Co-Simulation of the Borehole Thermal Energy Storage at the Solar District Heating System in Brædstrup, Denmark

Abstract

The solar share of district heating grids with integrated solar thermal collectors can be increased significantly by adding Borehole Thermal Energy Storages (BTES) to the system. The different project stages of such systems should be accompanied by numerical simulations to improve the resulting thermodynamic performance, cost effectiveness and environmental compatibility. While there are simulation tools available for the initial design to compare a great number of variants on a low level of detail, there is no single tool available yet for a highly detailed analysis of the whole system. Especially the modelling of the BTES is usually limited with respect to the accurate consideration of the local (hydro-)geology and resulting environmental impacts. To take the strong interdependencies between all system components into account, while simultaneously allowing for a high level of detail, a coupled simulation approach is used. The above ground heating system is modeled in Modelica, whereas the BTES is modeled with the Finite Element Method in FEFLOW. Both sub-models are run simultaneously. For the assessment and validation of the proposed methodology, operational data from the existing BTES in Brædstrup (DK) is used. Different sampling frequencies for the communication between the two sub-models are compared, considering the resulting computational effort and accuracy of the simulation process. The results demonstrate the possibility to predict the system behavior on a high level of detail, producing valuable information about environmental impacts and performance improvement potentials in advance.