"Compound" Parabolic Solar Cooker, Make a Template With Technical Drawing

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Introduction: "Compound" Parabolic Solar Cooker, Make a Template With Technical Drawing

Compound parabolic dishes have a super advantage over parabolic dishes. You rarely have to reposition them as the sun moves. So you can leave them unattended for long periods!
There are many possible compound parabolic dish configurations and rather than me try to satisfy everyone, I though it would be better to give you a method to make your own petal templates.
Then you can make a 1 hour, 2 hour 3 hour or 4 hour compound parabolic solar cooker.
Whatever you want is within your grasp! You can easily make petal templates for parabolic dishes with this method too.
For 12 petal templates the error in the method (because you are using flat material to make a surface curved in 3 dimensions) is about 1%. I believe that is acceptable.
If you do not want to read the whole instructable, just read the image notes on the first image.
It should suffice for many people.
Brian

Step 1: Compound Parabolic Concentrators, What Are They?

The typical compound parabolic concentrator is a fairly old device, They are shaped like a bicycle light or a tulip flower with a hole at the bottom and as long as the sunlight enters within the acceptance angle it gets reflected all the way down the concentrator and gets trapped there at the bottom. They are made by putting the focus for 2 identical parabolas on the curve of the other one. To do this you have to twist each parabola (which gives you the acceptance angle) and the resultant 2 dimensional shape is spun on its axis to get the flower shape of the compound parabola.

Many times the reflectors for box ovens are based loosely on this type of concentration. They are good because if you face the box cooker the right way it stays collecting sunlight well for a long time.
But I wanted something different. I set out to apply the same principle to parabolic dishes.
Parabolic dishes are great because they can focus a lot of energy but they quickly go out of focus and have to be readjusted. There is virtually nothing out there about applying the same principles to parabolic dishes so I had to start from scratch. My first compound parabolic dish was a great success but not many people were interested in copying my construction method. They prefer a template.

Step 2: How to Make the Compound Parabolic Shape

easy way to make a compound parabolic shape. You have a parabolic shape already with its focus.
All you need to do is rotate it as shown and you have the compound parabolic shape!
45 degrees acceptance angle gives a 3 hour solar cooking dish. The axis of the dish will be through the 22.5 degree line. Half of the acceptance angle.

Step 3: 45 Degree Compound Parabolic Petal Template

The first 2 images show the figures to make your own and the shape of the particular template.
Scale up until you get the right size.
Figures that are important for this template scaling are:
6.5 cm from top of the dish to the focus
The height of the dish is 11.7 cm and
the radius of the dish at the bottom is 12.3 cm
The cooking pot should fit in the scaled up 6.5 cm space between the focus and the top of the dish when the solar cooker is in operation.

Step 4: A Little About Compound Parabolic Dishes and Parabolic Dishes

For now I am just leaving an animated gif here to show the differences between parabolas and compound parabolas for dishes. Please note that this is very much a shot in the dark for me. There are likely to be errors (but not substantial ones) in my explanation. Eventually others may confirm or improve on what I explain but I wouldn't hold my breath. Please go with this until something better comes along.
Note. Parapolas are like circles in that there are large and small ones but the fundimental shape is exactly the same. Unlike circles, nobody has ever seen a complete parabola. (the ends go on infinitely).
Compound parabolic dishes can have their curves come in and meet up. I show the MAXIMUM EXTENT for useful curve for a 30 degree (green), 45 degree(blue) and 60 degree(yellow) compound parabolic dish in the animated gif. Note of course that making the maximum extent would be costly and you can just use a cost effective portion nearer the focal point just as people do with parabolic dishes!
For my compound parabolic dishes all I did was twist one half of the parabola round its focal point.
This concentrates the light in the area between the back of the dish and the focal point as long as the incoming light is within the acceptance angle of the dish.

Step 5: How Do We Test a Compound Parabolic Shape for Effectivness?

