Low-Temperature ATES in Germany: Demonstrating the Opportunities and Limitations in Berlin

Only a few low-temperature aquifer thermal energy storage sites (LT-ATES) exist in Germany. Our project aims to demonstrate the implementation of a LT-ATES system in an urban area motivated by the increasing demand for cooling in summer. The backyard of an office building complex in Berlin-Mitte was chosen as our test site. Due to the densely built-up conditions, it was not possible to implement a classic doublet system. A coaxial well with a drilling depth of 27 m and a filter distance of 6 m was therefore implemented, which can only be operated using a flow-through principle. In a coaxial well the groundwater is drawn from the lower end of the well system and fed to the heat pump via the inner pipe. After the thermal energy is extracted, the groundwater is re-injected via an outer pipe within the same borehole into a shallower layer. The injection of cooled or heated groundwater in winter and summer, respectively, results in vertical circulation of the groundwater within the aquifer.  The permissible temperature spread is limited to 3 K in Berlin. With reference to the local ambient groundwater temperature of around 13 °C, the thermal loading of the aquifer therefore can only vary between 10 °C and 16 °C. The well system was planned as an integrated heating and cooling source as part of the energy-efficient building refurbishment and is used to cover the base load with a maximum flow rate of 6 m³/h. The well went into operation at the end of June 2024.

We monitor the thermal-hydraulic, geochemical and ecological influences on the aquifer using three monitoring wells which are also located in the backyard. Fiber optic cables were installed for depth-differentiated temperature measurements. In addition, continuous measurements of groundwater level, conductivity and flow rates are carried out. To record the thermal influences on the groundwater ecology, groundwater samples are regularly taken at the site and compared with other samples in Berlin.

The potential for seasonal heat storage in the aquifer has been modelled indicating a potential thermal short circuit in the well configuration and raising the question of whether the well system is a seasonal regeneration of the aquifer rather than an active thermal storage. Increasing the distance between the filter sections might solve the problem. Preliminary monitoring results show a significant and increasing influence of the air temperature over the course of the summer and over the entire well length causing a interaction with well operation.