There has been an interesting discussion going on for the past few days in the Yahoo Solar Heat Group about the best type of black paint to paint a solar collector absorber with. Most absorbers are panted with flat black paint, but some arguments have been made for glossy paint being a better choice -- see this Solar Heat thread for the details....
In the test described below, I took some 6 inch square samples of thin aluminum and painted them with flat and gloss paint. Then pointed them at the sun, and using surface mount thermocouples stuck to the back of each panel, measured the temperature at the center of the panel. Idea being that the panel with the highest temperature is either absorbing more radiation or emitting less radiation (or some combination of the two) is the best absorber paint candidate?
I also added a sample that was painted with Thurmalox spray paint. This is a product intended for painting collector absorber plates and is said to provide a somewhat selective finish (ie a finish with high visible and near IR absorbency, but low emissivity in the far IR).
Based on John C's suggestion, the test was done for both near direct incidence, and also with the samples set up with an azimuth angle of about 45 degrees to the sun.
I enclosed the samples in a cover that was a good deal larger than the samples and that has a clear acrylic front. The idea was to reduce the variability in the sample temperatures that even very light breezes cause. This seems to have worked in that successive readings were quite consistent.
The bottom line results (at least for this test) is that the flat gives a consistently higher temperature:
For near direct incidence of sun on the 6 by 6 samples, the flat averaged about 7.3F hotter than the gloss sample.
Flat average = 155.4F, gloss average = 148.15F
For the test with the sun at an azimuth angle of about 45 degrees, the flat averaged abut 7.5 F hotter than the gloss sample.
Flat average = 153.58F, gloss average = 147.13 F
The Thurmalox was intermediate between the two -- perhaps closer to the flat
The paint thickness measured in one place on each sample (it no doubt varies over the sample):
Gloss - 0.7 mil
Flat - 0.6 mil
Thurmalox - 0.6 mil
In all cases, the paint appeared to provide a fully opaque coating. My aim in the painting was to apply the thinnest coating possible that was fully opaque.
I'd be interested to hear what people conclude from this, but my tentative conclusion is that I'll probably stick with using the Rust Oleum flat black high temperature BBQ paint.
Just as a side issue, the flat black paint also seemed to provide better adhesion -- I noticed this while removing the paint from a small patch on each sample to measure the thickness.
I'm not quite sure what to make of the Thurmalox results -- I had expected it to have the highest temperatures. I may have applied it in to thick a coat to get the beneficial effects. I have done a small panel test with the Thurmalox -- in that test, it appeared to just a touch better than flat black paint, but the difference was not great.
|Direct incidence test
||Sun at 45 degree azimuth test
These pictures show the test setup:
If you see anything that looks wrong, please let me know.
Samples from left to right are Thurmalox, flat black spray paint, gloss enamel spray paint.
The three paints.
The cover is intentionally leaky to allow a little air circulation, but to keep any light breezes away from the samples.
This picture from the previous day test shows how the surface mount thermocouples are stuck to the back of the samples.
The thermocouples come into the TC switch at the left, and the temperature values are read by the Onset Computer logger (little white) box, and then read on the little laptop. The switch is needed because the logger only reads one thermochouple input at a time.
So, readings were taken by switching to each thermocouple in succession -- cycling through all three takes about 10 seconds. The cycle was repeated about 10 times to insure consistency. While the average temperatures drifted up a bit during the test, the differences between flat and gloss were quite consistent.
The samples and enclosure rotated to produce an about 45 degree sun azimuth angle.
Air temperature inside the enclosure was about 93F. Ambient temp started at 60 F and was rising during the test period.
The sky was nearly clear with some high, thin, wispy clouds.
|Comparison of absorber paints|
|Gloss = Rust-Oleum Gloss Protective Enamel 7779 Gloss Black|
|Flat = Rust-Oleum Flat Protective Enamel 7776 Flat Black|
|Thurmalox = Thurmalox Solar Collector Coating 250 Selective Black|
|Weather was quite clear with some very thin lacey widely dispersed clouds, Tamb 60F at start|
|Air temperature in enclosure was 90F near end of test.|
|Near direct incidence test|
|45 degree azimuth test|
September 27, 2010