This is part of my continuing attempts to understand what I am doing!
Below are pictures of a 60 degree compound parabolic model that I made around January 18th 2009
As you can see, depending on the angle at which the picture was taken, the reflection of the apple or little ball was very different. (The apple happened to be closest to the size of the distance between the focus and the bottom of the dish). I think the photos are a clue to how effective the dish can be.
I also used a lazer spot to test its effectiveness.
I have not done anything scientific just yet but it is a start! As you get closer to directly over the dish, you get an increase in the size of the reflection that probably coresponds to how effective the dish will be when the sun is in that posititon.
Brian

Step 6: Known Issues

I. When I rotate the parabola around its focus for say a 45 degree compound parabolic dish, I am left with a 45 degree"gap" which I just fill with and arc of a circle. But an arc is probably NOT the best way to fill this space to reflect the light towards the cooking vessel. A line or perhaps a dimple pointing at the focus might be better and will allow for greater concentration of light and a smaller cooking vessel.
A circle is good enough for me for the moment. I am sure someone will find a more accurate solution in the future. This will allow for more concentration of energy and larger reflector to cooking pot ratios while maintaining the long unattended cooktimes.
2. I do not know exactly how the light reflects off the dish as the sun moves across the sky.
Some light may miss the cooking pot when the sun is not directly over the focus.
3. A large (2 square meter) stationary dish (perhaps 60 degree compound parabolic) may be suitable for same time every day cooking all year long if aimed at the equinox sun at its highest point.
but I do not know how it would behave when the sun traverses 23 degrees above the aimed direction (high summer) and 23 degrees below, (mid winter) and the tropical equivalents of these times.
This awaits software modeling or small physical reflector models and laser tests.
Thank you Brian White

Step 7: Other People's Work

Other people have traveled the compound parabolic road.
Anyone who designs a flashlight reflector or lampshade reflector to spread out the light from the bulb is doing something similar, just doing it in reverse!
A guy called Roland Winston had patents for compound parabolic troughs. The patents have now expired and are online. He is a well respected scientist. He did things probably the perfectly mathematically correct way so I will put one of his patent drawings here. I found it online on about the 14th of January 2009. (Pity I did not see it earlier)
Here is a picture of one of them from the patent application itself! As you can see, he has a bump or dimple under the receiver. Once again, I do not know how this will work when we make a dish.
He seems to use 2 parabolas without a combined focus. Please take a look.
I do not know if I am allowed to show the entire patent (I have it in pdf format on my computer) it is easy to search for it, if you just put in the patent number and winston in your search.
As you can see, it is fairly similar to what I came up with but it is a trough not a dish.
Brian

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19 Comments

Regarding the cone by Ronald Winston: The dimple at the bottom
allows the heated item to be inside the cone and heated from all sides.
If the target was at the bottom of the cone, some heat can conduct out
the bottom.

Another key part of the design is to have the light
reach the target with just one reflection or bounce. With each
reflection, a small amount of light is lost, this adds up relatively
quickly so ensuring only one bounce is required over the acceptance
angle provides the best heating. If you look closely at the angles,
you'll be able to see that a ray just grazing the upper edge of the cone
that hits near the dimple will be reflected to the target with a single
bounce.

Finally, I've heard that using a string and pencil will
also allow you to trace parabolic cones like the ones in the patent. One
end is fixed to your target, and the other end the other end represents
the light source. It may require some playing around to get curves that
look right and you may need to reposition the 'light source' end to
keep the incoming 'rays' parallel (Just keep the light source end the
same 'hight' from the target otherwise the curve will be wrong). You can
use this technique to create unique collectors, I saw one that was
almost like a nautilus seashell.

Hi, I have gone further along since this to the solar design T-square and clam shaped collectors. You are not correct about 2 bounces. Sure you lose a bit of power (to the dish metal that will re radiate it) but the gain in acceptance angle can be worth it. It is a trade off. One bounce, move the dish lots, multiple bounces, move it less. The only way to have a dish collector with a greater acceptance angle than a parabolic dish and a decent heating time between moves is to have more than one bounce. I wanted a dish for solar cooking. The main trap that every researchers falls into is that they make dishes with a wide acceptance angle in all directions. But for solar cooking you only need a collector that you aim at the sun and it accepts for about an hour. In an hour the sun moves (like a spotlight tilting) in a very specific way so a dish should have its acceptance angle in line with that tilting. So all the designers should be taking this into account. Not a single one does, except perhaps the designers of the butterfly and papiollon reflectors. I pointed this out 3 or 4 years ago and nobody "got" it. One of the emperor has no clothes type statements. So flipping obvious that only a child would get it. I made 2 models with a "claymation technique" and scaled one up to a full size reflector (something like 1 meter primary collector. I cooked potatoes in a big pot with it in february. Aimed it at the sun just once. I have pics of the cooked potatoes but they are not on this computer. After that I gave up. Nobody is really interested. Everyone comes back to parabolic dishes and anybody who solar cooks will tell you that parabolic dishes are not good for solar cooking because you have to attend to them all the time. And it is dangerous to be out in the heat like that.

