## 4 Conclusions  
The density and viscosity of nanofluids increase with volume fraction. Graphite nanofluids showing a greater increase in density compared to fullerene nanofluids. Specific heat capacities decrease as the volume fraction increases, with the base fluid (H\({}_{2}\)O) exhibiting a higher specific heat capacity at zero volume fraction compared to the corresponding nanofluids with graphite and fullerene nanoparticles. TC generally increases with volume fraction; however, for the fullerene nanofluid (C\({}_{60}\)), TC decreases due to the lower TC of C\({}_{60}\) nanoparticles (0.4 W/mK) compared to the base fluid (0.605 W/mK). Thermal diffusivity of both graphite and fullerene nanofluids increases with volume fraction. Additionally, as volume fraction increases, both the Nusselt number and the convection coefficient increase. Notably, the Nusselt number for fullerene is higher compared to graphite, but the convection coefficient for fullerene is lower than that of graphite across the evaluated volume fractions. This discrepancy is attributed to the lower TC of fullerene compared to graphite. From the evaluation of heat transfer in FSCs, using graphite and fullerene nanofluids, it can be concluded that adding these nanoparticles improves the convection coefficient by 40% and 30%, respectively, with 10% nanoparticles. From the performance results of the FSCs, it can be concluded that graphite and fullerene nanoparticles can enhance the efficiency of FSCs by 2% and 1.5% more than base fluid. From the evaluation of FSCs using hybrid carbon nanofluids presented here, it can be concluded that while the performance of the solar collector is not significantly improved, the density of the nanofluid decreases, which reduces the pressure drop in fluid flow. As the decrease in efficiency using fullerene with magnesium oxide is less than 0.85%, fullerene hybrid nanofluids could still be used in FSCs. Conceptualization, J.M.C., S.A.C., C.J. and A.Z. ; methodology, J.M.C., S.A.C., C.J. and A.Z. ; software, J.M.C., S.A.C., J.J.J.R., R.B. and A.Z. ; validation, G.R., J.J.J.R., R.B., A.L. and A.Z. ; formal analysis, J.M.C., S.A.C., G.L.G.-U. and A.Z. ; investigation, J.M.C., S.A.C., G.R., J.A.J. and A.Z. ; re-sources, C.J., G.R., J.A.J. and A.Z. ; data curation, J.M.C., S.A.C., G.L.G.-U., C.J., G.R. and A.Z. ; writing--original draft preparation, J.M.C., S.A.C., C.J. and A.Z. ; writing--review and editing, J.M.C., S.A.C., G.L.G.-U., C.J., J.J.R., A.L. and A.Z. ; visualization, G.R., J.J.R. and A.L. ; supervision, A.Z. and G.L.G.-U. ; project administration, A.Z. ; funding acquisition, G.R., J.J.R., J.A.J. and A.Z. All authors have read and agreed to the published version of the manuscript. This study was provided financial support from the science research grant SIP20240435, from the Instituto Politecnico Nacional, IPN; it is greatly appreciated. Figure 12: Efficiency of hybrid carbon nanofluids with respect to volume fraction. **Institutional Review Board Statement:** Not applicable. **Informed Consent Statement:** Not applicable. **Data Availability Statement:** The original contributions presented in the study are included in the article, further inquiries can be directed to the corresponding author. **Acknowledgments:** The authors also acknowledge the financial support given to the postgraduate students by the National Board of Science and Technology, CONACYT, of Mexico. **Conflicts of Interest:** The authors declare no conflicts of interest.