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Research Papers

# Analysis and Design of a Paraffin/Graphite Composite PCM Integrated in a Thermal Storage Unit

[+] Author and Article Information
R. Pokhrel, T. Hight

Department of Mechanical Engineering, Santa Clara University, Santa Clara, CA 95053

J. E. González1

Department of Mechanical Engineering, City College of New York, New York, NY, 10031gonzalez@me.ccny.cuny.edu

Department of Chemistry, Santa Clara University, Santa Clara, CA 95053

1

Corresponding author.

J. Sol. Energy Eng 132(4), 041006 (Sep 03, 2010) (8 pages) doi:10.1115/1.4001473 History: Received November 18, 2009; Revised February 02, 2010; Published September 03, 2010; Online September 03, 2010

## Abstract

The addition of latent heat storage systems in solar thermal applications has several benefits including volume reduction in storage tanks and maintaining the temperature range of the thermal storage. A phase change material (PCM) provides high energy storage density at a constant temperature corresponding to its phase transition temperature. In this paper, a high temperature PCM (melting temperature of $80°C$) made of a composite of paraffin and graphite was tested to determine its thermal properties. Tests were conducted with a differential scanning calorimeter and allowed the determination of the melting and solidification characteristics, latent heat, specific heat at melting and solidification, and thermal conductivity of the composite. The results of the study showed an increase in thermal conductivity by a factor of 4 when the mass fraction of the graphite in the composite was increased to 16.5%. The specific heat of the composite PCM (CPCM) decreased as the thermal conductivity increased, while the latent heat remained the same as the PCM component. In addition, the phase transition temperature was not influenced by the addition of expanded graphite. To explore the feasibility of the CPCM for practical applications, a numerical solution of the phase change transition of a small cylinder was derived. Finally, a numerical simulation and the experimental results for a known volume of CPCM indicated a reduction in solidification time by a factor of 6. The numerical analysis was further explored to indicate the optimum operating Biot number for maximum efficiency of the composite PCM thermal energy storage.

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## Figures

Figure 1

Sample setup in DSC for axial and radial thermal conductivities depending on direction of heat flow with respect to fiber direction. Fiber is oriented normal to the direction of applied force while preparing the graphite matrix.

Figure 2

DSC curve with heat flow versus sample temperature for (a) PCM, (b) CPCM 1, and (c) CPCM 2, respectively

Figure 3

Specific heat with temperature for both solids and liquids for (a) RT-80, (b) CPCM1, and (c) CPCM2, respectively

Figure 4

DSC curve for gallium showing the melting temperature of 30°C

Figure 5

(a) Thermal conductivity evaluation for PCM showing DSC curve for RT-80. The blue line indicates heat flow and the red line indicates constant slope at the phase transition of gallium. (b) DSC curve for thermal conductivity evaluation for CPCM 1. The axial thermal conductivity is represented by A and the radial thermal conductivity by B, respectively. (c) DSC curve for thermal conductivity evaluation for CPCM 2. The axial and radial thermal conductivities are represented by A and B, respectively. The different phase transition temperatures of the gallium on top of the sample depend on the height of the sample, and specific heat and thermal conductivity of the sample, neglecting the losses at the interface. A small amount of gallium (0.015 g) on top of the PCM required a longer time to melt completely, whereas 0.3 g of gallium melts completely at 5.3 min, 5.0 min, 4.5 min, and 4.5 min for CPCM 1 and CPCM 2, respectively, i.e., total melting time decreases with an increase in thermal conductivity. The values of thermal conductivity are tabulated in Table 2.

Figure 6

PCM in a copper tube inside a storage tank

Figure 7

Subdivision of r-t domain with constant Δr

Figure 8

Idealized semi-infinite case to test numerical formulation

Figure 9

(a) Melting PCM in a constant water bath of 90°C. (b) Solidifying PCM in an quiescent air of 25°C.

Figure 10

Experiment data for heating and solidification of PCM

Figure 11

Nondimensional temperature of the PCM and CPCM during solidification in air

Figure 12

Experimental results versus numerical analysis for PCM with the phase change, assuming h=6 W/m2 K for the numerical analysis phase change

Figure 13

Experimental results versus numerical analysis for PCM with the phase change, assuming h=8 W/m2 K for the numerical analysis phase change

Figure 14

Experimental results versus numerical analysis for CPCM 1 with the phase change, assuming h=6 W/m2K for the numerical analysis

Figure 15

Experiment results versus numerical analysis for CPCM 1 with the phase change, assuming h=8 W/m2K for the numerical analysis

Figure 16

Numerical results for solidification rate for CPCM 1 with the phase change at different Biot numbers

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