Selective Solar Absorber Coatings Research Articles

BiFeO3 perovskite-based all oxide ambient stable spectrally selective absorber coatings for solar thermal application

Enhancements in absorbance and thermal stability are essential for solar selective absorber coatings (SSACs) to convert solar energy efficiently into thermal energy.

Thermal stability of AlCrO antireflection layer for high-temperature cermet-based solar selective absorber applications

Highly stabled antireflection layers play a crucial role in enhancing the optical and thermal performance of cermet-based solar selective absorbing coatings. Few state-of-the-art antireflection layers are qualified for high-temperature (>873 K) utilizations due to the insufficient thermal stability and increased infrared emissivity. Herein, a highly stable multilayer cermet-based absorber with an AlCrO antireflection layer is developed using cathode arc ion plating. The deposited coating exhibits a solar absorptivity of 0.908 ± 0.005 and a thermal emissivity of 0.144 ± 0.004 even after annealing at 923 K for 2000 h in air, compared to the pristine coating with a spectral selectivity of 0.898 ± 0.002/0.186 ± 0.003. After undergoing the extensive annealing treatments, abundant particles with an average size of approximately 8 μm have been detected on the coating surface. However, the AlCrO layer exhibits a remarkably flawless and dense surface topography devoid of any cracks or voids. Based on the GI-XRD and TEM evaluations, the particles have been identified as Mn2O3 nanocrystals. The formation of Mn2O3 particles originates from the diffusion of Mn atoms from the stainless-steel substrate to the surface of the AlCrO layer, where they undergo oxidation. The presence of Mn2O3 can enhance the solar absorptivity by increasing transmittance in the visible light spectrum and reduce the emittance by enhancing reflection of infrared radiation. Then, the impact of Mn2O3 on spectra selectivity and thermal stability is discussed. These results indicate that the AlCrO layer exhibits remarkable durability when subjected to high-temperature annealing in an air environment, thereby significantly enhancing the thermal stability and optical performance of cermet-based solar selective absorbing coatings. These findings suggest that the AlCrO layer exhibits exceptional durability under high-temperature annealing in an air environment, thereby significantly enhancing the thermal stability and optical performance of cermet-based solar selective absorbing coatings.

Facile dip-coating assisted preparation of reduced graphene oxide-copper oxide nanocomposite thin films on aluminum substrate for solar selective absorber

Reduced graphene oxide integrated copper oxide nanocomposite thin film (rGO-CuO NC TF) solar selective absorber coatings were devised on aluminum substrate using dip-coating procedure. The prepared coatings were subjected to characterization by scanning electron microscopy, X-ray diffractometry, energy-dispersive X-ray spectroscopy, X-ray photoelectron spectroscopy, PL spectroscopy, UV–vis–NIR spectrophotometer, Emissometer and Raman spectroscopy. XRD and Raman analyses disclosed presence of mono-phase with highly crystalline monoclinic structured CuO in the rGO-CuO NC TFs. The SEM images revealed grain-like morphology of CuO, which are randomly distributed on rGO sheets. EDX and XPS investigations confirmed presence of anticipated chemical constituents and chemical composition, respectively in the NC TFs. The rGO-CuO NC TF with 1.0 wt% GO disclosed absorptance (α) of 79.92% and emittance (ε) of 4.2% with a highest selectivity (ξ) of 19.03 that suggests its promising prospect for solar absorber coating in solar thermal devices.

