The use of salinity contrast for density difference compensation to improve the thermal recovery efficiency in high-temperature aquifer thermal energy storage systems

Abstract

The efficiency of heat recovery in high-temperature (>60°C) aquifer thermal energy storage (HT-ATES) systems is limited due to the buoyancy of the injected hot water. This study investigates the potential to improve the efficiency through compensation of the density difference by increased salinity of the injected hot water for a single injection-recovery well scheme. The proposed method was tested through numerical modeling with SEAWATv4, considering seasonal HT-ATES with four consecutive injection-storage-recovery cycles. Recovery efficiencies for the consecutive cycles were investigated for six cases with three simulated scenarios: (a) regular HT-ATES, (b) HT-ATES with density difference compensation using saline water, and (c) theoretical regular HT-ATES without free thermal convection. For the reference case, in which 80°C water was injected into a high-permeability aquifer, regular HT-ATES had an efficiency of 0.40 after four consecutive recovery cycles. The density difference compensation method resulted in an efficiency of 0.69, approximating the theoretical case (0.76). Sensitivity analysis showed that the net efficiency increase by using the density difference compensation method instead of regular HT-ATES is greater for higher aquifer hydraulic conductivity, larger temperature difference between injection water and ambient groundwater, smaller injection volume, and larger aquifer thickness. This means that density difference compensation allows the application of HT-ATES in thicker, more permeable aquifers and with larger temperatures than would be considered for regular HT-ATES systems.

Analysis of the impact of storage conditions on the thermal recovery efficiency of low-temperature ATES systems

“…13), in contrast with the simulations with the highest ambient groundwater flow (50 m/y). Also the A/V ratios calculated for earlier simulation studies and experiments without ambient groundwater flow (Caljé, 2010;Doughty et al, 1982;Lopik et al, 2016) strongly correlate with the observed efficiencies in these studies. Like in this study, the results from Lopik et al (2016) and Doughty et al (1982) consist of a series systematic changing boundary conditions which allows for verification of the relations found in Fig.…”

“…13. Results of both Lopik et al (2016) and Doughty et al (1982) show a linear relation with similar slope between the surface area over Fig. 10.…”

“…Also, Doughty et al (1982) uses no dispersion, which explains why their simulations show the highest efficiency. Lopik et al (2016) uses shorter and less storage cycles as well as a slightly smaller dispersion coefficients compared to this study. From these (small) differences can be seen that at simulations with higher dispersion, the A/V -efficiency relation becomes steeper, small systems which have a larger A/V ratio then suffer relatively more, confirming the earlier observation from Fig.…”

“…E.g. ; both Doughty et al (1982) and Lopik et al (2016) used an axisymmetric model and a finer vertical spatial discretization compared to this study, resulting in differences in numerical dispersion. Also, Doughty et al (1982) uses no dispersion, which explains why their simulations show the highest efficiency.…”

“…While for high temperature (> 45°C) ATES systems, the negative impact of the buoyancy of the stored hot water on thermal recovery efficiency typically needs to be considered (Lopik et al, 2016;Zeghici et al, 2015), this can be neglected for low temperature ATES systems (Doughty et al, 1982;Zuurbier et al, 2013). However, as these low temperature ATES systems are typically targeting relatively shallow aquifers, the impact of stored volume displacement by ambient groundwater flow requires consideration.…”