Almost free garage heat – just drink a lot of soda

I’ve had a few days during the HMX build while I’m either waiting for parts or waiting for something to dry and had some free time. I’m not exactly one to sit and watch TV when I have nothing planned, so I set out on another project.

While I have electricity out to the garage now, heat has been an issue all winter long. Mattar graciously lent me his kerosene heater, which did an okay job of taking the bite off the chill. Insulating the garage would go a long way to help keep the bitter Vermont cold out, but that’s a project for another day. I decided instead to take advantage of the south-facing side of the garage and build a solar furnace to collect some of that sunshine just bouncing straight off my garage. My dad built one years ago and said he recorded a 110-degree temperature differential between inlet and outlet. And I had enough scrap materials around the basement to do something similar to what my dad built.

I started with some 2x4s and plywood to build a simple box. I’m no carpenter, but I learned that if it’s wobbly, just add more nails.

I actually built the box to certain dimensions, based on what scrap materials I had and on the dimensions of my heat collection method – aluminum cans. That sure was a lot of Sprite. Fifty cans in five columns of 10 will funnel the air upward.

Sealed the box using adhesive caulk, just to keep any heated air from escaping the box.

So you may have already thought, “How can air climb the columns of cans when there’s no hole at the bottom of the can?” Answer: drill press and 3/4-inch bit. Times 45.

The last five cans, the bases of each column, will sit on the bottom of the box and thus will be unable to draw air from underneath, so I poked holes in the sides of each of the five.

Stack the cans with liberal doses of adhesive caulk. Give them enough time to dry.

Once they’re dry, I painted each column with black BBQ paint. Black to best absorb the sun’s heat, BBQ paint to keep from flaking off the cans. At the top, I drilled an outlet hole. I left an inch or two of space between the tops of the columns and the top of the box to permit air to flow out of the columns.

I drilled the outlet hole based on the diameter of some wet-dry vacuum hose I picked up, about 1-1/2 inches in diameter.

At the bottom, I used another wet-dry vacuum attachement that would more evenly disperse the incoming air. Screwed it in at each end, then caulked the seal.

Then started to caulk the columns in place. At the bottom, you can see the inlet hole I drilled. At about this point, I realized that a better place for the inlet would have been through the plywood at the bases of each column. In this location, the air can simply pass over the cans (there’s about 1/2 to 3/4 of an inch between the cans and the upper edge of the 2?4 frame) and not really pick up that much heat. If I were to relocate the inlet, it would force all the air to pass through the cans and pick up the absorbed heat. Next time.

Had some red paint left over from one of Heather’s previous projects, so slapped on a couple coats to at least keep the weather off the bare wood.

The caulk is pretty strong. Kept the cans from falling out while I had the box inverted.

Also had some 3/4-inch PVC pipe from another previous project. Bought a couple elbows and T-fittings and whipped up a simple frame to keep the box off the ground and to angle it upward toward the sun. Didn’t give the exact angle too much thought.

Caulked a clear plexiglas cover on the front and sat the furnace out in the sun for a full day over the weekend to see how it would work.

Using some advanced technological equipment, such as this precisely calibrated pyrometer, I determined the intake air temperature, which should have been the same as the ambient air temperature, to be about 80 degrees.

Using the same equipment and methods, I determined the outlet temperature to be about 95 degrees – thus a 15 degree temperature differential. Not 110 degrees, but not bad , considering I didn’t even break $50 in materials – most of that being the plexiglas window.

Obviously don’t have the inlet and outlet attached to the garage – figures that the day I finish the furnace, it’s 80 degrees and sunny and it looks like we’re finally done with winter. Dad recommends wiring a pusher fan at the end of the inlet tube to keep the air circulating through the furnace.

Were I to do this again, I’d first make the furnace larger. As I recall, Dad’s measured something like four feet on each side. Obviously, the more surface area, the more heat you’ll pick up. Second, as mentioned above, I’d relocate the inlet to the back of the box to direct all the air through the cans. Or at least I’d cut a piece of aluminum to act as a baffle and prevent the air from rising straight up. Third, I might use those small soda cans I’ve seen in the grocery stores lately, just to get more surface area.

Fourth, I’d finish the build at the beginning of winter, not the end.

UPDATE: Welcome, MAKErs. I appreciate your comments and suggestions on improving the design of the box. I also appreciated the comments over at a similar project page on Instructables. Version 2.0 will be a lot better, so thank you all.

UPDATE UPDATE: The response on this has been fantastic. Thank you all for your comments and feedback. If I didn’t have the HMX to finish, I’d already be working on the next version of this box. By the way, I’m no engineer and only have the vaguest understanding of thermodynamics. I know how old cars work, that’s about it. But common sense tells me to build this thing bigger, to insulate it, to add a fan and to snake the air sideways as a few of you have suggested. Keep sharing your ideas and your successes in building your own boxes.

UPDATE (22.June 2010): I’ve taken many of the below ideas into consideration and finally finished a second version of the soda can solar heater. From 180 cans, I’ve so far achieved a 120-degree temperature differential.