Solar Selective Coatings Importantance in CSP Technology Ambesh Dixit SCHOTT Solar Inc. Indian Institute of Technology Jodhpur Parabolic Trough: an example SCHOTT Solar Inc. Radiation from the Sun transformed into thermal energy Used for Heating air or water/fluid media Presentation flow Solar thermal applications A bit about receiver tube and its design Spectral selectivity Selective absorbers with examples Mechanisms for solar spectral selectivity Solar absorber design constraints Physical process (RF/DC magnetorn sputtering) Chemical process (Sol-gel process) Surface engineering for enhanced solar absorption Conclusions Temperature ranges for solar thermal applications Low temperature (< 100 0C) Water heating and swimming pools Medium temperature (< 350 0C) Space heating or cooling and water desalination High temperature (> 350 0C) Mechanical energy production and catalytic dissociation of water, CSP (concentrating solar power ~ 500 0C or more) Receiver is an important Component in Parabolic Trough Collectors A receiver should comply with Low thermal losses ( vacuum, absorber with low thermal emittance) High solar absorptance ( efficient absorber, highly transmitting outer glass tube ) evacuated annulus getter to maintain vacuum cover tube with anti-reflective coating selective absorber coating on steel, glass-to-metalseal bellow to compensate expansion For power plant with a life span of more than 20 years is required to Match the long operational sustainability. Keep maintenance costs low during operation. During operation receivers are mechanically and thermally stressed. Most important issues are: Durability of glass-to-metal seal Stability of vacuum (low hydrogen permeation, appropriate getter) Durability of absorber coating (only small degradation of efficiency acceptable) Abrasion resistance of anti-reflective glass coating. Selective Absorber with Multilayer CERMET for High Temperatures Performance data: Temperature stable up to 500 °C Solar absorptance >= 95 % Thermal emittance <= 10% at 400°C Material: Polished low-carbon steel as substrate material Absorber coating 1,0 W-Al2O3 Multilayer Cermet coating 0,9 = 0,95 = 0,13 @ 400°C 0,8 AR-coating cermet reflectance 0,7 0,6 0,5 0,4 AM 1.5 (norm.) 0,3 BB, 400°C (norm.) 0,2 0,1 steel 0,0 0,4 0,6 0,8 1 2 4 6 8 10 wave length / µm 20 SCHOTT Solar Inc. Spectral selective surface: Non-selective surfaces Moderate selective surfaces Selective surfaces Performance quantification: Solar absorptance: Absorbed fraction of incoming radiation Thermal emittance: Emitted fraction of absorbed energy through infrared radiation Selective absorbers can accomplish this requirement by having (i) high solar absroptivity and (ii) high thermal reflectivity simultaneously Different mechanisms for solar spectral selectivity (i) Semiconductor with suitable band gaps (ii) Optical interference effect of a multilayer stack of thin films (iii) Materials, which are black for solar wavelengths but transparent for heat like metal-ceramic nanocomposites (called CERMET) (iv) Metallic surface with designed roughness Multiple reflections of the light inside surface groves -> enhanced solar absorption Examples: Black chrome Black zince, cobalt, nickel Copper oxide, iron oxide, aluminum oxide Electroplating Technique Solar absorption ~ 0.9 Thermal emittance ~ 0.1 Material Absorptance () Emittance () Break down temparature Comments (°C) Black silicon paint 0.86-0.94 0.83-0.89 Black silicon 0.9 0.5 350 Stable at high temperature paint Black copper 0.85-0.9 over copper 0.08-0.12 Black chorome over nickel 0.07-0.12 0.92-0.94 Slicone binder 450 Patinates with moisture 450 Stable at high temperatures Jan F. Kreider et al Solar Design (1989) As a designer for solar absorbers: A serious look into solar irradiance & Black body radiation @ 300 0C: BB radiation 2 mm – 30 mm No overlap between these two curves Possible to prepare surfaces that may absorb the soalr wavelengths and emitt poorly at thermal infrared wavelength. 