Thermal Energy Storage in Borehole Heat Exchanger Arrays · Thermal Energy Storage in Borehole Heat...

Thermal Energy Storage in Borehole Heat Exchanger Arrays

John S. McCartney, Ph.D., P.E. Associate Professor

University of California San DiegoSymposium on Energy Geotechnics, UPC Barcelona

June 2nd, 2015

Acknowledgements

• Sponsor: NSF SEP Grant CMMI 1230237 • Collaborators:

– Ning Lu, Colorado School of Mines (geotech. eng.)– Shemin Ge, University of Colorado Boulder (hydrogeologist)– Kathleen Smits, Colorado School of Mines (env. science)– Adam Reed, University of Colorado Boulder (energy policy)

• Graduate Students:– Tugce Baser, UC San Diego– Nora Catolico, CU Boulder

2

2

Presentation Overview

• Overview of Soil‐Borehole Thermal Energy Storage (SBTES) systems

• Simulation of Drake Landing Solar Community SBTES system using TOUGH2

• Design simulations of a small‐scale SBTES in Golden, CO using COMSOL

• Field data from small‐scale SBTES in Golden, CO• Simulations of the scalability of SBTES systems• Upcoming large‐scale SBTES system in San Diego, CA

3

3

District Heating using Solar Thermal Energy

Challenge: Heat storage

4

Drake Landing, Canada

Braedstrup, Denmark

Heat Storage Option: Geothermal Boreholes

5

Sheffield, UK

Drake Landing, Canada

SBTES System Operation

6

Heat Injection Heat Extraction

SBTES Systems within the Vadose Zone

7

Operation of SBTES Systems

8

Overall study goals:1. Understand the role of heat

exchanger array geometry 2. Calibrate models using field-

scale data from existing and new SBTES sites

3. Evaluate coupled heat transfer and water flow processes in the vadose zone

4. Understand scalability of SBTES arrays

5. Evaluate ways to improve the efficiency of heat extraction

Drake Landing Solar Community (DLSC)

Drake Landing Solar Community (DLSC) Okotoks, Alberta,

Canada

35mx35mx35m SBTES used to provide 95% of the heat to 52 homes

9

DLSC SBTES Numerical Model Domain

10

DLSC Numerical Model (TOUGH2)

11

Boundary Conditions and Calibrated Parameters

12

Parameter Value Unit

Soil Particle Density ƚ 2480 kg/m3

Soil Permeability 1.5x10‐14 m2

Soil Thermal Conductivity ƚ 2.03 w/m°C

Soil Porosity ƚ 0.50 m3/m3

Soil Heat Capacity ƚ 935.80 J/kg°C

Fluid Density 1000 kg/m3

Fluid Heat Capacity 4183 J/kg°CU‐tube Thermal Conductivity 0.51 w/m°C

Insulation Layer Thermal Conductivity ƚ 0.23 w/m°C

U‐tube Radius 0.055 mvan Genuchten m 0.5van Genuchten a 0.01 kPa‐1

DLSC Simulated Seasonal Ground Temperatures

13

DLSC Simulation vs. Measured Values

14


15


16

DLSC Efficiency of Heat Extraction

17

Groundwater Flow Effects on SBTES Systems

18

DLSC Parametric Evaluation (Perimeter Boreholes)

19

Convection Effects on SBTES Systems

20

Saturated Soil Conditions with a Water Table at the Surface

Convection Effects on SBTES Systems

21

Saturated Soil Conditions with a Water Table at the Surface

Pilot SBTES System at Colorado School of Mines

22

Simplified Numerical Analysis for DesignAssumptions‐ Heat transfer is governed by

conduction

23

⋅

Boundary Conditions‐ Constant heat flux applied to the

boreholes‐ Heat flux estimated as follows:

‐ Thermally insulated layer at top‐ Initial soil temperature of 10°C

Model Geometry

Baseline Model Inputs ( = 30 W/m)Parameter

Volumetric flow rate, 0.3 (m3/s)

Temperature difference, ΔT 2 °C

Borehole length 10 m

Heat exchanger diameter 0.025 m

∆2

Results: Impact of Borehole Spacing24

5

10

15

20

25

30

35

0 10 20 30 40

Soil

tem

pera

ture

(°C

)

Distance (m)

1 23 45 7.510

q = 30 W/m2

90 daysSpacing (m)

.

