EFFECT OF THERMAL DIFFUSION AND TRANSPORT OF OPERATION MODES
ON UNDERGROUND THERMAL ENERGY STORAGE
Qing Gao, Ming Li, Yan Jiang , Ming Yu
Jilin University
Changchun 130025, China
Tel: 86-431-85094241
[email protected]; [email protected]
Y.Y. Yan
University of Nottingham,
Nottingham, UK
ABSTRACT
This paper aims to propose an advantageous operation mode for the underground thermal
energy storage (UTES) by reformatting the ground temperature distribution, and clarify the
effects of an actual application. In this study there were four typical operation modes, such as
uniform mode, concentrative mode, intermittent mode and heat shield mode, which based on
the different load allotment similar to the practical boreholes of underground heat exchanger.
Especially, author presented the innovational heat shield mode, that usually the rise of
temperature on the area apron will shape an isothermal vertical wall to block heat flowing. We
have concluded that there is a more profitable operation which can store more energy stored
in the ground. The reformation of temperature distribution by suing of allotting load can
realize a satisfactory effect and get energy conservation longer. Thus, an efficient operation
will be obtained and it will promote the practical maneuver.
1．
INTRODUCTION
The performance of underground source heat pumps (GSHP) is affected by imbalanced
cooling and heating, which will be much more serious in the north of China where
predominately heating or in the south of China where predominately cooling. Therefore, the
popularization of GSHP is limited greatly. As we know, the underground thermal energy
storage (UTES) is one of the best way to supply and reuse the energy by storing energy in
underground. Additionally, it is also an outstanding type of seasonal thermal energy storage.
Thermal energy storage (TES) in the soil and rock involves complicated unsteady process and
its effect and capability depends on the controlled operation mode. Furthermore, the ground
temperature field after storing will affect on the stored energy diffusion and its loss seriously,
and its reformation results from an artificially allotting the different loads of borehole in the
area of ground heat exchanger.
Energy storage technology has been the focus in energy development program which was laid
out in 1990s by International Energy Agency (IEA). Some demonstration projects in EU have
facilitated the fundamental research and the advance of science and technology. Recently Gao
and Cui have been paid more attention to the operation mode of UTES system. However, it is
found that numerous efforts continue to improve the heat transfer of UTES, fewer efforts have
focused on manners of its operation and control of the field of multi-borehole for seeking a
higher efficiency. Therefore, authors presented new ideas to deal with the controllable heat
diffusion, the load distribution, the temperature field regeneration and the thermal shield to
improve the efficiency greatly and facilitate next application.
The interest of this paper lies in the analysis of the preliminary experimental results in the
bench test soil tank. Furthermore, the performance characteristics of underground thermal
energy storage and the behavior of thermal diffusion are investigated to explore the effect of
operation modes on the thermal diffusion and the stored energy loss in UTES and to realize
the higher efficiency and the longer period of energy conservation, which are expected to be
helpful for the coming improvement work.
2.
EXPERIMENTAL APPARATUS AND NUMERICAL CALCULATION
Experiment
UTES is a complex process which is infeasible to be analyzed by vast in-situ experiments
because it is difficult to detect the temperature in practical engineering project. However, the
parameters which are difficult to be detected in-situ can be measured in the soil tank, a
simulated test system. Much more researches on the features and characteristics of UTES can
be made by the soil tank. Therefore a multi-functional test soil tank was built up, which
consists of sixteen heat sources, one water piping system, and measurement system, as shown
in figure 1.
(a) Heat sources and temperature detectors (b) Sources and detectors in the soil
(c) In-situ apparatus
Figure 1. Test soil tank
There are 27 temperature detection locations in the soil
tank which are symmetrized in the arrangement to reduce
the detection locations. According to the same rule, heat
source positions are symmetric. Some feature temperature
detection locations were selected including heat source
wall location N, geometric center location A, inside ring
spacing location B, outside ring spacing location C,
margin spacing location D, spacing location E between
inside and outside, and adjacent heat source wall location
F, as shown in Figure 2.
Figure 2. Heat source and
temperature detector locations
Numerical Calculation
Numerical analysis has been regarded as an effective way to simulate complex experiments in
order to save time and money. Numerical experiments can also extend beyond the limitations
of experiments and predict and design a hypothetical system under any working conditions. In
the present study, in parallel with the experimental test, the ground temperature around ground
heat exchangers is studied numerically by finite element software. Simultaneously,
experiments validated the numerical model and calculation for the further use of numerical
analysis. In this study of heat transfer in multi-sourse field, the cylindrical heat source
methodology was taken and used in the cylindrical coordinate system.
