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JoeK

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Posts: 135
Reply with quote  #51 
Haha, math can do that to ya... That's a nice illustration of the benefits, thanks for making the nice visual aid. But if you simply spaced the Spouts closer together, the shaded area would shrink to nil just as you get close enough to shade the neighboring spout. At that optimal space the neighboring spout itself would reflect light into the "lee" side, shady corner. the one that you have covered by the V reflector.  Also this configuration would end up with more downspouts total ie more 3 Dimensional  absorber area.  I'm not about to do that math, but you ARE likely gaining more surface total by using the V reflectors as you suggest instead of moving the spouts closer and simply adding one or two more.  How much difference would there be though? worth the extra work? You would save some cost on DS perhaps but I think you might end up with a higher cost by the time your all said and done adding the V.  More downspouts also should assist with airflow and has the advantage of greater Heat transfer surface as well.

Of course you do not want to place them so close that the spouts shade one another, but just at that limit I believe is optimal spacing and already achieves much of the backside coverage your seeking.

KevinH

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Posts: 534
Reply with quote  #52 
Would you run air under the V?

If I were building a DS collector today, I would not leave a gap between the downspouts.  Over on BIS, Gary did some testing that showed that the heat conducts around to the back and sides.
http://www.builditsolar.com/Experimental/AirColTesting/DownSpoutCol/DownspoutCoFlowDistributionlTest/Downspout.htm
Even if it only goes down the sides, that is 7" of surface on a 3"x2" DS that is heating the air.  As long as the DS surface is hotter than the air flowing over it, there will be heat transfer.  I would leave the bottom of the downspouts unpainted (would try to get the existing paint off down to bare aluminum) to reduce the emissive loss, leave an air gap below the downspouts, and leave the polyiso reflective below the downspouts.  I would also try to round off the top surface using a wood form forced through the downspout (bottom would stay flat).  That would angle the sides in a little and create a narrow V light trap between each downspout.

Until someone does side by side testing on various ideas, however, it is all just speculation (always fun to brainstorm, though).

Kevin H
MN
JoeK

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Posts: 135
Reply with quote  #53 
I think that you would be better off by not inverting the V, and placing it under the spouts something like this.  The cost of the change I see is adding height to the collector, but I think you would see even greater absorber surface than either the inverted V, or no V and closer spouts. Plus it seems easier to assemble against the backplate.
DSconfig2.jpg 


Or maybe even better yet angle the DS themselves as well so there is only a corner touching the backing, could have V absorbers in between still too. something like this? 
DSconfig.jpg 
This exposes even more surface of the spout as absorber area and could also especially be of benefit in a collector with thin insulation as it reduces the thermal bridge through the back of the collector.  Adds even further height though.

JoeK

Registered:
Posts: 135
Reply with quote  #54 
Kevin, wouldn't you WANT to leave a small gap between the spouts to encourage heating the sides of the spout? greater absorber area and thus the spouts would get hotter, especially down the sides.  better heat transfer than relying on conduction alone to carry heat down the sides?

I'm (slooowly) putting together a DS collector and was thinking about how to determine optimal spacing, and then leaving the reflective foil on the polyiso unpainted as you suggest. I don't know if I wanna get any more complicated by adding V's. Thanks to this discussion I'm toying with the prospect of angling the spouts though...
KevinH

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Posts: 534
Reply with quote  #55 
I was thinking of a final shape something like this after running the form through the downspout.  Rounding the tops pulls the sides in a little.  Just a brainstorming idea.  I have not tried this, so I don't know how well a form would work along a full length of downspout.  And again, without testing I can't say this is better or worse than the square downspouts side by side with or without a gap.  No matter what we do, there will be shading and light hitting at different angles during the day.  Something that works better at noon may not work as well at 2:00 or vice versa.

 Rounded DS.jpg 
Another option would be to leave a small gap between the square downspouts and seal the gap on the bottom with foil.  I don't want any light going all the way past the downspouts to the polyiso where it likely goes to waste and I would rather have an insulating dead air space behind the downspouts.  The air is being heated by the downspout in the areas where the downspout is hotter than the air passing over it.

Are those unpainted aluminum downspouts in your pics?

Kevin H
MN


Rick Stone

Registered:
Posts: 121
Reply with quote  #56 
Here is something similar to your drawing.

http://www.solarairsystems.com/shop/item.aspx?itemid=17


Rick Stone
Talmo, Ga.

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cwwilson721

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Posts: 303
Reply with quote  #57 
The idea wasn't "the DS gets heat from the backplane", or "The DS gets heat transferring around the entire DS perimeter from a hot corner"

It was "pump MORE heat into the DS tubes than the backplate and/or just DS can provide".

