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