Broadband Solar Thermal Absorber Based on Optical Metamaterials for High‐Temperature Applications

Abstract

ible and near-infrared (IR) spectral region is critically important. On the other hand, in the mid-IR spectral region, surface emittance should be lowered in order to minimize the radiation loss in the form of thermal emission for high-temperature applications, such as concentrated solar tower. Therefore, wavelength-selective coatings are often applied to spectrally tune the optical property of the collector surface. [ 2 ] Recently, optical metamaterials have drawn much attention due to potential applications in thermophotovoltaic energy conversion, [ 3 ] solar cell, [ 4,5 ] solar thermal application, [ 6 ] and plasmonic sensors. [ 7 ] Regarding solar thermal applications, achieving perfect absorption in the visible and near-IR spectral region would be of great interest. For example, split-ring resonators, [ 8,9 ] 1D grating structures, [ 10,11 ] and cross-shape grating structures [ 12 ] were proposed to achieve perfect absorption. However, these studies showed only narrowband absorption; thus, their applicability as a solar thermal absorber is hardly possible. In order to achieve broadband absorption, various types of structures were proposed, such as photonic crystals, [ 13,14 ] nanowire and nanohole arrays, [ 15 ] and nanowire forests. [ 16 ] Alternatively, geometric combination of gratings can also attain a broadband absorption. For example, Yang et al. [ 17 ] investigated the resistor mounted crosswire and split-ring-resonator structure for the broadband and polarization-insensitive absorber in gigahertz frequency regime. In the visible and near-IR spectral region, Wang and Wang [ 18 ] showed that the metamaterial structure composed of two different-sized grating elements within one period on top of SiO 2 spacer and tungsten substrate can exhibit broadband absorption. In addition, saw-tooth [ 19,20 ] and pyramidal [ 21,22 ] shaped structures also demonstrated ultrabroadband absorption. Although several other techniques, such as a microencapsulation of phase change material, [ 23 ] a quasi-periodic nanocone, [ 24 ] a fi lm coupled nanoparticle, [ 25 ] and graphene nanodisc, [ 26 ] have been proposed for achieving broadband absorption, most of the structures are too complicated to be fabricated for real-world applications. In order to overcome the fabrication diffi culty, a relatively simpler type of tandem grating structure was proposed for demonstrating broadband absorption in the visible spectrum by exciting coupled surface plasmon and magnetic resonance; [ 27 ] however, experimental demonstration has not been made yet. It should be also noted that the temperature-dependent radiative properties In this work, a tandem grating solar absorber is proposed, which can be easily fabricated on a wafer scale and is thermally stable up to 800 K. The base of the solar thermal absorber consists of a tungsten substrate, SiO 2 spacer, and 2D tungsten nanohole array fi lled with SiO 2 . On top of the base structure, a 2D tungsten nanodisc array is coated with an additional SiO 2 spacer, forming the tandem grating structur...

Design of a Broadband Solar Thermal Absorber Using a Deep Neural Network and Experimental Demonstration of Its Performance

“…Notice that the Cr thin-film structure is nothing but a Fabry-Pérot structure made of a refractory metal 28 , which can also exhibit an excellent absorption performance in the solar spectrum. By opening about 21% of the surface area in the Cr film (i.e., [1 − w /Λ] 2 ), we can achieve a 5% enhancement of the solar absorptance, suggesting that the enhanced absorption by employing the Cr pattern is due to the near-field interaction of diffracted evanescent waves with the subwavelength-sized nanostructures 5,14 .…”

“…By inducing electromagnetic resonance, subwavelength-sized nanostructures have great potential for use in tailoring the radiative properties of system, especially the absorption spectrum 1–4 . Well-designed nanostructures with desirable radiative properties can be utilized as selective 1,3,4 and broadband absorber 5 . In designing subwavelength-sized nanostructures, precise prediction of the corresponding radiative properties resulting from light-structure interactions is crucial.…”

High-Temperature Solar Thermal Energy Conversion Enhanced by Spectrally-Selective Metafilm Absorber under Concentrated Solar Irradiation

“…One of the most common coatings in high-temperature CSP systems is Pyromark 2500 [6], which loses a significant amount of heat through emission in the IR range due to the lack of high spectral selectivity in spite of high solar absorption and thermal stability at elevated temperatures [7]. Artificial composites or micro/nanostructured metamaterials have been recently developed as selective solar thermal absorbers [8][9][10][11][12][13][14] such as subwavelength gratings [15][16][17][18][19][20], nanocomposites [21] or nanoparticles [22], cermet [23][24][25], photonic crystals [26][27], and multilayers [28][29][30][31]. We recently reported the design and fabrication of an ultrathin multilayer selective solar absorber [30], namely metafilm absorber, which is thermally stable in air up to 600C, while thermal cycle testing revealed its long-term thermal stability at 400C in ambient conditions.…”

Wavelength-selective mid-infrared metamaterial absorbers with multiple tungsten cross resonators

“…Since the resonance wavelength of a single resonator highly depends on its geometrical design, methods have been developed by combining dual or multiple resonators with different sizes in one unit cell in order to obtain the multi-band or broadband absorption, including multi-width strips [27], multiple patches [28], cross resonators [26], disks [31] and mixture of cross and disk resonators [32], as well as stacked double ring resonators [33]. The metal-dielectric multilayers are also used to realize ultra-broadband absorption with either 1D gratings [34] or 2D trapezoid cavities [35] based on the stop-light waveguide theory and structured metamaterial absorber through multiple overlapping resonances [36,37].…”