Thermal Conductivity of High Temperature Multicomponent Materials with Phase Change

Transcription

1 Thermal Conductivity of High Temperature Multicomponent Materials with Phase Change 1 Kátia S. do Couto Aktay*, Rainer Tamme, Hans Müller-Steinhagen German Aerospace Center DLR, Institute of Technical Thermodynamics, Pfaffenwaldring 38-40, Stuttgart, Germany * katia.aktay@dlr.de This work focuses on the investigation of the effective thermal conductivity (λ eff ) of heterogeneous materials consisting of a phase change material (PCM) and expanded graphite (EG). These composites may be employed in latent heat storage systems, where a PCM stores energy by being heated to a temperature higher than its melting point (T m ), and releases it during solidification. For the determination of λ eff the steady-state comparative method was used and modified to measure composite samples at temperatures above T m. Results were compared with the thermal conductivity of the pure PCMs and a significant enhancement could be observed. The dependence of λ eff on temperature was examined as well as the influence of material microstructure on the improvement of λ eff. 1 Introduction Composite materials are very important in many engineering areas, because they can be designed to exhibit the best characteristics of their individual constituents. They are ideally suited in modern technologies that require materials with an unusual combination of properties that cannot be met by conventional single-phase materials. Their properties are denoted effective properties (P eff ) and are defined by a linear relationship between an average of a generalized local flux (F) and an average of a generalized local or applied intensity (G) (Torquato 2002): F P eff G For conduction problems, F represents the average local heat flux and G the average local temperature gradient. The effective property is the effective thermal conductivity λ eff and depends on the phase properties and microstructural information, including the phase volume fractions, orientations, sizes, shapes and spatial distribution of the phases and their connectivity (Torquato 2002). The materials investigated in this work were composites consisting of a phase change material (PCM) and expanded graphite (EG) to be employed in high temperature latent thermal energy

2 2 storage systems. These systems operate typically at temperatures between 100 and 400 C. They require storage materials with thermal conductivity preferably in the range 5-20W/mK and additionally high volumetric heat capacity to guarantee high discharging power and high storage density. The storage principle relies on the usage of a PCM, which undergoes a phase transition, usually solid-liquid, and energy is stored upon melting and released during solidification. In comparison with storage of sensible heat, storage of latent heat has the advantage of a high storage density, due to the enthalpy of fusion, and because phase transition occurs at almost constant temperature, the storage system can be operated within small temperature variation. Latent thermal energy storage (LTES) finds application in solar energy heating and cooling, thermal protection of electronic devices, transport of temperature sensitive materials, building materials, etc. (Zalba et al. 2003, Farid et al. 2004). At temperatures above 100 C applications are mainly solar thermal power generation and utilization of waste heat (Tamme et al. 2003). Materials used as PCM must have an appropriate melting temperature, according to the operation temperature of the storage system, and have preferably high latent heat of fusion. For hightemperature LTES, potential PCMs are mainly inorganic anhydrous salts, which have very low thermal conductivity (below 1W/m.K) and consequently cannot fulfill the abovementioned requirements. It was therefore proposed to use expanded graphite, in form of flakes and as a porous matrix, to increase the thermal conductivity of the PCMs. The prepared PCM/graphite-composites are denominated Composite Latent Storage Materials (CLSM). They are expected to have their thermal conductivity basically determined by the graphite content and/or structure and the adequate melting temperature and high storage capacity of the PCM (do Couto Aktay et al. 2005). The present work reports on the measurement of the effective thermal conductivity (λ eff ) of such composites in a temperature range below and above the melting temperature of their PCMs. 2 Materials Composites were fabricated by infiltration and compression using PCM and expanded graphite, with a PCM content of 85weigth%. Two alkali nitrate salt systems were employed as PCM. Their composition and melting temperatures are summarized in Table 1.

