Selective Solar Absorber Coatings Research Articles

Solar selective absorbing coatings TiN/TiSiN/SiN prepared on stainless steel substrates

TiN/TiSiN/SiN solar selective absorbing coatings (SSACs) with and without a copper underlayer were deposited on stainless steel (SS) substrates via dc reactive magnetron sputtering. In this SSAC, SS/Cu or SS, TiN, TiSiN and SiN act as the metal reflector layer, high metal volume fraction layer, low metal volume fraction layer and anti-reflection layer, respectively. An absorptance (α) of 0.95 and a total hemispherical emittance (ε (70 °C)) of 0.04 were achieved in coatings with Cu underlayer. On the other hand, the SS/TiN/TiSiN/SiN SSAC showed a similar absorptance (∼0.95) but a poorer emittance and solar selectivity. X-ray diffraction analysis showed that the Cu layer is polycrystalline, while TiN, TiSiN and SiN are amorphous. The chemical-bonding states of each layer of the TiN/TiSiN/SiN SSAC were analyzed by using X-ray photoelectron spectroscopy. The solar selective absorbing performance of the Cu/TiN/TiSiN/SiN SSAC did not show significant changes after they were heat-treated in vacuum up to 700 °C. Raman spectra and atomic force microscopy images were collected to illustrate the structural and morphological stabilities of this SSAC upon heating in vacuum. Lastly, heat treatments in air for 150 and 300 h based on the PC nomenclature were carried out to demonstrate long term thermal stability of SS/TiN/TiSiN/SiN.

Solar selective absorbing coatings based on AlSiN/AlSiON/AlSiOy layers

Coatings with a double absorbing layer based on AlSiN/AlSiON were deposited on stainless steel substrates by magnetron sputtering technique, with different Al:Si ratios. A tungsten layer was used as a back reflector and AlSiOy or SiOx thin films were used as antireflection top layers. Prior the structure design, several single layers were deposited on glass substrates by varying the reactive gases flows, which allowed the stacking of a series of layers with different optical properties. Experimental transmittance and reflectance were modelled for the assessment of the spectral optical constants, which were then used to design a coating stack with optimized solar absorptance and thermal emittance. Optical properties, microstructure, morphology, composition and chemical bonding were investigated by employing optical spectroscopy, scanning electron microscopy, energy dispersive X-ray spectroscopy, X-ray diffraction and X-ray photoelectron spectroscopy. The samples were annealed in air at 400°C and vacuum at 580°C with the purpose to evaluate their oxidation resistance and thermal stability, which was subsequently correlated with the Al:Si ratio. Optimum results were achieved for an Al:Si ratio of 2.3:1, whereas for significantly higher Si content resulted in detrimental performance. The solar absorbance and thermal emittance for the optimized multilayer selective coatings is 93–94% and 7–10% (at 400°C), respectively.

Annealing effect on the optical properties and interdiffusion of MgO/Zr/MgO multilayered selective solar absorber coatings

Selective solar absorber coatings based on MgO/Zr/MgO multilayered coatings were prepared onto Zr coated stainless steel substrates using e-beam evaporation. It was found that MgO/Zr/MgO multilayered solar absorber coatings exhibited a good spectral selectivity of 0.91/0.12. The effect of annealing on the optical properties and interdiffusion between the layers was investigated. Annealing the multilayered coating in air up to 300°C did not show significant change on the spectral selectivity as well as on the interdiffusion between the layers indicating the coating’s thermal stability up to that temperature. Annealing in air at 400°C leads to decrease in solar absorptance and an increase in thermal emittance considerably, which was due to the interlayers appeared between MgO and Zr layers, oxidation of Zr layers, and change in both thickness and composition of the layers as investigated by RBS analysis. The MgO/Zr/MgO MSSACs were found to be thermally stable up to 250°C in air for 24h.

