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LONG TERM PERFORMANCE PREDICTION OF A
BOREHOLE AND DETERMINATION OF OPTIMAL
THERMAL RESPONSE TEST DURATION
MURAT AYDIN
ALTUG SISMAN
AHMET GULTEKIN
ISTANBUL TECHNICAL UNIVERSITY, ENERGY INSTITUTE
NEW ENERGY TECHNOLOGIES RESEARCH GROUP
BRIEF INFORMATION ABOUT GROUND SOURCE HEAT PUMP TEST AND
RESEARCH LABORATORY AT ITU ENERGY INSTITUTE
Vertical Ground Heat Exchangers:
Boreholes having different
• Depths (50m, 100m)
• Pipe diameters (Ø25, Ø32, Ø40)
• Number of U-tubes (1U, 2U, 3U)
• Shank spaces (LS97mm, LS135mm)
• Distance from each others (3m, 5m, 7m, 10m)
Laboratory facilities:
Horizontal Ground Heat Exchangers:
• Snail type (depth:2m, total length:100m)
• Slinky
• Vertical (depth:2m, total length:100m)
• Horizontal (depth:2m, total length:100m)
• Helix
• Vertical (depth:1.5m-4.5m, total length:40m)
• Horizontal (depth:1.5m, total length:40m)
GroundTemperature Measurement System
Depth:20m
Sensors: 15
Some other sensors in ground for different aims
For results : web.itu.edu.tr/murataydin/taso13.html
Thermal ResponseTest System
BRIEF INFORMATION ABOUT GROUND SOURCE HEAT PUMP TEST AND
RESEARCH LABORATORY AT ITU ENERGY INSTITUTE
Depth
(m
)
Temperature (C)
CONTENT
Thermal Response Test System
Introduction of Analytical Model
Experimental Results and Long Term Predictions
Optimum Test Duration
Conclusion
Thermal Response Test System
THERMAL RESPONSE TEST
In Ground Source Heat Pump (GSHP) applications, 75-80% of heat transfered to the buildingcomes from ground,
Determination of thermal properties of ground is an important issue,
Thermal Response Test (TRT) is used to determine thermal properties and then thelongterm performance predictions of a borehole can be made for a GSHP application,
Thermal Response Test Methods,
Constant Heat Flux Method
Constant Temperature Method
CONSTANT TEMPERATURE TRT
ADVANTAGES AND DISADVANTAGES
Advantages
• Flexible test temperatures
• Better accuracy
• Unlimited test duration
• Possibility of test of more than one boreholes simultaneously
Disadvantages
• High cost to build the test system
CONSTANT TEMPERATURE THERMAL RESPONSE TEST SYSTEM
WORKING DIAGRAM
WATER TANK
Electrical Resistance 3 x 6kw
Min
i P
um
p
Ground Return Collector
Flo
wm
ete
rs Ø
25
Exp
an
sio
n V
essel
Au
to.A
ir P
urg
e
Bore
hole
1
Bore
hole
2
Ground Inlet Collector
PID
CONTROL
PANEL
PT
1000
TE
MP
ER
AT
UR
E
SE
NS
OR
Bore
hole
3
T
DATA
LOGGER
Pump Filter
Bore
h. 1
Bore
h. 2
Bore
h. 3
T T
T T T
32
1
4
By p
ass lin
e
5
6
7
500lt Valve with
temp.sensor Ø25
Valve with
temp.sensor Ø25
TO MEASURE UNDISTURBED GROUND TEMPERATURE
CONSTANT TEMPERATURE THERMAL RESPONSE TEST SYSTEM
WORKING DIAGRAM
WATER TANK
Electrical Resistance 3 x 6kw
Min
i P
um
p
Ground Return Collector
Flo
wm
ete
rs Ø
25
Exp
an
sio
n V
essel
Au
to.A
ir P
urg
e
Bore
hole
1
Bore
hole
2
Ground Inlet Collector
PID
CONTROL
PANEL
PT
1000
TE
MP
ER
AT
UR
E
SE
NS
OR
Bore
hole
3
T
DATA
LOGGER
Pump Filter
Bore
h. 1
Bore
h. 2
Bore
h. 3
T T
T T T
32
1
4
By p
ass lin
e
5
6
7
500lt Valve with
temp.sensor Ø25
Valve with
temp.sensor Ø25
FOR PREPARING THE SYSTEM TO TEST
WATER TANK
Electrical Resistance 3 x 6kw
Min
i P
um
p
Ground Return Collector
Flo
wm
ete
rs Ø
25
Exp
an
sio
n V
essel
Au
to.A
ir P
urg
e
Bore
hole
1
Bore
hole
2
Ground Inlet Collector
PID
CONTROL
PANEL
PT
1000
TE
MP
ER
AT
UR
E
SE
NS
OR
Bore
hole
3
T
DATA
LOGGER
Pump Filter
Bore
h. 1
Bore
h. 2
Bore
h. 3
T T
T T T
32
1
4
By p
ass lin
e
5
6
7
500lt Valve with
temp.sensor Ø25
Valve with
temp.sensor Ø25
CONSTANT TEMPERATURE THERMAL RESPONSE TEST SYSTEM
WORKING DIAGRAM
TESTING PROCESS
CONSTANT TEMPERATURE THERMAL RESPONSE TEST SYSTEM
PICTURES
Analytical Model
For
Constant Temperature TRT
ANALYTICAL MODEL FOR CONSTANT TEMPERATURE TRT
2
outin TTT
p
b
g
br
rln
k
qTT
22
Fluid InletTemperature :
Fluid Outlet Temperature :inT
outT
Mean fluid temperature :
Borehole wall temperature :
outinp TTcmL
Q'q
Unit heat transfer rate :
Flow-rate : m
Measured Quantities
(Under steady state approx.)
