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solardan1959

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Reply with quote  #21 
Jimmy,
Quote:
And my idea was that it just blocks some light to be able to touch the surface of the absorber


forget about the back absorber, the two layers of screen would be the absorber. focus on the gap between the screens, (or close the gap between the screen and your current absorber).  You never did specify what that gap was now, I just guessed at three inches.  If this is too wide the air will just flow between them and not properly scrub the heat.  You could fairly easily build a 1 to 2 inch wooden frame that fits in your collector that fits in the width and is just short of your inlets.  Then stretch your screen on that and then lay that on the back of your collector for the second layer.  A piece of flashing, (thin piece of metal) could be used to deflect the air when it comes into the collector and force the flow towards the two screen channel.
You may find your current collector meets your needs even with the small vents but of course bigger is going to provide more heat.
Dan

gbwillson

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Reply with quote  #22 
Jimmy-

By far the easiest way to measure airflow is with an anemometer. They are as cheap as $15. 

As Dan mentioned, the screen IS the absorber. The air passes between the screen layers, so in effect, there in an absorber both above and below the air moving through the collector. As far as having too much air, you can always reduce the CFM output by using a fan speed control or reducing the amount of intake air. In any case, having too much air is easier to correct than having too little. 

As far as insulation, stay away from using foil covered wood, as the wood will dry out. Fiberglass would be a better choice, Also stay away from polystyrene, as it will melt at fairly low temps. If you are still not sure, show us what insulation is available to you locally, I'm sure we can find the best one for your build.

Greg in MN
Jimmy

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Reply with quote  #23 
Thanks gents!

Yes, I haven't specified the air gap between the layers. It is about 1cm. (3/8")
My calculation was to have the same area between the 2 layers as the inlet area.

So now I feel that I've nearly 'destroyed' it by removing the lower layer. [frown]

And yes, I've polystyren under the 3mm wood (MDF) backplate which is under the reflector foil.
I'm searching for polyiso but here the common inulation is EPS, XPS and rockwool.
I've found rockwool with AL layer on it:
rockwool.jpg 

But if the Al layer doesn't matter I can buy normal rockwool anywhere.


gbwillson

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Reply with quote  #24 
Jimmy-

The foil coated rock wool should work. I would worry about the dust from the uncoated rock wool. You CAN use polystyrene as long as there is a layer of the rock wool or polysio between the heated area and the polystyrene. The biggest problem of the rock wool is you will need to add a few hard mounting points for thermometers, switches, etc. Small blocks of wood that has been painted would work. I've seen several collectors that have been mounted directly to the side of the house that have NO insulation on the back since the walls of the house are already insulated. In that case, only the sides need insulation.

As far as the "best" screen gap between the layers there isn't one. Or should I say that if the gap is too small in relation to the ducting, the collector won't breath well due to the restriction. The velocity plays a big part too, since the higher the velocity, will have a tougher time moving easily through a narrow screen gap. If the screen gap is too large, the air passing through may be too far from the heated screen surfaces to warm the air efficiently. So if you are using 4" ducting, which has an area of roughly 12.5 Sq in, the screen gap should be an area of no less than 12.5 inches. Depending on the actual width of your screen gap(45", for example) the ⅜" gap would work since the area of the HxW would be roughly 17 Sq in. 

On the chance you end up needing to increase the ducting to 6", the same rules apply. So you may want to not only have a simple way to open up the collector, but a way to change the size of the gap should it be needed. My current ZP has a 1" gap, with a 45" width. My next ZP will have an 8" gap, but I would likely increase the gap to just over 1". 

Greg in MN
solardan1959

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Reply with quote  #25 
Jimmie,
  3/8 is too narrow of a gap and will impede the flow.  yes that may be close to equal for your inlet size but the two layers of screen that close will cause a lot of resistance to the flow.  Regardless of the inlet size,  (if you opened it up to a 6 inch vent you more than double the flow), The screen gap is going to have to be closer to an inch and actually sounds like it should be your goal.  I actually figured you had too wide of a gap before, especially when you removed the back layer. If you don't have that deep of a collector you can lay the screen on the back wall to get as much spacing as possible.  Some insulation should be used on the back of the collector, that stuff shown looks pretty good.  If the normal rockwool is more available and you want to save money you could just lay your current foil/absorber over the normal rockwool.  But now it seems like you may not have the depth to install this stuff, maybe you should just put the normal stuff behind the wood back.
Have you heard of the plastic bag method of measuring airflow?

From Scott Davis's post under Solar Hot Air Collectors > Airflow targets and measuring in a hot air collector.

