Development of Selective Solar Absorber Coating by・ Stratified Integration of Mesoscopic Structures

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It is the time to promote the energy shift from fossil fuel to natural alternative energy resources. The photothermal energy conversion is one of the solutions. Surfaces of this kind of solar absorber coating require a high solar absorptance (α) and a concomitant low thermal emittance (ε), i.e., solar selectivity. In order to improve photothermal energy conversion efficiency (η), the mesoscopic structure such as nano-particles and rough surface are especially concerned in this study.First of all, optical properties and thermal stability of the nano-structured Ag are evaluated. Based on the result, the high performance of photothermal conversion (η=0.72 at 373K for AM1.5 and ultimate temperature of 550K) is successfully realized.We recently developed a solar absorber coating with high a induced by surface roughness. This is the Al-N film prepared by reactive rf sputtering. We clarified that one of the reasons for the high a is the gradient-refractive index caused by the surface roughness. Unfortunately, the t was rather high and this should be improved.Therefore, in this study, a solar selective absorber coating with surface roughness is fabricated. First, the Al film with proper surface roughness, the degree of which is prepared by a method of two-step deposition of an Al layer at two different substrate temperatures, is prepared as an infrared reflector, which contributes to the low e. Then the Al-N film as an absorber layer is deposited on this Al reflector. In addition, a transparent AlN film is examined for use as an anti-reflector in the solar selective absorber with surface roughness. As the result, high performance of α=0.92(AM1.5),ε=0.06(373K), and then η=0.86 have been realized.As the other method, the courting by the Sn doped indium oxide (ITO) film on the AI-N film was examined. By the deposition of ITO film at 298K followed by the annealing at 773K, the emittance could be successfully suppressed.