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th
International Conference on Soil Mechanics and Geotechnical Engineering, Paris 2013
occasions excess solar thermal energy can be stored in the
ground.
There is, however, one clear example of heat extraction
visible in mid-October (Figure 6). Here the central thermistor
readings dip markedly to around 10
o
C on average. The
corresponding change in the mean outer thermistor readings is
much smaller, indicating that much of the heat energy required
has actually been extracted from that temporarily stored in the
pile concrete, rather than from the surrounding ground.
5 DISCUSSION
An important aspect of pile heat exchanger behavior is
illustrated by the data presented in Section 4. As has been
shown theoretically by Loveridge, 2012, large diameter piles
can take many days to reach a thermal steady state. Therefore,
for a heating/cooling demand which is varying on an hourly (or
less) timescale the pile concrete will rarely be at thermal steady
state. This is illustrated in Figure 6, which shows that the
temperature change near to the pile edge is significantly damped
compared to that close to the U-tubes and some subsidiary
peaks/troughs are not reflected at all. If the pile was at steady
state, as is assumed by all traditional design methods, then the
temperatures near the pile edge would reflect all the temperature
changes at the pipes.
This transient thermal behavior shown by the pile concrete
is important for a number of reasons. First, if the pile is
assumed to be at a thermal steady state then any ability of the
pile concrete to store energy (rather than just transfer it to the
ground) is being neglected. As a consequence steady state
design will either 1) overestimate the temperature change
predicted at the pile-soil boundary for a given heat flux, or 2)
underestimate the available thermal energy capacity for a given
temperature change. While this provides a safe conservative
design it will significantly under-predict the thermal efficiency
of the pile heat exchanger.
Taking a transient view of the pile concrete behaviour also
shows there to be a reduced risk of extreme temperatures
developing in the ground. Current practice (eg NHBC 2010;
SIA 2005) tends to recommend that the lower limit on the heat
transfer fluid temperature in pile heat exchangers should be kept
above freezing with a 2
o
C margin of error. However, given that
the largest dips in the central thermistor temperatures shown in
Figure 6 are not reflected to the same degree in the temperature
changes of the outer thermistors, this approach clearly would be
conservative in this case. This real behaviour is similar in
nature to recent theoretical studies (Loveridge et al. 2012)
which show that, for large diameter piles at least, temperatures
lower than 0
o
C can be sustained within the heat transfer fluid
for short periods and have no detrimental effects on the ground.
Similar conclusions were reached by Brandl in his Rankine
Lecture (Brandl 2006), but do not seem to have been acted upon
in general practice.
5.1 Further Work
The data presented in this paper is the beginning of a long term
monitoring programme. The temperature data in the pile will
subsequently be supplemented by energy data, both with respect
to the heat transferred to the instrumented pile and for the
balance of thermal energy between the different renewable
energy systems in the building. This is essential for fuller
interpretation of the pile data and will allow linking of the
energy demand and pile temperature changes. This will provide
a valuable dataset for validation of pile heat exchanger thermal
design methods.
6 CONCLUSIONS
Temperature sensors have been installed within a working
foundation pile which is also used as a heat exchanger within a
ground energy system. Initial data from the pile is now
available and demonstrates the transient nature of the heat
transfer within the pile. This is significant, as most existing
design methods for the thermal capacity of piles assume that the
pile is at a steady state. For large diameter piles such as the one
instrumented in this scheme, this is clearly not the case. Instead
the largest fluctuations in temperature at the centre of the pile
close to the pipe U-tubes are not reflected closer to the edge of
the pile. This is due to the thermal buffering provided by the
pile concrete, which acts as a short term energy store during
short duration peaks in thermal demand.
The consequence of neglecting this short term concrete
thermal storage is that design becomes over conservative and
underestimates the thermal capacity of the pile. It also leads to
an over estimation of the risk of ground freezing for large
diameter piles.
7 ACKNOWLEDGEMENTS
The authors would like to thank Joel Smethurst and Harvey
Skinner for assistance with installing the thermistors and
datalogger respectively. The work would not have been
possible without initial introductions and enthusiasm from Arup
and Balfour Beatty Ground Engineering and subsequent support
and commitment from Siemens Plc. We are also indebted to site
support from ISG, Foundation Developments Ltd and
Geothermal International Ltd. This work forms part of a larger
project funded by EPSRC (ref EP/H0490101/1) and supported
by Mott MacDonald, Cementation Skanska, WJ Groundwater
and Golder Associates.
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