3 Table 1: PCMs used for manufacturing of composites (do Couto Aktay et al. 2005). Composition PCM 1 PCM 2 68weight% KNO 3, 32weight% LiNO 3 54weight% KNO 3 46weight% NaNO 3 Melting Point 133 C 220 C 3 Expanded graphite is manufactured from graphite flakes and presents a vermicular structure as shown in Figure 1. It has a very low bulk density and can be compacted into stable graphite sheets without addition of binding materials, to form high porous graphite matrices with anisotropic thermal properties. For fabrication of compressed samples, ground expanded graphite flakes are more adequate; their structure is also shown in Figure 1. Detailed preparation of samples is described in do Couto Aktay et al. (2005). Fig. 1: Scanning electron micrographs of vermicular expanded graphite (left) and ground expanded graphite flakes (right). 3 Microstructure 3.1 Infiltrated Composites Composites fabricated by infiltrating molten PCM 1 into a porous expanded graphite matrix had a graphite content of ca volume%, 65-78volume% PCM and some residual porosity. Because of the layered structure of the matrix, these composites can be idealized as simple laminates and the treatment presented in Torquato (2002) can be applied for predicting λ eff. Laminates consist of alternating layers of phases 1 and 2, graphite and PCM respectively, of random thickness, with volume fractions φ 1 and φ 2 and thermal conductivities λ 1 >>λ 2. These composites are macroscopically anisotropic and the effective thermal conductivities along the principal axis in the laminates, as depicted in Figure 2, are given by:

4 4 λ ( a b plane) = λ1φ1 + λ 2φ 2 the arithmetic average of the phase conductivities and the harmonic average of the phases. λ λ 1 2 λ ( c axis) = λ1φ1 + λ 2φ2 λ1φ2 + λ 2ϕ1 Fig. 2: Idealized laminate composite with alternating layers of phases of random thickness (Torquato 2002). The arithmetic average corresponds to conduction along the slabs (a-b plane), where the phases are connected and the heat flow is relatively unimpeded, even if one phase is a poor conductor. The harmonic average, on the other hand, corresponds to conduction perpendicular to the slabs (c-axis) and the heat flow is relatively impeded in this direction (Torquato 2002). Fig. 3: Theoretical effective thermal conductivity along the slabs of a laminate for different phase thermal conductivity ratios.

5 5 Because λ 1 is not known, the enhancement of λ eff due to addition of graphite could be estimated considering the simple laminate model and different λ 1 /λ 2 ratios. Figures 3 and 4 show that laminates with φ 2 between 0,6 and 0,8 have an enhanced effective conductivity along the slabs of ca. 20 to 40% of the graphite conductivity and perpendicular to the slabs below 20% of λ 1. Fig. 4: Theoretical effective thermal conductivity perpendicular to the slabs of a laminate for different phase thermal conductivity ratios. 3.2 Cold Compressed Composites These composites were fabricated by compressing PCM 2 and ground expanded graphite under room temperature, consisting of ca. 80volume% PCM. Scanning electron micrographs in Figure 5 depict the distribution of the phases in the material. On the left micrograph, the material after manufacturing is shown, where the white, light grey and dark phases represent sodium nitrate, potassium nitrate and graphite, respectively. After cycling (on the right micrograph), potassium und sodium nitrate do not appear as separate phases, PCM and graphite are distributed differently and the material presents some porous and solid regions. The composite has interpenetrating phases and exhibits a significant change in microstructure after thermal cycling. Cold compressed composites are therefore expected to behave macroscopically isotropic.

6 6 Fig. 5: Scanning electron micrographs of material after fabrication (left) and after being thermally cycled (right). The aforementioned laminate model can be considered as upper and lower bounds for macroscopically isotropic composites (Torquato 2002). The arithmetic average overestimates λ eff and represents an upper bound, while the harmonic average underestimates λ eff and is a lower bound on the effective conductivity. The enhancement due to graphite can be thus predicted to lie between the limits determined by the simple laminate model, given in Figures 3 and 4 for different phase conductivity rations. 3.3 Warm Compressed Composites These composites differed from the cold compressed ones only in that the fabrication process was carried out at 180 C. They consisted of ca. 80volume% PCM 2. The distribution of the phases in the material is depicted in Figure 6. The left micrograph shows that the phases are less homogeneous in the composite after manufacturing. After thermal cycling (right micrograph), PCM exists as a single phase and the microstructure changes; some local porosity can be observed. It is expected that also these composites have also an isotropic thermal behavior, but λ eff may be lower than for cold compressed samples due to the poorer interconnectivity of graphite in the material.

