Expanded graphite as thermal conductivity enhancer for paraffin wax being used in thermal energy storage systems

Gulfam Raza, Yongming Shi, Yuan Deng*

Beijing Key Laboratory of Special Functional Materials and Films, School of Materials Science & Engineering, Beihang University, Beijing, 100191, China

*Email: dengyuan@buaa.edu.cn

Abstract

Phase change material such as paraffin wax has very low thermal conductivity which leads to many defects upon its practical utilization in thermal energy storage system. In this paper, study on paraffin/expanded graphite (EG) composite has been carried out to enhance the thermal conductivity of pure paraffin (base material). EG (supporting material) with its worm-like structure has been introduced as thermal conductivity enhancer in paraffin/EG composite. A type of paraffin wax with melting temperature (Tm = 60-62 0C) has been investigated. Three samples of paraffin/EG composite have been prepared with weight percentage of EG as 5%, 10% and 15%. Self-absorption technique has been introduced by preparing the samples in a cylindrical vessel at temperature slightly more than melting temperature of paraffin for a certain period oftime until maximum incorporation of paraffin wax was achieved into the porous structure of expanded graphite. Thermal characteristics have been investigated by differential scanning calorimeter which showed no change in the melting temperature of base material, however aslight change in phase transition temperature was observed, and measured latent heat of the composite was a little lower than theoretical latent heat. Thermal conductivity of each sample ofparaffin/EG composite determined by laser flash method has been achieved 4 times, 6 times and 6.5 times higher than that of pure paraffin for 5%, 10% and 15% paraffin/EG composite, respectively, and as quantity of EG was increased, thermal conductivity of the composite got increased as well. Results obtained by scanning electron microscopy indicated uniform mingling of paraffin wax and expanded graphite in the composite. Form-stability has been confirmed through liquid leakage test which showed no leakage of paraffin from composite. This work isequally significant to be engaged in thermal energy storage systems, cooling of electronic devices and thermal management of batteries.

Keywords: Paraffin wax; Expanded graphite as thermal conductivity enhancer; Self-absorption phenomenon; Thermal energy storage

I. Introduction

Increasing thermal efficiency of thermal energy storage (TES) systems is the burning issue of the era. As applications of thermal energy storage systems are diverse with respect to various fields, so a robust system with appropriate materials is indispensable to make this concept effective and

possible [1-3]. Research delineated that Phase Change Materials (PCMs) are the most conducive for storing huge amount of thermal energy on behalf of their high natural latent heat values. Although not all PCMs but many of them have got unique qualities that fit the best for development of thermal energy storage media. PCMs have been best classified into organic, inorganic and eutectics whereas organic PCMs (especially paraffin) have been proved to be most promising as thermal energy storage materials owing to their excellent thermal stability, non-toxicity, non-corrosiveness and non-sub cooling facets. There are plenty of organic PCMs but paraffin have got the most charming characteristics such as they are non-toxic, inert and stable, less expensive, non-corrosive, come up with high latent heat, show little volume change and low vapor pressure on melting. However, low thermal conductivity and poor long-term stability of PCMs put forth a chief hindrance in using them for operating systems [4-7]. Three basic thermal energy storage methods have been investigated i.e. sensible heat, latent heat and chemical energy storage methods whereas latent heat storage method has been the most reliable and stable due its excellence of higher thermal energy storage density than others [4]. In order to minimize the major shortcoming of the PCMs i.e. low thermal conductivity, a number of additives have been utilized, and can be named as thermal conductivity enhancers (TCEs). TCEs have been classified as nano-additives, metal fins, metal foams and carbon foams/fibers [8]. A lot of research has been carried out since 2005 at nano-structured materials such as metallic nanoparticles (Ag, Cu & Al), metal oxides (CuO, TiO2, MgO & Al2O3) and carbon-based nanomaterials (graphene flakes, nano-platelets & nano-fibers) [9]. Investigation has been carried out at nanomagnetite/paraffin composite. Nanomagnetite was prepared by sol-gel method and added into paraffin by dispersion technique. Results indicated that, upon addition of 10 weight % nanomagnetite in pure paraffin, thermal conductivity got increased by 48 % and latent heat value got enhanced by 3.3 % than original values [10]. Analysis has been done by developing a composite of carbon foam, paraffin wax and multi wall carbon nanotubes for thermal management of electronic devices where carbon foam provided the base structure and multi wall carbon nanotubes were introduced as thermal conductivity enhancer. Three thermal management modules i.e. pure carbon foam module, carbon foam + paraffin wax module, and carbon foam + paraffin wax + multi wall carbon nanotubes module. Results showed that inclusion of multi wall carbon had a profound effect in improving thermal response of module by increasing its thermal conductivity [11]. Effect of paraffin and aluminium foam composite has been analyzed for thermal management of lithium-ion battery. Heat storage analysis has been carried out along X-direction, Y-direction and Z-direction. Results indicated that uniformity of temperature in pure paraffin has been improved in all directions upon addition of Al foam, and upon applying paraffin and aluminium foam composite in thermal management of Li-ion battery at 1 0Ccharging rate, paraffin/Al foam composite brought about 62.5 % drop in maximum temperature rise of the battery, hence paraffin/Al foam composite caused to provide cooling effect to Li-ion battery during discharging mode [12]. Simone et al. studied about paraffin wax and copper foam composite. Solid-liquid phase change modes of three paraffin waxes at different heat fluxes have been investigated. Low thermal conductivity of paraffin wax was found as the main drawback

