Diapositive 1

1. IntroductionThermal energy storage plays an important role in an effectiveuse of thermal energy and it has applications in various areas, suchas building heating systems and cooling systems, solar energy collectors andindustrial waste heat recovery . Thermal energy can be stored as a change in internal energy of a material as thermo-chemical reaction, sensible heat or latent heat .

explanation of sensible heat and latent heat :

Latent heat is the energy absorbed by or released from a substance during a phase change from a gas to a liquid or a solid or vice versa.

Sensible heat is the energy required to change the temperature of a substance with no phase change.

In this work, we are interested in the latent heat storage method through phase change materials (PCM).

Aphase-change material(PCM) is a substance with a highheat of fusionwhich, melting and solidifying at a certain temperature, is capable of storing and releasing large amounts of energy. Heat is absorbed or released when the material changes from solid to liquid and vice versa; thus, PCMs are classified as latent heatstorage

Recent studies have been focused on the solidliquid phase change because of its high storage capacity and nearly isothermal heat storage or retrieval process .

Parafn is taken as the most promising phase change material because it has a large latent heat, low cost, little or no super cooling, low vapor pressure, good thermal and chemical stability, nontoxic and noncorrosive.However, parafn suffers from a low thermal conductivity (0.21 0.24 W m-1 K-1 ) . These drawbacks reduce the rate of heat storage and extraction during the melting and solidication cycles and restrict their wide applications, respectively. Consequently,

more working effort has been focus to improve the thermal conductivity of PCMs, by dispersing of high conducting particles within .The use of graphite particles has advantages such as high thermal conductivity, low density in contrast to metals and high resistance to corrosion. The scope of this study is to make an experimental investigation on the effect of graphite on thermal conductivity, diffusivity and specic heat of parafn/graphite composites.

Two kinds of graphite were used to enhance thermal conductivity of the parafn: Synthetic graphite (Timrex SFG75) and graphite waste obtained from damaged Tubular graphite Heat Exchangers. For this experiment , Parafn/graphite phase change composites with the masse fraction of 5%, 10%, 15% and 20% were prepared by cold uni-axial compression method. in this method parafn powders and graphite particles are mixed together, then the obtained mixture (parafn + graphite) is poured into a stainless steel mould followed by a uni-axial compression (80 bar) at ambient temperature.

5This technique leads to an anisotropic Cylindrical composite structure of parafn/graphite whose porosity is partially occupied by parafn grains The thickness and the diameter of all this specimens were 5 mm and 60 mm respectively.Then, neatly cut of the cylindrical samples to obtain a parallelepiped-shape specimens , with dimensions(42 mm 42 mm 5 mm) for thermophysical property measurements.

A periodical method was used to estimate simultaneously thermal conductivity, diffusivity and specic heat of parafn/graphite composite materials at room temperature . This method isbased on the use of a small temperature modulation in the parallelepiped-shape sample. The advantage of this method is that allows estimating simultaneously the effectivethermal conductivity and diffusivity with their corresponding statistical condence bounds . It is well suited for polymers and composite materials with a thickness between 1 and 10 mm.

In the conguration used the sample is sandwiched between two metallic plates. A thermal grease of high conductivity is applied on the contact surfaces between the sample and the metallic plates to ensure good thermal exchange between the various elements.The front side of the rst metallic plate (brass) is also xed to heating device (thermoelectric cooler) and heated periodically using a sum of ve sinusoidal signals. The whole device is placedin a vacuum chamber connected to a pumping system. The rear side of the second metallic plate (copper) is in contact with air at ambient temperature and high vacuum. The temperature is mea-sured with thermocouples placed inside both front and rear metallic plates.

Theoretical partThe system under study is composed of several layers with different thermophysical properties. A heat transfer function is dened at each frequency as the ratio between the Fourier transforms of temperature at the xr and xf point of the x-axis. We assume that the temperature of the front side of the metallic plate is modulated. The heat transfer exchanges on the rear face are taken into account trough a global exchange coefcient, which is considered constant during the experiment andthe thermal properties of the sample are supposed constant.By assuming a one-dimensional heat transfer in the x-direction, the conservation of energy equation in each layer can be written as:

In the case in periodic heating, the heat transfer equation is dened by:

This system is modeled with one dimensional quadrupoles the-ory i.e. matrices with two inputs and two outputs. It is also possibleto obtain the expression of the theoretical heat transfer functionusing the quadrupoles method. Let (Tk(s), k(s)) be the temperatureand ux at the rear side of the k-th layer of homogeneous material.With this k-th layer is associated a quadupole matrix .A quadrupole matrix [Q] is dened by:

Where = e/a : the Fourier time , R= e / : the thermal resistance . So that :

With :

is the Fourier transform of the temperature.