Increasingspacing

Results: Impact of Boundary Heat Flux 25

5

10

15

20

25

30

35

10 15 20 25 30

Soil

tem

pera

ture

(°C

)

Distance (m)

2025303540

90 daysλ = 1.0 W/mKSpacing: 1 m

q (W/m2).

5

10

15

20

25

30

35

10 15 20 25 30

Soil

tem

pera

ture

(°C

)

Distance (m)

2025303540

90 daysλ = 1.0 W/mKSpacing: 2 m

q (W/m2).

Spacing: 1.0 m Spacing: 2.0 m

Results: Impact of Heating Duration26


5

10

15

20

25

30

35

10 15 20 25 30

Soil

tem

pera

ture

(°C

)

Distance (m)

90100110120

q = 30 W/m2

λ = 1.0 W/mKSpacing: 1 m

Time (days).

5

10

15

20

25

30

35

10 15 20 25 30So

il te

mpe

ratu

re (°

C)

Distance (m)

90100110120

q = 30 W/m2

λ = 1.0 W/mKSpacing: 2 m

Time (days).

Results: Impact of Thermal Conductivity27


5

10

15

20

25

30

35

10 15 20 25 30

Soil

tem

pera

ture

(°C

)

Distance (m)

1.0

1.5

2.0

q = 30 W/m2

90 daysSpacing: 1 m

λ (W/mK).

Storagezone 5

10

15

20

25

30

35

10 15 20 25 30So

il te

mpe

ratu

re (°

C)

Distance (m)

1.0

1.5

2.0

q = 30 W/m2

90 daysSpacing: 2 m

λ (W/mK).

Storagezone

Performance Variables: Temperature Density

28

Temperature Density, TD ( /m3) is defined as follows

is the average temperature of the soil ( )(m3) is the heat storage volume

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0 2 4 6 8 10

Tem

pt. d

ensi

ty, T

D (°

C/m

3 )

Array spacing (m)

1.0

1.5

2.0

q = 30 (W/m2)90 days

λ (W/mK).

Performance Variables: Heat Loss29

0.0

1.0

2.0

3.0

4.0

5.0

0 30 60 90 120

Late

ral h

eat l

oss (

GJ)

Time elapsed (days)

12345

q = 30 W/m2

90 daysλ = 1 W/mK

Spacing (m).

0.0

1.0

2.0

3.0

4.0

5.0

0 30 60 90 120La

tera

l hea

t los

s (G

J)

Time elapsed (days)

1.0

1.5

2.0

q =30 W/m2

90 daysSpacing: 1 m

λ (W/mK).

Lateral heat losses for: Different array spacing (left)

Different soil thermal conductivity values (right)

Field Data from the CSM SBTES

Goals: 1. Perform a long‐term thermal response test on the system (Summer 2014)2. Monitor ambient cooling of system to evaluate losses (Fall 2014)

30

Construction Pictures

Geothermal boreholes

Foam insulation

60 mil HDPE hydraulic barrier

Site soil

Manifold

31

Heating Test Plan at Colorado School of Mines

32

Borehole 2 inletBorehole 2 outlet





Results: Ambient and Fluid Temperatures

33

0

2

4

6

8

10

12

14

10

15

20

25

30

35

40

45

50

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ΔT (°C)

Flui

d te

mpe

ratu

re (°

C)

InletOutletΔT

BH-2

0

2

4

6

8

10

12

14

10

15

20

25

30

35

40

45

50

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ΔT (°C)

Flui

d te

mpe

ratu

re (°

C)

InletOutletΔT

BH-5

0100200300400500600700800900

1000

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Fluid flo

w ra

te (m

L/s)

BH‐1BH‐2BH‐3‐4‐5

0

2

4

6

8

10

12

14

10

15

20

25

30

35

40

45

50

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ΔT (°C)

Flui

d te

mpe

ratu

re (°

C)

