Theoretical Design And Optimization For Radiative Cooling Performance Of Polymers

Due to the increasing power consumption for air-conditioning,the power-free passive radiative cooling technique has drawn great attentions in recent years.The spectral characteristic of materials plays a vital role for radiative cooling.To achieve sub-ambient temperature,materials should both emit strongly in the primary atmospheric window(8-13 μm)and reflect strongly in the solar regime(0.4-2.5 μm).Selective emitter(8-13 μm)and Broadband emitter(4-25 μm)are the two major types of emitters reported in current works.In this work,we put forward a new type of emitter which has high emissivity in the two atmospheric windows(8-14μm&16-25μm)while maintaining high reflectance at other wavelengths.The proposed emitter has greater cooling potential than both selective and broadband emitters according to our calculations.Nowadays,polymer-based emitter has become a hot topic,we designed PDMS/PS multilayer structure with a layer thickness gradient to achieve both high infrared emittance and solar reflection,without a metal substrate.The thermal equilibrium model between emitter and surroundings was proposed,and the contribution of the secondary atmospheric window(16-25 μm)to net cooling power was calculated to be 25%under ideal condition.The cooling performance of the newly-proposed emitter under different conditions was calculated and compared to those of the two traditional emitters,and was proved to be better,except for extreme sub-ambient conditions(hc<1.0 Wm2K-1).The emissivity of PDMS films with different thickness was calculated according to Fresnel equation.A three-layer structure(selective membrane reflector/broadband emitter/metal substrate)was proposed to realize the dual-window selective emission characteristic,the guided-mode resonance in two-dimensional photonic crystal was utilized to filter thermal emission between 14-16 μm.FDTD simulation was conducted to get fit geometry parameters for the silicon photonic slab with square lattice.The angular emissivity and physical mechanism were studied carefully,and a ε8-13μm valued 0.86 and a R14-16μm valued 0.75 were achieved.The temperature drop of the designed emitter was expected to be 15 K when Ta=300 K,Psun=30 Wm-2 and hc=2.0 Wm-2K-1.The genetic algorithm was combined with transfer matrix method,to get a proper thickness sequence for the PDMS/PS multilayer structure.A ε8-13μm over 0.9,and a R0.4-2.5μm up to 0.98 were achieved under 200 total layers,and the results were independent to angle and polarization to a large degree.The daytime cooling performance of the multilayer was expected to be 12.7 K when Ta=300 K,hc=6.0 Wm-2K-1.