| dc.description.abstract | In storing heat during summer for use in winter, the ground provides a better source/sink
of heat than the outside air in regards to heat pump efficiency, being cooler than the outside air in the summer and warmer in the winter. Due to their good efficiency, the use of geothermal energy is often encouraged; however, two issues arise in the long-term use of ground for thermal purposes: the sustainability and impact of these systems on the
environment. Studies show the potential of the geothermal heat exchangers for environmental impacts such as undesirable temperature rises from these systems in
temperature-sensitive regions. Furthermore, interference between adjoining installations
is being reported, raising issues of sustainability in terms of performance and equitable
sharing of natural resources.
The temperature of the soil surrounding the ground heat exchangers (GHEs) and the heat
flows in this region are the key factors in determining environmental impacts and their
potential thermal interaction. In this study, analytical and numerical models of vertical
heat exchangers are presented. First, the effect of system parameters such as borehole
spacing on the transient response of two GHEs is described. Second, a numerical finite
volume method in a two-dimensional meshed domain is used to evaluate the temperature
rise and the heat flows in the soil surrounding borehole systems over the long term.
Finally, to examine the effect of temperature rise in the soil surrounding a vertical GHE
on the performance of an associated ground heat pump, a reversible heat pump model is
coupled to the heat exchanger analytical model via the heat exchanger running fluid
temperature. The heat exchanger running fluid temperature, wall temperature and its heat
load profile are the main coupling parameters between the three models. The results of
the analytical model are compared with ones of a finite volume numerical model.
Below a certain depth, the temperature of the ground remains almost unchanged
throughout the year. This phenomenon can be exploited by placing a heat exchanger in
the ground and coupling it to a heat pump to store heat in the ground during summer for
use in winter. The ground provides a better source/sink of heat than the outside air in
regards to heat pump efficiency, being cooler than the outside air in the summer and
warmer in the winter. Due to their good efficiency, the use of geothermal energy is often
encouraged; however, two issues arise in the long-term use of ground for thermal
purposes: the sustainability and impact of these systems on the environment. Studies
show the potential of the geothermal heat exchangers for environmental impacts such as
migration of thermal plumes away from these systems which may cause undesirable
temperature rises in temperature-sensitive regions. Furthermore, interference between
adjoining installations is being reported, raising issues of sustainability in terms of
performance and equitable sharing of natural resources. With increasing interest in
installing such systems in the ground and their potential dense population in coming
years, regulations need to be implemented to prevent their thermal interaction and their
possible negative effects on the design and performance of nearby systems.
The temperature of the soil surrounding the ground heat exchangers (GHEs) and the heat
flows in this region are the key factors in determining environmental impacts and their
potential thermal interaction. Modeling such systems is important for understanding,
designing and optimizing their performances and characteristics. In this study, a number
of analytical and numerical models of vertical heat exchangers are presented. Through
these models, the temperature of the soil surrounding the GHEs and the heat flows in this
region can be determined. Thus, the effect of possible thermal interaction between these
systems on their coupling heat pump as well as their environmental impacts can be
studied.
First, the two-dimensional transient conduction of heat in the soil around single and
multiple GHEs is discussed via numerical and analytical methods. The effect of system
parameters such as borehole spacing as well as heat store capacity on the transient
response of two GHEs is described. The results of the temperature response of the soil
around a borehole, calculated with an analytical line source theory, are compared with the
soil temperature rise calculated numerically. In addition, a three-dimensional numerical
study is performed to examine the axial heat transfer effects in heat conduction in the soil
surrounding a borehole and especially near its top and bottom.
Second, the long-term performance of multiple vertical GHEs is investigated in order to
examine their interaction as well as migration of thermal plumes away from these systems
which may cause undesirable temperature rises in temperature-sensitive regions. A
numerical finite volume method in a two-dimensional meshed domain is used to evaluate
the temperature rise in the soil surrounding multiple borehole systems over the long term,
for a period of 5 years. A heat flux from the borehole wall is assumed, reflecting the
annual variation of heat storage/removal in the ground. By choosing a heat boundary at
the borehole wall, it is assumed that the inlet temperature of the circulating fluid running
in the U-tube inside the borehole will be adjusted according to the flow rate. The selection
of the sinusoidal function is based on the heat pump power consumption and building
heating and cooling needs gained via the bin method for a typical building in Belleville,
IL.
Next, to account the variation in heating strength along the borehole length due to the
temperature variation of the fluid flowing in the U-tube, the finite line source model and
the numerical model in a three dimensional domain are both coupled to the model inside
the borehole and the results are compared in terms of the soil temperature rise and the
borehole wall heat flux. Thus, critical depths at which the maximum heat flow rate
occurs, resulting in thermal interaction, can be determined.
Finally, with some improvements to the coupling procedure, the coupled model is used to
investigate interacting borehole systems with a periodic heat flow rate in the long term
system operation (30 years). To examine the effect of temperature rise in the soil
surrounding a vertical GHE on the performance of an associated ground heat pump, a
reversible heat pump model is coupled to the model inside the borehole via the running
fluid temperature in the U-tube inside the borehole. The running fluid temperature, the
borehole wall temperature and the heat load profile are the main coupling parameters
between the three models. The results of the analytical model are compared with ones of a
finite volume numerical model. | en |
