3371
Performance of Piled Foundations Used as Heat Exchangers
Performance des fondations sur pieux utilisées comme échangeurs thermiques
Loveridge F., Powrie W.
Faculty of Engineering and the Environment, University of Southampton, Southampton, UK
ABSTRACT: In closed loop ground energy systems, used to provide renewable heat energy to buildings, heat is transferred between a
heat pump and the ground via ground heat exchangers. Fluid is circulated through pipes within the heat exchanger. Including these
pipes within the concrete of piled foundations can give economies of excavation and materials. However, there remain few validated
design approaches for determining the heating and cooling capacity of pile heat exchangers. There have also been concerns that
inappropriate operation may cause extreme temperatures to develop in the ground, leading to loss of geotechnical performance. As a
result most current design is conservative and does not fully utilise the thermal capacity of the pile-ground system. To permit
validation of new design methods, instrumentation has been installed within a pile heat exchanger at a site in London. Initial results
are showing the pile concrete to be making a substantial contribution to the short term storage of thermal energy. This is significant,
as most design methods assume that the pile concrete merely acts to transfer heat to the surrounding ground. This heat storage also
acts to protect the ground against larger fluctuations in the fluid temperature and is therefore beneficial for geotechnical performance.
RÉSUMÉ : Dans les systèmes de transfert d’énergie du sol en circuit fermé, utilisés pour la fourniture d’énergie thermique durable
aux immeubles, les échangeurs thermiques transmettent la chaleur du sol à une pompe à chaleur. On peut obtenir des économies lors
de l’installation si les tuyaux de l’échangeur thermique se trouvent dans le béton des fondations sur pieux. Il n’existe cependant guère
d’approches conceptuelles validées pour la détermination des capacités thermiques des pieux à échangeurs thermiques et des
préoccupations existent sur leur utilisation inappropriée ; la plupart des conceptions actuelles est donc conservative. Pour la validation
de nouvelles méthodes de conception, on a installé de l’instrumentation dans un pieu à échangeur thermique sur un chantier à
Londres. Les premiers résultats démontrent que le béton du pieu offre une contribution substantielle sur l’accumulation à courte durée
de l’énergie thermique. Ceci est important pour l’amélioration de la performance géotechnique.
KEYWORDS: ground energy systems, piling, renewable energy, temperature effects
1 INTRODUCTION
It is becoming increasingly common for ground energy systems
to utilise building piled foundations as the heat exchanger part
of the system, facilitating heat transfer to the ground. In this
arrangement, small diameter (<30mm) plastic pipes are cast into
the piles, before being connected via larger header pipes to a
heat pump. This forms the “source” side of the ground energy
system. On the “delivery” side of the system, the heat pump is
connected to heating and air conditioning units, which should be
a low temperature system to maximise efficiency.
Design of pile heat exchangers is split into two main aspects.
First the calculation of the thermal capacity of the pile heat
exchanger, ie what heating and cooling power can be achieved
from the pile or pile group. Secondly, it is important that
additional checks are performed with respect to the geotechnical
design so that any additional concrete stresses or displacements
resulting from temperature changes induced in the piles can be
taken into account in the structural design. These two design
aspects do not, however, exist in isolation. The range of
temperatures at which the heat pump system operates will
directly influence both the available thermal capacity and the
geotechnical design. It is therefore important that appropriate
temperature limits are agreed between all parties. From a
geotechnical perspective it is essential that the pile-ground
interface does not reach freezing temperatures, while extreme
higher temperatures may affect the efficiency of the heat pump.
While procedures for geotechnical design of pile heat
exchangers are developing rapidly (GSHPA 2012; Amatya et al.
2012; Ouyang et al. 2011; Knellwolf et al. 2011), there remain
few datasets available with which to permit validation of design
methods for the heating and cooling capacity of piles
(Loveridge & Powrie 2012). To address this important
knowledge gap, the University of Southampton has commenced
a programme of in situ monitoring of pile heat exchangers. This
paper will present details of an instrumented pile heat exchanger
in East London, along with initial data from the first few
months of operation of the energy system.
2 SITE DESCRIPTION
The instrumented pile is part of the foundations for “The
Crystal”, Siemens’ new landmark building adjacent to Royal
Victoria Dock in East London. The Crystal is a multi-use
development and contains an interactive exhibition on
sustainable technologies as well as office space. It has been
designed to be an all electric building, utilising solar power and
ground source heat pumps to generate all the energy for the
development.
The source side of the ground energy system comprises 160
pile heat exchangers and a field of 46 150m deep boreholes.
The piles are 600mm, 750mm or 1200mm diameter and were
constructed using contiguous flight auger techniques. Each pile
was installed with a pair of plastic U-pipes, which were inserted
into the centre of the pile (Figure 1) after the pile cage had been
plunged into the concrete. The U-pipes were then connected
together in series and usually joined into a single circuit with a
neighbouring pile, before the pipework is continued to the
header chamber and then on via larger pipes to the plant room
for connection to the heat pumps.
