Solar Can Heater Build Part 2

Last time, (Part 1) I covered the theory and plan for turning my boxes of aluminum soda cans into a solar heater.  I tested the first tube after painting it black and found that the air flow was unsatisfactory.

Testing Prototype Tube 1 in an East Facing Window

 So for the second prototype tube, I took a can opener and removed the tops as well as widened the bottom openings.

Widening Holes

Off with their heads!
(Also titled "going topless" but the wife vetoed that one)

I tried a couple of different methods to enlarge the punched holes.  Forcing them open with pliers just warped the bottom ring on the can and made for a bad fit when placed in the jig for assembly.  I settled on cutting and folding back the segments of the bottom.   I should point out that this adds a tremendous amount of labor and risk for injury to the build.

Ouchy!

This naturally got me to thinking.  Here I am, going to a lot of trouble to make what amounts to some thin metal tubes.  I could probably save myself a lot of time (of which this has already consumed a lot) if not money (band-aids ain't cheap these days!) by just buying some prefab tube or thin sheet stock and fabricating the tube myself.   With the availability of dryer vents and elbows, I've got a lot of the parts I want just at too large a diameter.

So, I've decided to take the cans to the recycling bin and start checking out my options at the local home center.  I also have thought a lot about the air flow issue.  I had previously surmised that the baffling created by the small openings would slow the air flow and allow for more heat absorption.  But as it is, Aluminum doesn't hold much heat being thin in the case of the cans.  So slow air or fast air, my temperature measurements at the top of the stack were about the same.  But with the baffles widened, and air flow increased, I at least could feel the flow of air with my fingers when testing prototype 2.  So I concluded that what I really want is not resistance but length of run.  The longer the run, the more surface area the air can exchange heat with and hopefully come out the top much warmer.

So, back to the drawing board.  Instead of columns of cans, I plan a switchback path of tubing that will allow an always upward rise for the air but will provide one long run at high volume compared to 4 or 5 narrow, comparatively short, runs at negligible volume.  Hopefully I'll have more to report later this summer as I'd like to have this done before next winter but summer activities will take precedence.