by CE Kennedy · Cited by 656 — Spectral performance of an ideal selective solar absorber. 1. Figure 2. Spectral Emittance as a function of sample temperature for aluminum.

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July 2002 Ł NREL/TP-520-31267 •Review of Mid- to High-Temperature Solar Selective Absorber Materials C.E. Kennedy National Renewable Energy Laboratory 1617 Cole Boulevard Golden, Colorado 80401-3393 NREL is a U.S. Department of Energy LaboratoryOperated by Midwest Research Institute Battelle Bechtel Contract No. DE-AC36-99-GO10337

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July 2002 Ł NREL/TP-520-31267 •Review of Mid- to High-Temperature Solar Selective Absorber Materials C.E. Kennedy Prepared under Task No. CP02.2000 National Renewable Energy Laboratory 1617 Cole Boulevard Golden, Colorado 80401-3393 NREL is a U.S. Department of Energy LaboratoryOperated by Midwest Research Institute Battelle Bechtel Contract No. DE-AC36-99-GO10337

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NOTICE This report was prepared as an account of work sponsored by an agency of the United States government. Neither the United States government nor any agency thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States government or any agency thereof. Available electronically at Available for a processing fee to U.S. Department of Energy and its contractors, in paper, from: U.S. Department of Energy Office of Scientific and Technical Information P.O. Box 62 Oak Ridge, TN 37831-0062 phone: 865.576.8401 fax: 865.576.5728 email: Available for sale to the public, in paper, from: U.S. Department of Commerce National Technical Information Service 5285 Port Royal Road Springfield, VA 22161 phone: 800.553.6847 fax: 703.605.6900 email: online ordering: Printed on paper containing at least 50% wastepaper, including 20% postconsumer waste

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Table of Contents 1. Introduction 1 • 2. Characterization of Selective Surfaces. 2 • 3. Description of Types of Absorbers . 4 • a. Intrinsic or fimass absorbersfl5• b. Semiconductor-metal tandems.5• c. Multilayer absorbers 6• d. Metal-dielectric composite coatings..6• e. Surface texturing8• f. Selectively solar-transmitting coating on a blackbody-like • absorber..9• 4. Temperature Range of Absorber Materials. 9 • 4.1. Mid-temperature selective surfaces (100ºC < T<400ºC).13 • 4.2. High-temperature selective surfaces (T>400ºC..19 • Conclusion. 31 • References. 33 • Appendix. 46 • List of Symbols ..47 • List of Abbreviations ..48 • Material Abbreviations49• Definitions..51• i

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List of Figures Figure 1. Spectral performance of an ideal selective solar absorber 1 • Figure 2. Spectral Emittance as a function of sample temperature • for aluminum 3 • Figure 3. Schematic designs of six types of coatings and surface • treatments for selective absorption of energy 5 • Figure 4. Schematic designs of multilayer absorber film structure 6 • Figure 5. Schematic designs of two different metal-dielectric • solar selective coating 7 • Figure 6. Schematic design of double-cermet film structure 8 • List of Tables Table 1. Mid-Temperature Selective Surfaces 11 • Table 2. High-Temperature Selective Surfaces 12 • Table 3. Multilayer Selective Surfaces 20 • Table 4. Composition and Properties of Selected MCxOyNz absorbers 27 • ii

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1. Introduction Concentrating solar power (CSP) systems use solar absorbers to convert sunlight to thermal electric power. The CSP program is working to reduce the cost of parabolic trough solar power technology. One of the approaches is to increase the operating temperature of the solar field from approximately 400C to 500C (or higher). To accomplish this, new more efficient selective coatings are needed that have both high solar absorptance and low thermal emittance at 500C. Although designs are likely to use coating in evacuated environments, the coatings need to be stable in air in case the vacuum is breached. Current coatings to not have the stability and performance desired for moving to higher operating temperatures. For efficient photothermal conversion solar absorber surfaces must have high solar absorptance () and a low thermal emittance () at the operational temperature. A low reflectance ( 0) at wavelengths () 3m and a high reflectance ( 1) at 3m characterize spectrally selective surfaces, as shown in Figure 1. The cutoff may be higher or lower as it is dependent on the temperature. The operational temperature ranges of these materials for solar applications can be categorized as low temperature (T<100ºC), mid temperature (100ºC400ºC). CSP systems operate at high-and mid-temperatures. For CSP parabolic trough applications, the ideal spectrally selective surface would be low-cost and easy to manufacture, chemically and thermally stable in air at elevated temperatures (T 500ºC), and have a solar absorptance 0.98 and a thermal emittance 0.05 at 500ºC. However, a high temperature coating may require tradeoffs in performance as the high solar absorption and low emittance may be mutually exclusive. 1.00 2500 0.75 t Absorptance or Emittance 450°C Blackbody Direct AM 1.5 Terrestrial Solar S 2000 1500 0.50 1000 0.25 500 0.00 0 0.1 1.0 10.0 100.0 Wavelength () Figure 1. Spectral performance of an ideal selective solar absorber. Absorptance or Emittance Irradiance (W/m2-) 1 •

