Thermal Response Test – Current Status and World-Wide Application

To design borehole heat exchangers (BHE) for Ground Source Heat Pumps (GSHP) or Underground Thermal Energy Storage (UTES), the knowledge of underground thermal properties is paramount. In small plants (residential houses), these parameters usually are estimated. However, for larger plants (commercial GSHP or UTES) the thermal conductivity should be measured on site. A useful tool to do so is a thermal response test, carried out on a BHE in a pilot borehole (later to be part of the borehole field). For a thermal response test, basically a defined heat load is put into the hole and the resulting temperature changes of the circulating fluid are measured. Since late 1990s, this technology became more and more popular, and today is used routinely in many countries for the design of larger plants with BHEs, allowing sizing of the boreholes based upon reliable underground data. The paper includes a short description of the basic concept and the theory behind the thermal response test, looks at the history of its development, and emphasizes on the worldwide experience with this technology. INTRODUCTION The knowledge of underground thermal properties is a prerequisite for correct design of borehole heat exchangers (BHE). The most important parameter is the thermal conductivity of the ground. This parameter is site-specific and cannot be influenced by engineering. The thermal contact from the borehole wall to the fluid inside the pipes, however, is controlled by borehole diameter, pipe size and configuration, pipe material, and the filling inside the annulus. These items are subject to efforts in order to reduce the thermal resistance between borehole wall and fluid, usually summarised in the parameter “borehole thermal resistance”. Since the mid 90s a method has been developed and refined to measure the underground thermal properties on site, and mobile equipment for these measurements has been built in several countries. The Thermal Response Test (TRT, also sometimes called “Geothermal Response Test”, GRT) is a suitable method to determine the effective thermal conductivity of the underground and the borehole thermal resistance (or the thermal conductivity of the borehole filling, respectively). A temperature curve is obtained which can be evaluated by different methods. The thermal conductivity resulting is a value for the total heat transport in the underground, noted as a thermal conductivity. Other effects like convective heat transport (in permeable layers with groundwater) and further disturbances are automatically included, so it may be more correct to speak of an “effective” thermal conductivity λeff. The test equipment can be made in such a way that it can be transported to the site easily, e.g. on a light trailer (fig. 1). Figure 1: Swedish test rig, coupled to a borehole heat exchanger DEVELOPMENT OF THE THERMAL RESPONSE TEST The theoretical basis for the TRT was laid over several decades (e.g. by Choudary, 1976; Mogensen, 1983; Claesson et al., 1985; Claesson and Eskilson, 1988; Hellström, 1991). In the 90s the first practical applications were made, e.g. for the investigation of borehole heat storage in Linköping (Hellström, 1977). In 1995 a mobile test equipment was developed at Luleå Technical University to measure the ground thermal properties for BHE between some 10 m to over 100 m depth (Eklöf and Gehlin, 1996; Gehlin and Nordell, 1997). A similar development was going on independently since 1996 at Oklahoma State University in the USA (Austin, 1998). The first TRT in Germany were performed in summer 1999 (Sanner et al., 1999). A somewhat different test rig was developed and tested in the Netherlands (van Gelder et al., 1999): This rig uses a heat pump instead of electric resistance heaters, in order to be able to also decrease the temperature inside the BHE. This method, however, has intrinsic problems because of the dynamic behaviour of the heat pump and the need for a heat source/sink, and should only be used where testing with extracting heat has to be done explicitly. Beside the Dutch test rig, at least two other have a heat pump system, on in Germany and one in Sweden. Sanner, Hellström, Spitler and Gehlin 2 As to the information available to the authors, there are test rigs operational today in the following countries: Canada Chile (experimental) China Germany (4) Netherlands Norway South Korea Sweden (several) Switzerland Turkey United Kingdom USA (several). OPERATION OF THE TEST The general layout of a TRT is shown in fig. 2. For good results, it is crucial to set up the system correctly and to minimize external influences. This is done easier with heating the ground (electric resistance heaters) than with cooling (heat pumps). However, even with resistance heating, the fluctuations of voltage in the grid may result in fluctuations of the thermal power injected into the ground. Figure 2: Test setup for a Thermal Response Test (drawing UBeG GbR, Wetzlar) Another source of deviation are climatic influences, affecting mainly the connecting pipes between test rig and BHE, the interior temperatures of the test rig, and sometimes the upper part of the BHE in the ground. Heavy insulation is required to protect the connecting pipes (fig. 1), and sometimes even air-conditioning for the test rig is necessary, as was done in USA (fig. 3). With open or poorly grouted BHE, also rainwater intrusion may cause temperature changes. A longer test duration allows for statistical correction of power fluctuations and climatic influence, and results in more trustworthy evaluation. Typical test curves with strong and with low climatic influence are shown in fig. 4. With the increasing commercial use of TRT, the desire for a shorter test duration became apparent, in particular n the USA. A recommendation for a minimum of 50 hours was given (Skouby, 1998; Spitler et al., 1999a), which is compatible with the IEA recommendations (see below), but there is also scepticism (Smith, 1999). A test time of ca. 12 hours is desired, which also would allow not to have the test rig out on the site over night, In general, there are physical limits for the shortening of the measuring period, because a somewhat stable heat flow has to be achieved in the ground. In the first few hours, the temperature development is mainly controlled by the borehole filling and not by the surrounding soil or rock. A time of 48 h is considered by the authors as the minimum test period. Figure 3: OSU test apparatus on site in Nebraska 10 12 14 16 18 20 22 24 26 28 30