Underground Thermal Energy Storage and its Impact on Groundwater Based on Experimental Mock-Ups

Abstract

Heat storage systems are crucial for the energy transition, providing a more sustainable energy supply and mitigating peak demands, particularly in urban areas. Current developments focus on underground installations that use the subsurface for thermal storage, to reduce land use and sealing. However, these structures are impacting the subsoil and the groundwater, potentially degrading groundwater quality and therefore complicating the approval procedure and construction of underground thermal energy storages (UTES). This paper introduces a controlled experimental concept for analysing the effects of UTES on aquifers. Previous research primarily relies on numerical modelling, which offers limited legal validity for project approvals but serves as a foundation for optimization and monitoring strategies. The proposed approach enables the controlled reproduction of thermal plumes under variable hydraulic gradients, soil permeabilities, and storage geometries. Furthermore, a simplified construction method is presented to facilitate urban heat storage implementation. Initial tests under dry soil conditions confirm the setup's capability to capture high-resolution temperature fields using distributed fibre optic sensing (DFOS), with a maximum temperature difference of 42.4 °C recorded at the storage–soil interface. Complementary numerical pre-studies were used to identify suitable hydraulic gradients and thermal boundary conditions for further experiments and will support calibration and upscaling of the modelling framework. Subsequent experiments under saturated conditions with induced groundwater flow are to be performed to quantify plume migration and evaluate mitigation strategies.