Open Journal of Modelling and Simulation, 2013, 1, 1-6
http://dx.doi.org/10.4236/ojmsi.2013.11001 Published Online January 2013 (http://www.scirp.org/journal/ojmsi)
Research on Performance of Ground-Source Heat Pump
Double U Underground Pipe Heat Exchange
Youyin Jing1, Jing Hou2, Peng Yang3
1Power Engineering Department, North China Electric Power University, Baoding, China
2NORINDAR International Ltd. , Shi jia zhuang, China
3Hebei Institute of Architectural Design & Research Co., Ltd., Shijiazhuang, China
Received November 25, 2012; revised December 28, 2012; accepted January 15, 2013
ABSTRACT
This paper uses FLUENT software building the three-dimensional unsteady state model of ground source heat pump
single U and double U underground pipe to study on heat exchange of underground pip e system in the condition of un-
steady state long-term continuous running, analyzes the change of soil temperature filed around underground pipe and
performance of underground pipe heat exchange between single U and double U pipe system. The results show that
double U pipe system is better than single U system, which can improve unit depth heat exchange efficiency, reduce the
number of wells and reduce the initial investment.
Keywords: Ground-Source Heat Pump; Double U Unde rground Pipe Heat Exchange; Soil Temperature Field
1. Introduction
Ground source heat pump technology is a gradual rise
of th e en erg y savi ng techno logy along w ith glob al en erg y
crisis and the emergence of environmental issues. Geo-
thermal energy is a low grade energy, and a green re-
newable energy that human can use [1]. Conventional air
conditioning high energy consumption and environ-
mental pollution problem can be solved. The new geo-
thermal energy could ease dependency of coal and oil in
our country, and achieve the purpose of energy diver-
sification. Currently some of residential districts, office
buildings, hospitals and other kinds of buildings use the
ground source heat pump system to heat and cool, and
obtain good results. Ground source heat pump’s cha-
racteristic is high initial investment, but low operating
costs than conventional air conditioning system, so re-
ducing the initial investment is very important to ground
source heat pump. Double U pipe system can improve
heat exchange performance, red uce the initial investment
[2].
2. Introduction Model
2.1. The Theoretical Basis for Modeling
U-tube heat exchange process was divided into convec-
tive heat transfer between fluid and pipe wall, thermal
conductivity between pipe wall and fill material, thermal
conductivity of the fill layer, thermal conductivity be-
tween fill layer and the soil, thermal conductivity of the
soil [3]. In this paper, the model was simplified in the al-
lowable range, making the following assumptions:
 Thermal conductivity coefficient, density, specific
heat capacity and other physical parameters of soil is
uniform.
 When the system is running, ground water flow of
soil around underground pipe lead to change of heat
and moisture, the model ignore the change [4].
 Ignore the change of soil temperature with depth.
2.2. Physical Parameters
Parameters that affect the heat transfer include density,
specific heat capacity, thermal conductivity of soil, fill
layer, the fluid. In this model, pipes materials are high-
density polyethylen e pipe, backfill materials in clude 20%
bentonite and 80% SiO2 sand, the soil parameters is the
weighted average of many local soil materials [5]. The mate-
rial propertie s paramet ers of the m odel are shown in Table 1.
2.3. Geometric Model
This paper establishes the heat transfer model of the
three-dimensional unsteady state of single U pipe and
double U pipe, studies on the change of pipe outlet tem-
perature, temperature difference of inlet to outlet, the
mount of heat transfer and the change of soil temperature
filed around underground pipe. U tube long for 100 m,
tube diameter for 26 mm, tube wall thickness for 3 mm,
C
opyright © 2013 SciRes. OJMSi

