How To Build A Solar Cooker

Save money and cook outdoors with this DIY home project.
March/April 1974
http://www.motherearthnews.com/diy/how-to-build-a-solar-cooker-zmaz74zhol.aspx
The sun's rays are reflected to a single point for quick, fuel-less heating.


D.S. HALACY

Special Note: Copyright 1959 by D.S. Halacy Jr., and originally published by the Macmillan Company as a chapter of the book, Fun With The Sun. Reprinted with permission of the author.  

A stove made of paper sounds about as practical as a pitcher carved from ice, but this reflector cooker - constructed almost entirely of cardboard - will broil steaks, grill hot dogs, fry bacon and eggs and make hotcakes and coffee. It will also heat water for doing the dishes. All that's necessary to make it work is clear weather, because this stove cooks with sunshine! 

Stop to think about it for a minute and you'll remember that every time we cook - be it with gas, electricity or charcoal - we indirectly use the sun's energy, which has been stored up and reconverted to heat. Basically, then, our solar stove's fuel is nothing really new. Even the use of direct sun heat for cooking goes back many years. Sun-dried foods have long been eaten, and crude solar stoves were built a century ago. Besides, who hasn't heard of cooking an egg on the sidewalk on a really hot day? 

In recent years, however, many advances have been made in the design of solar cookers. Today there are commercial models on the market that are fine for campers or for patio use. One umbrella-like design folds up for easy carrying and storage, and also provides an answer for the skeptic who wants to know what you do when it rains! Such a cooker is just the thing for trips. If you're dubious about how well the sun can cook a meal, or if you don't have the cash to buy a ready-made stove, get busy and build the one described here. At most, it will cost five dollars. If you use discarded cartons and other salvage material, the outlay will be only a fraction of that. 

Making a Solar Cooker

Materials
3/16-inch thick cardboard (as required)
2 sheets of poster board 
1 roll of aluminum foil 
1 18-by-24 inch piece of plywood 
64 inches of 3/4-inch aluminum tubing
1 3/4-inch mounting flange
1 hand grill
1 small telescoping curtain rod
4 feet of 1-inch broomstick or dowel rod
1 foot of clothesline
Glue (as required)
Masking tape (as required)
1 set of 3/16-by-1-inch bolt with wing nut
 
 

The reflector framework is cut from fiberboard, approximately 3/16 inch thick, the kind large cartons are made from. Some poster board and aluminum foil will complete the cooker itself. A grill (for hot dogs, hamburgers, or pans) is made from plywood, some tubing and an inexpensive hand grill that costs about 50 cents. 

Study the plans in the image gallery first to get the overall picture, and to see how much material will be needed. If you want to buy new fiberboard, two sheets of 4 by 8 feet will be plenty. These cost about 80 cents each at a box factory or supply house. The other items will be easy to find. Get everything you'll need together and then begin construction. An eager beaver can do the job in a day or so and begin sampling outdoor cooking á la sun right away. 

Directions1Cut a base piece 4 feet square from the 3/16-inch cardboard. We'll mark the layout of the reflector ribs right on this base. With a pencil and a piece of string, draw a 48-inch diameter circle. This is the size our finished cooker will be. Next draw two lines through the center of the base, perpendicular to each other as shown on the plans. These mark the location of the main ribs, which we will make next. 

A word about the principle of our reflector cooker will be helpful before we proceed any farther. The sun stove simply focuses all the sun's rays that strike its surface onto the bottom of the grill. Even on a clear winter day, the 12 square feet of area in our cooker collects a lot of warmth which, when shrunk into the 1-foot area at the grill, becomes concentrated heat.  

The giant solar furnaces used by some research labs (such as the Mont Louis installation high in the Pyrenees mountains of southern France) use curved reflectors too. They generate thousands of degrees of heat at their focal points, using the same principle. To do this they must be very accurately made and of parabolic shape. This specially shaped curve reflects all the rays onto one tiny spot and gives the furnace a concentration ratio of many thousands to one. Obviously we don't want such high temperatures, for they would melt our pans! 

Our reflector will use a radius of 36 inches instead of a true parabolic curve. This results in a larger spot at the focal point. Besides this, we will use a number of wedge-shaped sections instead of one bowl-shaped reflector. Thus our focal spot will be roughly the size of the cooking pan, which is ideal for our purposes. 

