Abstract
The standard approach to Thermal Response Test, based on conduction heat transfer in the ground, turns out to be unsuccessful under significant groundwater flow. The applicability of the Moving Infinite Line Source model to interpret the TRT in this case still needs to be proved. In order to study the TRT in the presence of a groundwater flow, an original laboratory apparatus has been developed. The Sand Box design is based on a heat transfer similitude between the real scale TRT problem and the laboratory scale one. The Sand Box sizes (1,2 m x 0.6 m x 1.0 m) are then set in order to keep the boundaries unaffected by the heat source during the TRT. The U-pipe heat exchanger is reproduced through a twocables electrical resistance 1 m long. A hydraulic loop with a peristaltic pump allows to obtain a Darcy velocity across the sandy soil up to 6,7510-5 m/s. The measurement system consists in several thermocouples in the porous medium and in a flow meter. The TRT results at null groundwater velocity allow to derive a reference thermal conductivity. The first tests with groundwater flow show the suitability of the apparatus and allow to derive some preliminary considerations.
Highlights
Thermal Response Test (TRT) is a well-known experimental procedure allowing to measure in situ the average ground thermal conductivity and the borehole thermal resistance, namely key parameters for the correct sizing of the borehole heat exchangers
The Sand Box apparatus developed in this research allows to perform TRT under groundwater flow in the Darcy velocity range from zero up to about 7 10-5 m/s at the laboratory scale, corresponding to 7 10-6 m/s at the field scale
Preliminary tests executed at null velocity and at the maximum velocity prove that the Sand Box boundaries are not affected by the heat source for the typical TRT duration and that the apparatus is properly sized
Summary
Thermal Response Test (TRT) is a well-known experimental procedure allowing to measure in situ the average ground thermal conductivity and the borehole thermal resistance, namely key parameters for the correct sizing of the borehole heat exchangers. Chiasson and O’ Connell [8] compare the MLS, the groundwater g-functions and a mass-transport solution including thermal dispersion as analytical models for TRT analysis in the presence of groundwater flow They find that only the mass-heat transport analogy yields a favorable comparison to field test data, implying that thermal dispersion is an important parameter, at least for relatively high groundwater velocities. They find that this combined hydro/thermal test is more sensitive to the ground thermal conductivity than to the groundwater velocity, the latter being derived via the differences of temperatures read by the sensors. This paper describes the development of a Sand Box apparatus designed for this purpose, as well as the first tests executed through it
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