Selective Solar Absorber Coatings Research Articles

A multilayer solar absorber coating based on NbMoTaW refractory high entropy alloy: optical properties, thermal stability, and failure mechanism

The newly increasing development of abruptly high entropy alloy nitride (HEAN), especially the refractory HEAN, manifests excellent mechanical, corrosion resistance, and optical properties, which exhibits a wide application prospect in the field of coatings. Herein, we develop a high entropy nitride NbMoTaWN-based solar selective absorber coatings (SSACs), providing a potential to strengthen structural and optical advance through component selection and structure design. The coating exhibits a high solar absorptance (α) of 0.936 and a low thermal emittance (ε) of 0.126 at 82 °C. Investigation on structural and optical robustness demonstrates that the NbMoTaW HEAN-based SSAC could endure heat treatment at 500 °C for 10 h in vacuum environment. In addition, through the evolution of XRD and Raman spectra after annealing, the failure mechanism of coatings annealed in vacuum at 500 °C for 10 h and 400 °C in air for 5 h are obtained. Obviously, the advantages such as low-cost preparation, easily scalable production, convenient technology process, and repeatability in this work promise potentially valuable applications for photothermal conversion techniques.

Nanometer-Thick High-Entropy Alloy Nitride Al0.4Hf0.6NbTaTiZrN-Based Solar Selective Absorber Coatings

Solar selective absorber coatings (SSACs), with the characteristics of efficient harvesting and conversion of solar energy into the heat work fluid, are considered to be one of the key components f...

Spectrally Selective Solar Absorber Coating of W/WAlSiN/SiON/SiO2 with Enhanced Absorption through Gradation of Optical Constants: Validation by Simulation

The properties of spectrally selective solar absorber coatings can be fine-tuned by varying the thickness and composition of the individual layers. We have deposited individual layers of WAlSiN, SiON, and SiO2 of thicknesses ~940, 445, and 400 nm, respectively, for measuring the refractive indices and extinction coefficients using spectroscopic ellipsometer measurements. Appropriate dispersion models were used for curve fitting of Ψ and Δ for individual and multilayer stacks in obtaining the optical constants. The W/WAlSiN/SiON/SiO2 solar absorber exhibits a high solar absorptance of 0.955 and low thermal emissivity of 0.10. The refractive indices and extinction coefficients of different layers in the multilayer stack decrease from the substrate to the top anti-reflection layer. The graded refractive index of the individual layers in the multilayer stack enhances the solar absorption. In the tandem absorber, WAlSiN is the main absorbing layer, whereas SiON and SiO2 act as anti-reflection layers. A commercial simulation tool was used to generate the theoretical reflectance spectra using the optical constants are in well accordance with the experimental data. We have attempted to understand the gradation in refractive indices of the multilayer stack and the physics behind it by computational simulation method in explaining the achieved optical properties. In brief, the novelty of the present work is in designing the solar absorber coating based on computational simulation and ellipsometry measurements of individual layers and multilayer stack in achieving a high solar selectivity. The superior optical properties of W/WAlSiN/SiON/SiO2 makes it a potential candidate for spectrally selective solar absorber coatings.

Scalable and Ultrathin High‐Temperature Solar Selective Absorbing Coatings Based on the High‐Entropy Nanoceramic AlCrWTaNbTiN with High Photothermal Conversion Efficiency

High‐entropy ceramics, especially high‐entropy alloy nitrides (HEANs), have attracted increasing attention in recent years due to their unique structural characteristics that result in their potential use in correlated high‐temperature structural materials. However, research on their optical properties, especially the spectral selectivity, is still in its infancy. According to Carnot efficiency, which demands an extremely high working temperature to elevate the performance of a concentrating solar power (CSP) system, excellent thermal stability is valuable to meet the requirements of solar selective absorbing coatings (SSACs). Herein, the high‐entropy ceramic AlCrWTaNbTiN is introduced as effective absorption layers by adjusting the nitrogen content to design and fabricate an ultrathin multilayer solar absorbing coating, which exhibits a high solar absorptance of 93% and a low emittance of 6.8% at 82 °C. Coating design (CODE) software is used to design and optimize the multilayer solar absorbing coating. Moreover, the optical properties, microstructure, thermal stability, and failure mechanism are investigated. Finite‐difference time‐domain (FDTD) calculations are used to understand the nature of the optical absorption consisting of intrinsic absorption and extinction absorption.

