Modelling heat transport in a High Temperature ATES system

Abstract

In order to create a sustainable planet, human kind has set ambitious goals to reduce CO2 emissions in the near future. About 40% of the global energy consumption is used in heating and cooling in the built environment and the bulk of this energy is produced from fossil fuel burning. Aquifer Thermal Energy Storage (ATES) is a sustainable way for space heating and cooling. Surplus heat is stored in a subsurface groundwater aquifer during summer and reproduced in winter when heat demand is higher. Interest is aroused in High Temperature ATES (HT-ATES), where injection temperatures are higher (>25oC) than in regular ATES (<25oC). However, injection of warm water in a colder (~12oC) subsurface may thermally affect surrounding layers and induce processes that reduce the thermal recovery efficiency of the HT-ATES system. This research aims to find the processes and dominant hydrogeological and operational parameters controlling 1) the thermal effects of High Temperature ATES on overlying layers and 2) the thermal recovery efficiency of a HT-ATES system. To this end, and to provide more insight in the thermal transport in HT-ATES systems, a numerical 2D axisymmetric SEAWATv4 model was built to perform a sensitivity analysis around a reference Case Study scenario. The results showed that the thermal impact on an overlying aquifer mainly depends on the water injection temperature and the thickness of the cap layer that separates the overlying aquifer from the injection aquifer. The 1D steady state heat conduction theory provides an analytical solution that gives a good first order approximation of the nearly linear vertical temperature distribution in the overlying layers that can be expected on the long term. Density driven flow in the injection aquifer resulted in a larger radial extent of thermal impact on the overlying layers. Heat effects from the well casing was limited to locations close to the well. For low injection temperatures, heat conduction is the main process responsible for efficiency losses and optimizing the area over volume ratio increases the efficiency. At higher injection temperatures density driven flow also contributes to heat losses. For the modelled scenarios with higher injection temperatures, exceeding a critical injection aquifer thickness greatly increased density driven flow and decreased HT-ATES efficiency. Increasing yearly injection volume always benefits the efficiency of a HT-ATES system. These findings are valuable to identify under what conditions a secure and efficient realization of HT-ATES systems is possible.