Study On Integrated Utilization Of Spectrally Self-adaptive Photo-thermal And Radiative Sky Cooling Based On Vanadium Dioxide

Cold and heat are critically important energy resources as they occupy up to 51%of the total yearly energy consumption of the general population.However,the supply of cold and heat mainly depends on traditional fossil fuel combustion,which will undoubtedly further aggravate environmental problems.In this context,it is of great significance to develop renewable energy supply and heating in carbon neutralization and to protect the ecological environment.Compared with the Earth’s environment,the outer space(3 K)and the sun(5800 k)are an inexhaustible heat sink and heat source,respectively.The radiative sky cooling(RSC)and photo-thermal(PT)technologies directly utilize these energy sources,respectively,to obtain clean cold and heat.RSC refers to applying radiative heat transfer into the cold outer space through the high transmittance atmospheric window of 8.0-13.0μm to achieve the cooling effect.The photo-thermal refers to using a material that can efficiently absorb solar radiation(0.3μm to 2.5μm)and convert it into heat.These two technologies are similar in terms of the working principle and the physical device.If one device can both obtain cold energy by nighttime radiative sky cooling and heat by daytime photo-thermal,it will have higher energy efficiency than the single function device.However,the spectral selectivity ranges of radiative cooler and solar absorber are vastly different.Some researchers have applied a static spectral coupling coating with high emissivity in both solar radiation band and atmospheric window to realize the functions of daytime photo-thermal and nighttime radiative sky cooling.However,the spectral characteristics of the static spectral coupling coating are fixed,where the high emissivity in the atmospheric window is not conducive for highly efficient photo-thermal conversion.Based on this,this dissertation is focused on the comprehensive utilization of high efficiency daytime photo-thermal and nighttime radiative sky cooling(PT-RSC).The main work is as follows:1.The comprehensive utilization of spectral adjustable PT-RSC is proposed.That is,the spectral selectivity of coating can be dynamically controlled,which makes the daytime photo-thermal and nighttime radiative sky cooling modes correspond to the spectral selectivity required by their own.In daytime,the PT-RSC coating is photo-thermal mode,it has high absorptivity in solar radiation band(0.3-2.5 pm),low emissivity in mid infrared band(2.5-25.0μm);in nighttime,the PT-RSC coating is radiative sky cooling mode,it has high emissivity in the atmospheric window.2.Based on the change of optical properties of metal-insulator transition of vanadium dioxide(VO2),a spectrally self-adaptive PT-RSC coating(VO2-PT-RSC coating)with "Al/Al2O3/VO2/Al2O3" structure was designed and prepared.The spectral selectivity of the VO2-PT-RSC coating is automatically adjusted according to the different temperature in daytime and nighttime to meet the spectral requirements of PT and RSC modes;In the nighttime radiative sky cooling mode,the temperature of the coating is lower than the critical temperature of VO2,the VO2 is insulation state,the emissivity of the coating in the atmospheric window is 0.73,and the absorptivity in the solar radiation band is 0.82;In the daytime photothermal mode,the coating temperature will increase due to solar absorption and VO2 eventually converts to metal state,the absorptivity of the coating in solar radiation band is 0.88 and that of the mid-infrared band is 0.22.3.Optimize the thermal structure of PT-RSC device,design and build vacuum PT-RSC device to reduce the heat loss of the coating,including:(a)The rigid cover of multi-spectral zinc sulfide(ZnS)with full wavelength transmission is designed and prepared,the transmissivity of the ZnS cover in solar radiation band and atmospheric window is 0.85 and 0.75 respectively;(b)The design and optimization of thermal insulation structure of vacuum device.The total thermal loss coefficient of coating in the vacuum device is 0.3～0.7 W/(m2-K)in daytime photo-thermal mode,and 0.2～0.4 W/(m2-K)in the nighttime radiative cooling mode,which is significantly lower than from the traditional PT-RSC device.The on-site thermal performance of the VO2-PT-RSC coating in the vacuum device was measurement.In Urumqi,when the solar irradiance is 790.4 W/m2,the coating temperature can reach 187.2℃,which is 174.6℃higher than the ambient temperature;In the nighttime sky radiative cooling mode,the coating temperature is-13.3℃,about 20.0℃ lower than the ambient temperature.4.The heat transfer model of the vacuum PT-RSC device under static conditions was established.The daytime photo-thermal performance and nighttime radiative sky cooling effect of the VO2-PT-RSC coating were simulated and analyzed.Under the reference condition,the maximum cooling power of VO2-PT-RSC coating at night is 78.1 W/m2,the lowest stagnation temperature is 25.8℃ lower than the ambient temperature.The photothermal efficiency is 59.0%when the coating temperature is 80℃ in the daytime.The annual performance of VO2-PT-RSC coating in different climatic regions such as Beijing,Jiuquan,Hefei,and Phoenix in Arizona,USA is performed.The results show that PT-RSC performed the best in Pheonix,which implies it is suitable for use in hot and dry continental climates.