High-temperature Solar Absorber Research Articles

Bulk ceramics of lanthanum hexaboride with enhanced spectral selectivity and photothermal efficiency for novel hybrid thermal-thermionic solar absorbers

Lanthanum hexaboride (LaB6) single-crystals are known from a long time for their excellent thermionic and field-assisted electron-emitting properties. Recently, multi-crystalline ceramics, which offer several technological advantages over single-crystals, have been studied as well, proving a low work function and Richardson constant, in addition to favorable optical properties for high-temperature solar radiation absorbers. However, to fully disclose to LaB6 ceramics the novel application of a double-function bulk thermionic emitter, enabled by the direct absorption of solar radiation, and, at the same time, thermodynamic solar absorber for Concentrating Solar Power (CSP), their optical properties should be further optimized. The ideal result, consisting in the selective increase of the solar absorptance and decrease of the thermal emittance, thus increasing both the spectral selectivity and the photothermal efficiency of the absorber, has been obtained in this work by realizing Laser-Induced Periodic Surface Structures (LIPSS) on the ceramic surface by means of femtosecond-laser treatments. LIPPS formation has been investigated as a function of the accumulated laser fluence.

Processing and optical behavior of dense (Hf,Zr)B2 solid solutions for solar energy receivers

While individual borides and, more recently, quinary High Entropy Transition Metal Borides have been investigated, the fabrication and characterization of bulk binary to quaternary solid solutions are barely explored. In this work, dense (Hf0.5Zr0.5)B2 is produced by Spark Plasma Sintering (SPS) from powders prepared by Self-propagating High-temperature (SHS). SHS produced a multiphase product, whose secondary phases are fully converted into the desired diborides during the subsequent SPS step. Optical properties of (Hf0.5Zr0.5)B2 are evaluated for the first time with focus on the possible use as novel high-temperature solar thermal absorber, by hemispherical reflectance measurements and calculation of solar absorptance, temperature-dependent spectral selectivity and absorber opto-thermal efficiency at various solar concentration ratios. To optimize the material, a chemically etched surface texture was realized to modify the optical properties. The etched sample showed a higher solar absorptance (0.71) and a lower spectral selectivity than the unetched one, with consequent higher opto-thermal efficiency at all temperatures for solar concentration ratios 1000 ÷ 3000, while at lower concentration ratios and temperatures >1400 ÷ 1600 K, the unetched sample shows the highest efficiency. These results show the promising properties of binary diborides for solar thermal applications.

Stability and characterisation of Pyromark 2500 cured at different temperatures

Pyromark 2500 is a silicone-based high-temperature solar absorber coating commonly used for concentrating solar power receivers. It has good optical efficiency in its pristine condition but is prone to optical and/or mechanical degradation and failure after years of operation. This work studies the effect of curing temperature on the long-term stability of Pyromark. Pyromark painted on Inconel 625 was cured at 750℃, 850℃, and 950℃, and then isothermally aged at 900℃ for 3000 h. Pristine Pyromark cured at higher temperatures was shown to have a lower solar absorptance, but better optical and mechanical stability over the testing period. For example, the solar absorptance of pristine Pyromark cured at 950℃ was 1.2 % and 1.0 % lower than when cured at 750℃ and 850℃, respectively, but due to better optical stability during aging, had the highest solar absorption of all cases after completion of the 3000 h of testing. In terms of physical failure, spallation occurred for Pyromark cured at 750℃ and significant detachment between layers occurred for Pyromark cured at 850℃, whereas there was no sign of these failure modes for samples cured at 950℃. Therefore, despite the disadvantage of lower initial optical efficiency, curing Pyromark at a higher temperature appears to bring advantages in terms of long-term mechanical stability and a reduced optical degradation rate.

TiN-based metasurface absorber for efficient solar energy harvesting

Solar absorber, which is widely used in concentrated solar thermal systems and solar photovoltaics, has attracted great attention in recent years. Concentrated solar irradiation not only saves production costs, but also effectively improves the photothermal conversion efficiency of the system. However, the huge radiative heat flux will cause the system temperature to rise rapidly, which poses significant challenges for designing solar absorbers. Here, a metasurface absorber based on cylinder array structure is proposed for efficient solar energy harvesting, using the high-temperature resistant materials TiN and SiO2. The total solar absorption of the absorber is up to 0.94 at wavelengths of 300–2500 nm. The high absorption performance of the metasurface absorber can be explained as the coupling effect of surface plasmon resonance and Fabry-Pérot resonance, which is confirmed by the electric field distribution. Moreover, the effect of geometric parameters on absorption performance is analyzed. Finally, we discuss the influence of incident angle on the solar absorber. We believe this work could deepen the understanding of coupling resonance mode and guide the design of high-temperature solar absorbers.

