This little solar oven, which originally appeared in a 1959 book Solar Science Projects by D.S Halacy, Jr., will work on any sunny day at any time of the year. It works like a greenhouse: rays from the sun can pass through the glass front, but a good deal of the longer waves can't find their way back out. The result: heat is trapped inside. There is no real guarantee how hot your finished oven will heat, but if well constructed they typically heat to around 350 degrees F (175 degrees C).
Please keep in mind that this is from a classic article, and some things have changed since it was written. For instance, contemporary designers usually work in simpler materials, such as cardboard. Also, the interior spaces on contemporary solar cookers are not always painted black, as they are with this design. Black walls make the walls heat up, not the food, and besides many paints are toxic and can give off fumes when heated. Better to use a dark cooking pot instead. Also, fiberglass insulation is no longer recommended. If insulation is used at all, it is now usually paper or natural fibers. Many designers question the need for insulation in the first place! That said...
Basically the oven consists of an insulated metal box with a glass front to allow heat in. Surrounding the glass front are aluminum reflectors. The oven and reflectors are constructed from sheet galvanized metal or aluminum, so some knowledge of metal work is required. If you prefer, you can have a sheet metal shop do your cutting and bending for you. Note that metric measurements were not included in the original design; however these can be easily worked out if needed. You will need:
- 28-gauge galvanized iron (or aluminum) - 16 square feet
- No. 6, 3/8-inch sheet metal screws - approximately 24
- 2-inch fiber glass insulation - 12 square feet.
- Double strength window glass - 22 x 24 inches
- Drawer pulls - 3
- Flat black paint - 1 spray can
- 2-inch roofing nails - six
- Sealer strip - eight feet
- Aluminum sheet .025 by 22 by 24 inches - four pieces
- Hinges (see text)
- Galvenized wire, app. 10 feet
The bottom of the box is a rectangle of metal 25 1/2 inches by 19 3/4 inches. The corners are notched so that the sides can be bent up as flanges. The back and two side flanges are 3/4 inch wide and are bent up 90 degrees. The front flange is 1 inch long and bent to a closed 45 degrees. Once the flanges are bent up, the dimensions for the bottom should be 24 inches by 18 inches.
The two side panels are triangular and can be cut from the same piece of metal. They are 18 inches high and 18 inches deep, not including the flanges (see illustration). The flanges (only 2 sides) are 3/4 inches and bent to 90 degrees. Make sure to make the bends on these two side pieces opposite so that you don't wind up with two left or right sides!
The back has 3/4 inch flanges on each side and a hole for the door. The finished door opening will be 8 by 12 inches with 1 inch flanges for strength. Notch the corners of the door opening and bend in the flanges to 90 degrees in the same direction as the side flanges.
The top of the oven box has 3/4 inch flanges on all four edges. The back and side flanges are bent to 90 degrees, and the front flange is bent to 45 degrees open to match the slope of the glass.
Next you will make two retaining angles to hold the glass in place. These are strips of metal 18 inches long and folded lengthwise so that one side is 1 inch wide and the other is 3/4 inch. The angle of the bend needs to be 45 degrees.
The door is a bit more difficult to make. The 3/4 inch flanges are folded completely over, then 1/4 inch is folded back so that is at a 90 degree angle from the door (see illustration). The dimensions of the "standing" flange are 8 inches by 12 inches and should fit snugly into the door opening. The tighter the ft of this door, the better seal it will make and the better the oven will hold heat.
Now make a false bottom to prevent the food from collapsing the insulation on the bottom. It must be just small enough so that it does not contact the sides, front or bottom. This would cause heat loss due to conduction.
Once all of the pieces have been cut and shaped, it is time to begin assembly. The simplest way is to use 3/8 inch no. 6 sheet metal screws. Mark a pencil guideline 3/8 inch from the bottom edge of the side panels, spaced as shown in the illustration. Center punch the holes and drill with a number 40 drill. Set the bottom panel on a flat surface. Hold a side panel in place against the bottom panel. Drill through the holes in the side panel into the flange on the bottom panel. Add a screw as each hole is drilled to make sure the panels remain aligned. Repeat for both side panels.
The back can now be screwed on to the bottom and sides in the same manner as the sides were attached to the bottom. Before the top can be added, the glass needs to be installed.
