Thermal energy storage with Phase Change Materials (PCMs) is one of the most potential technologies for energy storage. However the low thermal conductivity of PCMs reduces the heat exchange rate during melting and solidification cycles. This paper studied the effects of two hybrid Carbon Nano-additives (CNs) fillers, that is, Expanded Graphite-Multi-walled Carbon Nano-tube (EG-MWCNT) and Expanded Graphite-Carbon Nano-fiber (EG-CNF), on the thermal conductivity of Paraffin-HDPE SSPCM. From the viewpoints of synergistic thermal enhancement effect and the interfacial thermal resistance, the principle of enhancing thermal conductivity of Paraffin-HDPE/EG-MWCNT and Paraffin-HDPE/EG-CNF composite PCMs was analyzed. A modified Maxwell-Garnett model with a synergy factor η was proposed, which found excellent agreement between model prediction and the experimental data. Compared with the 5wt% loading of single CN additive EG, the thermal conductivities of hybrid CNs fillers (EG-MWCNT and EG-CNF) Paraffin-HDPE SSPCM had increased by 60% and 21.2% respectively. Within the scope of mass ratios of hybrid CNs fillers in this paper, Paraffin-HDPE/EG-MWCNT composite PCM exhibited superior performance than Paraffin-HDPE/EG-CNF in thermal conductivity with the optimal mass ratio of EG and MWCNT being 4:1.
Thermal energy storage using phase change materials (PCMs) is of great interests in many fields, especially in solar thermal applications. To overcome the leakage problem caused by phase change and the low thermal conductivities of most PCMs, especially paraffin, the current study prepared and tested two Paraffin-HDPE-based composites by adding two hybrid Carbon Nano-additives (CNs) fillers: Expanded Graphite-Multi-walled Carbon Nanotube (EG-MWCNT) and Expanded Graphite-Carbon Nanofiber (EG-CNF). A comprehensive evaluation method was first proposed based on the Efficacy Coefficient Method (ECM) to assess the thermal performance of Paraffin-HDPE shape stabilized PCM (SSPCM). Seven individual indexes, including phase change temperature, latent heat, thermal conductivity, leakage rate, specific heat of both solid and liquid phase, and heat storage/heat release rates, were measured and applied to obtain the overall efficacy coefficients of the prepared SSPCM composites. The results showed that in all studied composites, n-octadecane-HDPE/EG-MWCNT composite exhibited the best comprehensive thermal performance with an optimal mass ratio of EG and MWCNT being 4:1.
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