Synthesis of therminol‐graphite nanofluids and photo‐thermal conversion properties

Summary Therminol‐based nanofluids with graphite nanopowders (GNs) were proposed as an efficient working fluid in direct‐absorption solar collectors (DASCs), especially for elevated‐temperature applications. In this work, Therminol VP‐1/GN nanofluids (TG nanofluids) were prepared in two steps with surfactant (Triton X‐100). In the synthesis procedure, the vortex generator for coating GN with sticky surfactant (μ = 240 mPa⋅s) and the ultra‐sonicator for diluting GN/Triton X‐100 mixture with Therminol VP‐1 are used. We have demonstrated excellent dispersion stability up to 4 weeks and even after heating over 100°C without any surface modification of nanoparticles. The feasibility of using the synthesized TG nanofluid as a working fluid in DASCs was experimentally demonstrated by measuring its absorption coefficient as well as by characterizing photo‐thermal conversion performance. The absorption coefficient of the TG nanofluid at 0.004 wt.% of GNs was measured to be 1.12 cm−1, which is more than eight times enhanced from that of bare Therminol VP‐1 (ie, 0.13 cm−1) in the wavelengths from 350 to 1600 nm. Finally, the temperature increment of 43°C was achieved after t = 3600 s under the 5‐sun condition in the static photo‐thermal conversion experiment. Moreover, the low viscosity of TG nanofluids (only 2.2% higher than that of the bare Therminol VP‐1) supports the superiority of TG nanofluids to be applied as working fluid in DASCs. Dispersion stability, optical property, and photo‐thermal conversion efficiency of the synthesized Therminol‐graphite nanofluids.