Finite element software is employed to resolve the transient temperature field around the
vertical heat exchanger pipe. The element type and the grid density were selected according to
the sensitivity of temperature quantity, so that the calculation can adapt to the actual situation
and reach a high level of accuracy. Because the temperatures change more sharply around the
cylinder heat source (borehole), the grid is designed to be denser in that area, while it is
sparser far away from the central heat source.
3.
MODE AND ANALYSIS
Uniform load mode
The uniform load mode is a basic operational control mode, which means that every heat
sources applied same continuous heating load. The temperature variation and restoration
characteristics were observed in the experiment to clarify the thermal diffusion during heat
storage. In this mode the load was 15W and the heating period was 6 hours. This uniform
mode was referred as a basic operation mode to be compared with others.
As can be seen from figure 3, most of heat is accumulated around the heat source during the
continuously heating process, and the temperature around the heat source rises sharply
because of very low heat transfer coefficient of soil. The profile of the temperature
distribution after the 6 hours of heating indicates a big temperature gradient around the heat
source, as shown in figure 3(a). The peak temperature is about 64℃, while the edge
temperature (apart from heat source) only increased about 0.3℃. The result shows that there
is too much heat accumulated around the heat source, and the soil space is not used
sufficiently and the heat does not spread abroad. It is implied that continuous heating is not
the most advantageous operation mode. Accordingly, this mode adapts well to a lower load
and a shorter period of heating.
(a)
(b)
Figure 3-1 Experiment result
(a) - At the heating end (after 6 hrs)
(a)
(b)
Figure 3-2 Numerical calculation result
(b) - After 29 hrs
Figure 3. Temperature distribution in a uniform load mode
23 hours after the heating period, the peak temperature fell down significantly, as shown in
3(b). Actually, the peak temperature around heat source was decreasing, while the temperature
of the area apart from heat source was increasing gradually. Simultaneously the temperature
gradient of the whole area in the soil tank was smaller and smaller, the temperature field
became more even. The protrusive centre temperature means that the heat was centralized and
the thermal energy enriched the area for storage. The centre temperature had a largest
increment and increased about 2.9℃, while the edge temperature just increased less than 1℃.
Commonly, all heat sourses in the field were located symmetrically, so a quarter of whole
field were selected for the numerical calculation. Thus the figure showing of the numerical
calculation result of temperature distribution was made in a partial field. Comparing results of
numerical calculation with experimental ones, both temperature distributions appear better in
In fact because the space in soil is limited,
the quantity of temperature measurement
location can not be sufficient and the shape
of the temperature distribution surface in the
figure 3-1 is not very smooth. Otherwise, it
is more difficult to detect the real
underground temperature so the numerical
simulation will be more necessary for
further and more extensive investigation.
Temperature(℃)
accord in the figure 3. For example, the heat source wall location N the numerical calculation
result is accorded with the experimental result in figure 4.
65
60
55
50
45
40
35
30
25
20
TN-test
TN-simulaiton
0
500
1000
1500
Time(min)
2000
2500
As we know, the heat of all heat sources
Figure 4. Temperature of heat source
diffused not only towards the centre of soil
wall location N
tank but also towards the edges of soil tank.
Finally the centre temperature becomes the highest and the centre area comes into being a
congregate “heat pool”. As a computation, supposing whole heat is equally diffused equally in
tank, the temperature should increase 1.8℃. This value defined in a limited space may be
taken as a criterion of the heat loss.
Concentrative load mode
In concentrative load mode, the heat source load of the inner internal ring was different from
outer one. The concentrative load mode kept a more concentrative heating. In this test there
were four heat sources in the the inner ring whose load was 25W, and there were twelve heat
sources in the outer ring whose load was 5W. For the sake of that the total load maintained the
same with other modes during heating period, while the heating time in this mode was 9 hours.
Normally, every mode takes different heating time. The temperature variation and distribution
is shown in figure 5 and figure 6 respectively. As the data clearly shows, the concentrative
load mode made its temperature distribution higher in the inner ring area and the thermal
energy was to be stored in the middle center.
Temperature(℃)
When the heating process of the
100
TA
TB
TC
TD
concentrative load mode lasted for 9 hours, the
TE
TF
TN
total heating quantity was same with that of
80
basic operation mode, the extreme temperature
of wall location N rushed over 90℃, which was
60
about 30℃ higher than that in the basic
40
operation mode. The edge temperature increase
by about 0.7℃, which was 0.3℃lower than that
20
in the basic operation mode. As shown in figure
0
500
1000
1500
2000
2500
6(a), the temperature of internal four heat
Time(min)
sources was far higher than that of outer twelve
Figure 5. Temperature curves
in concentrative load mode
heat sources, so that the concentrative
temperature field would weaken the thermal diffusion outward and maintain much more heat
energy in the area placed ground heat exchangers.
