Building Thermal Management Materials By Micro-nano Photonic Structures

Recently,due to the impact of global climate change and the increasing energy consumption of building cooling and heating,thermal management without any energy input has attracted more and more researchers’attention.Thermal management is mainly used to control the spectral characteristics of materials in the solar（0.3-2.5μm）and long wave infrared（LWIR,2.5-18μm）bands through micro-nano optical structures to manage the indoor temperature of buildings.Therefore,the application of this thermal management technology on the roof and exterior walls of buildings can improve the cooling and heating efficiency of traditional electric thermal management systems,reduce electric consumption,and achieve energy saving and emission reduction in buildings.At present,the research work of thermal management is mainly focused on a single function（cooling or heating）,which cannot adapt to the dynamic climate change in the four seasons of the year,thus greatly limiting the energy saving and emission reduction effect of thermal management.Therefore,this work starts from the direction of dynamic thermal management.First,the micro-nano photonic structure is used to realize the radiative cooling and selective solar heating performance respectively,and finally,this work combines the radiative cooling and selective solar heating materials to achieve the dynamic thermal management effect.（1）Structural design and performance investigation of radiative cooling materialsFirst,it is found that the self-thermal management performance of Bombyx mori cocoon and Pieris rapae butterfly wing.Based on the refractive index difference（Δn=0.56）between the structural components and the air,the micro-nano scale structure could realize the strong Mie scattering of sunlight at the interface between the structure and air,thus achieving high solar reflectance.In addition,according to the reciprocity of absorption and emission,its constituent materials have high absorptance in the LWIR band（especially in the the atmosphere’s LWIR transmission window,from 8-13μm）,resulting in high LWIR emittance.Inspired by the biophotonic structure,two photonic structures,a porous structure,and a fiber structure,are proposed.To realize the highly-efficient selective radiative cooling performance,polypropylene（PP）,which only has an absorption in the atmospheric window band,is selected as the polymer matrix.It is found that the air pores with a diameter of 10-50μm can achieve high reflectance in the LWIR band but have low scattering efficiency in the solar band,thus affecting the solar reflectance.For this reason,melt-blown PP film（MB-PP）uses the fiber and air pore to respond to the solar light and LWIR bands respectively,which can realize the precise control of the solar light and LWIR spectrum at the same time,and finally presents the selective radiation characteristics.Due to the weak absorption of PP in the atmospheric window band,PDMS is used to build a thermal radiative layer on the MB-PP surface to strengthen the atmospheric window emissivity to 0.82,thus achieving the daytime cooling effect of 4℃ below the ambient temperature.Due to broadband radiative characteristics of PDMS modified MB-PP,this work further studies and realizes the selective radiative characteristics of MB-PP for the better cooling effect.Therefore,based on the morphological hierarchical structure design and lamination technology,silicon dioxide（SiO₂）and silicon nitride（Si₃N₄）dielectric particles with high absorption characteristics in the atmospheric window are added to LWIR transparent polytene（PE）to prepare a selective radiative layer.According to the simulation results of absorption efficiency,the size of SiO₂ particles should be controlled at 2μm,and the size of Si₃N₄ particles should be controlled at 1μm is the best.Through the combination of SiO₂ and Si₃N₄ particles,the emittance of the atmospheric window band is increased from 0.50 to 0.82 without affecting other bands,thus achieving the daytime cooling effect of 6℃ below the ambient temperature.Based on the above research work,PE/SiO₂/Si₃N₄ film with selective radiative characteristics and polydimethylsiloxane（PDMS）coating with broadband radiative characteristics are respectively constructed on two sides of MB-PP,which can realize bidirectional asymmetric radiative performance.Its selective radiative surface can achieve 94%solar reflectance and 0.82 selective emittance,while the broadband radiative characteristic layer can achieve 0.8 broadband LWIR absorptance.Due to this bidirectional asymmetric radiative characteristic,the skin temperature can be maintained at 34℃ under outdoor sunlight.（2）Structural design and performance investigation of selective solar materialsA selective solar heating material is proposed to realize the building’s thermal management in cold areas or seasons.Based on the surface plasmon resonance effect of nano-metal particles,a layer of nano-Cu structural layer is deposited on the surface of the Zn layer through the Cu-Zn replacement reaction,to achieve a high absorptance of 90%in the solar band and a low emittance of 0.04 in the LWIR band.Therefore,under the same solar absorptance,the average temperature of Cu-Zn selective solar heating material is 2-3℃ higher than that of broadband solar heating material.（3）Structural design and energy saving of dynamic thermal management deviceTo meet the demand for dynamic thermal management of buildings,the selective radiative cooling material and the selective solar heating material are integrated to produce a Janus blade,replacing the traditional shutter blade.By rotating the blades,the switch between radiative cooling,solar heating,and solar transmitting functions can be realized,and the passive,intelligent,and dynamic thermal management effect can be achieved.According to Energyplus simulation,the total energy saving of the multimodal thermal management device in one year is up to 38%in the United States and 48% in China.