Enhanced high-temperature thermal stability of the novel MoAlSiN-based solar selective absorbing coatings by optimizing the multilayer structure

Solar selective absorbing coating (SSAC) plays an important role in the high-temperature concentrated solar power system. The high thermal stability of SSAC above 600 °C has been a big open question for molten salt concentrated solar power projects. In our work, the TiN/Mo/MoAlSiN/MoAlSiON/SiO2 and the TiN/Mo/MoAlSiN/Si3N4/SiO2 SSACs are developed for getting high-temperature thermal stability, besides excellent spectral selectivity. The solar spectral absorptivities of the MoAlSiN/MoAlSiON and MoAlSiN/Si3N4 SSACs are 95.2% and 93.2%, respectively. While the infrared emissivities of the MoAlSiN/MoAlSiON and MoAlSiN/Si3N4 SSACs are controlled below 10.6% and 10.5% at 400 °C, respectively. The novel MoAlSiN/Si3N4 SSAC deposited on quartz can maintain thermal stability even after heat treatment at 800 °C for 200 h in vacuum, which is one of the best results in the research field for high-temperature SSACs. It is found that the choice of suitable materials without oxygen and the elimination of interfacial thermal stress are very important for improving thermal stability. Furthermore, the practical total energy conversion efficiencies of the MoAlSiN/Si3N4 SSAC at 600 °C–800 °C are always above 57%. When the activation energy is assumed as 180 kJ/mol, the estimated service life of the MoAlSiN/Si3N4 SSAC at 600 °C is more than 25 years.

Realizing an Optical Micro‐Cavity in a CuCo2O4‐W‐CuCo2O4 Thin Film Stack for Spectrally Selective Solar Absorbers

AbstractTapping the potential of solar thermal energy requires spectrally selective solar absorber coatings, which lead to high photothermal efficiency. Micro‐cavities are known to dramatically enhance or suppress light absorption and emission in a directional manner and are used in nanophotonics, photodetectors, lasing, etc. Here, a dielectric‐metal‐dielectric multilayer stack with an optical micro‐cavity of a 12 nm tungsten layer formed between two dielectric layers of transition metal oxide CuCo2O4 (CCO) is designed. The CCO‐W‐CCO thin film multilayers deposited on stainless steel substrate by DC‐radio frequency magnetron sputtering achieve a 90% solar absorptance for the entire solar spectrum and a thermal emittance of 19% for the infrared spectrum. Optimization of thicknesses of individual layers is achieved by numerical simulations done in COMSOL Multiphysics, thereby reproducing the optical properties of each layer. The simulation results satisfactorily reproduce the experimental reflectance and demonstrate maximum power dissipation in the metallic layer in visible as well as NIR regions. Nevertheless, CCO material characterizations reveal that the enhanced absorption over the entire solar spectrum could be attributed to the underlying spinel crystal structure and the nanostructure film morphology.

Improved optical and mechanical characteristics of multilayered Cu/TiON/CrO2 coating via heat treatment for solar absorbing applications

In the last few decades, growing interest has been shown in the development of new solar selective coatings based on transition metal nitride and/or oxinitride for solar absorbing applications. Solar thermal collectors are well thought out to be the most effective process of converting and harvesting solar radiation. In this investigation, Cu/TiON/CrO2 multilayered solar selective absorber coatings have been coated onto Al substrates using the dip-coating process followed by an annealing process at (400, 450, 500, 550, and 600 °C. The XRD analysis showed excellent crystalline quality for the prepared thin films along with enhanced surface features as proved by FESEM images, and the grains are in the range of (27–81) nm. The optical investigations revealed that the film annealed at 600 °C exhibits improved solar absorptance, thermal emittance, and solar selectivity of ~ (90.7 %), (5.8 %) and 15.6, respectively. Also, the highest values of hardness ~ (26.8 GPa) and Young’s modulus ~ (250 GPa) were assigned for the film annealed at the highest temperature. Calculated optical band gaps of fabricated thin multilayered Cu/TiON/CrO2 films were found to be in the range of (1.8 – 2.10) eV.