7 7 Fig. 6: Scanning electron micrographs of material after fabrication (left) and after being thermally cycled (right). λ eff can be also predicted considering the upper and lower bounds given by the laminate model in Figures 3 and 4. 4 Measurement of Thermal Conductivity 4.1 Requirements The measurement of the thermal conductivity of composites comprising PCM and graphite imposes some practical problems. Specimens should be measured in their dimensions without need of machining so that an integral behavior of the material, including local volume fraction fluctuations, can be considered. For thermal storage, changes in thermal conductivity of the storage materials due to melting/solidification cycles must also be known. Regarding composites with partial phase change, their thermal behavior must be understood for the whole temperature range of operation of the storage, which can be divided into three stages with the following characteristics: (1) below melting point T m, where two solid phases exist; (2) at T m, where the graphite content does not change phase and the PCM exists in solid and liquid phase and (3) above melting point, where one solid and one liquid phase exists. In this last step, it must be also considered that the PCM may not be entirely in the liquid phase and that the transfer of heat in the composite may be good enough not to allow that the entire PCM content to melt, which would result in loss of storage capacity. Furthermore specimens do not have the same dimensions and surface characteristics, and may also have different radiative properties; some of them may become porous and others may lose structural integrity during the measurement, demanding the usage of a container. To understand these various effects and fulfill all discussed requirements one single method may not be sufficient and different alternatives may be used for determining λ eff and evaluate the different effects on it.

8 8 4.2 Measurement Methods Many techniques exist for measuring the thermal conductivity of solid materials. They apply either a time-dependent temperature response of the material for calculating its thermal conductivity or the measurement of the temperature difference under steady-state conditions. Three methods were available for the present work and will be discussed briefly concerning applicability to the specimens under investigation Transient Plane Source Technique (TPS) The TPS Technique, also known as Hot Disk, is described by Gustafsson (1991). It is a transient method based on an electrically isolated plane resistive element used as heat source and temperature sensor, which is sandwiched between two specimen halves. A constant current is applied to the specimen and the transient temperature increase in the sensor is recorded, from which both thermal conductivity and thermal diffusivity can be evaluated. Considering our composite materials thi technique has the major advantages in comparison with other transient methods that specimens with different dimensions can be measured and both the thermal conductivity and the thermal diffusivity are evaluated simultaneously, without need of additional accurate sample properties. Use of a container for high temperature measurements implies some practical restrictions regarding adaptability to the Hot Disk setup. Measurements above the melting temperature of PCM 2 face two additional limitations: one imposed by the contact of molten salt with the high temperature Mica sensor and if using Kapton sensors, the one imposed by the thermal stability of the Kapton foil Laser Flash Technique The Laser Flash technique, first described by Parker et al. (1961), is also a transient technique whereby the front face of a sample is heated by a short laser pulse, eliminating the problem of the thermal contact resistance. The temperature rise on the rear surface of the specimen is measured over time and used to calculate the thermal diffusivity. Its advantage relies in the non-contact measurement principle, where no limitation due to the sensing element exists, and on the possibility of measuring our specimens above the melting temperature of the PCMs. The method has the disadvantage of requiring additional accurate data for the calculation of the thermal conductivity of the specimens. The required very small specimens represent another major limitation, because thermal cycle effects cannot be investigated with the entire composite sample.

9 Steady-State Comparative Method This method is described by Tye (1992) and uses the comparison of parameters of an unknown specimen with those of materials of known properties. It consists basically of a specimen joined directly to or sandwiched between one or two reference materials and surrounded by a guard cylinder. A temperature gradient is established along the test tack and longitudinal heat flow is assured or maximized by adjustment of either the temperature gradient in, or the isothermal temperature of, the guard cylinder. Concerning the present composites, this method has the advantage of being adaptable to the different specimen dimensions and shapes. Disadvantages are mainly due to the interfacial thermal resistance between reference samples and composite specimens with different surface finishing, long measurement time and lower accuracy in comparison with more sophisticated methods. 5 Experimental Apparatus For the determination of λ eff of the composite materials the steady-state comparative method was chosen. For the purpose of this study the method allows measurement with available sample dimensions and composites can be measured with suitable modifications over melting temperature. Because materials do not need to be machined for the measurement, it is possible to cycle the samples in an external oven and measure them again. The advantage of simple mathematical evaluation of results avoids additional uncertainties of other not well-known properties of the composites. To fulfill the previously mentioned requirements a compromise must be found between the acceptable accuracy that can be reached with the materials studied and the practical attempts. On account of this the comparative method seems to be the most adequate for an initial understanding of transfer of heat in the PCM/graphite-composites. The designed apparatus consisted of a stack of cylindrical samples, as depicted schematically in Fig. 7. The specimen of unknown thermal conductivity was placed between two similar samples of reference material. Above the upper reference sample heat was generated in pencil heaters fitted in a copper sample. The bottom reference sample was placed on a water-cooled cold plate. The sample stack was surrounded by fiber glass insulating material, 65mm thick, and with thermal conductivity λ I of 0,034 and 0,042W/m.K at 20 and 100 C, respectively. It was finally enclosed within a cylindrical metal guard heated by a three-zone controlled heater.

