Solar Parabolic Trough Charcoal Oven

[GREENPOWERSCIENCE]

One large spot Fresnel lens 30" x 40"or larger would easily convert a small portion in a small oven/pipe 4" or smaller. The problem with Fresnel energy is heat dissipation. Because you are outside any heat captured is lost very quickly. Even a 1-2 mph breeze can cause a fast temperature drop. I am sure you have figured this out by now but insulation and limited wind exposure would help a lot. (I am working on a cheap evacuated tube-container process that can take Fresnel heat), with that, one lens would bake a lot of material.

I have heated the bottom of a pot before by bouncing the beam off of a glass mirror. 10 " below the pot. This exposed the mirror to a much weaker focal point preventing the mirror from getting damaged and placing about 90% of the beam energy directly on the bottom of the pot (10% loss due to mirror). This works best as heating the top of a pot or vessel does nothing for what is inside unless it is a liquid with no air space (direct contact). It also allows for top insulation where most heat is lost.

I will try and post a youtube video today of several mirrors directing sun into one Fresnel. We discovered that mirrors the same size as the lens produces a beam with 80-90% of the heat as direct sunlight. Nothing special until you add 2 mirrors 160%, 3 mirrors 240% etc. You cannot get all the beams to focus on one spot this way (which you would not want anyway unless you trying to obliterate something), but you can get them very close. I have a video with 6 mirrors and 6 spots which I thick would be close enough to fit on one oven or pipe. NOTE: mirror quality makes a big difference.

Trough vs. Fresnel. My personal experience is that Troughs are great for long stretches of water but not so good for higher heat unless you have an optically correct reflector $$$$$$$$$$$$$. Fresnel lenses are cheap in comparison. I will also post a simple way to make a Fresnel reflector. Nothing special and very simple but it woks. Cheaper than a parabolic mirror though not nearly strong (only 400-600 ? F)

[Nitack]

So I just had a thought, and feedback would be much appreciated. Right now we are trying to use the fresnel lens to transfer heat to a container (dutch oven) and from there to our organic matter. But why do we need the dutch oven? Why not just use a glass box/tube? With tempered glass the light can pass through right to our organic matter. If it is packed there is not much oxygen and what there is will quickly burn off, from there the chared matter will conduct heat very well and will pass it through contact with the rest of the material. Of course we would need an outlet for gas and such. But why do we need the dutch oven if with the fresnel we can heat the medium directly?

[freeztar]

@GPS
Thanks for the info and for posting your results. I look forward to seeing the videos.

@Nitack
Your idea makes sense, but I wonder if glass is up for the task. I cracked pryex in a matter of 15 secs. with only a 8"x10" lens. I think it cracked because of the rapid heat change. Perhaps the tube could be pre-heated with an unfocused beam to prevent the cracking from a sudden temperature spike.

[Nitack]

Okey dokey, here is what I have found.

Our ideal working material would be VYCOR. 96% pure silica and can withstand temperatures into the 2k range. At almost a thousand dollars for a 50 ml crucible with lid it is not going to happen!

Pyrex could handle it, but not your standard pyrex, we are talking the Pyrex VISTA stuff. top operating temperature in the 800s. It would be pushing the envelope and we would have to get the lighting just right or risk cracking the damn thing.

[freeztar]

Originally Posted by Nitack

Pyrex could handle it, but not your standard pyrex, we are talking the Pyrex VISTA stuff. top operating temperature in the 800s. It would be pushing the envelope and we would have to get the lighting just right or risk cracking the damn thing.

Yeah, that's pretty close to the 700 degrees we are looking to obtain. Not only that, but there is pressure to deal with as well which seems like it would compromise the system with such a small margin of error, even with a vent.

What about a solar oven for the recepticle? In other words, a box within a box with the outer box having a small window of glass where the concentrated beam can penetrate and heat the black box inside. The outside box acts as insulation for the inner box and should not get as hot as the inner box.

[GREENPOWERSCIENCE]

Consider a cement casted box with a glass covering. To test it and save time just use a standard cement block or corner block. Pour cement in the bottom to make it air tight and lay clear sheet of glass on top. Cement is a good insulator. If you need more depth to save the glass, cement 2 blocks together. A cement block will hold water for several hours. That should be air tight enough. If this works, cement ovens are easy to make.

[freeztar]

Originally Posted by GREENPOWERSCIENCE

Consider a cement casted box with a glass covering. To test it and save time just use a standard cement block or corner block. Pour cement in the bottom to make it air tight and lay clear sheet of glass on top. Cement is a good insulator. If you need more depth to save the glass, cement 2 blocks together. A cement block will hold water for several hours. That should be air tight enough. If this works, cement ovens are easy to make.

That sounds like it might work. I'm assuming you have experience making these? Were you able to get a temperature reading inside the box using this method?

[GREENPOWERSCIENCE]

Leave or drill a small hole, place a probe thermometer (High Temp Model) in and close up with cement paste. I have a simple design for the oven. I built one for just cooking/using yard debris for boiling water, it worked great. I have since recycled it and it is now filler for a poured cement wall. That one was big 3' x 3' x 2.5' x 2" thick and it took heat fine.

[GREENPOWERSCIENCE]

It works.

[Nitack]

Originally Posted by GREENPOWERSCIENCE

It works.

WOO HOO! Happy days are here again... now please elaborate on what exactly it is that works

Have you tested a cement oven and gotten it up to the required temperature? Have you produced coal? Have you found a way to focus the light of a hundred Fresnel lenses into one small SOLAR DEATH RAY?