Folie 1 > PCM-Graphite Composites for High Temperature Thermal Energy Storage > Thomas Bauer

PCM-Graphite Composites for High Temperature Thermal Energy Storage

Thomas BauerDLR – German Aerospace Center

Institute of Technical Thermodynamics – StuttgartCo-Authors: Rainer Tamme, DLR; Martin Christ, Oswin Öttinger, SGL Technologies GmbH

The Tenth International Conference on Thermal Energy Storage, Atlantic City, 31. May – 2. June 2006

Folie 2 > PCM-Graphite Composites for High Temperature Thermal Energy Storage > Thomas Bauer

Contents

Presentation is directed to fundamental aspects and correlation between material properties and manufacturing routes of PCM-graphite compositesPart of the work is funded by the Federal Ministry of Education and Research (BMBF) under contract 03SF0307A-F (LWSNet)

IntroductionCharacterization of alkali nitrate saltsPreparation and characterization of PCM-graphite compositesConclusions

Folie 3 > PCM-Graphite Composites for High Temperature Thermal Energy Storage > Thomas Bauer

Applications for PCM-Graphite Composites

320°C

130°C

High temperature applicationsImproved energy efficiency (Industrial process heat, CHP)Renewable energy (e.g. Solar thermal power)

Low temperature applications (e.g. Comfort Heating)

Folie 4 > PCM-Graphite Composites for High Temperature Thermal Energy Storage > Thomas Bauer

Materials for High Temperature Thermal Storage

Sensible heat storage materials

Phase change materials (PCM)Single or 2-phase heat transfer fluidVirtually isothermal operationHeat transfer limitations

Increased surface areaComposites

Alkali nitrate salts as PCMGraphite to enhance heat transfer

Enth

alpy

Temperature

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Selected Alkali Nitrate/Nitrites as PCMs

0

50

100

150

200

250

300

350

400

100 150 200 250 300 350Temperature [°C]

Ent

halp

y [J

/g]

KNO3

NaNO3NaNO2

KNO3-NaNO3

LiNO3-NaNO3

KNO3-LiNO3

KNO3-NaNO2-NaNO3

LiNO3

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KNO3-NaNO3 – Phase Diagram

Experimental DSC data by: Kramer, C.M., Wilson, C.J. (1980) Thermochimica Acta, 42, S. 253-264.

0

50

100

150

200

250

300

350

0 0,1 0,2 0,3 0,4 0,5 0,6 0,7 0,8 0,9 1Mole fraction NaNO3 [1]

Tem

pera

ture

[°C

]

LiquidSolid+Liquid Solid+Liquid

SolidEutectic composition:- Melting temperature 220°C- Enthalpy 100 J/g

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Eutectic KNO3-NaNO3 – Heat Capacity cpDifferential Scanning Calorimeter (DSC)

1

1,1

1,2

1,3

1,4

1,5

1,6

1,7

1,8

1,9

2

0 50 100 150 200 250 300 350 400Temperature [°C]

Hea

t cap

acity

[J/(g

.K)]

VoskresenskayaVoskresenskaya liquidKamimotoCarlingTufeuJanz 1982Nguyen-DuyDLR

Error bar 10%

Tm

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0,1

0,15

0,2

0,25

0,3

0,35

0,4

0,45

0,5

0 100 200 300 400 500Temperature [°C]

Ther

mal

diff

usiv

ity [m

m2/

s]

GustafssonTufeu (exp.)Tufeu (comp.)KobayasiOdawaraOhtaZhangKatoDLR 1DLR 2DLR 3

Error bar 5%

NaNO3 – Thermal Diffusivity αLaserflash

Tm

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Alkali Nitrate Salts – Thermal Conductivity pck ⋅⋅= ρα

Tm0

0,2

0,4

0,6

0,8

1

1,2

0 100 200 300 400 500Temperature [°C]

Ther

mal

con

duct

ivity

[W/(m

K)]