The pic above is GREAT. It show no shadows (imagine the solar energy being the lines in the flooring) at that solar angle, or greater towards solar noon. BUT,at any angle (time of day less than or greater than that angle) out of the narrow range of the angles between the tubes, you get shadowing and lowered efficiency.

The inverted V captures and transfers the solar energy that would otherwise not get utilized, either by shadowing of the DS material, or thru transfer of the heat by the backplate (if it's shadowed, heat doesn't get captured), in a spaced DS configuration.

In my previous drawing, note that in low sun angles (early AM, late evening) shadows and lower incidence angles shadows alot of the DS tubes surface (up to 50%, even if well spaced), plus no solar energy hitting alot of the backplate.

If you increase heat into the DS tubes in that 'cold' 50% with the inverted V's, you increase collector efficiency.

If you space the DS tubes with no spacing, you have a "backpass" or a "black/empty box" collector, which have already been tested against the 'standard' DS collector, and are not as efficient.


solardan1959

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Posts: 1,845
Reply with quote  #58 
Rick,
That is the site I was looking for.  I feel they would work good and could be simulated many ways such as inverted Vs or upside down gutters etc.

cwwilson721,
   One fault with your drawing is the air under the ^goes to waste.  With the tubes like Rick listed the entire area under the tubes is heated and you have no wasted space like with downspouts or downspouts and inverted Vs.
While you do kind of fix the shaded area it does waste a lot of space that could be heating up air more efficiently compared to just the heat transfer from the metal to the tube. Another fix is to also flow air though the inverted V
Quote:
The pic above is GREAT. It show no shadows (imagine the solar energy being the lines in the flooring) at that solar angle, or greater towards solar noon. BUT,at any angle (time of day less than or greater than that angle) out of the narrow range of the angles between the tubes, you get shadowing and lowered efficiency.

maybe round tubes instead of square downspouts, 1/2 inch apart, with a reflective back.

KevinH,
Almost everything you said echos my thoughts except I still would leave a 1/2 inch gap between spouts.
Dan
mocars2

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Posts: 173
Reply with quote  #59 
So we're trying to use the "V"s to get more reflection to the sides of the DS - which should heat up the DS to a greater temp in the early and later part of the day when the sun isn't directly right at it - Right?

Instead of all this work of cutting "V"s why not just use reflective materials shining on the collectors like mylar or mirrors and have them angled for the early and late sun?
Reading the thread KevinH on reflective mylar/snow -
JoeK

Registered:
Posts: 135
Reply with quote  #60 

I'm looking back and re-reading the last couple pages of this. I see I was misinterpreting your idea, cwwilson, as adding the V to reflect light into the DS. Your intention was to absorb light from between the spouts and transfer the heat into the DS via conduction. This does seem a better notion.  It also makes the Post about Dovey slightly off topic, but I still think he's onto a very interesting design, would love to see how it compares in efficiency to some of the standards around here.  Having gotten reflection off the brain a bit though it does seem clearly better to incorporate black metal absorber material between DS's rather than leave it reflective insulation foil coating, especially at noontime when the angle of incidence is highest.

What I see as a major distinction, among several, between many of the recent brainstorm ideas has to do with the angle of incidence you mention, and what times of day the collector is at peak efficiency.  As your illustration showed the inverted V's have improved angle of incidence during the shoulder hrs of morning and evening, but they have a worse AOI during the peak noon hrs compared to the DS top surface and flat backing. 

So the question, at what point does the increase in SA overcome the decrease in AOI effect (absorption efficiency of given SA).  Is the total heat collection more efficient by optimizing the absorber for morning/evening efficiency, or for noontime efficiency? Or is it just a wash? Seems like if you can create greater (or even equal) SAxAOI using some or all traingular absorber then having increased efficiency earlier in the morning, and later in the evening might yield a higher comfort factor as compared to production heavily peaked at noon.

It also is starting to seem like your primary notion of adding triangles in between the flat, square DS's could be a nice compromise. The link from Rick perhaps similarly so.

I was still really interested in your idea about Skipping the DS altogether and simply using triangular absorber surface area. So I did a little math...
DSareaCalc.jpg 

So the extra transfer area on the DS is always a big factor, but The absorber area is more complex.  does this look on track?

For morning/evening sun the angle absorber has 2.82"xL" exposed at good AOI, but 2.82" are also shaded.  The downspout has 3"absorber and a growing portion of the 2"sidewall both exposed at mediocre AOI, with 3+" shaded.  Triangle has much better AOI, but DS has more area, and still more transfer area as well.

For noon sun the angle absorber has 5.64" exposed at mediocre AOI, and no shading. while the DS has 4"exposed at good AOI, and 4" shaded. roles have reversed, but still the exchange area is major advantage to the DS.


I do suspect that in general there is a higher potential for gain during the peak hrs and that optimizing the collector to perform best at noon will give a greater overall daily gain.

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