Radiative Properties And Photothermal Conversion Performance Of Ionic Liquid Based Nanofluids

Photo-thermal conversion is one of the most practical and effective ways for the unitization of solar energy. Solar collectors are the key components in any solar thermal utilization systems. Compared with the traditional solar collectors, direct absorption solar collectors(DASCs) have the advantages of low heat loss and high conversion efficiency and thus have been considered as a promising type of novel solar collectors. This thesis focuses on developing novel heat transfer fluids for mid-temperature DASCs. Ion liquids(ILs) with wide liquid temperature range and high decomposition temperature have been chosen as the base liquids to prepare nanofluids by adding different kinds of nanomaterials in the ILs, and their radiation and thermophysical properties and photo-thermal conversion performance have been systematically studied.Three kinds of sphere nanoparticles with the same average sizes of 40 nm including Cu, Ni and carbon-coated Ni were dispersed into [HMIM][NTf2], respectively. The transmittance spectra of the obtained nanofluids were measured, and the effects of light distance, types of nanoparticles and their mass fractions on the transmittance of the nanofluids were investigated. It is shown that the addition of the nanoparticles in the IL has the function of increasing its optical absorption property, even at a loading as low as ppm level. The transmittance of the nanofluids decreases with the increase in light distance and mass fraction of the nanoparticles. The extinction coefficients of the nanofluids containing different nanoparticles were obtained from their transmittance according to the Beer-Lambert law, and compared with the calculated values based on the Rayleigh scattering model. It is found that the extinction coefficient of carbon-coated Ni nanofluid is higher than those of Cu and Ni nanofluids, owing to the good optical absorption property of the graphite carbon coated on the Ni nanoparticles in the visible-light and near-infrared range.Considering the excellent optical property of graphite carbon materials, three kinds of carbon nanomaterials including graphite nanoparticles(GNPs), single-wall carbon nanotubes(SWCNTs) and graphene(GE) were dispersed into [BMIM]BF4 to prepare carbon nanofluids, respectively. The thermophysical properties including thermal conductivity, specific heat and viscosity of the obtained carbon nanofluids were measured in the range from room temperature to 150 oC, and their photo-thermal conversion performance were evaluated by stagnation experiment. It is shown that the addition of the carbon nanomaterials obviously increase the thermal conductivity of the IL. At the same loading, the GE nanofluids exhibit the highest thermal conductivity as compared with the nanofluids containing the GNPs and SWCNTs. The specific heat values of the carbon nanofluids are slightly lower than that of the IL, due to the lower specific heat of carbon nanomaterials compared to the IL. The viscosity of the IL and its nanofluids remarkably decreases with the increase in temperature, which helps to their applications in solar energy mid-temperature utilization. During the illumination under a solar simulator, the carbon nanofluids exhibit higher rising rates in temperature than the IL, owing to the high thermal conductivity and excellent optical absorption property of the carbon nanomaterials;The nanofluid containing 0.01 wt% GE reaches the highest temperature of 103.5 oC at the light intensity of one sun, corresponding to 26.6% in photo-thermal conversion efficiency. These results reveal that GE/IL nanofluids show promising for use as the heat transfer fluids in DASCs.