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Research Papers: Thermal Systems

Heat Transfer in Double U-Tube Boreholes With Two Independent Circuits

[+] Author and Article Information
Parham Eslami-nejad

Département de Génie Mécanique, École Polytechnique de Montréal, Case Postale 6079, Succursale “Centre-ville,” Montréal, QC H3C 3A7, Canada

Michel Bernier1

Département de Génie Mécanique, École Polytechnique de Montréal, Case Postale 6079, Succursale “Centre-ville,” Montréal, QC H3C 3A7, Canadamichel.bernier@polymtl.ca

1

Corresponding author.

J. Heat Transfer 133(8), 082801 (May 03, 2011) (12 pages) doi:10.1115/1.4003747 History: Received March 11, 2010; Revised March 01, 2011; Published May 03, 2011; Online May 03, 2011

This study presents the development of an analytical model to predict steady-state heat transfer in double U-tube geothermal boreholes equipped with two independent circuits. Such boreholes can be used for heat extraction in one circuit, combined with a heat pump, for example, and simultaneous thermal recharging in the other circuit. The model accounts for a thermal interaction among pipes, and it predicts the fluid temperature profiles in both circuits along the borehole depth, including the exit fluid temperature. Different circuit configurations are assessed under typical borehole operating conditions. For a typical borehole geometry, results show that double U-tube boreholes with two independent circuits connected to a relatively low temperature heat source are superior to single U-tube and regular (one circuit) double U-tube boreholes. The axial variation in fluid temperature and the heat exchange among pipes show that most of the heat transfer occurs in the downward legs. Furthermore, in some cases, the fluid in the heat extraction leg gets cooled as it flows upward, which is contrary to the desired effect.

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Copyright © 2011 by American Society of Mechanical Engineers
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References

Figures

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Figure 1

Schematic representation of conventional U-tube configurations

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Figure 2

Schematic representation of a double U-tube borehole with two independent circuits

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Figure 3

Cross section of a double U-tube borehole with two independent circuits

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Figure 4

Possible piping configurations

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Figure 5

Presentation of terms used in Eq. 1

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Figure 6

Borehole cross section for three different configurations

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Figure 7

Dimensionless temperature profiles along the borehole depth

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Figure 8

Local and cumulative heat exchanges for all three configurations

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Figure 9

Impact of different parameters on borehole heat pump leg outlet temperature for different configurations

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Figure 10

Dimensionless temperature profiles along the borehole depth for ṁ=0.05 kg/s and H=200 m

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Figure 11

Fluid temperature profiles and borehole depth required to have Tin,hp=3.6°C for three configurations

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Figure 12

Comparison of the required depth of independent double U-tube boreholes with single and parallel double U-tube boreholes under different conditions

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