Separation of copper-oxide nanoparticles from nanoparticle enhanced phase change material

Abstract

Phase change materials (PCM) are used in many energy storage applications. Energy is stored (latent heat of fusion) by melting the PCM and is released during re-solidification. Dispersing highly-conductive nanoparticles into the PCM enhances the effective thermal conductivity of the PCM, which in turn significantly improves the energy storage capability of the PCM. The resulting colloidal mixture with the nanoparticles in suspension is referred to as nanoparticle enhanced phase change materials (NEPCM). A commonly used PCM for energy storage application is the family of paraffins (CnH2n+2). Mixing copper oxide (CuO) nanoparticles in the paraffin produces an effective & highly efficient NEPCM for energy storage. However, after long term application cycles, the efficiency of the NEPCM may deteriorate and it may need replacement with fresh supply. Disposal of the used NECPM containing the nanoparticles is a matter of concern. Used NEPCM containing nanoparticles cannot be discarded directly into the environment because of various short term health hazards for humans and all living beings and un-identified long term environmental and health hazards due to nanoparticles. This problem will be considerable when widespread use of NEPCM is practiced. It is thus important to develop technologies to separate the nanoparticles before the disposal of the NEPCM. This is the motivation behind this study. The primary objective of this research work is to develop methods for the separation and reclamation of the nanoparticles from the NEPCM before its disposal. It is aimed to find or design separation methods which are simple, safe, and economical. The specific NEPCM considered in this study is a colloidal mixture of dodecane (C12H26) and CuO nanoparticles (1% - 5% mass fraction and 5-15 nm size distribution). The nanoparticles are coated with a surfactant or stabilizing ligands for suspension stability in the mixture for a long period of time. Various methods for separating the nanoparticles from the NEPCM are explored. The identified methods include; (i) distillation under atmospheric and reduced pressure, (ii) high speed centrifugation, (iii) destabilization of the nanoparticles by adding chemical agents thereby inducing gravitational precipitation, (iv) silica-column chromatography, (v) silica adsorption and (vi) nanofiltration These different nanoparticle separation methods have been pursued and the results are presented with detailed process description, analysis, and conclusion.