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Comparison of two different models for pile thermal response test interpretation

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Abstract

Thermal response tests (TRTs) are regularly used to characterise the thermal resistance of borehole heat exchangers and to assess the thermal conductivity of the surrounding ground. It is becoming common to apply the same in situ testing technique to pile heat exchangers, despite international guidance suggesting that TRTs should be limited to hole diameters of 152 mm (6 in.). This size restriction arises from the increased thermal inertia of larger diameter heat exchangers, which invalidates the assumption of a steady state within the concrete needed to interpret the test data by traditional line source analysis techniques. However, new methods of analysis for pile heat exchangers have recently been developed that take account of the transient behaviour of the pile concrete. This paper applies these new methods to data from a multi-stage TRT conducted on a small diameter test pile. The thermal conductivity and thermal resistance determined using this method are then compared with those from traditional analytical approaches based on a line source analysis. Differences between the approaches are discussed, along with the observation that the thermal resistance may not be constant over the different test stages.

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Abbreviations

Fo:

Fourier number (αt/r 2b )

G :

G-function

H :

Pile or borehole length

h i :

Heat transfer coefficient

Q :

Total heating power (W)

q :

Heating power per unit length (W/m)

R :

Thermal resistance (mK/W)

R b :

Pile thermal resistance (mK/W)

R c :

Concrete resistance (mK/W)

R p :

Pipe resistance (mK/W)

r :

Radial position (m)

r b :

Pile radius (m)

S c :

Specific heat capacity (J/kg K)

S cv :

Volumetric heat capacity (J/m3K)

T :

Temperature (K or C)

ΔT :

Change in temperature (K or C)

t :

Time (s)

α :

Thermal diffusivity (m2/s)

γ :

Euler’s constant

λ :

Thermal conductivity (W/mK)

b:

Pile or borehole

c:

Concrete

f:

Fluid

g:

Ground

i:

Pipe inner dimensions

in:

Inlet

o:

Pipe outer dimensions

out:

Outlet

p:

Pipe

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Acknowledgments

The authors would like to thank Concept Consultants Ltd, Marton Geotechnical Services Ltd and Gecco2 Ltd for their roles in construction of the test pile and carrying out the TRT. The work of Jasmine Low (University of Southampton) and Echo Ouyang (University of Cambridge) in the installation are also gratefully acknowledged. The lead author was funded by the Engineering and Physical Sciences Research Council (research grant number EP/H049010/1).

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Authors and Affiliations

  1. Faculty of Engineering and the Environment, University of Southampton, Highfield, Southampton, SO17 1BJ, UK

    Fleur Loveridge & William Powrie

  2. Arup, 13 Fitzroy Street, London, W1T 4BQ, UK

    Duncan Nicholson

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  1. Fleur Loveridge
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  2. William Powrie
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  3. Duncan Nicholson
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Correspondence to Fleur Loveridge.

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Loveridge, F., Powrie, W. & Nicholson, D. Comparison of two different models for pile thermal response test interpretation. Acta Geotech. 9, 367–384 (2014). https://doi.org/10.1007/s11440-014-0306-3

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