High-temperature Solar Selective Absorbers Research Articles

A high-temperature solar selective absorber based on one-dimensional multilayer nanostructures

Selective absorbers play a crucial role in harvesting solar energy and realizing effective solar-thermal energy conversion in concentrated solar power systems and thermophotovoltaic systems. This study focuses on obtaining a high-temperature solar selective absorber which can well balance its thermal performance as well as structural complexity. Based on this, a selective absorber with simple one-dimensional multilayer nanostructures is designed to adapt to the high-temperature operation environments. The obtained absorber exhibits a total solar absorptance of 0.9504 to the AM1.5 solar radiation. Taking into account the infrared radiation loss, the solar-thermal efficiency of the absorber is found to be 91.2% under 1000 suns at 1273 K, and the total emittance is 0.1504 at the corresponding temperature. The absorber shows high insensitivity to both incident polarization angles and wide-angle incidence. Analysis of impedance and electromagnetic field distribution indicates that the selective absorption performance of the absorber is attributed to its impedance matching properties, resulting from the coupling effect of surface plasmon polaritons and magnetic polaritons.

AlCrO protected textured stainless-steel surface for high temperature solar selective absorber applications

The diffusion of substrate material into absorbing layer and oxidation of metal substrate or cermet metal nanoparticles at high temperatures are known as inevitable problems of the solar selective absorbers. In this study, we consider the use of textured stainless steel (SS) surface coated with a protective AlCr oxide layer as a high temperature solar selective absorber. The textured SS surface was prepared by ion etching techniques and AlCr oxide protective layer was deposited by RF magnetron sputtering. The absorptivity and emissivity of the as-prepared absorbers were 0.86–0.92 and 0.151–0.168, respectively. In order to evaluate the thermal stability, the absorbers were annealed at 600–800 °C for different time in ambient atmosphere. Absorbers demonstrated a red shift of the onset of the reflectivity at all annealing temperatures. The high activation energy of 315 kJ/mol was calculated. In terms of thermal stability, the service lifetime of the absorbers at 500 °C was estimated to be about 100 years and at 700 and 800 °C the absorbers were stable at least 50 and 1 h, respectively. A detailed examination of the annealed absorber surface revealed growth of surface Mn3O4 nanocrystals, which resulted in observed change of the reflectance spectra, while the textured surface morphology had no significant change. The results show that the protective textured surface has much higher thermal stability in air than iron-based cermet absorbers.

Ultrabroad wavelength absorption in high-temperature solar selective absorber coatings enabled by high-entropy nanoceramic AlTiZrHfNbN for high-performance solar-thermal conversion

A tailored synthesis method is used to yield AlTiZrHfNbN based coatings with excellent spectral selectivity, showing an ultrabroad absorption wavelength range at or beyond the level of solar selective absorber coatings.

Thermally stable Ni@SiO2 core-shell nanoparticles for high-temperature solar selective absorber

Improving the thermal stability of solar absorbers persists to be the challenge for solar-thermal technologies operated at high temperatures, such as concentrated solar power trough system. Herein, we report a rationally designed solar selective absorber based on Ni@SiO2 core–shell nanoparticles with significantly improved thermal stability at temperatures up to 700 °C. Ni nanoparticles were encapsulated into dense ceramic SiO2 nanoshells, which effectively prevented the formation of Ni-alloys and oxides by inhibiting diffusion of both inclusion Ni and metallic atoms from the substrate at high temperatures. Besides, the spectral absorption properties of the Ni@SiO2 solar absorber can be optimized by tuning the sizes of Ni cores and absorbing structures. As a result, high solar absorptance (∼0.913) but low thermal emittance (∼0.085) have been achieved for a double-layer Ni@SiO2 absorber with Ni cores of 40 nm and 80 nm. The excellent thermal stability and high absorption selectivity of the Ni@SiO2 solar absorber indicate its great potential for high-temperature solar-thermal conversion applications.

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.

A novel TiC–ZrB2/ZrB2/Al2O3 multilayer high temperature solar selective absorbing coating: Microstructure, optical properties and failure mechanism

A novel multilayer nanostructured TiC–ZrB2/ZrB2/Al2O3 solar absorber coating has been developed for Concentrated Solar Power. The TiC–ZrB2 and ZrB2 work as the absorptance layers. The SS/TiC–ZrB2/ZrB2/Al2O3 solar absorber coatings exhibit a high spectral selectivity of 0.92/0.10. The sample is annealed in vacuum to test the thermal stability. After keeping the as-deposited samples and the samples annealed in vacuum at room temperature for two years, the coatings show different colors. Through the SEM, XPS and Raman analysis, we have discussed the corresponding failure mechanism. For the purpose of clarifying the failure mechanism more clearly, corresponding failure model is established.

