Self-Assembled Needle-Like Spinel Composites on Alloy Surface as Specular Absorber
Abstract
Spectrally selective absorbers used in solar collector are known to improve efficiency of solar-thermal conversion. A desirable selective absorber is characterized by maximum absorption (Į) over the solar spectrum (0.3 ~ 2.5 ȝP) and low thermal emittance (İ) at operating temperature(Salmi et al. 2000). This is realized by low reflectance of absorbing surface in the solar main irradiation spectrum (nearly zero) and high reflectance (close to one) in the IR region. For practical reason, a good selective coating features optical property of Į > 0.9 and İ < 0.2. Of all types of selective absorbers, the surface texturing exhibits superior optical properties and thermal stability. Needle-like, dendrite, conic, or porous microstructure is constructed on a single-material surface called intrinsic materials, such as W, Mo, Si, etc. Rephaeli et al. (2008) have recently shown the wide angle absorption of tungsten pyramid structure as selective absorber through optical modeling. The textured surface on intrinsic materials is amazing because of its high absorption, stability at high temperature and convenient optimization through modeling. One problem concerning the textured surface is the difficulty of fabricating the desired structure as demonstrated (H Sai et al., 2001). Another is the limited absorption of a single material in the whole solar spectrum due to its intrinsic optical properties. In this paper, we have presented a selective absorbing surface of spinel materials fabricated on metal alloy surface. The surface features organized conic geometry of nano-scale, efficient for light trap. Both the period and height could be justified to optimize the absorbing selectivity. Materials constructing the surface are composites of spinel ferri-, nickel oxides. Diffuse reflection of the film is measured on a Lambda 950 spectroscopy in 0.3 ~ 2.5 µm and FIRT in 2.5 ~ 25 µm. Both optical selective absorption properties and thermal stability at high temperature renders this selective surface superior candidate for receiver at mid-high temperatures.