10 10 Fig. 7: Scheme of comparative apparatus. Considering the wide range of thermal conductivities, from less than 1W/m.K for the pure PCMs to 30W/m.K in radial direction for the porous graphite matrix, stainless steel AISI321 (DIN ), with thermal conductivity λ R around 15W/m.K (at room temperature) was chosen as a suitable reference material. λ R was obtained from data published by Perovič et al. (1995/1996) and is plotted as a function of temperature in Figure 8. Specimens were 40mm in diameter with length varying between 10 and 20mm and therefore top and bottom reference samples were 30mm long. Fig. 8: Thermal conductivity of reference material AISI 321 (Perovič et al. 1995/1996).

11 11 Temperature was measured by fine gauge metal sheathed chromel/alumel thermocouples with an overall diameter of 0,25mm using Agilent 34970A Digitalmultimeter. In both reference samples and specimen three thermocouples were placed at equidistant positions. They were fitted tightly into small holes, 0,30mm in diameter and 3mm deep, drilled longitudinally in the materials. For the evaluation of λ eff, steady-state condition was reached for a temperature drift per hour smaller than the thermocouple uncertainty. λ eff was calculated considering the temperature differences measured in the stack and an average apparent heat flux: λ eff 1 = 2 λ TR ΔT ΔL TR TR + λ BR ΔT ΔL BR BR ΔL ΔT where ΔT and ΔL are the temperature difference and the distance between thermocouples, respectively. The subscripts TR, BR and S stand for top reference sample, bottom reference sample and composite specimen. For measurements at higher temperatures the apparatus was modified following the test cell idea reported by Tye et al. (1977). A reference sample was fabricated from stainless steel AISI321 with a cavity to be used to contain the specimen. The dimensions of the reference sample were 44mm in diameter and 40mm high including the cavity 42mm in diameter and 10mm high. Thermocouples were placed at equidistant position in the wall of the cavity and in equidistant holes along the reference material. In order to check the apparatus a graphite specimen, previously measured with the steady-state comparative method standardized in the Norm DIN51908 was tested. S S 6 Results and Discussion Measurements with the described apparatus were made at successive increasing temperatures, from room temperature up to some 50 C above melting point (T m ). Measurements showed that an isothermal guard heating with a temperature slightly above the specimen temperature enabled measurement with data deviation below 1%. Deviation between apparent heat fluxes were kept below 20%. The graphite sample used for testing the apparatus showed a deviation of 12% from the value previously measured with the method described in the Norm DIN

12 12 Table 2: Comparison of thermal conductivity of graphite sample measured according to DIN and by the comparative apparatus. DIN Comparative Apparatus deviation 93W/m.K (20 C) 82W/m.K (30 C) -12% Measured specimens are summarized in Table 3. Table 3: Composites measured with the comparative apparatus. Composite No. Phase Change Material Fabrication Process Thermal Cycling Specimen Length 1 PCM 1 infiltration - 20mm 2 PCM 2 cold compression - 10mm 3 PCM 2 cold compression 6 10mm 4 PCM 2 warm compression - 10mm 5 PCM 2 warm compression 2 10mm Experimental results are presented in Figures 9 and 10. The values of the thermal conductivity of the pure alkali nitrates used for comparison were from literature. Thermal conductivity of lithium nitrate, sodium nitrate and potassium nitrate in the solid phase are published in Tye et al. (1977), Yoshida and Sawada (1960), White and Davis (1967), respectively. Data for all nitrates above melting point are from McDonald and Davis (1970). Composite 1, consisting of PCM 1 infiltrated in the porous expanded graphite matrix, was measured perpendicularly to its layers. Because of its anisotropy the effective conductivity measured corresponds to the axial effective thermal conductivity of the material. For comparison, the thermal conductivity of PCM 1 (λ PCM 1 ) was considered around 1W/m.K (similar to that of the component in higher content, i.e. KNO 3 ) in the temperature range ±50 C from T m (approximate operating temperature of a latent thermal storage unit). Results show that λ eff of Composite 1 is approximately five times higher than λ PCM 1 and that λ eff is relatively stable over the temperature, with a decrease of about 11% above T m. Assuming a phase conductivity ratio λ 1 /λ 2 of 50, λ eff represents 10% of the conductivity of the more conductive graphite phase. Comparing with predictions using the idealized simple laminate model, our experimental results lie above the estimated λ eff /λ 1 (Figure11).

