Design, construction, analysis and promotion of solar cookers
Sixteen Sided EB800D 0.8 Meter Diameter
The sixteen-sided EB800D evolved from our twelve-sided EB30D.
The number of sides was increased from 12 to 16 because with twelve sides a small portion of the reflected energy from the sun will miss the target.
We designed a cooker with all metric dimensions because we believe that many of the people who may want to build one of our cookers will live where the metric system is used.
We chose 0.8-meter (800mm, 31.5in) diameter because it is large enough to cook well, but small enough so that it is easy to move and to store. For permanent installations we recommend a larger cooker for faster cooking.
The 0.8-meter diameter EB800D has an aperture of 0.5m2. If the solar insolation is 1000W/m2, the cooker will intercept 500 Watts of solar power. The solar radiation consists of direct and diffuse components. If we are conservative and assume the direct component of solar insolation is about 700W/m2, the cooker will intercept about 350 Watts of solar power. When coking, we recommend enclosing the cooking pot in a greenhouse container to minimize heat losses from the pot. An oven-roasting bag probably is the easiest greenhouse container to obtain. Figure 1 shows the finished cooker with the cooking pot inside a glass greenhouse container
Figure 1: Finished Sixteen Sided EB800D Cooker
Figure 2 shows the profile of the EB800D. It is similar to other EB Cookers where the target is placed near the mouth of the cooker so that the cooker will primarily heat the bottom and sides of the pot, while most panel cookers heat the top and sides of the pot. The target is a 160mm diameter sphere. If a pot with smallest dimension at least 160mm is used, all of the reflected energy should hit the pot when the cooker is pointed directly at the sun. We recommend using a somewhat larger pot to insure that all of the reflected energy will hit the pot even when the cooker is pointed a few degrees from the sun.
Figure 2: Profile View of EB800D
We chose to make the sixteen-sided double cone for this prototype from bright sheet aluminum. Many other materials could be used. The cardboard with aluminum foil that we used for our 12-sided EB30D prototype probably is the least expensive option, but we prefer more durable materials.
The reflective cone can be cut from a single sheet of material as shown in Figure 3, with one of the sixteen sides shown in Figure 4. However, since we did not have a large enough piece of aluminum sheet, we made the cone from two pieces and spliced them together. If you do splice two or more pieces together, we recommend making the splices between cone sides along a radial line.
Figure 3: Layout for EB800D Reflector
Figure 4: One of Sixteen Sides
Layout of Reflector Cone
We recommend laying out the reflective cone by following the following steps.
Establish a center point and draw four concentric circles of radius 113.4, 143.4, 339.4, and 565.7mm. One easy way to draw the circles is to make a template by starting with a thin piece of material at least 600mm long (A meter stick could be used if you have one in which you are willing to drill small holes.). Drive a small nail near one end of the stick. Then mark and drill holes just large enough for the tip of a pencil or pen 113.4, 143.4, 339.4, and 565.7mm from center of the nail. Then use the template by driving the nail into your reflector material where you want the center, and use a pencil or pen that fits snugly into the holes you drilled in the template to draw the circles. Keep the template in case you build more cookers later.
Mark sixteen 156.7mm long cords along the largest circle.
From each mark on the largest circle, draw a radial line to the center of the circle.
Locate the point where each of the lines drawn in step 3 intersects the smallest circle. Make marks on the inner circle 9mm on each side of each of these points.
Draw a line from each point on the inner circle that was marked in step 4 to the point where the corresponding radial line intersects the next to the largest circle. This completes the layout.
Cutting Out and Assembling the Reflector Cone
With the layout completed, the next job is to cut out the reflector cone by cutting along the lines on the layout. The best way to cut the material clearly depends upon the material you are using for the reflector. Since we were using sheet metal for this prototype, we used tin snips for the cutting. If using metal, be sure to wear gloves, and use protective safety devices to minimize cuts and other injuries from removed sharp bits of reflector material.
