Assessment of the impact of ambient groundwater flow on High Temperature Aquifer Thermal Energy Storage (HT-ATES) performance and thermal spreading
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High Temperature Aquifer Thermal Energy Storage (HT-ATES) is a technology where groundwater of up to 90 ˚C is stored in the subsurface which is used as a source of heat for spatial heating. By utilizing high temperatures, the use of a fossil-fuel heat pump as in low temperature (< 25˚C) ATES is excluded. Before realizing HT-ATES, a basic framework of the storage specific and site-specific hydrogeological conditions that effect the thermal recovery efficiency and thermal spreading of heat in the subsurface is needed. This study has provides a part of such framework by the hand of numerical simulations, specifically looking at the impact of ambient groundwater flow (AGF) on HT-ATES systems in scenarios with the presence of an overlying aquifer. Additionally, theoretical interpretations of the results help to explain the interaction between various processes modelled. The results show that for HT-ATES systems in sites with ambient groundwater flow, thermal recovery efficiencies alter slightly due to the interaction between significant density-driven flow and ambient groundwater flow. However, the trend in ambient groundwater flow losses, where losses increase when decreasing the storage size or increase the AGF velocity, is similar to that of LT-ATES. Therefore, analysis whether AGF losses play a significant role at a certain site can still be performed with the analytical formula of Boemendal & Hartog 2018 that calculates efficiency losses due to ambient groundwater flow. In terms of spatial spreading, ambient groundwater flow causes for an increase in spreading in direction of the flow and increases when utilizing a larger storage aquifer. Additionally, flow in the storage aquifer results in more conduction losses to the overlying aquifer. The presence of an overlying aquifer causes for an increase in thermal spreading in vertical direction when significant density driven flow is present. Density driven flow in the storage aquifer results in more heat to conduct upwards which then spreads vertically in the overlying aquifer as well due to density driven flow. Having flow in the overlying aquifer does not result in major losses in efficiency but does reduces the spreading vertically and increases it horizontally, causing a shift in spatial impact of the heat. Additionally, dispersion causes reduction of overall temperatures in the overlying aquifer, therefore decreasing the impact on the subsurface in terms of high temperatures. The insights of this study have provided a basic understanding of how HT-ATES interplays with site specific hydrogeological conditions and therefore how these must be taken into account when realizing HT-ATES.