Additionally, as can see in figure 5, the temperature far from heat source had a lag time, such
as the location A, D, E. It takes longer time to reach their peak temperature. Because the peak
temperature of outer ring was not very high, it is hardly falling down. This behavior is caused
by great amount of heat accumulated in centre area, and it needs longer time to transfer
outward, so that the concentrative load mode can decelerate and prolong the diffusion and
lower the loss of stored energy than that of uniform load mode.
After the heating ceased for 20 hours (equivalently, after total of 29 hrs), the whole
temperature field was transformed into a more protuberant shape of inner high and outer low,
as shown in figure 6(b). The temperature in central area of the concentrative load mode was
higher than that of the basic operation mode. The temperature of the geometric center location
A, inner ring spacing location B, outer ring spacing location C increased 5.1℃, 4.8℃,
2.2℃respectively, and was 2.9℃, 2.8℃, 2.4℃ higher than counterparts of the basic operation
mode respectively. Results show that the concentrative load mode can maintain more heat in
the ground up to the same time. It was benefit for energy storage.
(a)
(b)
Figure 6-1 Experiment result
(a)
(b)
Figure 6-2 Numerical Numerical calculation result
(a) - At the heating end (after 9 hrs); (b) - After 29 hrs
Figure 6. Temperature distribution in a concentrative load mode
As we know, it should be pointed out that the concentrative load mode need higher heat flux,
and higher temperature of heat source. If solar energy is used in heat storage, this mode
mostly should occur in the hottest season or the mid-noon of a day.
Intermittent load mode
The intermittent load mode also applied same load to each heat source, but with periodical
heating. In this test the load was 15W, and the intermittent period was 3 hours which consists
of 1 hour heating and 2 hours pausing. When the operation lasted for 6 cycles, the total
amount of heat stored in soil was same to other modes. The variation of the temperature is
shown in figure 7.
In the intermittent load mode, the peak temperature on the wall location N was less than that
of basic operation mode about 6.4℃, and also less than that of concentrative mode about
32.4℃. Noteworthily, the requirement temperature of energy source in the intermittent load
mode is lower than other modes, and it is beneficial for utilization of low grade heat source,
such as lower solar heat, wasted heat, etc. Comparing with other modes, the intermittent load
mode makes a potential to use the low grade thermal energy widely.
In fact the experimental results also indicated that the temperature gradient approaching to the
heat source is so high that a serious heat transfer clogging happens. The temperature increases
fast, while the heat transfer quantity hardly rises due to the low heat transfer coefficient of soil
and rock. Therefore, it needs enough time to meet the transfer and diffusion of heat. The
intermittent load mode can supply a requirement of solving the heat clogging. However this
problem will be diminished by the intermission by means of sufficient heat diffusing in the
pause period and the prompt rise of temperature will be controlled effectively.
As shown in figure 7, it is found that the temperature variation of soil approaching to the heat
source is very apparent and similar to the temperature variation of the heat source, and more
close more alike. But, the fluctuation of
temperature far from the heat source is more
and more slight. Its “peak-valley” profile is
less and less obvious, the phase of the
fluctuation is latter and latter. The
temperature far from the heat source seams
to take a longer time to keep continuous and
smooth increment or decrement. Clearly, we
can understand that during the intermittent
mode of UTES, the phase is very important
Figure 7. Temperature curves in an
to a large scale multi-heat source system
intermittent load mode
(multi-borehole exchangers). The reason is
that the heat overlap and interference exits among heat sources with alternation load. It is
necessary for designers to pay more attention to both cycle and phase of the fluctuation to
make better use of the transport of a fluctuation wave besides the intermission and avoid the
obstructive effect of the adverse carrier wave to the heat.
Generally, a higher load always results in larger temperature gradient in the underground, and
always needs higher grade energy source, so it is necessary to introduce an intermittent
process.
Heat shield load mode
The heat shield is a potential operational control mode in the UTES. Authors presented this
innovational mode to diminish the heat diffusion outward by using a circle of heat fense or
“heat wall”. The heat shield used a non-uniform load mode, which means that after the
principal part of heat storage, heat sources of the outer ring continue to heat for a longer time
under a lower load, while the inner heat sources stop heating. Usually the rise of temperature
on the area apron will result in an isothermal vertical wall to block heat flowing and prevent
the heat stored in the central area from diffusing outward, therefore, more energy is kept
inside effectively.
100
Temperature(℃)
In the test the heat loads of the inner heat sources
were 25W, while the outer heat sources were
provided a lower load, 2.7W. After all heat
sources ran for 9 hours, the inner heating stopped,
while the heating of outer heat sources kept
running at 2.7W for another 7.67 hours until the
total heat was same to other modes. Experimental
results are shown in figure 8 and 9.
