Effect of Carbon Nanotube Diameter on Phase Change Thermal Energy Storage Properties of Paraffin Wax

To solve the problems of poor thermal conductivity and low thermal energy storage efficiency of paraffin wax, carbon-containing materials with high thermal conductivity are used to improve its thermal energy storage performance. Three carbon nanotube powders with different diameters, 8-12 nm, 10-20 nm, and 20-40 nm, are added to paraffin wax at a mass fraction of 5% to evaluate the influence of the carbon nanotube diameter on the thermal energy storage properties and the anisotropic thermal conductivity of carbon nanotube/paraffin wax composite phase change material (CPCM). The experimental results show that the specific heat capacity and latent heat value of CPCM decreases as the diameter of the carbon nanotubes decreases. The phase change temperature decreases by approximately 1 ℃; the latent heat value decreases by 0.5%-3.8%, and the specific heat capacity decreases by 16.2%-16.5% compared with those of pure paraffin wax. The improved thermal conductivity and thermal diffusivity shorten the paraffin wax melting time, and the melting time of 8-12 nm composites is reduced by 18.2%-46.7%. Temperature uniformity in the vertical direction was also effectively improved. At the external measurement points, the temperature fluctuation of 8-12 nm composites was 47.7% compared with pure paraffin, whereas at the internal measurement points, the temperature fluctuation of 10-20 nm was 32.9% of pure paraffin. The diameter of the carbon nanotubes significantly improved the thermal energy storage properties of the CPCM.