2. Now that we know the why of what we're doing, let's draw two main ribs as shown on the plans. Cut these carefully, using a sharp linoleum knife, pocketknife or modeler's razor knife, and be sure to plan ahead so as not to waste material. Each of the main ribs has a notch at the center. Notice that one is on the top and one on the bottom so that they'll interlock. 

3. Using a full rib as a pattern, mark out 12 half ribs as shown on the plans. Before cutting these, cement the full ribs to the base plate on the lines previously drawn. Model airplane glue or a good household cement will work well. While the parts are drying, cut out the remaining ribs. 

4. Three half ribs fit between each quarter section of the circle. Glue these in place, lining up the end of each one with the circle we drew on the base plate. While they're drying, cut the rectangular filler pieces of cardboard. As the plans show, these fit between the outer tips of the ribs to complete the framework. 

When the framework is thoroughly dry, we're ready to put on the wedge-shaped pieces of poster board. Since these form the curve that will reflect the sun's rays, we must use poster board that is thin enough to bend easily, yet has sufficient body to hold the proper shape. Lighter cardboard would have a tendency to ripple and wave. 

5. By means of cut-and-try methods, trim one piece of poster board so that it covers the space between two ribs, with about 1/8-inch overlap all around. Do not cement this in place yet; it will be our pattern for 15 more pieces. Cut the additional sections carefully, making sure they will cover any of the spaces between ribs. (In spite of care, there may be slight inaccuracies in the framework.) It's better to have the poster board pieces a bit too large than too small. 

6. With all the pieces cut we can now begin to cement them in place. Since butting the joints smoothly against each other would be difficult, we will glue eight sections into alternate spaces first. Spread glue along the tops of two ribs and the intervening filler piece, then lay the poster-board wedge in place and carefully press down so that it touches the ribs at all points. The glue will dry well enough in a minute or two so that you can go on to the next piece. Don't forget to leave every other section open. 

Now we can cover the remaining spaces with our second eight wedges of poster board. These will of course lap over the edges of the pieces already glued in place, thus making a strong joint. If you run into difficulty at the center where all the points come together, simply trim them off an inch or two. The hole left can later be covered with a separate piece of poster board. 

For added strength, seal all the joints with masking tape. While this isn't absolutely necessary, it will make a sturdier cooker. The reflector is now ready for application of the aluminum foil that will give it the mirror-like finish we need to collect heat for cooking. 

7.Cut out 16 pieces of smooth-surfaced aluminum foil, the kind used in the kitchen for wrapping food. These should be slightly larger than the poster-board wedges to assure complete coverage of the reflector surface. Use rubber cement to stick the foil to the poster board, and be sure to place the shiny side up. Work carefully and try to keep the foil smooth, but don't worry if the finished job isn't perfect. The cooker shown in the illustrations has a few ripples but works well anyway. 

8. We will now install a marker for the focal point of the reflector so that we'll know where to place the grill for the fastest cooking. This marker is simply a small, inexpensive curtain rod of the type used on kitchen doors. It consists of two tubes, one fitted inside the other. Cut a short length of the larger cylinder and insert it into a hole punched in the center of the reflector. Better still, use a drill the same size as the outside section of the curtain rod (or slightly smaller) to give a snug fit. Now cement the tube in place.. 

The smaller tube will fit into this "holder" and can be removed for easier handling when not needed. As we mentioned before, the proper place to mount our grill is one focal length from the reflector. With a spherical reflector that distance is half the radius or, in this case, 18 inches. As a double check, aim the reflector at the sun and adjust the tilt until there is no shadow visible from the pointer rod. Then hold a piece of wrapping paper with a small hole punched in it right at the tip of the pointer. Move the sheet toward the reflector and then away until the smallest spot of light is observed on the paper. This is the actual focal point, and our pointer rod should be trimmed to this length. 

9.Cut out two squares and one rectangle of cardboard for the adjustable support and cement them to the back of the cardboard base. The squares go first, and then the rectangle. After these are well dried, run a short length of clothesline through the slot and tie the ends in a square knot. Drill holes through a 48-inch length of 1-inch dowel (broomstick or tubing), spacing the holes about an inch apart halfway down the dowel. Insert a nail to engage the loop of clothesline. We can now set up our reflector so that it will stand alone. 

