The results from the evaluations of the monitoring data of three large-scale pit thermal energy storages in Marstal, Dronninglund and Gram and one borehole thermal energy storage in Brædstrup in general prove the efficiency and reliability of the presented storage technologies. The results mostly show good agreements with the design figures in terms of storage efficiency, usable temperature ranges and contributions to the heat supply of the connected district heating networks. Deviations are explainable by different operational conditions or other site specific effects.
Especially the example of the PTES in Dronninglund shows a high storage efficiency, which is on the one hand a result of the good technical quality of the storage construction that leads to low thermal losses, on the other hand the storage has a large energy turnover as it is used for seasonal storage and for short-term storage simultaneously. In addition, the low temperatures in the storage in the winter period result in negative thermal losses in the bottom parts of the storage.
All of the considered systems have a heat pump included in the system concept that enables a discharging of the storages below the temperature levels of the district heating return lines. This allows for a nameable increase of the usable temperature differences of the storages and by this smaller storage volumes with the same usable heat capacities than without heat pumps.