Mother's Flat Plate Solar Collector

Here's an easy-to-build flat plate solar collector that's bound to cut your water heating bills!

| January/February 1981

067 flat plate solar collector - four collectors

Three different flat plate solar collectors were compared with a commercial collector (the rectangular unit at right) for performance.


One of the most important considerations in designing a multi-tube solar collector is the establishment of even flow through all the lines in the device. Because of pipe friction and related hydraulic concerns, it's all too easy to end up effectively bypassing one portion of a heater's tubes, while letting the fluid move too quickly through the other parts of the collector, to be thoroughly warmed. Considerable research has been dedicated to overcoming this difficulty. The success or failure of such efforts is one of the factors that separate a top-notch collector from a mediocre one.

So when MOTHER EARTH NEW' research staff set out to build a solar water heater that could be easily duplicated by the average handyman, they elected to avoid the problem entirely! Rather than attempting to mass a large number of copper tubes inside an insulated box—which is the most frequently used approach—our workers decided to assemble a single-pass, in-line flat plate solar collector that can be duplicated as many times as is necessary to get a particular water-heating job done.

Hot Stuff

During the course of building and testing several different models—which were compared both with each other and with a standard commercial collector—our team not only developed an impressively effective water heater, they also discovered some interesting time- and money-saving shortcuts. For instance, a unit that was constructed from copper pipe with copper fins soldered to the tubes actually achieved only a 1°F higher temperature than did a similar collector built with recycled .007" aluminum printing plates sandwiched and stapled over the pipe ... and the rate of temperature rise was nearly the same!

Most important, the collector performed almost as well as a costly commercial unit that had been purchased for comparison purposes. When we linked three of our homemade jobs together to achieve a square footage similar to that of the "benchmark" collector, the water temperature rose only 1°F per hour more slowly in our setup. And our "did 'em ourselves" devices produced water temperatures that peaked only 7°F lower than did those of the liquid heated by the store-bought model ... at an amazing 140°F!

Build It

You can prepare for the (roughly) half a day's time needed to duplicate our collector by gathering up everything you'll need in advance. Consult the list of materials at the end of this article to determine what you'll have to buy at the building supply store. (Although we've chosen a 10' unit, you can build the heater in almost any length that's convenient.) Then locate a saber saw, tinsnips, a hammer, a heavy-duty stapler, a utility knife, a caulk gun, some 50-50 solder, and a propane torch.

Start by cutting the Thermax to match the width of your 1 X 12, and securing the foam board to the wood with panel adhesive. Then carve two grooves—down the length of the insulation—about 1/4" deep and 6 inches apart on center. (A sharp utility knife will do the trick for cutting foam insulation.)

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