended significantly in order to be able to cover a larger proportion of the required heat with solar energy. To make this energy available for use in winter, a seasonal borehole thermal energy storage system with 50 borehole heat exchangers made of PE-Xa will be constructed. This system stores the excess thermal energy at a depth of approximately 48 metres.
The way the storage system works is not very complicated: In summer, water heated up to 85 °C by the solar energy system, circulates in the borehole heat exchangers. The adjacent terrain is heated up in the process, creating a gigantic thermal energy store. In winter, when the thermal energy is required for the town's district heating supply system, the heat is transferred back to the circulating water and extracted via a heat pump.
Gigantic insulated water storage systems or ground water storage reservoirs are an alternative to borehole thermal energy storage systems for seasonal storage of thermal energy. Projects in Germany or Canada have however demonstrated that borehole thermal energy storage systems are the most economic choice when geological conditions are suitable.
Extension continues
The solar collector area will be extended to 18,000 m² in the first extension phase of the project. The combination with the borehole thermal energy storage system makes it possible to increase the solar contribution to about 20%. But that's not all: Extension of the solar collector surface area to a total of 60,000 m² and installation of 300–400 borehole heat exchangers for thermal energy storage, is planned for the final extension phase of the project. This is expected to result in a solar contribution to the total annual energy demand of 60%.
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