Performance of tantalum-tungsten alloy selective emitters in thermophotovoltaic systems

Abstract

A tantalum tungsten solid solution alloy, Ta 3% W, based 2D photonic crystal (PhC) was designed and fabricated for high-temperature energy conversion applications. Ta 3% W presents advantages compared to the non-alloys as it combines the better high-temperature thermomechanical properties of W with the more compliant material properties of Ta, allowing for a direct system integration path of the PhC as selective emitter/absorber into a spectrum of energy conversion systems. Indeed metallic PhCs are promising as high performance selective thermal emitters for thermophotovoltaics (TPV), solar thermal, and solar TPV applications due to the ability to tune their spectral properties and achieve highly selective emission. A 2D PhC was designed to have high spectral selectivity matched to the bandgap of a TPV cell using numerical simulations and fabricated using standard semiconductor processes. The emittance of the Ta 3% W PhC was obtained from near-normal reflectance measurements at room temperature before and after annealing at 1200• C for 24h in vacuum with a protective coating of 40 nm HfO 2 , showing high selectivity in agreement with simulations. SEM images of the cross section of the PhC prepared by FIB confirm the structural stability of the PhC after anneal, i.e. the coating effectively prevented structural degradation due to surface diffusion. The mechanical and thermal stability of the substrate was characterized as well as the optical properties of the fabricated PhC. To evaluate the performance of the selective emitters, the spectral selectivity and useful emitted power density are calculated as a function of operating temperature. At 1200• C, the useful emitted irradiance is selectively increased by a factor of 3 using the selective emitter as compared to the non-structured surface. All in all, this paper demonstrates the suitability of 2D PhCs fabricated on polycrystalline Ta-W substrates with an HfO 2 coating for TPV applications.

Refractory Ultrathin Nanocomposite Solar Absorber with Superior Spectral Selectivity and Thermal Stability

“…In this angular range, we observed the strongest (110) reflection of α‐W, overlapped with the strongest (210) reflection of β‐W, and an additional non‐overlapped (200) reflection of β‐W, thus confirming the polycrystalline nature of tungsten infrared reflective layer. [ 28 ] Similar reflections are also observed in the XRD spectrograph of full stack before annealing in Figure 6d. Annealing at 900 K in vacuum and air has not caused any change in the crystallinity of the samples, thus proving the thermal/oxidative stability of the samples.…”

“…To fabricate refractory nanocomposites, tungsten with high melting temperature is a good candidate, as it also exhibits high reflectance in near infrared region (R% = 94.6 at λ > 2.5 µm). [ 27,28 ] Besides, it also possesses considerable intrinsic absorption in the visible range; thus, based on the Fabry–Perot cavity, [ 29 ] ultrathin film of tungsten has been used as plasmonic absorptive material. In our case, tungsten nanoclusters in the SiC matrix contribute to broadening the intrinsic narrow absorption band in the UV‐visible‐near infrared (UV‐vis‐NIR) range, while tungsten reflective layer contributes in the low emissivity in the NIR and mid‐infrared region, as shown in Figure 1e.…”

Practical emitters for thermophotovoltaics: a review

“…The common metric spectral selectivity or spectral efficiency η sp , which is the fraction of the radiated energy that is convertible by the PV cell, can be calculated using the hemispherical emittance: 94,95 E Q -T A R G E T ; t e m p : i n t r a l i n k -; e 0 0 3 ; 1 1 6 ; 5 3 6…”

“…73 • Thermal expansion could lead to the cracking of a material. 67,68,95,103 Also, emitters with interfaces between different materials are at risk of delamination because different materials have different thermal expansion coefficients.…”

Performance analysis of solar thermophotovoltaic conversion enhanced by selective metamaterial absorbers and emitters

“…Besides photonic crystals [8][9][10][11], selective absorption/emission for the absorber-emitter module can also be obtained with multilayer cavities [12][13][14][15], nanowire [16][17][18] and nanoparticle based structures [19,20]. Recently, film-coupled metamaterials with selective radiative properties have been investigated.…”