Ground thermal conductivity

Keywords Geothermal response test, ground thermal conductivity, line source, GENOPT, TRNSYS, borehole heat exchanger... 

With the line source approach it is possible to calculate from the experimental data the ground thermal conductivity and borehole resistance. At the start of the experiment, before an energy pulse has been applied, the undisturbed ground temperature is measured. 

Figure 50. CUSUM test, stability and convergence of the estimates of ground thermal conductivity as a function of starting time and amount of data included, obtained in the reference...

During the many tests we performed we found that that ground water flow will significantly affect results of the In Situ Geothermal Response Test. This is especially clear in the CUSUM graphs, that show an increasing estimate of ground thermal conductivity with time. The reason for this is that the effect... 

The complete data series is used to calculate the temperature response, but only certain parts of the experimental data are used to calculate the error. An example of a calibration run is given in Figure 53, the final calibrated TRNSYS model run is shown in Figure 54. Using the first part of the data (with constant heat flux) an estimate of ground thermal conductivity of 2.15 was obtained. Yavatzturk s method yielded an estimate of 2.18, while the estimate obtained with the TRNSYS parameter estimation method was 2.10. 

In principle all parameters of the model can be entered in the parameter estimation procedure. For the time being we limit the parameters to be calibrated to the ground thermal conductivity, ground heat capacity and borehole filling conductivity. 

The first hours were run with a constant heat flux, allowing an estimate with the line source model. Results of the line source model are an estimated ground thermal conductivity of 2.05 W/m K and a borehole resistance of 0.12 K/(W/m), taking into account fluid properties, flow conditions and average shank spacing this borehole resistance equates to a conductivity of the borehole material of about 2.0 W/m K. 

With the exception of the last calibration run (calibrating the parameters in series) all ground thermal conductivity and borehole conductivity values are very comparable. The ground thermal conductivity values estimated are significantly lower than the estimates obtained with the line source method (on the first 40 h of data). When the parameters are estimated separately, estimated ground thermal conductivity is higher. 

In this paper we present for the first time a test that combines heat extraction and heat injection pulses in one experiment. It is expected that differences in the ground thermal conductivity, when different data windows are used to obtain an estimate, can be related to advection and convection of ground water. The real ground conductivity should be derived from the experimental data where the response is close to or lower than the natural ground temperature, minimizing effects of advection and convection. First results, for a case of no ground water flow, show that estimates of ground thermal conductivity are very comparable for the different data windows. 

Witte, H.J.L., A.J. van Gelder, and J.D. Spitler, 2002. In situ measurement of ground thermal conductivity The Dutch perspective, ASHRAE Trans., 108 (1). 

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