10. To make the grill, first cut an 18-by-24-inch plywood base. Any thickness from 1/2 to 1 inch will do. Mark the center of this base and install a mounting flange for the 3/4-inch aluminum tubing vertical support (which is 40 inches long). 

11. The adjustable arm is also aluminum tubing, 24 inches long. Flatten one end and bend it around a piece of pipe or a broomstick to make the collar, which fits over the vertical support. Drill a 3/16-inch hole as shown in the image gallery and insert a bolt capped by a wing nut. The other end of the adjustable arm may now be flattened. Be careful to keep the flat area at right angles to the collar so that the grill will be horizontal when installed. Slide the grill in place and the solar cooker is complete.  

Using the Cooker

Now that the work is done, the fun starts. Positioning the reflector is simple if you follow these directions.  

1. Stand behind the collector and face it right at the sun. Then tilt the reflector back until the shadow of the pointer rod vanishes as it did when we checked for focal length. This means that the collector is aimed perfectly and that all the sun's rays will be bounced right where we want them. 

2. Holding the reflector in this position, slip the dowel or broomstick through the rope loop and put the nail through the hole just below the loop. With the collector standing on its own feet you can now put the grill in place. Loosen the wing nut on the adjustable arm and move it up or down until the grill rests just above the tip of the pointer rod. As a double check, pass your hand quickly just above the grill. It should be hot, ready for you to start cooking. 

The grill surface itself is fine for cooking hot dogs, burgers or steak. Grease will drip onto the reflector but will not harm it. For bacon and eggs, hotcakes and the like, place a skillet on the griddle. And, if you like your steaks seared quickly to keep in the juice, use the skillet for them too. By putting it on the grill a few minutes early you can store up extra heat that will cook the steak more rapidly. 

Water for coffee, tea or dishwashing can be heated in a kettle or pot. To get the maximum efficiency from your solar cooker, use blackened utensils; however, just about any kind of utensil works satisfactorily. For variety try using a pressure cooker. 

3. Because the sun moves across the sky, the position of the reflector must be changed as time passes. In the early morning or late afternoon it will be nearly vertical, while at noon you will have to place it practically flat on the ground. That's why we drilled so many holes in the support rod. If you plan to boil beans or make stew, occasional adjustment of the reflector will be required to keep the hot spot where it will do the most good. The shadow from our pointer rod is the thing to watch. For bacon and eggs, hot dogs or steak, one setting will usually do the trick. 

At first the reflection from your cooker might be bothersome, and a pair of sunglasses will be handy. After practicing a while, however, you'll find out where to stand so there isn't any glare and by then you'll have noticed how nice it is not to have your eyes full of smoke. Solar cooking is cool cooking too, because the heat goes into the food on the grill and doesn't roast the chef as well. 

After cooking your meal and washing the dishes, remove the grill from the aluminum tube and clean it too. Then wipe off the reflector's surface with a paper towel or damp cloth and that's all there is to the job of solar cooking. 

Of course, solar stoves won't take the place of other kinds of cooking all the time. When the sun goes down you had better be through cooking, and on a rainy day the reflector is not much use except maybe to crawl under to keep dry! But properly used in clear weather this reflector stove will amaze the most skeptical observer.  

The Advantages of Solar Cooking

As you discovered when you held your hand close to the focal point, there is no warming-up period with a solar stove; it gets hot right away! By the time the fellow with the charcoal brazier works up a good bed of coals you'll be doing the dishes. Besides, he paid for his fuel while yours was free for the taking. And solar energy is available any place the sun shines mountains, desert, beach or your own backyard. 

You won't need matches to get your cooker going, either, and there's no danger of setting anything on fire with the unit. Lastly, there are no ashes or soot to contend with. And if someone complains about the lack of that charcoal or hickory taste, I suppose you can always provide him with a bottle of liquid smoke! 

Seriously, you should have a lot of fun cooking with sunshine. It's safe, it's clean and it's free. Chances are you'll like it enough to want a portable cooker for your next camping trip. That way you won't be tied down to a fireplace and the bother that goes with it. So save up for one of the commercially manufactured folding cookers, or you might even put your ingenuity to work and make a collapsible version of the cardboard unit you've just built.