Presentation on theme: "Solar Selective Coatings Importantance in CSP Technology"— Presentation transcript:
1 Solar Selective Coatings Importantance in CSP Technology Ambesh DixitIndian Institute of Technology JodhpurSCHOTT Solar Inc.
2 Parabolic Trough: an example SCHOTT Solar Inc.Radiation from the Sun transformed into thermal energyUsed for Heating air or water/fluid media
3 Presentation flow Solar thermal applications A bit about receiver tube and its designSpectral selectivitySelective absorbers with examplesMechanisms for solar spectral selectivitySolar absorber design constraintsPhysical process (RF/DC magnetorn sputtering)Chemical process (Sol-gel process)Surface engineering for enhanced solar absorptionConclusions
4 Temperature ranges for solar thermal applications Low temperature (< 100 0C)Water heating and swimming poolsMedium temperature (< 350 0C)Space heating or cooling and water desalinationHigh temperature (> 350 0C)Mechanical energy production and catalytic dissociation of water, CSP (concentrating solar power ~ 500 0C or more)
5 Receiver is an important Component in Parabolic Trough Collectors A receiver should comply withLow thermal losses ( vacuum, absorber with low thermal emittance)High solar absorptance ( efficient absorber, highly transmitting outer glass tube )
6 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.
7 Selective Absorber with Multilayer CERMET for High Temperatures Performance data:Temperature stable up to 500 °CSolar absorptance >= 95 %Thermal emittance <= 10% at 400°CMaterial:Polished low-carbon steel as substrate materialW-Al2O3 Multilayer Cermet coatingsteelAR-coatingcermetSCHOTT Solar Inc.
8 Spectral selective surface: Non-selective surfacesModerate selective surfacesSelective surfacesPerformance quantification:Solar absorptance:Absorbed fraction of incoming radiationThermal emittance:Emitted fraction of absorbed energy through infrared radiationSelective absorbers can accomplish this requirement by having(i) high solar absroptivity and(ii) high thermal reflectivity simultaneously
9 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 heatlike metal-ceramic nanocomposites (called CERMET)(iv) Metallic surface with designed roughnessMultiple reflections of the light inside surface groves -> enhancedsolar absorptionExamples:Black chromeBlack zince, cobalt, nickelCopper oxide, iron oxide, aluminum oxideElectroplating TechniqueSolar absorption ~ 0.9Thermal emittance ~ 0.1
10 MaterialAbsorptance()Emittance()Break down temparature(°C)CommentsBlack siliconpaint350Slicone binder0.90.5Stable athightemperatureBlack copper over copper450Patinates with moistureBlack chorome over nickelStable at high temperaturesJan F. Kreider et al Solar Design (1989)
11 As a designer for solar absorbers: A serious look into solar irradiance &Black body 300 0C:BB radiation 2 mm – 30 mmNo overlap between these two curvesPossible to prepare surfaces thatmay absorb the soalr wavelengthsand emitt poorly at thermal infra-red wavelength.Different names:Bandpass reflection filtersBlack infrared mirrorsSpectrally selective absorbers/coatingst = Transmissivityr = Reflectivityag = Absorptivity
12 As a designer for solar absorbers: Number of choices to fabricate solar selective coatingsCombination of various mechanisms to control and improve the optical property of an absorber layer such asTextured surface with required spectral selectivity, graded cermet or double cerment structureEquiped with an anti-reflectition layer may exhibit enhanced spectral selectivitySuch 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.
13 As a designer for solar absorbers: Solutions:Improve the selectivity of cermet based absrobers in single layer geometrysurface roughness on the absorber/air interface (laser structuring)Easy thin film process such as sol-gelfor quick fabrication of thin films and tunabilityusing stable colloidal suspensiions of nano-powders for cermat composites
15 Advantages Disadvantages Low pressure coating processes in which the coating flux is produced by a physical process.There are two main types:EvaporationSputteringAdvantagesExcellent process controlLow deposition temperatureDense, adherent coatingsElemental, alloy and compound coatings possible DisadvantagesVacuum processes with high capital costLimited component size treatableRelatively low coating ratesIn 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
16 RF/DC magnetron sputtering process Physical:RF/DC magnetron sputtering processMain sputtering processes:DC diode sputtering(for conducting targets)RF sputtering(for insulating targets)When energetic ions strike a surface, material is ejected by the transfer of momentum from the ion to the target atoms (akin to billiard ball collisions at the atomic scale). This can be conveniently achieved in a low pressure glow discharge of an inert gas such as argon.In such a process the target material is made the cathode and is raised to a potential of several hundred volts. Electrons leaving the cathode stream out into the gas phase where they can impact with argon atoms, ionizing them. The positively charged argon is then accelerated to the cathode where it impacts and sputters away material.The sputtering yields of different elements for given impact conditions do not vary very much so target alloy compositions can be maintained in the coating except in cases where there are large differences in the atomic weights of alloy constituents.
17 Mostly used for low deposition temperatures 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 sputteringMetal oxide such as aluminum oxide, copper oxide, iron oxide etcMetal nitrides such aluminum nitrides, titanium nitrides etceasy to dope simultaneously during growth.
18 Numerous materials: Systems of choice- Our Target: High solar absorptance (~ 0.95 or more) and low emittance (~0.05 or less) for high tempe-rature applicationsSystems of choice-Aluminum nitride (AlN) based cermets coatings using RF/DC sputteringStable at high temperature (> 500 0C), radiation resist, high absorptance and low emittance
20 Chemical: Sol-gel process Disadvantages Advantages Film quality is not comparable with physical processHeat treatment is necessary to develop the desired material stoichiometry and propertiesAdvantagesLow temperature treatmentEasy synthesis processCan coat complex shapes uniformlyHard particles can be incorporated to increase hardnessCan coat most metals and insulators
21 Numerous materials- Systems of choice- Our Target: High solar absorptance (~ 0.95 or more) and low emittance (~0.05 or less) for moderate temperature applicationsSystems of choice-Chromium oxide (Cr2O3) based cermets coatings using solution processEasy to fabricate, state at intermediate temperature, high absorptance and low emittance
22 Surface engineering by AR Coating with High Solar TransmittanceSol-Gel coating for borosilicate glass based on alcoholic dilutions with SiO2 nano particles for improved abrasion resistanceSolar transmittance of > 0,96 achievedChallenges in production: homogenous and stable coating of long glass tubes automated high precision solar transmittance test for long glass tubesOnly glass: = 92%With AR-coating : > 96%
23 ConclusionsSolar 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.
24 Acknowledgement Prof. Rajiv Shekhar (a driving force) Dr. Laltu ChandraMr. Ritesh PatelFunding agency- MNRE