A highly thermally stable Mn–Cu–Fe composite oxide based solar selective absorber layer with low thermal loss at high temperature

A highly thermally stable Mn–Cu–Fe composite oxide based solar selective absorber layer for high temperature applications has been developed on a special variety of alloy substrate (SS-J4) using an economical process of controlled thermal oxidation. This novel type of composite oxide layer exhibits a high thermal stability and low emissive property even in an open air atmospheric condition. It is very contrary from previously known selective solar absorber coatings exhibited strong thermal stability along with low emissive property only in the vacuum environment. A solar absorptance of 91.5% (αAM1.5; 300–2400nm) and a spectral emittance of 0.14 (3–25μm) have been achieved for an absorber layer developed by the best optimised conditions. An open air high temperature cyclic test of the optimised composite layer was also investigated at 500°C and the stability of the coatings with respect to optical properties was established. Moreover, a constant value of thermal emissivity, 0.19 at 500°C, was measured repeatedly by the thermal emittance measurement as well as a low value of performance criterion (PC), <0.01, was calculated after thermal cycle test, indicating a high thermal stability and long durability of the absorber layer.

Derivation of a Levelized Cost of Coating (LCOC) Metric for Evaluation of Solar Selective Absorber Materials

A new metric, the Levelized Cost of Coating (LCOC), is derived in this paper to evaluate and compare alternative solar selective absorber coatings against a baseline coating (Pyromark 2500). In contrast to previous metrics that focused only on the optical performance of the coating, the LCOC includes costs, durability, and optical performance for more comprehensive comparisons among candidate materials. The LCOC is defined as the annualized marginal cost of the coating to produce a baseline annual thermal energy production. Costs include the cost of materials and labor for initial application and reapplication of the coating, as well as the cost of additional or fewer heliostats to yield the same annual thermal energy production as the baseline coating. Results show that important factors impacting the LCOC include the initial solar absorptance, thermal emittance, reapplication interval, degradation rate, reapplication cost, and downtime during reapplication. The LCOC can also be used to determine the optimal reapplication interval to minimize the levelized cost of energy production. As a result, similar methods can be applied more generally to determine the levelized cost of component for other applications and systems.

Study of the Stability of a Selective Solar Absorber Coating under Air and High Temperature Conditions

Abstract Selective solar absorber material is one of the major components in the Concentrated Solar Power (CSP) technologies. Its optical properties determine the efficiency of the energy conversion from the concentrated sun irradiation to the heat recovered by the heat transfer fluid. Development of the CSP requires costs reduction for this technology. To achieve this goal, one way is the increase of the work temperature up to 400 – 500 °C or more. For economic reasons, the bankable of CSP technology requires a lifetime over 25 years for components and so for solar absorbers. The development of solar receiver, able to operate under air, instead of vacuum conditions, is a challenge to reduce the costs. Along the 25 years of a CSP plant lifetime, solar absorber material is daily exposed to high levels of stresses: high solar flux, high thermal gradient. As the oxidation is one of the main factors involved in degradation processes air stability of the absorber coating under high temperatures should be studied as it could be a critical point in the solar field maintainability in case of accidental vacuum loss. In this article we present a durability study of selective solar absorber from Archimede Solar Energy (ASE) exposed in air at high temperature (up to 500 °C). The results show a good stability of this materials up to 450 °C during 3000h without change of solar absorptance and emittance. An analysis of the degradation process at higher temperature is presented.

Thermal stability of multilayered Pt-Al2O3 nanocoatings for high temperature CSP systems

This contribution reports on the effect of thermal annealing on sputtered Pt–Al2O3 multilayered selective solar absorber coatings deposited onto stainless steel substrates with a Mo IR reflecting buffer layer. The Pt–Al2O3 cermet nanocoatings were annealed at different temperatures for different durations in vacuum. The spectral selectivity of the optimised Pt–Al2O3 was found to be thermally stable up to 900 °C in vacuum for 2 h with solar absorptance of 0.944 and thermal emittance of 0.11. At 1000 °C, its spectral selectivity decreased significantly to 0.892/0.13, which is attributed to the inter-diffusion between the layers, and formation of η-Fe3Mo3C phase. Annealing the Pt–Al2O3 nanocoatings for long term showed that the cermet nanocoatings were thermally stable up to 800 °C in vacuum for 24 h having spectral selectivity of 0.938/0.10.

