I now think that "compound" parabolic dishes have numerous advantages. (A compound parabolic solar cooker can also be made with the mathematician and I can add instructions later.)
Use hardiplank, cardboard, plaster concrete or cob to make any size or shape parabolic reflector with this device! Solar cookers can be made from waste cardboard boxes covered with kitchen foil. Mostly people use math to figure out the parabola or use a template. Lots of mcguyver types are not into math and waste cardboard comes in all shapes and sizes with cuts all over the place so templates are not always suitable..
This method of making a solar cooker is suitable for a one off or assembly line and will work with ANY cardboard Box. Please try one and spread the word about this way of making parabolic cookers.
I cook in dark glass containers and use an oven bag to reduce thermal heat loss.
Currently, I use My cob solar cooker to steralize soil. At 5 pm when i come home, my soil is still at about 60 C (after cooling down for about 3 hours!) And suitable for growing seedlings.
I feel that this method of making solar cookers will prove very adaptable, very affordable and find use all across the world.

Step 1: Introducing the Mechanical Mathematician!

Parabola's are hard to make unless you have some fairly fancy math skills.
But there is another way! The mechanical mathematician!
This little genius solves the parabola for all points on the curve! Using some string to help him. The string is attached at the focus of the parabola and goes through a point on the parabola (on a piece of metal for hanging curtains) and ends up tied at the pipe joiner.
There are many ways to make the mechanical mathematician, and the other one was used to make a cob solar cooker. Which works good.
It was made from old metal chairs and junk!
<p>I seem to remember that if you have two solid points with a somewhat loose string around it and you use a pencil that will pull the string tight while drawing around those two points you will have a perfect parabole. As I understand this may sound a bit cryptic, I added a drawing</p>
<p>Hi, no, sorry, your diagram will draw an ellipse not a parabola. .An ellipse is an important mathematical shape too but it is generally not used for solar cooking, or light concentration. I sometimes use the above method to make curved walls. </p>
<p>Actually I can think of one application that uses on ellipse for light concentration. Ruby lasers (or any optically pumped laser) They use a flash tube (like a camera flash) and a lasing rod or tube with an elliptical reflector around them with the two tubes at the foci. All the light from the flash tube converges simultaneously on the lasing tube. I think this is also the premis behind whispering domes.</p>
<p>my mistake. my math has become a bit rusty I guess</p>
<p>No problem. Thanks so much for going to the trouble to check it out! I have always learned more from my mistakes or from an experiment that when &quot;wrong&quot; than from things that went smoothly</p><p>. </p>
<p>no experiment ever goes wrong, it is just a way of discovering what doesnt work</p>
<p>While a parabola is ideal for an infinitely distant light source, an ellipse is ideal for a light source of a finite distance (such as a fire or lamp) where the two fixed points are the foci. Now if you had a really tall tower (say for a windmill or a water tower) and you could hang a string from it and place your other fixed point closer to the ground then you could draw a section of an ellipse that was pretty darn close to a parabola. Probably just as good as the precision of the Mechanical Mathematician. Of course you would need a tower shielded from the wind... </p><p>On the other hand you could use this method to draw an ellipse in your camp-site with the fireplace and your chair as the foci and place a bunch of mirrors on the ellipse then you would only need a small fire to keep yourself warm, or just a couple mirrors on the ellipse behind you to keep your back warm.</p>
<p>I seem to remember that if you have two solid points with a somewhat loose string around it and you use a pencil that will pull the string tight while drawing around those two points you will have a perfect parabole. As I understand this may sound a bit cryptic, I added a drawing</p>
In this post i came to know more about Solar parabolic cooker which i was searching since long. so thanks for the excellent description and detailing diagram for solar cooker<br><br><br> http://www.thehomeappliances.net/solar-cooker-review-of-smart-sun-solar-cooker.html
Honestly, it is not that good. <br>I have continued my &quot;research&quot; and things are now much better.<br>The clam shaped solar cookers are still a work in progress but are probably a whole lot better than this.
one also covers the satelite dish with foil.At my previous job we played around with that.almost melted the plastic rubbish bin.Haha
nice! <br>One that does not want to construct the dish can always use an old satelite dish used to watch cable tv as you guys would call it. <br>You can also build a mechanical device which you can control with a sunseeker circuit.Which is nice if you want the dish to be in the sun the whole day cooking or heating something up.
Also, if you measure the length of string used to draw the parabola you'll have the focal length. You can even choose a length of string in order to choose your focal length accurate to about 1/2 cm. or better.
