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!
Step 1: Problems with parabolas!
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!
Step 2: Searching for a solution
But the cusps did not concentrate much while the winston concentrators might be suitable for a box oven but not for what I had in mind. They concentrate in an area behind the compound parabola.
Eventually I decided that if I rotated a parabola round its focus by a certain number of degrees and joined the 2 inside curves It might work for me.
Using that shape focuses the light in an area at or below the focal point of the combined parabolas.
I have added image notes to the 2 pictures below to explain it a bit. Shape 1 is the parabola for a parabolic dish.
Shape 2 shows how to turn it to make a 45 degree compound parabolic dish (suitable for up to 3 hours of solar cooking without moving the dish!)
Step 3: Making the dome
I chose a pot, painted it black and used it in a model to decide on a good compound parabolic dish.
I saw that I could make a 3 hour 45 degree compound parabolic dish to shine the light on this pot.
so i set about making a mold for the dish.
First I marked out the piece of a parabola on a piece of wood and cut the wood to that shape.
Then I placed the wooden curve on a nail on a central axis.
I added rubble to under it to bring it close to the wooden form.
Finally I cobbed out the the wooden form all round.
Cob is a mixture of sand clay straw and water.
I had to let it dry for about a week.
Step 4: Making the dish
I had some old sign corugated plastic and I decided to use this for my first dish.
First I cut it so that it would fold down somewhat over the mold.
I held it down with rocks and the sledgehammer.
Look near the sledgehammer. See how the cuts overlap when it gets held close to the dome?
Wherever they overlap, you cut along again and remove the undercut.
Then you hold it all down with rocks or whatever and tape the sections together.
Step 5: Adding the mylar
I did not have enough material to cover the entire dome but who cares?
I cut pieces of mylar to cover the inside of the dish. I glued them on with potato paste.
I used big or small pieces depending on the curve. Smaller in the more curved sections to avoid wrinkles.
I let it dry and then tested it.
I hung a 7 liter pot over it and adjusted the dish until I thought it was pointing at the sun.
Later I put a turkey bag over the pot to conserve heat.
Step 6: Cooking!
Steamed soil is better for seedlings because it has no bugs and weed seeds are killed by the steam.
For cooking i used a little brown ceramic pot.
For a test I stewed some apple.
I forgot about it and it boiled over in the cooker.
The thermometer goes to HI when it is over 100C
Step 7: Conclusions, Template and pdf of my results
What I have made is a much more usable stand alone solar cooker!
The compound parabolic shape means that it does not need to be pointed exactly at the sun. The dish has to be in an area from the focus to the back of the dish. In my case the focus is at 10 or 11 inches from the back so my pot is almost perfect.
Here is a template so that you can make your own.
I think if you make the template from plywood, then use the plywood to cut the shapes in mylar, you can easily make 12 segments. Then just tape them together at the back and you should have a umbrella cooker the same shape as mine.
Use thin wire to keep your umbrella open (Up to you to figure out how!) and start cooking.
Graphs of my results in 2 tests heating 7 liters of water in a pot are shown in the other photos.
I also include a draft pdf file so you can check out my results.
Please note that I have a new instructable that is devoted to making your own template using basic technical drawing skills or a computer design program. If you can read a house plan and elevation, you can probably make a template!
Step 8: First attempt with the template. Cheap emergency solar cooker!
I decided to make an umbrella cooker even though it is almost October and results are unlikely to be useful. First I cut out the 12 segments, then masking taped them together. I cheated a little on the next stage, the wiring. First i duct taped along the rim so that the wire would have something substantial to stick to.
I placed the umbrella over the cob dome and made the rim of wire right there round it and then ducttaped over the wire. Then with the umbrella still on the dome, I cut 2 cross wires to go over it and bent them to hold onto the rim wire.
In hindsight I should have had 4 wires to hold it to shape!
I also redid my first compound parabolic with kitchen foil to see how that would work.
I will just put in some pictures to show the results (next page). I only got to about 85 C but as you can check, that is sufficient to boil potatoes and to bake them too!
Probably not many people thought of baking potatoes in a black pot! It works, even on Sept 29th.
My umbrella cooker was a first attempt and I did not in enough brace wires to keep the umbrella up. I just had the rim wire, and 2 wires criss crossing across the top. I think it needs 4 criss cross wires to be effective. Also, I had that tiny glass pot where I needed the big pot to grab all the light.
It still worked though!
Brian White 1st October 2008
Step 9: Cooking and Baking
I boiled potatoes in the emergency umbrella cooker and I baked in the big black pot in the (now coated with kitchen foil) solar cooker I made on the cob mold.
I was not happy with the performance of the emergency cooker but it clearly worked.
Partly the small pot was a problem but also there were lots of creases that probably bounced light in wrong directions.
As you can see both worked on the 29th of September. The temperature in either solar cooker did not go much over 80C.
I thought it was innovative of me to raise up the potatoes for baking!
I imagine in future designs that the potatoes can be put on little metal skewers on the insides of "baking" pots so that the heat can go straight into the middle of the potatoes for excellent baking!
