MoBTES – A New Borehole Thermal Energy Storage Model in Modelica

Abstract

The modeling language Modelica has gained a lot of popularity over the last years due to its conceptual advantages including an object-oriented equation-based design and multi-domain applicability. It has been frequently used for modeling Solar District Heating Systems. However, the consideration of geothermal storage systems is complicated due to a lack of open source and tool-independent Borehole Thermal Energy Storage (BTES) models. To address this problem, a new Modelica BTES model – MoBTES – was designed, using the superposition of the local thermal diffusion process around a single borehole heat exchanger (BHE) and the global temperature field of the storage. The model is radial symmetric and requires therefore much less computational effort than fully discretized 3D models. Moreover, to allow for the consideration of dynamic storage operation scenarios, a Thermal Resistance and Capacity Model (TRCM) of the BHEs is implemented. All model components are derived from elements from the Modelica Standard Library and a strict object-oriented approach was realized, resulting in a tool-box for thermal underground storage systems with a high reusability and extensibility. First simulations show a good fit to the results of established FEM models, especially for large BTES systems, while reducing the computation time significantly.