Borehole
r
θ
U-tube
Ground
inT outT
L
inT outT
brbT
ANALYTICAL MODEL - NONDIMENSIONALIZATION
To find the temperature distribution around the
borehole following expression should be solved
t
T
r
T
rr
T
112
2
bT,trT b
T)T(r,0
T,t)T(
2
bbb
b
r
tt~
;r
rr~;
TT
TT
Nondimensionalization
trrr~~~
1~ 2
2
0)~
,1( t
Initial Condition
Boundary Conditions
1)0,~( r
1)~
,( t
ANALYTICAL MODEL - SOLUTION
Solution:
rdJrYYrJr.dYJ
Jr~YYr~Je)t
~,r~(
r
t~
10000
0 2
0
2
0
0000
2
dYJ
JrYYrJetr
t
0 2
0
2
0
0000
~ ~~2)
~,~(
2
2
ANALYTICAL MODEL - HEAT TRANSFER RATE (HTR)
Heat Transfer Rate (HTR) per unit borehole lengthbrr
bdr
dTπkrq
2 1
2
r~
br~d
dθTTπkq
12
r~b r~d
dθ
TTπk
qq~ Nondimensionalization for unit HTR value
dβYβJ
βYβJβJβYe
r~d
dθ'q
~
0β 2
0
2
0
1010
t~
β
r~
2
1
Dimensionless unit Heat Transfer Rate
ANALYTICAL MODEL - DATA FITTING
Fitting the model to the experimental data
TT
tqk
r
tq
bb
2
)(~2
Exp. ResultsModel
k (Thermal Conductivity)
(Thermal Diffusivity)
A REPRESENTATIVE EXPRESSION FOR
For a borehole, variation of with is shown in the following figures:q~ t
~
0 2 4 6 8 10 12 14
4
2
0
2
4
0 200000 400000 600000 800 000 1.0 106 1.2 106 1.4 1060.10
0.12
0.14
0.16
0.18
0.20
0.22
0.24
Therefore a representative expression is needed to fit the model to results in an easy and fast way
2~brtt
(W/m)q~ q
~ln
t~
ln
q~
Fitting process is a time consuming process due to numerical integration
dβYβJβ
βYβJβJβYe
π
2tq
0β 2
0
2
0
1010
tβ2
~
~~
04920284000156000040 23 .)t~
(ln.)t~
(ln.)t~
(ln.q~
ln
0492028400
0156000040
2
2223
.)r/tln(.
)r/t(ln.)r/t(ln.expq
~
b
bb
Representative exp. of q~ Fitting a cubic polynomial
expression
TT
Tcmk
r
tq
b
p
b
2
~2
Short term
experimental
data fitting
Determining
k and α ,k
Long term
performance
prediction of
borehole
2
2b
br
tq~
)TT.(k'q
Repr.exp.
True exp.
Experimental Results
REPRESENTATIVE EXPRESSION FOR FITTING
Following figure shows a comparison of true and repr. equations
0 200000 400000 600000 800 000 1.0 106 1.2 106 1.4 1060.10
0.12
0.14
0.16
0.18
0.20
0.22
0.24
(W/m)q~
2~brtt
True expression
Repr. expression
Experimental Results and
Long Term Predictions
A TEST STUDY
Properties of Borehole and Test Conditions
Borehole diameter 0.17 m
Borehole length 50 m
Total test duration 240 hours
Ground inlet temperature 40.0 oC
Ground avg. outlet temperature 37.5 oC
Flow-rate 25.4 lt/min
Average unit HTR value 88.0 W/m
Experimental Results
Time [hours]
(W/m)'q
0 10 20 30 400
50
100
150
200
A TEST STUDY
DATA FITTING
Time [hours]
(W/m)'q
keff= 3.8 W/mK
αeff=0.7x10-6m2/s
0 10 20 30 400
50
100
150
200
0 50 100 150 200 2500
50
100
150
200
A TEST STUDY
LONG TERM PERFORMANCE PREDICTION
Time [hours]
(W/m)'q
Time [hours]
(W/m)'q12 days prediction 4 months prediction
Experimental Results
Fitted curve to test results
Prediction curve
0 500 1000 1500 2000 25000
50
100
150
200
Optimum Test Duration
0.00
0.50
1.00
1.50
2.00
2.50
24 48 72 96 120 144 168 192 216 240
% D
iffe
ren
ce
Test Duration
Variation of % Difference of Long Term
Predictions with Test Duration
OPTIMUM TEST DURATION
Test
Duration
[W/m]
prediction of unit
HTR value after 4
months non-stop
working
% Difference of
predictions
24 62.8 2.03
48 63.1 1.56
72 63.4 1.09
96 63.6 0.78
120 63.7 0.62
240 64.1 0
'q
Even 24 h test duration seems to be enough.
Reference
Test Duration
Conclusion
CONCLUSION
• A process to make long term predictions for unit HTR value of a borehole is developed,
• Optimum test duration is examined for constant temperature TRT and it seems that even
24h is enough,
• Long term predictions are made by using the experimental data for a single borehole,
• This process can be used to determine total length of boreholes for GSHP applications.
ThankYou For AttentionThis project is supported by
• BAYMAKA.Ş. and
• Republic of Turkey, Ministry of Science, Industry and Technology.
TB VARIATION DURING THE TEST
0.00
5.00
10.00
15.00
20.00
25.00
30.00
35.00
40.00
0.00 24.00 48.00 72.00 96.00 120.00 144.00 168.00 192.00 216.00 240.00 264.00
Tem
pera
ture
[C]
Time [hours]
Tb
SOME PICTURES
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