Thermal performance enhancement of evacuated tube solar collector using MWCNT, Al2O3, and hybrid MWCNT/ Al2O3nanofluids

Nanofluids have numerous applications in heat transference procedures due to their exceptional thermal characteristics. The most desirable parameter to enhance the solar collector's performance is the enhancement of the convective heat transfer coefficient between the working fluid tubes and the absorber. As a result, nanofluids have gained prominence as working fluids in solar thermal systems. The trendsetting review reveals that mostly the nanofluids in solar collectors are based on water employing nanoparticles of Al2O3, TiO2, SiO2, and CuO. Besides, nanoparticle concentration is a challenging factor in using nanofluids. In this research, under controlled conditions, the working fluids multi-wall carbon nanotube, Aluminum Oxide, and hybrid MWCNT/Al2O3 50:50% were experimentally examined for the thermal efficiency enhancement of the evacuated tube solar collector. For each type of nanofluid, four volume concentration percentages (0.5%, 0.025%, 0.01%, and 0.005%) were examined along with three distinct mass flow rates. According to the findings, using hybrid MWCNT/Al2O3 50:50% delivers an efficiency boost of about 20% overusing Al2O3, as was previously reported. Finally, it was found that the utilization of 0.5% MWCNT/water nanofluid at 3.5 L/m can enhance the ETSC's energy and exergy efficiency to reach 73.5% and 51% respectively while reaching approximately 60% and 44% for AL2O3, and 69% and 38% for hybrid MWCNT/Al2O3 (50:50%) under the same test conditions.