Improved heat recovery from paraffin-based phase change materials due to the presence of percolating graphene networks

Presently, engineers are unable to fully utilize the high thermal energy storage capacities of paraffin-based phase change materials (PCMs) in electronics cooling and waste heat recovery applications due to their inherently low thermal conductivities, which result in slow melting and solidification rates. In order to increase the paraffin's thermal conductivity, several groups have implanted nanoparticles within PCMs. Despite remarkable improvements in their thermal conductivities, however, it is expected that less thermal energy can be harvested during the nanocomposite PCM's solidification period due to the removal of some PCM mass in favor of the nanoparticles. In this study, a heat exchanger system is used to extract thermal energy that has been stored within paraffin nanocomposites embedded in a thermal containment unit (TCU). We find that the amount of thermal energy that can be harnessed from MWCNT, Al or TiO2 nanocomposite PCMs (at 20 v.% concentrations) is approximately 15-17% lower than the amount that can be extracted from the base paraffin during its period of solidification, as expected. However, the amount of thermal energy that can be harnessed from paraffin in the presence of graphene nanoparticle networks (15 nm thickness, 15 mu m diameter, at 20 v.%) is found to be nearly 11% greater than for the base paraffin. Based on these results, it is posited that the paraffin's alkane molecules are beginning to align across the face of the graphene nanoparticles, resulting in a more crystalline paraffin structure at the graphene-paraffin interface and a higher absolute phase change enthalpy. Consequently, it is expected that this work will open up new avenues of research for the creation of advanced nanocomposite PCMs with exceptionally high thermal conductivities and thermal energy storage capacities. (C) 2014 Elsevier Ltd. All rights reserved.