Toward high-temperature thermal tolerance in solar selective absorber coatings: choosing high entropy ceramic HfNbTaTiZrN

The most recent advances in high-entropy materials provide impetus for the development of high-performance materials, simultaneously providing high-temperature robustness and excellent functional properties owing to the high configurational entropy and distorted lattices.

Nanoparticles Based Single and Tandem Stable Solar Selective Absorber Coatings with Wide Angular Solar Absorptance

Nanoparticles Based Single and Tandem Stable Solar Selective Absorber Coatings with Wide Angular Solar Absorptance

Greatly enhanced solar absorption via high entropy ceramic AlCrTaTiZrN based solar selective absorber coatings

High entropy alloys (HEAs), which is at the expense of high cost compared to traditional alloy, should not be confined to the mechanical properties, but should be employed to devise a novel combination with unique functional and mechanical performances. In this work, high entropy alloy nitride (HEAN) is utilized as a novel double absorption layer to improve solar absorption in the high temperature solar selective absorbing coatings (SSACs). Our primary motivation is to lower thermal emittance (ε) and enhance solar absorptance (α). In order to realize this goal, coating design (CODE) software is employed to design and optimize the proposed HEAN based SSACs using appropriate dielectric function model. The ultimate as-deposited coating shows good optical properties with a high α value of 0.965 and a low ε value of 0.086 (at 82 °C). The estimate of thermal stability tests indicates that HEAN based SSACs has the ability to resist instability in high working temperature, which keeps good optical properties (α = 0.925, ε = 0.070) after annealing at 600 °C for 10 h.

Study on the solar selectivity and air thermal stability of cobalt–nickel–iron oxide coating fabricated by spraying method

Abstract Solar selective absorber coatings (SSACs) with transition metal spinel structure were fabricated with Fe-, Ni- and Co-based nitrate salts by facile sol-gel method and fast spraying process that facilitates large-area coating preparation. By optimizing the doping ratio of three transition metal nitrates, the number of spraying layers and the annealing temperature, the SSAC with solar absorptance of 0.93 and vertical emittance of 0.11 were achieved. The actual and comprehensive heat collection performance was studied by placing the coating samples directly in the sun. The surface temperature of the coating reached 72.8 °C when the outdoor temperature was 33 °C and the sunshine duration was 30 min. The thermal stability of the two-layer Ni0·9Fe0·1Co2Oy coatings was evaluated by continuously heating at 500 °C in the air for 48 h. The solar absorptance remained basically unchanged at 0.93, while the emissivity gradually increased from 0.11 to 0.30. The experimental results also show that the sintering temperature has a great influence on the absorptance of the coating, while the heating duration time has a greater influence on the vertical emittance. It indicates that the Ni0·9Fe0·1Co2Oy coating prepared by fast sol-gel spraying method is a promising SSAC which is easily prepared and applied on complex surface.

Thermal Stability of AlCrON-Based Solar Selective Absorbing Coating in Air

Thermal Stability of AlCrON-Based Solar Selective Absorbing Coating in Air

High-temperature solar-selective coatings based on Cr(Al)N. Part 2: Design, spectral properties and thermal stability of multilayer stacks

