Absorber Coatings Research Articles

Polyhedral oligomeric silsesquioxanes as protective monolayer coatings against the high-temperature corrosion of concentrating solar power absorber surfaces

The third generation of concentrating solar power (CSP) hybrid absorbers with enhanced solar absorptance and low thermal emittance (spectrally selective) is designed to have a high solar-to-thermal energy conversion efficiency, in spite of robust, long-term resistance to environmental loads. In this study, we report on the deposition, testing, and antioxidation properties of ultra-thin (1–3 nm) mono-, di-, and tri-layers of hybrid organic‒inorganic polyhedral oligomeric silsesquioxanes (POSS) on Inconel 617. The latter is a well-known metal alloy, which is used as a CSP absorber material. Ultra-thin POSS layers were prepared by Langmuir‒Blodgett (LB) deposition. After deposition, the detailed signature characteristics of the POSS monolayer were investigated using X-ray photoelectron spectroscopy (XPS), infrared reflection‒absorption spectroscopy (IRRAS), and contact angle measurements. The XPS results with element concentration profiles reveal that the LB method enables the formation of a dense ultra-thin POSS coating on the Inconel surface, which eventually improves the antioxidation properties. To further test the corrosion resistance improvement of the developed coatings, electrochemical impedance spectroscopy (EIS) and potentiodynamic tests were performed in 0.5 M NaCl solution. The as-obtained data were fitted with the corresponding equivalent circuit, which shows an increased charge transfer resistance (Rct) for coated samples. Tafel polarization plots show suppressed cathodic and anodic reaction rates for samples coated with ultra-thin POSS coatings. We envision that such coatings will have significant potential as a primer for spectrally selective absorber coatings for CSP technology.

Characterization of Solar Absorber Coated by Reduced Graphene Oxide Polymer Composite on Metal Sheets

This paper presents the characterization of solar heat absorber coatings on metal sheets using reduced graphene oxide polymer composites. The reduced graphene oxide (rGO) polymer composite material was evenly coated by the silk-screen method on three types of metal sheets. The thickness of the coated sheet samples was 0.25 mm which were exposed under solar radiation and the variations in temperatures versus time were recorded. The influence of solar radiation and angle of incidence during peak hours was recorded. The results show that stainless steel coated by rGO absorber gain more heat than other metal sheet absorbers. These coatings have a promising scope in solar thermal applications and the technique is cost-effective to characterize efficient solar absorber coatings.

Influence of High-Enthalpy Atmospheric Plasma Spraying Process Parameters on Microwave Dielectric Properties of Y2O3 Coatings

The low-dielectric inorganic adhesive is an important component of electromagnetic wave absorption coatings due to impedance matching with the atmospheric environment. Herein, high-enthalpy atmospheric plasma spraying (HE-APS) equipment is utilized to prepare Y2O3 ceramic coatings, and the influence of spraying power on dielectric and physical properties of Y2O3 ceramic coatings is systematically investigated. The surface and cross-sectional SEM images of Y2O3 coating show that the overall coating morphology is smooth and less porous at high spraying power. This result is consistent with the trend of coating density and porosity. The XRD pattern demonstrates that the Y2O3 coating possesses a stable cubic structure. The experimental results reveal that the dielectric properties of Y2O3 coating are mainly affected by the porosity. According to the dielectric mixing rule of composite materials, Y2O3 coating exhibits the minimum density of 4.86 g/cm3 and permittivity of 9.70 under the spraying power of 55 kW. The current work highlights the feasibility of Y2O3 as a low-dielectric inorganic adhesive in the field of electromagnetic wave applications.

Energy and Exergy Analysis of a Vertical Solar Air Heater with Nano-Enhanced Absorber Coating and Perforated Fins

Energy and Exergy Analysis of a Vertical Solar Air Heater with Nano-Enhanced Absorber Coating and Perforated Fins

Low reflectance of carbon nanotube and nanoscroll-based thin film coatings: a case study.

Research on carbon material-based thin films with low light reflectance has received significant attention for the development of high absorber coatings for stray light control applications. Herein, we report a method for the successful fabrication of stable thin films comprised of carbon nanotubes (CNTs) and nanoscrolls (CNS) on an aluminium (Al) substrate, which exhibited low reflectance of the order of 2–3% in the visible and near-infrared (NIR) spectral bands. Changes in the structural and chemical composition of pristine single-walled carbon nanotube (SWCNT) samples were analyzed after each processing step. Spectroscopy, microscopy and microstructural studies demonstrated emergence of CNS and multi-walled carbon nanotubes (MWCNTs) due to the sequential chemical processing of the sample. Transmission electron microscopy (TEM) and scanning electron microscopy (SEM) studies revealed the formation of CNS via curling and folding of graphene sheets. Microstructural investigations including SEM and atomic force microscopy (AFM) confirmed the presence of microcavities and pores on the surface of the film. These cavities and pores significantly contribute to the observed low reflectance value of CNTs, CNS compound films by trapping the incident light. Fundamental space environmental simulation tests (SEST) were performed on the coated films, that showed promising results with reflectance values almost unaltered in the visible and NIR spectral bands, demonstrating the durability of these films as potential candidates to be used in extreme space environmental conditions. This study describes the preparation, characterization, and testing of blended CNT and CNS coatings for low-light scattering applications.

