Study On Thermal Properties Of Paraffin And Its Composite Phase Change Materials

In this paper, octadecane was selected as the phase change material and Alnano-particles were used to improve its thermal conductivity according to the ambienttemperature of the Solar Energy Generating System (SEGS) with vertical collectordesigned by our group. In order to reduce the price of materials, mixed paraffin madeof low melting paraffin and high melting paraffin was used instead of octadecane, andthermal properties of mixed paraffin were studied experimentally.The nano composite phase change materials (NC-PCMs) were prepared bycombining chemical dispersants and ultrasonic oscillation, with Al nano-particles massfraction of1%,2%,3%,4%,5%,respectively. The octadecane and composite phasechange materials with1%Al nano-particle were tested on the use of fourier transforminfrared absorption spectroscopy, then their infrared absorption spectra were compared;The results showed that there are only physical composition relationship between Alnano-particles and octadecane. The thermal conductivity of NC-PCMs was measuredwith the method of transient thermal probe. The results showed that the thermalconductivity of NC-PCMs increased with the increase in the mass fraction of Alnano-particles, but the increasing trend slowed down with the sustaining increase inthe that. which was even more pronounced while the NC-PCMs were in its liquid state.The softwares of Materials Studio and Packmol were used to create molecularmodels of octadecane and the NC-PCMs which the content of Al nano-particle were1%wt,2%wt,3%wt,4%wt,5%wt, respectively. Then the LAMMPS was used tocalculate the thermal conductivity of the NC-PCMs. At last, the numerical results werecompared with the data measured by experimental measurements; The results showedthat the thermal conductivity of the NC-PCMs appeared to be rising in the simulationwith the mass fraction of Al nano-particle increasing, and the analog value of thethermal conductivity in liquid state was less than that in solid state; The analog valueof the thermal conductivity of the compound phase change material was closer to thatmeasured by experiment in liquid state, but in solid state, the numerical results had bigdifferences with experimental values. These were due to the size of molecular model,selection of Lennard-Jones potential function parameter and the effects ofmicro-convection could not be calculated. Mixing paraffin was prepared by mixing low melting point paraffin (hexadecane,octadecane) with high melting point paraffin (56#paraffin,62#paraffin) in the moltenstate. Differential Scanning Calorietry (DSC) was used to analysis the phase transitionproperties of the Mixing paraffin. The results showed that the phase transitiontemperature begins to decrease, after adding the paraffin with low melting into thatwith high; when the content of the low melting point paraffin was about60%wt, themelting point of the mixture would be close to the phase transition temperature of thelow melting point paraffin. The latent heat of the mixed paraffin was between that ofthe high and low melting paraffin, which had a small range of variation; with thenumber of cycles increasing, latent heat and phase transition of the mixing paraffintemperature fluctuated in the tiny range, indicating that the composite phase changematerial had a favorable thermal cycling stability. The thermal conductivity of mixedparaffin was measured by transient thermal needle method, the results showed that thethermal conductivity of the mixing paraffin changed relatively small. The low meltingparaffin is expensive, and the high melting point paraffin has a low price; the phasechange material which can meet the need of energy storage would be gotten by mixingthe low melting paraffin and the high melting paraffin with different proportions.