Thermal aging test of AlCrNO-based solar selective absorbing coatings prepared by cathodic arc plating

A novel AlCrNO-based solar selective absorbing coating was prepared by cathodic arc plating. The coating exhibited excellent selective absorbing properties with an absorptance of 0.93 and an emittance of 0.20. To test the in air thermal stability of this coating, the sample was annealed at 500°C for 100h in air. Experimental results indicated that the coating was highly stable in air at 500°C, as the absorptance/emittance of the coating kept almost unchanged (0.94/0.22) after 100h annealing. The microstructure of the coating was investigated by scanning electron microscope (SEM), energy dispersive spectrometer (EDS), atomic force microscope (AFM), and X-ray diffraction (XRD).

Effects of substrate temperatures on the thermal stability of AlxOy/Pt/AlxOy multilayered selective solar absorber coatings

We report the effects of substrate temperatures on the thermal stability of AlxOy/Pt/AlxOy multilayered selective solar absorber coating (MSSAC). The samples were deposited at different substrate temperatures (from room temperature up to 250 °C), and then annealed at various temperatures (300–600 °C) in air for 2 h. Characterizations are made via X-ray Diffraction (XRD), Scanning Electron Microscopy (SEM), Energy Dispersive X-ray Spectroscopy (EDS), Atomic Force Microscopy (AFM), Raman Spectroscopy, UV–Vis and emissometeric measurements. These coatings were found to be thermally stable up to 500 °C with good spectral selectivity of 0.930/0.11. Furthermore, the observed decrease in the spectral selectivity 0.883/0.13 at 600 °C is attributed to the diffusion of Cu and the formation of CuO phase. Such phase formation was confirmed using XRD and Raman spectral analysis. The insensitiveness of the thermal stability of such coatings on the substrate temperature is demonstrated.

Control of optical properties of TiN_xO_y films and application for high performance solar selective absorbing coatings

TiNxOy films with controllable optical properties have been fabricated by reactive mid-frequency magnetron sputtering from titanium nitride target. The optical and electrical properties were studied as a function of the reactive gas flow and were correlated with the film stoichiometry. The results showed that the behavior of TiNxOy films can be adjusted from metallic to dielectric by increasing oxygen content, which is of great significance to their extensive applications. Owing to the accurate control of optical properties, a TiNxOy based solar selective absorbing coating has been designed and prepared with the aid of TiO2/Si3N4/SiO2 antireflection layers. Its solar absorbance is as high as 97.5% and thermal emissivity is 4.3% with total thickness of 230 nm. The solar absorbance can maintain above 90% for a broad incident angle range from 0° to 65°.

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.

A Tantalum diffusion barrier layer to improve the thermal stability of AlxOy/Pt/AlxOy multilayer solar absorber

The effect of a thin Tantalum layer on the thermal stability of AlxOy/Pt/AlxOy multilayered selective solar absorber coatings has been investigated. The resulting samples were annealed at different temperatures for different durations in air. These were characterized by X-ray diffraction, scanning electron microscopy, energy dispersive X-ray spectroscopy, Atomic force microscopy, Raman spectroscopy, UV–vis–NIR spectrophotometer and emissometer. The Cu/Ta/AlxOy/Pt/AlxOy multilayer solar absorber coatings were found to be thermally stable up to 700°C in air for 2h with good spectral selectivity (α/ε) of 0.932/0.10. At 800°C, the spectral selectivity decreased to 0.870/0.12, which is attributed to the diffusion of Cu and formation of CuO phase. The formation of CuO phase was confirmed by X-rays diffraction, energy dispersive spectroscopy and Raman spectroscopy. Long term thermal stability study showed that the coatings were thermally stable in air up to ∼550°C for about 24h.

