| In recent years,with the huge consumption of fossil resources and the resulting environmental pollution,it has become an urgent problem to find new clean alternative energy sources,recycle waste heat and improve energy conversion efficiency to reduce the over-dependence on traditional energy.As an effective energy conversion device,thermal photovoltaic system can make full use of waste heat to generate electricity,reduce losses and improve conversion efficiency,and it has potential to become one of the best solutions to solve the problem.However,due to the mismatch that arises between emission spectrum of emitter and absorption spectrum of PV cell in TPV system,the performance of the system is greatly limited.Microstructure surface exhibits unique spectral wavelength selection,which can be used as an effective selective emitter in TPV system to improve the power generation efficiency of the PV cell.In this paper,different types of 2D periodic microstructures were proposed.The microstructure radiation characteristics were thoroughly studied,and the parameters of the proposed microstructure were optimized by genetic algorithm.In this paper,the spectral emission properties of 2D periodic microstructured surface,composed of tungsten and aluminum oxide,are studied based on the finite difference time domain(FDTD)method.We firstly focus on the single grating structure,by controlling variables in the case of same initial geometric parameters.The effects of period,grating width,grating height and dielectric layer thickness on the thermal radiation characteristics of the microstructure are analyzed.It was found that the geometric parameters can affect the spectral emissivity,the location of the emission peak and the emission bandwidth.The analysis of the radiation characteristics under different resonance peaks indicated that the high emittance can be attributed to the excitation of magnetic resonance,surface plasmon polarition and the intrinsic energy absorption of tungsten.Besides,by adding another metal-dielectric stack upon the single grating structure,we form a composite structure,and the radiation characteristics are studied.Compared to the simple grating spectral emissivity,the emissivity of the composite structure is further enhanced,and the normal emissivity generally exceeds 0.9 in the target spectrum.The distribution of the electromagnetic field at the resonant wavelength showed that the composite structure can exhibit equivalent characteristics as the single structure,and similar magnetic resonance phenomena can be observed in the overlay layer.Under different polar angles and polarization states,the emissivity of the composite structure exhibits insensitive to direction and polarization.The proposed microstructure which shows good wide wavelength selection and radiation characteristics,as a selective emitter,could play a good role in improving the performance of TPV system.In order to achieve the matching between the selective emitter and different PV cells,the optimization method combining FDTD method with genetic algorithm is applied to three microstructures in different shapes as selective emitters,to optimize the structural parameters.we established an appropriate objective function through the theoretical model of the TPV system for the problem to be solved,and an optimization method was proposed to maximize output power of the TPV system while limiting the losses due to thermal leakage.The emission spectrum of the optimal designs obtained using this method for emitter temperature of 1500 K matches well with the EQE of PV cell,all selective emitters with different shapes limit the radiant energy to a very low value at longer wavelengths when the power output of the GaSb and InGaAsSb cells is maximizing.This study has a good reference value for the design and optimization of selective emitter for TPV systems. |
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