Solar Air Heater - DIY

[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

[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]

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

[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]

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!

[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!

[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.

[Mike90250]

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.

I'd like to see a reference for this !

The type of surface - it's emissivity is what controls absorption / radiation .

The base material is what controls the CONDUCTION

best to worse :
silver
copper
aluminum
the rest aren't worth mentioning.

[russ]

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

Check out simplysolar - they have lots of wild ideas and some really fruitcake suggestions.

The wire thingy sounds like a waste to me.

[Crumb]

I'd like to see a reference for this !

The type of surface - it's emissivity is what controls absorption / radiation .

The base material is what controls the CONDUCTION

best to worse :
silver
copper
aluminum
the rest aren't worth mentioning.

I have seen this laid out my way with references, but it was years ago and it would take me all afternoon to find it.

I think we might be agreeing, at least partially.
Copper has the better conduction, no doubt - we agree there.
I am pretty sure that aluminum, while being a less efficient conductor of heat, is marginally better at wanting to give it up. I think it was by using a different measurement than surface/emissivity like you were using.

I havn't looked in years, but I think it had something to do with (specific?) heat capacity and maybe there was a caveat in there of "by weight" or somesuch?
Would that make better sense to you if that were the case? Again, its been years since I saw the references.

I didn't write the book on thermodynamics, nor did I read it - I am just instinctively good with it. I can't be positive, but I remember having the same surprise that you did- when I heard it. The explanation was convincing enough for me. I remember seeing graphs showing aluminum just a whisker better in some cases.

I think the study I saw was about this: If you want a heat sink based on thermal mass alone, (big dumb lump) you want copper.

If you want a finned heatsink in an airflow with lower thermal mass and lower overall weight and not a giant lump of copper you are better off with aluminum- And it wasn't much - A few tenths? A few percent? In those terms - it made total sense to me.

I welcome any references to the contrary, because what I know might not specifically apply here using the different set of measurements you are talking about.

In our scenario copper wire should be okay and work nearly as well, but if you are going to all that trouble - why not built in a little more heat transfer if you can help it?

The performance is close either way, so I certainly wouldn't frown on a system using copper, I just wouldn't do it that way and would recommend otherwise.

[russ]

For any others following this - you will be far better off looking at www.builditsolar.com and using one of their plans.

This is getting a bit strange for virtually zero gain.

[Crumb]

Check out simplysolar - they have lots of wild ideas and some really fruitcake suggestions.

The wire thingy sounds like a waste to me.

I am willing to test it in the real world, side by side, later this fall when I build it.

One straight black pipe. One pipe with aluminum windings, one with copper windings.
Just for fun we can do black plastic sewer pipe with windings too to specifically measure the effect of the windings regardless of the aluminum gutter? We'll see.

I doubt I have anything accurate enough to measure the difference, however, I will report back.

Most of my temperature measuring stuff is +/- 3%, But - hey, I like the idea enough to try to take it a little further.

EDIT: May also try slots with L shaped strips of aluminum on alternating diagonals. One surface gets black and put on the sun facing surface. This may be less tedious than the wires. I am excited about the idea but have my hands full on a solar build right now.

Update 9/7 - Project abandoned. My surface requirements are too great due to thinner air above sea level to be worthwhile. Thinner air means less thermal mass to move the heat into, and even less efficiency. Even radiators and fans are less effective out here. I forget this often.

Now, solar-to-water might be a better candidate but that's another headache for another day.

By the way - Tests on black spraypainted cardboard, cardboard with a copper "stitch" and aluminum "stitch" - favored the copper wire by quarter to half a degree hotter on the backside of the corrugated cardboard than aluminum or nothing. Meter was +/- 3% so I really can't conclude much other than a "maybe/dunno." Did not do any timing to see which cooled quickest, but they were roughly the same in just a quick check after about 15 seconds out of the sun.

If I ever tackle any of this I'll put the heat into water first, then into the air - it's a little different out here than most places.

[CarrieTaylor]

i can advise you on that i would do differently and would cause a big difference would be to actually pull the warm air from inside the house then warm it up more and pump it back in, at the moment i am getting the outside cold air and warm it up and pump it into my house and usually has a guideline to my system if its 15c outside(air intake) its pumping 30c into my house

Thank you!

[JennyRox02]

The Solar Sponge is a low cost Do-It-Yourself active Solar Air Heater designed to heat your home using energy from the sun. The sun provides heat all year round, so why not use solar in the winter too! Solar air heating can supplement your regular heating system and can dramatically reduce your heating costs. Air in your home is circulated through a collector on the exterior wall where it can gain up to 30 degrees before being vented back into the room. These 4'x 8' collectors look like large doors and should be mounted on a sunny south facing wall or roof.

[nobody]

i'm in the stages of doing another one of these for myselves. we have two that are made on 4x8 plyboard. i was wondering if it might help to heat the air more if some how you put a U shaped tube connecting the can roles. say the first role of cans you leave open in the return header space. then you link the first role to the second, on the other end of the second you link it to the third role and so on till the last role is open on the end the hot air is piped out of. the air remains in the painted flat black cans longer getting heated more and more as it moves through the cans.

the thing we keep talking about is there has to be a happy medium for the output fan size. to big a fan and the air doesn't have enough time to heat up enough. to slow of a fan the air gets very hot, but it doesn't blow out into the room to be heated well.

this may have been bruaght up before and if it has i'm sorry. just me thinking again. the first one i did is hooked up on my chciken coup in the winter monthes. yep. my chickens live a pampered life. the second one i gave to a friend for his cabin. this one will go on my out biulding. i have been using small dc computer fans and a small solar panel to run them. they turn on in the day and off on thier own at night.

i have the intake hooked up so that the heated air is being reheated again and again. it seems to work alot better than pulling cold air in and trying to heat it up. i have the output hose run to the furthest side of the chicken coup. when i do my out building i will do it the exact same way. also the intake is high up on the roof of the chicken coup. the output is down next to the floor.