Multi-Scale Femtosecond-Laser Texturing for Photothermal Efficiency Enhancement on Solar Absorbers Based on TaB2 Ceramics.

Tantalum boride is an ultra-refractory and ultra-hard ceramic known so far for its favorable high-temperature thermo-mechanical properties and also characterized by a low spectral emittance, making it interesting for novel high-temperature solar absorbers for Concentrating Solar Power. In this work, we investigated two types of TaB2 sintered products with different porosities, and on each of them, we realized four femtosecond laser treatments differing in the accumulated laser fluence. The treated surfaces were then characterized by SEM-EDS, roughness analysis, and optical spectrometry. We show that, depending on laser processing parameters, the multi-scale surface textures produced by femtosecond laser machining can greatly increase the solar absorptance, while the spectral emittance increase is significantly lower. These combined effects result in increased photothermal efficiency of the absorber, with interesting perspectives for the application of these ceramics in Concentrating Solar Power and Concentrating Solar Thermal. To the best of our knowledge, this is the first demonstration of successful photothermal efficiency enhancement of ultra-hard ceramics using laser machining.

Photothermal performance of three chromia-forming refractory alloys for high-temperature solar absorber applications

Spectral hemispherical emissivity is a crucial material characteristic that determines radiation heat transfer. In high-temperature solar thermal applications, it affects not only the efficiency of the solar energy absorption but also the heat losses caused by thermal radiation and the radiative heat transfer within the receiver. Due to the limitations of the working temperature of existing solar absorber coatings, the spectral hemispherical emissivity of the oxidized surface is a key performance indicator for evaluating the potential of a candidate refractory alloy for high-temperature (> 1000 °C) solar receiver/reactor designs. In this work, we systematically studied the photothermal performances of the oxidized surfaces of three widely used high-performance commercial chromia-forming alloys (Haynes 230, Hastelloy X, and SS 253MA) by analyzing the spectral hemispherical reflectance in the band 0.25–25 μm. The stability of the optical properties of the formed oxide layers have also been studied by exposing the three alloys at 1150 °C in air for three different exposure periods (10 h, 100 h, and 200 h). The results show that the solar absorptivity of all the samples is in the range of 0.800–0.855, with SS 253MA showing the best performances in offering both high and stable solar absorptivity in the range of 0.837–0.855. The evaluation of the photothermal performances suggest the potential of these three alloys in solar-thermal applications.

Optical properties and thermal stability of Gd1-xNdxBaCo2O5+δ ceramics for high temperature air-stable solar thermal conversion

The efficiency of the concentrated solar power (CSP) framework is significantly influenced by the operating temperature. The solar thermal conversion material with high efficiency and good stability at high temperature in air environment is of importance to obtain the high operating temperature. Here, the optical properties and thermal stability of Gd1-xNdxBaCo2O5+δ (0 ≤ x ≤ 0.4, GNBCOx) ceramics were studied to assess their possibility to be applied as high temperature solar absorbers. Results presented that GNBCOx exhibited high absorptance in the solar spectrum and minimal radiation heat loss at infrared wavelengths. In addition, the GNBCOx ceramics demonstrated excellent stability at 800 ℃ in air, indicating that the GNBCOx ceramics could be applied as solar thermal conversion material for upcoming concentrated solar power systems.

Optical properties of bulk high-entropy diborides for solar energy applications

So far, the studies regarding the innovative High-Entropy Borides (HEBs), which belong to the more general class of Ultra-high temperature ceramics (UHTCs), have been entirely confined to their fabrication or characterization from the microstructural, mechanical and oxidation resistance viewpoints.In this work, the optical properties of two members of HEBs, i.e. (Hf0.2Zr0.2Ta0.2Mo0.2Ti0.2)B2 and (Hf0.2Nb0.2Ta0.2Mo0.2Ti0.2)B2, are evaluated for the first time to assess their possible utilization in the thermal solar energy field. The bulk samples (96.5 % and 97.4 % dense, respectively) are obtained as single-phase products by Spark Plasma Sintering (1950 °C/20 min/20 MPa) starting from powders previously synthesized by Self-propagating High-temperature Synthesis (SHS). The optical characterization, whose results are discussed by comparing HEBs to the individual borides, shows that they are characterized by intrinsic spectral selectivity and low thermal emittance, resulting therefore interesting for high-temperature solar absorbers applications.