Extracting Optical Parameters of Cu–Mn–Fe Spinel Oxide Nanoparticles for Optimizing Air-Stable, High-Efficiency Solar Selective Coatings

High-temperature Cu–Mn–Fe spinel-oxide nanoparticle solar selective absorber coatings are investigated experimentally and theoretically. A general realistic approach to evaluate absorption coefficient spectra from the optical measurements of the nanoparticle-pigmented coatings is developed by solving the inverse problem of the four-flux-radiative transfer model. The derived absorption properties are utilized to elucidate the direct and indirect bandgap transitions in the visible spectral regime as well as the sub-bandgap absorption in the infrared range. Furthermore, the optical properties of NP materials can be directly applied to optimize the solar selective coating design and analyze thermal degradation mechanisms. The analysis reveals that the Cu–Mn–Fe spinel oxides are semiconductors with an indirect bandgap ranging from 1.7 to 2.1 eV, while iron-free CuMn2O4 is a direct bandgap material with Eg = 1.84 eV. With the same coating thickness and nanoparticle load, the solar absorptance ranks in the order of Mn2O3 < MnFe2O4 < CuFe2O4 < CuFeMnO4 < CuMn2O4. The optimized spray-coated iron-free CuMn2O4 NP-pigmented coating demonstrates a high solar absorptance of 97%, a low emittance of 55%, a high optical-to-thermal energy conversion efficiency of ∼93.5% under 1000× solar concentration at 750 °C, and long-term endurance upon thermal cycling between 750 °C and room temperature in air. The optical parameter analysis approach can be easily extended to other material systems to facilitate the search and optimization of high-temperature nanoparticle-pigmented solar selective coatings.

High-temperature thermal stable solar selective absorbing coating based on the dielectric-metal-dielectric structure

Obtaining high-temperature thermal stable solar selective absorbing coatings is a big challenge for ultra-high temperature solar-thermal conversion applications. In this paper, a dielectric-metal-dielectric (DMD) structured solar selective absorbing coating (SSAC), AlN/TiB2/AlN/Mo/AlN/substrate is proposed and then prepared by direct current magnetron sputtering. The as-deposited SSACs have a high absorptance of 93.4% and a low emittance of 6.9% at room temperature. The SSACs with the DMD structure deposited on the stainless steel (SS) substrate can maintain stability at 800 °C for 200 h in vacuum. The performance criterion (PC) value, a key factor to evaluate thermal stability, is only 0.010 for the coatings after heat treatment. The SSACs deposited on the quartz substrate can even remain stable at 900 °C with a low PC value of 0.008. Our findings will have great potential applications in ultra-high-temperature solar-thermal conversion, and satisfy fully the requirements of thermal stability for current CSP projects.

Research on the Thermal Stability in High-Temperature Air of Cr-Fe Composite Oxide Solar Coatings by Chemical-Colored of Stainless Steel

The stainless steel chemical coloration demonstrates excellent repeatability of process when adopted to fabricate solar selective absorber coatings (SSACs) on TTS445J1 stainless steel base material. The optical performance, morphology, and composition of coatings are characterized by UV-3600, IR-Affinity-1, SEM, EDS, etc. The experimental results suggest that the Cr-Fe composite oxides with coatings in a spinel structure obtained by chemical coloration on a stainless steel surface exhibit outstanding spectral selectivity, with α/ε = 0.9334/0.1326. The coatings were generated by the direct reaction between the stainless steel substrate composition and the coloring solution, which changes the traditional way of combining the coating with the substrate by physical methods. Comparing the SEM images of the coatings before and after aging at 500 °C in air, we noticed no significant changes at the interface between the coatings and the substrate, indicating excellent coating adhesion. At the same time, the substrate grains did not change much after the chemical reaction of the stainless steel substrate, indicating that the oxidation resistance of the stainless steel substrate was not weakened. Finally, the Cr-Fe composite oxide exhibited excellent thermal stability in air. Based upon a microstructure analysis, the Performance Creation (PC) is 0.01 after aging at 500 °C for 200 h in high-temperature air, primarily because of the loss of H2O molecules from the hydrates in the coatings. After aging for 800 h, PC = 0.0458. After the aging hours are extended to 1000 h, PC = 0.0762. During the aging process at high temperature, the coatings of the Cr-Fe composite oxides maintained stable composition and phase structures. The decay in optical performance is due mainly to the reconstruction of the surface morphology of the coatings as a result of the largening of grains.