Two multilayer solar selective absorber coatings [Al/CrN0.95/Cr0.96Al0.04N1.08/Cr0.53Al0.47N1.12/Al2O3 (stack #1) and Cr0.96Al0.04N0.89/Cr0.62Al0.38N1.00/Cr0.53Al0.47N1.12/Al2O3 (stack #2)] were deposited on 316L steel by combining direct current (DC) and high power impulse magnetron sputtering (HiPIMS) technologies with the aim of increasing the working limit temperature. The composition and thickness of the constituent layers were optimized using CODE software to achieve a high solar absorptance (α) and low values of thermal emittance (ε) in the infrared region. The deposited multilayered stacks were heated during 2 h in air at 600, 700 and 800 °C to study their thermal stability and optical performance. Compositional, structural and optical characterization of the stacks (as-prepared and after thermal treatment) was performed. Both stacks presented a good solar selectivity with α > 95% and ε < 15%, were stable up to 600 °C and fulfilled the performance criterion PC < 5% after 600 and 700 °C treatments. Despite the stacks suffered chemical transformations above 600 °C, partial oxidation (stack #1) and Cr2N formation (stack #1 and #2), the optical properties were optimum up to 700 °C for stack #1 (α = 94%, ε(25 °C) = 12%) and 600 °C for stack #2 (α = 93%, ε(25 °C) = 13%). The solar-to-mechanical energy conversion efficiencies (η) of the as-deposited and annealed (600 and 700 °C) samples were up to 20% points higher than the absorber paint commercially used (Pyromark). At 800 °C, they underwent a further structural transformation, provoked by the oxidation of the inner layers, and they consequently lost their solar selectivity.

Enhanced spectral selectivity of HfC based high temperature solar absorbers with the addition of Mo

Simple technological process, low cost and high stability are important for large-scale production of the high temperature spectral selective solar absorber coatings. In this study, a simple Mo-HfC/Al2O3 coating that only contains an absorber layer (Mo-HfC) and an antireflection layer (Al2O3) deposited onto stainless steel substrate, which exhibits a high spectral selectivity (αs/εT) of 0.92/0.11. The experimental results indicate that the addition of Mo could greatly enhance the spectral selectivity of the coating. Thermal stability test demonstrates that the spectral selectivity of the coating remains unchanged even after annealed it at 600°C for 100 h in vacuum. Obviously, the coating shows a promising prospect in high temperature application. In addition, the optical degradation mechanism is also discussed in this work.

Temperature-induced degradation of bilayer MoOx-based solar absorbers under air annealing conditions

Thermal stabilities of solar selective absorber coatings in operating environment are crucial to evaluate the durability, which are directly related to the optical properties. Degradation induced by temperature is an important guarantee for assessing long service life. Here, the sputtered bilayer MoOx cermet-based solar selective absorber coatings with an absorptance of 0.91 and an emittance of 0.06 are investigated under air annealing conditions. Thermal aging test shows no degradation of the MoOx/SiO2 absorber with a selectivity of 0.904/0.06 when annealing up to 250 °C for 200 h. In contrast, the MoOx/MoOx absorber shows significantly poor thermal tolerance at 200 °C in aerobic environment. As extending the temperatures and durations, the solar absorber is severely degraded owing to the diffusion and oxidation of Mo atoms induced by temperature, which is also responsible for the optical and structural deterioration of the absorber. These results highlight that the as-obtained absorber coatings have good possibility for solar thermal harvesting application.

Thermal annealing characteristics of solar selective absorber coatings based on nano-multilayered MoOx films

Designing double metal-dielectric (cermet) solar selective absorber coatings (SSACs) often requires complex co-sputtering techniques with multiple targets. This inevitably limits the simple and low-cost industrial fabrication. Here, we develop novel nano-multilayered MoOx-based SSACs by simple and stable one-step reactive magnetron sputtering process using single molybdenum target. The proposed multilayer SSACs exhibit good solar absorptance of 0.93 and low thermal emittance of 0.06. Owing to the temperature-induced oxygen diffusion and oxidation phenomenon the as-sputtered SSACs have a poor thermal tolerance under air atmosphere, and after annealing at 200 °C for 150 h, the resulting absorptance is diminished from 0.93 to 0.90. However, the optical performance of the annealed SSAC is relatively stable in high-vacuum environment, even after annealing at 450 °C for 200 h, it still displays an ideal spectral selectivity of 0.92/0.07. With above properties, the resulting MoOx-based SSAC is a promising absorber for enduring thermal harvesting in solar vacuum collectors.