1 t r g Different names: t = Transmissivity r = Reflectivity g = Absorptivity Bandpass reflection filters Black infrared mirrors Spectrally selective absorbers/coatings As a designer for solar absorbers: Number of choices to fabricate solar selective coatings Combination of various mechanisms to control and improve the optical property of an absorber layer such as Textured surface with required spectral selectivity, graded cermet or double cerment structure Equiped with an anti-reflectition layer may exhibit enhanced spectral selectivity Such structures may result in good solar absorptance ~ 0.98 and poor thermal emittance ~ 0.02 or less, yet these structures are complicated and thickness sensitive. As a designer for solar absorbers: Solutions: Improve the selectivity of cermet based absrobers in single layer geometry surface roughness on the absorber/air interface (laser structuring) Easy thin film process such as sol-gel for quick fabrication of thin films and tunability using stable colloidal suspensiions of nano-powders for cermat composites Thin film Coating Process Physical Vapour deposition Thermal evaporation e-beam evaporation Chemical vapour deposition Chemical Electrodeposition Chemical deposition Spraying Sol-gel Physical vapour deposition Metal organic Molecular beam epitaxy deposition (MOD) RF/DC magnetron sputtering Pulse laser deposition (PLD) Low pressure coating processes in which the coating flux is produced by a physical process. There are two main types: Evaporation Sputtering Advantages In both cases the source material is a solid (metal or ceramic). A reactive gas may be used in the deposition chamber to deposit compound coatings from an elemental source or maintain the stoichiometry of coatings from compound sources. Typical coating thicknesses range from 1-5mm Excellent process control Low deposition temperature Dense, adherent coatings Elemental, alloy and compound coatings possible Disadvantages Vacuum processes with high capital cost Limited component size treatable Relatively low coating rates Physical: RF/DC magnetron sputtering process Main sputtering processes: DC diode sputtering (for conducting targets) RF sputtering (for insulating targets) Mostly used for low deposition temperatures. No post deposition heat treatment required. Fine thickness control. Easy to dope with noble metals. The coating rate scales with the electrical power used to sustain the discharge. The coating rate also depends on the plasma density, so techniques to increase this (e.g. by confining the electrons close to the target using magnets) will increase the coating rate. However, as much as 95% of the power is dissipated as heat in the target so good cooling is essential. Materials may be deposited using sputtering Metal oxide such as aluminum oxide, copper oxide, iron oxide etc Metal nitrides such aluminum nitrides, titanium nitrides etc easy to dope simultaneously during growth. Numerous materials: Our Target: High solar absorptance (~ 0.95 or more) and low emittance (~0.05 or less) for high temperature applications Systems of choiceAluminum nitride (AlN) based cermets coatings using RF/DC sputtering Stable at high temperature (> 500 0C), radiation resist, high absorptance and low emittance 20 30 40 50 60 70 80 2 (degree) % R (arb. units) Intensity (arb. units) Glass substrate AlN/Glass (DC sputtered) AlN/glass DC sputtered 800 1600 2400 3200 4000 -1 Wavenumber (cm ) Chemical: Sol-gel process • Advantages • • • • Low temperature treatment Easy synthesis process Can coat complex shapes uniformly Hard particles can be incorporated to increase hardness • Can coat most metals and insulators • Disadvantages • Film quality is not comparable with physical process • Heat treatment is necessary to develop the desired material stoichiometry and properties Numerous materialsOur Target: High solar absorptance (~ 0.95 or more) and low emittance (~0.05 or less) for moderate temperature applications Systems of choice- Intensity (arb. units) Chromium oxide (Cr2O3) based cermets coatings using solution process Easy to fabricate, state at intermediate temperature, high absorptance and low emittance Cr O 2 3 (From Dip Coating) 20 30 40 50 60 70 80 2 (degree) Surface engineering by AR Coating with High Solar Transmittance Sol-Gel coating for borosilicate glass based on alcoholic dilutions with SiO2 nanoparticles for improved abrasion resistance Solar transmittance of > 0,96 achieved Challenges in production: - homogenous and stable coating of long glass tubes - automated high precision solar transmittance test for long glass tubes Only glass: t = 92% With AR-coating : t > 96% Conclusions Solar selective coatings are important for numerous solar thermal applications. Stable high temperature solar selective coatings are essential to realize CSP applications. Nitrides based CERMET coatings may be promising candidates for CSP applications, where temperature may go beyond 500 0C. Sol-gel process may be explored for development of oxide based CERMET coatings. Surface engineering may enhance the solar absorption beyond the material’s intrinsic limit enhancing multiple reflection assisting absorption by reducing bulk reflection. Acknowledgement Prof. Rajiv Shekhar (a driving force) Dr. Laltu Chandra Mr. Ritesh Patel Funding agency- MNRE Thank you & ????