Depending upon the thickness of the material, we recommend cutting along slightly different paths. The difference involves the small triangles between the two middle-sized circles. For thicker material, the triangles are cut out, while for thinner material cut is made along the radial line and the excess material overlaps adjacent material.
6. Options: For THIN material ( < 2mm)– continue to step 7 OR for THICK material (>= 2mm) go to step 19.
7. Cut around the outside of the piece following the largest circle, the smallest circle, and the radial lines at the edges. Be sure to include a trapezoidal tab about 25 mm wide by 226 mm long located along the radial line at one edge as shown in Figure 3.
8 . At each intersection of two lines where a cut will be made, drill a 2mm diameter hole through the material to relieve the stress in the material.
9. Cut a slit along a radial line from the smallest circle to the second largest circle. There are 15 places to make this cut. (Before making this cut, make sure that there is a 2mm diameter hole at the end of the cut.)
1 0. Cut a short slit along the second smallest circle to the point where the circle intersects the line drawn in step 4. (Each cut will be about 9mm long. There should be a 2mm diameter hole at the end of each cut.) There are 32 of these cuts. These cuts define one edge of each of the 32 tabs. Make each of these slits into a V by clipping off a triangular segment on one side of the slit.
11. Drill a 3mm hole in the center of each of the 32tabs for a wire to hold the tabs in place.
12. Once all the cuts have been made, use a fine file or a piece of emery cloth or sandpaper to deburr the reflector. The goal is to remove all sharp edges.
13. If the reflector base material does not have a reflective surface, glue a reflective layer onto the backing material, and then trim the reflective material.
14. Bend up the 32 tabs. Be sure to carefully make the bends so that the alignment of the reflectors surfaces will be correct.
15. Fold the smaller 16 reflector sections along the circumferential edges to create the inner parabolic geometry of the reflector then bend the16 outer reflector segments along the radial lines.
16. Use the large tab and four fasteners to join the outside edges together.
17. Place a flexible wire through the bent up tab holes. Tie the ends of the wire together and tighten as appropriate to assure properly aligned reflector tabs and reflector geometry. Figure 5 shows the rear of the completed reflector. (In Figure 5, a shoestring was used threaded through the tabs. However, since the shoestring stretches with time, we recommend replacing it with a wire)
18. Go to step 28.
19. Cut around the largest circle.
20. Add a trapezoidal tab about 25mm wide by 226 mm long as shown in Figure 3, along one outside radial line. Cut around the tab and continue along the radial line. Cut around the smallest circle, and the other edge along the radial line to the outside. Cut out the smallest circle. This leaves a hole. (For very thick material you may want to eliminate the tab and instead make a butt joint with a strap.)
21. At each intersection of two lines where a cut will be made Drill a 3mm diameter hole through the material to relieve the stress in the material. (This step may not be necessary for cardboard and some other materials.)
22. Cut a slit along a radial line up to the second largest circle. There are 15 places to make this cut.
23. Cut out the triangles between the two center circles by first cutting a 9mm slit along a circumferential line. Then cut along the long side of each triangle.
24. If the reflector base material does not have a reflective surface, glue a reflective layer onto the backing material, and then trim the reflective material.
25. Drill a 3mm hole in the center of each of the 32 tabs for a wire or shoe lace in the situation where corrugated material is used to hold the tabs in place.
26. Bend up the 32 tabs. Be sure to carefully make the bends so that the alignment of the reflectors surfaces will be correct.
27. Fold the smaller 16 reflector sections along the circumferential edges to create the inner parabolic geometry of the reflector then bend the16 outer reflector segments along the radial lines.
28. Use the large tab and four fasteners to fasten the outside edges together. (If using a butt joint, join the edges with a strap.)
29. Place a flexible wire through the bent up tab holes. Tie the ends of the wire together and tighten as appropriate to assure properly aligned reflector tabs and reflector geometry.
30. This completes the reflector.
Figure 5: Rear of Reflector
Base, Support Structure and Pot Holder
Since the base, support structure and potholder for this cooker are the same as for the twelve-sided EB30D, the descriptions are not repeated here. Refer to the descriptions in the twelve-sided EB30D document