Y. Y. JING ET AL.
2
Table 1. Physical parameter table.
Materials Density
kg/m3 Specific heat
capacity J/(kg·K) Thermal conductivity coefficient W/(m·K)
Ter 998.2 4183 0.599
Pipe 950 2300 0.45
Backfill
materials 1900 900 2.2
Soil 1600 1800 2
two tube feet spacing for 80 mm; double U model, two U
tube parallel, four tube feet distribute square, edge long
for 80 mm; drilling (filled layer) model is a cylindrical
shape using two tube Center as circle, diameter for 140
mm, high for 101 m; soil model is also a cylindrical
shape using the same circle, radius of 5 m, high for 105
m. The initial temperature of soil is 290 K, buried pipe
inlet temperature is 310 K. The velocity is 0.4 m/s.
3. Numerical Simulation of Double U System
and Single U System
3.1. Soil Temperature Distribution
Figures 1-8 show soil temperature field of double U and
single U pipe system run for 10 days, 30 days, 60 days,
90 days. In 30 m depth, intercept a square soil using tube
centre as square centre, and side long is 2 m, research
distribution of soil temperature field. Comparative analy-
sis double U pipe and single U pipe effect on soil tem-
perature field.
Figure 1 shows soil temperature field of double U and
single U pipe system run 10 days. Compare Figures 1
and 2, it can be seen from the figure double U pipe im-
pact on the surrounding so il temperature is slightly larg er
than the single U pipe system, after 10 days, soil tem-
perature far from the pipe centre 1 m is 291.75 K, the
temperature of the same locate in the single U system is
291.25 K. The soil temperature around the double U pipe
growth rate is higher then the soil around the single U
pipe. The soil temperature difference between double U
system and single U system is 0.5 K in the radius 1 m.
Figure 2 shows soil temperature field of double U and
single U pipe system runs for 30 days. System continu-
ously runs for 30 days, soil temperature distance from the
pipe centre 1m is 293.75 K, the temperature of the same
locate in the single U system is 293.25 K. The soil tem-
perature around the double U pipe growth rate is also
higher then the soil around the single U pipe. The soil
temperature difference between double U system and
single U system is 0.5 K in the radius 1 m.
Figure 3 shows soil temperature field of double U and
single U pipe system run 60 days. System continuously
runs for 60 days, soil temperature distance from the pipe
centre 1m is 296.5 K, the temperature of the same locate
in the single U system is 295.75 K. The soil temperature
around the double U pipe growth rate is also higher then
the soil around the single U pipe. The soil temperature
difference between double U system and single U system
is 0.75 K in the radius 1 m, slightly increase the tem-
perature difference.
Figure 4 shows soil temperature field of double U and
single U pipe system running 90 days. System continu-
ously runs for 90 days, soil temperature distance from the
pipe centre 1 m is 298.25 K, the temperature of the same
locate in the single U system is 297.5 K. The soil tem-
perature around the double U pipe growth rate is also
higher than the soil around the single U pipe. The soil
temperature difference between double U system and
single U system is 0.75 K in the radius 1 m.
System continuously runs for 90 days in summer, to
complete the summer cooling conditions. It can be seen
from the figure, the two systems continuously run for 90
days, the double U system effects on soil temperature
field is more than single U system. We can clearly see
that there are two high temperature inlets and two rela-
tively low temperature outlets in the double U plan.
There is a high temperature inlet and a low temperature
outlet in the single U plan. Double U system has two U
shape pipes, four tube feet, influence on surrounding soil
is bigger than single U system, with more heat transfer,
so it can rapidly change surrounding soil temperature,
with wide temperature range. But running after 3 months,
surrounding soil temperature of double U pipe and single
U pipe is little different, which is 0.75 K.
3.2. Comparison of Pipe Heat Exchange
Figures 5-7 show that single U system and double U
system outlet temperature changes with time. It can be
seen from the figure, each U shape pipe outlet tempera-
ture basically same in double U pipe. Single U pipe out-
let temperature and double U outlet temperature are same
in the initial operation time; after that, double U pipe
outlet temperature rise faster then single U pipe; run after
1 day, outlet temperature difference between single U
pipe and double U pipe reach maximum value, and later
gradually reduced. Running after 1 day outlet tempera-
ture of the single U pipe and the double U pipe are
302.31 K and 304.68 K, temperature difference is 2.37˚C.
Running after 90 day outlet temperature of the single U
pipe and the double U pipe are 305.61 K and 307.03 K,
temperature difference is 1.42 K.
Figures 8-10 show that single U system and double U
system change of temperature difference with time. It can
be seen from the figure, two branch pipe temperature are
basically the same in the double U system, temperature
difference between inlet and outlet in the double U sys-
tem is significantly lower than the temperature dif-

Y. Y. JING ET AL.
3
Figure 1. Soil temperature field of double U and single U pipe system run 10 days.
Figure 2. Soil temperature field of double U and single U pipe system run 30 days.
Figure 3. Soil temperature field of double U and single U pipe system run 60 days.

Y. Y. JING ET AL.
4
Figure 4. Soil temperature field of double U and single U pipe system run 90 days.
290
292
294
296
298
300
302
02040 60 80100120
time(min)
temperature(K)
singleU doubleU1 doubleU2
Figure 5. Run 2 h the change of pipe outlet temperature.
299
300
301
302
303
304
305
246810 12 14 1618 20 22 24
time(h)
temperature(K)
singleU doubleU1 doubleU2
Figure 6. Run 2 h - 24 h the change of pipe outlet tempera-
ture.
302
303
304
305
306
307
308
0 102030405060708090
time(d)
temperature(K)
singleU doubleU1 doubleU2
Figure 7. Run 1 d - 90 d the change of pipe outlet tempera-
ture.
8
10
12
14
16
18
20
0 2040608010012
time(min)
temperature(K)
0
singleU doubleU1 doubleU2
Figure 8. Run 2 h the chan ge of pipe temperature difference.
4
5
6
7
8
9
10
11
12
24681012 1416 18 2022 24
time(h)
temperature(K)
singleU doubleU1 doubleU2
Figure 9. Run 2 h - 24 h the change of pipe temperature
difference.
2
3
4
5
6
7
8
0 102030405060708090
ti me(d)
tem per atu re(K)
sin
g
leU doubleU1 doubleU2
single
U
doubleU2doubleU1
Figure 10. Run 2 h - 24 h the change of pipe temperature
difference.

