Developing High-performance Water Based Nanofluids For Direct Absorption Solar Collectors

Direct absorption solar collectors(DASCs),a new concept of solar collectors,have the merits of low heat loss and fabrication cost,making them a promising type of high-performance solar collectors.The core in a DASC is its working fluid that functions as both a light absorbing medium and a heat transfer fluid,and thus its characteristics greatly determine the performance of DASCs.Nanofluids are a kind of promising working fluids for DASCs,but they should meet the requirements of long-term stability,good optical absorption property,high thermal conductivity and excellent photo-thermal conversion performance.This thesis focuses on developing high-performance water based nanofluids for low temperature DASCs.Specifically,graphene oxide(GO)was chosen as the nanoadditive for preparing water based nanofluids owing to its excellent dispersion in water.And the effect of the mass fraction of GO on the dispersion stability,optical absorption property,thermal-physical property,and photo-thermal conversion performance of the GO/water nanofluids was systematically investigated,with the purpose of optimizing the mass fraction of GO.Furthermore,in order to further increase the photo-thermal conversion performance of the water based nanofluid,a novel and simple route was presented,which involved directly irradiating the GO/water nanofluid with the optimal mass fraction using a UV lamp to prepare reduced GO(RGO)nanofluids.A series of GO/water nanofluids with the GO mass fractions ranging from 0.001 % to 0.1 % were prepared,and their Zeta potentials were measured.It is shown that the maximum Zeta potential(?)was achieved by the nanofluid containing 0.02 % GO,which exhibited excellent long-term stability and could keep stable at elevated temperatures.The optical absorption property of the GO/water nanofluids was superior to water and increased with the mass fraction of GO.Compared with water,the GO/water nanofluids exhibited enhanced thermal conductivity,and the enhancement ratio increased with the mass fraction of GO.The photothermal conversion performance of the GO/water nanfluids were experimentally evaluated,and it is found that the photo-thermal conversion performance of the GO/water nanofluids was higher than that of water,and the highest efficiency was achieved by the one containing 0.02 % GO,which was 97.45 % at 30 ? and 48.92 % at 80 ?.The GO/water nanofluid at a loading of 0.02 % was irradiated under UV light for different times to prepare a series of reduced graphene oxide(RGO)/water nanofluids.X-ray photoelectron spectroscopy(XPS)analyses revealed the reduction degree of GO increased with the UV irradiation time.Zeta potential measurements suggested that the RGO/water nanofluids obtained under the irradiation time of 340 s or less can keep stable at elevated temperatures.The transmittance of the RGO/water nanofluids was quite less than that of the GO/water one,indicating that the reduction of GO leaded to the enhancement in optical absorption property.Compared with the GO/water nanofluid,the RGO/water nanofluids possessed higher thermal conductivity and lower specific heat.It is found that the photo-thermal conversion efficiency of the RGO/water nanofluid obtained under the irradiation time of 340 s was 96.93 % at 30 ? and 52 % at 75 ?,higher than those of the GO/water one(95.51 % at 30 ? and 46.2 % at 75 ?).The results suggested that the method for preparing the reduction GO/water nanofluids by irradiating the GO/water nanofluid under UV light for a suitable time can not only keep the good dispersion stability of the GO one,but also enhance its optical absorption property and thermal conductivity,resulting in the superior photo-thermal conversion performance of the RGO/water nanofluid.In this thesis,the dispersion stability,optical absorption,thermos-physical property and photo-thermal conversion performance of the GO/water and RGO/water nanofluids were experimentally investigated,and the obtained results shed light on the development on high-performance nanofluids for use as working fluids for low-temperature DASCs.