Solar Hot Air Collectors

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I'm starting a new set of solar hot air collection panels to supplement my propane heater. A number of years ago I made a large hot air collector for home heating. I have corrugated steel siding on my building (that I turned into my house), so I simply framed up a 10' x 10' area of the south wall, caulked it, painted it flat black, installed some air baffles and glazed it in a heavy clear vinyl sheeting. It worked quite well, heat coming out of the 6" vent was 120 to 130 degrees and it pretty much heated my house when the sun was shining. But the vinyl only lasted about a year and a half, I tried replacing it with heavy poly plastic, but of course it didn't last long either. So now I'm set to make a new solar hot air collector, the key is that I have four 75" x 33" glass patio doors to use as glazing.

I'm divided on whether surface area or efficiency is the key, so with this build I'm looking for both. Enough area to compensate for low efficiency, but still get reasonable efficiency. Of course I'd also like them to last a few years with little maintenance. I looked at a home solar collector build on the web, a collector the same size as mine will be was putting out just over 500 watts in peak sun, that seems reasonable to me, with a glass area of 1.596 square meters that's about 31% efficiency. I'm planning on 4 of these collectors, which should give me some 2,000 watts in peak sun. I have used electric base heating in the past, and a single 2,000 watt electric heater was enough heat in all but the worst of conditions.

I have no grand vision of these panels providing all my heating needs, far from it. So I've got to keep cost in line with what returns I expect to see from these panels. If I average 4 hours of sun a day, that gives me about 27,300 BTU's of heat per day, or just over 10% of my actual normal heat use of 264,000 BTU's per day in the winter. Assuming I have 5 months (152 days) out of the year the panels are producing heat when needed, that's 4,149,600 BTU's of heat per season. With propane at $1.50 a gallon my savings would be 45.43 gallons or $68.15. Not a huge dent in overall energy bill, but it's enough to make it viable if the panels last for 10 years. I'm not taking into consideration the cost of electricity to run the fan to move the hot air. I'm hoping an efficient DC fan can run off maybe a 120 watt panel. For now, still having grid power in my workshop, I'll use grid power and the fan setup I used in the past. Since a PV solar panel should easily outlive the hot air system, and the PV panel can be reallocated to battery charging 7 months out of the year, I'll leave out that cost in my final cost assessment when the project is done.

With home built solar hot air collectors there are really two types. One type has the absorber sealed air tight, and the air moves on the bottom side of the absorber, the other type has the absorber positioned in the air tight box and air is allowed to move both above and/or below the absorber plate. The sealed absorber plate design is thought to be slightly more efficient, since the moving air doesn't come in contact with the glass surface and cool it. The bottom side of the glass plate also won't get dusty, since it is sealed from the moving air. I'm not utterly convinced the sealed absorber design is better, but I also don't like the idea of the stagnant hot air under the glass of that design. So my plan is to use an absorber plate positioned in the middle of the sealed box, with air allowed to flow both above and below the absorber plate.

From here on, a lot of the details will come with pictures.

Here's the start of the first panel. I debated on what material to use as the back. I was going to use exterior plywood, but the particle board was a lot cheaper. Since the back side doesn't see a lot of weather (and I'll paint it), I went with the cheaper particle board. To introduce air into the system, I used a 2.25"x14" sheet metal vent to couple to a 6" diameter duct. I want to keep the flow as unrestricted as possible, the vent has an area slightly larger than the surface area of the 6" duct. The framing is standard 2x4's and the particle board is 7/16".



I caulked all the framing then screwed it together using decking screws. Then I sealed all the inside edges with caulk for insurance. I don't want any air leaks, so it's crucial at this point to have everything well sealed. I even caulked up the seams on the sheet metal duct.



At this point I've primed and painted with outdoor house paint. The foil insulation was a last minute decision at the store. I looked at 1/2" foil backed foam, then saw this "space age" insulation. It's foil on both sides with a 1/4" of fiberglass inside, it's supposed to have a higher R value than the 1/2" foam. It's stapled in around the edges and the seam in the middle. Notice I used foil duct tape on the air inlet, both to seal it better but also to keep the insulation from getting pulled up at the air inlet.



You can see the air outlet on the lower right, this will mate up with the next panel in the series. I've also installed a couple of .75" x 1.75" boards on the outside edges to support the heat absorber. The board in the middle is the deflector plate that sends the air up the left side, through a 7.5" opening at the top of the deflector then back down the right side.



The absorber plate is corrugated steel painted with Rustoleum high temp flat black paint. The steel panels were only 24" wide so I had to rip a length and pop rivet the 2 pieces together. It took a full can of spray paint to do the one panel. The absorber plate is screwed into the boards on the sides, I also added 2 blocks on both the top and bottom to help support the ends of the absorber plate.



Here is a side view of looking into the air outlet. You can see the absorber plate mounted in the middle of the box so air flows both above and below the plate. I got a little lazy cutting the air outlet hole, I used a hole saw at either end and cut between the two with a circular saw. Since this is covered by the next mating panel it really doesn't have to look pretty.



The upper side baffle has been installed and foam tape applied to the top to seal it to the glass cover.



Here's a first look at the glass on the panel. I was waiting to see how it all fit together before I decided on how I was going to seal the glass to the panel. I think I'll use some foam tape on the wood frame, I was worried about rain getting in, but the aluminum frame of the door fits pretty well to the wood panel frame. I'll caulk the top so rain doesn't get in easily, but I want to be able to remove the glass for cleaning or servicing in the future. I'd also like to install the panel without the glass in place, so the foam tape seems the best bet.