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cwwilson721

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Reply with quote  #41 
I was thinking something thin that wouldn't add thermal mass, as opposed to actual 'angle', that would add thermal mass, would be best (something along the lines of the fins for the Solar Hot Water tube collectors)

The 'inverted v' would actually capture the solar heat and transfer it, rather than reflect and lose it, so should be black. If mylar coated, the heat would be 'lost' Remember, the airspace around the tubes doesn't move much, and that reflected heat potential could be lost thru the glazing if reflected, rather than absorbed. Also, the 'v' would have to be physically connected to the downspout tubes.

The v fins would also be a greater collector area than the panel-back material. Ex: 2 pieces of 2" material, connected at the top at 90 degrees, (has 4" of area). The lower leg (the 'panel backing') would be 2.82" (a sq + b sq = c sq). You would effectively add approximately 50% of the absorption area. So, lose the backing.

The fins need to be 'raised' enough to catch as much light as possible, but angled and low enough not to block the light from hitting the actual DS, thus why I showed the 'lower' connection. If spacing allows it, all the way on the bottom is best. But, if the DS tubes are too close together, and get "shaded" by it's neighbor alot, mount the fins higher, and change their angle.

So, yes. The angle would be determined by the spacing between the tubes. Total absorber size would increase, but how much is also determined by the tube spacing.

cwwilson721

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Reply with quote  #42 
Holy Cow! BRAINSTORM!

Forget the DS's all together.

The 'bottom' of the collector could be all 90 degree v's, and push the air thru THEM, or thru the v's AND the airspace.

Imagine a 7.5'x8' absorption area in a 4'x8' box.

Pros: Real easy to construct, no "shadowing" the v next to it (no shadows, greater heat gain), can make the collector MUCH thinner.

Cons: Don't know. Would it be more efficient than the DS? I know you'll REALLY improve on the backpass and "blank box" in comparison...

Testing time!
gbwillson

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Reply with quote  #43 
Think of a quality furnace filter. The corrugated filter greatly increases the surface area of the filter in a given space.

Greg
solardan1959

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Reply with quote  #44 
cwwilson721,
   We have talked about doing that before and the upside down gutter was a variation of that.
Solar Web Page > Forums > General Discussion and Chat Page > How big should my plenum be in a downspout collector? 

  There also is a manufacturer that makes a 1/2 circle tub that snaps together to make an absorber, cant find it right now but it kind of looks like the one below.
http://www.siliconsolar.com/shop/solar-store/solar-hot-water-heaters/solar-hot-air-heaters/

or these "shutters" can go on each side of a window for a nice clean stealthy look.
http://www.solarairsystems.com/images/solar%20air%20heater1.jpg

But a corrugated sheet of tin will do the same thing a whole lot easier.  Sorry I did not see your original drawing and was responding to Greg's post but it is nice we are all thinking along the same lines.

(also read about your honeycomb but think it is too expensive, along with the rock wool)
Dan
cwwilson721

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Posts: 303
Reply with quote  #45 
I have GOT to get my bending brake built...

In reference to my post#42 above, I have some perforated sheets just sitting here...Make them 'corrugated', blow air thru it/over it...

But that's another discussion.

I REALLY think the V Fins will help with gathering heat on the DS collector...

But, what would be the best method of attaching them? Preference would be a continuous attachment, but that would be a bugger...

Screws, I believe, while they would give a great mechanical bond, and heat conductance, might interfere with airflow.

Staples, maybe? Pretty good bond, and could be thin enough to not disturb the airflow too much.

Best would be solder/weld joint, but I shudder at the labor of that..

Probably silicone caulk and staples would be 'best', at least for DIY.

I also believe the inverted vee, as a continuous unit, compared to fins 'meeting' at the apex, might be better for heat conductance, too... 

 
cwwilson721

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Reply with quote  #46 
Quote:
Originally Posted by solardan1959

...

(also read about your honeycomb but think it is too expensive, along with the rock wool)
Dan


I'll answer that in the threads those are in...I hate hijacking threads...lol
Rick Stone

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Posts: 121
Reply with quote  #47 


But, what would be the best method of attaching them? Preference would be a continuous attachment, but that would be a bugger...

Screws, I believe, while they would give a great mechanical bond, and heat conductance, might interfere with airflow.

Staples, maybe? Pretty good bond, and could be thin enough to not disturb the airflow too much.

Best would be solder/weld joint, but I shudder at the labor of that..

Probably silicone caulk and staples would be 'best', at least for DIY.