Scalable and highly efficient high temperature solar absorber coatings based on high entropy alloy nitride AlCrTaTiZrN with different antireflection layers

A high entropy alloy nitride AlCrTaTiZrN nanofilm is employed to develop a novel duplex spectrally selective solar absorber coating, which leads to a scalable, highly efficient and cost-conscience structure.

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

A super-hydrophobic composite coating with near-infrared absorption properties

Through reasonable coating structural design, a polydimethylsiloxane (PDMS) modified polyurethane (PU)/Sm2O3 composite coating with super-hydrophobic and near-infrared absorption properties was prepared by glass rod scraping method using nano-SiO2, PDMS modified PU, and Sm2O3 particles as micro-nano structural modifier, adhesives, and functional pigments, respectively. The effects of curing temperature, Sm2O3 addition, and mass ratio of Sm2O3 and nano-SiO2 on the coating properties were discussed. The results show that the Sm2O3 addition has an important effect on the coating properties. When the curing temperature of the coating is 80 °C and the addition amount of Sm2O3 is 50 wt%, the reflectivity of the coating to 1.06 μm near-infrared light can be as low as 36%. The water contact angle (WCA) of the coating can reach 115°, which is significantly higher than traditional PU-based near-infrared absorption coatings. The super-hydrophobic properties of the coating can be achieved by adjusting the mass ratio of Sm2O3 and nano-SiO2 in the coating. When the mass ratio of Sm2O3 and nano-SiO2 in the coating is 7:3, obvious papillary micro-nano rough structure can appear on the surface of the coating, so that the coating can exhibit super-hydrophobic properties. The WCA of the coating can reach 153°, the sliding angle can reach 7°, and the reflectivity of 1.06 μm near-infrared light can be as low as 59.3%. The coating has good super-hydrophobic and near-infrared absorption properties at the same time.

Extremely high temperature stable nanometric scale multilayer spectrally selective absorber coating: Emissivity measurements at elevated temperatures and a comprehensive study on ageing mechanism

Spectrally selective W/WAlSiN/SiON/SiO2 solar absorber coatings were sputter-deposited on stainless steel and silicon substrates. The optimized as-deposited sample exhibits a high solar absorptance of 0.955 and a low thermal emissivity of 0.10 at 82 °C. The coating exhibits a very low reflectance of 1.2% in the wavelength range of 0.5–1.5 μm in the solar spectrum. The spectral emissivity measurements at various operating temperatures and with varying emergence angles were investigated. Also, the impact of emissivity on the photothermal efficiency at different temperatures of the developed solar absorber coating was calculated. The calculated optical properties of the as-deposited sample exhibited low thermal emissivity of 0.157 (at 500 °C) and a high heliothermal efficiency of 89.5%. Angular reflectance measured at room temperature illustrates an insignificant change in the hemispherical and near normal emissivities of the samples. In addition to these studies, the ageing tests of the as-deposited samples at various operating temperatures were studied in detail. The thermal ageing tests of the samples in air and vacuum environments indicated excellent thermal stability at elevated temperatures, i.e., in air at 400 °C for 500 h, at 450 °C for 175 h and at 500 °C for 100 h, whereas, in a vacuum it was stable at 700 °C for 200 h. The top anti-reflection layers and the fine nano-multilayers of WAlSiN (W2N and AlSiN) prevent the inward diffusion of oxygen and thereby improve the overall thermal stability of the tandem absorber.

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.

Stabilization of a Two-Dimensional Hypersonic Boundary Layer Using a Shallow Cavity

The stability of a two-dimensional hypersonic flat-plate boundary layer with a shallow cavity is investigated at Mach 6 using direct numerical simulations. The boundary layer is perturbed by a single-frequency wall blowing–suction actuator. The results indicate that the second mode is damped when the cavity is placed slightly downstream of the synchronization point (SP) of mode F and mode S, whereas the effect is reversed if it is located upstream or further downstream. The effect of the cavity depth is also studied. Strong damping of the second-mode disturbance is observed across the cavity. Energy budget analysis reveals that the presence of a shear layer bridging the leading and trailing edges of the cavity significantly reduces the work done by the Reynolds stress on the mean velocity gradient, which derives energy from the base flow and is the major contributor to the second-mode amplification near the flat-plate surface. It is suggested that a boundary layer dominated by second-mode instabilities can be efficiently stabilized by placement of a shallow cavity in the near-downstream region of the SP associated with the most amplified frequency. The damping effect depends on the cavity depth with a trend similar to that for hypersonic boundary layers over ultrasonically absorptive coatings, despite the fact that the flow structures can be very different.