Improvement of thermal stability of ZrSiON based solar selective absorbing coating

Solar selective absorbing coatings (SSACs) are required to have not only excellent optical property but also outstanding thermal stability for high temperature applications. The optical properties of Mo/ZrSiN/ZrSiON/SiO2 SSAC had been optimized successfully before. Herein, we are focusing on the evaluation and mechanism of thermal stability of this multilayer coating for its potential applications in concentrated solar power (CSP) systems. Fortunately, the coating exhibits excellent thermal stability after aging at 400 °C for 1500 h in vacuum. At aging temperature of 500 °C for 1000 h or 600 °C for 300 h in vacuum, the slight inter-diffusion between Mo layer and stainless steel (SS) substrate occurs. At higher aging temperature of 700 °C for 100 h in vacuum, the serious inter-diffusion between Mo layer and SS substrate leads to invalidation of the coating, which has been evidenced by Rutherford backscattering spectrometry (RBS) and X-ray diffraction (XRD) technologies. Additionally, this coating also has an outstanding thermal stability after aging at 400 °C for 300 h in air. A heating-cooling cycling (HCC) treatment evidences the good thermal stability of this coating working in cold environment (−60 °C). The results reveal that this coating can be a promising candidate not only for CSP system in high temperatures but also for usage in cold environment.

Thermal Stability of Chromium-Iron Oxidation Mixture Cermet-Based Solar Selective Absorbing Coatings.

The solar selective absorber coating (SSAC) are at the core of the efficient solar-thermal system. In this paper, for the first time, the Chromium-iron oxidation mixture cermet was successfully prepared on the surface of ultra-pure ferritic stainless steel by chemical coloring as SSAC. The coating surface has an optical trap structure, and the chromium-iron oxidation mixture cermet is used as an absorption layer to realize solar-thermal conversion. The solar absorptance (AM1.5) of the coating reached 93.66, and the thermal emittance was less than 13. After thermal shock tests at 25/300 °C done 32 times (accumulated 812.8 h), the Performance Criterion (PC) of the coating was 0.01375 < 0.05, showing outstanding thermal stability.

Structural, Optical and Electrical Properties of Electron Beam Evaporated Tioxny Films as Selective Solar Absorber Coatings

Titanium oxinitride (TiOx Ny ) solar absorber coatings were deposited at different oxygen partial pressures onto Cu, Si and glass substrates using electron beam evaporation technique. XRD diffraction patterns evidenced (111), (200) and (220) orientation of TiNx phase. The preferred orientation of the films changed with oxygen partial pressure. XPS revealed the intensity of both Ti 2P3/2 and Ti 2P1/2 increases as a function of oxygen flow, and also shifted towards higher binding energy, indicating more oxidized state of Ti species than that of TiO2 due to incorporation of nitrogen atoms. Formation of uniformly distributed spherical like particles and an increase in surface roughness of the TiOx Ny films were observed as a function of oxygen partial pressure as depicted from SEM and AFM, respectively. Ellipsometric and resistivity measurements showed a shift from metallic to semiconductor behaviour of the TiOx Ny films as oxygen flow changed. A solar absorptance value of 0.94 in the solar spectrum region and a low thermal emittance value of 0.05 were achieved for the TiOx Ny solar absorber coatings prepared at the oxygen partial pressure of 7.5x10-5 Torr due to both interference and intrinsic absorption. This study confirmed that a single layer of TiOx Ny film can be a good candidate as selective solar absorber.

