Study On The Photo-thermal Conversion Performance Characteristics Of Therminol~?66 Oil-based Nanofluids In Direct Absorption Solar Collectors

This thesis focuses on the solar thermal utilization of oil-based nanofluids in direct absorption solar energy systems.The basefluid used in nanofluids is Therminol^?66,and nanocomposites of graphene oxide—muti-walled carbon nanotubes?GO-MWCNTs?and core-shell Ag@Al₂O₃ NMs were used as nanomaterials?NMs?.The oil-based nanofluids were prepared by two-step method,and oleic acid was selected as the dispersant.The optical absorption characteristics of oil-based nanofluids were measured by UV-vis spectrophotometer.The photo-thermal conversion performance of oil-based nanofluids was investigated based on indoor and outdoor photo-thermal conversion experiments.In addition,the change of Therminol^?66 before and after photo-thermal conversion experiment was analyzed and discussed,and its influence on the photo-thermal conversion performance of nanofluids was evaluated.The main work and results are obtained as follows:?1?Compared with pure Therminol^?66,there was a sharp decline for the transmittance of oil-based GO-MWCNT nanofluids in the wavelength range of 200²500 nm,and the optical absorption performance of oil-based nanofluids increased with the increase of nanofluid concentration.The photo-thermal conversion performance of nanofluids increased first and then decreased with the increment of mass fractions of GO-MWCNT NMs.An optimum concentration of 100 ppm was observed with respect to photo-thermal conversion performance in this experiment.After an irradiation time of 45 min,the temperature of 100 ppm nanofluid was increased by 11.6 ^oC,which is higher than that of pure Therminol^?66.Besides,the receiver efficiency was increased from 52%?pure oil?to70%?100 ppm?.The outdoor experimental results showed that a highest temperature of153 ^oC was obtained at about 13:00 pm by the 100 ppm nanofluid.?2?The absorbance of oil-based core-shell Ag@Al₂O₃ nanofluids is near the maximum in the wavelength range of 190³50 nm.In the visible and near-infrared bands,the absorbance of Therminol^?66 and nanofluids decreased drastically,and the absorbance of the nanofluids increases with the increase of the NMs'concentration.In the wavelength range of 350⁴50 nm,a"red shift"occurred as the mass fraction of Ag@Al₂O₃ NMs increased,indicating that the localized surface plasmon resonance?LSPR?of core-shell nanofluids was enhanced with the NMs'concentration.Thus,the broad-band solar absorption by nanofluid could be achieved.The temperature rise of core-shell nanofluids was generally higher than that of pure Therminol^?66,which indicates that the excitation of LSPR of core-shell Ag@Al₂O₃ nanoparticles is beneficial to enhance the absorption characteristics of Therminol^?66 to solar energy spectrum and enhance the photo-thermal conversion performance of nanofluids.The receiver efficiency increased with the increase of NMs'concentration,and it increased first and then decreased with the irradiation time.The maximum receiver efficiency value was 82.7%for 0.05 wt%nanofluid at an irradiation time of 10 min.?3?It was found that the minor color change of Therminol^?66 was observed before and after the photo-thermal conversion experiment,and the color of Therminol^?66 sample became darker with the increase of heating temperature.UV-visible spectrophotometer and nuclear magnetic resonance?NMR?measurements were carried out on three samples?before experiment,after indoor experiment and after outdoor experiment?.The results showed that thermal oxidation degradation of Therminol^?66 appeared after the experiment.The nanofluid after photo-thermal conversion experiment has better long-term stability than that before experiment.In the wavelength range of 350⁴50 nm with a significant drop of absorbance,the structural change of Therminol^?66 after photo-thermal conversion experiment partially enhanced the optical absorption of oil-based core-shell plasma nanofluid.The absorbance of core-shell nanofluids in the 450¹150 nm band is relatively stable,and the illumination has little influence on the absorbance of nanofluids at low concentrations.However,for the high concentration of 0.05 wt%,the illumination effect on Therminol^?66 is not conducive to the improvement of optical absorption performance of oil-based nanofluids.