Model-based comparison of operation modes for water-gravel-filled multi-storage basins operated for seasonal thermal energy storage

Seasonal thermal energy storage (sTES) using artificial basins filled with water or water-gravel mixtures and embedded in the ground has been investigated for several decades, primarily in the context of district-scale solar thermal applications. Over time, innovation steps have led to the classification of sTES technologies into distinct generations. Building on this evolution, a fifth sTES generation has been introduced by Bott et al. (2019). It is based on the integration of multiple, interacting storage units forming combined multi-storage systems. This concept responds to the fact that, ideally, heating and cooling systems are not static but evolve over time (Müller et al. 2025). In modern mixed-use districts, storage capacities, system configurations and operational requirements may change, including temperature levels, charging/storage/discharging periods and control strategies for heat and cold. As a result, sTES must be designed for adaptability rather than optimised for a single, fixed operation mode.

Our contribution presents numerical simulation results for water-gravel thermal energy storage (WGTES) configured as multi-cascaded, multi-purpose and multi-functional systems. The analysis focuses on key operational parameters, such as temperature ranges, fluid circulation concepts and cascading versus parallel operation. System performance is evaluated in terms of storage efficiency and thermal losses (to the surrounding ground and to the other units). A set of generalised scenario analyses based on representative cases is used to identify robust storage characteristics and to derive conclusions that are transferable beyond specific case studies.

Bott, C., Dressel, I., & Bayer, P. (2019). State-of-technology review of water-based closed seasonal thermal energy storage systems. Renewable and Sustainable Energy Reviews, 113, 109241.

Müller, S., Bott, C., Schmitt, D., Faigl, M., Göttl, K., Strobel, R., Bayer, P., Schrag, T. (2025). Implementation of an Expanding Thermal Source Network as a Step Towards CO₂-Neutral Industry. Energy, 330, 136766.