My passive beer can solar furnace
In the cold winter days the temperature in the storage building can fall below freezing point. I could use the spare electric oil heater I happen to have, but keeping it running for long months would cost a fortune. It did not take long until I found the famous passive soda can / beer can / pop can solar heaters / furnaces. This is just an other DIY, sharing my experiences during the building process.Since the project is far away from ready, this post will be kept up-to-date to reflect the progress.
The first is the most joyful part: drink all the beer! An important thing to remember is not make the same mistake as I did and collect hundreds of cans without moving to the next step! Oh, and always keep your liver in mind 
Once a handful of cans are empty, cut the top and bottom sides open. I used two different size circular saws, but the same can be achieved with a can opener or a utility knife. Very important to wear correct protection! The thick rubber gloves not only provided an excellent grip, but repelled the circular saw a couple of times when it jumped out from it's track.
This is an excellent time to put all the cans in a disinfecting and cleaning bath, since the liquid will be able to reach the whole surface more easily. I left them in a general purpose household cleaner for a day, shaking the bucket a couple of times. After the bath rinse the cans with clean water and put them aside to dry.
Sand them carefully to provide the paint a better surface to stick onto. This is a ridiculously boring part, so be prepared. I used a 60-grain sandpaper, and did all of them by hand. Again, the gloves protected my hand against the sharp edge of the cuts. After sanding, I cleaned the top and bottom parts with alcohol, and glued the cans together with heat-resistant silicone caulk. I made the stands from scrap wood and laminated flooring which will help the cans to stand straight until the silicone sets; a good, air-tight seal is VITAL here:
On the left side there is a column which is properly sealed. As the cans are heating the air up, it rises and exits through the top. On the right side, the column is missing proper sealing, so a part of the warm air can escape, and is being replaced with cold air. Most of the airflow will exit through the top, but there will be a significant difference between the temperature.
The stands I previously created were able to hold only 6 cans but my furnace will need absorbers as tall as 10. I had to make bigger and smaller pieces first and glue them together. Since all everything got dried last night, I modified one stand to support this and set the first full-size absorber.
After the last absorber is done, wash them with soapy water and rinse them carefully. Set them aside to dry somewhere warm and clean. This is the perfect time to do some math and find out how big air in- and outlets will be needed, which will not cause a bottleneck the airflow. The surface of the inlet should be equal to the total surface of the absorbers:
I used a 44mm diameter circular saw for the top and a 52mm one for the bottom of the cans. In this case I have to consider the smallest radius, which is 22mm. I'll have 12 columns, so the radius of my outlet (and inlet) should be ~76mm. This is just WAY too big, so I'll use 2 10cm pipes for both the inlet and the outlet.
