In this study, the efficiency of a flat plate and U-tube solar collector as a function of the concentration of Al2O3 and CuO nanofluid and the size of nanoparticles was investigated experimentally. Moreover, the efficiency improvement of a flat plate and U-tube solar collector with Al2O3 and CuO nanofluid as the working fluid was compared to that with water.
The thermal conductivity of the Al2O3 and CuO nanofluid was found to increases with increase in its concentration and decreases with the nanoparticle size. But it''s increasing rate decreased. As an experimental result, when the inlet fluid and ambient environment were the same temperature, the flat plate and U-tube solar collector with 1.0vol% Al2O3 nanofluid, 20 nm-nanoparticles and a mass flow rate of 0.047 kg/s showed the highest efficiency, which was 17.6% and 17.9% higher than that of the solar collector with water. In addition, the efficiency of the flat plate solar collector with 1.0vol% Al2O3 nanofluid was 3.54% and 5.53% higher than those with 1.5vol% and 0.5vol% nanofluid, respectively. For the use of CuO nanofluid, the flat plate and U-tube solar collector with 0.4vol% CuO nanofluid, 40 nm-nanoparticles and a mass flow rate of 0.047 kg/s showed the highest efficiency, which was 7.29% and 7.27% higher than that of the solar collector with water.
As the experimental result, the value of the flat plate collector was relatively larger than that of the U tube type solar collector. It is predictable that U-tube solar collectors will have higher performance at high temperatures and wide operating temperature range. However, the maximum efficiency of flat plate solar collector was higher than that of U-tube solar collector.
The synthesize results of this study are to increase solar radiation absorption and increase the efficiency of solar collectors, the nanofluid must be uniformly dispersed in the base fluid. Besides, in order to improve the performance of the solar collector, the optimal nanofluid concentration according to nanofluid should be considered appropriately. One more thing, the size of the nanoparticles has a significant effect on the performance improvement of the solar collector and the smaller nanoparticles increased the more active the Brownian motion and the higher the effective thermal conductivity. In conclusion, the concentration of nanofluid and nanoparticle size are important factors to improve the performance of the solar collector. When nanofluids are used as working fluid in the solar collector. In addition, nanofluids can make better the efficiency in the solar collector more than water because of their superior ability to absorb solar energy and heat transfer. Therefore, Al2O3 and CuO nanofluid are effective to the increasing of the efficiency in the solar collector.