Theoretical Investigation of Radiative Transport and Heat Transfer of Nanofluids in a Direct Solar Absorption Collector

Conversion of solar energy into heat is a common way to explore the clean and renewable energy source. Generally, a spectrally selective coating should be deposited to the solar-absorbing surface of the solar collector in order to reduce thermal loss emitted away from the surface in the form of thermal radiation. Nanofluids can be filled into a flat plate solar collector to absorb the solar radiation directly. The absorbance of the nanofluid in the solar spectrum may be tuned by various nanoparticles of different materials, sizes and mass fractions. In this paper, a theoretical investigation is carried out to model absorption, scattering and extinction of solar radiation within the nanofluid and to model the heat transfer within the liquid film in the direct solar absorption collector. The predicted extinction coefficient is much lower than the measured value in visible spectrum for the SiO₂-water nanofluid. The calculated average temperature increase in the outlet of the collector using the measured radiative properties agrees with the measured value while that applying the predicted radiative properties is significantly lower. Nevertheless, the agreement in the predicted and the measured extinction coefficients of the TiO₂-water nanofluid is reasonably good within the solar spectrum. The calculated average temperature increase is close to the measurement value. This work is helpful to understand radiative transport and heat transfer of direct solar absorption collectors using nanofluids.