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stmbtwle

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Reply with quote  #31 
Well it's INTENDED to act as "glazing', but in my limited experience it doesn't work like that. If the space in front of the screen is HOTTER, what's the point? That hot air is against the twinwall and IMO would INCREASE heat loss. You are always losing heat through the glazing, so it should be cooler if the screen is doing it's job.

An aluminum backsheet isn't going to help. Aluminum is a conductor, but where do you want to conduct that heat to? You want the heat to remain right there on the surface where the air can pick it up. A sheet of Thermoply would be better (cheaper too). Now if you are flowing air behind the backsheet as in a dual pass, aluminum would be good.

I am not impressed with the rear screen. It allows sunlight to penetrate to the backsheet heating it up, but at the same time keeps the airflow away. This seems to me to be counterproductive.

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Willie, Tampa Bay

gbwillson

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Reply with quote  #32 
Wille-

I wouldn't disagree a bit with your thoughts regarding the rear screen being raised off the back surface versus flat against the back. You could be right. We usually double up the back screen layer to reduce the amount of sun passing through. Using solar screen would work too. The space behind the back screen could be small, and if space inside a collector were tight, I'd have no problem eliminating it altogether as opposed to the space between the screens or the space between the upper screen and the glazing being too small. Being that a little bit if air DOES move outside the screen gap, that means there is bit of turbulence in and out of the screen gap, which couldn't occur if the screen were placed directly on the back surface. And as you had mentioned previously, a small gap under the back screen may add a bit of insulating value.

I'll have to see if we have time to test that out, or if the test is fair as our test box has no insulation. I'm sure Craig wants his driveway back as fishing season approaches.

Greg in MN
stmbtwle

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Reply with quote  #33 
Oh yes the fishing season! I just wish the CATCHING season would get here!

It just seems to me that when I raised my back screen off the backsheet and replaced it with standard (as opposed to shade) screen my production went down. It wasn't a scientific test though and there were too many other variables. It will be interesting to see what your tests show.

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Willie, Tampa Bay
Rick H Parker

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Reply with quote  #34 

the better they are at breaking up the laminar flow “

 

Break up the laminar flow or or separate it from the screen?


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Rick H Parker
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gbwillson

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Reply with quote  #35 
Hi gang-

It's done! Maybe a half hour to tweak a few things and attach the fan and such. But as you can see, we finished up too late in the day for any tests. And the current forecast is for cloudy skies with a chance of showers the next two days. Not a big deal as Craig is picking up his boat tomorrow anyway. But he wanted the test collector out of the driveway before the boat came home. Anyway, when the skies clear, we'll be ready...

Greg in MN


Below are the three test channels from left to right: 1", 1.25", and 1.5" high.
IMG_1096.jpg 

Plastic cover is attached with minimal staples until the film relaxes a bit before the final stretch and seal.
IMG_1097.jpg 

gbwillson

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Reply with quote  #36 
Hi gang-

We got a late start here in testing day as you can see the skies are pretty cloudy. It cleared up nicely as the afternoon moved on. 

Anyway, right around solar noon we started running our tests. The results between the 3 channels were as expected, with the exception being the large margin of victory. The best screen gap produced 30% more BTU's than the worst! Anywho, tomorrow we hope to test the current winning screen gap and tweak it by ⅛" to see if the results can be improved even further. 

The forecast tomorrow is for cloudy skies, but the three days after that look to be sunny. Hopefully I'll have some final numbers for you very soon.

Greg and Craig in MN

IMG_1098.jpg 

KevinH

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Reply with quote  #37 
Thursday is supposed to be sunny.

How well did the air flow balance out?

Is it easy to adjust the screen gap in the test box?  If so, a final test could be to set all 3 channels to the same optimal gap.  That way you could be sure that the channel location in the box was not a significant factor (all 3 should give about the same result).

Kevin H
MN
gbwillson

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Reply with quote  #38 
Kevin-

The airflow was surprisingly quick and easy to balance with two people. One person read off the output speeds of the three channels, and the other tweaked the "special" intake dampers until flow was balanced. It only took maybe 8-10 attempts to balance the three within 1/10 of a MPH. Several years ago when I tried to balance two collectors for testing I was using the fans to try match the speed. Very frustrating and time consuming. There were a lot of times where the adjustment took 45 minutes. The fan speeds would waver slightly or a gust of wind hits the face of the panel. The fan speed controls were VERY touchy and the fans themselves were soft start so you had to allow a moment for any fan adjustment to stabilize. But in our case we have only one fan, three channels, with only two of the channels needing to be adjusted to the third channel. Easy-Peasy.

Unfortunately, the screens are not so easy to change. Each channel is just too narrow for an easy change. And you would need to replace all the screen material. Even with the ⅛" adjustment, we may be splitting hairs, there may not be a measurable difference. With only 9" from side to side, there is no issue with screen sag. But over a wider distance, there could very easily be a ⅛" sag. Kinda hard to plan on a specific amount of screen sag as a way to create the desired gap. 

Greg


Rick H Parker

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Reply with quote  #39 
"With only 9" from side to side"

I cannot get three 9" channels to add up to 48"  Unless you are using 2.5" width screen mounting rails. From your pictures your screen mounting rails are clearly not 2.5".

Lumber is 1/2" smaller then the size described, the 2 in 2XL is 1.5" A 2X4 is 1.5" X 3.5".

48" - 4*1.5" = 42"
42"/3 = 14"
14" - 9" = 5"
5"/2 = 2.5" screen mounting rail width.

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Rick H Parker
Kansas, USA
Electronics Engineering Technologist
KevinH

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Reply with quote  #40 
Greg,
Looking forward to the final results and winner.


Rich,
The test box is narrower with 9" channels.  The references to the 4 foot width in this thread were for their completed 4 x 16 ZP collectors (like the one shown on the roof in the photo).

Kevin H
MN
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