Abstract
Two methods are currently available to estimate in a relatively short time span the subsurface heat capacity: (1) laboratory analysis of rock/soil samples; (2) measure the heat diffusion with temperature sensors in an observation well. Since the first may not be representative of in-situ conditions, and the second imply economical and logistical issues, a third option might be possible by means of so-called oscillatory thermal response tests (OTRT). The aim of the study was to evaluate the effectiveness of an OTRT as a tool to infer the subsurface heat capacity without the need of an observation well. To achieve this goal, an OTRT was carried out in a borehole heat exchanger (BHE). The total duration of injection was 6 days, with oscillation period of 12 h and amplitude of 10 W m−1. The results of the proposed methodology were compared 3-D numerical simulations and to a TRT with a constant heat injection rate with temperature response monitored from a nearby observation well. Results show that the OTRT succeeded to infer the expected subsurface heat capacity, but uncertainty is about 15% and the radial depth of penetration is only 12 cm. The parameters having most impact on the results are the subsurface thermal conductivity and the borehole thermal resistance. The OTRT performed and analyzed in this study also allowed to evaluate the thermal conductivity with similar accuracy compared to conventional TRTs (3%). On the other hand, it returned borehole thermal resistance with high uncertainty (15%), in particular due to the duration of the test. The final range of heat capacity is wide, highlighting challenges to currently use OTRT in the scope of ground-coupled heat pump system design. OTRT appears a promising tool to evaluate the heat capacity, but more field testing and mathematical interpretation of the sinusoidal response is needed to better isolate the subsurface from the BHE contribution and reduce the uncertainty.
Highlights
The thermal conductivity (TC) was evaluated via the recovery period in order to avoid the effect of the of the borehole thermal resistance andlocation, cable location, which can induce in the temborehole thermal resistance and cable which can induce noise innoise the temperature perature signal of the heat injection period
A fully transient numerical simulation would likely achieve closer values, but the results show that the analytical scheme to implement the borehole heat exchanger (BHE) in FEFLOW is comparable to the field case
Borehole thermal resistance can be evaluated by means of Thermal response test (TRT), which can help evaluate the performance of borehole heat exchangers (BHE)
Summary
Thermal response test (TRT) is the most common field method to estimate the subsurface thermal conductivity (TC) and the borehole thermal resistance for ground-coupled heat pump systems (GCHP). Other than being very compact and needing only 120 V power, the heating cable unit can provide a TC profile of the ground with several temperature sensors at depth, or with a fiber-optic cable [4,5]. It does not require a BHE since it can be performed in open wells, provided that water is present to ensure thermal contact with the subsurface. Electric cable with heating and non-heating sections were tested, but significant free convection occurs in the pipe (or well) according to the Rayleigh number stability criterion, allowing only 15% accuracy in TC estimation [4]
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