It is trial and error and it will change a LOT before I can include it in the instructable.
I really hoped that someone else would do this and save me the trouble. I might not have time to use it but hopefully the pictures will explain what I am trying to do.
I am 99% sure that it will work well. (If not for me, then for someone with greater technical skill).
A great Austrian humanist called Wolfgang Scheffler thought up a new design for a solar concentrator while daydreaming in university class in Germany. He could not afford to make his design a reality in Germany or Austria so he left for Kenya (and its different labor and price structure) and after about 2 years the design became reality. It is still something they can make cost effectively in Kenya but not in Germany. As soon as he built the first one, he was asked to build another and pretty soon he was asked to help design more in India. So off he went and now there are thousands of Scheffler solar kitchens in developing countries around the world! But very few in rich countries due to subsidized fossil fuels here.
Basically Scheffler solves a problem with seasonal adjustment of parabolic dishes by warping the dish dish a little every day and effectively making a new parabolic dish every day as the sun's path changes with the seasons! His dish is on equatorial mount and turns at 15 degrees per hour (Just like telescopes following the sun).
Basically the task here is to design a parabolic dish that works on equatorial mount but does not need to be a shape changer. If we can do this we can have powerful effective parabolic solar cookers stamped out by the millions for next to nothing by the type of machine that turns a piece of metal into a car body.
Step 1: Problems
(which must be on the axis of rotation to keep the timing right)
And that is a big problem!
And another big problem is that every time you change your dish you screw up the center of gravity and that makes it harder to move the thing at the exact speed you want.
Also if you make your dish bigger to collect more heat, your poor cook can no longer reach the cooking pot without extremes of posture, etc. Scheffler dishes are 2 sq meters in size and bigger. Most other parabolic solar cookers are 1 or 1.5 sq meters.
How can we get a big fixed shape dish like Scheffler without running into these problems?
Step 2: Solutions
Having 2 dishes is no problem near the equator but at higher latitudes like Toronto it becomes a little difficult. You might need a stepladder to reach the second dish. (Or you might use it as a water heater).
Remember that this is just a model and the concept has evolved further.
More in the next couple of days
Step 3: How do we make the reflective dish? Without wasting too much material.
I even made one from cob about 4 years ago. I stuck the reflective material straight onto the cob.
But normally the idea is to use lightweight reflective material and bend and it into the dish shape. I tried petals cut to the right angles and lengths so that I could tape them together and make the shape that way. That was not a success for me. It tends to curve in 2 planes and that wrinkles the reflective material. I also made a big dome mold and bent corrugated plastic sheet over it and randomly cut and then cut again until it fitted snugly. then taped the lot together and that worked reasonably well.
I thought for this one I would try something different. A guy called Magnar owns solarcookers at ning and he has lots of information about 3 and 5 "cone" parabolic dishes. He cuts his plastic sheet into semicircular bands, and then tapes them together to approximate a parabolic dish. 5 or 6 pieces instead of 12 or 20 petals appeals to me!
The less cuts the better! I have been studying it today and yesterday and this is what I come up with.
First, decide how concentrated you want your parabolic dish. Do you really need a 2 inch ball of fire? Or will 6 or 8 inches wide do the work? Then decide the width you want.
And then away you go.
I started under the target with my first attempt to design it.
I now think that is a mistake. I think the best place to start is straight across from the focal point. You put a cone at 45 degrees there.
I will put explanatory diagrams in soon.
Step 4: Amazing luck!
I am still really surprised about how little material is wasted.
I ended up moving the corugated plastic sheet to the left to increase the size of the green strip and join the 2 green strips to make just one.
Step 5: Gaining rigidity.
I think a combination of ribs, at the back, wire through the corrugations, strings through the corrugations and strings and light bars across the face of the dish will keep it rigid enough.