How do you input a TRT?

In this short article, you will learn what a TRT-test is and how you can input its data into GHEtool.

What is a TRT-test?

TRT stands for Thermal Response Test. It is an in-situ test to measure the ground thermal properties at a specific location. Since all the tabulated ground data is an oversimplification of reality and ground properties play an important role in the final design of the borefield, it is recommended to use TRT for big projects.

Note

Practically, a TRT-test takes place after the first borehole has been drilled. This borehole should be representative for the whole borefield. A monitoring machine is connected to the pipes and a fluid starts circulating, measuring the initial ground temperature. After a while, the machine start injecting a constant power into the ground and measuring the temperature evolution of inlet and outlet. (Based on (François et al., 2016) 1)

A TRT test measures three different things:

  1. The initial (undisturbed) ground temperature

  2. The ground thermal conductivity

  3. The borehole equivalent thermal resistance

How to enter this into GHEtool?

Since a TRT measures 1) the ground temperature at infinity (i.e. the undisturbed ground temperature), 2) ground thermal conductivity and 3) the borehole thermal resistance, we have to enter all those parameters in GHEtool.

Ground properties

For the ground properties, you go to the Earth tab and you select Measured as the source for the ground temperature data. The ground thermal conductivity you can enter straight away.

In the example below, the TRT measured a thermal conductivity of 1.85W/mK and an undisturbed ground temperature of 11.83°C.

Earth properties

Borehole thermal resistance

To set the value of the borehole thermal resistance, you go to the Options tab and select constant for the borehole resistance. Next, you go to the Thermal Resistance tab and you enter the value from the TRT.

Warning

Using TRT-data for the aim of determining the required depth is dangerous, since the measured borehole equivalent thermal resistance is only valid for one specific depth (since this parameter is a function of the depth). Since this aim changes the depth of the boreholes the borehole resistance will change as well. This effect can be very significant if you have a fluid regime on the border of laminar/turbulent flow (see also What is the Reynolds number?).

Please use this only as a first estimate and for final depths that are in line with the depth of the measured borehole.

References

1

François L., Van de Bossche P., Van Lysebetten, G. (2016) Technische voorlichting (259): Ondiepe geothermie, ontwerp en uitvoering van bodemenergiesystemen met U-vormige bodemwarmtewisselaars (in Dutch).