BASIMO – Borehole Heat Exchanger Array SIMulation and Optimization Tool

Abstract

Arrays of borehole heat exchangers (BHE) are an increasingly popular source of renewable energy. Furthermore, they can serve as borehole thermal energy storage (BTES) systems for seasonally fluctuating heat sources in district heating grids like solar thermal energy or industrial waste heat. Projects generally tend to grow in size and capital cost with more and deeper BHEs. Thus, a priori simulations become an essential part of the planning process. In contrast to single borehole ground source heat pump systems, many mathematical simplifications are not exploitable for modeling and simulation of BHE arrays as they result in too large errors. Thus, current simulation tools cannot – or only to some extent – describe BHE arrays in sufficient detail. Key features like stratigraphy dependent thermo-physical and hydrogeological properties can only be captured in more complex and more sophisticated simulations. Arbitrary bore paths for each individual BHE or partly insulated boreholes can even require fully discretized models of the borehole heat exchangers. However, fully discretized models often come at a high computational cost, especially for large arrays of BHEs. We give an update on the development of BASIMO: a tool, which uses one-dimensional thermal resistance and capacity models for the BHEs coupled with a numerical finite element model for the conductive and convective subsurface heat transport in a dual-continuum approach. An unstructured tetrahedral mesh bypasses the limitations of structured grids for bore path geometries, while the thermal resistance and capacity model is improved to account for BHE properties changing with depth. Thereby, partly insulated boreholes can be considered in the model. Furthermore, BASIMO can be used to improve the design of BTES systems: the tool allows for automated parameter variations and is readily coupled to other code like mathematical optimization algorithms. Optimization can be used to determine the required minimum system size or to increase the system performance.