A Study of the Transport Phenomena of Porous Thin Plate Drying Using Solar Energy

• Main Authors: SEKONE Abdoul Karim, 西肯
• Other Authors: 李明蒼
• Format: Others
• Language: en_US
• Published: 2016
• Online Access: http://ndltd.ncl.edu.tw/handle/20035552199616905833

博士 === 國立中興大學 === 機械工程學系所 === 104 === The improvement of the performance of thermal drying systems is an important issue. The introduction of new materials such as nanostructures in solar thermal systems and a better control of the drying conditions are of great importance to attain good drying systems. Silicon nanowire, for example, possesses a great potential for solar energy harvesting and conversion owing to its unique thermal and surface properties. The behavior of silicon nanowire to thermal radiation at different wavelengths in the solar spectrum needs to be exploited for this purpose. Furthermore, analysis of the drying process, from the transport phenomena of the drying material to the energy conversion efficiency of the dryer, is a means to determine the performance of drying systems and to provide comprehensive insights to designers, researchers and engineers. In this research project, spectral hemispherical reflectivity and transmissivity of the black silicon nanowire array and of plain silicon wafer were measured. It was observed that the reflectivity of the black silicon nanowires array was lower in the visible range but higher in the infrared range compared to the plain silicon wafer. The significantly reduced spectral reflectivity of silicon nanowire to visible light makes it even more attractive in solar energy applications. Drying experiments and theoretical calculations were carried out to directly evaluate the effects of the trade-off between scattering properties of silicon nanowires at different wavelengths. Results of this study shows that a 17.8% increase in the harvest and utilization of solar thermal energy could be achieved using a silicon nanowire array on silicon substrate as compared to that obtained with a plain silicon wafer. In addition, a porous thin plate was dried in a convective dryer at different drying conditions to analyze the drying process and conduct the energy and exergy analyses. Unlike the conventional methods used in most literatures where constant moisture transport coefficients were usually assumed, moisture content (ϕavg) dependent mass transfer parameters (diffusivity Deff and convective mass transfer coefficient hm) were proposed in the current work for the moisture transport analysis. Results of the moisture-dependent mass diffusion coefficient (Deff) and the mass transfer coefficient (hm) were also determined for the cases in other achieved studies for comparison. It is shown that the moisture-dependent mass transfer parameters that derived from the proposed method in the current study can better represent and reflect the physics of the transport phenomena in porous material drying. Analysis of the thermal performance of the convection dryer in the current study indicated that a significant amount of the energy provided to the system was not utilized for drying especially for high drying rate dryers. It was also shown that by adjusting the configuration of the air inlet and flow rate, the time average exergetic efficiency of the dryer can be significantly increased from 0.5% to 10%. Based on these findings, the current study aimed to provide useful and valuable insights and directions to attain high efficiency drying systems.