An analysis of the accuracy and computational efficiency of the use of one-dimensional fluid models in borehole heat exchangers

• Novel methodology of modelling borehole heat exchangers to reduce computational time within COMSOL Multiphysics. • Symmetry planes with equation adjustments implemented with one-dimensional linear fluid elements to provide accurate results and reduced computation efforts. • Comparison of full three-dimensional CFD model in ANSYS CFX solving Navier-Stokes and energy equations within the fluid. • Models validated with experimental thermal response test data. This paper compares the accuracy of a one-dimensional fluid model to that of a fully three-dimensional model for the simulation of a thermal response test performed on a single borehole heat exchanger. The simplification of the fluid domain within the one-dimensional model allows for reduced computational time while still maintaining an accurate prediction of transient fluid temperature. The model uses a simplified one-dimensional fluid model while solving the full three-dimensional transient heat conduction equations in the borehole heat exchanger and surrounding ground. A symmetry plane is implemented to further reduce the computational effort, and the model and equation adjustments necessary to merge the use of symmetry planes and 1D linear elements along the central plane without loss of model accuracy is explained in detail. The proposed model is compared to a full CFD model and validated using experimental data for a constant heat rate test, commonly known as a thermal response test, to ensure no accuracy is lost due to model adjustments. Additionally, the computation times are compared for each case to quantify the time savings that result from model implementation.