High-temperature aquifer thermal energy storage (HT-ATES) is a heat storage technology utilising the subsurface, which can reduce greenhouse gas emissions in renewable-dominated heating sectors. Since the temperature difference between the surrounding groundwater and the injected water (> 50 °C) leads to density differences, HT-ATES can induce buoyancy flow. This process results in uneven heat distribution across the aquifer thickness, lower storage efficiency, and increased thermal impacts. The occurrence and intensity of buoyancy flow depends on, among others, vertical and horizontal permeability.
The proposed HT-ATES storage site in Hamburg, Germany, utilizes the Miocene Lower Braunkohlensande (brown coal sands) as the storage aquifer. This geological formation was formed in a coastal transition regime between terrestrial and shallow marine settings and is primarily composed of sands. Layers of brown coal, silt, and clay, embedded in the main storage aquifer, as identified from borehole information, were formed from peat swamps and lagoons and may impede convection due to their low permeability. The influence of these low permeability layers, also considering their lateral extension and position relative to the warm well, on induced convection and on HT-ATES efficiency and thermal impacts is examined in this work by employing a site-specific numerical HT-ATES model representing the coupled thermo-hydraulic processes.
Results show, that including even thin low permeability layers can effectively hinder temperature induced thermal convection, thus increasing thermal efficiency of a HT-ATES. The scenario simulations also show, that convection is already significantly dampened if the layers extend only up to the thermal radius.