which was solved upon addition of Cu foam, hence heat transfer capability of paraffin wax was improved alongwith achievement of uniform distribution of temperature inside of paraffin wax[7]. Expanded graphite (EG) powder is a porous material with micro-pores down to several microns, and has high thermal conductivity, worm-like structure, high specific surface area and larger volume. Expanded graphite is usually obtained from expandable graphite by subjecting it to high temperature for certain time [13]. Investigation has been done on tetradecanol and expanded graphite (EG) composite where expanded graphite (EG) served as an excellent thermal conductivity enhancer. EG was prepared from expandable graphite through microwave irradiation. Results indicated that thermal conductivity of pure tetradecanol has been increased from 0.433 W/m.K to 2.76 W/m.K upon addition of 7 weight % of EG, and 5.71 W/m.K upon 40 weight % of EG [14]. Expanded graphite has been added in RT100. XRD and FTIR spectra showed that EG had no chemical reaction with PCM which confirmed that PCM just got absorbed on EG. An average melting enthalpy and solidification enthalpy of RT100/EG composite has been reported as177.3 J/g and 173.3 J/g, respectively, which are slightly lower than calculated values. Maximum thermal conductivity of RT100/EG composite equal to 6.792 W/m.K was achieved at packing density of 671.7 kg/m3 [15]. Paraffin and expanded vermiculite composite has been studied. Expanded vermiculite has been used as supporting material to enhance thermal conductivity of paraffin, however thermal conductivity hasn’t been calculated. Latent heat of melting of paraffin/expanded vermiculite composite has been reported as 101.14 J/g [16]. Expanded perlite has been introduced in paraffin as an additive which provided effective latent heat, good form-stability and good thermal reliability to the pure paraffin. It has been reported through leakage test that expanded perlite don’t let the paraffin to be leaked out of its porous structure [17]. Two types of graphite i.e. waste graphite and timrex (SFG75) graphite, have been used as thermal conductivity enhancer in paraffin wax with melting temperature of 52 0C. Uniaxial compression method has been employed to prepare paraffin/graphite composite. Thermal conductivity of pure paraffin wax enhanced from 0.233 W/m.K to 0.906 W/m.K upon 20 weight % addition of timrex graphite. Timrex graphite has been reported to show better results than waste graphite [18]. Experimental and simulated studies have been carried out for paraffin/EG composite with investigation of optimal saturation sorption capacity of EG. It has been reported that composite with 94 weight % of paraffin and 6 weight % of EG showed thermal conductivity 20 times more than that of pure paraffin [13]. Composite of waste graphite and paraffin has been investigated numerically and experimentally. Increased thermal conductivity has been reported as 0.401 W/m.K upon 20 weight % addition of waste graphite, and latent heat has been reported as 114.899 J/g upon 20 weight % addition of waste graphite. Reduction of latent heat has been attributed to possible restriction of molecular thermal motion of paraffin due to presence of graphite matrix [19]. A lot of work has been done at paraffin/EG composites. First main issue is to come up with a kind of good mixing method by whichmaximum incorporation of PCM into pores of EG can be achieved, and the structure of EG should also be kept from eruption. Second main issue is to keep the latent heat from lowering down due to increment in the quantity of EG and reduction in the quantity of PCM. In this paper,

paraffin wax with melting temperature of 60-62 0C has been investigated. Paraffin wax with melting temperature of 60-62 0C in combination with EG has also been investigated by Jin et al. [20]. Latent heat capacity has been determined by means of differential scanning calorimeter(DSC). Thermal conductivity of pure paraffin and paraffin/EG unified composite has been testedthrough LFA method using laser thermal conductivity analyzer. It provided favorable results with enhancement of thermal conductivity of paraffin/EG thermal composite. Microstructural examination and integration of paraffin/EG composite have been done by scanning electron microscope (SEM). Additionally, form-stability has been confirmed through liquid leakage test by making discs through mold and dry pressing device.

