Spectrally Selective Absorbing Nanocomposite Coatings For Solar Thermal Applications

With the depletion of fossil fuel,energy crisis has become a severe problem.At the same time,the use of fossil fuel has also brought a series of environmental problems.One possible approach to address these problems is to develop renewable energy to partially replace traditional energy sources.Solar energy is a promising source of renewable energy,as the hourly incident solar flux on the surface of the earth is greater than annual global energy consumption.Efficient harvesting of solar energy for various applications can solve the problems of energy crisis and environmental contamination.There are three ways of using solar energy:solar chemical,photovoltaic,and solar thermal applications,among which the solar thermal conversion has the highest efficiency up to 80%.In the solar thermal application,the surface of solar collector is covered by a spectrally selective absorbing coating with a high solar absorptance and low thermal emittance.The performances of the coating is the key to the efficiency of solar collector.There are a variety of spectrally selective coatings,among which the metal cermet coating,consisted of metal particles embedded in dielectric matrix,have the most wide applications.These coatings can sustain high solar thermal efficiency when working at temperature below 500 ?,while subjected to performance degradation at higher working temperatures due to aggregation of the metal particles.In order to meet the challenges of high efficiency and high working temperature in applications,we designed a series of metal cermet coatings with a layered nanocomposite structure in which the metal particles are well-separated by amorphous dielectric layers,to suppress the aggregation of the metal particles,thus improving the thermal stability of the metal cermet coatings.Meanwhile,we systematically investigated the surface radiative properties of the coatings under different deposition conditions in order to design spectrally selective absorbing surface with excellent solar thermal conversion efficiency.(1)Radiative properties of CrAlO-based nanocomposite coatings with single phase nanoparticles for spectrally selective absorbing surface.A serious of CrAlO-based nanocomposite coatings were synthesized by cathodic arc evaporation method under different oxygen flowrates,then these coatings were subjected to various characterization techniques including XRD,TEM,XPS,and RBS.We also used spectrophotometer to measure the reflectance and transmittance spectra of the coating,and based on the dielectric function model of nanocomposite materials,we calculated the optical constants of these coatings at different deposition conditions.Thus,the relationship between microstructures and optical properties of the coatings could be obtained.Based on the optical constants of single layer coatings,a four-layered stack absorber was designed and the effects of composition and thickness of individual layer in the stack on reflectance spectra was systematically investigated.We fabricated a spectrally selective absorbing surface using the optimized four-layered structure.In order to investigate the thermal stability of the surfaces in high-temperature air,the surfaces were annealed at temperatures of 500,600,700,and 800? for different durations.Then we studied the changes of the structures and optical properties of the coatings to obtain the degradation mechanism at high temepratures.Based on the Finite Difference Time Domain(FDTD)method,we systematically investigated the effects of sizes,distribution and numbers of metal particles on the extinction property of the coatings.In order to improve the thermal stability of the CrAlO-based coating,we doped N elements into the CrAlO-based coatings to form CrAlON-based coatings.We systematically investigated effects of the dopant N on the microstructures,optical properties and thermal stability of the spectrally selective absorbing surface.(2)Radiative properties of TiAlON-based nanocomposite coatings with two-phase nanoparticles for spectrally selective absorbing surface.A serious of TiAlON-based nanocomposite coatings were deposited by cathodic arc evaporation under different N2/O2 flowrate ratios from 0.5 to 7.5,while the O2 flowrate was fixed at 20 sccm.We used XRD and TEM to characterize the microstructures of the coatings,and used spectrophotometer to measure the reflectance and transmittance of the coatings.Then we fitted the reflectance and transmittance spectra using dielectric function model of nanocomposite materials to obtain optical constants of the coatings.Based on the optical constants of the single layer coatings,we designed and fabricated a spectrally selective absorbing coating,and the effects of different ratio of TiAl3N and TiN content on the optical properties were investigated experimentally and theoretically.(3)In order to improve the solar thermal efficiency of traditional solar desalination system,we proposed a bio-inspired capillary-driven pump for solar vapor generation.The pump was mainly consisted of a TiAlON-based solar absorber,a porous NiO wick,and a one-dimensional channel for water-supply.The purpose of designing one-dimensional channel is to limit heat conduction to bulk water,thus reducing the thermal loss of the pump.The porous NiO wick with low thermal conductivity had the TiAlON-based solar absorber on its surface could efficiently capture solar radiation.The porous NiO could wick water through the one-dimension channel to the surface for evaporation.We fabricated the capillary-driven pump,then measured the evaporation rates,temperatures of evaporation surface,and solar thermal efficiency of the pump under solar intensity from 1 sun to 4.8 suns.We also investigated the energy loss of the pump through radiation and convection,and we could obtain the solar thermal efficiency based on the energy conservation law,in order to prove the accuracy of the measurements.