CNT-Based Solar Thermal Coatings: Absorptance vs. Emittance

A novel approach for fabricating selective absorbing coatings based on carbon nanotubes (CNTs) for mid-temperature solar–thermal application is presented. The developed formulations are dispersions of CNTs in water or solvents. Being coated on stainless steel (SS) by spraying, these formulations provide good characteristics of solar absorptance. The effect of CNT concentration and the type of the binder and its ratios to the CNT were investigated. Coatings based on water dispersions give higher adsorption, but solvent-based coatings enable achieving lower emittance. Interestingly, the binder was found to be responsible for the high emittance, yet, it is essential for obtaining good adhesion to the SS substrate. The best performance of the coatings requires adjusting the concentration of the CNTs and their ratio to the binder to obtain the highest absorptance with excellent adhesion; high absorptance is obtained at high CNT concentration, while good adhesion requires a minimum ratio between the binder/CNT; however, increasing the binder concentration increases the emissivity. The best coatings have an absorptance of ca. 90% with an emittance of ca. 0.3 and excellent adhesion to stainless steel.

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.

The impact of Fe3O4 on the performance of ultrathin Ti/AlN/Ti tandem coating on stainless-steel for solar selective absorber application

Successful deployment of large scale concentrating solar power (CSP) systems require spectrally selective absorber coatings with high absorptance and low emittance characteristics in the visible and infrared regions of solar spectrum respectively. In this paper, we synthesize and study the properties of Ti/AlN/Ti multilayer selective absorber coating deposited on Fe3O4 layered stainless steel (SS) substrate using direct current/radiofrequency (DC/RF) magnetron sputtering. Simple annealing process was used to create a layer of Fe3O4 on SS substrate. Several characterizations tools such as XRD, UV–Vis-NIR, FESEM, AFM and FTIR were employed. High solar absorptance of 0.89 and low thermal emittance of 0.19 were achieved. This high absorptance value is attributed to multiple reflections of the incident light, induced by interference mechanism and surface roughness of the coating caused by high deposition rate of Ti and the formation rough oxide layer of Fe3O4 as a result of annealing the substrate prior to deposition.

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

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

Fabrication of Self‐Cleaning CNT‐Based Near‐Perfect Solar Absorber Coating for Non‐Evacuated Concentrated Solar Power Applications

A near‐perfect black solar absorber made of carbon nanotubes (CNTs) prepared by a low‐cost wet‐deposition method on a reflective metal surface for mid‐temperature non‐evacuated concentrated solar power (CSP) applications is demonstrated. The dispersed CNTs in an alumina–silica matrix exhibit an absorptance of 0.985 in the entire solar spectrum and emittance of 0.90 in the infrared (IR) region. The coating shows high durability and is super‐hydrophilic (0° contact angle) after plasma treatment, without affecting the solar absorptance and excellent coating adhesion. The efficiency of the coating is evaluated by analytical models, which implies that it has higher efficiency at low temperature and at a high solar concentration ratio than that of previously reported selective coatings.

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.

Portable Microwave Reflection Coefficient Meter of Coatings

In this study, we propose a new design of a portable meter of reflection coefficients of radar absorbing coatings. An experimental comparison of the reflectivity meters of the new and traditional types is performed. The technical, operational, and cost advantages of a meter of a new type are shown. The efficiency of the meter calibration system was developed and proven, and the optimal geometric parameters of the antenna probe that ensures high-accuracy measurements of the reflection coefficient of radar absorbing coatings were determined.

Physical models of the bidirectional reflectance of solar receiver coatings

We present measurements of the Bidirectional Reflectance Distribution Function (BRDF) of the state-of-the art solar receiver coating, Pyromark® 2500, and show how the data can be accurately described with physical models. In the conducted measurements of the receiver coating, a peak shift of the radiation away from the direction of ideal specular reflection was observed. Both the Cook–Torrance and the He–Torrance models reproduce this behaviour. For the application of the former model, an empirical angular dependence of the parameter describing the surface roughness has to be assumed in order to obtain an adequate fit to the data. The latter model is less exact in a statistical sense, but it is physically based, with six meaningful fit parameters. As we found that the He–Torrance model with its original form of the specular reflection yielded physically incorrect results for incidence angles larger than 80°, a modification is proposed to overcome this deficiency.Overall, the fitted models are able to provide a good description of the BRDF of the receiver coating over three orders of magnitude. Our results can play an important role in facilitating camera-based flux control or optical simulations of solar tower systems. Moreover, we think that our findings can serve as a sound starting point for the physical modelling of other absorber coatings currently being developed.