New way to form parabola for solar cookers

[gaiatechnician]

Check out "mechanical mathematician" on the net. It is basically a simple way to make a parabolic dish. It requires no great knowelege of math and (for poor people) the only outside material needed is kitchen foil. I used it to make a 4 by 4ft solar cooker from cob. (sand clay and straw mix).
I see it as an excellent helper for people to make large parabolic cookers with minimal outside help and interference.
I think it could also be useful to make deep parabolic solar ovens and combination parabolic box cookers (with the parabolic reflector under a table and the box upsidedown on the table recevieving the light from underneath.
The section of Parabola could be designed in place under the table with the mathematician!
Check it out. I also have video. It was a one person operation so video quality is poor. And some video is long. Nearly 10 minutes showing the full process for making the cob parabola.
Brian White

Victoria bc Canada

[Johnny Electriglide]

Nice cheapo project. The problem is the structural need of a tripod hook to cooker mount to stay at the focal point and the ability to track with a structurally attached two way pivot and attach. No wonder the commercially available parabolic tracking cookers/boilers are so expensive. Kind of like my full tracking Wattsun 8-75 watt panel system.
I use the Global Sun Oven, and have two of them. In summer they reach 300*F in 20 minutes and can be propped up with the back locking tube and hand pivoted every hour. Actual cooking time compared to a conventional oven is about +40%. I actually cooked a Thanksgiving turkey in one, but it took two days. Cakes 1.5 hours, whole chicken 2.5 hours, salmon half .5 hour, 3 quart stew 3 hours. I live close to 9,000 feet so that adds to the time. Clouds can ruin the timing or even stop the cooking altogether.
Similar reflector ovens from cardboard covered with aluminum foil work at lower temperatures and for smaller amounts of food.
The commercial parabolics reach several hundred degrees C quickly. There are giant reflector cookers, too. I think the cob or mortar stiffened cardboard parabola of yours could work better with an integral adjustable lifter on a point on the edge that would be kept aligned with the center toward the sun, going up and down by adjusting to the suns angle. Similar to how the Global Sun Oven works, but with a large campfire tripod suspending the dark pot to the focal point. The parabola should really be full circular for maximum concentration, and have to be smaller than the tripod's base. The cooking pot shades the center of the parabola, so a post mount with the circular pot holding top of the "T" at the focal point and black(maybe a welded tipped T), the base hole in the parabola large enough for typical mid day adjustments at the latitude. The parabola would have to have a focal point similar to the equilateral triangle edge to edge plus center dip of the commercial ones. A plywood structure could be scribed with the device for a say 6" height at the rim and cut and glued parabolic plywood sections (say 3/8" spruce 4 ply), then the card board with foil glued to that. Once the pattern for the full diameter center piece is made the radial sections would be scribed "halves" center cut to fit an expanding hub section (scribed to a butt cut and glued). 12" or less around the outside rim. First two -3/16, second four -9/32, third 8 -1/2, etc. to maintain a perfect circle. Keep up the good work!!!!

[gaiatechnician]

I am working on another similar idea. A mechanical mathematician to make a large parabolic dome. Then when the dome is made, you just use it as a form for making parabolic cardboard reflectors.
Perhaps someone can try making parabolic reflectors on the back of a satalite dish? It should be really easy to do. YOu just make the first bunch of radial cuts, fold down to overlap and then cut the overlapping slices off.
I think telescope trackers are the way to go. Something like 30 dollars for an equatorial moung tracker (if memory serves me correctly).
This would be set up and probably need adjusting once a week for changes in solar zeneth.

