Research On The Selective Cover Shield Of Nanoporous Materials For Radiative Cooling

Energy crisis and climate warming have threatened the survival of mankind.To achieve sustainable development,it is imperative to find an energy-saving cooling method.Radiative cooling is a passive cooling technology without energy consumption and environmental pollution.The cover shield is the key technology to achieve sub-ambient radiative cooling.A spectral selective cover shield can not only suppress the influence of non-radiative heat exchange but also effectively reflect sunlight,so that emitters without solar reflection can achieve daytime radiative cooling,reducing the requirements for spectral characteristics of emitters,which is of great significance to the development of radiative cooling.This article focuses on the selective cover shields for radiative cooling.The main research contents and achievements are as follows:(1)A theoretical model for radiative cooling based on the spectral characteristics of the cover shield was established.Considering the energy transfer between the cover shield and emitter,the model is more accurate and solve the problem of traditional model without cover shields.And a simulation model was built using the full implicit difference method.Then the working mechanism of the selective cover shield to improve radiative cooling performance was discussed,and the influence of environmental parameters on the cooling performance was analyzed using the proposed model.(2)Based on the optical theory of transfer matrix method,Lorenz-Mie scattering theory,and finite difference time domain method,the effects of nanoporous material on the spectral properties were studied.The specific parameters of the selective cover shield were determined.Then the nanoporous composite film(NCF)was prepared by thermally induced phase separation method.The NCF has a solar reflectance of 94.5%and an atmospheric window transmittance of 84.9%,providing a new way to deal with the sunlight for daytime radiative cooling.Furthermore,the NCF has good environmental stability,that can meet the application requirements for outdoor radiative cooling technology.(3)An outdoor experimental system was built using the NCF as the cover shield for the radiative cooling devices.Then,the cooling effect and cooling power of the experimental device were tested.The study found that NCF could help the emitters without solar reflection to achieve daytime sub-ambient radiative cooling.The average temperature was 4.9℃ lower than the ambient temperature,the net radiative cooling power was 85.9 W/m~2 in a low-humidity environment.And the cooling performance was further improved for solar-reflective emitters.Moreover,the cooling capacity of the selective cover shield in different areas of China was discussed,and the regional applicability of the technology was analyzed.(4)Applying the NCF to the exterior surface of the building roof,a dynamic heat transfer model of the building coupled with the radiative cooling roof was established according to the heat capacity mass point system method.Then the cooling potential of direct application and indirect application in a prefabricated house of steel plates and a single-layer building were analyzed.It was found that NCF can not only reduce the roof temperature,indoor temperature,and air conditioning cooling capacity,but also delay the turn-on time of the air conditioning,delay the peak occurrence time,and effectively reduce the cooling energy consumption of the building.