Solar Parabolic Trough Charcoal Oven

[freeztar]

I like the "greenhouse" idea. It's like a solar oven. I wonder how hot those can get?

[Nitack]

Originally Posted by freeztar

I like the "greenhouse" idea. It's like a solar oven. I wonder how hot those can get?

I actually thought of a passive design with the same setup. Put your vessel in the center, put angled mirrors on all four sides to redirect light, and a sheet of glass up top to trap heat but let light through.

Interesting thought going back to the parabolic mirror and pipe design...

Why not run the pipe through a cinder block structure like this? One of the main reasons that the pipe did not work was probably it's being exposed to wind, however it was still able to get up over 200 degrees... so why not shield it from the wind?

[MarkB]

Originally Posted by Nitack

I actually thought of a passive design with the same setup. Put your vessel in the center, put angled mirrors on all four sides to redirect light, and a sheet of glass up top to trap heat but let light through.

Interesting thought going back to the parabolic mirror and pipe design...

Why not run the pipe through a cinder block structure like this? One of the main reasons that the pipe did not work was probably it's being exposed to wind, however it was still able to get up over 200 degrees... so why not shield it from the wind?

This seems the right idea to me. The sunlight is providing an energy (heat) input, to get a high temperature you just need to insulate the system well enough that it loses heat slower than it comes in.

I would be trying to design something with roughly 2 boxes, one above the other :
- the bottom one would be enclosed on 3 sides (maybe partly dug into the ground?) with a glass cover on the 4th side. Light would enter through here and roughly focus on a metal receiver which would form a pillar of some kind inside the box.
- the box on top would be your chamber to put stuff in and would sit on top of the heated pillar. This would be heavily insulated on all sides and the top to retain the heat rising from the heating element. You would need a air escape somewhere and I imagine that would be best as some kind of hole/pipe at the bottom of the oven chamber leading down to minimize hot air loss.

The idea is that light doesn't have to be accurately focused on the heating pillar - my understanding is that it doesn't make any difference how tight you focus it, a given size of lens is going to supply the same total energy whether it's going into a half-inch spot or into a 4 inch spot ... the point is that you want to keep that heat in there. So you enclose the heating element behind glass/similar and cover the oven itself up completely. (I'm not good on insulation at these temps, so suggestions gratefully received.)

A question from me - how many hours would this need to be getting the sun? I'm wondering if you could half-dig this into a South or SW facing slope to help with the insulation? I'm thinking maybe a foot wide window, with an 8 inch heating element a couple of inches behind it, would allow a fair range of angles, assuming your lens arrangement could be moved around it ...

MB

[freeztar]

Mark, I would need to see a diagram to fully envision what you are proposing. I think I have the idea, but I'm not quite sure.

Originally Posted by MarkB

A question from me - how many hours would this need to be getting the sun?

It depends on lots of factors such as efficiency of the system, wind, cloudiness, etc.

I'm wondering if you could half-dig this into a South or SW facing slope to help with the insulation? I'm thinking maybe a foot wide window, with an 8 inch heating element a couple of inches behind it, would allow a fair range of angles, assuming your lens arrangement could be moved around it ...

Ah yes, the moving problem...
When I was doing my initial trials, I had to move the stand and or lens every couple of minutes to maintain a strong beam. We've proposed solutions to this earlier in the thread, but all propositions are quite complex/expensive. If you have any ideas on this, I'd sure like to hear them.

Welcome to Hypography btw.

[Turtle]

Originally Posted by freeztar

...Ah yes, the moving problem...
When I was doing my initial trials, I had to move the stand and or lens every couple of minutes to maintain a strong beam. We've proposed solutions to this earlier in the thread, but all propositions are quite complex/expensive. If you have any ideas on this, I'd sure like to hear them.

Welcome to Hypography btw.

