Interpretation of ongoing thermal response tests of vertical (BHE) borehole heat exchangers with predictive uncertainty based stopping criterion

This paper investigates the applicability of currently available analytical, empirical and numerical heat flow models for interpreting thermal response tests (TRT) of quadratic cross section precast pile heat exchangers. A 3D finite element model (FEM) is utilised for interpreting five TRTs by inverse modelling. The calibrated estimates of soil and concrete thermal conductivity are consistent with independent laboratory measurements. Due to the computational cost of inverting the 3D model, simpler models are utilised in additional calibrations. Interpretations based on semi-empirical pile Gfunctions yield soil thermal conductivity estimates statistically similar to those obtained from the 3D FEM inverse modelling, given minimum testing times of 60 hours. Reliable estimates of pile thermal resistance can only be obtained from type curves computed with 3D FEM models. This study highlights the potential of applying TRTs for sizing quadratic, precast pile heat exchanger foundations.

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“…Equivalent pipe model EQpipe by [54]. The model presented in [55] is used in this study. The model neglects the finite length of the pile.…”

Section: Numerical Approachesmentioning

confidence: 99%

“…It does not consider the finite length of the ground heat exchanger but it does consider its thermal mass. The model used in this paper is the one presented in [55]. The model takes into account heating rate variations by temporal superposition.…”

Section: Numerical App Equivalent Pipe Model (Eqpipe)mentioning

confidence: 99%

Comparing heat flow models for interpretation of precast quadratic pile heat exchanger thermal response tests

Alberdi-Pagola

Poulsen

Loveridge

et al. 2018

Energy

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“…Shonder and Beck 1999;Javed and Claesson 2011;Beier 2014). Analytical solutions are principally used for interpretation of thermal response tests (Sass and Lehr 2011;Li and Lai 2012;Zhang et al 2014;Poulsen and Alberdi-Pagola 2015) and to a lesser extent for large-scale applications. Introducing simplifying assumptions, analytical solutions can for example account for either interaction of multiple BHEs (e.g.…”

Section: Introductionmentioning

confidence: 99%

Efficient simulation of multiple borehole heat exchanger storage sites

Boockmeyer

Bauer

2016

Environ Earth Sci

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In this paper, an adapted model is developed for borehole heat exchangers (BHEs) to simulate geothermal applications such as heat storage on a large scale efficiently and with high accuracy. The adapted numerical model represents all BHE components, allowing for a detailed representation of the governing processes. The approach is calibrated and validated for a single U-tube BHE using a high-resolution experimental data set from a laboratory thermal response test. It is found that the computational effort can be reduced by factors of *50, *50 and *25 for single U-tube, double U-tube and coaxial BHEs, respectively, if an absolute deviation of less than 1 % compared to a conventional fully discretised model is allowed. Computation times can be reduced further by accepting higher deviations. The adapted modelling approach allows for a detailed and correct representation of the temporal and spatial temperature distribution under highly transient conditions by applying it to a high-temperature heat storage scenario using multiple BHEs. The model is especially suited to represent coupled flow and heat transport processes, to account for groundwater flow in the BHE region as well as geological heterogeneities and especially interaction between a large number of BHEs.Keywords Borehole thermal energy storage Á Borehole heat exchanger Á Numerical simulation Á Fully discretised models Á OpenGeoSys List of symbols aSolid compressibility (Pa -1 ) bFluid compressibility (Pa -1 ) cq Volumetric heat capacity (J m -3 K -1 ) dThickness (m) D Heat diffusion dispersion tensor g Gravitational acceleration (m s -1 ) k Intrinsic permeability (m) L Length (m) m Side length (m) n Porosity (-) p Pressure (Pa) Q Sources and sinks (kg m -3 s -1 ) Q H Heat sources and sinks (W m -3 ) rRadius (m) R th Thermal resistance (K W -1 )Transport velocity (m s -1 ) z Depth (m) a Dispersivity (m) kThermal conductivity (W m -1 K -1 ) l Fluid dynamic viscosity (N s m -2 ) q Density (kg m -3 )

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“…This first approach to linearize the infinite line source model requires rejecting the early measurements for the subsequent test interpretation. TRT duration is usually established between 36-72 h, but this duration has been thoroughly discussed in the past [10] and at the moment is an area of active research [11][12][13][14][15]. Despite the errors these test could involve [4], the high accuracy of ground thermal conductivity results represent essential information in the corresponding geothermal loop sizing.…”

Section: Introductionmentioning

confidence: 99%

Comparative Analysis of Different Methodologies Used to Estimate the Ground Thermal Conductivity in Low Enthalpy Geothermal Systems

et al. 2019

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In ground source heat pump systems, the thermal properties of the ground, where the well field is planned to be located, are essential for proper geothermal design. In this regard, estimation of ground thermal conductivity has been carried out by the implementation of different techniques and laboratory tests. In this study, several methods to obtain the thermal properties of the ground are applied in order to compare them with the reference thermal response test (TRT). These methods (included in previous research works) are carried out in the same geological environment and on the same borehole, in order to make an accurate comparison. All of them provide a certain value for the thermal conductivity of the borehole. These results are compared to the one obtained from the TRT carried out in the same borehole. The conclusions of this research allow the validation of alternative solutions based on the use of a thermal conductive equipment and the application of geophysics techniques. Seismic prospecting has been proven as a highly recommendable indicator of the thermal conductivity of a borehole column, obtaining rate errors of below 1.5%.

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Interpretation of ongoing thermal response tests of vertical (BHE) borehole heat exchangers with predictive uncertainty based stopping criterion

Cited by 13 publications

References 18 publications

Comparing heat flow models for interpretation of precast quadratic pile heat exchanger thermal response tests

Comparing heat flow models for interpretation of precast quadratic pile heat exchanger thermal response tests

Efficient simulation of multiple borehole heat exchanger storage sites

Comparative Analysis of Different Methodologies Used to Estimate the Ground Thermal Conductivity in Low Enthalpy Geothermal Systems

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