NaNO3 McDonald

NaNO3 Bloom

NaNO3 White

NaNO3 DLR

Error bar 5%

0

0,2

0,4

0,6

0,8

1

1,2

0 100 200 300 400 500Temperature [°C]

Ther

mal

con

duct

ivity

[W/(m

K)]

NaNO3 McDonaldNaNO3 BloomNaNO3 WhiteNaNO3 DLRKNO3 McDonaldKNO3 YoshidaKNO3 WhiteK-NaNO3(eu) McDonaldKNO3(40Gew%)-NaNO3 ZavoicoKNO3(45Mol%)-NaNO3 BloomK-NaNO3(eu) TufeuK-NaNO3(eu) Fossnas

Error bar 5%

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Number of Pipes vs. PCM Thermal Conductivity

Pipe number can be reduced significantly for PCM conductivities from 5 to 15 W/mK

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Types of Graphite and Composite Preparation Route

1. Vacuum/pressure infiltration process using Natural graphite

2. Infiltration process using Compressed expanded graphite plates

3. Compression using Ground expanded graphite

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Characterization of the Composites, Position and Orientation of the Laserflash Discs

25,4 mm12,7 mm

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Natural Graphite Composite Thermal conductivity

0

1

2

3

4

5

6

7

8

0 50 100 150 200 250 300 350 400Temperature [°C]

Ther

mal

con

duct

ivity

[W/m

K]

Through-plane, Pos. 1Through-plane, Pos. 2Through-plane, Pos. 3Through-plane, Pos. A, cycledThrough-plane, Pos. B, cycledK-NaNO3(eu)NaNO3KNO3

Tm

30wt% graphite

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30wt% graphite

Natural Graphite Composite SEM Pictures

Before thermal cycling After 10 thermal cycles

Graphite

KNO3-NaNO3

Folie 15 > PCM-Graphite Composites for High Temperature Thermal Energy Storage > Thomas Bauer

0

5

10

15

20

25

30

35

40

0 100 200 300 400Temperature [°C]

Ther

mal

con

duct

ivity

[W/m

K]

30%, Orientation 1, Pos. 130%, Orientation 1, Pos. 230%, Orientation 2, Pos. 130%, Orientation 2, Pos. 220%, Orient. 1, Pos. with little salt20%, Orient. 1, Pos. with plenty salt20%, Orientation 1, Pos. 120%, Orientation 1, Pos. 220%, Orientation 1, Pos. A, cycled20%, Orientation 1, Pos. B, cycled10%, Orientation 110%, Orientation 1, cycledK-NaNO3(eu)NaNO3KNO3

Compressed Composite Thermal conductivity

Tm

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Compressed Composite SEM Pictures

10wt% graphiteBefore thermal cycling After 10 thermal cycles

NaNO3 prilled

KNO3

GFG500

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Composite made of the infiltrated MatrixThermal conductivity

0

1

2

3

4

5

0 50 100 150 200 250 300 350 400Temperature [°C]

Ther

mal

con

duct

ivity

[W/m

K]

In-plane, Pos. 1In-plane, Pos. 2In-plane, Pos. 3Through-plane, Pos. 1Through-plane, Pos. 2Through-plane, Pos. 3Through-plane, Pos. A, cycledThrough-plane, Pos. B, cycledThrough-plane, Pos. C, cycledK-NaNO3(eu)NaNO3KNO3

Tm

Compression

Source:

5wt% graphite, densityof the matrix 0,07 g/cm3

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Composite made of the infiltrated MatrixSEM Pictures 5wt% graphite, density

of the matrix 0,07 g/cm3Before thermal cycling After 10 thermal cycles

PorosityPorosity

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Conclusions

PCM storage attractive for 2-phase heat transfer fluid systemsSingle salt properties mostly in agreement with literature values

With PCM-Graphite composites suitable effective thermal conductivities can be achievedSEM and Laserflash examinations indicate thermally stable graphite structure in the composite

Of the investigated manufacturing routes, the infiltration route has the potential of high effective conductivities with a low graphite fractionInfiltration route needs still further R&D to realize composite materials with sufficient PCM content

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