Assessing the worthwhileness of efficiency enhancement methods mitigating buoyancy flow in high-temperature aquifer thermal energy storage

Abstract High-temperature aquifer thermal energy storage (HT-ATES) presents a promising approach for achieving a temporally and spatially balanced, continuous energy supply. A key challenge in HT-ATES systems is the density-driven buoyancy flow, which can result in considerable heat losses and reduced energy efficiency, or thermal recovery efficiency. This study explores two strategies for mitigating the effects of buoyancy flow: adjusting the working fluid’s salinity and modifying the production well screen length, under varying intensities of buoyancy flow. Numerical simulations are employed to evaluate the recovery efficiency and pumping energy requirements across a range of fluid salinities and screen lengths over multiple HT-ATES operational cycles. Under conditions of high intensity of buoyancy flow, the impact of different fluid salinities on thermal front evolution and associated hydrodynamic processes is analyzed. Recovery efficiency exhibits a nonmonotonic response to fluid salinity, with the maximum improvement reaching 36.5% at an increase in salinity of 6%. At a moderate intensity of buoyancy flow, altering fluid salinity proves ineffective due to the formation of a buffering layer, where cooled saline water separates the injected fluid from the surrounding groundwater. Comparatively, modifying fluid salinity is found to be more effective than adjusting the production well screen length. For both strategies, the additional pumping energy required is negligible relative to the gain in recovered heat. The number of operational cycles has minimal influence on the heat plume’s location and morphology. Furthermore, owing to the additional preferred pathways and irregular-shape fronts, the recovery efficiency enhancement methods are found more effective in heterogeneous aquifers.