PCM-Graphite Composites for High Temperature Thermal Energy Storage

Transcription

1 PCM-Graphite Composites for High Temperature Thermal Energy Storage Thomas Bauer DLR German Aerospace Center Institute of Technical Thermodynamics Stuttgart Co-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 1 > PCM-Graphite Composites for High Temperature Thermal Energy Storage > Thomas Bauer

2 Contents Presentation is directed to fundamental aspects and correlation between material properties and manufacturing routes of PCMgraphite composites Part of the work is funded by the Federal Ministry of Education and Research (BMBF) under contract 03SF0307A-F (LWSNet) Introduction Characterization of alkali nitrate salts Preparation and characterization of PCM-graphite composites Conclusions Folie 2 > PCM-Graphite Composites for High Temperature Thermal Energy Storage > Thomas Bauer

3 Applications for PCM-Graphite Composites 320 C 130 C High temperature applications Improved energy efficiency (Industrial process heat, CHP) Renewable energy (e.g. Solar thermal power) Low temperature applications (e.g. Comfort Heating) Folie 3 > PCM-Graphite Composites for High Temperature Thermal Energy Storage > Thomas Bauer

4 Materials for High Temperature Thermal Storage Sensible heat storage materials Phase change materials (PCM) Single or 2-phase heat transfer fluid Virtually isothermal operation Heat transfer limitations Increased surface area Composites Alkali nitrate salts as PCM Graphite to enhance heat transfer Enthalpy Temperature Folie 4 > PCM-Graphite Composites for High Temperature Thermal Energy Storage > Thomas Bauer

5 Selected Alkali Nitrate/Nitrites as PCMs LiNO Enthalpy [J/g] KNO 3 -LiNO 3 KNO 3 -NaNO 2 -NaNO 3 LiNO 3 -NaNO 3 NaNO 2 KNO 3 -NaNO 3 NaNO 3 KNO Temperature [ C] Folie 5 > PCM-Graphite Composites for High Temperature Thermal Energy Storage > Thomas Bauer

6 KNO3-NaNO3 Phase Diagram 350 Temperature [ C] Solid+Liquid Liquid Solid Solid+Liquid Eutectic composition: - Melting temperature 220 C - Enthalpy 100 J/g ,1 0,2 0,3 0,4 0,5 0,6 0,7 0,8 0,9 1 Mole fraction NaNO 3 [1] Experimental DSC data by: Kramer, C.M., Wilson, C.J. (1980) Thermochimica Acta, 42, S Folie 6 > PCM-Graphite Composites for High Temperature Thermal Energy Storage > Thomas Bauer

7 Heat capacity [J/(g.K)] Eutectic KNO3-NaNO3 Heat Capacity c p Differential Scanning Calorimeter (DSC) 2 1,9 1,8 1,7 1,6 1,5 1,4 1,3 1,2 1,1 1 Voskresenskaya Voskresenskaya liquid Kamimoto Carling Tufeu Janz 1982 Nguyen-Duy DLR T m Error bar 10% Temperature [ C] Folie 7 > PCM-Graphite Composites for High Temperature Thermal Energy Storage > Thomas Bauer

8 Thermal diffusivity [mm2/s] NaNO3 Thermal Diffusivity α Laserflash 0,5 0,45 0,4 0,35 0,3 0,25 0,2 0,15 Error bar 5% Gustafsson Tufeu (exp.) Tufeu (comp.) Kobayasi Odawara Ohta Zhang Kato DLR 1 DLR 2 DLR 3 0, Temperature [ C] T m Folie 8 > PCM-Graphite Composites for High Temperature Thermal Energy Storage > Thomas Bauer

9 Alkali Nitrate Salts Thermal Conductivity 1,2 k = α ρ c p Thermal conductivity [W/(mK)] 1 0,8 0,6 0,4 0,2 0 NaNO3 McDonald NaNO3 Bloom NaNO3 White NaNO3 NaNO3 DLR McDonald KNO3 NaNO3 McDonald Bloom KNO3 Yoshida KNO3 NaNO3 White White K-NaNO3(eu) McDonald NaNO3 DLR KNO3(40Gew%)-NaNO3 Zavoico KNO3(45Mol%)-NaNO3 Bloom K-NaNO3(eu) Tufeu K-NaNO3(eu) Fossnas Error bar 5% Temperature [ C] T m Folie 9 > PCM-Graphite Composites for High Temperature Thermal Energy Storage > Thomas Bauer

