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Research Papers

Infrared-Reflective Coating on Fused Silica for a Solar High-Temperature Receiver

[+] Author and Article Information
Marc Röger

Plataforma Solar de Almería, Institute of Technical Thermodynamics, German Aerospace Center (DLR), Apartado 39, E-04200 Tabernas, Spain

Christoph Rickers, Frank Neumann, Christina Polenzky

 Fraunhofer Institute for Thin Films and Surface Technology IST, Bienroder Weg 54E, D-38108 Braunschweig, Germany

Ralf Uhlig

Institute of Technical Thermodynamics, German Aerospace Center (DLR), Pfaffenwaldring 38-40, D-70569 Stuttgart, Germany

J. Sol. Energy Eng 131(2), 021004 (Mar 24, 2009) (7 pages) doi:10.1115/1.3097270 History: Received August 28, 2007; Revised March 17, 2008; Published March 24, 2009

In concentrating solar power, high-temperature solar receivers can provide heat to highly efficient cycles for electricity or chemical production. Excessive heating of the fused-silica window and the resulting recrystallization are major problems of high-temperature receivers using windows. Excessive window temperatures can be avoided by applying an infrared-reflective solar-transparent coating on the fused-silica window inside. Both glass temperatures and receiver losses can be reduced. An ideal coating reflects part of the thermal spectrum (λ>2.5μm) of the hot absorber (1100°C) back onto it without reducing solar transmittance. Extensive radiation simulations were done to screen different filter types. The examined transparent conductive oxides involve a high solar absorptance, inhibiting their use in high-concentration solar systems. Although conventional dielectric interference filters have a low solar absorption, the reflection of solar radiation, which comes from various directions, is too high. It was found that only rugate filters fulfill the requirements for operation under high-flux solar radiation with different incident angles. A thermodynamic qualification simulation of the rugate coating on a window of a flat-plate receiver showed a reduction of almost 175 K in mean window temperature and 11% in receiver losses compared with an uncoated window. For the configuration of a pressurized receiver (REFOS type), the temperature could be reduced by 65 K with slightly reduced receiver losses. Finally, a 25μm thick rugate filter was manufactured and optically characterized. The measured spectra fitted approximately the design spectra, except for two absorption peaks, which can be avoided in future depositions by changing the deposition geometry and by using in situ monitoring. The issue of this paper is to share the work done on the choice of filter type, filter design, thermodynamic evaluation, and deposition experiments.

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Copyright © 2009 by American Society of Mechanical Engineers
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References

Figures

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Figure 1

Pressurized high-temperature receiver (REFOS)

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Figure 2

Geometric receiver configuration (flat plate) and coating operation principle

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Figure 3

Index profiles of conventional (left) and rugate filter (right) in comparison

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Figure 4

Reflection spectra of conventional (left) and rugate filter (right) in comparison. The apodization leads to a suppression of ripples.

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Figure 5

Absorptance, reflectance, and transmittance spectra of the uncoated GE 214 substrate, TCO filter system, and conventional and rugate interference filters for a flat receiver under diffuse radiation. Glass side, left and coating side, right. Filter details, see Table 1.

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Figure 6

Solar spectrum weighted absorptance, reflectance, and transmittance; glass side

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Figure 7

1100°C-blackbody spectrum weighted absorptance, reflectance, and transmittance; coating side

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Figure 8

Thermodynamic model of REFOS receiver

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