Investigation Of The Performance Of Nanofluid-Based Direct Absorption Solar Collectors

Optical and thermal radiative properties of nanofluids are much different from those of the base fluid because of the dispersed nanoparticles. Therefore, nanofluids may become another kind of efficient absorbing material and have a good prospect in solar utilization. Due to their optical and thermal radiative properties, nanofluids can be applied into direct absorption collectors to improve the thermal efficiency and to achieve an efficient use of solar energy.The objective of this paper is to investigate the optical and thermal radiative properties of nanofluids and to evaluate the thermal performance of direct solar collectors using nanofluids. Furthermore, it is important to find nanofluids suitable for direct solar collectors. In this paper, the radiative properties of SiO₂-water nanofluid, TiO₂-water nanofluid and ZrC-water nanofluid are investigated. Results show that SiO₂-water and TiO₂-water nanofluids selectively absorb thermal radiation, however ZrC-water nanofluid is completely absorbing.On this base, the paper studies and evaluates the performance of direct solar system using nanofluids by experimental investigation and theoretical analysis. The experiment is carried out to test the working fluid temperature at the stagnation status and the temperature increase of nanofluids at the circulation status. It is found that ZrC-water nanofluid has the highest stagnation temperature and circulation temperature increase while the temperatures of SiO₂-water nanofluid are the lowest. Moreover, the fluid temperature decreases with increasing of fluid flow rate. Then, a theoretical investigation is set up for direct absorption solar systems to model absorption, scattering and extinction of solar radiation within nanofluids and to model the heat transfer in the fluid film. Spectral extinction coefficients of nanofluids, temperature distribution within the nanofluid film and thermal efficiency of the system are obtained by simulation. The results show that the theoretical extinction coefficient of TiO₂-water nanofluid is significantly higher than those of SiO₂-water nanofluid.In a word, the thermal efficiency of ZrC-water nanofluid solar absorption system is highest, TiO₂-water nanofluid is lower and SiO₂-water nanofluid is lowest. Therefore, ZrC-water nanofluid is most suitable for the direct solar absorber, and will improve the system efficiency greatly.