Novel ceramic fibre - Zirconium diboride composites for solar receivers in concentrating solar power systems

Ultra-high temperature ceramic matrix composites (UHTCMCs) are a new class of composites which combine boride phases with carbon fibres to achieve new structural and functional properties and were originally designed for aerospace applications. For the first time, this work investigates the optical response of UHTCMCs in view of their possible application as high temperature solar absorbers in Concentrating Solar Power systems. Carbon fibre-reinforced ZrB2 pellets with different fibre type and arrangement were manufactured and characterized from the point of view of optical properties and surface microstructure. The composites had high fractions of ZrB2 (∼40 vol%) and carbon fibres (up to 60 vol%). Spectral hemispherical reflectance in the range 0.3–16 μm wavelength was assessed and compared to pure boride and graphite phases. Irrespective of the fibre architecture, the solar absorptance significantly increased with respect to the pure boride, reaching or even overpassing the solar absorptance of silicon carbide. Thermal emittance, calculated from 800 K to 1500 K temperature, increased as well in comparison to the pure boride, but always remained lower than that of SiC. These optical properties, combined with structural damage tolerance and light weight, suggest that UHTCMCs are of great interest as high temperature solar absorbers, overpassing the weaknesses of current solar receivers, as well as those of the most promising alternative materials to date.

A three-material chimney-like absorber with a sharp cutoff for high-temperature solar energy harvesting

For high-temperature solar absorbers, the inconspicuous cutoff in absorptivity gives rise to substantial self-radiation loss, reducing the heat transfer efficiency of solar energy conversion systems. To further improve thermal efficiency, we designed a broadband (300–1500 nm) absorber with a sharp cutoff based on chimney-like metamaterials. The design involves three different materials, each with unique optical properties, which work together to achieve the desired absorption spectrum. The proposed absorber provides a high absorptivity over 0.95 at 340–1400 nm with a narrow cutoff bandwidth of less than 300 nm. Additionally, this design can maintain good absorption performance over a wide angular range and at high temperatures. Based on theoretical calculations, the proposed absorber achieves an excellent thermal efficiency of 91.62% under 1200 K and 1000 solar concentrations, which is better suited for high-temperature solar energy harvesting.

Structure and thermal stability of a novel nanocomposite solar selective absorber coating

Improving the utilization efficiency of solar energy requires effective high-temperature solar absorbers. However, the most current absorbers degrade at high working temperatures. Here, we report a novel TiN/(TiN/AlTiSiN)3/AlTiSiON/AlTiSiO tandem absorber with excellent thermal stability up to 650 °C in air. The absorber was designed according to the optical constants of each layer which were obtained by modeling their experimental reflectance and transmittance spectra on the quartz substrates. Then the designed absorber was fabricated using arc ion plating, and its optical performance was manipulated by tuning the thickness of AlTiSiN. An optimized solar selectivity of 0.905/0.146 could be achieved on the as-deposited absorber, and the optical performance only slightly decreased with a performance criterion (PC) of 0.0215 after annealing treatment at 650 °C in air for 2 h. The dense amorphous AlTiSiO and AlTiSiON films can effectively hinder the oxygen diffusion to the inner absorber, and the nanocomposite and multilayer structure in the TiN/AlTiSiN absorber layer can limit atomic diffusion and grain growth, resulting in high thermal stability. These results indicate that the absorber can be a suitable potential candidate for high-temperature solar energy conversion applications.

Formation, structure, and optical properties of copper chromite thin films for high-temperature solar absorbers

CuCr2O4 thin films grown by physical vapour deposition were studied in order to evaluate their potential as absorber material for the next generation of concentrated solar power plants. A series of Cu-Cr-O thin films was deposited by reactive ion beam sputtering. The Cr/Cu ratio in the sputter target is demonstrated as the most important parameter to achieve the intended film stoichiometry. In-air annealing at 800 °C leads to structural transformations of the as-deposited films and results in phase compositions according to those expected from the ternary Cu-Cr-O phase diagram. Tetragonal CuCr2O4 with 98.6 at.% phase purity regarding the solid film constituents is obtained for the appropriate Cr/Cu ratio in the sputter target. CuCr2O4 thin films absorb light in the entire solar spectral range from 300 to 2500 nm. Their energy gap is found to be < 0.5 eV, and their solar absorptance is estimated to be (0.85 ± 0.03). The dense microstructure with good thermal conductivity, full adhesion to the substrate, and a relatively low surface roughness are discussed as technological advantages of CuCr2O4 thin films grown by physical vapour deposition.

