Abstract
It is well known that heat recovery of high-temperature aquifer thermal energy storage (HT-ATES) systems is negatively impacted by buoyancy-driven flow of the injected hot, low-density water during storage. Density differences, however, also affect pressure head conditions within the well and therefore pressure head differences with the aquifer. Thus, it was anticipated that nonuniform flow distribution across the well screen may occur even when homogeneous aquifer permeability is assumed. In this study, the extent and conditions under which nonuniform flow across HT-ATES well screens occurs, influenced by both variable density and viscosity, were evaluated using both numerical and analytical approaches. Results show that with larger density differences (injection temperatures up to 90 °C) and lower well pressure heads, flow distribution is increasingly nonuniform, with the highest flow rates across the top part of the well screen, up to multiple times the average flow rate. Nonuniformity is described analytically by the maximum depth of injection ( D inj,max ) and the maximum normalized flow at the top of the aquifer ( q n,max ). Decreasing viscosities with higher temperatures further amplify buoyancy-induced nonuniform flow distribution, with flow rates up to 3.7 times the average. Flow distribution is shown to be asymmetrical during injection and extraction, and besides contributing to losses, the extent to which flow distribution is nonuniform, consequently also results in substantially higher maximum velocities across the borehole wall, which is a critical design parameter to prevent clogging. A literature screening of HT-ATES storage conditions in practice highlights the relevance of considering buoyancy-induced nonuniform flow distribution.