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Measuring soil thermal properties for use in energy foundation design
La mesure des caractéristiques thermiques du sol pour la conception des fondations énergie
Low J.E., Loveridge F.A., Powrie W.
Faculty of Engineering and the Environment, University of Southampton, Southampton, UK
ABSTRACT: Energy foundations incorporated into ground source heat pump systems provide a viable alternative to conventional
building temperature regulation systems in the move towards sustainable building solutions. To design such a system, it is important
to accurately model the heat transfer process between the foundations and the soil, which is largely governed by the soil thermal
conductivity. This paper compares two laboratory test methods for determining soil thermal conductivity: the thermal cell which is a
steady state method, and the needle probe which is a transient method.
RÉSUMÉ : Pour l’orientation vers des immeubles durables, les fondations énergie incorporées dans des systèmes de pompe à chaleur
géothermique fournissent une alternative viable aux systèmes conventionnels de régulation de température des immeubles. La
conception d’un tel système implique le modelage précis du processus, qui est en grande partie déterminé par la conductivité
thermique du sol, de transfert thermique entre les fondations et le sol. Dans le texte qui suit l’on compare deux méthodes d’essai de
laboratoire pour la détermination de la conductivité thermique du sol : la cellule thermique, méthode de régime établi, et sonde à
aiguille, méthode de régime transitoire.
KEYWORDS: soil thermal conductivity, thermal cell, needle probe
1 INTRODUCTION
Ground source heat pump systems provide a viable alternative
to conventional heating and cooling systems in the move
towards sustainable building solutions (Banks, 2008). Heat is
transferred between the ground and the building by means of a
refrigerant which is pumped through a series of pipes buried in
the ground. To minimize initial construction costs, the pipes can
be cast into the foundations, eliminating the need to make
further excavations. These systems are known as energy
foundations. To design such a system, it is important to
accurately model the heat transfer process between the
foundations and the soil. This is largely governed by the soil
thermal conductivity.
There are several different methods of measuring soil
thermal conductivity (Mitchell and Kao, 1978). They fall into
one of two categories: steady state or transient methods. At the
laboratory scale, steady state methods involve applying one-
directional heat flow to a specimen and measuring the power
input and temperature difference across it when a steady state is
reached. The thermal conductivity is then calculated directly
using Fourier’s Law of heat conduction. Transient methods
involve applying heat to the specimen and monitoring
temperature changes over time, and using the transient data to
determine the thermal conductivity. This paper compares the
two approaches using a thermal cell (steady state) and a needle
probe (transient) apparatus. The tests were carried out on U100
samples of London Clay upon which a full soil classification
was afterwards conducted.
2 BACKGROUND
There are several methods of measuring thermal conductivity
which are considered as suitable for use with soils. For this
study, the needle probe and thermal cell methods were chosen
due to the simplicity of the apparatus.
2.1
Needle probe
The measurement of thermal conductivity using the needle
probe method is based on the theory for an infinitely long,
infinitely thin line heat source (Carslaw and Jaeger, 1959). If a
constant power is applied to the heat source, the temperature
rise Δ
T
at time
t
after the start of heating, at a radial distance
r
from the heat source, is:
t
r Ei
q T
4
4
2
(1)
where
q
is the power per unit length of heater,
λ
is the
thermal conductivity,
α
is the thermal diffusivity and
Ei
is the
exponential integral. After the power is switched off, the
temperature difference is given by:
heat
t t
r
Ei
t
r Ei
q T
4
4
4
2
2
(2)
where
is the time at which the power is switched off.
Equations (1) and (2) cannot be solved for
λ
and
α
explicitly, so
a simplified analysis approximating the exponential integral is
used which leads to (ASTM International, 2008):
heat
t
t
q T
ln
4
(3)
heat
t t
0
heat
t t
t
q T
ln
4
(4)
heat
t t
The needle probe used is the TP02 probe produced by
Hukseflux Thermal Sensors (2003). It is 150mm long with a
diameter of 1.5mm, and encloses a 100mm long heating wire
with a thermocouple located midway along this heater
measuring the temperature (see Figure 1).
