Instructables

Making and using a "compound" parabolic solar cooker (Large project)

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Best shape for solar reflectors?
Parabolic dishes to a point ONLY when they are pointed EXACTLY at the sun. But the sun moves and that point quickly spreads all over the place unless your are watching over your solar cooker all the time.
My conclusion after a couple of months of working with a prototype is that a parabolic dish is the wrong shape!
This instructable is about the shape I found to replace the parabola and how you can use it to make a simple reliable "Compound" parabolic solar cooker. Mine will focus the heat on your cooking pot for up to 3 hours without moving it!
http://www.youtube.com/watch?v=uX9Z-nsUHiA Here is a video link to help too.
You can skip to the 7th page if you just want to see a template for the cooker.
Use the template to make "petals" from any sheet material.
Then tape the template together with masking tape or duct tape and stick your reflective coating on it.
Finally a use for corrugated election signs when the election is over!
 
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Step 1: Problems with parabolas!

Why I needed a new shape
The tracking solar accumulator
It was a solar cooker that consisted of a pot of material and a parabolic reflector that turned slowly to track the sun on an equatorial mount. This parabolic reflector was not the bottom of a parabolic bowl as most reflectors are but rather, a section from the side of the bowl.
I did this daily tracking with a low tech tracker that I made.
It was just an adjustable waterclock (also on instructables)
The sun moves higher and lower in the sky with the seasons and I intended to do this adjustment twice a week to keep my dish pointed exactly at the sun and the light exactly on my pot of food. I had to make a "mechanical mathematician" for making my section of a parabolic dish, and I had to make a low tech tracker too.
One problem was that every time I adjusted for the sun's change for the season, the center of gravity changed and I had to readjust the balance of the dish. Time consuming!
Another is that parabolic dishes are precision devices.
If you aim slightly wrong at the sun or if your tracking mechanism is slightly inaccurate, or even if you bump your dish a few times (as I did) you will not get the hoped for results.
This are some of the reason that I needed a more forgiving shape than the parabola!
warengadia4 years ago
ihave idea you put inside the box or card board
gaiatechnician (author)  warengadia4 years ago
Yeah, you could have a box cooker up where my oven bag and pot are.
This was the first test of the idea and the second test is beginning soon with completely redesigned equipment.
Hi Brain,
I would like to know the type of leaser level that you used for the that interesting compound parabolic experiment? can you also tell me what is the aproxx cost of that type?
gaiatechnician (author)  Krishnashaw3 years ago
Is that the experiment with strips of mylar? It was a mastercraft laser level.  Laser levels are much harder to find these days in Canada.  The el chepo ones are gone. Basically any laser level that does a line will work. You just have the line pointing straight up and down.  Mostly they have adaptors to do the line and you just plug that in.  If you buy a laser level (for general use), it is far more important to have the dot than the line. The dot can be seen much further away.  This is probably my most useless laser level.  It only does a line and even though it is pretty powerful, it is pretty much useless outdoors, in daylight because a line fades very quickly further than 10 or 12 ft from any laser level. My best laser level was a jobmate 9 inch one that I got for 15 bux.  It has a better brighter laser than laser levels costing 4 times as much. (And just as accurate). By the way, laser levels can be checked with a water level for accuracy.  Some are not so accurate when you check them!
Hi Brain..
Thanx for these interesting tips.
I am actually working on compound parabolas, but could not able to get it right...the compound parabola that you have shown is continuous.(above video).
but i have found few site in net that shows some different picture for it..
(url:http://www.fossilfreedom.com/increase-output.html)
I am totally confused can you plz help regarding this.