Step 10: The future of the compound parabolic solar cooker?
The compound parabolic solar cooker is part of my seeking after my holy grail.
A low tech, tracking solar cooker. I think it brings it way way closer!
But it is a lot more than that. It is a pretty good reliable and easy to make stand alone solar cooker.
Even the little one I made puts about 200 watts into the food. On an engineering forum, a guy calculated that it could pay for itself in electricity costs in 50 meals. (assuming the material cost 30 dollars).
He calculated that my 15 watt solar panel would pay for itself in "wait for it" Wait, WAIT ______
Wait 70 YEARS!!!!!!
If anyone can make the umbrella cooker a bit more close to the proper shape (without wrinkles like I had), the cost comes right down to under 10 dollars!
That could be a life saver all over the world when people go hungry.
The compound parabolic shape means that you can use pretty low tech tracking and still get good performance. The seasonal change in the suns altitude in the sky is 46 degrees.
That means that my cooker (if on a single axis tracking system), could cover almost the entire YEAR!
That is incredibly valuable. Accurately Tracking the sun is criminally hard to do.
With this little thing you do not have to be really accurate.
I also see this compound parabolic dish (in a half dish version) as being super useful for upside down solar ovens on tables. You have your tracking half dish under the table focusing the light through a hole in the table into the underside of the highly insulated solar oven.
So you can get higher heat and a greater transfer of energy too.
Some people think me weird to want to use it for soil steaming. I spoke on an online forum and some gardeners are quite liking the idea.
Including one with a tv show! Expanding the uses of solar cooking is good in my opinion!
Brian (Diagrams to come soon)
Step 11: Scaling up
For instance, If you took all the lengths in the dish and the pot and just made them twice as big?
I have checked some of it. And here are some guidelines.
The surface area of the pot increases at the same rate as the area of the dish.
But if you double the surface area of the pot you increase the volume by about 2.8 times!
This means that twice the size of device will put exactly the same solar flux on the surface but there will be a lot more capacity inside.
I think that this means that compound parabolic solar devices will have more or less ideal sizes for specific uses.
Also this aspect, the volume increasing faster than the collecting area, might mean that 15 degree and 30 degree compound parabolic dishes will become the norm in larger units.
Solar cookers come in a huge variety of shapes and sizes and lots of them are great when pointed directly at the sun but performance becomes poor very quickly as the sun moves. I have been on the lookout for software to help with design since august 2008 or earlier. I finally found something useable by me. Art of illusion. With it, I compared a parabolic dish, various compound parabolic dishes and a hemisphere as the sun moves in 5 degree increments. (20 minute incriments). It is only preliminary results of course but the hemisphere wins as the best stationary dish!
Who would have thought! Parabolic dish came --- LAST!
In fairness my test was pretty crude. I just set up 7 cameras to render the scenes from directly overhead to 30 degrees off. (2 hours later). I used a red ball to represent the cooking vessel and I reason that the size of its reflection in the dish will approximate the cooking power.
Nothing was optimised. I put the ball at 000 at the xyz cooordinates and moved the dishes until the reflection image filled the dish with the camera overhead. Then I made all the other dishes invisible and rendered the scene with each camera in succession. Finally I put the 35 images in one jpg file for people to do their comparison. I have also uploaded a slightly better version of my art of illusion scene file for anyone else who wants to try this. I got a lot of help in the art of illusion forums with this and I thank them for it. The help thread is
If you use my scene file, you need to reduce the magnification from the default 100 to 1 or 2 to see the dishes The scene file should be .aoi when you download it. I have been told that it can download as .temp (Just change to .aoi and it should be fine)
Explanation of picture. With the sun overhead, all the dishes show a full red image, meaning that the sunlight will be all reflected to the red ball at the focus! So they are all equally good when pointed at the sun! By 10 degrees off (40 minutes later) a black cresent appears in the parabolic dish reflection meaning that this dish is much poorer in those conditions. It reflects conciderably less light to the red ball than do the other dishes! By 30 degrees off target, pretty much all the dishes are useless, the only red that we can see is the red ball itself. The dishes just reflect the sunlight back into space. (The people at art of illusion are not too impressed with my poor choice of magnification) etc. I should have had the red discs much bigger. Sorry
16th feb, I got help with a new scene file. If you use it, you can render a stop motion movie of jpg files of each dish very quickly, you can also move the movie timeline and your camera moves quickly.( I have video on youtube to help you get started with this.
http://www.youtube.com/watch?v=y-s2N_8tzQo is part one of software
http://www.youtube.com/watch?v=obMlr2pXXgY is part 2 It is poor quality but if you get through it, it is worth it.)
The scene file for making movies is pretty neat. It is called dishs all same size nullworks.aoi I used one frame per second and 48 frames in total
i left the 7 unused cameras so you can use them as markers as your move camera 1 in an arc over the scene
20th feb I added the 5 .mov files here for download so you can inspect how the dishes reflect light over time.
Depending on the dish, the reflection pattern is very Different! I urge people to download and inspect these quicktime movie files. They show a dish from ovehead to 45 degrees off the side.