Misc. My new solar cooker/solar drier 1/5 scale model: It's smoking!

[Brian in Victoria BC]

TL;DR Summary

Its a model of an upcoming solar cooker. Its made of wood and 3 silvery plastic plates. Might be useful as a school science project.

I "designed" this idea over a decade ago but never build a working model. The idea is to make something much simpler and easier to use than typical parabolic dish solar cookers. The biggest issue is that people use the full dish and then it is very easy to get a flash of sunlight in your eyes when you are stirring the pot. This doesn't have that issue.

It's a solar cooker with a reflective parabolic dish on equatorial mount that if it moves at 15 degrees per hour keeps the light focused (more or less) on the same spot all day. The focused spot will be either under a cooking pot or directed into the bottom of a solar dehydrator or solar box cooker. The box cooker will be opened and closed from the back so nobody gets flashes of stray light in their eyes. Same with the pot, I will have to make some sort of shield to protect peoples eyes as they put the pot on and off the hot spot.

This is made for my latitude, 48 degrees north. At the north pole, it would stand vertical and at the equator it would be fully horizontal. (The angle of equatorial mount is the same as your latitude) Anyway, I wasn't expecting it to burn paper or wood so it actually get quite hot.

Last year's tracking solar cooker won a prize on Instructables, but I made it specifically not to get very hot in one spot, and let it be hot all around a big cook pot. Now I realize that this was a mistake. If you get a smaller hot spot and have it on the bottom, you will get more rapid heat transfer. This is like using high voltage versus low voltage.

Anyway, the full scale version will have a reflector about 4 ft by 4 ft. I'm just working out the size now so I can cut it out of a 4 by 8 sheet of chloroplast corrugated plastic without too much waste. I think this would be a great science project for a school.

 

[Brian in Victoria BC]

I made the model to make sure no material was wasted when scaling up. And it was set up very quickly. A few days later, I noticed a glaring mistake in the set up! Hint: it is equatorial mount and it was around April 4th. Can you spot my mistake?

 

[DaveC426913]

I made the model to make sure no material was wasted when scaling up. And it was set up very quickly. A few days later, I noticed a glaring mistake in the set up! Hint: it is equatorial mount and it was around April 4th. Can you spot my mistake?

I get the basic idea - kind of a "Fresnel mirror" that tracks the sun - but none of your pics shows any of the mount config, so I don't know what pitfall you encountered (eg. does the whole contraption - mirror and target rotate? Or just the mirror?).

I can't think of any significant geometry peculiar to circa April 4, unless you mean Daylight savings - but that would have no effect on your contraption unless it used a timer of some sort. Something to do with the analemma path?

Oh wait. I get it. Easter. You suddenly have a 20lb. turkey to cook in your 1/5 scale cooker.

 

[Brian in Victoria BC]

I had adjusted the model down to do the declination, instead of up. So in the demonstration, the spot of heat would have been more concentrated and hotter if I had done it correctly.

 

[sophiecentaur]

I had adjusted the model down to do the declination, instead of up. So in the demonstration, the spot of heat would have been more concentrated and hotter if I had done it correctly.

If you want 'normal cooking' temperatures and if you can produce a few hundred W of heat then you don't need good focussing. If the main part of the reflected image of the Sun is well within the area of the cooking pot then you will avoid burning and minimise lost heat (IR) from the local hot spot. A Fresnel reflector works well, as you will no doubt have seen in your searches. A simple tracking solar mount should need minima long -term adjustment. You can easily correct for the analemma problem (if necessary with a fuzzy heated spot) when you want to set up each cooking operation.

TL;DR Summary: Its a model of an upcoming solar cooker. Its made of wood and 3 silvery plastic plates. Might be useful as a school science project.

I think this would be a great science project for a school.

I like your offset reflector design; very basic and well adequate. I would warn against a four foot square for a school project; nearly 1kW would be a safety hazard. To make the system as light as possible (to allow minimal motor power) then you could keep the target pot near the axis of rotation and it could be stationary. That may call for a longer focal length for the reflector.

 

[Brian in Victoria BC]

If you want 'normal cooking' temperatures and if you can produce a few hundred W of heat then you don't need good focussing. If the main part of the reflected image of the Sun is well within the area of the cooking pot then you will avoid burning and minimise lost heat (IR) from the local hot spot. A Fresnel reflector works well, as you will no doubt have seen in your searches. A simple tracking solar mount should need minima long -term adjustment. You can easily correct for the analemma problem (if necessary with a fuzzy heated spot) when you want to set up each cooking operation.


I like your offset reflector design; very basic and well adequate. I would warn against a four foot square for a school project; nearly 1kW would be a safety hazard. To make the system as light as possible (to allow minimal motor power) then you could keep the target pot near the axis of rotation and it could be stationary. That may call for a longer focal length for the reflector.

Hi Sophie The target spot is at the axis of rotation and the target spot stationary. The reason for the offset paraboloid is so that there is easy access to the target spot at all times. The amount of power used for tracking is tiny. Instead of a motor to run the rotation, I use an Airlift pump/waterwheel/winch. I use low pressure air that steadily increases in pressure to do the timing. Ideally, a battery operated "thing" or clockwork with a stepper motor could do the timing but I cannot make it. The timer needs to have adjustable speed because I use wooden wheels of slightly different diameters. If a company made my type of solar reflector, they could standardize the timer.

 

[Brian in Victoria BC]

If you want 'normal cooking' temperatures and if you can produce a few hundred W of heat then you don't need good focussing. If the main part of the reflected image of the Sun is well within the area of the cooking pot then you will avoid burning and minimise lost heat (IR) from the local hot spot. A Fresnel reflector works well, as you will no doubt have seen in your searches. A simple tracking solar mount should need minima long -term adjustment. You can easily correct for the analemma problem (if necessary with a fuzzy heated spot) when you want to set up each cooking operation.


I like your offset reflector design; very basic and well adequate. I would warn against a four foot square for a school project; nearly 1kW would be a safety hazard. To make the system as light as possible (to allow minimal motor power) then you could keep the target pot near the axis of rotation and it could be stationary. That may call for a longer focal length for the reflector.

The model has roughly 1 sq ft reflector area. I think that would be ok for a school project. There are lots of plastic Fresnel lenses of similar size. I think just making a lens of that size using plastic plates from the dollar store might empower the children. Making the entire model would empower them more. Designing the whole thing including the size of target, even more.

 

[sophiecentaur]

I think just making a lens of that size using plastic plates from the dollar store might empower the children.

That could be a good project if the plastic reflector plates are conveniently flexible enough and if you could give them an already shaped fresnel shaped former to mount their strips. That would mean you do the actual hard work but they get the resulting reflecting structure. So many kids are unused to practical things these days and their building skills are usually limited to Lego models. They can be brilliant at that but you may find 'assembly' is more successful than 'fabricating' for your school exercise. Your kids may be different though, lucky for you if they are.
I acquired an old satellite reflector dish and stuck kitchen foil strips on it. The resulting optics was quite adequate for concentrating sunlight and the dish itself was pretty lightweight - suitable for your wooden structure. It's always a matter of grabbing what you have to hand and making as little as possible. A few years ago there were loads of OHP table fresnel lenses to play with but no more., alas.