gimplayerstsquare.jpgIMG_0956.JPGIMG_0949.JPG

Hi all,

I read carefully all information posted by gaiatechnician.

I found this webpage that might be interesting for those of you reading this post:

http://solarcooking.org/plans/parabolic-from-flat-sheet.htm

Cheers!

I was wondering if you experiment with the compound parabola. Where I live if there is sunshine, on any given day there is enough of it, there's no need to go with extra ordinary measures. A blessing in Winter, a curse in the summer.

I have done hardly any experimenting this year due to workload. I gave a better and far more useful reply on your other comment, check it out.
Thanks Brian.

Great article, thanks!
Please post the patent number for the work you mention by Roland Winston, I'd like to see what he did.
I wrote a freeware program that calculates parabolas of any depth and diameter, you can see or download it here . I'm thinking of writing a calculator to help people make compound parabolic collectors as you've described in this article, they are extremely useful because they don't have to be aimed as parabolic reflectors do, looking at his patent might help me get a few ideas about how to approach it.
Thanks, Mike

Hi, Mike, glad to hear from you. I gave a talk in February and in the research for it I found a very good pdf about Winston and other curves. (All seem to be for troughs). Anyway, several might be good, winstons with a 47 degree acceptance angle, (horizontal and aimed at the equinox sun would work), and there are 2 "seashell" troughs, and the trombe minell cusp. I don't remember the patent no. but if you send me your email address, I can send you the "interesting curves" PDF, etc.
It contains some of the math that you need.
Anyway, the seashell troughs are completely new to me but they help show that I was on the right track with my "clam shaped" solar cookers. (Even if I only ever made one).
Have you seen the solar design T-square? Thats what I used to make the clam shaped design. Maybe you could do a better design and more repeatable design with your grasp of mathematics? Brian

Patents themselves are public domain, you can always copy and publish them (the documents themselves), even if they have not expired. The idea behind patents is that by telling others how to make and use the invention, they (the others) will be able to use that knowledge to improve other things. There is an incredible amount of information residing in the U.S. patent database, just waiting to be rediscovered and/or reapplied.

However, if the patents have not expired, you just have to be careful about making, using, offering to sell, selling or importing anything that is covered by the patent's claims. All modern U.S. patents have claims. See 35 U.S.C. 271.

would like to make one to accept a 9.5 in. pot .Just scaling up approx 49 in petal. can scale up be used or do i have to try to start from scratch.

I misspoke I intend to take half of template and cut a curve as in the template using 3/8 ply. then duplicate this curve 6 timesto create a half circle. On half circle i will place formica to make a crved half circle,affixing formica as needed with fir strips for backing to hold curve and half circle, repeat for other half making of course a ltittle overlap to connect the two half circles . My computations using your template shows me that this will create shape that is approx 31 1/2 inches in diameter at top and approx 48 inches in height along centerline(focus line ). will line surfaces with mylar. Will also affix a swivel platform for pot and add a side door opening to allow access to food pot . Mount side brackets to allign parabolic to sun and put on a swivel base for any other adjustments to alignment. It seems that this shape is very close to what you would make with wet clay. If you take this concept further you could use these curves to make your cob dome. My real concern is have i done the scaling right. . all i did was measure and expand by 10x. Surly do arrpeciate ypu taking the time to respond. I am a almost total novice . Have built large solar oven that gets to 400 on a nice day . This should have about 800 sq in surface and be suitable for cooking . Question "is this doable?