3343
Numerical Modelling of Ground Heat Exchangers with Different Ground Loop
Configurations for Direct Geothermal Applications
Modélisation numérique des échangeurs de chaleur souterrains avec différentes configurations de
boucles pour les applications géothermiques directs
Bidarmaghz A., Narsilio G., Johnston I.
Department of Infrastructure Engineering, The University of Melbourne, Australia
ABSTRACT: The design of ground heat exchangers (GHEs) involves the selection of detailed configuration options. However, there
is limited understanding of the relative importance of different design choices on performance. This study investigates the effects of
different design parameters such as pipe configuration and fluid flow rate on the heat extraction rate, and will be helpful to design a
system which is energy efficient and cost effective. Different pipe configurations in vertical grouted boreholes including single U-
pipe, double U-pipe, and double cross U-pipes for small diameter boreholes, and spiral and multiple U-pipes for larger diameter
boreholes, are modelled in detail using state-of-the-art finite element methods. The effects of GHE configurations and fluid flow rate
on system efficiency is determined and contrasted. Numerical results indicate that the thermal performance of the system is enhanced
by transitioning from laminar to turbulent regime, and by increasing the volume of carrier fluid inside the pipes for a given GHE
length (i.e., single versus double pipes). However, in larger diameter boreholes, GHE’s thermal performance does not change
significantly for different pipe configurations with similar pipe lengths inside the borehole (i.e., spiral versus multiple U-pipes).
RÉSUMÉ : La conception des échangeurs de chaleur souterrains (ECS) nécessite un choix parmi différentes configurations.
Cependant, la compréhension de l'importance relative des différents choix de conception sur les performances est limitée. Cette étude
examine les effets des différents paramètres de conception, tels que la configuration des tuyaux ou encore le débit du fluide sur le taux
d'extraction de la chaleur. Elle sera utile pour concevoir un système éco-énergétique et rentable. Différentes configurations de
tuyauterie dans des forages verticaux injectés, y compris le simple U-tube, le double U-tube et le U-tube en double croix pour forages
de petit diamètre, et de multiples spirales et U-tuyaux pour forages de grand diamètre, sont modélisés en détail en utilisant des
méthodes aux éléments finis. Les effets de la configuration d’un ECS et le débit du fluide sur l'efficacité des systèmes sont déterminés
et comparés. Les résultats numériques indiquent que le rendement thermique est accru par la transition du régime laminaire au régime
turbulent, et en augmentant le volume de fluide porteur à l'intérieur des tubes d’ECS d'une longueur donnée. Toutefois, dans les puits
de grand diamètre, la performance thermique d’un ECS ne change pas de façon significative pour les configurations de tuyaux
différents avec des longueurs de tuyaux semblables à l'intérieur du puits (par exemple, en spirale ou multiples U-tubes).
KEYWORDS : Direct Geothermal Energy, Vertical Ground Heat Exchangers, Ground Loop Configurations, Numerical Modelling
1 INTRODUCTION
In recent years, geothermal energy has become an alternative
energy source with great environmental and economical
benefits. Geothermal energy sources range from shallow depths
to hot water and hot rocks a few kilometers below the ground
surface. Ground source heat pump (GSHP) systems use shallow
geothermal energy sources for heating, cooling or even hot
water supply of commercial, industrial and residential buildings.
The ground temperature below about 5 to 10 meters depth is
nearly constant over the year and is close to the mean ambient
temperature. Therefore, the ground is warmer than the
atmosphere in winter and cooler in summer. GSHP technology
takes advantage of this relatively constant ground temperature.
In winter time, heat is extracted from the ground and transferred
to the indoor area via GSHPs. This process is reversed in
summer.
A critical part of GSHP systems is the ground heat
exchanger (GHE), with vertical GHEs being a common choice
due to their reduced footprint and significantly higher energy
performance characteristics in comparison to horizontal systems
due to smaller temperature fluctuations in the ground at depth
(Banks 2008). The performance of GSHP systems depends on
the amount of the heat transferred between the ground and the
carrier fluid which circulates within the pipes embedded in the
GHEs. Several design choices are required; however, only a
relatively limited number of numerical, analytical and
experimental studies have been conducted to assist in
optimizing the design parameters.
Pipe loop configuration, fluid flow rate and pipe separation
are some of many design parameters which affect system
efficiency and they are numerically modelled here. Heat transfer
and fluid flow are the two main physical processes combined in
the numerical model. Heat exchange rates, which arise from
temperature distributions in the ground, at the borehole wall and
in the carrier fluid in different ground loop configurations, are
discussed for a variety of ground loop configurations and
operating conditions.
2 MODEL DESCRIPTION
A model of GHEs was developed from first principles,
accounting for fluid flow and heat transfer through the various
components of the GHE. The model represents GHEs that
consist of grouted boreholes placed vertically in the ground,
with water circulating within the pipes of these GHEs. Details
of these models follow.
2.1
3D finite element model
The motion of the carrier fluid in the pipes is described by the
well-known Navier-Stokes equations (NS). These equations are
the formulation of the continuity law for an incompressible flow
which represents the conservation of mass, and the formulation