II. Experimental

A. Material

Paraffin wax was purchased from company, Shanghai Huayong Limited, China. Thermophysical properties of paraffin wax have been given in the table 1.

Table 1. Thermophysical properties of paraffin waxMaterial Melting Temperature

(0C)Thermal Conductivity

(W/m.K)Thermal Diffusivity

(m2/s)Latent Heat

(kJ/kg) Paraffin

Wax 60-62 0.36101 1.49x10-7 188.96

B. Methodology and elaboration of paraffin/EG composite

A novel method has been adopted to prepare paraffin/EG composite by using a sealed cylindrical vessel. Blocks of paraffin were cut into fine pieces by cutting tool. Certain quantity of paraffin and EG was weighed through electronic balance. At the bottom of cylindrical vessel, spread a layer of EG, and upon it, spread a layer of paraffin. By following the same way, paraffin and EG were spread layer by layer in the cylindrical vessel. This cylindrical vessel was sealed and kept in electric oven at temperature 65 0C which was slightly more than melting temperature of the paraffin. Cylindrical vessel was taken out of electric oven after 12 hours. Sample was subjectedto electric heating with constant stirring by hand stirrer until EG became entirely wet due to incorporation of liquid paraffin into its fine pores. Method has been explained in Fig.1. This method has been adopted by considering following factors; maximum incorporation of paraffininto the pores of EG, keeping the loose structure of EG in pristine state while in composite,keeping the latent heat close to the original value of paraffin upon addition of EG and reduction of paraffin’s weight in the composite. Maximum incorporation of paraffin into pores of EG has been achieved by self-absorption phenomenon which was, firstly controlled by appropriate setting of electric oven’s temperature to a certain value so that paraffin could keep melting slowly, and secondly by provision of sufficient period of time which was almost 12 hours so that EG could get suffice time to impregnate with melted paraffin. Hence, three samples were prepared with mass fraction of EG as 5%, 10% and 15%, termed as S1, S2 and S3, respectively.

Fig.1. Schematic view for preparation of paraffin/EG thermal composite

III. Testing and characterization

A. Liquid leakage test and microscopy

Formability of self-absorbed paraffin/EG thermal composite has been analyzed through liquid leakage test. The test has been performed for each thermal composite sample containing 85 weight%, 90 weight% and 95 weight% paraffin wax. Three square-shaped blocks of three paraffin/EG thermal composites were obtained by using mold followed by dry pressing device.The mold has been compressed at 2 bar using dry pressing device. Square mold apparatus followed by dry pressing device gave square-shaped blocks of paraffin/EG thermal composites.Then, each block of paraffin/EG thermal composite was put upon the filter paper and subjected to heating at 70 0C through electric oven for 1 hour. Temperature was suitably selected so thatthe complete melting of paraffin could be achieved. Scanning electron microscopy (FEI Siron 200) has been used for microstructural analysis of expanded graphite and paraffin/EG thermal composite.

B. Thermal transport properties and LFA method

Thermal transport properties of paraffin/EG thermal composite such as thermal conductivity and thermal diffusivity has been measured using laser thermal conductivity analyzer (LFA 427)which made use of laser flash (LFA) method. Unlike transient plane source (hot disk, TPS)method, laser flash method gives thermal conductivity values indirectly. Additionally, laser thermal conductivity analyzer made use of thin disc sample with diameter of 12.7 mm and thickness of 1.5 mm~2 mm. Thin disc samples of paraffin/EG thermal composite were prepared through disc mold having diameter of 12.7 mm followed by dry pressing device at compression rate of 2 bar, and great care should be taken while applying pressure through dry press to obtain feasible thickness of thin discs. Hence, thermal diffusivity and specific heat capacity of

paraffin/EG thermal composite were directly determined at room temperature by using a singlethin disc of each sample. Results of each sample of paraffin/EG thermal composite obtained through laser thermal conductivity analyzer have been shown in table 2.

