Development and validation of a new model for system-integrated shallow borehole heat exchanger fields with irregular geometry

In the context of reducing greenhouse gas emissions and mitigating climate change, accurate and fast borehole heat exchanger (BHE) field models that can be integrated into the broader system level are required for the design and operation optimisation of shallow geothermal district heating and cooling grids. Available system-integrated BHE field models need adaption in order to fulfil all requirements (high short- and long-term accuracy, low computational cost, varying inlet conditions, irregular field geometries). In this study, a new BHE field model is developed and implemented in TRNSYS. The heat transfer inside the borehole is modelled by adapting an existing thermal resistance and capacitance model for high short-term accuracy for double U-tubes. The borehole wall temperature is prescribed by a combination of g-functions. The proposed model (DynIBF), including various modelling aspects, is validated using existing measured data and models. DynIBF shows to be an adequate supplement to the existing models for irregular BHE fields. The results in the more irregular case are distinctly improved (mean absolute error (MAE) $-0.63$ K from the next best result). On the given hardware, DynIBF is between 8% and 41% faster than the examined existing TRNSYS models.