Developments in the synthesis of flat plate solar selective absorber materials via sol–gel methods: A review

There is a great demand for low-cost and environmentally friendly techniques for synthesizing high quality solar selective absorber (SSA) coatings. Such coatings are capable of absorbing most of the incoming solar radiation (high absorptance) without losing much of the thermal energy through re-radiation from heated surface (low emittance). Sol–gel techniques are promising synthesis methods for these SSA coatings. The optical properties and durability of the SSA coating can be easily controlled by fine-tuning relevant design parameters such as heating temperature or precursor concentrations in the synthesis process. In light of this, there are many knowledge gaps that need to be filled in the context of technicalities regarding the sol–gel processes and the optical and morphological characteristics of these coatings. Comprehensive understanding of these characteristics is a vital component in the optimal design of SSA coatings and therefore, the aim of this paper is to identify these technical issues and review developments in the synthesis of flat-plate SSA materials produced by sol–gel methods.

Study on the Microstructure of Solar Selective Absorbing Coating Prepared by Magnetron Sputtering

The solar selective absorbing coating is the key component of solar energy collector which determines the heat conversion efficiency. The coating materials mentioned in this paper were prepared by Al as substrate, Al2O3-Pt as absorbing layer and metal Ag as infrared reflector. We got three kinds of coatings by adjusting the magnetron sputtering process. Respectively testing and analyzing the performance and microstructure showed that element content of Al, N, O in the coating are of important influence on its performance, meanwhile, the better the crystallization of coatings surface, the more helpful to improve the performance of its corrosion resistance and its moisture and heat resistance.

Selective Solar Absorber Coatings on Receiver Tubes for CSP – Energy-efficient Production Process by Sol-gel dip-coating and Subsequent Induction Heating

The energy efficiency of production processes for components of solar energy systems is an important issue. Other factors, which are important for the production of black selective solar coatings include production speed, cycle time and homogeneity of the coating, as well as the minimization of the quantity of the needed chemical precursors. In this paper a new energy-efficient low-cost production process is presented for production of optically selective coatings for solar thermal absorbers.The used method to produce such coatings is sol-gel dip-coating for which all the solutions have been synthetized at the Solar Energy and Building Physics Laboratory of EPFL. The used precursors are tetraethyl orthosilicate, manganese acetate tetrahydrate, copper chloride dihydrate and cobalt chloride hexahydrate. Solutions were obtained by dissolving these precursors in a solution based on a mixture of absolute ethanol, nitric acid and demineralized water.The layers deposited on sheetlike substrates were annealed in a benchtop furnace. For 2 meter long austenitic stainless steel tubes, a novel, fast and energy-efficient process based on induction heating was developed for the thermal annealing. An induction coil passes along the tube. An alternating current flowing in the coil induces a current in the tube. By resistive heating, the temperature of both the metallic tube wall and the deposited film increases. The homogeneity of the temperature distribution of the tube after a single passage of the coil was monitored by infrared imaging.The optical and morphological properties of the Cu-Co-Mn-Si-O based triple layer have been characterized by spectrophotometry, transmission electron microscopy (TEM), time-of-flight secondary ion mass spectroscopy (ToF-SIMS) and X-ray photoelectron spectroscopy (XPS). After optimization of the multilayer design, a solar absorptance of 0.95 and a thermal emissivity of 0.12 at 100°C have been achieved. The intermediate Cu-Co-Mn-Si-O layer was analyzed by transmission electron microscopy. The likewise obtained images show an agglomeration of crystalline grains with 5-20nm in diameter. Therefore, we can consider that the Cu-Co-Mn-Si-O phase is nanocrystalline.In order to roughly estimate the corrosion resistance of the coating in an acidic environment, a simple corrosion test in harsh conditions confirmed the durability of the novel sol-gel coating. Moreover, the excellent stability at elevated temperatures in ambient air makes the coating an interesting candidate for solar applications involving concentrated solar radiation, such as the generation of solar electricity (concentrated solar power), industrial process heating and solar cooling. For that reason, prototype coatings consisting of stacks of three individual layers were deposited on 2 meter long stainless steel tubes.

