Manipulation And Application Of Selective Absorption/Emission

For the sustainable development of human society,the development and utilization of renewable energy has become an important research direction for researchers.Passive radiative coolers,transferring heat to outer space through radiation,have been continuously developed and become one of the emerging renewable energy technologies.It is known that solar energy is abundant,widely distributed,and can be converted into different available energies.As one of the most effective ways to utilize solar energy,solar thermal conversion technology has also attracted much attention.Both radiative cooling and solar heating require devices to have spectrally selective absorption/emission.This thesis presents selective absorbers/emitters based on simple double-layer planar structures for daytime radiative cooling and a nanostructured selective absorbers for solar thermal conversion.First,we experimentally demonstrate a selective absorber based on a very simple polar dielectric/metal(SiO2/W)double-layer planar nanostructure without photolithography.The absorber is fabricated by growing tungsten and silicon dioxide successively.It can achieve dual-band perfect absorption in the 8-13 μm atmospheric window band.Due to the excitation of the Berreman mode and constructive interference resonance,this absorber can achieve a greatly enhanced absorption at wavelengths of～8 μm and～10 μm,respectively.According to Kirchhoff’s law,this simple SiO2/W double-layer planar structure has the ability to transfer heat energy to outer space through an atmospheric transparent window of 8-13 μm,and is expected to realize radiative cooling passively.Next,we investigate selective absorbers/emitters based on SiO2/Ag and PDMS/Ag planar structures in sequence,and explore their daytime radiative cooling performances.The PDMS/Ag double-layer planar structure achieves excellent daytime radiative cooling with a PDMS thickness as thin as～16.2 μm,much thinner than previously-reported PDMS-based coolers.Based on the measured optical spectra from 0.3 to 20 μm,its cooling power up to 76.84 W/m2 is calculated.Considering the influence of convective heat transfer coefficient of 12 W/(m2*K),our device is predicted to be 5.2℃ and 3.7℃ cooler,when the ambient temperature is set to 300 K and 283 K,respectively.Experimentally,～1.8℃ lower than the ambient temperature(only～10℃)is achieved experimentally in a clear winter day,and is expected to achieve a better cooling effect in summer when the ambient temperature is higher.The daytime radiative cooler based on the PDMS/Ag double-layer planar structure is simple,easy to prepare,low in cost,and has great advantages in practical applications.Finally,a distinct nanostructure is prepared based on self-assembled polystyrene nanospheres for manipulation of the selective absorption/emission spectra.Due to the combined effects of surface plasmon polaritons on the interface of tantalum(Ta)and germanium(Ge)and the unconventional interference inside the ultrathin lossy Ge film,our spectrally selective absorber achieves a higher solar absorption than its planar counterparts with Ge/Ta double-layer and Ta/Ge/Ta three-layer configurations.In the long wavelength region,the thermal emission is well suppressed,and thus the solar heat is accumulated.Consequently,our solar selective absorber has a temperature as high as 172.8℃ under 7-sun sunlight,which is much higher than the commercial black paint and its planar counterparts.It is easy to prepare,low in cost,and therefore is expected to be potentially applied to solar thermal systems.