13 13 Fig. 9: Axial effective thermal conductivity of the composite material consisting of PCM1 versus temperature (for comparison the thermal conductivity of pure lithium nitrate and potassium nitrate). Measurements on Composites 2 and 4 were made at temperatures where the PCM was in the solid state. The samples had a microstructure similar to that depicted previously in the left micrographs in Figures 5 and 6, i.e. they represent the composites after manufacturing. Composites 3 and 5 were six and two times thermally cycled, respectively, before measurement. The thermal conductivity of PCM 2 (λ PCM 2 ) was considered for comparison between 0,5 and 1W/m.K for the relevant temperature range around T m. λ eff of composites is significantly higher than that of the single PCMs, but decreases with increasing temperature. This confirms the importance of measurements at temperatures around T m and if values obtained at room temperature are used for design purposes, λ eff is consequently overestimated. The influence of thermal cycling on cold compressed composites is stronger than on warm compressed composites. This can be explained by the changes in microstructure. Assuming a phase conductivity ratio λ 1 /λ 2 of 50, λ eff of cycled composites at the temperature range around T m represents 5-6% of λ 2. Our experimental results lie between predicted upper and lower bounds given by the simple laminate model (Figure11).

14 14 Fig. 10: Effective thermal conductivity of composites consisting of PCM 2 versus temperature (for comparison the thermal conductivity of pure sodium nitrate and potassium nitrate). Fig. 11: Comparison between results and upper and lower bounds assuming λ 1 /λ 2 = 50.

15 7 Conclusions and Outlook 15 The comparative method was chosen for the determination of the effective thermal conductivity of PCM/graphite-composites, also denoted Composite Latent Storage Materials. The use of expanded graphite as flakes and porous matrix and the different composite manufacturing processes resulted in materials with different microstructures and thermal properties. With the comparative apparatus designed for this study, the effective conductivity of composites could be measured over the melting point of their PCMs. In comparison with the pure PCMs composites have an effective conductivity three to five times higher. Current research activities focus on the measurement of λ eff of infiltrated composites in radial direction over the melting temperature with the comparative method. Further investigation is planned, in which the laser-flash technique will be used. Due to faster measurement times more samples may be studied. Lack of data about the thermal conductivity of the binary alkali nitrate systems also motivates their measurement. Acknowledgements We are thankful to the Bavarian Center for Applied Energy Research (ZAE Würzburg) for the measurement of the thermal conductivity of the fiber glass insulation material. We also thank SGL Technologies for supplying graphite samples. References do Couto Aktay K S, Tamme R and H Müller-Steinhagen, 2005, in Proceedings of the 2 nd Phase Change Material and Slurry Conference, Yverdon-les-Bains, June pp Farid M M, Khudhair A M, Razack S A K and Al-Hallaj S, 2004, Energy Conversion and Management McDonald, J and H T Davis, 1970, J. Phys. Chem. 74 pp Perovič N L, Maglić, K D, Stanimirović and Vuković, G S, 1995/1996, High Temp. - High Press. 27/28 57 Tamme R, Steinmann W-D and Laing D, 2003, in Proceedings of the 9 th International Conference on Thermal Energy Storage Futurestock 2003, Warsaw, 1-4 September 2003 pp Torquato S, Random Heterogeneous Materials Microstructure and Macroscopic Properties (New-York: Springer-Verlag, 2002)

16 16 Tye R P, 1992 in Compendium of Thermophysical Property Measurement Methods Vol. 1, Eds. K D Maglić, A Cezairliyan and V E Peletsky (New York: Kluwer Academic / Plenum Publishers) pp Tye R P, Desjarlais A O and Bourne J G, 1977, in Proceedings of the 7 th Symposium on Thermophysical Properties ASME, pp White and Davis, 1967, J. Chem. Phys Yoshida and Sawada, 1960, J. Phys. Soc. Japan 15, pp Zalba B, Marín, J, Cabeza, L F and Mehling H, 2003, Applied Thermal Engineering