Solar Parabolic Trough Charcoal Oven

[Turtle]

Originally Posted by GAHD

This is where a squeegee could come in handy, eh gost?

Or possibly a brayer.

Originally Posted by gost

It occurs to me that a layer of some kind of insulation could be put on the top of the pipe to help retain the internal heat. This would reduce the total solar gain, of course, but the overall conservation of heat might be worth it. Any chance you could test this idea on your trough Turtle?

The thought did occur to me to insulate the top portion of the receiver tube that lay above the plane of the trough. Any thoughts on what material? Fiberglass wool? Asbestos? Cogitating.

[redgreenblue]

Originally Posted by Turtle

The thought did occur to me to insulate the top portion of the receiver tube that lay above the plane of the trough. Any thoughts on what material? Fiberglass wool? Asbestos? Cogitating.

Perhaps you could even use a layer of clay to act as a thermal mass (with fiberglass wool on the outside of this?).

[Turtle]

Originally Posted by redgreenblue

Perhaps you could even use a layer of clay to act as a thermal mass (with fiberglass wool on the outside of this?).

Clay is a good idea, but I thought more on the fiberglass & decided a layer thick enough to make much difference would stick up & throw a big shadow on the reflector.
Maybe some of those formed clay tiles they sell in garden shops?
I also considred a layer of pumice between a couple pieces of screen/hardware cloth. Use the dark pumice they sell for bonsai & make a 3/4" thick shield that covers the top 30° of the receiver.
That's all I have for now; still no reply from my enquiry to the commercial builder. Anyone else written any enquiries? Keep us posted.

[cwes99_03]

Might I recommend a simple test before going to the next step of actually insulating.
Find a way to shade just the tube from the sunlight above it and after an hour in the shade feel the tube. If it is still hot, then you have a good reason for insulating. If it is cooler to the touch than the underside of the tube, then insulating will actually hurt your end result (because the black paint on the tube is doing more to warm it than insulation will do).
Even black stone keeps the ground under it cool. Rake away the stone and the ground underneath it is generally cool. Five minutes later once the sun has been active on it, the once covered ground is hot.

[Turtle]

Construction notes:
I left my reflector outside a couple nights & it is now nicely rimpled. Nice rimpling=diminished efficiency. Fortunately my corrugated paperboard structure only needed to establish some baseline data points for trough width to temperature ratio.

Since gost is using plywood & masonite for a trough 2 times the width of mine, I recommend paint or varnishing the exposed surfaces.
I think the most practical design once the optimum size is determined is an all aluminium structure. The plywood cutouts you use as a form for the trough supports, and thin sheet alumnium for the reflector surface. Polish the reflector to as high a sheen as possible.
Make it collapsible & modular so you can transport it. Then take it on site (garden, forest, farm, etc.) bake up the charcoal from on site materials, discharge the charcoal, collect your bill, & then load up & head to the next job.

[Michaelangelica]

Originally Posted by gost

The problem with trapping heat inside of the trough is that it would then require the entire contraption to be made from non-combustible materials. The temperature we're trying to reach is around 470F.

Temperatures of 350-400C would be optimum for Terra preta.
A random thought:-
Could you modify this design to make a solar water still?

[Turtle]

Originally Posted by Michaelangelica

Temperatures of 350-400C would be optimum for Terra preta.
A random thought:-
Could you modify this design to make a solar water still?

That is a higher temp than I (we?) set as a mark, i.e. 450° -500°F (232°-260°C).
Yes, the design is adaptable to a solar still. It's a matter of changing the receiver setup. Here is a link to a group using them to make dry ice:
http://www.thesustainablevillage.com...ise/display/14

[Michaelangelica]

See pages 34 -38?
of
http://www.energy.gatech.edu/presentations/dday.pdf

Sorry thought you were talking C not F
200C might be OK look at graphs from page 34 on
Temperature
The last website I posted had an interesting graph:-
275-400c best to maximise carbon storage for mankind and optimal zone for energy extraction
400-600C best for microbial life but complete devloatilization -requires addition of energy &/or oxygen
400C chars very gradually released its ammonia
I can't see how Amazonian Indians could control temperature anyway. Perhaps they had a mixture of all temps?

Thanks for the tip about solar stills.
I am very interested in low tech designs
I might need one soon if the drought here gets any worse!

[SolarFreak]

this is great work.

is there a forumal for how much surface area your parabolic needs to be reach the temp needed?


is only the very bottom edge of your pipe in focal point?

I would think that lowering the pipe just a little, would give it more surface area for the sunlight to hit.

also on the top side of the pipe I would use some type insolation to keep the heat from radiating. or cover the top though with glass so the pipe is not cooled by a breaze.

I know that in building solar ovens to cook food the key to getting them up to temp was keeping the wind and open air off the cooking area. most times this best results I have gotten is to make the area inside air tight.

[Turtle]

Originally Posted by SolarFreak

this is great work.

is there a forumal for how much surface area your parabolic needs to be reach the temp needed?

If there is, no one has offered it & I haven't found it. I wrote to a commercial manufacturer to ask & have as yet received no reply. [Perhaps they are too busy designing/making charcoal ovens now? ]

Originally Posted by SolarFreak

is only the very bottom edge of your pipe in focal point?

No; roughly 2/3 is immersed.

Originally Posted by SolarFreak

also on the top side of the pipe I would use some type insolation to keep the heat from radiating. or cover the top though with glass so the pipe is not cooled by a breaze.

Acknowledged.

As my oven is too small for the required temp., I have just been waiting to hear from gost on his larger trough. Thanks for your interest SolarF.