To measure is to know!
The thermal properties of the ground are among the most critical parameters in the design of a ground heat exchanger. Moreover, it is very difficult to estimate these parameters with sufficient accuracy.
The thermal properties can be estimated using available literature values, but the range of values found in literature for a specific soil type is very wide. Also, the values specific for a certain soil type need to be translated to a value that is representative of the soil profile at the location. The best method is therefore to measure directly the thermal soil properties as well as the properties of the installed heat exchanger.
This test is used to measure with high accuracy:
- The temperature response of the ground to an energy pulse, used to calculate:
- the effective thermal conductivity of the ground
- the borehole resistance, depending on factors as the backfill quality and heat exchanger construction
- The average ground temperature and temperature - depth profile.
- Pressure loss of the heat exchanger, at different flows.
- Soil stratigraphy, dril log and first estimate of the thermal parameters of the soil profile.
- Test procedure.
- Test results (thermal conductivity, borehole resistance, ground temperature).
- Quality of the results, stability of the obtained estimate of conductivity (convergence).
- Evaluation of possible effects of ground water flow.
The default "classical" test (based on the line source model) requires a constant energy flux. However, it is possible to generate a second energy step (at a higher flux rate) that can be analyzed using numerical estimation techniques. This makes it possible to determine the borehole resistance with greater accuracy as well as quantify better the effects of ground water flow.
References
Austin, W.A. 1998. Development of an In situ System for Measuring Ground Thermal Properties. MSc. Thesis, Oklahoma State University, USA. 164 pp.
Gehlin, S. 1998. Thermal Response Test, In-Situ Measurements of Thermal Properties in Hard Rock. Licentiate Thesis, Luleå University of Technology, Department of Environmental Engineering, Division of Water Resources Engineering. 1998:37. 41 pp.
Van Gelder, A.J., Witte, H.J.L., Kalma, S., Snijders, A. and R.G.A. Wennekes 1999. In-situ Messungen der thermische Eigenschaften des Untergrunds durch Wärmeentzug. IN: T. Hitziger (Ed): OPET Seminar "Erdgekoppelte Wärmepumpen zum heizen und Klimatisieren von Gebäuden". 109 pp.
Witte, H.J.L., Gelder, A.J, van & Spitler, J.D. 2002. In-situ measurement of ground thermal conductivity: The dutch perspepctive. ASHRAE Transactions, Volume 108, No. 1.