Study On Selective Absorption Characteristic Of Nanofluids Under Solar Radiation

Compared to ordinary materials, nanoparticles have different characteristic of light absorption under solar radiation because of its small size effect. When particles are dispersed in different medium, light transmission, scattering and absorption characteristics of the dispersed system which is also different from ordinary dispersed system vary according to properties of medium, diameter of particles, optical constants of particles, concentration of dispersed particles, space distribution of particles and thickness of medium. Solar energy PV/T system increases the comprehensive utilization efficiency of solar energy by combining solar cell unit with solar energy collector and integrating utilization of whole solar energy wavebands.Fisrt, this paper studies the radiation characteristic of nanofluids, overviews the research about the thermophysical of nanofluids and systematically introduces theoretical modes for radiation transmitting in medium. Effective medium theory can be used to explain the radiation mechanics of nanofluids when the scattering of particle can be neglected. If not, we should use Mie theory and Far-field, Near-field approximation.Then, this paper shows some research on radiation characterisitc of nanofluids. High performance nanofluids are prepared by microfluidizer method. The size and size distribution are analyzed by Laser Particle Size Analyzer. Results indicate that transmissivity of SiO₂ nanofluids are affected by the size, sort and concentration of particles. Transmissivity of SiO₂ nanofluids are again tested when the thickness of medium varies. Results show that the change of transmissivity of water with 7nm diameter particles suspended with the variation of optical path fits the labbert-beer law. But when the diameter of particles raises to 40nm, the labbert-beer law is no longer valid. Because attenuation of light transmitting in nanofluids is caused by both absorption and scattering, attenuation coefficient is a function of wavelength and thickness of medium. A method is proposed to amend lambert-beer law for nanofluids.Third, solar radiation is simulated by Solar Energy Simulate Generator. Experiments on the performance of solar cell with nanofluids on the surface have been done to analyze the influence of nanofluids on opening circuit voltage, the maximum power and the temperature of solar cell. When PV/T system reaches to stable state, calculation model is established to calculate the efficiency of PV/T system when battery-length, mass flow of cooling fluid. intensity of radiation and inlet temperature of cooling fluid vary. This paper defines the concept of critical length and generalizes the ways to improve the system efficiency, providing reference for the construction of PV/T system.This paper aims at using special optical characteristics of nanoparticles to change the radiation characteristic of medium with certain wavebands and exploring certain nanofluids which can fully transmit solar visible light and absorb other wavebands to improve efficiency of utilization of solar energy for PV/T system. Reserches indicate that properties of medium, concentration of dispered particles, diameters of particles, optical constants of particles and surface configuration of particles are the five main factors that influence the radiation characterictis of nanofluids.