Cermet Solar Absorber Research Articles

Solution-based Ni-Al2O3 Solar Selective Coating using Convective Deposition

Abstract In the past century, an interest in solar energy grows rapidly due to its renewable resource and pollution-free operation. One of the technologies to accumulate solar energy is a concentrating solar power (CSP) system which contains a solar absorber as a key part. A role of solar absorber is to convert solar energy into heat. In this study, a technique called rapid convective deposition was used to produce a thin film of cermet solar absorber. The Ni-Al2O3 cermet solution was prepared with chelating and wetting agents. The rapid convective deposition was used for coating a cermet thin film onto a 304 stainless steel substrate. This technique has capability to control the thickness of coating with less amount of substances compared to other coating techniques. Finally, the coated solution was treated at high temperature where the aluminum ions were converted into a matrix of Al2O3, and nickel ions were converted into nickel nanoparticles embedded in the matrix. The important parameters including mole ratio, total metal ion concentration, and coating speed were examined. The optical properties including solar absorptance and thermal emittance were tested and calculated for the solar selectivity to represent solar-to-thermal energy conversion efficiency. The solar selectivity was enhanced from 5.6 to 6.5 with increasing the mole ratio of nickel and aluminum ions. However, the total metal ion concentration did not significantly improve the solar selectivity. The slower coating speed resulted in higher solar selectivity. Overall performance indicated that the cermet and the rapid convection deposition can potentially be exploited for industrial production of selective absorber.

Electrochemical deposition of Ni-Fe alloys into porous alumina for solar selective absorbers

The morphological, structural and optical characterizations of Al/Al2O3/Ni-Fe cermet solar absorbers have been investigated. The X-ray diffraction patterns indicated the presence of Ni-Fe alloys with variable composition. However, the formation of Ni-Fe amorphous phases was detected for compositions as high as 73% of Fe. X-ray photoelectron spectroscopy measurements revealed the presence of superficial binary and ternary oxides such as NiO, Fe2O3, and NiFe2O4. Furthermore, the properties of solar absorptance and thermal emittance were calculated for the cermets. It was found that the presence of Fe decreased the solar absorptance of the cermets up to 5% and decreased the emittance up to 10%. However, an excessive content of Fe increases the emittance ∼5%. Finally, it was demonstrated that cermets containing Ni-Fe alloys can be useful to enhance the optical and spectral selectivity properties of solar absorbers.

Interfacial engineering of solution-processed Ni nanochain-SiOx (x < 2) cermets towards thermodynamically stable, anti-oxidation solar selective absorbers

Cermet solar thermal selective absorber coatings are an important component of high-efficiency concentrated solar power (CSP) receivers. The oxidation of the metal nanoparticles in cermet solar absorbers is a great challenge for vacuum-free operation. Recently, we have demonstrated that oxidation is kinetically retarded in solution processed, high-optical-performance Ni nanochain-SiOx cermet system compared to conventional Ni-Al2O3 system when annealed in air at 450–600 °C for several hours. However, for long-term, high-temperature applications in CSP systems, thermodynamically stable antioxidation behavior is highly desirable, which requires new mechanisms beyond kinetically reducing the oxidation rate. Towards this goal, in this paper, we demonstrate that pre-operation annealing of Ni nanochain-SiOx cermets at 900 °C in N2 forms the thermodynamically stable orthorhombic phase of NiSi at the Ni/SiOx interfaces, leading to self-terminated oxidation at 550 °C in air due to this interfacial engineering. In contrast, pre-operation annealing at a lower temperature of 750 °C in N2 (as conducted in our previous work) cannot achieve interfacial NiSi formation directly, and further annealing in air at 450–600 °C for >4 h only leads to the formation of the less stable (metastable) hexagonal phase of NiSi. Therefore, the high-temperature pre-operation annealing is critical to form the desirable orthorhombic phase of NiSi at Ni/SiOx interfaces towards thermodynamically stable antioxidation behavior. Remarkably, with this improved interfacial engineering, the oxidation of 80-nm-diameter Ni nanochain-SiOx saturates after annealing at 550 °C in air for 12 h. Additional annealing at 550 °C in air for as long as 20 h (i.e., 32 h air annealing at >550 °C in total) has almost no further impact on the structural or optical properties of the coatings, the latter being very sensitive to any interfacial changes due to the localized surface plasmon resonances of the metal nanostructures. This phenomenon holds true for Ni nanoparticle diameter down to 40 nm in Ni-SiOx system, where the optical response remains stable for 53 h at 550 °C in air. The oxidation vs. time curve also shows saturation behavior deviating from the kinetic Deal-Grove oxidation model. These results strongly suggest a promising approach to thermodynamically stable, anti-oxidation Ni/SiOx cermet absorbers via interfacial engineering.

Polychromic Al–AlN cermet solar absorber coating with high absorption efficiency and excellent durability

Al–N cermet solar selective coatings with double absorbing layers were deposited on Al substrates via a home-made DC magnetron sputter system. The as-deposited coatings showed a high solar absorptance of 0.942 and a normal emittance of 0.066 at 80°C. Thermal accelerated aging tests results showed that the coatings are durable for at least 25 years. Neutral salt spray test showed that the coating presents excellent corrosion stability because the optical performance (absorptance and emittance) of the coating remained almost unchanged after a 200h test. The structural, chemical, and optical evolution of the coatings with heat treatment in air was comprehensively investigated. Our research clearly indicated that the as-deposited double cermet layer Al–AlN solar absorber coatings can be used for solar-thermal flat plate collectors. Furthermore, we obtained different colored Al–AlN cermet coatings in a wide range only by tuning the thickness of the Al–AlN (LMVF) and AlN anti-reflective (AR) layers, while maintaining high absorptance (>0.90) and low emittance (<0.10). These coatings are colored blue, purple, red, yellowish orange, and yellow, can be easily produced via a commercial production process, and are suitable for building integration solar energy saving systems.