This form is used for thermal modeling of plates of brass and copper, and for the sample.The grease layer at each interface is represented by a thermal contact resistance.

The relationship between input variables and output is writtenas follow

Where ; are the quadrupoles associated to brass, grease, sample and copper

respectively.Thus, a theoretical heat transfer function can be obtained as:

The determination of the composite density is important not only for the specic thermal capacity estimation but for checking the quality of the samples. It is clear that if composite samples are well processed, i.e., good homogeneity is reached without air bubbles in the sample as well as without unlled pores at the parafn/graphite interface, the specic density of the composites should have a linear dependence upon the volume fraction, according to the simple rule of mixtures given by:

Results and discussion Scanning electron microscopy (SEM) was used to observe the morphology of parafn/graphite composites also pure parafn and graphite (SFG75 and graphite waste).The morphology of graphite SFG75 is illustrated in Fig. a and b.

Fig. c and d show the SEM photographs of the used graphite waste and the parafn respectively.

Fig. e and f illustrated composite with 10 % and 20 % of SFG75 respectively. Also, the morphology of the parafn/ graphite waste composite with 10 % and 20 % of graphite waste aredemonstrated in Fig. g and h respectively.

This gures exhibit different type of morphology and shows the presence of two different regions represents two different materials, namely parafn and graphite (SFG75 or graphite waste).It can be observed that the graphite (SFG75 and graphite waste) is dispersed into the parafn used as matrix material. We can easily recognize the graphite dispersed in the parafn by the darker region and by its uniform shape, this dispersion enhances the thermal properties of the matrix.Observed phase separation in composites was caused due to unlike chemical nature of parafn and graphite.

3.4. Thermal diffusivity and thermal effusivity:We note, as in the thermal conductivity, a non linear raise in the composite thermal diffusivity by increasing the graphite mass fraction for the two kinds of composites paraffin/SFG75 and paraffin/graphite waste. As compared with thermal diffusivity of pure paraffin (1.309 107 m2 s1), we can see that thermal diffusivity of the paraffin/graphite composite increase from 47.1% to 382.5%, with increasing of the mass fraction of graphite Timrex (SFG75) and a smaller increase can be seen for graphite waste composite with an increasing rate from 51.3% to 157.4%.Observation and comment:The third parameter is the thermal effusivity of the composites samples. It can be calculated using the thermal conductivity and diffusivity values as follows:

Thermal effusivity and diffusivity are summarized in Table 2:

observation and comment:We can note that the thermal effusivity varied from a paraffin/graphite composites to another due to their differing ability to transfer heat. This is due to differences in heat transfer through and between particles, and is therefore a function of density andmorphology.

The figure below shows that the measured thermal conductivity of paraf n/graphite composite is greatly inuenced by the graphite addition and increased with increasing the mass fraction of graphite.This increase is due to the higher thermal conductivity the graphite. In the case of parafn/SFG75, we observe a non linear increase of the thermal conductivity with increasing synthetic graphite (SFG75) mass fraction

The behavior of thermal diffusivity (aeff) of parafn/graphite composite is depicted in Fig. 8 as function of the graphite mass fraction.

We made on the Abaqus software an analysis of the behavior of a paraffin block under a thermal stress , one side of the paraffin block is subjected to a temperature of 100 c when the other side is left at room temperature . We observe clearly that the paraffin block shape changes as the temperature passes through .

Conclusion

In this work, composites based on a phase change material (PCM) and graphite were elaborated using cold uniaxial compression method. In such a composite, the parafn serves as a latentheat material and the graphite as an effective heat transfer promoter.

Two types of graphite were compared: graphite waste and graphite synthetic Timrex (SFG75). The thermophysical properties of composites are characterized using the periodic temperature method and the values are compared to the results of different analytical models.

Results show that, increasing the mass fraction of graphite from 0% to 20% gradually increased the thermal conductivity and diffusivity of parafn/graphite composite.

A non linear increase of the thermal conductivity and diffusivity with increasing graphite synthetic (SFG75) and a linear increase for the case of graphite waste was observed for 20% of mass .

Two experimental methods were adopted for measuring the densities of parafn/graphite composites. A good agreement between experimental and theoretical values according to the rule mixture is obtained.