Table 2. Thermal transport properties of paraffin/EG composite by LFA methodSample # Paraffin/EG Mass % Specific Heat Cp (kJ/kg.K) Thermal Diffusivity (m2/s)

Pure 100% Paraffin 1.922 1.49 x10-7

S1 95% Paraffin+5% EG 1.849 9.23x10-7

S2 90% Paraffin+10% EG 1.838 1.39x10-6

S3 85% Paraffin+15%EG 2.105 1.28x10-6

C. Thermal energy storage propertiesThermal energy storage properties like latent heat and phase change temperature have been carefully determined through differential scanning calorimeter (METTLER TOLEDO DSC 1)which was operated under nitrogen atmosphere. 5~10 mg of each sample of paraffin/EG thermal composite was taken in a crucible, and programmed temperature was set including initial and final temperature at 20 0C and 100 0C, respectively with heating rate of 2 0C/minute. Latent heat values were calculated for pure paraffin and for each sample having 5%, 10% and 15% mass fraction of EG in paraffin/EG thermal composite.

IV. Results and discussion

A. Morphology analysis

Morphology of paraffin/EG thermal composite has been characterized through SEM, and results are depicted in fig.2. It can be observed from fig.2 that loose and porous structure of expanded graphite is its excellent quality because of which impregnation of paraffinic hydrocarbons becomes quite easy and stable. Fig.2 showed appropriate mingling and absorption of paraffin wax and expanded graphite. It can be seen that paraffin/EG thermal composite has achieved uniform and homogeneous microstructures, and self-absorption of paraffin into the pores of expanded graphite is quite clear.

Fig.2 (a) 95% paraffin/EG thermal composite, (b) 90% paraffin/EG thermal composite, (c) 85% paraffin/EG thermal composite

B. Formability analysis

Samples of paraffin/EG thermal composites were analyzed two times each after 30 minutes to note the commencement of leakage. No leakage was observed from paraffin/EG thermal composites except those composites which had concentration of EG lower than 5 weight percent.Hence, it proved that cohesive affinity of paraffin towards expanded graphite is really effective and promising, and thus paraffin/EG thermal composite is form-stable to be used as thermal energy storage system.

C. Thermal conductivity analysis

Thermal conductivity of disc-shaped specimen of each paraffin/EG thermal composite has been calculated numerically using following equation; ( ) = ( ) × × where is thermal diffusivity, is the specific heat capacity and is the density. Calculations have been shown in the table 3.

Table 3. Numerical calculation of thermal conductivitySamples Weight

(kg)Thickness

(m)Diameter

(m)Volume

(m3) (kg/m3)Cp

(kJ/kg.K) (m2/s)K

(W/m.K)Pure 0.00039 0.0022 0.0132 3.06 x10-7 1260.62 1.922 1.49 x10-7 0.361S1 0.00028 0.0022 0.0131 3.02 x10-7 920.24 1.849 9.23 x10-7 1.571S2 0.00026 0.0023 0.0132 3.17 x10-7 831.54 1.838 1.39 x10-6 2.127S3 0.00024 0.0021 0.0130 2.77 x10-7 863.19 2.105 1.29 x10-6 2.337

Experiment was performed at room temperature through laser thermal conductivity analyzerwhich made use of laser flash method. Thermal conductivity of pure paraffin has been reported as 0.361 W/m.K which is really very low and limits the applicability of paraffin wax to be used as thermal energy storage material for thermal management of a system. In fact, materials with low thermal conductivity can’t be able to achieve uniform heat distribution among their constituents. This aspect causes to generate heat spots which are really detrimental for any practical system. As thermal conductivity is the measure of how fast heat gets transferred in a material from one spot to another, hence high thermal conductivity bring about not only fast heat transfer but also uniform distribution of heat. This is the main reason to introduce thermal conductivity enhancers (TCE) in paraffin waxes so that their thermal transport properties can be improved. Graphite, a form of carbon which has crystalline structure, acts as TCE on behalf of its free electron in valence shell while expanded graphite comes up with improved aspects such as loose and porous structure which can absorb more quantity of paraffin per unit volume than graphite, so expanded graphite is recommended. In this paper, three samples with 5 weight%, 10 weight% and 15 weight% of EG have been tested by laser flash method at room temperature, and thermal conductivity has been calculated numerically and shown in table 3. Results indicated that thermal conductivity of pure paraffin wax got increased from 0.361 W/m.K to 1.571 W/m.Kupon 5 weight% addition of EG and 2.337 W/m.K upon 15 weight% addition of EG which went in favor of uniform heat distribution. Hence, a general analysis can be made as following; “Kpcm OMEG i.e. quantitative thermal conductivity of paraffin wax is directly proportional to the quantity of EG until optimal mass (OM) fraction of EG in paraffin/EG thermal composite is achieved.” A little agglomeration was also observed with 5 weight% of EG which suggested that use of EG with 5 weight% will remain feasible for uniform mingling of EG and paraffin wax.

