Solar Air Heaters

[VersysVsWhat]

My dad is on the poorer side. This past winter, I made him a Solar Air Heater to help supplement his main oil heating. It was partially altruism, but to be honest, I think I was living a bit vicariously through him because I've wanted to make these for our house for a long time, but just don't have enough consistent sun near our windows for it to work well (also, my spouse would probably veto it based on the "looks").

There were a few differences from the typical ones you see on YT and the like. One, I made a double pane. The inner layer was polycarbonate because it can take quite a bit of heat, and the outer layer was acrylic because it has good light transmission and holds up really well to UV exposure. There was about a 1.5" air gap between the two.

And while I used dark aluminum window screen as the primary heat absorber (which is fairly common), I combined it with some vertical, curved "fins" made out of Al flashing, that loosely redirected air (very loosely, I didn't want to block or strongly slow down the air). And for the top layer of the bottom of the collector, I adhered activated carbon/charcoal powder. The idea behind the latter was it would slightly slow down, and more importantly, mix the air a bit better than a smooth surface.

My dad's window had a good set up for this, because he had a regular window and storm window, so at night when not using it, he could just close the main window over it. He said that on most days, it kept his living room pretty toasty (he has a small house) even when he turned the central heat way down. (Note: acrylic is kind of a pain to bond well. If I ever make another, I would probably use super glue rather than the silicone and toughed epoxy.)

Originally I was going to set it up with having an air intake from the house, but after having a dream about the plastic melting, I decided to not do that (air intake is from the bottom, screened off for bugs and critters, with an Al flashing shroud for rain). But yes, for the most efficient design, you would want the air intake to come from within the house (I just couldn't trust my dad to open and close the windows at optimal timing. He's not as conscientious and analytical about this kind of stuff). I also thought about putting reflectors on the sides, but for the same reason above, decided to not do that either.

 

[VersysVsWhat]

I made this as low cost as possible, and with materials I had lying around from other projects. The outer perimeter frame was made of 2" XPS foam. But because that kind of foam can't take much heat, I lined it with cardboard and on the cardboard was adhered aluminum foil to reflect heat away (the combo keeps it much cooler than it would be normally). The backing was cardboard reinforced with fiberglass cloth and Titebond III wood glue on the outside and inner part of the cardboard (exposed to light and heat) was treated with thinned silicone. There were two sheets of treated+reinforced cardboard spaced apart with cut bamboo sections, and in between the cardboard was sprayed expanding foam (this I would change for future iterations, because it was hard to get it equal/level).

So it is quite well insulated. If money was less of an object, would have used wood and also would have used hydrophobic coated fumed silica as the primary insulation. I make homemade vacuum insulation panels and might some of those in there as well.

 

[Cujet]

Very interesting. I live in South Florida so there is not much need for heat. However, a planned move to TN will change that. I've seen a version that uses old aluminum soda cans painted black. And of course, I've watched a number of YouTube videos on the subject. I recall one video where they tested various methods and came to the conclusion that there was not much difference as long as the airflow is free. A black surface absorbed solar heat and it would heat the air flowing through it.

This video shows some steel cans with small holes. It has a bit less output. I would guess a non restrictive airflow setup is better.

If I remember correctly, the DIY panels seem to be about 60% efficient, and each square meter can produce about 2000BTU per hour when it is sunny in wintertime TN.

I am glad you brought this subject up. Something for me to plan.

 

[VersysVsWhat]

Awhile back, I looked at all the research I could find that tried various different absorbers, usually window screen was a bit more efficient as other methods tended to slow down the air flow too much. It's also the easiest and quickest to use/install by a long shot.

Some people have used fiberglass window screen, but I don't recommend that has the fiberglass is coated in vinyl and if it gets hot enough, the vinyl will start to offgass (some people have experienced losing all color on these, because all the vinyl off-gassed). You might pay a bit more for aluminum window screen, but it is worth it. The color is permanent as it is anodized on.

 

[JohnnyG]

You guys should look up Trombe wall.

 

[factorytuned2012]

they work i have seen them capable of heating bath water.