Preparation And Energy-saving Effect Of Transparent Radiative Cooling Materials For Energy-saving Windows

With the intensification of the greenhouse effect and the increase in the requirements for living environments,the proportion of buildings’cooling energy continues to rise in total energy consumption.As the main way of the energy exchange by heat radiation between indoor and outdoor,windows need special designs to save energy consumption.Now,there are lots of research and products about light modulating layers to reduce cooling energy consumption.However,due to the large absorptivity of the light modulating layers within the solar spectrum,the temperature of energy-efficient windows tends to increase,which will reduce the energy-saving effect by the secondary heat transfer of windows.For cooling the heated windows,the integration of radiative cooling,a passive process that can reduce its temperature without energy input,with existing energy-efficient window technology has a broad prospect.Nevertheless,most transparent radiative cooling materials are metamaterials with photonic crystal structures,which need complicated manufacturing processes.Meanwhile,the microstructure of photonic crystal will scatter visible-light and cannot be used on the window.For improving the energy-saving effect of energy-efficient windows and achieving industrial application,a cheap new radiative cooling material that has high transmittance in the visible-light region and prevents scattering is needed.To solve the above problems,based on the theory of interaction between materials and light,this article explores how to improve the radiative cooling performance of the material by changing its optical constants,and selects polydimethylsiloxane（PDMS）as the optimal transparent radiative cooling material.Through the spectral characterization,the actual test,and the energy consumption simulation,this article proves that PDMS is the optimal transparent radiative cooling material for substrates that have cooling demands and especially for the heated window.The main content and significance of this article are as follows:（1）A new method for selecting transparent radiative cooling materials is proposed.Based on the theory of interaction between materials and light,silica（Si O₂）which is the principal part of the existing transparent radiative cooling materials is retained.Based on its infrastructure Si-O bond,the optical constants are changed by introducing Si-C and Si-O-C bonds in Si O₂,and polydimethylsiloxane（PDMS）was selected as the optimal transparent radiative cooling material.Compared with the metamaterials based on Si O₂,spectral characterization proves that molecular design can improve the radiative cooling performance without scattering the visible-light and balance the cooling and lighting demands of energy-efficient windows.This method provides a new idea for the design and selection of radiative cooling materials for better performance.（2）The radiative cooling performance of PDMS films is proven by the actual tests.By testing the actual radiative cooling effect on various substrates,this article proves that the PDMS films can be combined with the Ag reflecting layer to cool down the substrates like metal Al that do not require sunlight,and the temperature drop can reach 17.9 ^oC.The PDMS films can also reduce the operating temperature of solar cells and low-e glasses without changing the spectral characteristics of the substrate within the solar spectrum,and the temperature drop can reach 5.7 ^oC and 8.7 ^oC respectively.The tests show PDMS films have a broad prospect.（3）The feasibility of PDMS films used for building energy saving is studied.In this article,Energy Plus is used to prove that the energy-saving effect of low-e glasses can be further improved by using PDMS films under the different weather conditions in multiple cities.Based on the differences in energy-saving effects of low-e glasses in different months after using PDMS,the importance of dynamically adjusting the mid-infrared emissivity of radiative cooling materials is proposed.Through the calculation,the optimal inner and outer surface mid-infrared emissivity combination of the energy-efficient window and the energy-saving effect in each month is given.It provides a new option for the further development of radiative cooling materials.