gaiatechnician (author)  Krishnashaw3 years ago
Hello, Krishnashaw, The fossil freedom thing is a compound parabolic TROUGH based on a winston patent that has expired but the picture is scaled WRONG. Do not use it! I scaled it up on paper and played with it a while before realizing that it is crap and the angles in it are useless!
My thing was the first part of my attempt to turn that trough into a dish. (Because parabolic dishes have such a short time before you have to move them and because dishes have greater concentration.
So basically the bad news is that my stuff here is just part of the process to making a better dish. It is probably a bit better for solar cooking than a parabolic dish but it is not as good as I had hoped. My latest version is the clam shaped solar cooker. That should be best of all but I still have a LOT of work to do on it. So search "solar design t-square and clam shaped solar cookers" for my most up to date stuff. (And please be prepared to work on my foundations, and also be prepared for mistakes by me) I am not perfect and I do not have the math for this but the trial I made of the clam shaped solar cooker seemed to work pretty well.
If you want to go into the theory of troughs and compound parabolic troughs, send me a message and I can send you pdf's with the correct dimensions and with several other trough designs too. (the pdf files are not my work but were obscure yet very interesting stuff and helpful for the hobby trough and dish maker). You might never find them on your own. Hope this helps
Brian White
Hi, Brain.. sorry for the late reply... i was actually busy with my exams..
it would be good if u can send me the materials you mentioned above..
the problem is that i have already done experiment on compound parabolas and don't want to leave this work with out seeing its end.. so i will do it and lets see if i can make it a little efficient..
i just need your materials to study..
my email address:
anilkrishnashaw@yahoo.co.in
Regards,
Anil.
rimar20006 years ago
Good work, and good intentioned. The earth needs more people as you. I am making a similar design, equatorial mounting included. I tried many methods to make the parabolic collector, and my conclusion was that the metalized plastics are not convenient because each wrinkle, large or small, behaves as a hole in the reflective surface. And there are thousands of them. Then, I am using glass mirrors. The problem with they is that it is necessary to calculate the form of each mirror with certain precision, for which I had to disinter my old know-how of trigonometry, and then to orient them carefully one to one so that they aim all the sunlight to the same place. They are 85, for an circular array of one square meter. These difficulties are real and despicable nothing, but once surpassed, the result is acceptable. You live as you say in a very high latitude (mine is 33º south), which obliges you to maximize the cares. In full summer the abundance of sunlight permits to cook an egg putting it inside the car with the glasses closed, but in winter one must have a parable or an arrange of mirrors well designed and well built. Otherwise, one wastes time trying to cause to boil a small pot of water. Pardon my English, I live in Argentina. I speak Spanish.
gaiatechnician (author)  rimar20006 years ago
I accept your argument about the wrinkles. I found kitchen foil produced more wrinkles than the metalized plastic. A bigger problem is that some of the metalized plastic lost its metal. I do not know if they are coated front and back or just at one side. I suspect that I laid some sheets upside down and the metal reacted with the wheat paste. But I have not looked into this. I simply cannot tell if they are coated both sides or just one side. But I also think that this time I produced acceptable results. i have them in the pdf file at the end. www.barli.org/documents-PDFs/solar-cookers2.pdf has results from a sk14 solar cooker in India heating water 10 liters in a 12 liter pot. So basically the solar cooker is about twice the size of mine heating 10 liters in a 12 liter pot. It reliably took just under 2 hours to bring the water to near the boil. Mine heated 7 liters in an 8 liter pot. My best results was just over 3 hours to get to a similar temperature. But remember it is September in Canada! So compared to one of the best that there is (The SK14) it did not do too bad at all. I love your version of English. The meaning is clear and it is almost poetic. Thanks for the encouragement. We try to do an acceptable job in the time available! Brian
i think there is a parabolic cooker potentially with a same unattended time http://www.solarcooking.org/unattendedparabolic.htm
gaiatechnician (author)  lsiyong3 years ago
Thanks, I retired my compound parabolic and made a clam shaped solar cooker for unattended use. I think that the clam shaped will easily beat the parabolic because it collects more sunlight. You do get a double bounce of light sometimes but the extra collection area more than makes up for any losses from the extra reflector.
Brian
The plastic sheet is metalized on one side only. You can detect it damaging the surface with the tip of a knife or something similarly sharp.

Another problem with the glass mirrors is the weight: one square meter of 2 mm thickness (the thinnest I can obtain and cut) it weighs 6 Kg, which obliges me to put a counterweight to balance the parable, and that at the same time obliges me to reinforce all the structure.

There are also light plastic mirror, but is very expensive for me. And besides the salesperson told me that it does not resist the elements a lot of time, it goes losing the shine quickly.

We continue insisting, that some day our efforts will be valued by who do not share the generalized attitude of spending and to waste the fuels.

if you want, you can see two brief videos of my first successful test of my arrangement of mirrors, winter 2006, in http://es.youtube.com/user/rimar2000 or, if you can't enter there, in http://es.youtube.com/watch?v=NAg16m_O0os and http://es.youtube.com/watch?v=Z08OqEaXS_M
Gentlemen, Y Caballeros...Here is a site to help, (Aqui' te' encuentreras un sitio que tal vez puede ayudarte).

http://www.cockeyed.com/incredible/solardish/dish01.shtml

Enjoy...(Disfrutela)
I have used this method for many years and it works nicely--Actually you only need one side of the Parabolic curve. You can copy the curve to the other side.

Hope this helps.

http://www.sciences.univ-nantes.fr/physique/perso/gtulloue/conics/drawing/para_string.html
warengadia4 years ago
iknow that solar cooker and how its work
wilson, te5 years ago
The template is difficult read. Reflective light makes it difficult. Is there another source for template? tkstaco
Here is a parabolic template which you may find useful

http://www.solreka.com/parabolic-solar-cooker-template-150cm.html
Plasmana6 years ago
This is really good, I was hopping to make a parabolic solar cooker a while ago, but found it too hard to make...
gaiatechnician (author)  Plasmana6 years ago
I am trying to do one just with mylar and my template today. I got 12 segments cut out of the mylar and I taped them together with masking tape. I had the mark the mylar first round the template with a marker and then slice along the lines with a scissors. It did not work just slicing round the template. You get a jagged cut that way. It is not great, probably need more segments to do a better job. I still have to figure out how to hold it in umbrella shape! But I bet it will work. If it works half way well, the umbrella one would be awesome for emergencys or camping because it stays cooking for 3 hours unattended. I think the key to getting these things working really well is NOT to use a parabloic shape. I think a hibred between a compound parabola and a cusp will end up being the right shape. Do my optics experiment and decide the best shape yourself! Brian
Okay, I will try my best soon!