Solar Mirrors: How Engineers Measure Mirrors

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Solar Mirrors How Engineers Measure Mirrors

• Terminology used
• A little optics
• Standards
• How it all relates to solar energy application

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Specular Reflectance

• Refers to reflectance as a function of wavelength
• In aluminum lighting market, the spectrum used is
  CIE (human eye response) visible light only
• In solar market, the spectrum used is the
  standard solar spectrum, often called the air
  mass 1.5 spectrum or AM1.5

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FROM WIKIPEDIA
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How Accurate Do Mirrors Need To Be?

• The suns disk appears to us on earth to be a
  light with a cone angle of plus or minus a
  quarter degree
• Mirror accuracy requirement depends on what kind
  of solar collector you are trying to make

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Parabolic Troughs Mirror Accuracy Requirements

• Depends on operating temperature requirements
• Typically want the mirror to make only small
  effect on the image at the receiver
• Goals of concentrator designers is typically to
  have total error under 10 mrad for high
  temperature systems
• Difficult to do with aluminum mirrors
• US DOE goal is lt7 mrad

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Aluminum Mirrors

• Typically have surface roughness on the order of
  10-30 mrad
• Depends on treatment of mirror, polishing,
  whether vacuum coat process used

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Angles for Typical Solar Trough
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What is AM1.5?

• The suns spectrum changes over day due to air
  absorbance
• Example sun appears orange at sunset
• Coloring is due to the atmosphere
• The standard air mass 1.0 is the spectrum you
  get at sea level with the sun directly overhead
• The air mass 1.5 spectrum is solar spectrum
  with sun at lower angle so light goes through 1.5
  time more air
• This is a typical solar spectrum over day

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Air Mass 1.5
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What is Wavelength and Light?
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What is the Suns Spectrum?
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What Causes Dips in Sun Spectrum?
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Total Hemispherical Reflectance What Does It
Mean?

• Total hemispherical reflectance is the
  reflectance of light off of a surface collected
  at all angles, - 90 degrees
• A 100 white object can thus have a total
  hemispherical reflectance of 100 and not look at
  all like a mirror
• Standard measurement equipment measures over a
  full hemisphere
• Doesnt tell you quite enough information about
  how mirror-like surface is, but still is used

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ACA Mirrors

• Lowest cost type does not use vacuum processes
• Somewhat lower reflectivity than other aluminum
  mirrors
• Still very useful for many applications
• Here is how our lowest cost mirror data looks
  relative to a somewhat higher cost vacuum
  deposited mirror

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ACAs Lower Cost Protectal Mirror

• AM 1.5 total hemispherical reflectance is around
  87-88

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Recent results-NREL Specula Reflectance with
Total Hemispherical Reflectometer Note
Consistencies of measurements is good
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What about specular reflectance of low cost
Protectal mirror?

• Now, we need to talk specifics of all the various
  specular reflectance standards
• Some standards use very narrow angle of
  measurement, on the order of 1 degree
• Other standards use larger angle of measurement,
  on order of up to 10 degrees
• Look at specific standards now

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Recent results-NREL Specula Reflectance with DS
Meter Note Inconsistencies of measurements
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One Type of Specular Measurement The DS Model 15
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Reflectance MetersDS Model 15 Used By NREL
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Competitor AlanodData Showing Many Reflectance
Measurements-Same Material
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