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.

Effect of surface roughening on increasingthe spectral selectivity of cermet solarselective absorbers

Abstract The spectral selectivity of Cu–Al2O3 cermet solar absorbers was studied theoretically and experimentally in different layer designs. The results showed clearly that, by increasing the substrate temperature during film deposition, the desired random roughness features will be created on the surface of the cermet, which leads to higher selectivity. Theoretical simulation showed that a single cermet structure with the assistance of an anti-reflection layer, results in the best optical properties. Instead of such a structure, which is sensitive to fine thickness control, it was tried experimentally to increase the selectivity of a single cermet structure as much as possible using a roughness template layer. Surface roughening using such a layer design increased the solar absorptance from 0.840 to 0.956 while keeping the thermal emittance almost unchanged at about 0.028.

High performance Al–N cermet solar coatings deposited by a cylindrical direct current magnetron sputter coater

High efficiency Al–N cermet solar selective coatings have been designed using a numerical computer model and deposited experimentally. In numerical modeling calculations, Bruggeman approximations of dielectric function for composite materials were employed. Aluminum is used as a metallic component and AlON as a ceramic component in cermet. Numerical calculation results show that the double cermet layer film structure has the highest photothermal conversion efficiency for the Al–AlON cermet solar selective coatings. The optimized film has a solar absorptance of 0.958 and a hemispherical emittance of 0.035 at 80 °C. The optimized film yields two cermet layers with metal volume fractions of 0.143 and 0.275, and with layer thicknesses of 43 and 80 nm, going from the antireflection coating to the infrared reflector layer. In the optimized double cermet layer film the solar radiation is efficiently absorbed internally and by phase interference. Thermal loss is also efficiently reduced by using thin cermet layers with such low metal volume fractions, which are substantially transparent in the main wavelength region of blackbody radiation within the temperature range of interest, 50–200 °C. The predicted film structure has been used as a guide for the experimental deposition. The Al–N cermet solar selective coatings with double cermet layer film structure have been deposited onto batches of solar collector tubes using a commercial-scale cylindrical dc magnetron sputter coater. Two Al–N cermet solar absorber layers are deposited by dc reactive sputtering in a gas mixture of argon and nitrogen. The different metal volume fractions in the cermet layers are achieved by changing the reactive nitrogen gas flow rate, while the sputtering current is fixed. A solar absorption of 0.96 and a normal emittance of 0.08 at 80 °C have been achieved for deposited Al–N cermet solar coatings. The baking process, for 1 h at 400 °C in vacuum, hardly affects solar absorptance, however the emittance reduces about 0.01 at 80 °C. These high-performance low-cost Al–N cermet solar collector tubes may be a less-polluting replacement for black chromium solar collectors produced by electrochemical methods.

High efficiency MoAl 2O 3 cermet selective surfaces for high-temperature application

Highly efficient MoAl 2O 3 cermet solar absorbers have been designed with a numerical model and deposited experimentally. The typical film structure is an Al 2O 3 anti-reflection layer on a double MoAl 2O 3 cermet layer on a Mo or Cu metal thermal reflector. In numerical calculations of the thermal emittance at high temperature for these selective surfaces, the temperature dependencies of the complex refractive indices of the metal reflector and cermet in the infrared region have been considered, and the dielectric functions of the cermet materials are evaluated using Sheng's approximation. An optimization calculation yields a photothermal conversion efficiency as high as 0.914 at 350°C for a concentration factor of 26 for the film structure consisting of a double cermet layer on a Mo metal thermal reflector with an Al 2O 3 anti-reflection coating. The corresponding normal absorptance and hemispherical emittance at 350°C are 0.96 and 0.11, respectively. MoAl 2O 3 cermet selective surfaces using the double cermet layer structure were deposited by vacuum co-evaporation, and an absorptance of 0.955 and near normal emittance of 0.032 at room temperature have been achieved. An emittance of 0.08 at 350°C is estimated based upon room temperature experimental data for the film structure of a double cermet layer on a Cu metal thermal reflector with an Al 2O 3 anti-reflection coating.

Ni-Al2O3 cermet solar absorbers by RF planar magnetron sputtering for high temperature applications

Ni-Al2O3 cermet films were sputtered from a hot pressed Ni-Al2O3 composite target, in a RF planar magnetron sputtering system. Coatings of various thickness were deposited on different substrates and their absorptance (α) and emittance (ε) were obtained in the solar and thermal infrared regions, respectively, to evaluate their use as spectrally selective surfaces. Films were also deposited on different substrates with SiO2 anti-reflection coatings and an α of 0.88 to 0.92 and ε of 0.07 to 0.33 were obtained. Films of about 700 Å thick deposited on molybdenum-coated nickel-plated stainless steel substrates with 600 Å SiO2 AR coating and annealed at 550°C in air for six hours have best selective properties with α=0.91 and ε=0.07. These coatings are stable up to 500°C in air for more than 1000 h. ESCA and Auger depth profilings were done to estimate the composition and its variation with depth.