“Solar Soda Can Furnace” or “FREE Solar Heat System YOU Can Build!

We’re not talkin’ photovoltaic panels for power, though they’re pretty sweet, too. I’m talking about a MUCH simpler system, this one is based on the principle of “Black things heat up in sunlight”. Direct is better than indirect, but that’s the beauty of this system… it scales. You’re not getting enough heat on chilly overcast days? Build MORE of these systems! Play with the pitch-angle, rig up some kind of stepper motor to angle the panel WITH the sun as it traverses the sky. There are many ways to make the system BETTER, but my goal is to make it SIMPLER. I take a page from Isaac Azimov’s book (giggle at my own pun), he rewrote hundreds of textbooks on science in an attempt to simplify the language. I’m not of HIS caliber, but I do my best to make the fundamental woodworking projects a little easier to swallow; so to speak (editors note: Do NOT Swallow woodworking). I want to make it as simple as I can so everyone can grasp the fundamentals, then the exploration is up to you. Sorry for the rant, onto SOLAR HEATING!
So I originally got this idea from a feller in Spaniards Bay, here in my beloved province of Newfoundland and Labrador. As it happens, my ancestors came over here from England about 250-300 years ago and settled in Spaniards Bay, before the four brothers parted and one of them came over to the West Coast of the island, and to whom I’m most directly descended. Anyway… This guy manufactures these systems, the sort of design that I used. It’s all open sourced, you can find hundreds of plans online with subtle changes, and some not-so-subtle, but all variations on a theme: Move air from your house, use solar energy to heat that air and move that air back into your house. Jim Meaney’s work is nice, I must admit, and you should really check him out if you have the cash to lay out for one of his systems to come in a package via freight and just plug the sucker into your house. I saw his work, figured it out, and saw some bells and whistles that, while awesome in their own right, I didn’t think were necessary for the hobbiest woodworker to play with fire (giggle, again).

Solar Soda Can Furnace - 1EnterSolar Soda Can Furnace - 2 MY project: My system is simple enough. Just a 4′ by 8′ sheet of 3/4″ plywood, with a box of 5.5″ wide 3/4″ pine around it’s edges. Follow along with the small images to the right, they are micro-nized for convenience, but clicking on them will bring up the full-sized image.

Solar Soda Can Furnace - 3Now that you basically have a big, shallow box, you can lay down the 2″ rigid insulation. The top and bottom pieces are 3 5/8″ (~92mm) wide, and 3′ 10 1/2″ (~1181mm) long. They had to be trimmed from the original width to accommodate the side-boards of the frame. The gap you cut- and throw-away is the width of the same 3/4″ (~19mm) board, but this one is full of holes. I used a 1.5″ (~38mm) diameter hole-saw bit  and punched Solar Soda Can Furnace - 4a hole every 2.5″ (~63.5mm) On Center (O.C.). Don’t forget the insulation on the sides!

Solar Soda Can Furnace - 5The soda can towers will fit into these, but depending on the type of cans you use, you may require a slightly wider or narrower bit. I use typical beer/soda cans, 8oz (355ml) at 4.75″ (~121mm) tall and 2.5″ (~63.5mm) wide. There are 17 towers of 18 cans, 306 in total. Use a hole-saw (or aviation sheers to cut away like the picture below) to cut away the bottoms (leave the tops as they are) of EVERY can (wheew) and let them soak in detergent for a few days to dislodge any dried syrup from whatever beverage was contained in side of it… usually beer in my case, it was a huge effort drinking it all, but I did it for my beloved blog audience. I’ll be using big plastic garbage bins to soak mine, remember to use LOTS of dish-liquid and as much hot water as you can handle in the bins, it’ll make the soap work better.

These cans must be caulked together using industrial adhesive or similar, and every tower must be painted a matte black. These slide into the holes top and bottom. The black paint will cause the cans to heat up and the air will circulate. Now the box on top and bottom of the frame are considered a manifold. You cut away the insulation and use a hand-saw to cut a very shallow track between every second gap in the bridge with all the holes for the cans. You slide the little panels of aluminum in the grooves so the air only moves down one can tower at a time. At the very end, you can use the same hole-saw to cut a space in the top corner and the opposite bottom corner. You can use the images as a guide. This way the air can leave one porthole and travel through EVERY Can, collecting heat energy. You left the TOPS of the cans in-tact so that the air would strike the can and baffle around, collecting more heat per can during it’s whole run. Ideally adapted for low thermal gain areas. You can use little tiny solar photovoltaic cells to power DC Computer fans to assist in the “draft”, like the picture here.

One really easy modification of my work is to put the hole in the SIDE of the device, and not the back. This way you could run a bunch of the machines in parallel (I believe you would lose efficiency in series, since the air will only heat up so much), the logic is like wiring seen here. By putting them in parallel, air can move into each system and relatively balance-out, heating a larger volume of air all at the same time. The final component is sealing the glazing, be it lexan/glass/plexiglass over the whole mess. MAKE SURE IT WORKS FIRST. 😛 If you make small screw holes every so far and use industrial caulking adhesive to make a good seal, you’ll NEVA Get it open. Better off setting it on fire and starting from scratch.

WHY This works is simple enough: heat rises, and hot air will rise up, sucking cooler air where the hot air was; called a convection current. As the cold air settles near the floor in your house, it’ll get pulled into the bottom of the assembly, assuming you have it directly attached to your wall. It is then heated up by zig-zagging through the cans and manifolds top and bottom until it gets as much as 30C hotter than when it went in. Now this has a HUGE variety of factors effecting it, from the angle of the array in correspondence to the average of the sun, to the transparency of your glazing, to the cloud-cover, to your latitude, to the size of the holes in your system, efficiency of any insulation you used, any number of issues could reduce the efficiency.

I want to build one of these out of pop cans, and build an IDENTICAL model, only changing the cans out for plastic pipe of equivalent size. Put a thermometer next to the intake and exhaust of each, and record the differences a couple of times per day over the course of a year. That’s a masters degree scholastic paper thesis, right there. “Thermal Efficiency Studied Across Multiple Models of Solar Furnaces in Northern Climates.” Build another bad boy out of pipe to circulate water, exact same form factor for the purposes of the study… shit, I smell a doctorate for this! I’m sure there’s a university who’d LOVE to publish a paper covering a massive MANOVA analysis of various sorts of off-grid heating solutions… hmm… where’s my bibliography…

2014-09-26 Edit: I’ve come to the conclusion that this has some SERIOUS potential in the outports here. I’ve begun the process of requesting research funding to build several of these models with subtle variations. The EXACT same box, but one variant will swap out the aluminum cans for copper tubing, another will swap it out for plastic PVC tubing, a third variant is being considered using multiple sheets of corrugated sheet metal. I can use thermal sensors attached to a tablet and hubs via USB to measure the temperature inside the garage, outside the garage, and inside the intake and exhaust of each system… as a measure of their thermal potential. I will probably seek the help of my professors at Grenfell Campus of Memorial University of Newfoundland, my Alma mater, to complete the study and present a formal academic paper proving (or DIS-proving, hey, this IS science) the viability of passive solar thermal furnaces in our climate. This paper will be the candidate to seek funding for building/manufacturing similar systems in the West Coast for our thermal heating potential. I may also contact CanSolair and inquire about franchising rights. Hmm… now THERE’s an idea…


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