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
Folie 5 > PCM-Graphite Composites for High Temperature Thermal Energy Storage > Thomas Bauer
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
Folie 6 > PCM-Graphite Composites for High Temperature Thermal Energy Storage > Thomas Bauer
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
Folie 7 > PCM-Graphite Composites for High Temperature Thermal Energy Storage > Thomas Bauer
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
Folie 8 > PCM-Graphite Composites for High Temperature Thermal Energy Storage > Thomas Bauer
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
Folie 9 > PCM-Graphite Composites for High Temperature Thermal Energy Storage > Thomas Bauer
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%
Folie 10 > PCM-Graphite Composites for High Temperature Thermal Energy Storage > Thomas Bauer
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
Folie 17 > PCM-Graphite Composites for High Temperature Thermal Energy Storage > Thomas Bauer
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
Folie 18 > PCM-Graphite Composites for High Temperature Thermal Energy Storage > Thomas Bauer
Composite made of the infiltrated MatrixSEM Pictures 5wt% graphite, density
of the matrix 0,07 g/cm3Before thermal cycling After 10 thermal cycles
PorosityPorosity
Folie 19 > PCM-Graphite Composites for High Temperature Thermal Energy Storage > Thomas Bauer
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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