HT-ATES system case study on TU Delft campus

Abstract

High Temperature Aquifer Thermal Energy Storage (HT-ATES) systems can be implemented to store geothermal energy, residual industrial heat and solar heat. Due to the large temperature differences between the injection fluid and ambient groundwater, buoyancy flow occurs, which causes heat losses which in turn results in poor efficiency of such systems. The heat losses can be reduced by applying density difference compensation, this entails injection water with a higher salinity, to compensate for the low density caused by the high temperature of the injection fluid.

The goal of this study is to get insight into the behaviour of a possible HT-ATES system on the campus of the TU Delft with the injection of saline groundwater from deeper layers for storage in both less deep and less saline aquifers. To achieve this different cases will be considered; a reference case, a theoretical optimum, density difference compensation of the TU Delft case and the optimum density difference compensation. For these cases, different scenarios are modelled in SEAWAT to identify the influence of injection temperature, injection volume, aquifer thickness, and hydraulic conductivity on the behaviour of the system.

Using the density difference compensation from the aquifer at TU Delft, with a salinity of 16.4 kg/m$^3$, a maximum improvement of 2.0\% can be achieved. Also, differences between the different scenarios for the TU Delft case were maximum 2.5\%, with exception for a thicker aquifer or smaller injection volume. Due to the large injection volume in thin aquifers, the hot water has a large volume, causing conduction losses to dominate in the system. For thick aquifers a lot of density driven flow occurs in the reference case. With optimum density difference compensation, having a salinity of 39.1 kg/m$^3$, the density driven flow can be significantly decreased and the efficiency increased with 21.5\%. Both aquifer thickness and salinity of the injection fluid have a negative correlation with salt recovery.

Thin aquifers in relation to injection volume are not sensitive to density driven flow, so density difference compensation is not useful for such HT-ATES systems. Density difference compensation in the case of the TU does give significant improvement in efficiency. The most influential parameter on the efficiency is aquifer thickness, for both the cold and warm well, in relation to injection volume. For thick aquifers with optimum density difference compensation the highest efficiency of 75.5\% is achieved after 10 cycles.