Nanoparticles based single and tandem stable solar selective absorber coatings with wide angular solar absorptance

Solar selective absorber coatings with wide angular solar absorptance aids in attaining high photo-thermal conversion efficiency for solar thermal systems. In this regard, nanoparticles-based absorber coatings were developed on stainless steel grade 304 by combining the impregnation method, solvothermal process, and dip-coating technique. Developed nanocomposite (SiO 2 nanoparticles in transition metal oxide matrix) based single layer absorber coating with nano void textured surface, exhibits solar absorptance of 0.92 and spectral emittance of 0.12. MgF 2 nanoparticles based anti-reflective layer on single-layer absorber coating improves the solar absorptance to 0.94 by destructive interference mechanism. Both nanoparticles based single and tandem absorber coatings show wide angular solar absorptance of 0.88 and 0.89, respectively, at an incidence angle of 50°. Besides, developed absorber coatings show lower thermal emissivity of 0.17 and good photo-thermal conversion efficiencies at high operating temperatures (400–500 °C). These developed absorber coatings offer excellent thermal stability at an open atmospheric condition till operating temperature, such as 400 °C for 100 h. Selective nature with wide angular solar absorptance and low heat loss behaviour of stable absorber coating show the photo-thermal conversion efficiency of 91% can improve the performance of receiver tubes in solar thermal systems. • Cost-effective and novel nanoparticles based single and tandem absorber coatings. • Nanocomposite based absorber coating exhibits wide angular solar absorptance. • Tandem layer (MgF 2 AR/nanocomposite) absorber shows solar absorptance of 0.94 • Absorber coatings shows excellent photo-thermal conversion efficiencies (>90%). • Absorber coatings are thermally stable at 400 °C for 100 h with good adhesion.

Angular absorptance and thermal degradation analysis of TiB2/Ti(B,N)/SiON/SiO2 multilayer solar absorber coating

AbstractMultilayer solar selective absorber coatings have been developed in the last few decades. The thermal stability in terms of microstructure gives an insightful understanding of the optical properties of such coatings. In this context, we extensively utilized transmission electron microscopy (TEM) analysis to establish the thermal stability of TiB2/Ti(B,N)/SiON/SiO2 coating, under thermal cycling/continuous heating to 500°C in vacuum for 250 h. In particular, this work reports the variation in the solar absorptance of TiB2/Ti(B,N)/SiON/SiO2 coating with different angles of incidence of the solar radiation. Extensive analysis using the TEM technique reveals the presence of oxide interlayers that act as diffusion barrier layers to enhance the thermal stability of the coating. Computational simulation using SCOUT software validates the measured reflectance spectrum of the developed multilayer coating. The minor changes in absorptance and emissivity after heat treatment in vacuum at 500°C, together with high solar absorptance over a broad angular variation, establish the potential application of TiB2/Ti(B,N)/SiON/SiO2 as a selective coating in concentrated solar power systems.

First principle calculation of thermal conversion properties of W-Al2O3 based solar selective absorbing coating

Al2O3 based solar selective absorbing coatings have been considered as a promising candidate for solar thermal conversions, but their properties are expected to be improved urgently. Herein, inspired by the composite materials design, an optimization method for optical performance and thermal stability of Al2O3 was explored by us, the W-Al2O3 based composite structures used for solar was developed, and its electronic structure and optical performance were investigated by the first-principles calculations. It was found that the band gap of the proposed composite structure can be successfully regulated and it decreases significantly with the increase of the mass fraction of W doped in Al2O3, which leads to the improvement of the thermal stability. The optical properties and solar thermal conversion properties of W-Al2O3 based selective absorption coating are accurately predicted. The results show that W-Al2O3 based solar selective absorption coating also has good absorption and reflection properties. The addition of W atom leads to the reduction of solar thermal efficiency. When the W mass fraction doped in W-Al2O3 is 23.92% at the temperature of 273 K, compared with pure Al2O3, the solar thermal conversion efficiency is reduced by 6.56%, and the corresponding absorption rate and thermal radiation was reduced by 6.54% and 1.99%, respectively.

