| Passive radiative cooling dissipates heat from Earth into outer space through the atmospheric transparency window(8–13 ?m).This technique can be useful for applications in passive building cooling,thermal photovoltaic energy conversion,renewable energy harvesting and passive refrigeration in arid regions.Therefore,the passive radiative cooling technique has been widely studied.In this dissertation,the multilayer emitters are studied.We both theoretically and experimentally demonstrate the spectral characteristics and radiative cooling performance of the structures.MATHEMATIC,COMSOL,and ESSENTIAL MACLEOD software are used to design the suitable structures,and the spectral characteristics of the structure are numerically simulated.At the same time,the cooling performance and the cooling mechanism are discussed by using FORTRAN language.In the experiment,a variety of multilayer emitters with passive radiative cooling performance are prepared.Samples are experimentally characterized by using an UV-VIS-NIR spectrophotometer and a Fourier transform infrared spectrometer,respectively.At the same time,the actual cooling performance of the structures is tested.The results show that the experimental results match well with the theoretical ones.The major contents of this dissertation are as follows:(1)We theoretically propose a design of quasi-periodic multilayer structure with high solar reflectance and perfect absorption in the atmospheric transparent window.The materials of silicon dioxide,silicon nitride,alumina and silver are used in this structure.And the absorption mechanism is analyzed.At the same time,the angle insensitivity of this structure is theoretically studied.The influences of the non-radiative heat exchange and the ambient air temperature on the cooling performance of the emitter surface are also theoretically analyzed.The radiative cooling performance of the structure is theoretically verified.(2)We construct an improved near-black infrared multilayer thin film emitter,which can remarkably reduce the absorption of solar radiation and achieve great daytime radiative cooling performance.The structure consists of five layers,which are polydimethylsiloxane(PDMS),titanium dioxide,magnesium fluoride,fused silica,and silver.The layer thickness is 100 ?m,36 nm,200 nm,500 ?m and 120 nm,respectively.The influences of some factors on the cooling performance are theoretically studied,such as the non-radiative heat transfer coefficient,the atmospheric transmittance and the ambient air temperature,etc.By using high vacuum coating technology,two samples are prepared for comparison.On-site measurements are conducted.The results show that the improved structure has better daytime radiative cooling performance.In addition,the influence of the polyethylene film on the experimental results is theoretically analyzed.(3)A simple dual-layer emitter,which is easy to prepare and can be applied to effective daytime radiative cooling,is experimentally demonstrated.The emitter consists of a 200-?m-thick PDMS film on top of a 120-nm-thick Ag film,which is coated on a fused silica wafer or a plastic substrate,respectively.Due to the high reflectivity from the visible to the near-infrared and near-black emittance in the mid-infrared,the experimental results show that both structures have similar daytime radiative cooling performance.Therefore,it is verified that the emitter is insensitive to substrates.In the presence of significant non-radiative heat exchange and non-ideal atmospheric conditions,theoretical simulations are in good agreement with the measurements.This work indicates that the dual-layer emitter proposed here can achieve a technically efficient production,and may be a key element in the realization of energy-efficient radiative cooling devices.In summary,this dissertation provides some ideas for the design of passive radiative cooling emitters which have selective emissivity in the atmospheric transparency window and broadband emissivity in infrared region.There are many potential applications of the proposed passive radiative emitter in various temperature-sensitive optoelectronic devices,such as thermophotovoltaics,photovoltaics and infrared detectors,which will stimulate the continuous interests of thin film structures,thermal nano-photonics,and metamaterials. |
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