Superior thermal conductivity and photo-thermal conversion efficiency of carbon black loaded organic phase change material

Abstract

Efficient energy-storage materials are necessary to reduce our excessive reliance on depleting fossil fuels and to address environmental issues. Phase change materials (PCM) have been promising candidates for efficient thermal management but their poor thermal conductivity remains as a major drawback for applications. We report superior thermal conductivity enhancement (~ 135%) and photo-thermal conversion efficiency (~ 84%) in paraffin wax (PW) based organic phase change materials (PCM) loaded with carbon black nano powder (CBNP) at low loading concentrations (2.5 wt%) for latent heat thermal energy storage. The large enhancement in thermal conductivity in the solid state is attributed to the formation of quasi-2D network of percolating structures of CBNP with high phonon mediated heat conduction efficiency. Besides, lower primary particle size, high compressibility and volume filling ability of CBNP with low thermal barrier resistance also contribute to the large enhancement of thermal conductivity in the solid phase. A good agreement between the experimental data and logarithmic heat conduction model for percolating structures is observed. The role of aggregation, at microscale, on thermal conductivity enhancement is also studied using optical phase contrast microscopy. Our investigations show an enhancement in photo-thermal conversion efficiency upon increasing the CBNP loading due to an increase in total extinction efficiency, attributed to the intense Mie scattering of the incident solar radiation by the micron sized CBNP clusters. The enhanced heat transfer rate, gain in cooling time, low density and low cost, make the CBNP loaded PCMs suitable for practical thermal energy storage applications.