Solar Hot Air

[denny]

Hello, I have been doing some research about building a solar hot air colector.The more reading I do, The more I am confused. This one says only use corrigated aluminum , that one uses old carpet painted black. What do you think is the best?

Secondly, I want to mount my collector on the frount porch roof. This roof only has a slope of about 5 degrees. Will that work? I can find no information on an installation like that. My latitude is 41' so the ideal angle is 56 degrees from what I have read. Most of these collectors are mounted on the wall at 90 degrees.

If 90 degrees is ok, is 5 degrees as good or will their be excessive reduction in heat gain?

Thanks for your time. I am looking forward to your input about this

Denny

[dudevato]

Denny, More will jump in and direct you, I'm sure. The closer to 90* you can make the collector to the sun the better. Here's another site you can look at:

http://www.mobilehomerepair.com/article17solar.htm

[spinningmagnets]

Have you seen the website " www.builditsolar.com "?

Theres a months worth of reading there!

I have not built any solar collectors yet, but everything I've read indicates that there is a very significant difference in how much heat you'll get depending on the angle of the collector. If it's a fixed-position collector, you should place it as close as possible to directly facing the sun at the time of day that you want the most heat input.

If the materials are almost free, and you don't want it to stand out (I have a wife) a non-tracking 5 degrees is better than nothing, but I'm certain you will be very disappointed.

[EMF]

Check out these DIYers at

www.solarsponge.com

[Stonebrain]

Hi Denny,

You need hot air in winter.

In summer you don't want it.

So the 5 degrees inclination is rather bad.

In winter the position of your panel must be closer to the vertical then to the horizontal.

I would say 60 degrees or more.90 degrees(=vertical)would be fine too.

For the materials,I would say black carpet it better then aluminium.

In my opinion,the best design is with black fibrous material and the air passing through the material.So the hot air is behind the absorbant layer,this gives the highest efficiency.I didn't find the ideal materialfor this.Maybe something like black blankets.

You will find some designs on the net.But you can do allways better iff you have a good understanding of the processus.

cheers,

stonebrain

[davidmlantz]

Hello all,

I read a couple of years ago a pdf about building a DIY solar air panel. Back was reflective faced foam insulation, sides were wood, from was greenhouse polycarbonate and the collector was black dressmakers felt.  The article said it was the best material.  Ductwork was done with small chimney pieces and the fan was a small PV powered fan, like the kind used to ventilate the engine rooms on yachts.  There was no controller, or thermostat, just a small PV panel tied directly to the 12v fan.  These were intended to be put on the side of a house, through a partially open window.

Hope this helps--Dave

[feral air]

I wonder if black "shade cloth" (google that) wouldn't work well. You could pleat it inside the box (think air filter, but looser) and since it has holes the air would pass through it fairly well. I imagine the lower the shade blocking value the better, that way the light gets down through all the layers...

..just another random thought.

[wooferhound]

I have been thinking about the same idea. But instead of pleating the Shade Cloth, I was just going to layer the cloth. Possibly as many as 30 layers. The cloth I've seen has only 30% blockage, so there should be plenty enough holes spaced randomly enough to let the air flow through easily. That should absorb plenty of Sun.

[ULR]

It doesn't matter very much what the material is, as long as it doesn't reflect much light, couples the resulting heat effectively to the circulating air, and doesn't stink up the house.

Clean new carpet should be fine.  Aluminum painted flat black should be fine.  Copper painted flat black should be fine.  Lots of other stuff would be fine, too.

Just remember that it's going to get really hot.  So you want something that doesn't melt easily.  And it's going to have room air past it, so it should smell nice and be easy to clean the dust out.  And it shouldn't imped the airflow too much, or you'll need a fan, or a bigger fan, rather than depending on convection or a small fan.

Folding things so they present the edges of deep cleats (like fan-folding), rather than a flat surface, to the incoming light, drastically reduces reflections if the paint isn't really, really black.  Specular reflections have to bounce back-and-forth a bunch of times, with a high percentage absorbed on each bounce, before they make their way back out.  (This is why black velvet is SO black, and colored velvet is SO strongly colored.)  But if your paint is already absorbing, say 95+% of the incident light above the near-infrared cutoff of the cover glass, you can get more benefit by making it 6% bigger in one direction than you'll get from all the cleating in the world - and all the extra material that requires.  (A little cleating parallel to the air flow may improve the coupling of heat from the absorber to the air.)

[denny]

but what do you feel about only a 5 degree tilt?

[clarsondd]

if 56 degree is optimal for you,

to get the same heat as at 56 degrees:

at 90 your panel would need to be 22% larger,

at 5  your panel would need to be 55% larger,

at 90 you can take advantage of the natural convention

to circulate the air; hot air rises.

at 5 your going to have to use a fan.

at 90 snow melts off

leaves, bird stuff, dust can all be washed

off with a hose

probably going to take more work at 5.

[DamonHD]

I know nothing, but I hear that 10* to 20* is the minimum tilt recommended to (for example) avoid rainwater pooling.

My investigations suggest that 20* gains a lot of benefit over 5* in terms of capturing sunlight, especially in winter with low sun, see the tables in:

http://www.earth.org.uk/note-on-solar-PV-for-diffuse-light.html

Rgds

Damon