Experimental results of a high-temperature aquifer thermal energy storage test site – storage efficiency and thermal impact on the environment

High-temperature aquifer thermal energy storage (HT-ATES) in the geological subsurface can help bridge the temporal mismatch between production and demand of energy from renewable sources. Despite great importance for energy system transformation in the heat supply sector, HT-ATES faces some challenges and risks such as regulatory challenges. The heat input experiments at the TestUM –Aquifer test site provide a basis for characterization and verification of hydraulic, thermal, geophysical, microbiological, and geochemical process understanding. A HT-ATES system was experimentally simulated at the field site, representing three phases with varying loading and unloading cycles at injection temperatures of 80°C. More than 500 thermocouples were used to record temperature data over a 579-day period between July 2021 and February 2023. A total of eleven operating cycles divided into three phases were performed, representing a total heat input of 155 MWh. The temperature records are highly resolved spatially, especially in the vicinity of the injection well, with intervals as low as 0.5 m in the vertical and horizontal directions, and a temporal resolution of 10 min. Thus, the temperature distribution in the subsurface and the position of the heat plume is well characterized at any time. Results show, that the temperature distribution is affected by density driven convection, caused by the temperature differences, as well as heat loss to the confining unit. The storage efficiency was determined by measuring return flow rates and temperatures, showing that storage efficiency decreases with cycle length and with downtimes between charging and discharging.