A comparative study of thermal conductivity and thermal emissivity of high temperature solar absorber of ZrO2 /Fe2O3 and Al2O3/CuO ceramics

Oxide ceramics are considered as promising high temperature solar absorber materials. The major aim of this work is the development of a new solar absorber material with promising characteristics, high efficiency and low-cost processing. Hence, this work provides a comparative and inclusive study of densification behavior, microstructure features, thermal emissivity and thermal conductivity values of the two new high temperature solar absorbers of ZrO2/Fe2O3 and Al2O3/CuO ceramics. Ceramic composites of ZrO2/(10–30 wt%) Fe2O3 and Al2O3/(10–30 wt%) CuO were prepared by pressureless sintering method at a temperature of 1700 °C/2hrs. Identification of the solar to thermal efficiency of the composites was evaluated in terms of their measured thermal emissivity. Thermal efficiency and heat transfer homogeneity were investigated in terms of thermal conductivity and diffusivity measurement. The results showed that both composites exhibited comparable densification behavior, homogenous and harmonious microstructure. However, Al2O3/10 wt% CuO composite showed higher thermal and solar to thermal efficiencies than ZrO2/Fe2O3 composites. It gave the lowest and the best thermal emissivity of 0.561 and the highest thermal conductivity of 15.4 W/m. K. These values proved to be the best amongst all those of the most known solar absorber materials made from the expensive SiC and AlN ceramics. Thus, Al2O3/CuO composites have succeeded in obtaining outstanding properties at a much lower price than its other competitive materials. These results may strongly identify Al2O3/CuO composites as promising high-temperature solar absorber materials instead of ZrO2 and the other carbide and nitride ceramics.

A novel multilayer high temperature solar absorber coating based on high-entropy alloy NbMoTaW: Optical properties, thermal stability and corrosion properties

With the development of new materials and technology, high entropy alloy (HEA) nitride films have attracted much attention of researchers due to their excellent optical properties and mechanical properties. Herein, a novel SS/NbMoTaWN (HEAN)/NbMoTaWON (HEAON)/SiO2 coatings are prepared, which shows a high spectral selectivity of α/ε = 0.944/0.12. The preparation and optimization of the coating are studied by combining experiments with ellipsometric program and CODE software. High temperature thermal stability test is performed in depth, which proves that the coating could bear 400 °C in air for 2 h, and 600 °C for 2 h in vacuum. Long-term thermal stability researches indicate that the SSACs still keep good optical properties (α = 0.902, ε = 0.106) even after annealing at 600 °C for 100 h. The failure mechanism is analyzed by XRD and Raman spectra. In addition, neutral salt spray test is performed to investigate the anti-corrosion ability, which indicates the coating has a good optical performance after soaking in 3.5 wt% NaCl solution for 30 days. Obviously, this work provides a new strategy to construct solar absorber coatings based on NbMoTaW high entropy alloy.

Heat Transfer Augmentation of Concentrated Solar Absorber Using Modified Surface Contour

This work aims to compare the cavity surface contour’s thermal performance to that of the solar absorber’s plain surface contour for Scheffler type parabolic dish collectors. The absorber is tested for the temperature range up to 600°C without working fluid and 180°C with the working fluid. The modified absorber surface's thermal performance is compared with the flat surface absorber with and without heat transfer fluid. The peak temperature reached by the surface modified absorber (534°C) is about 8.6% more than that of the unmodified absorber (492°C) during an outdoor test without fluid. The energy efficiency of cavity surface absorber and plain surface absorber are 67.65% and 61.84%, respectively. The contoured cavity surface produces a more uniform temperature distribution and a higher heat absorption rate than the plain surface. The results are beneficial to the design of high-temperature solar absorbers for concentrated solar collectors.

Heat Transfer Augmentation of Concentrated Solar Absorber Using Modified Surface Contour

This work aims to compare the cavity surface contour’s thermal performance to that of the solar absorber’s plain surface contour for Scheffler type parabolic dish collectors. The absorber is tested for the temperature range up to 600°C without working fluid and 180°C with the working fluid. The modified absorber surface's thermal performance is compared with the flat surface absorber with and without heat transfer fluid. The peak temperature reached by the surface modified absorber (534°C) is about 8.6% more than that of the unmodified absorber (492°C) during an outdoor test without fluid. The energy efficiency of cavity surface absorber and plain surface absorber are 67.65% and 61.84%, respectively. The contoured cavity surface produces a more uniform temperature distribution and a higher heat absorption rate than the plain surface. The results are beneficial to the design of high-temperature solar absorbers for concentrated solar collectors.