TA
TE
TB
TF
TC
TN
TD
80
60
40
20
0
500
1000
1500
Time(min)
2000
2500
As shown in figure 8, the temperature of location
Figure 8. Temperature curves in a
N is the highest, approaching 90℃.This
heat shield load mode
temperature gradient is the largest among all
modes (see figure 8). The profile of temperature field is similar to the concentrative load
mode at all heating end (after 9 hrs). Because of a heavy load of the inner four heat sources,
the temperature distribution has a typical shape of the inner high and outer low. However,
when the inner heat sources stopped heating and the heat sources of the outer ring continued
heating at the low load, the temperature in the area apron rose, as the temperature in the
central area descended gradually, as shown in figure 9-2. The heat shield “wall” was formed.
By heating in the outer ring of heat sources, the isothermal defense shield is formed for the
duration between both inside and outside of thermal energy storage area, which limits and
controls the heat flow outward. Finally, the heat shield has diminished the diffusion and loss
of heat stored in a limited underground area.
After another 12.33 hours of stopping all heating when the heat was preserved (equivalently,
after 29 hrs), the whole temperature field was transformed into a more protuberant shape of
inner high and outer low on the base of a little higher temperature, as shown in figure 9(c). In
the condition of both the same total amount of heat energy storage and same time (the total
time is 29 hours) during which injecting heat into ground and preserving heat in the ground
were going one after the other. The increment of temperature at the 29th hrs was selected as a
subject contrast for evaluating the effect of thermal storage. We can see, a higher increment in
the same period means better effect of underground thermal energy storage. The increment of
feature temperature of location A, B and C in four modes, ΔTA, ΔTB, andΔTC are shown
in table 1.
(a)
(b)
(c)
Figure 9-1 Experiment result
(a)
(b)
(c)
Figure 9-2 Numerical Numerical calculation result
(a) At all heating end (after 9 hrs); (b) At the outer ring heating end (after another 7.67 hrs); (c) After 29 hrs
Figure 9. Temperature distribution in a heat shield load mode
Comparing all modes mentioned above, the heat shield mode can keep more energy stored
effectively in the limited area, mainly in the central area. Its temperatures are the highest
among four modes. This situation indicates that more heat was maintained in the area of heat
source and a less heat was losen. In fact this behavior means that there is a better way to get a
high efficiency of energy storage by the control of operation, especially by allotting the load
of underground borehole exchangers. Furthermore, the control strategy can make well use of a
subjective control from allotting the load to reform and regenerate the ground temperature
field and its distribution for a advantageous effect of UTES.
Thermal Diffusion and Transport
Normally, the diffusivity should be larger during energy deposition and extraction, and it
should be smaller during energy conservation. Thus, whole UTES system maintains a high
efficiency for a long evolution. In fact, the UTES fronts with facilitating and restraining
energy transport. Next work should be based on the operation mode to try an exploration of
the dynamic control for the whole controllable heat flow by using initiatively the variable load
of every borehole (heat source). A dynamic temperature field has a potential of curbing the
energy flow. The controllable underground heat diffusion will be pursued to realize an
efficient utilization. Furthermore, novel concepts of the dynamic heat shield and the heat flow
surge will be defined and handled. This study will consummate the heat flow theory of UTES
and facilitate its development and application.
5.
CONCLUSION
(1) An efficient control strategy is discussed according to the analysis of temperature
distribution from the changeable heat load. A new idea on reforming the ground temperature
field and its distribution is presented tentatively, that predominates the heat diffusion and its
transport for high efficiency of UTES by subjective control of load distribution of heat
sources. Actually, the changeable heat load results in the different operation mode in the
UTES.
(2) Generally, higher load always results in larger temperature gradient and needs higher
grade heat source, so it is necessary to introduce an intermittent process and select a suitable
energy source. The heat shield mode diminished the heat diffusion outward since the
isothermal wall is formed continuously between inside and outside of the area, which resists
and obstructs the heat flow outward.
(3) The numerical calculation simulation was introduced for validation. Comparing results of
numerical numerical calculation with experiment, both temperature distributions appear better
in accord. In fact it is more difficult to detect the real underground temperature so the
numerical simulation will be more necessary for further and more extensive investigation and
study.
ACKNOWLEDGEMENTS
This paper is based upon work sponsored by the NSFC (National Natural Science Foundation of China) under
the grant No. 50576030, 50806028. The authors gratefully acknowledge the Department of Science & Technology
of the Jilin Province and Changchun Municipal Science & Technology Commissions. We would like to express
heartfelt thanks to Professor Jeffrey D. Spitler, Oklahoma State University USA, and Dr. Liu Xiaobing, Climate
Master Ltd. USA.
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