Optical Performance, Thermal Stability, and Failure Analysis of the WNx-Si3N4 Multilayer Solar Selective Absorbing Coatings

Optical Performance, Thermal Stability, and Failure Analysis of the WN_<i>x</i>-Si₃N₄ Multilayer Solar Selective Absorbing Coatings

A thermal efficiency-enhancing strategy of parabolic trough collector systems by cascadingly applying multiple solar selective-absorbing coatings

For a long time, the pathway for enhancing solar collector thermal efficiency of solar energy converting into heat has been blocked because state-of-the-art parabolic trough collectors (PTCs) suffer low thermal efficiency from severe reradiation heat loss. Based on the nonuniformity temperature distribution in the parabolic trough collector loop, a novel strategy of cascadingly applying multiple solar selective-absorbing coatings (SSCs) in different sections of the collector loop is proposed. Two systems were considered to implement the designed strategy: an ideal system with multiple ideal SSCs with related optimal cutoff wavelengths at various temperatures; and a multi-section system with multiple practical SSC materials. By setting up a two-dimensional heat transfer model, the thermal performance of the PTC systems was analyzed. The results showed that the ideal and the multi-section system had better thermal performance than a conventional system. The heat loss of the multisection system was reduced by 29.3%, and the thermal efficiency was enhanced by 4.3% with an operating temperature of 290–550 °C. This strategy is expected to provide a new possibility for the realization of cost-effective solar energy utilization.

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.

Structure, Optical Properties and Thermal Stability of All-Ceramic Solar Selective Absorbing Coatings: A Mini Review

Solar selective absorbing coatings (SSAC) harvest solar energy in the form of thermal energy. Traditional metal-rich SSACs like cermet-based coatings and semiconductor–metal tandems usually exhibit both a high solar absorptance and a low thermal emittance; however, metal nanoparticles can easily oxidize or diffuse at high temperature. Different from these SSACs, the all-ceramic SSACs can keep the superior optical performance at high temperatures by restraining oxidation and metal element diffusion. Besides, the facile and inexpensive fabrication of the all-ceramic SSACs makes it possible for commercial applications. These SSACs are usually a regular combination of transition-metal carbides and nitrides, which show great thermal stability and optical properties simultaneously. The structure design of the SSACs will affect the element diffusion, element oxidation, phase transition, as well as the spectral selectivity obviously. In this article, we review the structure designs of all-ceramic SSACs, and the optical properties and thermal stability of the all-ceramic SSACs in the latest literature are also compared. The purpose of this review is to identify the optimal structure design of the all-ceramic SSAC, and we also present an outlook for the structure design strategy for all-ceramic SSACs with high photothermal conversion efficiency and thermal stability.

Accelerated aging tests to selective solar absorber coatings for low temperature applications

The need of a higher role of solar energy within the energy mix in the coming decades obliges the collectors to increase their performance and reliability. It is demanded that the solar absorbers, as the key component of solar thermal collectors, should be low cost with high efficiency for extended lifetimes under different kinds of environments. Commercially, there are presently two main types of solutions as selective solar absorbers coatings for low temperature applications: coatings obtained by physical vapour deposition (PVD) and by paint coatings (PC). These coatings present different physical and chemical characteristics. Therefore, it is important to know how these commercial coatings degrade over time in different environments.