Y. Y. JING ET AL. 5
ference between inlet and out in the single U system,
because the number of tubes in the double U system
every well is the number of tubes in the single U system
every well, double U system have more heat transfer
with surrounding soil, temperature of surrounding soil
rapidly increases as time, temperature difference, heat
transfer is small, so the temperature difference between
inlet and outlet is small. However, heat transfer in the
double U system is the sum of two U pipe heat transfer.
Because the fluid flow in the double U pipe is twice of
single U pip e.
Figures 11-13 show that the change of unit depth heat
exchange in the single U pipe and the double U pipe as
time. Figure 14 is comparison chart about unit depth
heat exchange between single U pipe and double U pipe.
U1 is one of the double U pipe, U2 is an another U shap e
pipe in the double U pipe, U1 parallel U2 form doub le U
pipe, so double U pipe unit depth heat exchange is sum
of U1 pipe heat exchange and U2 pipe heat exchange.
We can see from figure that separate U1 and U2 pipe unit
depth heat exchange is less than single U pipe heat ex-
change, but double U pipe unit depth heat exchange is
the sum of U1 and U2 heat exchange, so heat exchange
in the double U pipe is larger than the heat exchange in
the single U pipe. Temperature difference between inlet
and outlet is small, leading to heat exchange of each
branch pipe is small, but double U pipe flow rate is sin-
gle U pipe flow twice, or have two temperature differ-
ence, so the total heat exchange in the double U pipe is
each branch heat exchange twice. Form Figure 14 we
can see that heat exchange of double U pipe is signifi-
cantly higher than heat exchange of single U pipe. After
system continuous operating 1 day, single U pipe unit
depth heat exchange is 68.2 W/m, double U pipe unit
depth amount of heat exchange is 94.3 W/m. After sys-
tem continuously operating for 10 day, single U pipe and
double U pipe unit depth amount of heat exchange are
53 .6 W/ m an d 74.9 W/m. After system continuous operat-
ing for 1 month, single U pipe and double U pipe unit
depth heat exchange are 45.6 W/m and 60.1 W/m. After
system continuous running for 2 months, single U pipe
80
100
120
140
160
180
200
220
240
260
280
300
320
340
020406080100 120
time(min)
heat of unit depth(W/m)
singleU doubleU1 doubleU2 doubleU
Figure 11. Run 2 h the change of pipe temperature dif-
ference.
40
60
80
100
120
140
160
180
246810 1214 16 18 20 22 24
time(h)
heat of unit depth(W/m)
singleU doubleU1 doubleU2 doubleU
Figure 12. Run 2 h - 24 h the change of pipe temperature
difference.
20
30
40
50
60
70
80
90
100
0 102030405060708090
time(d)
heat of unit depth(W/m)
singleU doubleU1 doubleU2 doubleU
Figure 13. Run 2 h - 24 h the change of pipe temperature
difference.
35
45
55
65
75
85
10 2030 4050 60 7080 90
time(d)
heat of unit depth(W/m)
sin
g
leU doubleU
singleU doubleU
Figure 14. Run 2 h - 24 h the change of pipe temperature
difference.
and double U pipe unit depth heat exchange are 40.3
W/m, 55.3 W/m. After system continuous run 3 months,
single U pipe and double U pipe unit depth heat ex-
change is 39.0 W/m, 52.6 W/m.
4. Conclusions
In this paper, using FLUENT software to simulate ground
source heat pump single and double U-pipe system the
long-term continuous operation in unsteady state con-
ditions, the surrounding soil temperature changes and
buried pipe heat exchange changes, get following conclu-
sions though comparativ e study:
1) After the system running for 3 months, the soil
temperature of double U system at the radius of 1 m rises
to 298.25 K, the soil temperature of single U system at
the radius of 1 m rises to 297.5 K. Double U system has
more impact on the surrounding soil than single U sys-

Y. Y. JING ET AL.
6
tem, but not much difference.
2) After the system running for 3 months, temperature
difference between inlet and outlet in the double U sys-
tem is less than the temperature difference in the single U
system, but the unit depth heat exchange is much larger
than single U system, because the flow rate in the double
U pipe is twice than the flow rate in the single U pipe, or
we can say that double U system has two temperature
differences, so unit depth heat exchange can greatly en-
hance, reduce the number of wells and reduce the initial
investment.
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