Thermal-optical Radiation Through Porous Materials And Photonic Crystals For Solar Energy Devices

Nowadays,new approaches aimed at the development of inexpensive energy devices have become essential to both science and industry.Metals as well as porous materials due to their remarkable mechanical,thermal and electrical insulation properties are used in an array of thermal and optical systems for energy issues.Inspired by these perspectives,this work is attempted as contribution to investigate with respect to the porosity effects on optical and thermal radiation characteristics through porous materials and photonic crystals?PhCs?for solar energy systems.This is to develop a comprehensive approach for the more accurate prediction in light manipulating as well as enhancement of heat transfer characteristics through such design in terms of radiative efficiency at known porosity.In achieving this,optical radiation through porous materials based PhCs have been extensively studied by numerical simulation employed the transfer matrix method?TMM?under"Matlab program"and the esteemed finite differential time domain?FDTD?tools under"Lumerical FDTD Solutions".However,near-infrared one-dimensional transmission characteristics of porous Si/Al₂O₃ materials based PhCs were studied.This provides new insights in efficiency of light transmission characteristics through 1D PhC which consisted of periodical pattern of porous silicon/porous alumina?p-Si/p-Al₂O₃?layers with different dielectric constants arranged in the same dimension.This was used to investigate at the normal incidence the effect of porosity on the transmission characteristics through such structure in near-infrared wavelength range of 800-2200 nm.The results have been compared with relevant works and agree with these existing published solutions in the literature.This study revealed that an increase in porosity which corresponds to a decrease in effective refraction index reduces the width of the transmission pass band.While,the appearance of the stop band is visibly apparent as porosity increased.In addition,increase in unit cells ensued apparent creation of transmission stop band.Furthermore,FDTD implements were applied to simulate the optical properties characteristics' through square and triangular lattices of pSi PhCs;which consisted of periodical patterns of circular air holes built into the pSi material.This was used to investigate the influence of porosity and lattice dynamic on the reflection,transmission and absorption characteristics through unit cell pSi PhC in the visible wavelengths domain of 400-700 nm.The numerical simulation was achieved using FDTD Lumerical solutions with periodic boundary conditions?PBC?and perfectly matched layers?PML?as the appropriate boundary conditions.The results revealed that the limitation of optical properties is dependent on porosity and the lattice dynamic in pSi PhC.This was presented by the trend;the higher the reflection the higher the porosity and a decrease in porosity led to an increase in absorption in both lattice considerations.It was discovered that attaining optimum properties for triangular lattice will entail considering porosities less than 50%and hole radius r to the lattice constant a ratio r/a beyond 0.3 for the absorption characteristic and behind 0.3 for the transmission characteristic.Therefore,triangular lattice can be adapted to improve the optical pattern through the PhC.In addition,the optimisation of these properties through pSi PhCs was achieved by controlling porosity and the ratio r/a.Besides,a novel composite of Ag/porous Si as metallo-dielectric based periodical lattice nanostructure was proposed and studied for improvement of the light manipulating efficiency through a proposed structure which can be integrated in solar devices.This provides new insights on numerical FDTD simulation and brings contribution to the analysis of the optical characteristics through the proposed structure at known porosity.This was used to investigate the effect of porosity and the number of layers?nLs?on the reflection,transmission and absorption characteristics through a proposed structure in a visible wavelength range of 400-750 nm.The results revealed that the higher the nLs,the lower the reflection.Also,the reflection increases with porosity increase.The transmission characteristics were the inverse to those found in the case of reflection spectra and optimum transmission was attained at high n Ls.Also,increase in porosity results in reduced transmission.Increase in porosity as well as in the nLs led to an increase in absorption.Therefore,absorption into such structure can be enhanced by elevating the n Ls and the degree of porosity in the visible wavelengths.On the other hand,the same study in such proposed design has been conducted in the near-infrared wavelength range of 750-3000 nm.The results revealed that these highlighted optical properties through the composite are influenced by variation in the n Ls and porosity in near-infrared wavelengths.The reflection and transmission spectra presented contrasting patterns.Optimum transmission was attained at 80%porosity with the thickness higher than 500 nm.For the composite Ag-pSi,all the peaks found in these optical properties tended towards higher wavelengths as compared to the case of the composite Ag-Air which had an empty structure.The optimum absorption was improved by 10%with the incorporation of pSi into the Ag material.Therefore,the enhancement of optical properties in this proposed design can be achieved by controlling both the nLs and porosity.Besides,the thermal radiation characteristics in porous materials based PhCs were studied.The pSi was selected as typical porous material for such investigation.However,a plan wave expansion?PWE?algorithm from wave equation based Fourier expansion method to the first kind of Bessel function was established.This was used to ascertain the effect of porosity on thermal radiation characteristics for enhancement of light absorption in pSi PhCs.Thus,2D concept of unit-cell for square and triangular lattice of circular air holes was formulated and solved.The spectral energy density?SED?of the thermal radiation at different porosity consideration of Si PhC was investigated for both mentioned lattices including the effect of these lattice structures in the creation of photonic band gaps?PBG?.It revealed useful to increase the number of frequency band gaps by subjecting the Ph C to a square lattice at low porosity.Moreover,a triangular lattice beyond 50%of porosity would be suitable in thermal radiation control where the band gap stabilization applications are required.Moreover,the prediction of thermodynamic properties performance in pSi PhCs was studied for enhancement of light conversion into different types of energy based thermophotovoltaic energy conversion system.The unit-cell for 2D square and triangular lattices of circular air holes was highlighted.This was used to investigate the thermodynamic properties as well as the influence of lattice dynamic on these properties at different porosity consideration in silicon PhCs;which are regarded as isolated and non-interacting particles systems.This was achieved by the link connecting the density of states?DOS?to the thermodynamic quantities as described in statistical physics.This revealed that irrespective of lattice type,increasing porosity ensued a decrease in thermodynamic properties.Novel insights and theoretical concepts that could be integrated in thermophotovoltaic system for responding in solar energy mitigation were revealed.Finally,analysis of heat transfer characteristics through porous materials based thermal lattice Boltzmann method?TLBM?has been conducted in highlighting the heat conduction process through porous materials.The polyurethane?PU?foam due to its high performance insulation under thermal protection was used in this analysis as a typical porous material.However,a novel algebraic model in investigating the effective thermal conductivity?ETC?was developed based on single relaxation time?SRT?approach of lattice Boltzmann method?LBM?.The temperature profiles and conductive heat transfer characteristics were examined using LBM algorithm with suitably selected boundary conditions and implemented in Matlab.The results were compared with existing prediction models and agree well with these relevant works of heat conduction characteristics in porous materials.It revealed that increment in the degree of material-porosity reduces the thermal conductivity.However,the mechanical strength of the material is adversely affected and therefore posing a challenge.