Enhanced thermal stability of Mo film with low infrared emissivity by a TiN barrier layer

The infrared reflective layer is one of the most important components of solar selective absorbing coatings (SSACs). The low infrared emissivity of the infrared reflective layer is necessary to reduce the energy losses caused by the thermal radiation from the SSACs. Also, the thermal stability of the infrared reflective layer has a crucial impact on the thermal stability of the entire coatings. In this study, a series of Mo films, as the infrared reflective layer, were deposited on stainless steel (SS) or glass substrates by DC magnetron sputtering. The effects of sputtering parameters on crystal structure, surface morphologies, film resistivity, and infrared emissivity of the Mo films are systematically investigated. The lowest infrared emissivity of the prepared Mo films is 0.06, which can be obtained by optimizing the deposition parameters. And the emissivity could be further decreased to 0.03 by annealing treatment. The insertion of a TiN layer between the Mo layer and the SS substrate can effectively inhibit the elemental diffusion from the SS substrate to the Mo film in vacuum until 700 °C, which improves effectively the operating temperature of the Mo film as an infrared reflective layer in SSAC.

Influence of operating conditions on the optical optimization of solar selective absorber coatings

In concentrated solar power (CSP), selective solar absorber coatings increase absorbed solar flux (high solar absorptance AS) while reducing radiative losses (low thermal emittance EBB). Coating developers usually maximize the selective ratio As/EBB (highest As, lowest EBB). The solar-to-heat conversion (heliothermal) efficiency Rh of a solar absorber is however a more relevant optimization target. It is deduced from As and EBB, but is more sensitive to As. It also strongly depends on operating conditions, solar concentration ratio C and absorber temperature TA, radically varying between CSP technologies. This allows different optimal designs and leads to more efficient selective coatings, adapted to their specific application. In this paper, the influence of operating conditions on the optimal selective coating design is investigated. A popular selective coating design, W/W–Al2O3/Al2O3, was chosen as illustrative example. Coating structure and composition, i.e., cermet composition and layer thicknesses, were optimized to maximize Rh for a wide range of operating conditions: concentration ratios from 10 to 300, absorber temperatures from 50 to 550 °C. Optimal coating characteristics are illustrated on 3D maps over the entire (C, TA) range, showing the existence of three operating regions. Each region gives an optimization priority on either As, EBB or a combination of both, leading to specific optimized coating characteristics for each region, which should be considered when designing selective coatings for given operating conditions. Based on one example, this study provides a better understanding of the optimal design and use of selective coatings in the different operating conditions of CSP technologies.

Ultrasound-assisted wet chemical synthesis of texturized Mo/MoO3 spectrally selective solar absorber coatings

A simple, low-cost Mo/MoO3 textured spectrally selective solar absorber coating prepared using an ultrasound-assisted wet chemical technique on a Mo substrate at different chemical reaction times is presented. The X-ray diffraction structural analysis of the textured coatings revealed only peaks related to Mo from the substrate, implying that the MoO3 coatings were amorphous at ambient temperature. X-ray photoelectron spectroscopy and energy dispersive X-ray spectroscopy confirmed the presence of Mo+6 ions representing MoO3 in the coatings. The topologic investigations using atomic force microscopy indicated texturized growth with an average surface roughness of 53.15 nm for the sample attacked for 4 h. Absorption bands related to M=O and Mo-O stretching bonds were evident from Fourier transform infrared measurements. Scanning electron microscopy investigations confirmed the presence of surface texturing and agglomerated nanoparticles in the coatings. Optical investigations of the MoO3 coatings on the Mo substrate revealed both a high solar absorptance of 0.91 and a low thermal emittance of 0.05 at 100°C indicating very good solar selectivity. These optical properties can arise from both light trapping which favors multiple absorptions, and intrinsic effects that result from the textured and intrinsic spectrally selective solar absorber design. These results demonstrate that Mo/MoO3 textured and intrinsic coatings deposited by using ultrasound-assisted chemical bath etching and oxidation are exceptional materials for selective solar absorber applications.