An anemometer will give you the most accurate results, but here is an inexpensive method that will give you a reasonable estimate of the cubic feet per minute (CFM). This spreadsheet will do all the calculations below for you: http://www.n3fjp.com/solar/AirflowBagTestCalculator.xls:

1. Get the largest trash bag that you can (we will assume 30 gallons for this exercise)

2. Quickly put the bag over the outlet of your system and time how long it takes to fill it

3. Divide the gallons by 7.48. There are 7.48 gallons per cubic foot, so a 30 gallon trash bag would hold 4 cubic feet of air volume (30 / 7.48 = 4)

4. Divide 60 seconds by the number of seconds it took to fill the bag and then multiply by the volume

If it takes 3 seconds to fill our 30 gallon bag, the air flow is calculated as follows:

60 seconds / 3 seconds to fill X 4 cubic feet of air volume = 80 cubic feet per minute
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Dan

Edited, Sorry, was doing this about the same time Greg posted and some of the information overlaps and may appear to disagree.  I also worried about breathing rockwool fibers but figured the current metal absorber would seal that, but may not be good enough to do that. I still feel the 3/8 provides to much surface resistance regardless of the flow area being the same of the input. (but I have never built one that narrow and Greg has experimented with different gaps and has built more zero passes)

gbwillson

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Reply with quote  #26 

Dan/Jimmy-

I changed my mind about Jimmy's collector and the ⅜" screen gap several times before posting. My reasoning is this...if his collector is roughly a 6'x6' square, that ⅜" screen gap would give him a screen gap area of roughly 25 square inches, which is double the area of a 4" duct. I think at that thin size, the gap will become a bottleneck, restricting a high velocity flow. But.....With the entire collector only 6' long, minus any space for the two manifolds, very high velocity would not be effective as the air passing through at high velocity wouldn't have time to heat up. Hence, a lower velocity would have to be used, in which ⅜" isn't too restrictive.

When I built my prototype ZeroPass several Winters ago, the screen portion was 4'x8' and when I reduced the screen gap down to ¾" at high velocity, the air flow "choked". But when I reduced the CFM's, the air flowed smoothly. It still produced heat at the higher velocity, but it was less than when I reduced speed. And at the lower CFM speeds, the narrower ¾" gap produced more BTU's, likely due to the closer proximity of the screens to the air flow.

In any case Jimmy, allow yourself some flexibility in your design for easy changes, if needed. Nobody has tried a screen gap so small, but at some point the gap will be too small for the amount of air being moved.

Greg in MN


Jimmy

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Reply with quote  #27 
I see my next project.
After ordering an anemometer I'll modify this single collector to be able to change the gap from outside without opening that up. [smile]
This current collector is 1m x 2m.
So I'm thinking about a range of 3/8 to 1" gap between the 2 layers.
I think there is currently 1" between the top layer and the glazing. And not sure about the total depth of the frame but I can increase it or even change it.
The garage has a 6cm insulation on it. So I might remove the one from the back and screw the panel directly to the wall.
I'm happy because I'm full with new ideas so let's play. [smile]
stmbtwle

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Reply with quote  #28 
I'm sure it would work, but as we've discovered that fan speed often needs to change with the amount of heat, you may find that the screen gap needs to change as well. Maybe automate the whole thing with an Arduino?

Hm I wonder if it would be possible to build a flexible screen mount that would open & close the gap according to fan air pressure?

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Willie, Tampa Bay
Jimmy

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Reply with quote  #29 
It seems that I can get some polyiso. But only in small sizes. 1,25m x 0,625m.
Is the plywood a must at the back? I'm asking because of the weight. But how do you fix the insulation without the plywood?
If the lower screen should be the absorber does it make sense to paint the surface of the polyiso to black?
If I get the polyiso I can start with the double size collector maybe next weekend. Just as it is rather cold out there I would skip the painting work if it is not a must.
solardan1959

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Reply with quote  #30 
Jimmy,
   Not just the lower screen but both screens are the absorber.  they both collect heat and as the air travels between them it extracts heat from the screens.  Many times the painting of the back has come up, I have built multiple layer screen collectors so the back could barely be seen and left it highly reflective with mylar.  My thoughts were it would keep the heat off the back and reflect it back toward the screens.  It worked pretty well but that does not mean it would work better than a black background.  other people have tried it and thought the black worked better.  With just two layers and a real shiny back it will reflect some light back out. Does going through essentially four layers of screen capture most of the heat?  I generally paint it black unless I use a 70 + percent solar screen which then it has no way of getting through the screen and back out.  any heat collected by the black back helps warm up the box and the inside air so painting can't hurt.
Plywood is not a must, you could even just have the outside frame and your cement wall as the back of the collector.  It may take the collector a little longer to heat up the cement wall but the screens will warm up very quick still and start heating the air that is outputted into the garage.
Dan
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