High Temperature ATES: Thermal impact and efficiency assessment with numerical simulations

High temperature aquifer thermal energy storage (HT-ATES) is a promising technology for mitigating the temporal disparity between availability and demand for heating energy supply. By applying seasonal storage, renewable or alternative sources like waste heat can be used, reducing the dependency on fossil fuels and avoiding CO₂-emissions.

HT-ATES uses external heat sources and stores heat in suitable formations in the geological underground by injecting hot water at temperatures of up to 90°C. Balanced energy injection and extraction however, is usually not feasible due to energy losses, leading to residual heat in the subsurface and maybe changing groundwater composition and quality. This study shows that numerical simulations can be used to quantify the thermal impact of heat storage on the geological storage formations as well as the subsurface space demand of such storage sites.

In a hypothetical scenario, a HT-ATES system is designed to store about 35 GWh/a of excess heat from solar thermal installations and a waste incineration plant, which would cover about 20 % of the heat energy needs of a typical city district. For this purpose, a three-dimensional numerical model of the HT-ATES is set up, which consists of six well doublets placed at 100 m depth in a typical northern German Pleistocene formation, a sand aquifer bounded by till layers at the top and bottom. The screen lengths of all wells cover the entire storage formation thickness of 20 m. The daily excess heat storage demand is derived from the estimated daily heat demand for space heating and hot water production for the city district, based on an available 3D building stock model and daily outside temperature data for 2018, combined with a supply curve for solar thermal heat production, which is based on available roof and open space area in the district and daily global radiation data for the location of the district from 2018.

Injection flow rates vary between 0 and 45 m³/h, while the injection temperature is assumed constant at 70°C. The extraction flow rates are controlled by a well doublet control module, which iteratively adapts the extraction flow rates according to the heat demand curve.

Results show that during the heating period from October to May, at least 21 GWh and up to 26 GWh after 30 years of operation or 12 - 15 % of total district heat demand can be supplied each year by the HT-ATES. Supply temperatures range from 70 to 39 °C at the start and at the end of the heating period, respectively. The storage efficiency increases from 65 to 74 to 78 % after 5, 15 and 30 years of operation, respectively. After 30 years, the HT-ATES operation affects an ellipsoid shaped volume of 28 Mio m³ with temperature increases of > 1 °C, which corresponds to the volume of a cube of approximately 300 m side length.