Structure, thermal stability and chromaticity investigation of TiB2 based high temperature solar selective absorbing coatings

Transition metal titanium diboride (TiB2) is a promising material for the solar selective absorbing coatings due to its favorable spectral selectivity and thermal stability. In this research, spectrally selective TiB2/Al2O3 tandem absorber is deposited on stainless steel (SS), which exhibits a high solar absorptance (αs = 0.93) and a low thermal emissivity (ε = 0.11). In this tandem absorber, the TiB2 acts as the main absorber layer and the Al2O3 acts as an antireflection layer. The tandem absorber coating is systematically characterized by ultra-high resolution scanning electron microscope (SEM), UV–vis-NIR spectrophotometer, Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS) and Micro-Raman techniques. The thermal stability in vacuum of the SS/TiB2/Al2O3 tandem absorber coatings is investigated in the temperature range of 400–800 °C for 2 h and 100 h, respectively. SEM and Raman spectra indicate that the change of surface morphology and chemical composition for the coating result in the degradation of optical performance. With the different annealed temperature, the SS/TiB2/Al2O3 tandem absorber coatings exhibit different color, which is further investigated using chromaticity diagram.

Structure, optical simulation and thermal stability of the HfB2-based high-temperature solar selective absorbing coatings.

Transition metal borides are a kind of potential materials for high-temperature solar thermal applications. In this work, a novel SS/HfB2/Al2O3 tandem absorber was prepared, which exhibited high solar spectrum selectivity (α/ε) of 0.920/0.109. The optical constants of the coating were obtained using spectroscopic ellipsometry, and the dispersion model of the HfB2 layer was modeled with the Tauc–Lorentz dispersion formula. In addition, the reflectance spectrum simulated by the CODE software corroborated well with the experimental results. The thermal stability test indicated that the HfB2/Al2O3 solar absorber coating was thermally stable in vacuum at 600 °C for 2 h. When extending the annealing time to 100 h, the coating could maintain high spectral selectivity after aging at 500 °C irrespective of whether in air or vacuum. All these results indicate that the coating has good solar selectivity and is a promising candidate for high-temperature solar thermal applications.

High temperature solar selective absorber coating deposited by laser cladding

In order to prepare high temperature stability cermets solar selective absorbing coating, single- layer Ni/Mo–TiC cermets coatings were firstly deposited on stainless steel substrate using laser cladding method and surface coating method. The result shows that the performance of the laser cladding coating is far superior to the coating fabricated by the surface coating method with thermal emittance decreased from 44.6% to 5.5%. Furthermore, the spectrally selective coating fabricated by laser cladding shows the excellent thermal stability. The solar absorptance and thermal emittance of the coating are 80.7% and 6.0% at 600 °C, 80% and 5.5% at room temperature, respectively. This result indicates that TiC-based cermets are more propitious solar selective absorber materials. More importantly, laser cladding, as a representative of new techniques, could be applied to the field of the solar selective absorber coating.

A High-Temperature Solar Selective Absorber Based upon Periodic Shallow Microstructures Coated by Multi-Layers Using Atomic Layer Deposition

Regarding the fabrication of solar selective absorbers, the ability to create microstructures on top of metal surfaces is a promising technology. Typically, these materials are able to possess spectrally-selective absorption properties for high-temperature usage. Solar-selective absorbers that function at temperatures up to 700 °C and possess shallow honeycomb cylindrical microcavities coated with a metal-dielectric multi-layer have been investigated. Honeycomb array cylindrical microcavities were fabricated on W substrate with interference lithography and multi-layers consisting of Pt nano-film sandwiched by Al2O3 layers were created for a uniform coating via atomic layer deposition. The absorbance spectrum of fabricated samples reveals results consistent with a simulation based on a rigorous coupled-wave analysis method. A solar absorbance value of 0.92 and a hemispherical total emittance value of 0.18 at 700 °C was determined from the fabricated solar-selective absorber. Additionally, thermal stability of up to 700 °C was confirmed in vacuum.

Microstructure, optical and electrical properties of CrAlN film as a novel material for high temperature solar selective absorber applications

The microstructure, optical and electrical properties of CrAlN and (Cr, Al) films synthesized by DC reactive magnetron sputtering method have been comparatively investigated. The results show that CrAlN film crystallizes in B1 NaCl-typed CrN structure with the preferred growth orientations of (111) and (220) and CrAlN grains show perfectly crystalline pyramid-like shape. The CrAlN film is determined as Al-doped CrN phase with the chemical formula of Cr0.9136Al0.0864N0.8999. It shows similar reflectance and absorptance characteristics to those of TiN based films in the solar spectrum region (300–2600nm) and exhibits semiconductor nature with a sheet resistance of 38kΩ/sq. Hence CrAlN can be used as a novel candidate material for high-temperature solar selective absorber coatings with good thermal stability and oxidation resistance.

