Graphene nanoplatelet nanofluids stabilised by hybridisation with graphene oxide: preparation, stability, and performance in flat plate solar thermal collector.

This study examined hybrid nanofluids of graphene nanoplatelets (GNP) and graphene oxide (GO) in terms of their stability and performance as a working fluid in a flat plate solar thermal collector. Instead of functionalisation, this experiment utilises the synergetic effect of GO, which stabilises GNP nanofluids by acting like an amphiphilic surfactant. Experimental results found that stability is significantly improved with a sufficiently high concentration of GO. Stability improved as the proportion of GO was increased. In particular, the sample with an equal 50:50 ratio of GNP and GO showed good stability in sedimentation measurements, retaining over 90% of its concentration over one month. Stability was also measured under working pipe flow conditions in a solar collector test rig. This type of data has not previously been reported in the literature. Measurement of the change in concentration showed that in addition to sedimentation, concentration loss can occur via an alternative mechanism: deposition of the nanoparticles inside the pipes. The pristine GNP sample lost over 80% of its concentration over one day of running in the test rig, despite showing moderate stability over one month under still conditions. This shows that measurement of stability in still conditions does not fully describe the stability of nanofluids intended to be working fluids in heat transfer. The sample with a 50:50 ratio of GNP and GO retained around 66% of its concentration over five days of running in the test rig, and the concentration seemed to stabilise, reaching equilibrium, implying no further decreases. Viscosity measurements showed a small increase. Despite the moderately encouraging stability results, however, solar collector testing showed no discernible change in heat transfer performance when using nanofluids compared to water, making these nanofluids unsuitable for this particular flat plate solar collector design. [ABSTRACT FROM AUTHOR]