2985
Numerical simulation of energy consumption of artificial ground freezing applications
subject to water seepage
Simulation numérique de la consummation d’énergie des applications pour la congélation artificielle
du sol soumise au flux de l’eau souterraine
Ziegler M., Schüller R.
Geotechnical Engineering, RWTH Aachen University, Germany
Mottaghy D.
Geophysica Beratungsgesellschaft mbH, Aachen, Germany
ABSTRACT: The energy consumption of artificial ground freezing applications results from the necessary (consumed) refrigeration
capacity. The determination of the refrigeration capacity requires the numerical modeling of heat transfer processes within the freeze
pipe. Therefore a new module has been implemented in the Finite Difference Program SHEMAT. An approach for calculating the
heat transfer processes and finally the refrigeration capacity is presented in this paper. Thus numerical simulations are carried out to
determine the influence of groundwater flow on the refrigeration capacity and the related energy consumption for artifical ground
freezing applications.
RÉSUMÉ : La consommation d’énergie des applications pour la congélation artificielle du sol résulte de la capacité nécessaire de
réfrigération. La détermination de cette capacité de réfrigération exige un modèle numérique des processus de transfert de la chaleur
dans le tuyau de congélation. C’est pourquoi un nouveau module a été implémenté dans le programme SHEMAT. Une approche pour
calculer le transfert de la chaleur et finalement la capacité de réfrigération est présentée dans cet exposé. Ainsi, des simulations
numérique ont été réalisées pour déterminer l’influence du flux des eaux souterraines sur la capacité de réfrigération et la
consommation d’énergie qui en résulte des applications pour la congélation artificielle du sol.
KEYWORDS: artificial ground freezing, refrigeration capacity, heat transfer, Nusselt number
1 INTRODUCTION
The ground freezing method aims to provide artificially frozen
soil which increases the strength of the ground and makes it
impervious to water seepage. In recent years the artificial
ground freezing method is more often used not only in
tunneling but also in the construction of common basement
stories. However, the regular application often fails as the
energy consumption and its related costs are expected to be
excessively high.
In the last few years several numerical simulations using the
program SHEMAT (Simulator for Heat and Mass Transport)
were carried out at the chair of Geotechnical Engineering of
RWTH Aachen University. The Finite-Difference Program
SHEMAT had been developed by a group of Prof. Clauser at
the chair of Applied Geophysics of RWTH Aachen University
for the simulation of geothermal processes in porous rocks
(Clauser 2003). Mottaghy and Rath (2006) implemented a phase
change model including latent heat effects due to freezing and
thawing of subsurface fluids, as well as temperature dependent
thermal properties. As part of the dissertation of Baier (2009),
this work has been extended in terms of varying freezing curves
and additional thermal and hydraulic ground properties and their
temperature dependence. The numerical simulations carried out
showed that significant reductions of the freezing time can be
achieved by flow-adapted freeze pipe arrangements
(Ziegler et al. 2009).
To increase the regular application of the artificial ground
freezing method not only the freezing time but also the energy
consumption has to be reduced. Therefore it is necessary to take
into consideration the operating phase which essentially affects
the total energy consumption of artificial ground freezing
applications. The energy consumption can be estimated by
determining the refrigeration capacity of artificial ground
freezing applications first.
The aim of this paper is to highlight the determination of the
refrigeration capacity by numerically modelling the heat
transfer processes in the freeze-pipe itself and at the transition to
the soil.
2 DETERMINATION OF REFRIGERATION CAPACITY
To ensure a realistic determination of the refrigeration capacity
the freeze pipe has to be examined in detail. That implies the
consideration of the so far neglected heat transfer within the
freeze pipe itself.
2.1
Basic heat transfer within freeze pipes
The heat transport mechanisms, occuring during the ground
freezing process, can be split into mechanisms in the soil, in the
freeze pipes and in the transition of both. In the soil the heat
transport consists of conduction and forced convection
(advection) due to water seepage. These processes are not part
of the current research and therefore not illustrated in detail. For
a detailled explanation see e.g. Baier (2009).
In the following, the heat transfer processes in the freeze
pipes are described in detail.
In general freeze pipes are coaxial pipes that consist of an
inner pipe of polyethylene where the refrigerant moves
downwards and an outer pipe of steel where the refrigerant
moves upwards. In most cases a calcium chloride brine is used
as refrigerant. The heat extraction of the surrounding soil warms
the refrigerant in the outer pipe moving upwards. Therefore the
refrigeration capacity can be determined from the temperature
difference of inlet and outlet refrigerant temperature, the pump
rate Q and the volumetric heat capacity of the refrigerant c
v
(see Eq. 1).
)
(1)
T T(QcP
outlet
inlet
V
For a realistic determination of the outlet temperature the
ongoing heat transfer processes (
e s
) within the freeze pipe
have to be considered (see Figure 1).
Q/Q
