Experimental determination of the g-functions of a small-scale geothermal borehole

•A fully instrumented small-scale borehole laboratory installation is described.•The g-function of the small-scale borehole is determined from measurements.•The experimental g-function validates the analytical g-function within the experimental uncertainty.•The air temperature variations are shown to have an effect on the borehole wall temperature.

An experimental setup was built to obtain the experimental g-function of a small-scale borehole. The experimental g-function is calculated from the measured borehole wall temperature and the net heat injection rate into a 400 mm long borehole inserted in a 2 m3 sand tank with known thermal properties. With such a small length it is possible to reach a steady-state condition after a 168 h long test. The experimental g-function of the borehole is compared to the g-function obtained from an analytical model based on the finite line source solution. The difference between the experimental and analytical g-functions is 4.7% after one week but the two curves remain within the experimental uncertainty bands. The air temperature variations during the test are shown to have a relatively significant effect on the results.