D. DSC analysis

Thermal energy storage characteristics have been analyzed through differential scanning calorimeter (DSC) and results have been presented in fig.3. Thermal properties like initial temperature (Ti), peak temperature (Tp), and final temperature (Tf), latent heat of melting (LHm), latent heat of freezing (LHf) and theoretical latent heat (LHt) during solid-liquid phase transitionhave been summarized in table 4a and 4b, for heating and cooling mode, respectively. It is observed that DSC shows two peaks in each mode referring to two prominent phase transitions i.e. solid-solid phase transition and solid-liquid phase transition. Solid-solid phase transition is

shown by smaller peak where transition occurred between two solid states. During solid-solid phase transition, only a small change occurred in the structure of solid paraffin which can be proved on behalf of small amount of latent heat absorption. In short, solid-solid phase transition is a low temperature and short-living phase which started at about 36 0C and ended at about 48 0C with a little value of latent heat during cooling and heating cycles. On the other hand, a huge amount of latent heat can be achieved in terms of heat storage during solid-liquid phase transition. This phase started a little before the achievement of melting temperature of the solid paraffin with initial temperature Ti=52.56 0C for S1, Ti=58.21 0C for S2, and Ti=60.1 0C for S3.It has been observed that the melting temperature (Tp) of paraffin showed a little variation after incorporation of EG. The probable reason for sharp commencement of initial temperature and a little variation in peak temperature may be referred to the integration between paraffin and EG. Solid-liquid phase change occurred on behalf of absorption of heat energy in paraffinic hydrocarbon which resulted in liquid phase at melting temperature of paraffin. The heat energy absorbed by the constituents of paraffin at the constant temperature is referred as latent heat of fusion. In heating mode, phase change is occurred from solid to liquid which means that energy has been absorbed in the liquid paraffin in terms of latent heat, and energy is released during cooling mode. As internal energy in liquid phase is always higher than solid phase due to weaker intermolecular forces and higher potential energy, so solid-liquid phase always gives higher latent heat storage capacity than solid-solid phase change. Latent heat storage also depends upon mass fraction of substance in the mixture. As mass fraction decrease, latent heat storage capacity also decreases. This is the reasons that latent heat storage decrease from S1 (having 95 % paraffin) to S3 (having 85 % paraffin).

Table No.4a Thermal characteristics of paraffin/EG thermal composite in heating mode

Samples T i(0C) T p(0C) T f (0C) LH m (J/g) LH t (J/g)

Pure 54.56 61.73 64.29 -188.96 -188.96

S1 52.56 63.73 67.45 -123.16 -179.512

S2 58.21 63.52 65.06 -104.27 -170.064

S3 60.1 63.59 65.15 -103.06 -160.616

Table No.4b Thermal characteristics of paraffin/EG thermal composite in cooling mode

Samples T i(0C) T p(0C) T f (0C) LH f (J/g) LH t (J/g)

S1 61.67 59.78 53.35 126.06 179.512

S2 62.49 58.36 52.18 107.59 170.064

S3 63.75 58.45 52.28 106.54 160.616

V. Conclusion

Paraffin wax and expanded graphite were intermingled to get thermal energy storage composite material. Low thermal conductivity of paraffin wax has been improved greatly through the application of highly conductive expanded graphite. Thermal conductivity of paraffin wax got improved from 0.361 W/m.K to 1.571 W/m.K, 2.127 W/m.K and 2.337 W/m.K upon 5%, 10% and 15% (by weight) addition of expanded graphite, respectively. Although latent heat storage of pure paraffin wax got reduced due to reduction of its mass upon addition of expanded graphite but enhancement of its thermal conductivity is much more conducive than a little reduction in the latent heat. Intermolecular forces and thermal motion of constituents of paraffin are affected by

Fig.3 (a) DSC curve of pure paraffin wax, (b) DSC curve of 95% paraffin/EG thermal composite, (c) DSC curve of 90% paraffin/EG thermal composite, (d) DSC curve of 85% paraffin/EG thermal composite

penetration of number of molecules of expanded graphite which caused to replace the number of molecules of paraffin, and confined the molecular movement; therefore, a little reduction in latent heat storage capacity of paraffin in presence of expanded graphite is sure to be observed. However, some other additives must be introduced along with thermal conductivity enhancers in order to keep the latent heat storage capacity of paraffin close to the theoretical latent heat.

Acknowledgement

Facilities and resources provided by Beijing Key Laboratory of Special Functional Materials and Films, Beihang University, Beijing, China, are highly appreciated.

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