I also believe the inverted vee, as a continuous unit, compared to fins 'meeting' at the apex, might be better for heat conductance, too... 

 
If you are using metal track as your manifold you could cut the inverted v up the middle about an inch and bend it at rt angles. You would then have two tabs that could be pop riveted to the face of each of your manifolds and go from end to end.
cwwilson721

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Reply with quote  #48 
Pop rivets...Didn't even think of those...

Would work on the downspout, too, not just the metal track.

Drill, pop rivet, move along.

BUT, then you'd almost need to install the entire assembly all at once...

But, if you dryfit, check layout, dryfit, check layout, dryfit...THEN check again, should be ok.

The vee would need to look like this:
        /\
       /  \
      |    |

so you'd have a good area to pop-rivet to the DS tube

Just make sure it's aligned before you pop-rivet. After it's all together, you can squeeze the assembly together to get it in the case, but you can't make it lay flat...
JoeK

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Posts: 135
Reply with quote  #49 
Have you seen this absorber design by Morris Dovey that he devised for high efficiency passive panels? http://www.iedu.com/Solar/Panels/

I think it has some unique advantages, which he explains well.  Particularly that photons reflecting between the fins create a zone of increased flux density that increases the heating of the air itself, as well as the fins.  Unfortunately this benefit does not translate the same to a DS collector because the air is segregated inside the DS. I think you would still see an increase in absorption as the fins would both conduct heat to the DS and reflect additional photons onto the DS. 

I was cutting some corrugated siding not long ago when I just happened to end up with a scrap piece that looked like this.
collectorfin.jpg  collectorfin2.jpg 
The light bulb went on. By cutting straight along each ridge (or valley) a corrugated roofing sheet could be cut up rather quickly and easily to form ~20 fins 4ft each. No bending, and thus no brake, needed.  I'm not sure about optimum spacing or how many fins you would need to fill a collector, but it sounds a heck of a lot easier than bending them out of flashing or what have you. It looks darn near perfect for Dovey's design. Might be a bit "rounded" for your DS idea, perhaps the smaller/finer corrugation sheets would be better?  I believe the size pictured is sold as 2-1/2" ribs.

All that said, I think you could be barking up the wrong tree in trying to fill up "waste" space between the DSs. Plus attaching those fins sounds like a couple hrs of tedious work.  That space is not exactly waste as I see it, unless the DS are spaced too far apart. A small space allows light to hit the sides of the DS, where if not absorbed it will still reflect into the neighboring DS, or onto the backing and (depending on backing material, sun angle etc.) then to the neighbor DS.  Also a slight spacing between DS allows the increased absorber area along the sides to take greater advantage of increased heat transfer surface area around the sides of the DS. As we well know there is an optimum spacing of DS that allows light to hit the sides at certain times of day, yet does not allow many photons to pass all the way to the backing.  By placing the  V shaped fins in that space I think you might make some gains by more efficient reflection (excluding the backing from absorbing light), but you also are forcing a lot of heat to tranfer via conduction first through the fins,then to the DS, and finally reaching the air inside. I'm no expert, but it seems like you may be better off (or just as well for much less work) focusing on optimum spacing so that light does hit the sides of the DS, but rarely the backing.

I'm glad your willing to put in the extra work to design and make a more efficient absorber though, otherwise we might all still be using those old crummy backpass units.  so I hope you do try it out and report back!

cwwilson721

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Posts: 303
Reply with quote  #50 
My thinking on the inverted V fins is two-fold:
  1. The angled fins would allow greater surface area on each DS with sun hitting it. Where it is mounted is crucial. You don't want to 'shadow' the DS next to it, and you want as much surface area exposed as much as possible to absorb and conduct heat into the 'shaded' side of the DS it is connected to.
  2. While the 'backplate' can, and does, collect and transfer heat, alot of it is totally shaded, and thus, not real efficient. With the inverted V, you avoid the shading, plus have a much more efficient angle to capture the maximum amount of energy possible (angle of incidence).

This might help:
incidence.jpg
This is just an example. I could pull out the physics, but the basic thought is correct. 

Also, with the inverted V, you have more area to absorb the solar energy, no matter the angle, as compared to the flat backplane.

Example: At 60 degree V, and 2x2" sides, the inverted V has 4 inches of exposed material, as compared to the flat backplane, that has 2".

Now, I gots me a head achin'.

"Ah, Math..Tho art a heartless <b-word>..." - Sheldon Cooper, "Big Bang Theory

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