I don't think that is exactly correct. But certainly, you can get the focal length you want by measurement at the central post. The length of string you need will be twice the focal length +the height to where it is attached on the saddle.<br/><a rel="nofollow" href="http://groups.google.ca/group/Sustain-the-development/web/the-mechanical-mathematician">http://groups.google.ca/group/Sustain-the-development/web/the-mechanical-mathematician</a> has some new info about making a mathematician and mould at the same time. <br/>(It is part of the accumulating barbecue project).<br/>I used a hinge and post and rested the arm on the frame for the mould to make a stronger construction with a much more steady mathematician. This should mean a more accurate dish (or part dish)<br/>I no longer make dishes. I make sections of dishes and there are a number of reasons to do this.<br/>
The string is intended to draw an arc for creating the curved surface. Once the focal length, and therefore the arclength, have been chosen and drawn, a piece of string half the length of the secant line of the arc can be used to draw the circumference. I'm not sure what is meant by "central post" and "saddle". Perhaps we're thinking of two different things.
I think we are thinking of 2 different things. I think secant generally refers to circles. The device above draws the surface of a parabolic dish, not a circle. The &quot;saddle&quot; is in blue in the diagram above and it slides back and forth. There is a picture of my latest mathematician at<br/><a rel="nofollow" href="http://www.appropedia.org/Image:Cobmould.JPG">http://www.appropedia.org/Image:Cobmould.JPG</a> and in that case, the saddle is made of wood, and it is on a wooden &quot;arm&quot; that is hinged to the central post. In that case, I am not trying to make a total dish, just a mould for a piece of a dish. <br/>(This might start to make sense if you look at the &quot;tracking solar barbecue, the wave of the future&quot; video which might be in the related column on your right.) <br/>
&nbsp;I don't understand why this makes parabola's<br /> to me it looks like it just makes circles, because the string keeps a constant radius, which makes a circle, while a parabola has to have a constantly changing radius.<br /> could you please explain why this works?<br />
The drawing is not clear enough but it&nbsp; has image notes to explain it.&nbsp; <br /> The string is attached at the focal point and at the saddle.&nbsp; <br /> It is not attached at the slider bar.<br /> String passes through an &quot;eye&quot; in the slider bar.&nbsp; <br /> As you move the saddle nearer or further from the central Post, string passes through the eye so it is not a constant radius. <br /> It traces a parabolic dish.<br /> <br />
thanks now i see what you mean
cool! i'm listed as a collaborator! ... but what do you need? was it a mistake?
Just a note that i have several video's on utube and on google video about this project now. If you know anyone who does misionary or relief agency work, please let them know about this and the videos. Some on google are longer and clearer quality (at least if you download them) so that you can better share the ideas. (Utube fogs up some videos a bit and you have to have the files small). If you are prepared to download large files, grab the explaination from google video. Please grab copys of the new start diagram too. And, anyone speak a foreign language? It would be excellent if it were translated into swahili, french, indian languages, tigali and spanish and portugeese. You have my permission to do so. Thank you Brian White
Another non math way to derive a parabolic curve is to get a length of dowel and just stretch a length of string from end to end. Just use the resulting curve as a profile, works ok. <br/><br/>Other non math solar cooker designs that may interest you<br/><strong>Harf </strong><br/><a rel="nofollow" href="http://www.thefocsle.net/solar/howtobuild.html">http://www.thefocsle.net/solar/howtobuild.html</a><br/><br/><strong>Parvati</strong><br/><a rel="nofollow" href="http://www.angelfire.com/80s/shobhapardeshi/ParvatiCooker.html">http://www.angelfire.com/80s/shobhapardeshi/ParvatiCooker.html</a><br/><br/>This may come in handy if you decide to make one of them, just print it out and stick it to something<br/><a rel="nofollow" href="http://www.lucasinjection.com/Degree_wheel_100.jpg">http://www.lucasinjection.com/Degree_wheel_100.jpg</a><br/>
I took a look at the harf link. I think that link is useful to parabola makers too. Your parabola does not need to be close to the focus. You can make the parabola a bit to one side of the focus if you wish! The advantage? Imagine aiming at a 9 inch round fish bowl. A bunch of the light from a 9 inch square directly under will miss it. But if your light is reflected at an angle, your square can be bigger and nearly all the light will hit the fish bowl. So less bends are needed.
Although it is a little harder initially to make the mechanical mathematician, the cardboard is more efficiently used by this method. More surface area is available to collect sunlight.<br/>Also, the mechanical mathematician is adaptable. If a larger piece of cardboard comes along, or a larger cooking vessel is available, You can reset the focus or the size of the segments to use that too. I have a video on utube about using the mechanical mathematician to make a parabolic oven with cob.<br/>These videos are pretty long though! Not for everybody's taste!<br/> <a rel="nofollow" href="http://www.youtube.com/user/gaiatechnician">http://www.youtube.com/user/gaiatechnician</a><br/>
errm.. Why do you sterilize soil if you don't mind my asking?
If you just use ordinary soil from your garden to germinate seeds you run into 3 problems. Weed seeds , microbial attack and bug attack. The chance of successful seedlings is greatly reduced. I find you only need to solar cook to about 60 centigrade to greatly improve your success rate. Solar cooked soil can also be used round transplants to stop weeds from germinating. I always apply water to the soil first so it doesn't burn in the solar cooker.

About This Instructable




Bio: 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 ... More »
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