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Superlattice photonic crystal as broadband solar absorber for high temperature operation

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Abstract

A high performance solar absorber using a 2D tantalum superlattice photonic crystal (PhC) is proposed and its design is optimized for high-temperature energy conversion. In contrast to the simple lattice PhC, which is limited by diffraction in the short wavelength range, the superlattice PhC achieves solar absorption over broadband spectral range due to the contribution from two superposed lattices with different cavity radii. The superlattice PhC geometry is tailored to achieve maximum thermal transfer efficiency for a low concentration system of 250 suns at 1500 K reaching 85.0% solar absorptivity. In the high concentration case of 1000 suns, the superlattice PhC absorber achieves a solar absorptivity of 96.2% and a thermal transfer efficiency of 82.9% at 1500 K, amounting to an improvement of 10% and 5%, respectively, versus the simple square lattice PhC absorber. In addition, the performance of the superlattice PhC absorber is studied in a solar thermophotovoltaic system which is optimized to minimize absorber re-emission by reducing the absorber-to-emitter area ratio and using a highly reflective silver aperture.

© 2014 Optical Society of America

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Figures (4)

Fig. 1
Fig. 1 Normal spectral absorptance for square lattice and superlattice with period a = 0.66 µm, radius r1 = 0.28 µm and r2 = 0.12 µm and depth d = 4.6 µm, and absorptance for bulk Ta at 1478 K [31] for comparison. Schematics of the lattices are shown for all PhC designs.
Fig. 2
Fig. 2 (a) Thermal transfer efficiency ηT and thermal emissivity ε at 1500 K and solar absorptivity α for PhC absorbers with r1 = 0.28 µm and increasing radius r2 of the superlattice cavities. (b) Thermal transfer efficiency at 1500 K and 250 suns for the superlattice absorber with period a = 0.66 µm and depth d = 4.6 µm in dependence of the cavity radii.
Fig. 3
Fig. 3 Normal spectral absorptance for: 1 the square lattice (cylindrical) PhC with a = 0.66 µm, r1 = 0.24 µm, and (polygonal) superlattice with 2: a = 0.6 µm, r1 = 0.27 µm, r2 = 0.1 µm, 3: a = 0.8 µm, r1 = 0.39 µm, r2 = 0.15 µm, 4: a = 1.0 µm, r1 = 0.49 µm, r2 = 0.24 µm; all with cavity depth d = 4.6 µm and antireflection coating of 40 nm HfO2, and solar spectrum AM1.5D (green trace).
Fig. 4
Fig. 4 (a) Schematic of the STPV configuration with area-optimized absorber/emitter pair and aperture. (b) STPV system efficiency dependence on incident irradiance for different AR using the polygonal superlattice PhC absorber (design 4) with Ag aperture and PhC emitter matched to an InGaAsSb PV cell (λPV = 2.3µm), and measured cell efficiency ηPV. System temperature T for the same system in dependence of irradiance (dashed lines, right axis). (c) Optical conversion efficiency ηopt of superlattice PhC, square lattice PhC and blackbody absorber with Ag aperture (solid lines) and without (dashed) for different AR at 1500K. (d) ηopt of superlattice PhC absorber for different STPV system configurations, all at 1500K with Ag aperture vs. absorber-to-emitter area ratio AR. The different cases were calculated for small scale 1 cm2 and large scale 100 cm2 device areas, and different reflection coefficients RPV on the PV cell side.

Equations (5)

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α ¯ = 1 H S 0 d λ ε ( λ ) S S ( λ ) ,
ε ¯ = 0 d λ ε ( λ ) / { λ 5 ( e h c λ k T 1 ) } 0 d λ / { λ 5 ( e h c λ k T 1 ) } ,
η T = α ¯ ε ¯ σ T 4 H s ,
η o p t = F V H S λ m i n λ P V d λ ε e f f ( λ ) 2 π h c ² λ 5 ( e h c λ k T 1 ) ,
η S T P V = η o p t η P V ,
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