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Precise calculation of the borehole length requires good estimation of the ground thermal conductivity. In practice, the ground t h ermal c ondu ctivi ty is measured in-situ at a specif ic location using what is ref erred to as a thermal response test (TRT) unit. This paper presents a novel virtual borehole (VB) concept for calibrating TRT units. The VB replaces a real borehole with an above-ground compact heat exchanger and a c h i l l e r un it t o mimic t h e thermal behavior of the ground with a user-set virtual ground thermal conductivity. In an attempt to develop the VB concept, three control scenarios are examined to emulate the ground thermal response for dif ferent thermal conductivity values. A test bench was built at the CanmetENERGY-Varennes research laboratory to validate the VB concept experimentally. A test is performed to calibrate a commercial l y availabl e TRT un it f or a t h ermal conductivity value of 3 W m K. The TRT unit connected to the VB reported a value of 3.18 W m K representing a 6% error. INTRODUCTION Ground source heat pump (GSHP) systems are receiving significant interests due to energy efficiency. However, to design them correctly, thermal properties of the ground where the ground heat exchangers (GHE) are installed, must be estimated precisely. Ground thermal conductivity (k), borehole thermal resistance and the undisturbed ground temperature (UGT) are among the most important parameters for design and are commonly estimated from in-situ thermal response tests (TRT) (Gehlin 1998). During a TRT, a constant and known thermal load is injected into the GHE and accurate measurements of the time history of the inlet and outlet fluid temperatures are recorded. The ground thermal conductivity is then deduced from these measured values. Thermal response tests include the measuring period and the subsequent data analysis. The latter requires a relatively precise GHE mathematical model. The accuracy of the model has a major impact on the thermal conductivity value reported by the TRT units. Different models have been used to evaluate the experimental data obtained from TRTs. Most models are based on either analytical approaches or numerical methods. Complete reviews of various analytical and numerica l models have recently been presented (Rees 2016; Spitler and Gehlin 2015). In brief, the modeling of the GHE has undergone many improvements since it was first suggested in 1983 by Mogensen (Mogensen 1983). Starting from the infinite line source (ILS) model (Sharqawy, et al. 2009), the infinite cylinder source (ICS) model (Eskilson 1986), the finite line source (FLS) model (Bandos, et al. 2011) going to more complex two and three-dimensional numerical models by Bozzoli, et al. (2011) and Marcotte and Pasquier (2008). TRT result analysis is based on the ILS and ICS models that represent relatively simple and rapid methods for estimating the ground thermal properties. However, numerical approaches are more accurate and can capture more complex phenomena in the ground, such as ground water movement and variable heat injection and fluid mass flow rates. For instance, Bandos, et al. (2011) proposed a correction method to account for heat losses to the ambient, Sass and Lehr (2011) account for ground water advection while other studies (Beier and Smith 2003; Choi and Ooka 2015) improved the analysis by using multiinjection rate thermal response tests. Concerning the TRT unit itself, significant improvements have been achieved since the design proposed by Mogensen (1983) including mobile TRT units allowing both heat injection and extraction. Some TRT units dev iate from the standard approach and use different measurement techniques to perform a TRT. For instance, Raymond, et al. (2015) proposed to use a heating cable without supplying any flow to the GHE; Witte, et al. (2000a) proposed a telemetry system; Acuña and Palm (2013) used a distributed ground temperature measurement technique using optical fiber; and Rohner, et al. (2005) proposed downhole measurements using light submersible wireless probes. Although research and development on TRT units has evolved significantly, conventional TRT units are still widely used in the industry. Individual measurements (temperature, flow rate, power input) inside the TRT unit are often calibrated. There has also been past attempts to perform round-robin testing of some TRT units. However, there is no method to calibrate a particular unit as a whole, including the data analaysis. The virtual borehole concept presented here proposes to simulate the behavior of a ground heat exchanger to calibrate TRT devices with userselected ground thermal conductivities. VIRTUAL BOREHOLE CONCEPT In a TRT unit, several measuring devices are used to collect the required data including fluid mass flow rate and inlet and outlet fluid temperatures to the borehole. These data are typically used in conjunction with a onedimensional heat transfer model (ILS for example) for evaluating the thermal conductivity of the ground. Both measurements and result interpretation introduce uncertainties in the process. The VB proposed here is shown in Figure 1. It consists of a compact heat exchanger and a cooling unit. The TRT unit to be calibrated is connected to the VB as it would be to a borehole in an in-situ TRT. The chiller and compact heat exchanger are carefully controlled using a pre-defined control scenario to mimic the thermal behavior of the ground heat exchanger. In other words, the VB creates a thermal condition for the TRT unit to react thermally just like if it were connected to a real borehole located in a medium with a known ground thermal conductivity. The control unit reduces the mass flow rate (using a 3-way mixing valve) or/and increases the temperature of the cooling loop to increase the average temperature of the heating loop (on the TRT side) over time exactly as it would happen during a real test. The thermal conductivity value interpreted by the TRT unit is then compared against the value set by the control unit. c o n tro l u n it c o o li n g u n it h e a t e x c h a n g e r T R T u n it V ir tu a l B o re h o le h e a t in g lo o p c o o lin g lo o p M ix in g 3 -w a y v a lv e

Virtual borehole for thermal response test unit calibration: Test facility and concept development