Microstructure, optical properties and thermal stability of ZrB<SUB>2</SUB> and Zr–B–N thin films as high-temperature solar selective absorbers

ZrB2 and Zr B N thin films were deposited by DC magnetron sputtering technique. The microstructure and optical properties of thin films were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy dispersive X-ray spectroscopy (EDS) and spectrophotometer. Thin films with high selectivity (a = 9526%IsRT = 5%) were achieved by optimizing the nitrogen content of absorbing layers and thickness of antireflective layer. Thermal stability test illustrated that Zr B N thin film with high nitrogen content showed weak thermal stability due to the loose morphology of thin films. Thus, thin films with low nitrogen content and suitable thickness of Si3N4 layer can exhibit superior solar selectivity (a 91.7%lem- = 4.1%) after annealing at 500 C/60 h in air. The XRD, cross-sectional TEM and EDS results of films annealed at 500 C/100 h in air demonstrated that the solar selectivity degradation of films was due to the surface oxidation of ZrB2 layer. Moreover, the solar selectivity was still kept at high level (a = 89.05%lsRT = 10.5%) after treating in neutral salt spray for 24 h. It can be deduced that ZrB2 thin film by sputtering is a superior candidate as high-temperature solar selective absorbers.

Improvement of Thermal Stability of ZrB2/Si3N4 Coatings as High-Temperature Solar Selective Absorbers

ZrB2 thin films were prepared by DC magnetron sputtering technique. The microstructure, thermal stability and optical properties of thin films were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and spectrophotometer. The compactness of ZrB2 thin films was studied to improve the thermal stability by optimizing the deposition parameters. The compactness and thermal stability of the coatings were improved with the increase of substrate temperature. However, these properties of the coatings were enhanced firstly and then weakened with the increase of substrate bias voltage. The selectivity of sample deposited at high substrate temperature and suitable bias voltage degraded slightly after annealing at 500 °C/100 h in air. This provided a new way to improve the thermal stability of high-temperature solar selective absorber.

Optical Characterization and Modeling of Coatings Intended as High Temperature Solar Selective Absorbers

SiCNH thin coatings synthesized by Plasma Enhanced Chemical Vapor Deposition (PECVD) are considered as high temperature ceramic materials for solar selective multilayer absorbers. As indicated by ellipsometry and spectrophotometry measurements, their optical characteristics make them suitable as solar absorbing and antireflective coatings in multilayer stacks and/or ceramic- metal composites (cermets). These measurements gave access to spectral values of complex refractive index N = n + ik, reflectance R and transmittance T of the coatings. R and T were also modeled from N, based on Fresnel equations, and a good agreement was found between simulated and measured reflectance in the visible region. A transfer matrix method was then applied to predict the optical response (R, T) of virtual multilayer stacks including SiCNH materials. W-SiCNH cermets were also considered as absorptive layers in these stacks and their refractive indices were calculated using Bruggeman effective medium approximation theory. Solar absorptance αS and thermal emittance ɛ(T) were estimated from simulated reflectance. Layer thicknesses and cermet composition were optimized using CODE software and a MATLAB in-house code, so as to maximize solar-heat conversion efficiency which relied on high αS (up to 95%) and low ɛ (down to 6% at 500°C). The corresponding global yield which could be attained in typical Fresnel and central tower solar plants using these selective coatings was also calculated and compared to current technology.

High-temperature Photonics Using Self-organization of Superalloys for Solar Selective Absorbers

Making surface microstructures on refractory metal is one of the promising technologies of controlling optical property at high-temperature condition. However, the technique of fabricating microstructures for refractory metal is limited, such as using semiconductor technologies. Therefore, the large-area fabrication of periodic microstructures on refractory metals is a key technology supporting the practical application of controlling optical property using surface microstructures. This report describes large-area fabrication of two-dimensional submicron quasi-periodic microcavities using self-organization on a nickel-based superalloy. The surface microcavities on a bulk metal were obtained by appropriate heat treatments and simple chemical etching process. The spectrally selective property attributed to the confined modes inside cavities is observed. The selective absorbing property and thermal stability are also confirmed at 973K. Therefore, this fabrication method can be applied for high temperature solar selective absorbers. The fabricated sample showed solar absorptance of 0.74 and emittance of 0.25 at 873K. The performance of solar selective absorber is verified by simple heating test using the sample with randomly arrayed microstructures. It is indicated that the temperature of microstructured sample shows 30oC higher than that of a black-painted sample.