Originally Posted by Johnny Electriglide

Nice cheapo project. The problem is the structural need of a tripod hook to cooker mount to stay at the focal point and the ability to track with a structurally attached two way pivot and attach. No wonder the commercially available parabolic tracking cookers/boilers are so expensive. Kind of like my full tracking Wattsun 8-75 watt panel system.
I use the Global Sun Oven, and have two of them. In summer they reach 300*F in 20 minutes and can be propped up with the back locking tube and hand pivoted every hour. Actual cooking time compared to a conventional oven is about +40%. I actually cooked a Thanksgiving turkey in one, but it took two days. Cakes 1.5 hours, whole chicken 2.5 hours, salmon half .5 hour, 3 quart stew 3 hours. I live close to 9,000 feet so that adds to the time. Clouds can ruin the timing or even stop the cooking altogether.
Similar reflector ovens from cardboard covered with aluminum foil work at lower temperatures and for smaller amounts of food.
The commercial parabolics reach several hundred degrees C quickly. There are giant reflector cookers, too. I think the cob or mortar stiffened cardboard parabola of yours could work better with an integral adjustable lifter on a point on the edge that would be kept aligned with the center toward the sun, going up and down by adjusting to the suns angle. Similar to how the Global Sun Oven works, but with a large campfire tripod suspending the dark pot to the focal point. The parabola should really be full circular for maximum concentration, and have to be smaller than the tripod's base. The cooking pot shades the center of the parabola, so a post mount with the circular pot holding top of the "T" at the focal point and black(maybe a welded tipped T), the base hole in the parabola large enough for typical mid day adjustments at the latitude. The parabola would have to have a focal point similar to the equilateral triangle edge to edge plus center dip of the commercial ones. A plywood structure could be scribed with the device for a say 6" height at the rim and cut and glued parabolic plywood sections (say 3/8" spruce 4 ply), then the card board with foil glued to that. Once the pattern for the full diameter center piece is made the radial sections would be scribed "halves" center cut to fit an expanding hub section (scribed to a butt cut and glued). 12" or less around the outside rim. First two -3/16, second four -9/32, third 8 -1/2, etc. to maintain a perfect circle. Keep up the good work!!!!

[Johnny Electriglide]

Now I know what to do with an old 30" satellite dish I've good. Next summer I will contact cement foil into it and see how it heats with a pan hanging from the existing focal point bent tube. Better yet, I will just use silver paint, no wrinkles. The other old style big ones could be had cheap at a garage sale, with a little luck.

[gaiatechnician]

I have a tracking solar accumulating barbecue project on the web and on the solar cookers international wiki. I havn't made one yet. (Awaits the nice weather).
Note that the center of a parabola is definitely not used in these barbecues! It will be a slice off the side of the parabolic dish.
Serious progress has been made on the project!
I have 2 proposals for tracking units processing away in my head at the moment. One might be uploaded to appropedia or to the solar cooking wiki by the time you read this.
These would be pretty simple and necessarially DIY units.
Once people make and demonstrate them, I am pretty sure the commercial guys will move in. Thats fine by me. The task is to get solar cooking popular and much more used than in the past. With your help, success can be achieved.

[windy1]

These solar cookers are a great idea, but I guess that they need to generate quite a large quantity of heat in order to cook something like a chicken, rather than just attain a high temperature.

What sort of diameter must the parabolic reflector be to make a reasonable cooker? Are there any dangers that the heat produced could burn the person using the oven?

[MartinSykes]

Coming to a garden near you - the solar-powered barbecue | the Daily Mail



Incidentally if making something yourself and you're painting the parabola, from research for my aquarium, bright white gloss paint is a better reflector than silver - think about how hot a piece of metal gets in the sun and why mediterranean countries paint their houses white rather than silver. A parabolic dish might give more heat concentrated at a point but it would be like burning paper with a magnifying glass. You could incinerate your food one sausage at a time. You need a less-focused heat as the picture shows.

[gaiatechnician]

About 1 meter wide should be fine. I am hoping that the more advanced ones will have a gauge on them. Say they reach 300 C in a 50 kg lump of rock salt at the end of the day, then the final temp of the 50kg + the turkey can be figured out (allowing for gradual heat losses from the insulated lump of course . I do not know the weight or heat capacity of turkey because I did not see it on wikipedia but once it is known, an approximation of the cooking time can be figured out and a meat thermometer can be used for certainty. Turkey needs about 80 degrees C to be fully cooked if I remember correctly.

[gaiatechnician]

The tracking solar accumulating barbecue will look nothing like that barbecue.
It does not heat the food directly, rather it accumulates the heat in a large insulated vessel (like how a hot water tank accumulates the heat from its element) Among the suitable materials are Iron aluminium, parafin, rocksalt, granite or limestone (or a mixture of these materials).
For instance an aluminium hotplate with legs extending into a big block of rock salt might diliver the heat quickly when needed.
The reflector will be a parabolic dish concentrating its heat on the bottom of the accumulator. The reflector will be on an equatorial mount and will turn at 15 degrees per hour. The focus (base of the accumulator) will be on a point on the axis of the equatorial mount so the focus remains constant all day.
Anyways, I got to post a timing mechanism for the tracker. Bye brian

[windy1]

I expect it will never be as easy to cook with a solar cooker as it is with electric or gas cookers, e.g. you can't just turn the heat up and down like turning a knob. There is also the problem that the cooker will be hottest when the sun is at its highest, and it may not be the best time to cook at midday.

I like the idea that the cooker can store the heat in rocks or a block of metal, and then be used to cook food later in the day. It will be interesting to see what ideas can be used to overcome the possible drawbacks of solar cookers.