Revised heat transfer modeling of double-U vertical ground-coupled heat exchangers

Conti, Paolo; Testi, Daniele; Grassi, Walter

doi:10.1016/j.applthermaleng.2016.06.097

Cited by 45 publications

(26 citation statements)

References 24 publications

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“…TRT in situ tests are widely used in industry and standardized today [7][8][9][10] and their basis is the comparison of the temperature response of a BHE to a constant heat injection or extraction, with the prediction of some simplified model of choice based on a few parameters to be fitted, including soil thermal conductivity. For reasons of simplicity and speed [11,12], among the widest used models [13], the basic reference is the Kelvin line-source theory, also termed the Infinite line-source (ILS) [14][15][16] model. Nevertheless, other approaches are becoming increasingly common, such as the finite line-source approach (FLS) [17][18][19], the cylinder-source model [14], commonly applied in Europe [11,[20][21][22] and North America [23][24][25], or other models based on the use of capacitance and resistance analogs (like CaRM [26]).…”

Section: Introductionmentioning

confidence: 99%

How Reliable Are Standard Thermal Response Tests? An Assessment Based on Long-Term Thermal Response Tests Under Different Operational Conditions

et al. 2018

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In this contribution, we analyze the results of a number of thermal response test (TRT) experiments performed during several years at the same location at our university campus in Valencia (Spain), a permeable saturated soil area with possible groundwater flow conditions. A combination of different heat injection rates, TRT operation times of up to 32 days, and various methods for parameter estimation of ground thermal properties have been applied to study their influence on the result and accuracy of TRTs. Our main objective has been to experimentally quantify the influence of groundwater flow heat advection using moving infinite and finite line-source theories, as well as to analyze the influence of factors such as test duration, sensor accuracy, and external thermal influences. We have shown that the traditionally used infinite and finite line-source models, as well as the moving line-source models, can accurately represent experimental temperature evolution, but that there are many caveats regarding the significance parameters extracted and its reproducibility and stability. These features can be improved if data from the first test days are disregarded for the analysis, obtaining a much faster convergence to the definitive soil parameter estimates, including the effective Péclet number that represents groundwater flow in our particular case.

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Section: Introductionmentioning

confidence: 99%

How Reliable Are Standard Thermal Response Tests? An Assessment Based on Long-Term Thermal Response Tests Under Different Operational Conditions

et al. 2018

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“… The two-dimensional multipole equation for double U-loop borehole GHEs suggested by Conti, et al (2016);  The equivalent diameter method proposed by Shonder and Beck (2000);  A two-dimensional numerical model of the GHE cross-section.…”

Section: Analytical and Numerical Borehole Thermal Resistancesmentioning

confidence: 99%

Assessment of Effective Borehole Thermal Resistance from Operational Data

Mikhaylova¹,

Johnston²,

Narsilio³

2017

Proceedings of the IGSHPA Technical/Research Conference and Expo 2017

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Ground source heat pump (GSHP) systems use the ground as a source of sustainable thermal energy for heating and cooling of buildings. Efficient design of the ground heat exchangers (GHEs) for these systems is important so that long-term operation is adequate, efficient and cost-effective. Several design methods have been developed to size GHEs, and many of these methods, including the widely used ASHRAE method, use an effective borehole thermal resistance to model thermal processes in boreholes. A correct estimation of this parameter is crucial for an adequate sizing of borehole GHEs. This study estimates an experimental effective borehole thermal resistance of the borehole GHEs of an operating GSHP system based on monitoring data collected during the Elizabeth Blackburn School of Sciences full-scale shallow geothermal operational study in Melbourne, Australia. The experimental resistance is compared with the resistances predicted using several analytical and numerical methods. It was found that the experimental resistance can be significantly different from the resistances predicted by these other methods. The paper discusses possible reasons for such differences.

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“…Despite the recent development of computer science and numerical methods (see, for instance, [13,14]), analytical models are able to provide useful, practical and general indications of the thermal behavior of the ground source with an appropriate tradeoff between implementation efforts and solution accuracy [3,15]. The advantage of the numerical approach, at least in theory, is the great variety of geometry, boundary conditions, length and time scales that could be analyzed.…”

Section: Introductionmentioning

confidence: 99%

“…According to [15,21], after time periods longer than t b = α g t/r 2 b (a few hours for standard BHEs), the heat transfer process within and outside a BHE can be decomposed into two distinct subsystems: the ground heat exchanger can be assumed as a pure resistance body (i.e., a thermal resistance), while the surrounding soil can be analyzed through a time-dependent model in which the BHE is replaced by a Neumann boundary condition (i.e., imposed heat flux). The general relationship between the mean temperature of the circulating fluid, T f , the undisturbed temperature of the soil, T 0 g , and the heat transfer at the BHE surface,Q b , is given by Equation (1) [21].…”

Section: Introductionmentioning

confidence: 99%

Dimensionless Maps for the Validity of Analytical Ground Heat Transfer Models for GSHP Applications

Conti

2016

Energies

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Abstract:This article provides plain and handy expressions to decide the most suitable analytical model for the thermal analysis of the ground source in vertical ground-coupled heat pump applications. We perform a comprehensive dimensionless analysis of the reciprocal deviation among the classical infinite, finite, linear and cylindrical heat source models in purely conductive media. Besides, we complete the framework of possible boreholes model with the "hollow" finite cylindrical heat source solution, still lacking in the literature. Analytical expressions are effective tools for both design and performance assessment: they are able to provide practical and general indications on the thermal behavior of the ground with an advantageous tradeoff between calculation efforts and solution accuracy. This notwithstanding, their applicability to any specific case is always subjected to the coherence of the model assumptions, also in terms of length and time scales, with the specific case of interest. We propose several dimensionless criteria to evaluate when one model is practically equivalent to another one and handy maps that can be used for both design and performance analysis. Finally, we found that the finite line source represents the most suitable model for borehole heat exchangers (BHEs), as it is applicable to a wide range of space and time scales, practically providing the same results of more complex models.

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Revised heat transfer modeling of double-U vertical ground-coupled heat exchangers

Cited by 45 publications

References 24 publications

How Reliable Are Standard Thermal Response Tests? An Assessment Based on Long-Term Thermal Response Tests Under Different Operational Conditions

How Reliable Are Standard Thermal Response Tests? An Assessment Based on Long-Term Thermal Response Tests Under Different Operational Conditions

Assessment of Effective Borehole Thermal Resistance from Operational Data

Dimensionless Maps for the Validity of Analytical Ground Heat Transfer Models for GSHP Applications

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