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1 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 0 2 4 6 8 101214161820 max for 149°C T = 25°C T = 149°C Wavelength () Figure 2. Spectral Emittance as a function of sample temperature for aluminum. Emittance is a surface property and depends on the surface condition of the material, including the surface roughness, surface films, and oxide layers [4]. Coatings typically replicate to some degree the surface roughness of the substrate. Therefore to facilitate development, it is important to measure the emittance of each coating-substrate combination as well as the uncoated substrate when developing a solar selective coating. Furthermore, selective coatings can degrade at high temperatures because of thermal load (oxidation), high humidity or water condensation on the absorber surface (hydratization and hydrolysis), atmospheric corrosion (pollution), diffusion processes (interlayer substitution), chemical reactions, and poor interlayer adhesion [5,6]. Calculating the emittance from spectral data taken at room temperature assumes that the spectral characteristics do not change with increasing temperature. This is only valid if the material is invariant and does not undergo a phase change (as do some titanium containing materials), breakdown or undergo oxidation (as do paints and some oxide coatings) at higher temperatures. It is important before using high-temperature emittance calculated from room temperature data, that the calculated data is verified with high-temperature emittance measurements for each selective coating. The key for high-temperature usage is low , because the thermal radiative losses of the absorbers increase proportionally by the fourth power of temperature; therefore, it is important to measure the emittance at the operating temperatures and conditions [2]. In addition to the initial efficiency, long term stability is also an important requirement for absorber coatings. At high temperatures, thermal emittance is the dominant source of losses, and the requirement of low emittance often leads to complex designs that are frequently susceptible to degradation at the working temperature. There is an International Energy Agency (IEA) Task X performance criterion (PC) developed for flat-plate collector selective absorber testing (i.e., non-concentrating, 1-2X sunlight intensity). The PC describes the influence in the change of solar absorption (s) and emittance () on the solar fraction: PC s 0.25 0.05, (6) Normal Hemispherical Emittance 3 •

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assuming a service lifetime of at least 25 years and a decrease in the annual solar fraction of 5%. Service lifetime testing for this criterion is performed by exposing the absorber coatings for 200 h at 250C. If the material survives, it is then exposed for 75 h at 300C, followed by 600 h at 40C/95% relative humidity (RH), then 85 h at 60C/95%RH [5, 7]. After exposure testing, the emittance is typically measured at 100C. No similar criterion has been developed for testing the service lifetime of high-temperature absorbers for CSP applications. Thermal stability is sometimes based on the thermal properties of the individual materials or the processing temperature parameters, and actual durability data are rarely known for high-temperature absorber coatings. Durability or thermal stability is typically tested by heating the selective coating, typically in a vacuum oven but sometimes in air, for a relatively short duration (100s of hours) compared to the desired lifetime (5-30 years). This procedure often masks cascaded processes and interactions during exposure [8]. Degradation of high-temperature absorbers usually causes increasing emittance; therefore, emittance is a sensitive indicator to monitor degradation in the normal case where emittance changes with exposure. In addition, while the emittance of many materials after exposure to high temperatures does not return to the original emittance measured (e.g., paint), for some materials (e.g., Boral, a malleable boron-aluminum alloy) the emittance changes at high temperatures and returns to the original value after cooling to room temperature. Therefore, it is important to verify for each selective coating that the emittance does not change during the heat cycle. The capability must be built to allow spectrally selective coatings to be exposed and measured at their operating temperatures and conditions for longer periods of time to determine the durability and thermal stability of the materials. Then a criterion needs to be developed for high-temperature selective surfaces applicable for concentrating applications. 3. Description of Types of Absorbers Selective absorber surface coatings can be categorized into six distinct types: a) intrinsic, b) semiconductor-metal tandems, c) multilayer absorbers, d) multi-dielectric composite coatings, e) textured surfaces, and f) selectively solar-transmitting coating on a blackbody-like absorber. Intrinsic absorbers use a material having intrinsic properties that result in the desired spectral selectivity. Semiconductor-metal tandems absorb short wavelength radiation because of the semiconductor bandgap and have low thermal emittance as a result of the metal layer. Multilayer absorbers use multiple reflections between layers to absorb light and can be tailored to be efficient selective absorbers. Metal-dielectric compositesŠcermetsŠconsist of fine metal particles in a dielectric or ceramic host material. Textured surfaces can produce high solar absorptance by multiple reflections among needle-like, dendritic, or porous microstructure. Additionally, selectively solar-transmitting coatings on a blackbody-like absorber are also used but are typically used in low-temperature applications. These constructions are shown schematically in Figures 3a-f, respectively, and are discussed in greater detail below. 4 •

