I've been working on building a solar air heater for the shop to help reduce the amount of kerosene I have to burn to keep the shop warm. The build of this was started about the time I found out about the Yahoo Simply Solar group and before I saw any of the testing results for different designs of solar air heaters.
The heater is based on a soda can design that I found here. After doing a lot of reading during and after the construction of this, I believe that there are several designs that are better performing and in some cases, less expensive to build. Those designs include the screen, downspout, and vented soffit.
Performance:
Based on calculations for solar performance from built-it-solar, the collector is putting out about 3600 btus/hr at an efficiency rate of 43%. I had to adjust the value for the density of air from the examples as they are based on a high elevation and I'm at sea level. I ended up using a value of 0.075 based on this information.
The snap disc controller turns the fan on when the temperature reaches 110 and off when it drops to 90. On a typical sunny day the fan will run for about 4-4.5 hours. Starting at 10:30 and going off around 3. There is one tree that partially shades the unit at around 2:30 and the fan will sometimes turn off then.
It's possible that I could get a little bit more run time out of the heater (by it starting earlier in the morning) if I had mounted it at a slight angle instead of flat against the building. As mounted, the unit is facing 18 degrees west of due south.
One of the downfalls that I see with a can based collector is the huge pressure drop that occurs as the air tries to make its way through all of the cans. For optimum performance the heat needs to be removed from the cans as quickly as possible to prevent heat from radiating back out through the glazing. The low air flow can probably be solved by using a blower fan instead of an axial fan, but that increases the cost tremendously and the problem can be solved by a better collector design.
Construction:
The 4x8 box is made of 3/4 x 4.5" oak. The back is a piece of 1/8" plywood and is just there to prevent racking of the box. The inside of the box is insulated with 1/2" polyisocyanurate with the foil side facing the inside of the collector. This was not painted black as I have seen done in a few other builds. The manifold at the top and bottom of the collector is ~6" high. One thing I wish I had done differently is the cover for the manifolds. It is currently a piece of black painted plywood insulated on the inside. This should really be a piece of black painted aluminum flashing and not insulated so that the manifold areas can function as extra collector surface. As it stands, only the cans act as the absorber plate for the collector with a surface area of 28 sqft.
The cans take a lot of time to prepare. There are 17 columns of cans with each column containing 17 cans for a total of 289 cans. The cans had the top/bottom cut out to open up the can as much as possible. A v-trough made from some scrap 1x6 was constructed so that the cans could be aligned and sealed together with silicone. The cans were then cleaned and painted with flat black high temperature grill paint. Note that although most cans look the same, they all differ ever so slightly and that difference added up over 17 cans leads to differences is the lengths of the stacks of up to 1/2".The fan I used is a 124 cfm axial fan. The dimensions made it difficult to work with and I ended up having to make a mounting plate for it that was fastened to a 6" to 4" reducer to feed into the inlet side of the heater. It's a good thing this is in the shop as aesthetic wise, this would not pass the "wife test" for being installed in the house. The glazing is 8mm twin-wall polycarbonate. I think it looks better than the single wall corrugated panels but it does add a bit to the cost.
Cost:
Wood for box, back, manifold: free, salvaged
Cans: free
Twin-wall glazing: $90
Silicone: $10
Paint: $15
Fan, snap disc controller: $50
Wire, ducting, screws/etc: $25
Insulation: $10
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