The following is an excerpt from "DIY Solar Projects" by Eric Smith (Creative Publishing, 2011).
Solar ovens are simple devices that capture heat from the sun with a reflective surface that’s angled or curved towards a cooking pot. Because they can be easily made from cheap materials like scrap cardboard and tinfoil, they are widely used in areas of the world where trees and fossil fuel are scarce or expensive. Once made, they can be used to cook food and boil water in a reasonable amount of time for absolutely no cost.
There are dozens of possible designs; some angle the rays down into a small center area, while others focus the rays upward toward the underside of a pot, like a reversed magnifying glass. You can also buy portable solar ovens assembled from polished metal online—they’re great equipment for camping. But if you’re serious about integrating free fuel from the sun into your cooking, the plan below features a solar oven that works beautifully and is also built to last. Plus, you can build it for a fraction of the cost of a purchased solar cooker.
Depending on variables like location, ambient air temperature and the angle of the sun, a solar oven can reach temperatures above boiling (212° F). In ideal conditions, some types can reach 300° or more. This temperature range is high enough that you can safely cook any food, including meat. Cooking times are longer, but because the temperature is lower there’s little danger of overcooking, and the food is delicious.
Solar Oven Types
Solar cookers can be made in a wide variety of designs. The main criteria is that they have a reflective side or sides that focus sunlight toward a heat-absorbing (usually black) pot or base.
Made from cardboard and aluminum foil, this solar cooker is still capable of heating food almost to boiling. Variations of this basic design are widely used in poor areas of the world that have abundant sunlight but limited fuel; their use helps preserve dwindling forests.
There are numerous ways to make a solar cooker—one website devoted to the subject has dozens of photos of different types sent in by people from all around the world—and all of them seem to work reasonably well. We settled on this model mostly because we’re carpenters and we like working with wood more than metal. Feel free to modify it as you wish.
The cooker is big enough to hold two medium-size pots. All the pieces are cut from one eight-foot-long 2 × 12 and a sheet of ¾" plywood. The cooker would work just as well with ¼" plywood, but we used ¾" because it made it simpler to screw the corners and edges together. The base is made from 1½"-thick lumber for ease of construction and for the insulation value of the thicker wood, but thinner material would also work.
The foil we used was a type recommended for durability and resistance to UV degradation by an independent research institute. Unfortunately, it was expensive, and if you’re just starting out you may want to do a trial run with heavy-duty aluminum foil. Although foil looks a little dull, it actually reflects solar rays almost as well as specially polished mirrors.
Tools and Materials
• Circular saw
• Jigsaw or plunge router
• Tape measure
• Drill/driver with bits
• Speed square
• Eye and ear protection
• #8 countersink bit
• ¾" × 4 × 8-ft. BC or better plywood
• 2 × 12 × 8-ft. SPF SolaRefle× foil or heavy-duty aluminum foil
• 1⅝ and 2½" deck screws
• Clear silicone caulk
• Contact cement, or white glue and brush, optional
• Mid-size black metal pot with glass top
• Wire rack
• ¼ × 17¼ × 17¼" tempered glass
• No-bore glass lid pulls (Rockler item no. 29132)
• ¼ × 2" hanger bolts with large fender washers and wingnuts
||1½ × 11¼ × 19"
||1½ × 11¼ × 16"
|| ¾ × 19 × 19"
|| ¾ × 10 × 17"
|| ¾ × 20 × 33¾"
|| ¾ × 10 × 25¼"
|| ¾ × 20 × 31¼"
|| ¼ × 17¼ × 17¼"
Sun rays reflect off the foil sides and are concentrated at the base of the cooker, where they are absorbed by the black pot. The glass cover (or clear oven cooking bag) helps hold heat and moisture in the pot. The cooker should face the sun. Raise or lower the box depending on the time of year so that you catch the sun straight on. Shim the wire rack as needed to keep the pot level.
How to Make a Solar Oven
1. Cut the four 2 × 12 base pieces to length according to the cutting list. Arrange the base parts on a flat work surface and clamp them together in the correct orientation. Check with a carpenter’s square and adjust the parts as needed. Then drill pilot holes and fasten the pieces together with 2½" deck screws. See photo of step 1.
2. Lay a 4 × 8-ft. sheet of plywood on the worksurface with the better side facing up. Select a good grade of ¾" plywood (we used BC) or you’re likely to have issues with parts warping, and you’ll find it difficult to drive screws into the edge grain of the plywood. Mark and cut the 19 × 19" bottom piece first. Rest the full sheet of plywood on a couple of old 2 × 4s—you can cut through them as you make your cuts without any need to move them out of the way. See photo of step 2.
3. To create the panels that form the reflector you'll need to make beveled cuts on the bottom and sides so the panels fit together squarely. With the best side of the plywood facing up, mark two 20" x 76" long pieces, measuring from the two factory edges so the waste will be in the middle. Set your circular saw base to 22½°, then cut along the line you drew at 20" (20" is the long side of the bevel). Cut the other piece starting from the opposite end of the plywood. You should end up with two mirror image pieces. See photo of step 3.