10 Number of Pipes vs. PCM Thermal Conductivity Pipe number can be reduced significantly for PCM conductivities from 5 to 15 W/mK Folie 10 > PCM-Graphite Composites for High Temperature Thermal Energy Storage > Thomas Bauer

11 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 Folie 11 > PCM-Graphite Composites for High Temperature Thermal Energy Storage > Thomas Bauer

12 Characterization of the Composites, Position and Orientation of the Laserflash Discs 12,7 mm 25,4 mm Folie 12 > PCM-Graphite Composites for High Temperature Thermal Energy Storage > Thomas Bauer

13 Thermal conductivity [W/mK] Natural Graphite Composite Thermal conductivity wt% graphite Through-plane, Pos. 1 Through-plane, Pos. 2 Through-plane, Pos. 3 Through-plane, Pos. A, cycled Through-plane, Pos. B, cycled K-NaNO3(eu) NaNO3 KNO3 0 T m Temperature [ C] Folie 13 > PCM-Graphite Composites for High Temperature Thermal Energy Storage > Thomas Bauer

14 Natural Graphite Composite SEM Pictures 30wt% graphite Before thermal cycling After 10 thermal cycles Graphite KNO 3 -NaNO 3 Folie 14 > PCM-Graphite Composites for High Temperature Thermal Energy Storage > Thomas Bauer

15 Thermal conductivity [W/mK] Compressed Composite Thermal conductivity T m 30%, Orientation 1, Pos. 1 30%, Orientation 1, Pos. 2 30%, Orientation 2, Pos. 1 30%, Orientation 2, Pos. 2 20%, Orient. 1, Pos. with little salt 20%, Orient. 1, Pos. with plenty salt 20%, Orientation 1, Pos. 1 20%, Orientation 1, Pos. 2 20%, Orientation 1, Pos. A, cycled 20%, Orientation 1, Pos. B, cycled 10%, Orientation 1 10%, Orientation 1, cycled K-NaNO3(eu) NaNO3 KNO Temperature [ C] Folie 15 > PCM-Graphite Composites for High Temperature Thermal Energy Storage > Thomas Bauer

16 Compressed Composite SEM Pictures Before thermal cycling 10wt% graphite After 10 thermal cycles KNO 3 NaNO 3 prilled GFG500 Folie 16 > PCM-Graphite Composites for High Temperature Thermal Energy Storage > Thomas Bauer

17 Composite made of the infiltrated Matrix Thermal conductivity 5wt% graphite, density of the matrix 0,07 g/cm3 Thermal conductivity [W/mK] T m In-plane, Pos. 1 In-plane, Pos. 2 In-plane, Pos. 3 Through-plane, Pos. 1 Through-plane, Pos. 2 Through-plane, Pos. 3 Through-plane, Pos. A, cycled Through-plane, Pos. B, cycled Compression Through-plane, Pos. C, cycled K-NaNO3(eu) NaNO3 KNO3 Source: Temperature [ C] Folie 17 > PCM-Graphite Composites for High Temperature Thermal Energy Storage > Thomas Bauer

18 Composite made of the infiltrated Matrix SEM Pictures 5wt% graphite, density of the matrix 0,07 g/cm3 Before thermal cycling After 10 thermal cycles Porosity Porosity Folie 18 > PCM-Graphite Composites for High Temperature Thermal Energy Storage > Thomas Bauer

19 Conclusions PCM storage attractive for 2-phase heat transfer fluid systems Single salt properties mostly in agreement with literature values With PCM-Graphite composites suitable effective thermal conductivities can be achieved SEM and Laserflash examinations indicate thermally stable graphite structure in the composite Thank You Of the investigated manufacturing routes, the infiltration route has the potential of high effective conductivities with a low graphite fraction for Your Attention Infiltration route needs still further R&D to realize composite materials with sufficient PCM content Folie 19 > PCM-Graphite Composites for High Temperature Thermal Energy Storage > Thomas Bauer