Article

High performance solar-selective absorbers using coated sub-wavelength gratings

Dept. of Electrical Engineering, Stanford University, Stanford, CA 94305 USA.
Optics Express (Impact Factor: 3.49). 03/2010; 18(6):5525-40. DOI: 10.1364/OE.18.005525
Source: PubMed

ABSTRACT Spectral control of the emissivity of surfaces is essential for efficient conversion of solar radiation into heat. We investigated surfaces consisting of sub-wavelength V-groove gratings coated with aperiodic metal-dielectric stacks. The spectral behavior of the coated gratings was modeled using rigorous coupled-wave analysis (RCWA). The proposed absorber coatings combine impedance matching using tapered metallic features with the excellent spectral selectivity of aperiodic metal-dielectric stacks. The aspect ratio of the V-groove can be tailored in order to obtain the desired spectral selectivity over a wide angular range. Coated V-groove gratings with optimal aspect ratio are predicted to have thermal emissivity below 6% at 720K while absorbing >94% of the incident light. These sub-wavelength gratings would have the potential to significantly increase the efficiency of concentrated solar thermal systems.

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    • "Micro/nanostructures of wide profile diversity are able to tailor thermal radiation by utilizing different physical mechanisms. Not only one-dimensional (1D) gratings [12] [13], V-groove gratings [14], and photonic crystals [15] [16] [17], but also various two-dimensional (2D) nano/microstructures have been investigated as promising selective TPV emitters. Heinzel et al. [18] manufactured 2D wavelength-selective emitters for the near-infrared spectral range, but the emittance exhibited directional dependence. "
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    • ", [2], [5], [12], [14], [15], [32]. As shown in Fig. 1, the geometry of the grating structure is characterized by the period , the grating ridge width and the grating thickness . "
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    • "Several challenges arise in the efficient numerical solution of grating scattering problems: (i) the period of the gratings can be many wavelengths in size, (ii) in many applications (such as photovoltaic [4] or solar absorber design [39]) solutions are needed at many incident angles and/or frequencies, (iii) the scatterer may have physical resonances in the form of guided modes (see Remark 2.1), which leads to ill-conditioned problems, and (iv) so-called Wood's anomalies may occur, that is, scattering parameters (incident angle and frequency) for which one of the Bragg angles lies precisely along the grating. Note that challenges (iii) and (iv) are distinct: (iii) is a physical resonance leading to an ill-posed problem (see the reviews [41] [30]), whereas Wood's anomalies do not cause ill-posedness—and yet they do cause problems for many numerical schemes. "
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