Influence of thermal energy storage basins on the subsurface and shallow groundwater

Seasonal thermal energy storage for heat and cold supply is of growing importance in modern energy systems. Yet, high thermal losses and inadequate storage efficiencies hinder their market maturity. Especially under challenging conditions, e.g., due to groundwater flow, the accuracy of planning predictions is affected. Moreover, for subsurface installations, interaction with the subsoil cannot be sufficiently modeled, while operational risks can result when legal environmental thresholds are violated. Addressing these shortcomings, this study presents a new modeling and simulation framework to improve the design and operation of ground-based, sensible, seasonal thermal energy storage systems. Coupling two customized models via a co-simulation approach, both the internal storage (i.e., storage medium/structure) behavior as well as processes in the surrounding environment are resolved in detail. In this way, previously unconsidered mechanisms are unraveled allowing in-depth analyses regarding environmental impacts and interactions under various conditions. The study firstly introduces the newly developed tool, then secondly benchmarks its applicability in a subsequent parameter study, examining impacts under different hydrogeological conditions. Results show a broad efficiency range of 13 %, despite thermal insulation. In the case of uninsulated storage, efficiency is reduced by up to 24 %. Introducing a novel spatial differentiated analysis, influences of the groundwater saturated zone are quantified and temperatures of >45 °C in the 5 m distance of the storage are observed after 10 years of operation, whereby increased groundwater flow velocities are favorable to reduce impact intensities but also increase the affected area and the thermal losses. Along with the results of the case study, the presented framework provides a valuable tool for planning recommendations for future installations while offering a comprehensive assessment of various perspectives that have not been covered so far.