This page describes experiments of the Durango Renewable Energy Group with a solar cooker made from a recycled satellite dish.
I have long wanted to try to convert one of the multitude of discarded old-style satellite dishes into a powerful reflective-style solar cooker. At our June 30th, 2002, meeting Ben Jason and Holly Nagler arrived with a sample dish to experiment with. This dish is about 7 feet (2.1 meters) in diameter. The reflective surface of the dish is a parabola in 12 wedges formed of expanded aluminum, attached to ribs of square cross-section aluminum tubing.
The satellite dish is an ideal raw material for a cooker project. In our town, these dishes are often offered free in the classified ads. The dish is a prefabricated parabola, a shape that focuses visible and infrared light (or, originally, microwaves) to a single point in space called the focal point. The focal point is located above the center of the dish, where the struts hold the receiver in place. We removed the receiver since it is in the spot where we want to put the food. This dish is about 7 feet (2.1 meters) in diameter, about 38.5 square feet (3.6 square meters) in area. At the surface of the earth, about 1000 watts of energy fall on every square meter of ground when the sun is overhead. This value is called the solar constant. Multiplying 1000 watts/square meter by our dish area of 3.6 square meters, we see that the dish should be able to collect and focus about 3600 watts of energy. This is a lot more than most cookers, and is about twice the power of a countertop hotplate or griller. Thus, I feel the dish should be able to be turned into a powerful cooker capable of heating water or grilling foods quickly. We'll see!
With the satellite dish being spray-painted, here are (L-R) Walt Venable (supporting dish), Ben Jason (painting dish), Calixto Cabrera, Holly Nagler, and Judith Harrison
The spray-painted dish did not generate much heat at the parabola's focus. I believe there may be some dish materials (meshes) that are more suitable to simple painting than others. Rather than the expanded metal of the dish we salvaged, perhaps we can find a dish that uses a continuous, curved sheet material, perforated with multitudes of tiny holes. Thus all of the material remaining would be locally flat and facing toward the focus of the dish, rather than facing in different directions as does the material whose orientation has been changed in the expanded metal. Expanded metal reflects very well at microwave frequencies, probably with a wavelength of several centimeters (perhaps, 2-6 cm). But I believe it is less effective at visible to infrared frequencies, 1000 to 300 micrometers. To these waves, each portion of the expanded metal surface acts as a mirror, reflecting in its local orientation, not necessarily toward the focus of the parabola. This is just my theory at this point, but it indicates the expanded metal types will probably have to be overlaid by an aluminum foil or mylar layer to be effective reflectors. Aluminum foil could be applied for a simple, permanent covering. Or, if we stitched Mylar (or "space blanket") panels into a silver umbrella shape, the reflective surface could could be stored when not in use and then be tied (or "Velcro"-ed) into the satellite dish quickly, requiring minimal setup.
This oven is no longer available, according to an article in Home Power Magazine, issue #88, April/May 2002. Unfortunately, at the time of our meeting the immense Missionary Ridge fire near Durango was burning intensely, and even though our home is perhaps 15 miles from the center of the fire, smoke shrouded the sun and the cooker did not receive enough energy to cook the rice (Ian had to take the rice inside and finish cooking it on our stove). In normal sunlight, I have seen this oven perform well when baking cookies. I did not observe it closely, but I believe it took about 15-20 minutes to bake a batch of cookies, 6 at a time. Its design does not allow for a large area for the cooking vessel (hence only 6 cookies fit on the tray that fit inside the cooking container). However, the cooking container is deep and the clear glass cover is domed to allow more room inside.
We may attempt to replicate this design, in a somewhat simpler if bulkier version. This new, build-it-yourself version would use three or four 1 foot by 4 foot ("full length") mirrors. You can pick up these mirrors at many garage sales for US$1 to US$3, a nominal cost when compared to new, high-quality custom mirrors. Each mirror would be cut in half to form two 1 foot by 2 foot pieces, and these pieces would form the sides of a simplified dish shape. Since these mirrors are larger than the eight main mirrors of the Solar Chef (which are only 8 inches wide), they eliminate the need to fabricate the eight smaller triangular corner pieces. This is a significant reduction in complexity of fabrication. We will also make the cooking chamber deeper so we can replace the cooking dome with a flat sheet of glass. I have seen two versions of the Solar Chef cooker covers. One is made up of cut triangles of glass joined at their edges to approximate a dome shape, and is shown here. Another used an actual dome of some temperature-resistant plastic. Both of these methods would be difficult for a do-it-yourselfer to replicate, so I would like to try substituting a flat glass plate instead, in a hexagon or octagon shape to match the mirror layout. This can be cut from a larger, similarly-sized piece of glass with 5 or 6 cuts, respectively.
So, the total number of glass cuts could be reduced to 8 or 10 if the dish has 6 or 8 sides, respectively. This compares to perhaps 15 cuts to make the mirrors of the Solar Chef shown in the picture (or 23 if you don't already have mirror material that's 8 inches wide), plus probably 11 cuts to make the central glass dome. The disadvantage of making these changes is that the cooker becomes a little wider and deeper, making it less portable.
With four strips of aluminum foil attached to the satellite satellite dish, here are (L-R) Walt and Ellen Venable, Ben Jason and Holly Nagler
Even with only this much of the dish's surface covered with foil, and the sun vastly diminished by smoke in the air, the dish generated signifigant heat at the parabola's focus. If you placed your hand there it was brightly illuminated even in the dim sun.
To attach the foil to the dish surface, we mixed up a solution of 1 part white ("school") glue and 1 part water. We stretched the foil out and tore it off at the proper size to span the width of the dish. Then we took a little scrub brush and used it to coat the back of the foil (the side that's not as shiny) with glue. Finally, we stretched the foil out again, touched down with the center of the strip first, and then smoothed it out toward the outer edges of the dish.
We decided to attach the foil in strips across the dish instead of trying to make triangular cutouts to match the wedges of the dish because it seemed easier and would have the same ultimate effect. Also, we left strips of open space between the strips of foil to allow air to pass through so the dish would not be as likely to blow around wildly.
To continue the development of the satellite dish cooker prototype, we will fabricate a simple holder to attach to the four struts, allowing us to suspend cooking vessels at or near the focus of the dish. Then we can perform tests such as heating water (or cooking a hot dog!). We may also opt to add two additional, shorter strips of foil at the outer edges of the dish. This would provide even more power.