Here's a design for an 83% efficient solar collector that uses old fluorescent tubes.
Here's yet another simple, easy-to-build, inexpensive, "make it from scrap" solar collector from the talented folks at MOTHER EARTH NEWS' Research Labs. Tests indicate that the unit operates at efficiencies as high as 83% ... and you can put one of the collectors together for only about $52 (less than $2.00 per square foot).
Start this project by rounding up a healthy supply of burned-out 96"-long fluorescent tubes (available for free from offices, factories, stores, shopping centers, dumps, etc., all over the continent). You'll need 29 of the tubes altogether for your finished collector, but it's a good idea to pick up a few extras in the beginning just to allow for breakage.
Puncture the metal or plastic ends of the long lights to relieve the vacuum inside, then use the special cutting tool described in MOTHER'S Fluorescent Glass Tube Cutter to remove one end from each tube. Next, pour some sharp sandblasting sand into the cylinders and tip them back and forth repeatedly until all the coating has been removed from their insides. Then cut enough off the other end of each cylinder to leave you with pieces of clean, transparent glass tubing, each 85" long and open on both ends. Finally, paint all 29 of your glass cylinders flat black inside and out (use a cotton pad, pulled through each tube with stiff wire, to spread the paint deep inside).
To help with the remainder of the assembly process, we've prepared this Fluorescent Tube Solar Collector diagram.
Next rout a 1/8" X 3/8"-deep groove in the sides of the collector box, rabbet its corners, trim the paneling to fit, and fasten the whole box together with wood screws. Cut one of the remaining 1 X 6's into four pieces and screw them to the bottom of the frame as additional support for the collector's back. Then, using water glass as glue, attach aluminum foil (shiny side up) inside the bottom paneling.
Cut the two 1 X 6 tube holders down to a width of 5 1/8", then draw a line down the middle of both boards and—starting 9/16" in from one end—mark the 29 center points (1 9/16" on centers) for the mounting of the modified fluorescent tubes. Next, drill out all the tube mounting holes with a 1 1/2" hole saw and cut the two boards apart on their center lines. One half of each holder should then be securely fastened inside the collector box (5 1/8" in from each end). Finally, lay a bead of silicone sealant along the surface of the holes in the mounted half-holders, set each cut-and-painted glass tube in place, spread sealant on the holes in the other halves of the holders, and—while the top halves of the holders are held snugly down on the tubes—attach the scalloped pieces of wood to the sides of the box with wood screws.
A length of T-bar (ordinarily used to support a suspended ceiling) is next added to the center of each completed tube holder to keep the 5-mil UV-X film that will be used to cover the collector from sagging down onto the tubes. Mortise a groove and cut a slot into the middle of each tube holder so the T-bar will set down flush with the tops of the pieces of wood. (Don't be afraid to file away the "bulb" on the bottom of the T-bar's "leg", if that's what it takes to make it fit down into the slot.)
Now's the time (before you close your collector up and make it too difficult to work on) to cut inlet and outlet air holes into one side of the box with a 3" hole saw. Smooth the edges of the holes with sandpaper, clean the sawdust out of the box, and slap a good coat of wood sealer onto all the wood—inside and out—that's still exposed on your collector.
Then stretch and staple the UV-X film to the box's top. The plastic covering should be further secured and protected—all the way around the face of the collector—with 3/4" X 3/4" outside corner molding. In addition (to keep air from blowing past, rather than through, the painted collector tubes), lengths of 1/8" X 3/4" screen molding should be cut to fit, laid down across the UV-X film, and securely screwed in place along the tops of the tube holders.
That's it. If you now hook a scrounged-up small (100- to 200-cubic-feet-per-minute) blower to your new collector's inlet hole, attach a flexible hose to its outlet, and aim the energy catcher at the sun. You'll find that the unit will give you up to 6,250 Btu's per hour on a clear day, which is enough to heat a room, dry your clothes, or dehydrate food in a bin. Experiment a little! And let us know how you come out.
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