Experimental validation of a hybrid model for vertical ground heat exchangers under on-off operation conditions

Abstract

This work presents an experimental validation of a two-dimensional hybrid model for vertical single U-tube ground heat exchanger. It is 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. 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. The model was experimentally validated by using reference data sets drawn from different tests at GeoCool plant, located at the Polytechnic University of Valencia (Spain). Validation process took into account the stop-start intervals of the geothermal heat pump operation. In a test of one week with 5-min time steps, model results agree very well with experimental data achieving RMSE values smaller than 1 °C for output fluid temperatures. The capability of the model to reproduce vertical distribution of borehole temperature was also studied. Furthermore, a comparison with the simulated results from a refined CFD model was made.