Technical Papers and Presentations

Transforming the energy sector worldwide towards renewable, climate-neutral energy sources requires innovative ideas and excellent engineering solutions. In particular, shallow geothermal energy utilization is currently showing a remarkable development. In highly urbanized regions, large numbers of borehole heat exchangers (BHEs) of different design parameters for single family houses of different heating demands have gained special interest. The large field of BHEs is composed of numerous shallow geothermal units (mostly two BHEs of double u-tube design) of small thermal power (<30 kW). Implementing numerically such a complex layout of BHEs with varying operational schemes and a seasonably-varying theoretical underground temperature field is an ongoing effort. Specially, underground temperatures in densely-populated cities are already significantly altered by human activity. Quantifying how much the underground temperature is affected by such a large field of single BHEs under different thermal and hydrodynamic loading conditions and how large is the affected domain are the main objectives of this work. We lay special emphasis on the thermal interaction between the tightly deployed BHEs as how to understand the optimal parameter setting for a sustainable operation of a large system of BHEs.

A detailed modelling, simulation, and optimization of a large configuration of BHEs of different dimensions in lithologically and hydrogeologically stratified porous media (Fig. 1) is a challenging computational task that involves several strategic steps within the numerical experimental design. An efficient workflow that considers building the static and dynamic model is described. Important aspects include building an optimal spatial discretization (Fig. 2) and setting the velocity field within the pipes (double u-tube utilization design). Based on data from quaternary and tertiary unconsolidated sediments from the North German Basin, a simplified layered-cake model that contains the lithological, hydrogeological and geothermal stratification is designed. The model is populated with parameters according to measured data. Thermal transient initial and boundary conditions are taken into account according to theoretical approaches (Figs. 3 and 4). Using the COMSOL Multiphysics® software, the short- and long-term thermal performance of a large array of shallow geothermal facilities is numerically simulated. We also consider the impact that coupled groundwater flow and transient heat transport has on the system performance. In this work, we present our latest modeling and simulation results.