Silicon dioxide coating nanocomposites and cellulose-modified stable nanofluid for direct absorption solar collection

•The SiO2@C nanoparticles were prepared by pyrolysis of melt-blown filter obtained from face masks.•The SiO2@C nanoparticles has excellent optical absorption performance.•The water-based SiO2@C nanofuilds has excellent stability due to the dispersion effect of CNF-C.•The thermophysical, optical and photothermal properties of SiO2@C nanofuilds was investigated.•The enhancement of photothermal conversion efficiency of the SiO2@C nanofluid relative to base fluid was 101.4%. Using nanofluids in direct absorption solar collectors (DASC) is a potential means of improving photothermal conversion efficiency. Nanocarbon materials, such as carbon black, graphene, and carbon nanotubes, are generally used to prepare nanofluids for DASC. However, carbon black is difficult to disperse, and graphene and carbon nanotubes areexpensivetoproduce. In this study, we reported a new silicon dioxide@carbon (SiO2@C) nanocomposite prepared by chemical vapor deposition (CVD), forming a core–shell nanoparticle. The carbon source was a melt-blown filter obtained from face masks. The SiO2@C nanocomposites have a broadband absorption range of 200–2500 nm. The water-based SiO2@C nanofluids were prepared using carbonylated cellulose nanofibers (CNF-C) as the dispersant. The nanofluids demonstrated excellent dispersion stability, heat transfer, and optical properties. The viscosity of SiO2@C nanofluids decreased with increasing SiO2@C mass fraction, which differs from previous research findings. The maximum photothermal conversion efficiency of the SiO2@C nanofluid reached 91.4%. In one hour, the enhancement of photothermal conversion efficiency of the SiO2@C nanofluid relative to deionized water was 101.4%, proving that the SiO2@C nanofluid shows great applicability to DASC.