3417
Technical Committee 307 + 212 /
Comité technique 307 + 212
Proceedings of the 18
th
International Conference on Soil Mechanics and Geotechnical Engineering, Paris 2013
the heat transfer fluid through one loop of absorber pipes closest
to the boreholes equipped with thermocouples. Two TRTs were
carried out by transporting the fluid through all three loops of
absorber pipes in a continuous series within the energy pile.
Inflow and outflow temperature of the heat transfer fluid,
ground temperature at every 2 m to 16 m depth within the two
boreholes located at 0.5 m and 2.0 m from the edge of the test
pile as well as the pile concrete temperature were recorded
continuously during the heating periods. The test pile and the
ground were cooled naturally by letting the induced heat
dissipate into the surrounding environment following each TRT.
The subsequent TRT did not start until the temperature readings
within the pile and the boreholes were returned, as close as
possible, to their initial undisturbed temperatures. The duration
of each TRT are summarised in Table 1.
Table 1. Duration of Thermal Response Tests
TRT Name
Test Duration
(Heating)
Rest After Test
(Cooling)
1 loop (3 days)
3 days
5 days
3 loop ST (9 days)
9 days
47 days
3 loop LT (52 days)
52 days
78 days
Field research of in-situ measurement of the soil thermal
conductivity was undertaken across Europe and USA on
borehole ground heat exchangers for a number of years.
Published literature (Gehlin, 2002; Austin, 1998) showed that
during a TRT, based on the line source method, a defined
energy was applied to the heat exchanger whilst the power input
and the inflow and out flow temperature of the heat transfer
medium was recorded. This measures the entire length of the
ground heat exchanger, providing an effective thermal
conductivity value whilst considering the borehole backfilling
(or pile properties), variable ground and groundwater
conditions. The effective thermal conductivity measured from a
field TRT can be calculated by Equation 1:
(1)
Where:
Q
= constant heat power (W)
L
b
= length of heat exchanger (m)
k
= logarithmic relationship (slope of curve)
between test duration (in log time), and the mean
temperature of the heat transfer fluid
In-
situ field estimation of the ground system’s effective
thermal conductivity consists of incorporating the energy pile
ground heat exchanger and the surrounding soils as a whole
system. This study presents an estimate of the effective thermal
conductivity utilising the three TRTs.
k
is found by plotting the
regression line derived from the time temperature series of a
TRT, during the steady increase period of the fluid temperature.
The average heat transfer fluid temperatures, with an applied Q
of 2.4 kW, were plotted against time for each of the tests and
the regression lines are shown in Figure 2.
The effective thermal conductivity calculated from
Equation 1 was based on the heat exchanger and its immediate
vicinity attaining steady-state conditions (Eskilson, 1987). This
requires a minimum time criterion, as shown in Equation 2, to
be satisfied.
(2)
Where:
t
=
“minimum
-
time” criterion for test duration (s)
r
b
= borehole or pile radius (m)
a
= thermal diffusivity (J/m
3
K)
The test data prior to this initial period,
t
= 100 hours for 3
loop TRTs, needs to be ignored to reduce errors as during this
initial heating stage, the thermal front gradually reaches further
beyond the heat exchanger wall. The average heating fluid
temperature rises rapidly during this initial period, then as the
thermal front travels further into the surrounding ground, the
increase in average fluid temperature becomes steady. However,
for the 1 loop TRT, the test was terminated after 3 days.
Therefore, the first 48 hours of test data was ignored for
comparison between the three TRTs.
Figure 2. Estimation of effective thermal conductivity
–
slope of
average fluid temperature vs. logarithmic of time
The results of effective thermal conductivity carried out in
the three TRTs were not consistent. The 3 loop ST TRT
achieved the highest value of 4.99 W/mK whilst the 3 loop LT
TRT achieved the lowest value of 3.75 W/mK and the 1 loop
TRT achieved an effective thermal conductivity of 4.19 W/mK
for the energy pile system.
Austin (1998) showed that the line source model utilised by
TRTs to estimate thermal conductivity were very sensitive to
the temperature fluctuations. Figure 2 shows that there were
fluctuations of the heat transfer fluid temperature during the
heating periods of each TRT. The HDPE absorber pipes were
insulated between the top of the energy pile to the testing unit
with a combination of insulation foam, aluminium foil and soil.
However, the top of the energy pile was exposed to the summer
environment and direct solar energy. The fluctuation of average
fluid temperature shown in Figure 2 was likely to be caused by
solar radiation. The direct sunlight would heat up the concrete
of the energy pile’s upper surface section whilst increasing the
average fluid temperature within the absorber pipes.
Subsequently, during cooler nights where solar radiation was
not present, the pile concrete cooled down significantly and
decreased the average fluid temperature.
The estimated effective thermal conductivity found in this
study is comparable to other published literature utilising energy
piles as the ground heat exchanger. Published data (Brandl et
al., 2006; Gao et al., 2008; Brettmann and Amis, 2011) shows
that whilst utilising energy piles of at least 0.6 m in diameter
during TRTs, effective thermal conductivity of the ground
systems reached between 4 W/mK to nearly 7 W/mK in sandy
and clayey soils. However, within smaller diameter piles the
effective thermal conductivity was found to be between
2 W/mK and 3 W/mK. The long term TRT (3 loop LT) carried
out over 52 days shown in this study is not a practical test to
carry out due to the length of the testing period.
b
eff
kL
Q
4
a
r t
b
2
5
