|Some more tests/observations on using regular window insect screen
for the absorber in solar air heating collectors.
The new items tested
We had a fully sunny day today -- one of not very many this winter.
The two test solar air heating collectors were in their collection positions facing roughly south at a tilt angle of about 70 degrees. The fans were not hooked up, but the ducts leading into the shop were open on the shop end, so there was just a little thermosyphoning going on - but not much.
I noticed when I walked into the shop a sort of burning plastic smell. On closer look it the smell was coming from the air collector ducts that lead into the shop. It was pretty strong when you got right near the ducts.
I went out to look at the collectors, and its very apparent that some deposits have been left on the inside of the glazing. I assume these are from the screen that was reaching near stagnation temperatures and out gassing stuff that then condensed on the glazing as a sort of scummy coating.
Here are some pictures of it -- click for full size
I took the glazing off of one of the two collectors, and the material is liquidy and wipes away with a finger easily. It is spread over nearly all of the inside surface of the glazing.
There is no apparent degradation of the screen itself, but it has only been exposed to this a few days.
My guess is that the odor and residue is coming from the black plastic coating that is applied over the fiberglass strands, as it seems like the FG should be fine at high temperatures.
On the 2nd collector, I installed a thermometer about a foot from the top of the collector, and with the probe going in just behind the first of three layers od screen.
Temperature gage -- near stagnation.
Probe on the temp gage extends about a
foot into the collector just under the layer of
screen closest to the glazing.
When the end of the duct in the garage was open and the collector could thermosyphon just a bit, the maximum temperature was 235, and when the duct in the garage was shut off completely for full stagnation, the temperature went up to 255F. This is with a 28F ambient with a bit of breeze.
This was with a temp of about 28F out -- in the summer, the outside temp could easily be 60F higher, which would raise the inside temp as well.
I had come around to the point where I thought that the fiberglass screen was the way to go as its nice to handle, cheaper, and it blocks a little more sun than the aluminum, but I am now thinking that the aluminum is a safer long term bet? Even it it holds up OK over time, I don't much like the idea of breathing the fumes from whatever is happening to the plastic?
Anyone have any thoughts on this?
This is the test Doug suggested with the screen held so that the sun is not coming straight into the screen -- which would be more typical of most of the day.
I just did an eyeball at a 45 degree angle.
I first did the sun straight on test, then did the eyeball 45 degree angle.
The 2 layers of screen with the sun straight on dropped pyranometer reading from 1053 w/sm down to 530.5 w/sm, or 0.503 absorption.
The 2 layers of screen with the sun making a 45 degree incidence angle with the screen was 1050.5 down to 466, or 0.445 absorption.
so, the screen does absorb somewhat more light -- about a 12% increase in absorption -- not as much as I would have expected, but significant.
Setup for 45 degree incidence angle test -- this looks like the 90 deg one that was done first.
Can't remember who suggested this, but the idea was to use two layers of screen, but instead of running both layers so that all the wires were in the vertical and horizontal directions, rotate one of the layers so that its wires were running diagonally.
So, starting with the frame that had two layers of aluminum screen on it, I pulled off the layers on one side, rotated it 90 degrees and stapled it back into place.
2 layers of screen with wires in both layers running vertical and horizontal -- dropped pyranometer reading from 1053 w/sm down to 530.5 w/sm, or 0.503 absorption.
2 layers of screen with wires running diagonally in 2nd layer -- dropped pyranometer reading from 1048.6 dwon to 510 w/sm, or 0.486 absorption.
Only about a 3% difference in absorption.
Not a great picture, but you can see the back layer of screen has been rotated 45 degrees from the front layer.
logger plot for the new two screen tests
Gary February 21, 2011