Abstract
The classical methodology to perform and analyze thermal response test (TRT) is unsuccessful when advection contributes to heat transfer in the ground, due to the presence of a groundwater flow. In this study, the applicability, the advantages, and the limitations of the moving line source model to interpret TRT data are discussed. Two real TRT case studies from the Italian Alpine area are reported and analyzed, with both the standard infinite line source approach and the moving line source one. It is shown that the inverse heat transfer problem is ill-posed, leading to multiple solutions. However, besides minimization of the error between measurements and modeling, physical considerations help to discriminate among solutions the most plausible ones. In this regard, the MLS approach proves to be effective in the advection-dominated case. The original time criterion proposed here to disregard initial data from the fitting, based on a resistance–capacitance model of the borehole embedded in a groundwater flow, is validated in terms of convergence of the solution. In turn, in the case when advection and conduction are competitive, the MLS approach results more sensitive to ground thermal conductivity than to Darcy velocity. Although in this case a limited impact of the uncertainty in the groundwater velocity on the boreholes sizing is expected, future studies should focus on the development of a successful TRT methodology for this condition.
Highlights
Thermal response test (TRT) is a well-known experimental procedure allowing to derive in situ fundamental thermal–physical properties of the ground and of the borehole
This study aims to investigate the applicability, the advantages, and the limitations of the Moving Line Source (MLS) approach to TRT analysis
The analysis of the two real TRT cases presented in this paper clearly shows that finding both the ground thermal conductivity and the groundwater flow velocity from the time profile of the thermal-carrier fluid temperature measured in the standard TRT is an ill-posed inverse problem leading to multiple solutions
Summary
Thermal response test (TRT) is a well-known experimental procedure allowing to derive in situ fundamental thermal–physical properties of the ground and of the borehole. Since its first developments in the mid-90s, it has spread rapidly, being available in about 40 countries worldwide (Nordell 2011; Spitler and Gehlin 2015). It consists in forcing a thermal-carrier fluid at constant flow rate in a test borehole and in regulating the fluid inlet temperature in order to inject into the ground (or extract from it) a constant heat rate for 2–3 days. Angelotti et al Geotherm Energy (2018) 6:12 problem, the average ground thermal conductivity, the borehole thermal resistance, and the ground undisturbed temperature can be assessed Such parameters are the essential inputs for the design of ground heat exchangers (Zhang et al 2014). Numerical models can be used, requiring more modeling and computational effort, but offering the possibility to analyze heterogeneous geological conditions, groundwater flow, or non-conventional TRT procedures (Signorelli et al 2007; Raymond et al 2011)
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