Numerical and experimental validation of a new hybrid model for vertical ground heat exchangers

• We have presented a new simplified model to simulate ground heat exchangers.
• Ground nodes were substituted with short and long-term g-functions.
• We validated the model numerically and experimentally by using reference data sets.
• We found very good agreement for outlet and borehole temperatures.

A numerical and experimental validation of a new simplified model to simulate single U-tube ground heat exchangers is presented in this paper. The model 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 has been validated experimentally by using the reference data sets drawn from two tests (constant heat input rate and interrupted tests) made under controlled laboratory conditions. The results have also been compared with the simulated data from a detailed Computational Fluid Dynamics model of the experimental system. A very good agreement has been observed for the modeled outlet fluid and borehole temperatures, with root mean square and relative error values smaller than 0.2 °C and 0.3%, respectively.