Solar Mirrors: How Engineers Measure
Mirrors
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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 Trough
10 mrad Slope Error Receiver Image Width vs vs
Position for Parabola with f=0.91 m
0.06
Image Width at Receiver (m)
0.05
0.04
0.03
0.02
0.01
0
0
0.5
1
1.5
Position (m)
2
2.5
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
Solar-weighted (DIRNOR15) hemispherical reflectance of ACA OverCoat/Anodized Al mirrors
after accelerated exposure in Blue M (dark/85oC/85%RH) and Ci65 WeatherOmeter (WOM)
(~3XOD; 1X light/60oC/60%RH) chambers and outdoor exposure in Golden, CO (NREL)
100
% Reflectance
80
NREL
60
BlueM
Ci65 WOM
40
20
0
0
3
6
9
12
15
Exposure Time (Months)
18
21
24
27
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
25-mrad at 660 nm Specular Reflectance of ACA OverCoat/Anodized Al mirrors after
accelerated & outdoor exposure.
100
25-mradian Specular Reflectance at 660 nm (%)
90
80
70
60
NREL
50
BlueM
Ci65 WOM
40
30
20
10
0
0
3
6
9
12
15
Exposure Time (Months)
18
21
24
27
One Type of Specular Measurement:
The D&S Model 15
Reflectance Meters
D&S Model 15 Used By NREL
Competitor: Alanod
Data Showing Many Reflectance Measurements-Same Material