Enhanced thermal stability of the TiB2–ZrB2 composite ceramic based high temperature spectrally selective absorber coatings: Optical properties, failure analysis and chromaticity investigation

The long-term thermal instability and low solar-thermal conversion efficiency are challenging issues to be addressed for the solar selective absorbing coatings (SSAC). Herein, we describe a novel design for a high-performance SSAC based on TiB2–ZrB2 composite ceramic. The TiB2–ZrB2/Al2O3 coatings which deposited on stainless steel (SS) substrates exhibit a high solar-thermal conversion efficiency (η600°C = 82.6%) at 600 °C black body radiation. After annealing at 600 and 700 °C for 100 h in vacuum, the coatings exhibit a higher solar-thermal conversion efficiency of η600°C = 84.3%. Unfortunately, the η600°C decreases to 67.5% when the coating is annealed at the 800 °C for 100 h in vacuum, which is put down to oxidation of the absorbing layer, as evidenced by scanning electron microscope (SEM) and Raman spectra. In addition, the color of the coating is changed from dark blue to bright yellow during the thermal stability experiments.

Thermostability and weatherability of TiN/TiC-Ni/Mo solar absorption coating by spray method-laser cladding hybrid deposition

A novel one-layer micrometre-sized TiC/TiN-based spectrally selective coating was designed and fabricated via spray method-laser cladding hybrid deposition in air. The laser cladding was used to obtain TiC/TiN-Ni/Mo cermet coatings with desirable optical properties. In particular, an absorptance (α) of ~84% and thermal emissivity (ε) of ~5% at 300 K were calculated at a TiC/TiN weight ratio of 1:1. In addition, the thermal stability of the coating was outstanding after heat treatment at 650 °C for 200 h. The absorptance and emissivity of the 30 wt.% TiC-30 wt. %TiN-20 wt.% Ni-20 wt.% Mo cermet coatings were respectively 82% and 4%, respectively, at 650 °C. Furthermore, the cermet coating also exhibited excellent weatherability, indicateing that TiC/TiN-based cermets are suitable for spectrally selective materials. Moreover, the laser cladding was demonstrated as an improved and novel preparation technology for manufacture of solar selective absorbing coatings.

Enhanced spectral absorption of bilayer WOx/SiO2 solar selective absorber coatings via low vacuum pre-annealing

Solar selective absorber coatings (SSACs) have been one of promising strategies for conversing solar radiation into heat source. To enhance the solar absorption properties, various strategies have been developed to fabricate the SSACs, involving multilayer structure, texturing surface, metal nitrides and carbides, etc., whereas these often entailed sophisticated processes and expensive material costs. Therefore, developing the simple method to yield platforms with excellent optical properties is highly desired. Here, we showed a simple and effective method to boost the optical absorptivity, namely low vacuum pre-annealing process. The resulting absorptance of WOx/SiO2 SSACs is significantly raised from 0.91 to 0.93, and the emittance remains around 0.055 only after annealing at below 300 °C for 10 h. The optical absorptivity of the pre-annealing SSAC is on par with most of absorbers, and it still remained excellent thermal stability in high-vacuum at 400 °C. This simple and efficient method could be a prospective strategy for booting the spectral absorption in the design of the desirable SSACs.

An Ultrathin, Nanogradient, and Substrate‐Independent WOx‐Based Film as a High Performance Flexible Solar Absorber

Advances in flexible and wearable energy‐related devices increase the need for highly efficient, low‐cost, ultrathin solar selective absorber coatings (SSACs). Herein, the fabrication of nanogradient WOx‐based SSACs with excellent properties, including a superior solar absorptance of 0.93, an outstanding thermal robustness of up to 300 °C, and substrate independence, is reported. More importantly, the thickness of WOx‐based SSACs is only approximately 100 nm, which is substantially thinner than all other reported SSACs. These features arise from the two intrinsically absorptive WOx layers on a thin nanoplasmonic W layer. The deposition process is based on self‐doped reactive sputtering via limited tungsten oxidation due to a small amount of oxygen. The WOx‐based SSACs on a flexible polyimide sheet demonstrate stable performance, strong adhesion, and bendable nature. The proposed self‐doped fabrication process provides a new way to design cost‐effective ultrathin SSACs to meet the demand for large‐scale flexible energy harvesting and supply applications.