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Intrinsic selective material Substrate a) Intrinsic absorber Antireflection coating• Semiconductor• Metal • b) Semiconductor-metal tandems Dielectric Metal Dielectric Substrate c) Multilayer absorbers Metal Dielectric Metal d) Metal-dielectric composite e) Surface texturing Metal SnO2:F– Black enamel– Substrate f) Solar-transmitting coating/blackbody-like absorber Figure 3. Schematic designs of six types of coatings and surface treatments for selective absorption of energy. a) Intrinsic or fimass absorbersfl Intrinsic or fimass absorbers,fl in which selectivity is an intrinsic property of the materials, are structurally more stable but optically less effective than multilayer stacks examples include, metallic W [9], MoO3-doped Mo [10], Si doped with B, CaF2 [11], HfC [12], ZrB2 [13], SnO2 [12], In2O3 [11], Eu2O3 [14], ReO3 [14], V2O5 [14], and LaB6 [15]. No naturally occurring material exhibits intrinsically ideal solar-selective properties, but some roughly approximate selective properties. Intrinsic solar-selective properties are found in transition metals and semiconductors, but both need to be greatly modified to serve as an intrinsic absorber. Hafnium carbide (HfC) could be useful as an absorbing selective surface at elevated temperatures because of its high melting point. However, HfC requires structural and/or compositional changes in the lattice or an antireflective (AR) layer composed of a quarter wavelength of a dielectric material to create the required properties. Single-layer AR coatings that have been used include SiO, SiO2, Si3N4, TiO2, Ta2O5, Al2O3, ZrO2, Nd2O3, MgO, MgF2, and SrF2 [16,17]. AR coatings can also be made from very thin layers of two materials having properly matched indices of refraction, for example, thallium iodide and lead fluoride [18]. Historically, research in intrinsic absorbers has not been very productive because there are no ideal intrinsic materials; but the intrinsic materials are finding increasing use as a component in high-temperature absorber multilayers and composite coatings. b) Semiconductor-metal tandems Semiconductors with bandgaps from about ~0.5 eV (2.5 µm) to 1.26 eV (1.0 µm) absorb short-wavelength radiation, and the underlying metal provides low emittance to give the desired spectral selelectivity to semiconductor-metal tandems. Semiconductors of interest include Si (1.1 eV), Ge (0.7 eV), and PbS (0.4 eV) [19]. Thin semiconductor films of high porosity or antireflection coatings are needed because the useful semiconductors have high refractive indices, which result in large detrimental reflectance 5 •

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losses. Si-based designs produced by chemical-vapor deposition (CVD) are well known that are suitable for mid-to high-temperature applications [20]. c) Multilayer absorbers Multilayer absorbers or multilayer interference stacks can be designed so that they become efficient selective absorbers. The selective effect is because the multiple reflectance passes through the bottom dielectric layer (E) and is independent of the selectivity of the dielectric. A thin semitransparent reflective layer (D), typically a metal, separates two quarter-wave dielectric layers (C and E). The bottom-reflecting layer (D) has high reflectance in the infrared (IR) region and is slightly less reflective in the visible region. The top dielectric layer (C) reduces the visible reflectance. The thickness of this dielectric determines the shape and position of the reflectance curve. An additional semitransparent (i.e., thin) metal layer (B) further reduces the reflectance in the visible region, and an additional dielectric layer (A) increases the absorption in the visible region and broadens the region of high absorption. The basic physics of the multilayer absorber is well understood, and computer modeling can easily compute the optical properties given by an optimum multilayer design of candidate materials [21,22]. Multilayer interference stacks have high solar absorption, low thermal emittance, and are stable at elevated temperatures ( 400ºC) depending on the materials used. Several multilayer absorbers using different metals (e.g., Mo, Ag, Cu, Ni) and dielectric layers (e.g., Al2O3, SiO2, CeO2, ZnS) have been cited in the literature for high-temperature applications [23]. A Dielectric• B Metal• C Dielectric• D Metal• E Dielectric• F Substrate• Figure 4. Schematic designs of multilayer absorber film structure. d) Metal-dielectric composite coatings Metal-dielectric composite coatings or absorber-reflector tandems have a highly absorbing coating in the solar region (i.e., black) that is transparent in the IR, deposited onto a highly IR-reflective metal substrate. The highly absorbing metal-dielectric composite, or cermet, consists of fine metal particles in a dielectric or ceramic matrix, or a porous oxide impregnated with metal. These films are transparent in the thermal IR region, while they are strongly absorbing in the solar region because of interband transitions in the metal and the small particle resonance. When deposited on a highly reflective mirror, the tandem forms a selective surface with high solar absorptance and low thermal emittance. The high absorptance may be intrinsic, geometrically enhanced, or both. The absorbing cermet layer comprised of inherently high-temperature materials can have either a uniform or graded metal content. The metal-dielectric concept offers a high degree of flexibility, and the solar selectivity can be optimized by proper choice of constituents, coating thickness, particle concentration, size, shape, and orientation. The 6 •

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