Study Of Nanofluids' Radiation Properties And Its Utilization In Photovoltaic/Thermal System

Energy plays an important role in the development of human society. However, over the past century, the fast development of human society leads to the shortage of global energy and the serious environmental pollution. All countries of the world have to explore new energy sources and develop new energy technologies to find the rode to sustainable development.Solar energy as the renewable and environmental friendly energy, it has produced energy for billions of years. Solar energy that reaches the earth is around 4×1015MW, it is 2000 times as large as the global energy consumption. Thus the utilization of solar energy and the technologies of solar energy materials attract much more attention. Nano-materials as a new energy material, since its particle size is the same as or smaller than the wavelength of de Broglie wave and coherent wave. Particles'cyclical boundary condition is changed. Additionally, as the particle diameter decreases, the surface effect of nanoparticle increases sharply. Therefore, nanoparticle becomes to strongly absorb and selectively absorb incident radiation. Researchers present the direct solar radiation absorption collection (DAC) system concepts 1980s. Based on the radiation properties of nanoparticle, the utilization of nanofluids in solar thermal system becomes the new study hotspot.Based on the review of nanofluids radiation, direct absorption collection systems and solar energy utilizations, first the thesis discussed the ratiative heat transfer of nanofluids, then designed and analyzed the new Photovoltaic/Thermal system using the DAC technology. Because the premise of nanofluids research is even distribution and no sedimentation of nanofluids, in the thesis we used the one-step method to produce even distribution nanofluids, and two-step method to produce nonhomogeneous distribution nanofluids by the M-110S Microfluidizer Processor. The stability of nanofluis and the size of nanoparticle and aggregate were also studied, which lays the foundation for the nanofluid radiation research.The thesis studied the radiation properties of nanofluids system and nanopartcles, and compared the radiation transfer equation (RTE) and the effective medium theories. The effective optical constant of nanofluids was simulated by different effective medium models. The validity of these models for various nanofluids were analyzed in combination with the experiments. As for the radiation of nanoparticles, the extinction and scattering experiments bench were set up, and the "Maximum Ratio Method" was proposed first time. The method avoids the dependence of the number of experimental nanoparticles for the simulation. It can be as the bridge between experiment and simulation, and provide an easy way to study the nanoparticles radiation. Based on the method, the effects of free electron term and interband or bound electron term on the nanoparticles radiation properties were investigated.Aiming to solve the practical problems, the thesis innovatively developed an inverse method for radiation. Since, the method is based on the classical dispersion theory, and adopted the PIKAIA genetic algorithm, the retrieved results of optical constant automatically satisfy the Kramers-Kronig relation, and the retrieving process is robust and fast. The method can be started from the desired radiation properties such as desired reflectivity and absorptance et al. The method can be also used to determine the complex index of refraction, and it can serve as a guideline for designing solar thermal systems.Moreover, the thesis innovatively expands the DAC nanofluids technology to the Photovoltaic/Thermal system, and presents a new Photovoltaic/Thermal system. The new system separately utilizes the solar radiation to produce thermal energy and electricity, thanks to the working fluid absorbing infrared solar radiation and the transmitted visible radiation by the solar cell. Furthermore, the system consists of a thermal unit placed above a PV module and separated by an air gap of arbitrary thickness. Thus, the thermal unit is no longer extracting heat from the PV module, and has no temperature limitation of the PV module. The PV module need not glued to the thermal unit to avoid delamination concerns. In addition, the system can produce high temperature thermal energy with high electricity efficiency.Using the inverse method, the effective complex index of refraction of working fluid in the system was retrieved and optimized based on the effective medium model. The absorptance and average absorptance of the thermal unit and PV module were also calculated. Furthermore, the working temperature of thermal unit and PV module, the electrical and thermal efficiencies and the exergetic efficiency were studied based on the radiation transfer and energy conservation equations. The results indicated that the new Photovoltaic/Thermal system separately using the infrared and visible light to avoid the five limitations of traditional systems, the system has high electricity and thermal efficiencies and exergetic efficiency.