Modelling and validation of a new hybrid scheme for predicting the performance of U-pipe borehole heat exchangers during distributed thermal response test experiments

The reliable design of Ground Coupled Heat Pumps (GCHP) requires accurate models for both predicting the long term behavior of the borehole (BHE) field and their sub-hourly response during the thermal response test experiments. Several literature models have been proposed in the past to cope with this goal: resistance/capacitance (RC) models, finite difference (FD) thermal descriptions of both inner BHE and surrounding ground, FEM models based on commercial software packages. The present model, as Fortran code, is a hybrid approach that employs an RC scheme (to single or double U pipes) BHEs and solves the full Fourier equation in the (even layered) ground volume. The fluid vertical energy transfers are treated with an upwind scheme. New thermal parameters have been introduced and used as indicators of the physical and geometrical conditions of the U-BHE. Crucial of the RC part is the proper estimation of local resistances and the correct position of the inner BHE thermal capacitance, to be located somewhere in between the pipe surface and the BHE periphery. The extensive validation with experimental TRT data allowed the model to be refined in order to provide very close agreement among predictions and measurements (root mean square error within 0.17 °C).