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Design and global optimization of high-efficiency solar thermal systems with tungsten cermets

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Abstract

Solar thermal, thermoelectric, and thermophotovoltaic (TPV) systems have high maximum theoretical efficiencies; experimental systems fall short because of losses by selective solar absorbers and TPV selective emitters. To improve these critical components, we study a class of materials known as cermets. While our approach is completely general, the most promising cermet candidate combines nanoparticles of silica and tungsten. We find that 4-layer silica-tungsten cermet selective solar absorbers can achieve thermal transfer efficiencies of 84.3% at 400 K, and 75.59% at 1000 K, exceeding comparable literature values. Three layer silica-tungsten cermets can also be used as selective emitters for InGaAsSb-based thermophotovoltaic systems, with projected overall system energy conversion efficiencies of 10.66% at 1000 K using realistic design parameters. The marginal benefit of adding more than 4 cermet layers is small (less than 0.26%, relative).

©2011 Optical Society of America

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Figures (8)

Fig. 1
Fig. 1 Schematic illustration of a solar TPV system. Sunlight is collected via optical concentrators and sent to a selectively absorbing surface. That structure is thermally coupled to a selective emitter, which in conjunction with a filter, thermally emits photons with energies matched to the semiconductor bandgap of the TPV cell receiving them.
Fig. 2
Fig. 2 Diagram depicting the layers of the cermet structures examined in this manuscript, which include a dielectric AR coating, back reflector, and 1–4 cermet layers in between.
Fig. 3
Fig. 3 Behavior of the dielectric constants associated with the Bruggeman approximation (a) as a function of metal volume fraction (λ = 2μm) (b) as a function of wavelength (real part) and (c) as a function of wavelength (imaginary part).
Fig. 4
Fig. 4 Emissivity spectrum of tungsten for various temperatures both in experiment (circles) and in our numerical model (lines).
Fig. 5
Fig. 5 The figure of merit for a two-layer cermet selective absorber as a function of cermet thicknesses for the first and second layers. The rest of the parameters are from the optimized two layer structure, with the other parameters set identical to the selective solar absorber at 400 K given in Table 3(b).
Fig. 6
Fig. 6 For silica-tungsten cermet selective absorbers with C=1 at 400 K: (a) Optimized reflection spectra for 1–4 layer structures (b) corresponding metal volume fractions as a function of thickness for optimized structures of 1–4 layers.
Fig. 7
Fig. 7 For silica-tungsten cermet selective absorbers with C=100 at 1000 K: (a) Optimized reflection spectra for 1–4 layer structures (b) corresponding metal volume fractions as a function of thickness for optimized structures of 1–4 layers.
Fig. 8
Fig. 8 For silica-tungsten cermet selective emitters at 1000 K: (a) Optimized emissivity spectra for 1–4 layer structures (b) Metal volume fraction as a function of thickness for optimized 1–4 layer structures.

Tables (3)

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Table 3 Parameters for 1–4 Layer Tungsten–Silica Cermet Structures at 400 K and 1000 K for (a) 1-Layer Cermets, (b) 2-Layer Cermets, (c) 3-Layer Cermets, and (d) 4-Layer Cermets (All Thicknesses in nm)

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Table 1 Spectrally Averaged Absorptivities α, Emissivities ɛ, and Thermal Transfer Efficiencies ηt for 1–4 Layer Tungsten–Silica Cermet Structures (Illustrated in Fig. 2) Compared to Other Selective Absorbers at 400 K with Unconcentrated Sunlight, and 1000 K with Concentration of 100 Suns

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Table 2 Overall Figures of Merit (Defined as Efficiency Times Power Output of System Based on Ref. [10]) for 1–4 Layer Tungsten–Silica Cermet Selective Emitter Structures

Equations (3)

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η t = 1 C I 0 d λ ɛ ( λ ) [ B d I d λ π h c 2 λ 4 ( exp ( h c / λ k T ) 1 ] B α ɛ σ T 4 C I ,
v= ε m ε ε m +2ε +(1v) ε d ε ε d +2ε =0,
Γ o ( T ) = Γ o ( T o ) ( T T o ) α .
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