A new RC and g-function hybrid model to simulate vertical ground heat exchangers

This paper presents a new hybrid two-dimensional model to simulate single U-tube ground heat exchangers based on the use of the electrical analogy to model heat transfer within the borehole and thermal response factors (short and long time-step g-functions) to estimate heat flow to the surrounding ground. Unlike other similar models, heat transfer equations are discretized only within the borehole domain, with two nodes for the fluid and two additional nodes for the grouting material, reducing the number of equations as ground temperature nodes are not considered. The substitution of ground nodes with short and long-term g-functions allows the simulation with short time steps keeping the possibility of simulating periods of several years. A complete analysis of the influence of characteristic parameters of the model in terms of the heat conduction path, vertical discretization level and equivalent heat capacity, has been carried out. The model shows a good performance for short simulation time-steps of five minutes for the prediction of both the short-term and long-term response. It has been successfully validated through comparison with a numerical computational fluid dynamic (CFD) reference model, achieving good RMSE values, which were smaller than 0.15 °C for water outlet temperature and borehole surface mean temperature.