High temperature and long-term stability of carbon nanotube nanofluids for direct absorption solar thermal collectors

Stable dispersions (nanofluids) are produced using plasma functionalized multi-walled carbon nanotubes (MWCNTs). To our knowledge, this study presents a first quantitative demonstration of nanofluid stability over extended periods of time (currently tested up to 8 months) and after intense heating. No agglomeration is found to occur in the water and glycol-based nanofluids after heating at 85 and 170 °C, respectively. Significant agglomeration does occur for suspensions produced using the non-polar Therminol® VP-1 heat transfer fluid. Optical characterization of the nanofluids demonstrates that the MWCNTs are highly absorbing over the majority of the solar spectrum, allowing for close to 100% solar energy absorption, even at low concentrations and small collection volumes. These absorption properties coupled with the stability of the nanofluids make them ideal candidates as direct solar thermal collectors.