Solar Air Heater - DIY

[Crumb]

I can suggest an improvement (how I would/will do it) :

I would use a fan and straight pipes because this will be on the restrictive side-
A small PV panel and a 12V fan with good pressure (InHg) is something I did this week- pretty easy and fairly cheap.

1. Don't bother painting it black. Smear the whole darn thing with thermal paste.
2. Big spool of aluminum hobby wire - 18 AWG: Wrap a few turns to start, then, drill a hole and poke it through the wide dimension. Wrap a few more turns and then poke it through the narrow dimension. You can do the whole heated area like this, or just the top half (Don't use thermal paste on the gutter area you won't be wrapping - I doubt it takes paint well.) Make sure the wire is tightly wrapped and has good contact with the surface, and itself between turns.

3. NOW paint it black. You increase the external collector surface area slightly AND increase transfer area into the air being heated. You also break up the airflow a bit with the + wire configuration (looking through the pipe) for nice mixing internally.

I would bet you lose very little because the pipe is corroguated and the wire does not make contact across the whole surface. I would also bet that the increase in efficiency more than makes up for it. Very interested in testing this in the fall.

Edit: If you are very clever and have lots of time and a very forceful fan, I suppose instead of a "+" configuration internally you could make grid somehow. Would take lots more wire, but transfer area would go way up.

Edit 2: Why aluminum wire when copper wire is easier to find and absorbs heat better?
It is a trade off, like anything else. Copper absorbs heat better than aluminum. Aluminum gives up heat better than copper. Sometimes it is just better.

Why hobby wire? It tends to be on the purer side than wire for other purposes.

Why such a small AWG? That sounds tedious - Yes, but while bigger wire might have a larger surface area in the transfer area inside the pipe, and will need less turns, it will also have less surface area in contact on the wrappings externally with the gutter- You just have to poke it through more to gain back the surface area inside. Again there is a sweet spot to all of these numbers, and it will probably need more fine-tuning.

[Crumb]

Moving heat is something I know a little about - just enough to be dangerous.

Actually this one is pretty obvious and fairly clever, but I had never considered it. Makes perfect sense and I tip my hat to the guy who came up with it. A can of flat black spraypaint and some aluminum gutter! I am impressed with the simplicity!

If you are going to use forced air flow anyway, you would be better served by zig-zags (think elbows - 45-90) to maximize transfer.
The thing is you might loose surface area overall for something window sized. If the fan dies it will be very ineffective because the airflow is more troublesome through the bends, and you really need a fan for that method.

Use straight pipes (as straight as possible) for passive without the fan, just make the intake and exhaust manifold longer and - just as a car, you get more "torque" to move the air. Leave both ends unpainted - the differential in heat will move the air for you. I would get the collector near the floor as possible and have the exhaust as near the celieng as possible, but maybe use a piece of white foamcore with aluminum tape all over it above the exhaust for insulation. No sense in heating the sheetrock in the ceiling. I think this guy has his hooked into his house ducting though. Probably need a fan there regardless.

Also, I am not sure if you gain more with forced flow or not- lower peak temperatures. Slow the airflow down and it has more time to heat up. There has to be a sweet spot in there somewhere. Not sure where it might be though - it would take lots of fine-tuning.

Once things slow down this fall I will totally try this. Thumbs up!

[russ]

Our glass and aluminum guy should be coming tomorrow (supplies and installs windows, handrails etc). I have to take this up with him and try to get him to make a panel for me.

I keep thinking about it but haven't taken any action!

[kyle]

I agree with russ. just place baffles on a rod inserted from one end and remember don't be so hard on it.

[russ]

Easy enough to place baffles on a rod inserted from one end. Standard with large commercial heat recovery bundles.

To make it properly flow would have to be balanced - again not hard but not automatic either.

Russ

[Naptown]

My only concern would be with laminar flow in longer collectors. He does have some flow issues between tubes but could be more of a function of the short lengths. no top plenum and very few tubes. The beer can collectors would have much more turbulent flow due to the treatment on the ends of the cans. Perhaps dimpling the tubes would help with creating turbulence.

[russ]

A different design that is simple for DIY has been posted on Build It Solar http://www.builditsolarblog.com/2011...r-heating.html

Gary used gutter down spout pipes to pass the air through. These are inexpensive and available in longer lengths. All the advantages of the coke can type but far easier.

Russ