Numerical simulation of borehole thermal energy storage in the geological subsurface
Seasonal storage of heat in the subsurface is an increasingly used technology for providing heat during winter by storing surplus energy from primary producers. Storage technologies such as high-temperature borehole thermal energy storage (BTES) could meet the heat demand in winter and recharge in summer when heating is not necessary. For a broad application of this storage technology, an adequate system knowledge is required in terms of storage characteristics, dimensioning and the induced effects. In this thesis, high-temperature BTES systems are studied with numerical scenario simulations using the scientific open source simulator OpenGeoSys. For gaining a thorough understanding of the coupled induced thermal and hydraulic processes with high temperatures, heat storage is simulated with a validated numerical model of a single borehole heat exchanger (BHE) in a homogeneous subsurface. This model accounts for the complex BHE geometry with all its components to ensure a correct representation of the steep temperature gradients. It is shown that thermal convection can occur in storage formations with high permeability and that this strongly affects the storage characteristics. In particular, heat convection reduces the storage capacity to the point at which heat storage becomes inefficient. For this reason, conditions that prevent thermally induced convection should be constraining the decision process of storage site locations. Further scenario simulations investigating high-temperature heat storage on a larger scale are thus placed in a conductive setting. A BTES model consisting of multiple BHEs is used to assess mutual BHE interferences, storage impacts and capacities. For this purpose, an adapted modelling approach is developed and validated for the comprising BHEs, making efficient yet accurate simulation of heat storage with feasible computational effort possible. Using this approach, storage scenarios are simulated employing sets of typical subsurface thermal parameters and storage cycle lengths to quantify their respective impact on storage features such as storage capacities and rates as well as induced temperature changes. Additionally, storage layouts are varied in terms of BHE number and distance to derive dependencies for storage dimensioning. The simulation results show that the effects of thermal parameters and cycle length on storage behaviour are fundamental for determining required dimensions of the BTES. BHE distance and number strongly affect storage features and induced impacts in the subsurface; they can further compensate for disadvantageous subsurface conditions. Optimal storage dimensions in terms of BHE distance thus have to be identified at the given storage site. The results show further that site-specific numerical simulations are necessary to identify temperature impacts on the subsurface at the selected storage site and to execute an ideal dimensioning before installing a BTES.
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