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PaulG

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Reply with quote  #11 
I'm just starting out with solar air. I'd discovered Desoto Solar's page before finding this forum, so had been thinking about slats/airfoils before hitting on aluminium screen. I've done a couple of basic small boxes, each 120x60cm (4' x 2'), 10cm (4") deep. Both are open across the bottom front. Neither have fans at this stage — purely passive air flow. Both are glazed with twinwall.

One has a three layer screen (dark grey aluminium), standard position (leaning forwards at the top, so air rises up the front and has to pass through to get to the outlet). The other has a commercial venetian blind (charcoal grey 25mm (1") aluminium slats) attached to the top (forwards position) and the bottom (at the back), again so the air has to pass between the slats to reach the outlet. I've set the tilt of the slats so the blind is almost closed (no light gets through), but the gap between them at the overlap is about 2mm (1/16").

I don't have a good setup for comparative output, but my initial measurements show the boxes give about the same temperature, with the venetian perhaps being a few degrees better. Using a plastic bag to measure flow, the venetian appears to have a slightly higher flow. So all in all it appears the venetian blind setup is promising, perhaps better than a three screen.

Note that I haven't painted either box black yet — I'm interested in isolating what the internals contribute, rather than the black box (again, the principle of heating the air, rather than the box). That might be why the venetian does so well - no light reaches the back of the box, as opposed to quite a lot (50%) getting through the 3 screens.

The venetian slats are a shiny dark grey, so I could probably improve it by painting them a flat black. I do have reservations about the venetians in terms of longevity though — will the venetian blind housing perish in the heat? (it's plastic). I could probably remove the housing and just use the blind part, but it would be fiddly. And more importantly, will the string/cord complex that holds the slats in position perish in the heat? If the string breaks, it will just be an almighty mess of slats in the box. Could I make a more durable framework to hold the slats in position? Something easy, ideally something off-the-shelf, since it would be enough of a job just to transfer all the slats into the framework.

In the end, it might all be a moot point — although the venetians setup shows (possibly) superior performance, it probably wasn't enough of an improvement to warrant the extra work. Screens setup wins with simplicity.

Next step is probably to change the three-screen to a variant of the zero-pass.

Paul G
Hobart (Tasmania, Australia).

gbwillson

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Reply with quote  #12 

Hi Paul-

Welcome to the forum! Interesting experiment with the innards of a solar heater. A solar collector box as small as a 2'x4' box might not give you definitive answers as to comparative performance, it should be able to quickly and inexpensively eliminate any non-contenders. As far as painting the inside of a box, remember that the back of the collector box also has a role to play in the overall performance of a collector. It is true that a collector absorber that prevents any sunlight from reaching the back of the collector. But any sun that hits the back of the collector through layers of screen also hits the screens a 2nd time. So the sun entering a 3 screen collector actually has to pass through 6 layers of screen before exiting back through the glazing. Or as per your example, 50% of the sunlight gets through the 3 screens on the way to the back surface of the collector. But that also means that 50% will be absorbed that reflected off the back meaning virtually 100% of the sun’s rays entering the collector box are absorbed by the screen mesh. 

But while you are trying to compare the difference in the performance of the various absorbers inside a collector you may want to consider painting the inside of all of the collector test boxes. The reason being is that some collectors use the heat absorbed by the black paint of the back of the collector more than others. For example, a three screen absorber can catch the suns rays both entering and exit the collector box. That’s effectively 6 layers of screen for the suns’ rays to be absorbed by the screen mesh. Not much solar energy will be lost after having to pass through 6 layers. Whereas an absorber made up from mini blind slats effective prevents virtually any solar energy to hit the back of the collector box. Yes, heating the actual collector box is not the goal, heating the absorber is. But when it is frigid on a winter morning, such a cold box will not run properly or at all until the interior of the solar heater box warms up. My suggestion would be to consider making a third test box, painted flat black, with no absorber of any type. This can be your control unit when testing various absorbers. 

A 2x4 passive collector box is a not lot of space for proper testing, especially being passive. But it is inexpensive, easy to build test unit. Once you have narrowed down possible top performing absorbers, you might consider building larger test units to give a more accurate indication of top performers as small test units can have very small, subtle differences in test temperatures that even slight variances during the build may enhance or negate changes to the absorber.

Again, welcome! And thank you for experimenting and sharing. The discovery, or chance of discovery is what makes this exciting!

Greg in MN[wave]

PS[tongue]lease forgive the rambling as I was quite distracted while writing and I was too tired to do any editing before posting.

Rick H Parker

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Reply with quote  #13 
Or as per your example, 50% of the sunlight gets through the 3 screens on the way to the back surface of the collector. But that also means that 50% will be absorbed that reflected off the back meaning virtually 100% of the sun’s rays entering the collector box are absorbed by the screen mesh.

If 50% of the sunlight gets through the 3 screens on the way to the back surface of the collector because the screens block only 50% of the total ares. Then 50% of 50% gets through on the way from the back to the glazing for a net of 75% adsorption.

If the back is black, 50% get adsorbed by the screens and 50% by the back plate for a net of 100%.


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Rick H Parker
Kansas, USA
Electronics Engineering Technologist
gbwillson

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Reply with quote  #14 
Yup, I forgot to add a sentence about the back absorbing a percentage of the incoming light in additions to the screen layers. The point being that any sunlight entering the collector has virtually no chance of exiting. 

Greg in MN
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