Danish Case on Large-scale Thermal Storage to enable a Flexible Energy Systems

Large scale thermal storages will be an important element in future efficient district heating systems because they can provide the electricity sector with flexibility services and at the same time store excess heat from industries and waste incineration and reduce consumption of fossil fuels in peak hours. For storages of more than 50,000 m3 PTES are of special interest because the investment costs for TTES are 2-3 times higher than for PTES with the same storage capacity.

Large-scale thermal energy storages (for district heating purposes) are normally based on water as the storage medium, but other storage media (soil and gravel) can be used. Storages are normally unpres-surised, and operating temperatures are below the boiling point of water (100°C). Large-scale thermal storages are commonly defined as storages with a storage capacity of at least 1 GWh. They can be utilized as short-term storages or long term/seasonal storages depending on their storage cycle duration.

There are four different types of large-scale thermal storages for district heating systems:

• Tank Thermal Energy Storages (TTES), which normally are steel tanks above ground,
• Pit Thermal Energy Storages (PTES), which normally are underground storages where excavated soil is used as surrounding embankments,
• Borehole Thermal Energy Storages (BTES), which consist of boreholes with closed loops of un-derground water pipes, and
• Aquifer Thermal Energy Storages (ATES), where an underground water reservoir is used as stor-age (open system).

In this report we will primarily present Danish experiences with PTES, but also explain the use of TTES in Danish district heating systems. One case of BTES will be presented, but since realized ATES plants in Denmark are only small-scale, ATES will not be presented.

The report is made for planners and policy makers to facilitate implementation of large thermal water storages in district heating systems. It is prepared for the Danish Energy Agency and LEA Hessen.