Preparation and characterization of CuO nanostructures on copper substrate as selective solar absorbers

Selective solar absorber coatings of copper oxide (CuO) on copper substrates are prepared by room temperature oxidation of copper at different alkaline conditions. The surface morphology and structural analyses of the CuO coatings are carried out by scanning electron microscopy (SEM), X-ray diffraction (XRD), energy dispersive spectroscopy (EDS) and Raman spectroscopy techniques. XRD and Raman studies indicated the single phase nature and high crystallinity of the prepared CuO nanostructures. Different CuO nanostructures, viz., nanoneedles, nanofibers and nanoparticles are formed at different alkaline conditions. The influence of reaction time on morphology of the CuO nanostructures is also studied. The thermal emittance values of these nanostructured CuO samples are found to be in the range of 6–7% and their solar absorptances are ranged between 84 and 90%. The observed high solar selectivity values (>12.7) suggest that these coatings can be used as selective absorbers in solar thermal gadgets.

Study on Thermal Stability and Salt Spray Resistance of SS-O Solar Spectrum Selective Absorbing Coating

The Cu-based SS-O solar spectrum flat-plate selective absorption coating system was prepared by the method of direct current reactive sputtering coating films, in which, two layers are SS-O absorbing layer and one is Si-O anti-reflection layer. The reflectance spectra of the absorption coating system was measured by UV3600 spectrophotometer, the reflectance and absorptance of the absorption layer and the anti-reflection layer were calculated, and certain experimental tests were conducted on the prepared absorption coating system in terms of the salt spray resistance performances, thermal stability and damp heat stability. The results show that the SS-O selective absorption coating which was prepared by magnetron sputtering technique possesses excellent processing property, corrosion resistance and thermal stability, whose absorption rate and the emission ratio have reached the level of the German blue tape series. It can meet the domestic demand for high-level flat-plate market.

Performance Forecast of Solar Spectrum Selective Absorbing Coating Based on RBF Neural Network

The paper researches on the coating performance prepared by sputtering stainless steel particles to Cu substrate, analyzes the effect of the six magnetron sputtering process parameters on absorption rate, such as Ar pressure, oxygen flow, sputtering vacuum degree, sputtering time, target voltage and target current with the artificial neural network technology, and forecasts the performance of the coating prepared by specific process parameters. The results show that during the process of magnetron sputtering when Ar pressure is within the range of 0.2 ~ 0.5 Pa, the oxygen flow is within the range of 0 ~ 20 sccm, and the target current is within the range of 380~ 430 A, the higher the sputtering vacuum degree and the target voltage, the superior the performance of the coating can be obtained. It is also found that the sputtering time has little effect on the coating performance, in the actual preparation process, on the condition that the performance can be guaranteed, the sputtering time should be as short as possible.

The Spectral Properties and Thermal Stability of AlCr-AlCrON Solar Selective Absorber Coating

Spectrally selective AlCr-AlCrON cermet solar absorber coating has been deposited on Cu substrate experimentally by reactive DC magnetron sputtering and heat-treated in air at 300°C for 2h. The spectral properties and thermal stability of the as-synthesized AlCr-AlCrON coatings were characterized. The solar absorptance and thermal emittance of the as-synthesized AlCr-AlCrON coating are 0.926 and 0.065, respectively. In addition, after heat treatment at 300 ° C for 500 h in air, the spectral properties of the as-synthesized AlCr-AlCrON coating is virtually unchanged. The results indicate that the as-synthesized AlCr-AlCrON coating has good spectral properties and thermal stability, which will be a good selective absorbing coating for mid-temperature solar vacuum collector tubes.

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.