Transcript Document 7581402

Can Irradiance be Found
Using Properties of Paint?
Jennifer Quincy, David Miller, Jake Edmunds
Objective:
Verify solar irradiance at
BYU experimentally.
SWKT
CLYDE
WILK
The Experiment
Step 1- Paint hollow steel cubes black and
white.
Step 2- Place cubes in foam insulation with
one face exposed to sun.
Step 3- Allow cubes to reach steady state
temperature.
Step 4- Measure temperature inside of
cubes.
Step 5-Determine the irradiance G from
Energy Balance.
Predictions for April 2007
• Average monthly maximum irradiance in
April at BYU is ≈1100 W/m2
• Mean monthly irradiance in April at BYU is
≈265 W/m2
• We will measure at noon so the irradiance
should be around 1000 W/m2
Theoretical Analysis
Energy Balance
qrad,abs – qrad,emit – qconv = 0
αG – εσTs4 – hconv(Ts - T∞) = 0
About Spray Paint
• Flat White and Flat Black differ slightly
– Flat White 6% Talc, 4% Titanium Dioxide
– Flat Black 4% Talc, .4% Carbon Black
– Everything else in spray paint is VOCs and
solvents that evaporate
• Absorptivity will be different, but emissivity
will be virtually the same (mostly IR)
Assumptions
• Identical and simultaneous setup allows
comparison of the results from the two
colors.
• Foam insulation negligibly participates in
heat transfer.
• Assumed material constants are accurate.
• At steady state, internal air temp of the
cubes is the same as the surface temp.
Material Properties
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αw ≈ .5
εw ≈ .98
αb ≈ .9
εb ≈ .98
These properties are very general; they can vary
widely even within the same brand of paint.
• Only paint intended for aerospace applications
has constant and well-researched properties.
Unknown Values
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Irradiance G
Steady State Temperatures Tw and Tb
Ambient Air Temperature T∞
Free convection coefficient hconv
Actual Setup
• JFSB, 3rd floor balcony, 12 am, 12 April
2007
• Thermocouples
in cubes to
measure temp
Observations
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Steady state was reached in 30 minutes
Light breeze present
Lightly overcast
Cool ambient air
Measured Temperature
• Steady State Temp
– Tw = 21.6°C
– Tb = 35.1°C
• Ambient Temp
– T∞ = 19.8°C
Convection
• For convection (v<20 m/s) we used
h = 10.45 − v + 10√v
• The light breeze was ≈1 m/s
• h ≈ 20 W/m2K
Solution
• G = [εsσTs4 + hconv(Ts - T∞)]/αs
• White
– G= (.98*5.67e-8*(273+21.6)4+20*(21.6-19.8))/.5
– G= 910 W/m2
• Black
– G= (.98*5.67e-8*(273+35.1)4+20*(35.1-19.8))/.9
– G= 900 W/m2
Conclusion
At 12:30 pm, the irradiance at BYU is ≈ 900
W/m2.
We confirmed this with the BYU ESC
Weather Station data.
We verified that the irradiance can be
accurately be found using known
properties of paint.
Recommendations
With a known irradiance, absorptivity of a
different color paint could be found.
Accurately calibrated thermocouples would
yield much greater reliability.
A transparent box could be used to reduce
convection to negligible levels.
Find more accurate values for ε and α.
References
• Absorptivity and Emissivity
www.solarmirror.com/fom/fomserve/cache/43.html
• BYU ESC Weather Station
marvin.byu.edu/Weather
• Krylon Spray Paint MSDS
www.kpg-industrial.com/krylon