Clean the glass thoroughly. Glue on a sealing strip (gasket) around the edge with appropriate cement. Once this is fully set, carefully slide the glass through the top. Lay the oven front (glass) side down on a flat level surface, being very careful not to break the glass.
The angles that hold the glass in place are now installed. Holes are drilled in the side of the box where the strips will go. Four screws should be sufficient. The 1-inch leg of the angle piece will go against the side, so the holes should be a little less than 1/2 inch from the corner formed by the side and its front flange. Slip in one of the angle pieces so that the 1-inch side lies flush against the side of the box and the 3/4 inch side lies against the glass. Reaching in from the top or back, press the angle lightly against the glass. Do not press so hard that it flattens the sealing strip; this strip both insulates and provides a cushion for the glass. While holding the angle in place, mark in pencil the positions of the screw holes through the holes previously drilled in the sides. Remove the angle piece, drill the holes as marked, then reinsert the angle piece and screw into place. Repeat for both angles.
Put the top piece into place. Note that the top fits over the side and top panels. Drill holes through the top and into the back and sides; screw into place.
A handle (drawer pull) is now screwed into place into the outside center of the door. Two more handles can be added to the sides for ease of transport.
Fit the door into place and mark holes for the turn button latches that will hold the door tight. Drill the holes using the appropriate size bit (depending on the bolts used for the turn buttons) and attach the turn buttons using nuts, bolts, and washers. The washers allow the turn buttons to clear the hemmed edge of the door.
The fiberglass insulation is now cut to fit the top, bottom, sides, and back. The front edge of the bottom piece can be beveled at 45 degrees to provide a neater fit. Keep in mind when cutting the side pieces of insulation that you will need to compensate for the thickness of the top, bottom, and back insulation. Paint the inside surfaces of the fiberglass using flat black enamel spray paint. Paint the false bottom piece as well.
Once the paint is dry, the insulation is now glued into the box using an appropriate cement. Start by removing the back of the box and laying the box face (glass side) down, being careful not to crack the glass. Glue in the top piece first and allow it to dry. Set the box right side up and cement in the bottom piece. Once dry, press five 2-inch roofing nails into the insulation and place the false bottom over them. Now glue in the side insulation.
Cement the insulation to the inside of the back panel. Cut out the rectangle for the opening and glue this insulation to the inside of the door. Carefully replace the back and its screws.
As is, the oven will only reach app. 250 degrees F. By adding aluminum reflector plates, more heat will be directed into the box. The 4 reflectors are made of aluminum sheets (Alclad) and are approximately the same dimensions as the front (glass) of the oven. When cutting these sheets, keep in mind that they will be hinged to the box and will fold over the face (to transport and to protect the glass), so the first to fold in (the bottom and sides) will be slightly smaller so as to fit. Rivet two hinges to each reflector. The side reflectors will have hinges along one side edge or the other; the top and bottom will have hinges either along the top or bottom edge. Attach the reflectors by screwing their hinges to the box sides and top, making sure the shinier sides will face the glass.
The top reflector should have a small round hole, about the size found in loose-leaf paper, punched in the center near the edge opposite the hinges. The side and bottom reflectors will each have two holes, one at each corner of the side opposite the hinges.
You should test your oven on a sunny day. To use, set it in the sun, facing the sun. It should be on a level surface off the ground; the edge of a picnic table works well. Loosen one screw in the top center edge of the back (you can also use an eye screw in place of this screw). Open all of the reflectors. Swing the top reflector up and down while watching inside the oven. You will know when it is in the correct position by observing the sunlight reflecting into the oven. To hold it in place, insert an piece of galvanized wire through the hole, bend it (glass side) to form a stop, then tie off the wire to the loosened screw or eye screw on the back. Open the side panels so that they are app. 30 degrees to the direction of the sun (120 degrees from the front of the oven). Attach each to the top reflector with a piece of wire. Swing the bottom reflector, watching the inside of the oven as with the top. When it is properly positioned, fix with two wires from it to the side reflectors.
It will be helpful to insert an oven thermometer in order to estimate cooking times. If you are baking for a long period of time, the top and bottom panels may need to be adjusted periodically.
Now that you have read this classic article, be sure and visit The Solar Cooking Archive for many more solar cooking ideas. Well worth the visit, as many of these are simple and efficient designs using materials that are very low cost and easy to work with.