Designing a parabolic solar cooker

[8thsinner]

Given the ease of which a solar cooker can be made I am thinking of trying my hand at making one. And I want to try a parabolic design as opposed to the more boxy like ones.
But What shape is best?
I am assuming the thing has basically got to be an off cut of a perfect sphere right?

If so, and for example lets say the sphere, is a 12" circle. How high from the ground does it need cut? 5", 6"?

Is a sphere this size capabale of utilising enough of the solar energy? Or do I need to think bigger? I know there are lots of box designs around 12 - 14" but these have much more reflecting areas than a sphere would...

If any one has suggestions or advice for solar cooker use please let me know.

 

[gregorach]

Have a look at this lot mate.

 

[jimford]

I am assuming the thing has basically got to be an off cut of a perfect sphere right?

Wrong, it needs to be a parabola - like a car headlamp.

Jim

 

[gregorach]

Yeah, but the actual practical difference between a parabolic and a spherical reflector isn't really that great. If astronomical telescopes can manage with spherical reflectors, I don't imagine it's going to make that much difference for an oven.

 

[jimford]

If astronomical telescopes can manage with spherical reflectors, I don't imagine it's going to make that much difference for an oven.

Astronomical reflecting telescopes use parabolic mirrors.

Jim

 

[8thsinner]

OKay those links were really helpful, but it also looks like the parabolic designs are a lot larger than I had though they would be. At least now I have a better idea of how it all works.

Now say I had a flexible plastic as a backing as opposed to cardboard, can anyone recommend a good glue which is felxible incase the thing gets bent? I am pressuming the heat won't really soak through the foil...

 

[gregorach]

Astronomical reflecting telescopes use parabolic mirrors.

Some do, but many don't.

As for flexible glues... PVA?

 

[jimford]

Some do, but many don't.

Interesting

But unless they're a parabola they won't bring parallel rays to a focus!

Jim

 

[gregorach]

Yeah, but at "long" focal ratios (generally longer than f/10), the spherical aberration is actually quite minor, whilst grinding a proper paraboloid is really hard. Even when the spherical aberration is significant, it's often easier to correct it using additional elements than to correct it in the primary reflector.

Optical design is always a matter of trade-offs, and no optical system is completely perfect.

 

[rich59]

I understand that the major astronomical telescopes use 2 hyperbolic mirrors.

Anyway - back to the solar oven - an approximate hyperbolic mirror made from 100 flat mirrors might do the job cheaply?

Would you want to make it track the sun?

 

[gregorach]

A cardboard box lined with foil does the job surprisingly well. I wouldn't bother going to all the trouble of making some super-duper complex solar cooker unless you've already tried a simple one. KISS principles apply.

Unless you really want to of course...

 

[8thsinner]

You have a good point gregorach, I do still intend to keep it simple though.
Thanks