Bio-based nanofluids of extraordinary stability and enhanced thermal conductivity as sustainable green heat transfer media

Abstract

Nanofluids (NFs) as a new generation of heat transfer media can be applied inter alia as engine coolants, in the microelectronic industry for the cooling of electronic components and systems, and in solar panels. In the present study, the extraordinarily, that is, more than 1 year, stable NFs composed of multi-walled carbon nanotubes (MWCNTs), biomass-derived 1,2- propanediol or 1,3-propanediol, and poly(N-vinylpyrrolidone) were created and studied. The thermal conductivity and density of NFs did not change over 8 months, and NFs did not sediment over 14 months. The real image of NFs determined using transmission electron cryo-microscopy allowed us to prove that the extraordinary stability and enhanced thermal conductivity were resulted by fully individualized MWCNTs in the continuous phase and MWCNTs stabilized in dispersions by shorter carbon nanoparticles and mostly homogenous poly(N-vinylpyrrolidone) coating. The maximum enhancement in thermal conductivity was 22 and 20% for NFs composed of 2 wt % MWCNTs in comparison with that of pure 1,2-propanediol and 1,3-propanediol, respectively. The improved thermal properties were accompanied by the practically Newtonian nature of all NFs. The cytotoxicity test on normal human dermal fibroblasts indicated that the use of diols diminished the toxicity of MWCNTs. Finally, the thermal conductivity and Prandtl number of bio-based NFs-as compared with those of commercial heat transfer fluids DOWCAL 200 and DOWCAL N-predestine them as superb green heat transfer media in sustainable energy systems.