InletOutletΔT

BH-1

0

2

4

6

8

10

12

14

10

15

20

25

30

35

40

45

50

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ΔT (°C)

Flui

d te

mpe

ratu

re (°

C) InletOutletΔT

BH-3

0

2

4

6

8

10

12

14

10

15

20

25

30

35

40

45

50

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ΔT (°C)

Flui

d te

mpe

ratu

re (°

C)

InletOutletΔT

BH-4

Results: Soil Temperatures

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10

15

20

25

30

35

6/19/2014 8/28/2014 11/6/2014 1/15/2015

Gro

und

tem

pera

ture

(°C

) T-A 2.3 mT-A 6.0 mT-A 7.8 mT-A 9.0 m

10

15

20

25

30

35

6/19/2014 8/28/2014 11/6/2014 1/15/2015

Gro

und

tem

pera

ture

(°C

) T-B 2.3 mT-B 6.0 mT-B 7.8 mT-B 9.0 m

10

15

20

25

30

35

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Gro

und

tem

pera

ture

(°C)

T-C 2.3 mT-C 6.0 mT-C 7.8 mT-C 9.0 m

10

15

20

25

30

35

6/19/2014 8/28/2014 11/6/2014 1/15/2015

Gro

und

tem

pera

ture

(°C

) T-D 2.3 mT-D 6.0 mT-D 7.8 mT-D 9.0 m

10

15

20

25

30

35

6/19/2014 8/28/2014 11/6/2014 1/15/2015

Gro

und

tem

pera

ture

(°C

) T-E 2.3 mT-E 6.0 mT-E 7.8 mT-E 9.0 m

-30

-20

-10

0

10

20

30

40

50

6/19/2014 8/28/2014 11/6/2014 1/15/2015

Air

tem

pera

ture

(°C

)MaximumAverageMinimum

Results: Temperature Profiles

35

0

2

4

6

8

10

0 5 10 15 20 25 30 35

Dep

th fr

om su

rfac

e (m

)

Ground temperature, T (°C)

6/19/147/1/147/14/148/8/149/2/149/27/1410/22/1411/16/1412/27/14

Time (days) T-A0

2

4

6

8

10

0 5 10 15 20 25 30 35

Dep

th fr

om su

rfac

e (m

)


6/19/147/1/147/14/148/8/149/2/149/27/1410/22/1411/16/1412/27/14

Time (days) T-B0

2

4

6

8

10

0 5 10 15 20 25 30 35

Dep

th fr

om su

rfac

e (m

)


6/19/147/1/147/14/148/8/149/2/149/27/1410/22/1411/16/1412/27/14

Time (days) T-C

0

2

4

6

8

10

0 5 10 15 20 25 30 35

Dep

th fr

om su

rfac

e (m

)


6/19/147/1/147/14/148/8/149/2/149/27/1410/22/1411/16/1412/27/14

Time (days) T-D0

2

4

6

8

10

0 5 10 15 20 25 30 35

Dep

th fr

om su

rfac

e (m

)


6/19/147/1/147/14/148/8/149/2/149/27/1410/22/1411/16/1412/27/14

Time (days) T-E0

2

4

6

8

10

0 5 10 15 20 25 30 35

Depth from

surface (m

)


06/01/1407/01/1408/01/1409/01/1410/01/1411/01/1412/01/1401/01/15

Tm,out = 12 °CTout = 15 °C = 0.54 W/mKCs = 1200 J/kgKs = 30 W/m2K=1.99×10‐7 s‐1

d=√(2a/)k=/(sd)=tan‐1(k/(1+k))

Results: Temperature Profiles with Radius

36

10

15

20

25

30

35

0 1 2 3 4 5 6

Gro

und

tem

pera

ture°

C

Distance from center borehole (m)

