Particularly in urban areas, heating and cooling is responsible for a large fraction of total energy demand. Due to the seasonal and diurnal variation in heat demand and for example that of solar heat supply, the storage of heat is critical for the development of sustainable heating.
Aquifer thermal energy storage (ATES) systems allow the storage of heat cost-efficiently and at a scale that is sufficiently large to allow seasonal heat storage from office buildings to regional district heating networks (DHN). Critical for the performance of ATES systems is the heat recovery efficiency. While for low-temperature (LT, <25°C) ATES systems, recovery efficiency mainly depends on conduction losses from and displacement of the stored volume, at higher storage temperatures density differences between the stored hot water and the lower native groundwater temperatures (e.g. ~10°C in NL), result in losses by buoyancy driven flow. Particularly for HT-ATES systems, also the effects of heat loss on the thermal, chemical and microbial quality of the surrounding groundwater and that in over- and underlying aquifers still need to be understood.
We therefore focused in this study, on the controlling factors for the heat recovery efficiency and thermal impact through integrated density-dependent, heat transport and groundwater flow modelling of ATES systems for a wide range of representative storage (e.g. T:15–90°C, storage volume: 10,000–1 Mm3) and hydrogeological conditions. Results showed that for storage temperatures up to 30°C, under all conditions tested, conduction losses were dominant in controlling recovery efficiencies (55–85%), as evidenced by its observed strong linear correlation with the thermal area over volume ratio (A/V) with an optimum ~2 for the aquifer thickness over thermal radius ratio (L/Rth) for a particular storage volume. For storage temperatures of 45 and up, the negative impact of density driven flow was significant and became the dominant control for lowering of the observed recovery efficiencies (18–75%) at 90°C. From 45 to 90°C, the recovery efficiency was progressively more negatively and non-linearly correlated with L/Rth, with the highest recovery efficiencies at the lowest tested L/Rth (0.1). In addition to storage temperature and volume, the thermal impact on the simulated overlying aquifer depended strongly on the thickness of the confining aquitard. For the heating of the aquifer overlying and surrounding the HT-ATES system, an analytical 1-D conduction and 2-D radial conduction expression was found to give a good first approximation, respectively, but deviated at higher temperatures due to the occurrence of buoyancy flow at higher storage temperatures.
Overall, it is shown that both the performance and impact of a particular HT-ATES system strongly depends on a range of site-specific storage and hydrogeological conditions. These need to be considered carefully for system optimization and assessment of business cases and potential thermal environmental impact.