Solar Mirrors: How Engineers Measure Mirrors

Solar Mirrors: How Engineers Measure Mirrors

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

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

FROM WIKIPEDIA

How Accurate Do Mirrors Need To Be?

• The sun’s 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

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 <7 mrad

Aluminum Mirrors

• Typically have surface roughness on the order of 10-30 mrad

• Depends on treatment of mirror, polishing, whether vacuum coat process used

Angles for Typical Solar Trough10 mrad Slope Error Receiver Image Width vs vs

Position for Parabola with f=0.91 m

0

0.01

0.02

0.03

0.04

0.05

0.06

0 0.5 1 1.5 2 2.5

Position (m)

Ima

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Wid

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at R

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er (

m)

What is AM1.5?

• The sun’s 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

“Air Mass 1.5”

What is “Wavelength” and Light?

What is the Sun’s Spectrum?

What Causes Dips in Sun Spectrum?

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

• Doesn’t tell you quite enough information about how “mirror-like” surface is, but still is used

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

ACA’s Lower Cost “Protectal” Mirror

• AM 1.5 total hemispherical reflectance is around 87-88%

Recent results-NREL Specula Reflectance with Total Hemispherical Reflectometer: Note Consistencies

of measurements is good

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

Recent results-NREL Specula Reflectance with D&S Meter: Note

Inconsistencies of measurements

One Type of Specular Measurement: The D&S Model 15

Reflectance MetersD&S Model 15 Used By NREL

Competitor: AlanodData Showing Many Reflectance Measurements-Same Material