# Advances in Thermal Response Tests

Classical Geothermal Response Testing depends on applying a constant energy forcing to a Borehole Heat Exchanger and analysing the temperature response in terms of ground thermal conductivity and borehole resistance using the line source or similar analytical approach. In the test the only heat transport mechanism accounted for is conduction, and the principle aim of the test is to measure accurately the ground thermal conductivity.

The TRT is based on the premise that the temperature response of a material, when a thermal energy flux (forcibly heating or cooling the material at a certain location) is applied, is proportional to the thermal conductivity of the material. This relation is expressed by Fourier's law of conduction. Using the Kelvin Line Source approach a method has been deviced to measure the conductivity of a material in a fairly straightforward way. However, the analysis of error (and therefore accuracy of results) as well as checking if the fundamental assumptions of the test holds is not so straightforward.

To address the issues around error sources and validity of assumptions we developed and patented an extended test method that uses a sequence of heating and cooling pulses and employs sophisticated numerical analysis of the results to more fully understand the thermal response of the ground.

Research topics include:

- Influence of ground water movement on TRT results and borehole heat exchanger modelling.
- TRT for different geometries
- Convection effects in borehole heat exchangers
- Performance of different borehole heat exchanger types

**References**

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.

Witte, H.J.L. & van Gelder, A.J., 2006. Geothermal response tests using controlled multi-power level heating and cooling pulses (mpl-hcp): quantifying ground water effects on heat transport around a borehole heat exchanger. ECOSTOCK 2006.

Witte, H.J.L., 2009. TRT: How to get the right number (Thermal Response Testing of borehole heat exchangers - a validation of assumptions). Geodrilling International, 151: April 2009.