A High-temperature, High-efficiency Solar Thermoelectric Generator Prototype

Solar thermoelectric generators (STEGs) have the potential to convert solar energy at greater than 15% efficiency. This project investigates the system design, the necessary thermoelectric and optical technologies, and the economic feasibility of the STEG approach. A STEG is a solid-state heat engine that converts sunlight directly into DC electricity through the thermoelectric effect. STEGs consist of three subsystems: the solar absorber, the thermoelectric generator (TEG), and the heat management system (insulation, heat exchanger, vacuum enclosure, etc.). This project will integrate several state-of-the-art technologies to achieve high efficiency, including next- generation materials for TEGs, high-temperature solar-selective absorbers, and thermal cavities. We will test STEGs at NREL's high flux solar furnace (HFSF) and perform analysis of parasitic losses and lifetime analysis to optimize prototype operation. Equally important for this technology is the development of a cost model to determine the economic competitiveness and possible application niches for STEG technologies. We report on first-order economic analysis to identify the most promising pathways for advancing the technology.

Spectral properties and thermal stability of solar selective absorbing AlNi–Al2O3 cermet coating

A new solar selective absorbing coating of AlNi–Al2O3 cermet multilayer is designed and prepared on Cu or stainless steel (SS) substrate by magnetron sputtering. The coating consists of an IR-reflective metallic layer, an absorption bilayer and an anti-reflection layer from substrate to top. The IR-reflective metallic layer is Cu or Mo. The absorption bilayer consists of a high metal volume fraction (HMVF) AlNi–Al2O3 cermet sublayer and a low metal volume fraction (LMVF) AlNi–Al2O3 cermet sublayer. The anti-reflection layer is Al2O3. The optical performance (α/ε) of the coating is 0.94/0.07 on Cu substrate and 0.95/0.078 on SS substrate by optimizing the metal volume fraction and thickness of each layer. The samples deposited on SS substrate were heated at 500°C for 138h and 284h in vacuum to evaluate the optical stability. Experimental results showed that the high temperature stable AlNi intermetallic compound, instead of Mo in the traditional Mo–Al2O3 coatings, displayed a good thermal stability at 500°C. There is no significant change appearing in the optical properties of the coating after the heat treatment. It means that the new coating is a good potential candidate as a high temperature solar selective absorbing coating for parabolic trough concentrated solar power (CSP).

Characterization of Pyromark 2500 Paint for High-Temperature Solar Receivers

Pyromark 2500 is a silicone-based high-temperature paint that has been used on central receivers to increase solar absorptance. The radiative properties, aging, and selective absorber efficiency of Pyromark 2500 are presented in this paper for use as a baseline for comparison to high-temperature solar selective absorber coatings currently being developed. The solar absorptance ranged from ∼0.97 at near-normal incidence angles to ∼0.8 at glancing (80°) incidence angles, and the thermal emittance ranged from ∼0.8 at 100 °C to ∼0.9 at 1000 °C. After thermal aging at temperatures of ∼750 °C or higher, the solar absorptance decreased by several percentage points within a few days. It was postulated that the substrate may have contributed to a change in the crystal structure of the original coating at elevated temperatures.

Optical modeling of multilayered coatings based on SiC(N)H materials for their potential use as high-temperature solar selective absorbers

SiCH thin films grown from Ar/tetramethylsilane plasmas (PECVD) were evaluated as absorber layers in selective coatings for high-temperature solar receivers. SiCH coatings are used in thermomechanical applications because of their good thermal stability. A SiCH film with an absorption index k(430nm)=0.11 was taken as a reference for optical simulations of refractory metal–SiCH multilayer stacks and cermets. The transfer matrix method and Maxwell–Garnett effective medium approximation were used to calculate the spectral reflectance and transmittance of the selective absorbers. Their solar absorptance and thermal emittance were also deduced. A seven-layer stack with alternating metal and SiCH layers was found to present a simulated solar absorptance of 0.92 and thermal emittance of 0.08 at 500°C.

The film thickness dependent thermal stability of Al2O3:Ag thin films as high-temperature solar selective absorbers

The monolayer Al2O3:Ag thin films were prepared by magnetron sputtering. The microstructure and optical properties of thin film after annealing at 700 °C in air were characterized by transmission electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy, and spectrophotometer. It revealed that the particle shape, size, and distribution across the film were greatly changed before and after annealing. The surface plasmon resonance absorption and thermal stability of the film were found to be strongly dependent on the film thickness, which was believed to be associated with the evolution process of particle diffusion, agglomeration, and evaporation during annealing at high temperature. When the film thickness was smaller than 90 nm, the film SPR absorption can be attenuated until extinct with increasing annealing time due to the evaporation of Ag particles. While the film thickness was larger than 120 nm, the absorption can keep constant even after annealing for 64 h due to the agglomeration of Ag particles. On the base of film thickness results, the multilayer Al2O3:Ag solar selective thin films were prepared and the thermal stability test illustrated that the solar selectivity of multilayer films with absorbing layer thickness larger than 120 nm did not degrade after annealing at 500 °C for 70 h in air. It can be concluded that film thickness is an important factor to control the thermal stability of Al2O3:Ag thin films as high-temperature solar selective absorbers.