Germany's pit thermal energy storage

District heating is widely recognised as an efficient way to distribute renewable heat, whether in large-scale city networks or smaller setups like in the town of Meldorf, Schleswig-Holstein, Germany.

Here, WIMeG Wärmeinfrastruktur Meldorf GmbH & Co. KG (WIMeG) is planning a new district heating network that will replace the individual heating systems from approximately 55 residential and commercial buildings. It is estimated that switching from individual heating systems to district heating will reduce annual carbon emissions by 1,000 tonnes. In addition to the district heating network, WIMeG is also building Germany’s largest thermal heat energy pit storage with a capacity of 43,000 m3 – equal to 17 Olympic swimming pools.

The pit storage adds flexibility to the system, as up to 1,500 MWh can be stored during the summer and in periods where there is a lower demand for heat. This stored heat is then redistributed to the network when demand rises, minimising waste heat and ensuring a high security of supply.

WIMeG appointed Ramboll to provide planning services for basic engineering and approvals. This includes hydraulic pipe design, preparing routing plans and planning the energy central including heat generation. In the first phase of the project, Ramboll was also responsible for planning and design of the storage pit which covered earth dams, culvert, steel, and pipe construction, sealing and floating cover.

Ramboll continues to support WIMeG by providing construction supervision and further consultancy services.

To ensure maximum efficiency and reduced emissions, the district heating network will primarily use waste heat from a nearby printing plant. This will be supplemented by biogas engines and peak-load gas boilers with the option to implement solar thermal energy in the future.

The pit storage works by heating water to maximum 90 °C through several integrated heat generation units. The water is then stored in the pit for future use. Heat loss is minimised by a heat-insulating floating cover. The liner itself does not have high requirements for thermal insulation.

Ramboll’s work on the Meldorf district heating system and the pit storage began in 2020. The pit storage is expected to be completed and ready for operation in early summer 2023, and the district heating system is expected to be operational by winter 2023.