I heard this guy on the radio the other night; he is using an array of fresnels on a single mount, and drives it by computer to track the Sun. He said his steam system operates at ~400 lb/in^2, and that he & his crew have custom made a turbine to drive a generator. You might get some ideas here: >> Xenotech Research Forum :: View topic - Solar energy

[Nitack]

I read Physics for Future Presidents by Richard Muller when it came out. Fascinating book. Among the many things I learned from the book, one was about the feasibility of a solar powered car. A car that ran directly off of sunlight is in fact not possible. The reason that it is not possible is that a square yard of surface receiving sunlight contains roughly the energy equivalent of one kilo-watt of power. This was disheartening for those dreaming of a direct solar car when you take a couple more steps down the line. A car has about 2 sq. meters of surface space. 1 KW is equal to 1 horse power. The most power you could get directly from the sun to power a car at 100% efficiency is 2 horse power, which any car enthusiast knows is no where near the power of even the smallest/weakest automobiles. So died my dream of an directly solar powered car. Doesn't exclude something with a lot of batteries and that has time to recharge between usages though.

This realization about the power contained in a square yard of sunlight has a great deal of relevance for those of us who are trying to produce solar powered charcoal makers. We are trying to focus a large area of sunlight into a small area in order to use the energy of a larger area to heat a smaller area to the temp we need.

So the question now may better be defined for us to figure out how to make a solar charcoal maker. How many KW/volume does it take to reach a high enough temperature to create charcoal. I think this also highlights the necessity of insulation in order to create a compounding effect. Additionally, is it still a solar powered charcoal maker if you use a retort and burn the wood gas once it starts being released?

[Turtle]

Originally Posted by Nitack

I... This realization about the power contained in a square yard of sunlight has a great deal of relevance for those of us who are trying to produce solar powered charcoal makers. We are trying to focus a large area of sunlight into a small area in order to use the energy of a larger area to heat a smaller area to the temp we need.

So the question now may better be defined for us to figure out how to make a solar charcoal maker. How many KW/volume does it take to reach a high enough temperature to create charcoal. I think this also highlights the necessity of insulation in order to create a compounding effect. Additionally, is it still a solar powered charcoal maker if you use a retort and burn the wood gas once it starts being released?

I agree that we know all the theoretical constants at play to derive a theoretical design for a charcoal cooker. Really that's the basis for our experiments. For my design starting this thread off, I further agree that insulation is very important. Here again, the theoretical/proper solution is a Dewar tube around the receiver holding the material for charcoalfication, but the practical problem is getting that made.

If it is the Sun making the wood gas, then yes the system is still 'solar powered'.

[freeztar]

Originally Posted by Turtle

I heard this guy on the radio the other night; he is using an array of fresnels on a single mount, and drives it by computer to track the Sun. He said his steam system operates at ~400 lb/in^2, and that he & his crew have custom made a turbine to drive a generator. You might get some ideas here: >> Xenotech Research Forum :: View topic - Solar energy

I found this somewhat randomly and found it especially relevant for this thread.

Solar PV tracker

[Michaelangelica]

Is this article any help?
(I haven't been following the discussion. I can't make anything that says together for more than 5 mins)


Solar Cooking Demonstration in San Diego
Solar Cooking Demonstration in San Diego : Eat. Drink. Better.
They say it gets up to 330 degrees but it dosen't say if that is F or C.
If it is Centergrade it is not far to go for charcoal production (+50-150?)

[Nitack]

So this may be the most lowtech idea of making my frasnel lens actually make charcoal, but I actually think it might work. This came to me as I was testing out a traditional burner and retort (which did not work because I used unseasoned, large pieces of wood). I never got the retort to fire, but I came up with an idea. I am going to take apple sauce jars, the big mothers, punch a few holes in the top, pack them with mulch, and then sit them in the fire to char and eventually give off wood gas to help fuel the burn for my 55 gallon drum.

But I was thinking, why wouldn't that also work with a Fresnel directly on the mulch in the jar? As long as I pack the much there should be relatively low oxygen. Additionally, as long as I don't completely focus the beam, it should get hot enough to char everything. The stuff towards the inside may not get hot enough for long enough, but the stuff around the outside should all char right?

I think the biggest mistake I was making with the Fresnel was to just try to make one point supper hot and then have conduction/convection heat the material. Too damn much heat was being lost to the air around me. Thoughts?

Edit: finding the most long and skinny jar I can should probably help as well.