Dual Parabolic Dishes on Equatorial Mount (and Their Descendants)

Introduction: Dual Parabolic Dishes on Equatorial Mount (and Their Descendants)

About: I am a stone mason. My hobby is making new solar cooking and gardening stuff. I have used solar heat to cook soil for a couple of years. In mother earth news in January, i read that their compost expert does...

September 5th I am working on a one dish version and you can see it on facebook at
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

One of the reasons for Schefflers solution is that as the seasons change you have to keep the parabolic dish pointed at the sun AND the focus pointed at the cooking pot or heat collector.
(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

This is a model of a solution. It is a little unusual in that there are 2 focal points and 2 parabolic dishes.
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.

First of all, there are numerous ways of making a parabolic dish.
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 didn't just come up with the image below.  I worked for several   hours yesterday with trial and error to find something that might work with my equipment and material.
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.

The dish is currently too floppy.  It is not a perfect parabolic dish. It needs rigidity and certainty about its shape.  How can we get this while keeping it as light as possible?
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.

Step 6: Old Stuff

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    6 Discussions


    9 years ago on Introduction

    thanks for the technical research. this has been really helpful. I'm trying to do a parabolic tracker to "fuel" a sterling engine. we'll see how it goes


    Reply 9 years ago on Introduction

    I just made a video about setting up the equatorial platform.  I guess a compass would work too but this takes its readings directly from the sun. 

    Reply 9 years ago on Introduction

    I am doing more today and I think it will be very helpful to you. One thing is collecting heat from my cone solar dish and the other is about setting up an "equatorial platform" and putting the cone solar dish on it. (I took a day off and it is sunny). A rare combination this year in Victoria! My cone dish took a beating in a recent windstorm and it is warped so don't put too much value on my readings. I have to take readings because there is thin but widespread interest in this. Anything helps people make their judgment. Anyway, I did a really crappy job with my dish. Bamboo stays were only added as an afterthought.
    If they were planned into the design from the start, it would have been much more rigid and would focus better too.

    I uploaded another video about the cone solar and confirmed the declination angle and even though the cone solar was not as successful as I had hoped, the mistakes are treatable, especially when I put it on the equatorial platform.

    9 years ago on Step 6

    I know that what you're trying to accomplish is well over my head, but wouldn't a satellite dish work? When I go to the local junk yard they are plentiful. I have often dreamed of putting them to use, and again I am probably way off base so just take my comments with a grain of salt. Hope you get your design off the ground, I love what you are doing.

    Cheers, William


    Reply 9 years ago on Step 6

    Hi, William, I think a satellite dish will work IF you can counterweight its center of gravity. I just work on this idea when time permits so unfortunately it goes really slow. And I am not great at making the stuff I dream up so worse again.
    The long term object is to have something that slowly rotates to follow the sun, and have the receiver ALWAYS in the one place. So you still probably have to lop off a chunk of the dish, or slices through it to attach it to the equatorial mount because it would get in the way at some parts of the seasons.
    My project stalled last September. (My backyard gets little light in winter). And this spring in Victoria has been the coldest wettest since the 1940's. I took it apart and rebuilt part of it recently but now that the sun has showed up, I am insanely busy at work.
    Finished at 7.40 today, 9.40 last night and about 8.40 the previous night. (That's burnout schedule for stonemasons!) Anyway, thanks for the comment. Glad someone is still checking the stuff out. Maybe one of you will make it a reality.