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.

Scalable-manufactured metal–insulator–metal based selective solar absorbers with excellent high-temperature insensitivity

Solar absorbers, harvesting solar irradiance in the form of heat, are extensively applied in the solar hot water systems and concentrated solar thermal systems such as concentrated solar power plants, solar thermoelectric generators, and solar thermophotovoltaics. It is of great significance to incorporate spectrally selective solar absorbers into solar thermal systems, especially at high operational temperatures to depress the thermal loss due to the thermal re-emission of high-temperature solar absorbers. This work computationally and experimentally demonstrates a new spectrally selective solar absorber consisting of a multilayered stack made of silica/alumina/tungsten/alumina/tungsten based on metal–insulator–metal resonance structures and fabricated by the magnetron sputtering method, which are angular insensitive and polarization-independent. The relationship between solar conversion efficiency, cut-off wavelength, operational temperatures, and concentration factor is theoretically investigated. An overall absorptance of 88.1% at solar irradiance wavelength, a low emittance of 7.0% at infrared thermal wavelength, and a high solar-to-heat efficiency of 82.5% are identified. Additionally, it shows the annealed samples maintain an extremely high absorption in solar radiation regime over at least 800 °C and a high concentration factor of over 100. The SEM topography images of the absorbers after thermal annealing at various temperatures demonstrates that the surface blisters and cracks result in the thermal degradation of the absorbers due to the dissimilarity between thermal expansion coefficients of tungsten and silica. The high-temperature insensitivity of the multilayer metal–insulator–metal-based selective solar absorbers will shed light on an alternative novel photonic metamaterial structure that can be scalable-manufactured to improve the energy conversion efficiency of solar thermal engineering.

Enhanced spectral selectivity of HfC based high temperature solar absorbers with the addition of Mo

Simple technological process, low cost and high stability are important for large-scale production of the high temperature spectral selective solar absorber coatings. In this study, a simple Mo-HfC/Al2O3 coating that only contains an absorber layer (Mo-HfC) and an antireflection layer (Al2O3) deposited onto stainless steel substrate, which exhibits a high spectral selectivity (αs/εT) of 0.92/0.11. The experimental results indicate that the addition of Mo could greatly enhance the spectral selectivity of the coating. Thermal stability test demonstrates that the spectral selectivity of the coating remains unchanged even after annealed it at 600°C for 100 h in vacuum. Obviously, the coating shows a promising prospect in high temperature application. In addition, the optical degradation mechanism is also discussed in this work.

Tailoring optical properties of surfaces in wide spectral ranges by multi-scale femtosecond-laser texturing: A case-study for TaB2 ceramics

The use of high power pulsed lasers is an effective tool for microstructuring material surfaces. It appears particularly useful when the material has some characteristics making difficult using other procedures (e.g. a high hardness). The present work reports on the femtosecond-laser treatment on tantalum diboride ultra-high temperature ceramics with different starting porosity fractions. The interaction with the laser beam creates a pattern with a complex multi-scale structure on the ceramic surface, whose characteristics depend on accumulated laser fluence and pristine porosity. Optical properties are significantly changed, allowing to separately optimize the interaction of the material with electromagnetic radiation spectrally located in different regions. As a case study, we apply the proposed strategy considering high-temperature solar thermal absorber applications, where the independent management of UV–Visible-Near IR radiation (sunlight) and Mid-IR (thermal radiation at the operating temperatures) are required. The correlation between the typical sizes of the realized multi-scale structures and the optical parameters (solar absorptance and thermal emittance in our example application) is discussed using an original predictive approach. The method here shown can be extended to every situation where materials are required to simultaneously interact with electromagnetic radiation in various spectral ranges. • Femtosecond laser-treatment creates a multi-scale surface pattern on ultra-hard TaB2 ceramics. • Local surface morphology and composition rely on accumulated laser fluence and pristine material porosity. • Optical properties are changed, with a remarkable increase of solar absorbance. • An original and simple method is described to correlate optical properties to the multi-scale surface characteristics. • The method is extremely useful to predictably optimize any surface interacting with light.