Performance Evaluation of a High Temperature Borehole Thermal Energy Storage Under Influence of Groundwater Flow

Abstract: Recent years have seen a growing interest in large-scale high-temperature borehole thermal energy storage (HT-BTES) as a means to store industrial waste heat and solar energy between the seasons. A profound understanding and characterization of the thermal and hydraulic processes involved in such systems is required for the optimal design as well as for environmental assessments of the storage. In this work, the importance of groundwater flow effects on the thermal performance of HT-BTES has been studied. The current research status on groundwater flow and transport modeling techniques applied in the field of shallow geothermal energy as well as in other geosciences disciplines has been reviewed. A finite element heat conduction model of an existing HT-BTES located in dry heterogeneous soil has been developed and validated against operational and monitoring data. The heat conduction model provided a basis by which to compare the behaviour and performance of the storage under influence of ground waterflow. Numerical experiments were conducted considering both pure heat conduction as well as different scenarios accounting for groundwater flow. A performance evaluation study based on key performance indicators in terms of energy and exergy efficiencies has been carried out to quantify the impact of groundwater flow on the amount and quality of the heat being stored and exchanged. The analysis shows that the presence of groundwater flow is in general detrimental to the energy and exergy performance of the HT-BTES. The results indicate, however, that small groundwater flow rates also can have a slight positive effect on seasonal energy and exergy efficiencies as compared to case of pure conduction. Further studies are though needed where a wider range of time scales, BHE designs, operation conditions and subsurface conditions are adressed. From the inherent uncertainties associated with subsurface flow and transport processes it follows that general guidelines on how and under what conditions groundwater flow may have impact on BTES design and performance are difficult to provide. The characteristics of these processes in porous, and particularly fractured, media are often very site-specific and scale dependent, making it a challenging task to select and use an appropriate modeling approach that can capture all relevant features of the problem. To face this challenge, various modeling approaches, typically based on deterministic and stochastic continuum or discrete fracture network concepts, have been developed within the field of subsurface flow and transport modeling. To widen the modeling framework typically employed in shallow geothermal energy applications, their applicability also in the context of BTES modeling could be explored.