Design Of Composite Films For Radiative Thermal Management And Their Applications In Energy-saving Windows

Windows are the important interface for energy exchange between indoor and outdoor environments,which have a great impact on the heating and cooling energy consumption in enclosed spaces（e.g.,buildings and vehicles）.The development of energy-saving windows has become an important boost to China’s“dual carbon”goals.Different from traditional energy-saving strategies,which focus on reducing the energy exchange through suppressing conduction and convection,a new way to further improve indoor energy efficiency is based on the spectral characteristics of windows.The key is to explore suitable materials for radiative thermal management to meet the practical requirements of windows.Nowadays,these materials are of great interest in the thermal management of exterior walls,roofs,and indoor occupants,but their applications in windows require more precise modulation of multi-band light from visible to mid-infrared radiation.It is still a challenge to establish the structure-activity relationship between materials and their spectral characteristics to achieve reasonable modulation of multi-band lights（visible,near-infrared,mid-infrared）,and to realize optimal thermal management in energy-saving windows that match different climatic conditions.In this thesis,three optimal radiative thermal control composite films are prepared for radiative heating,radiative cooling,and switchable dual-mode functions based on the theoretical guidance.Their corresponding energy-saving potentials in window applications are also evaluated.The main points are summarized as follows:（1）The influence of radiative-heating materials on indoor heating energy consumption is studied,which have high visible light transmittance,high near-infrared transmittance,and low mid-infrared emissivity.Based on the Drude model,the effects of carrier concentration and mobility of transparent conductors on solar transmittance and mid-infrared emissivity are expounded.By adjusting the hydrogen doping concentration in indium oxide thin films,the radiative-heating composite films with suitable carrier concentration(2.17×10²⁰ cm^-3)and high mobility(98 cm² V^-1 s^-1)are fabricated on glass substrates.This radiative-heating glass has high solar transmittance（0.836）and low mid-infrared emissivity（0.117）,which can effectively transmit solar radiation and suppress thermal radiation.Compared with the common low-emissivity glass,the radiative-heating glass can improve the temperature（～5°C）of the light-absorbing material below.Besides,the heating demand of a typical building model based on the radiative-heating window is evaluated under different climatic conditions.Results show that the radiative-heating window can save 82～348.4 MJ m^-2 per year,which accounts for2%～31.6%of annual heating energy consumption.（2）The material design principle of radiative-cooling energy-saving windows is studied and realized through the combination of high-emissivity film and near-infrared reflective film that are both visibly transparent.The former is selected based on a principle of functional group matching.A polydimethylsiloxane film is accordingly prepared with low solar absorption（～0.004）,high mid-infrared emissivity（～0.9）,and high visible-light transmittance（～0.94）,which can not only suppress the photothermal effect but also strongly emit mid-infrared radiation.Combining with a near-infrared reflective film,the radiative-cooling glass shows a cooling effect of～7°C lower than the ordinary near-infrared reflective glass through on-site temperature tests.The impact of a typical building model using radiative-cooling windows on the indoor cooling energy demand under different climatic conditions is evaluated.Results show that cooling energy consumption of 20～49 MJ m^-2 can be saved per year,accounting for 3%～8%of the annual cooling energy consumption.（3）The effect of dynamic modulation of mid-infrared emissivity on the energy-saving effect of windows is studied.Based on the above design principles of radiative-heating and radiative-cooling windows,the spectral characteristics of switchable dual-mode（heating/cooling）materials are expounded.The low mid-infrared emissivity,high visible-light,and near-infrared transmittance are required in the heating mode,while low visible-light and near-infrared transmittance,as well as high emissivity,are required in the cooling mode.A dual-mode thermal management composite film consisting of polydimethylsiloxane and poly（3,4-ethylenedioxythiophene）polystyrene sulfonate is fabricated.Under the axial strain,reversible wrinkles and cracks are formed on the surface,thus realizing the function switching.A multi-band light modulation mechanism is revealed,where the formed wrinkles reduce solar transmittance,while the formed cracks increase the mid-infrared emissivity.Thus,the design of synergistic modulation is realized.Experiments show that the temperature regulation ability of the dual-mode composite film to the closed cavity can be up to～4°C.The dynamic regulation ability can enhance the adaptability of typical buildings to changeable climates and reduce the total energy consumption of heating and cooling,which is assessed to be up to 45.5～166.5 MJ m^-2 per year,accounting for 2.5%～21.4%of the total annual energy consumption.