Study On The Cermet-based Quasi-optical Microcavity Solar Selective Absorber

As one of significant strategy to harvest solar radiation,photothermal technologies are widely applied due to the simple process,low cost and high conversion efficiency.In photothermal system,solar selective absorber is a crucial component that capture sunlight.Up to now,the traditional absorbers with cermet and dielectric-metal-dielectric structure have been extensively investigated.However,it still has a great challenge to obtain a higher solar absorptance in photothermal field.To pursue higher conversion efficiency,the operating temperature is elevated to enhance the Carnot efficiency.However,the thermal stability of solar selective absorbers at elevated temperature is also confronted with challenge.Considering the conceptions of improving solar absorptance,obtaining the robust thermal stability,and optimizing the conversion efficiency,in this article,we systematically carried out the work about how to obtain higher solar absorptance,the robust thermal stability,and the optimal conversion efficiency of solar selective absorber by designing a novel quasi-optical microcavity structure based on the combination of cermet and dielectric-metal-dielectric structure,and replacing the metal in cermet layer with the metal alloy.Improving the spectral selectivity of solar selective absorbers under certain optical concentration could boom the photothermal conversion efficiency.Here,the W-SiO₂ cermet based quasi-optical microcavity structure solar absorbers are fabricated on the mechanically polished stainless steel substrates by magnetron sputtering.The enhanced spectral selectivity via improving the solar absorptance in quasi-optical microcavity-based absorbers is ascribed to the synergistic mechanisms of cermet and traditional metal-dielectric-metal structure absorptances.The W-SiO₂ cermet based spectrally selective absorbing coatings with optimized quasi-optical microcavity structure demonstrate an excellent performance with solar absorptance of 0.96,thermal emittance of 0.044（82 oC）and superior thermal stability at 600 oC in vacuum.The suppressing heat loss without degraded solar absorptance is due to the better crystallization and the formation of α-W in infrared reflector layer upon annealing at this temperature.The total conversion efficiency of an ideal system using this coating from solar radiation to electricity would reach around 62.5% at 600 oC with the focus of 1000 suns.Controlling solar spectra via the optical design could realize the optimized utilization of energy.Achieving the appropriate transition wavelength can boom conversion efficiency for a certain application under the consideration of cost.Here,we experimentally demonstrated a kind of WTa-SiO₂ cermet-based absorbers with transition wavelength of around 1500 nm,2000 nm,and 2500 nm,which can meet the versatile applications under the conditions of different temperatures and different optical concentrations.All the absorbers possess a promising solar absorptance of above 95% and various emittance resulting from the optimized spectrum shifting,as well as the superior thermal stability at 600 oC due to the formation of tantalum oxide（Ta2O5）in WTa-SiO₂ cermet in the annealing process,and even illustrate the robust stability after annealing for 10 hours at elevated temperature of 800 oC in vacuum.