Highly-efficient nanofluid-based direct absorption solar collector enhanced by reverse-irradiation for medium temperature applications.

The direct absorption solar collector (DASC) with nanofluids is a promising solar energy collection technology. However, various studies have focused on low-temperature applications of nanofluids, and the medium-temperature collection system that involves high-grade energy is always neglected. This study examines the photo-thermal properties of titanium nitride nanofluids with thermal transfer oil as the base fluids under different solar irradiation intensities. The irradiation surface layer reaches ∼160 °C under 5 suns, and a high-temperature gradient develops within the working fluid, producing a low collector photo-thermal efficiency that is below expectation. To overcome these disadvantages, the heat transfer change from thermal conduction to free convection within the fluid is achieved via reverse irradiation direct absorption solar collector (RI-DASC). The performance parameters of this RI-DASC, including the optical properties of nanofluids, steady-state equilibrium temperature, photo-thermal conversion efficiency, and energy utilization distribution are investigated in detail. The experimental results demonstrate that the temperature difference between the irradiation and non-irradiation surfaces for ∼0.005 wt% under 5000 kW/m2 are ∼50 °C and ∼10 °C in DASC and RI-DASC, respectively. The collector photothermal conversion efficiency of DASC (∼40%) is improved to ∼50% for RI-DASC, and the steady-state temperature is enhanced to 165 °C in RI-DASC. Reverse irradiation can effectively improve the light-to-heat conversion efficiency and the temperature uniformity inside the direct absorption solar collector. Image 1 • Medium temperature nanofluids solar collector involving high-grade energy has been studied. • The reverse irradiation DASC produces 10% PTE improvement. • Uniform temperature distribution of nanofluid is obtained in reverse irradiation DASC. • Effect of temperature on optical properties of nanofluids has been explored. [ABSTRACT FROM AUTHOR]