Design, construction, and scientific monitoring of a Reno-sTES

Seasonal thermal energy storage (sTES) is an enabler for matching a temporal imbalance between thermal supply and demand in energy systems. This increases, among others, system efficiency and resilience. By reducing peak loads and fossil supply, sTES further supports decarbonization strategies, making it a cornerstone of long-term, climate-neutral thermal supply strategies. sTES can be realized through artificial systems (i.e., tank (TTES), water-gravel (WGTES), and pit thermal energy storage (PTES)) (Bott et al. 2019). In contrast to geological storage technologies, they store heat or cold in closed volumes separated from the ground. While they can achieve high efficiencies, new installations are often associated with high costs due to excavation, construction, sealing, and insulation. They amount to emissions (Weise et al. 2025), and their performance can be sensitive to thermal losses, groundwater interactions (Bott et al. 2024), and spatial constraints, limiting scalability in dense urban environments.The Reno-sTES (renovated sTES) concept addresses these challenges by reusing idle/decommissioned infrastructure. By integrating thermal storage into existing basins, Reno-sTES significantly reduces construction effort, material use, and economic risks, while improving environmental performance and accelerating time-to-operation. Further advantages include no further land use, soil sealing, and increased public acceptance: Instead of installing new, visually intrusive infrastructure, desolate installations are brought back to life. Typical candidates for Reno-sTES include basins for former (waste-) water treatment, swimming pools, gravel pits, stormwater retention, industrial cooling, or fire-water reservoirs, and abandoned industrial tanks.This study presents the first-time R&D-accompanied implementation of a Reno-sTES system at the incampus, Ingolstadt (Müller et al. 2025). Former water treatment basins of a refinery are being repurposed into a combined, multi-unit WGTES. Our contribution focuses on challenges and solutions during the design and construction/renovation phase, including questions related to handling complex geometries, choosing materials that balance thermal performance and durability, and tailored heat-exchanger designs for effective charging and discharging. Environmental constraints and the need for innovative, new planning/design and construction methods are addressed as well (Dahash et al. 2025). The study summarizes the scientific assessment of these aspects before commissioning, expected in Spring 2026, and includes comprehensive monitoring concepts (e.g. via active distributed temperature sensing) within the basins and surrounding ground. Also on this basis, a successful preparation and construction of this Reno-sTES represents an important contribution to the energy transition and forms the basis for further analyses within the Horizon Europe project INTERSTORES (INTERSTORES 2026).LiteratureBott et al. (2019). State-of-technology review of water-based closed seasonal thermal energy storage systems. Renewable and Sustainable Energy Reviews, 113, 109241.Bott et al. (2024). Influence of thermal energy storage basins on the subsurface and shallow groundwater. Journal of Energy Storage, 92, 112222.Dahash et al. (2025). Simulation-based planning for cost-effective and energy-efficient large-scale seasonal thermal energy storage systems. Renewable Energy, 258, 124813.INTERSTORES 2026. Available online: https://interstores.eu/. Müller et al. (2025). Implementation of an Expanding Thermal Source Network as a Step Towards CO₂-Neutral Industry. Energy, 330, 136766.Weise et al. (2025). Comprehensive life cycle assessment of selected seasonal thermal energy storage systems. Renewable Energy, 124232.