Radiative Properties Of Nanofluids And Its Tilization In Solar Thermal Power Svstems

Alternative energy is attracting a growing interest in solving the problems of global energy and environment, including nuclear, wind and solar energy. Solar energy is clean, abundant, and widely distributed, and solar power technologies have a great progress in recent years.Nanofluid is an engineered colloidal suspension of nanoparticles in a base fluid. Convective transport and effective thermophysical properties of nanofluids have been extensively studied in the last decade. As to the radiative properties, it is shown that nanofluids can strongly absorb or spectral selectively absorb solar radiations. Nanofluids are expected to apply in fields of automotive industry, disease treatment, cooling of electrical devices, solar energy and etc.The nanofluids were prepared though a two-step method in this thesis, e.g. TiO2, Al2O3, Ag, Cu, SiOa, graphite and carbon nanotubes, were added directly into Texaherm oil for preparing stable suspension colloids. In preparation, the mixtures were mechanically stirred, sonication, and oleic acid (OA), cetyltrimethyl ammonium bromide(CTAB) and hexamethyl disiloxane (HMDSO) were added as dispersing agents with SN ratio0.1-1. The equivalent diameters of the agglomerated clusters were detected by Malvern Zetasizer Nano ZS90to evaluate the stabilities of the nanofluids samples.The radiative properties of nanofluids are numerically and experimentally investigated. For nanofluids, scattering and absorbing of electromagnetic waves by nanoparticles, as well as light absorption by the matrix/fluid wherein the nanoparticles are suspended, should be considered. Five models for predicting apparent radiative properties of nanoparticulate media were compared and evaluated their applicability. Using spectral absorption and scattering coefficients predicted by different models, the apparent transmittance of a nanofluid layer was computed, including multiple reflecting interfaces bounding the layer, and compare the model predictions with experimental results in literatures and measured using SHIMADZU UV3150ultraviolet and visible spectrophotometer. Finally, a new method to calculate the spectral radiative properties of dense nanofluids were proposed for the radiative properties of nanofluids, which shows a quantitatively good agreement with the experimental results.Hunt introduced the small particle heat exchange receiver (SPHER) for solar-thermal electric power generation in1978, by dispersing small absorbing particles into a gaseous working fluid, and showed that this device offered an effective improvement. A nanofluid solar collector experimental model based on direct absorption collection (DAC) concepts was built to analyze influences of working fluids and operation conditions on system performances. By solving the radiative transfer equation (RTE) combined with conduction and convection heat transfer equations, system efficiencies and temperature distributions of the collector were modeled, whose results were well agreed with those of experiments. The study indicated that nanofluids were almost "black" on absorbing solar radiation even for low-content nanofluids, providing high outlet temperatures and system efficiencies.