Solar air heating collectors that use columns of aluminum soda pop cans or that use metal gutter downspouts as the absorber are quite popular.
In this design, the supply air flows into a plenum at one end and is distributed to the pop can columns or downspouts, the air then flows up the pop cans. The sun striking the outside of the black painted cans heats them up, and this heat is transferred to the air flowing up through the cans.
One of the challenges for an efficient design is to get the same flow in each of the pop can columns. Having unequal flows in the columns makes for a less efficient collector with some parts of the collector running hotter than they need to and losing heat out the collector glazing.
When I built a downspout collector for testing against other collector types, I was unable to come up with a plenum design that gave an even flow in the downspouts... I think that this may have been a significant part of why the collector did not do as well in the testing as the the screen absorber collector.
Sooooo, if you are building or have a pop can or downspout collector, how about testing the flow in each column, and see how the flow comes out on yours. If its not close to the same in each column, maybe you can experiment around with changes to the collector to get the flow to even out?
Testing for even flow is easy to do, and coming up with a collector/plenum design that provides even flow to all the can columns would be a real benefit to a lot of collector builders.
You can let us know how your testing came out in the Comments section below, or drop me an email -- Gary...
Update: August 6, 2013
I did a test of a pop can collector vs a screen absorber collector...
One thing that I observed was that the airflow distribution across the can columns of the pop can collector was pretty even -- much more even than the distribution in the downspout collector reported below. So, it looks like the extra resistance of the pop can columns makes for a more even distribution of airflow, and uneven airflow distribution may not be much of a problem for pop can collectors.
Some pictures of the collector that the flow distribution test was done on -- more details here...
There are probably many ways to measure the flow distribution -- the following shows how I did it.
I tested for the flow in each column by opening a small hole in each downspout and using a wind meter to measure the flow velocity.
Measuring the flow velocity in the downspout columns.
The results showed quite a bit of variation between the columns.
All the details on what I did and what the collector looked like here...
Another way to get an idea would be to use an IR temperature gun to measure the temperature of each pop can column. A column that runs hotter indicates that its not getting as much airflow as a cooler column. This, of course, has to be done with the glazing off. It would be good to get the temperature at (say) the bottom, middle, and top of each column. This method does not actually tell you how far off the flows are, but its easier and less destructive than the method above.
- This section on solar air heaters has several builds pop can and downspout heaters...
- Testing of a downspout heater compared to other air heating collectors...