6/19/147/1/148/8/149/2/1410/22/1411/16/1412/27/14

Time (days) 2.3 m from surface

10

15

20

25

30

35

0 1 2 3 4 5 6

Gro

und

tem

pera

ture°

C


6/19/147/14/148/8/149/2/1410/22/1411/16/1412/27/14


10

15

20

25

30

35

0 1 2 3 4 5 6

Gro

und

tem

pera

ture°

C


6/19/147/14/148/8/149/2/1410/22/1411/16/1412/27/14


10

15

20

25

30

35

0 1 2 3 4 5 6

Gro

und

tem

pera

ture°

C


6/19/147/14/148/8/149/2/1410/22/1411/16/1412/27/14


Heat Flux and Ground Properties

37

0

200

400

600

800

1000

1200

1400

6/19/2014 8/28/2014 11/6/2014 1/15/2015

Hea

t tra

nsfe

r rat

e (W

)

BoreholeAve. flow rate (0 to 49 days)

Ave. flow rate (49 to 75 days)*

Average heat injection rate

Thermal Conductivity (1‐4 days)

Thermal conductivity (12‐17 days)

Thermal conductivity(49‐75 days)

(ml/s) (ml/s) (W/m) (W/m K) (W/m K) (W/m K)1 500 300 18.6 0.48 0.52 0.552 50 30 18.5 0.45 0.55 0.543 150 83 23.1 0.56 0.54 0.664 150 83 19.4 0.55 0.57 0.555 150 83 19.3 0.54 0.57 0.55

0

10

20

30

40

50

1 10 100 1000

T flu

id, m

ean

(°C

)

Elapsed time (hours)

outinfff TTCVQ

1

ln4

tddT

LQ

Analysis: Numerical vs Experimental

38

Analysis: Heat Balance

39

Conservation of Energy

Injected Heat

Heat stored (Claesson and Hellstrom 1981)

Scalability of SBTES Systems

40

T T T

Distance from center



Single borehole

Smallest‐scale array

Full‐scale array

End of heat

injection period

End of rest period

Scalability of SBTES Systems: Arrays Considered

41


42

1011121314151617181920

0 5 10 15 20 25 30

Soil

tem

pera

ture

(°C

)

Distance (m)

InjectionRestInitial

q = 30 W/m2

90 daysλ = 1.0 W/mK

.

1011121314151617181920

0 5 10 15 20 25 30

Soil

tem

pera

ture

(°C

)

Distance (m)


q = 30 W/m2

90 daysSpacing: 2.5 mλ = 1.0 W/mK

.

1011121314151617181920

0 5 10 15 20 25 30

Soil

tem

pera

ture

(°C

)

Distance (m)


q = 30 W/m2

90 daysSpacing: 2.5 mλ =1.0 W/mK

.

Array 1

Array 3

Array 2


43

0

1

2

3

4

5

6

7

0.50 0.75 1.00 1.25 1.50 1.75 2.00 2.25 2.50

Hea

t sto

red

afte

r inj

ectio

n (G

J)

λ (W/mK)

13 BHs5 BHsSingle BH

q = 30 W/m2

90 days injection

.

0

1

2

3

4

5

6

7

0.50 0.75 1.00 1.25 1.50 1.75 2.00 2.25 2.50

Hea

t sto

red

afte

r res

ting

(GJ)

λ (W/mK)

13 BHs5 BHsSingle BH

q= 30 W/m2

90 days resting

.

Storage After Heat Injection Storage After Resting Period


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0

5

10

15

20

25

20 30 40 50 60 70 80 90 100

Hea

t sto

red

(GJ)

qin (W/m2)

13BHs90 daysSpacing: 2.5 mλ = 1.0 W/mK

New SBTES Site at UCSD

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Planned Borehole Array

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1.5 m

Borehole heat exchanger

Thermistor string

0.75 m

3.0 m

1.5 m

6.0 m

0.75 m

Planned Borehole Array

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Final Comments

• SBTES systems can effectively store heat in the subsurface

• Efficiency of heat extraction is low, but the heat source is renewable and nearly free

• Heat storage is best in soils with low thermal conductivity and with low permeability (low convection)

• Closer spacings (1‐2 m) will result in the greatest concentration of heat

• Sufficient boreholes are required in an array to retain elevated ground temperatures after a resting period

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Thermal Energy Storage in Borehole Heat Exchanger Arrays · Thermal Energy Storage in Borehole Heat...

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Transcript of Thermal Energy Storage in Borehole Heat Exchanger Arrays · Thermal Energy Storage in Borehole Heat...