4. Re-set your saw base so it's flat, then cut each 20"-wide panel in half so you have four 20 x 38" panels, each with one beveled 38" edge. With the beveled edge facing up and closest to you, draw a centerline at 18" on each panel, then make marks on the beveled edges at 8" on both sides of the centerline. Position a speed square so it pivots at the 8" mark, then rotate the speed square away from the centerline until the 22½° mark on the speed square meets the top of the beveled edge. Draw a line along the speed square as shown, then use a straightedge to extend the line to the other edge (the factory edge) of the plywood. Repeat at the other 8" mark, flipping the speed square and rotating it away from the centerline so the lines create a flat-topped triangle. Set the base of your circular saw at 40°, then cut along the angled lines (although it seems incorrect, 40° is the angle required to form a square corner when the pieces are assembled). Mark and cut the remaining three panels in the same fashion. See photo of step 4.
5. Finish cutting the reflector parts to final size and shape. TIP: Clamping or holding smaller parts for cutting can be tricky. Here is a useful trick: After you’ve laid out your cutting lines, set the workpiece onto a pair of old 2 × 4s. Tack the workpieces to the 2 × 4s with finish nails, ideally driven into the waste area of the panels. Keep the nails at least a couple of inches from any cutting line. Set your saw so the cutting depth is about ¼" more than the thickness of the workpiece and then make your cuts. See photo of step 5.
6. Assemble the reflector. Brace two of the reflector sides against a square piece of scrap plywood clamped to the work surface, then join the edges with screws driven into countersunk pilot holes. Repeat for the other two pieces, then join the two halves together with four screws at each corner, completing the reflector. The bottom edges should be aligned. The top edges won't match perfectly, so sand them smooth. See photo of step 6.
7. Make the adjustable leg, which contains parallel slots so the leg can move up and down over a pair of hanger bolts, raising and lowering the angle of the cooker so you can take full advantage of the direction of the sun's rays. Outline the slots in the adjustable leg of the oven so they are ⅜" wide (or slightly wider than your hanger bolt shafts). Locate a slot 2" from each edge of the adjustable leg. The slots should stop and start 2" from the top and bottom edges. Cut the slots with a jigsaw or a plunge router. See photo of step 7.
8. Screw the base and the plywood bottom together. Set the adjustable leg against one side of the base, then drill guide holes and install the hanger bolts so they will align with the slots. The centers of the bolts should be at the same height: roughly 2½" up from the bottom of the box. Use large fender washers and wingnuts to lock the adjustable leg in position. See photo of step 8.
9. Fasten the reflector to the base with countersunk 2½" screws. Angle the drill bit slightly as you drill to avoid breaking the plywood edge. Use two screws per side. See photo of step 9.
10. Cut pieces of reflective sheeting to fit the sides of the reflector as well as the base. You can use heavy-duty aluminum foil, but for a sturdier option try solar foil. The product seen here is essentially polyethylene tarp material with a reflective aluminum surface. Make sure to cut the pieces large enough so they overlap the edges and can be easily attached. See photo of step 10.
11. Glue the reflective sheeting inside the base and reflector, overlapping the corners so all bare wood is covered. Use contact cement or silicone caulk to adhere solar foil, and staple the edges to reinforce the glue; use diluted white glue with a paint brush instead of contact cement if you’re using aluminum foil. Pull or smooth out the reflective material as much as possible; the smoother the surface is, the better it will reflect light. See photo of step 11.
12. Take measurements to double-check the glass lid size. Ideally, the lid will fit in so it comes to rest about 1" above the top opening of the box. As shown here, a ¼ × 17 × 17" piece of tempered glass fits just right. Be sure to order glass with polished edges. You can also just use a clear plastic oven bag instead of the glass. Either will trap heat and speed up the cooking. See photo of step 12.
13. Caulk the joint between the angled top and the base with clear, 100% silicone caulk. Set a wire rack inside the oven to keep the cooking pot slightly elevated and allow airflow beneath it. See photo of step 13.
If you use rugged solar foil to create the reflective surface, you can glue it to the 2 x 12s and the plywood base prior to assembly. If you are using heavy-duty aluminum foil, which tears easily, you’ll get better results if you glue it to the wood surfaces after the box is assembled.
Getting a Handle on Glass
Since it is virtually impossible to lift the glass lid from above, you’ll need to install handles or pulls designed to attach to glass (available from woodworking hardware suppliers) to lift and replace the glass cover. The simplest of these require no drilling. You squeeze a bead of clear, 100% silicone into the U-channel of the lid handle, then slide the handle over the edge of the glass.
Cooking with a Solar Cooker
Anything that can be cooked in a slow cooker, including meat, can be cooked in a solar cooker (as long as the sun is out!). You can also make bread and other baked goods, rice, fish, potatoes, and dozens of other dishes. You’ll need to experiment a little with a cooking thermometer, because cooking times will vary depending on the time of year and where you live; most foods will need 2 to 4 hours. Other points to keep in mind when cooking in a solar oven:
• Be sure to adjust the back leg so there are no shadows in the cooker, and move the cooker every hour or so to face the sun directly.
• Since the cooking temperature is fairly low and the food is in a closed pot, it won’t overcook or dry out if you leave it in too long.
• You can use a candy thermometer or oven thermometer to find out how hot the oven is. This will help you determine cooking time.
• Avoid opening the lid unless absolutely necessary—it’s estimated that every time you open the lid you add 15 minutes to the cooking time.
• Wipe down the interior of the oven after every usage. Keeping the glass lid clean allows as much sunlight in as possible.
• You cannot cook in the oven without a dark pot with a lid. The dark metal of the pot is warmed by the sunlight and transfers its heat to the food.
• Do not allow children to use the solar oven unless they are under direct adult supervision.