Solar Parabolic Trough Charcoal Oven

[freeztar]

:) Holly crap, you don't do anything half-a$.

 

Nope. ;)

It was a deal on ebay. I got them all for about $2.00 a piece so I figured, why not?

I'm going to pack one away in my camping backpack to use as a firestarter. :(

I don't know where I read it, but seem to remember something about the heat at which the lenses start losing clarity (not sure about melting) and distortions ruin the focal power is in the high 100's, like around 180.

 

That's disappointing news. I wonder if it is higher for glass?

There is a different kind of fresnel that might work well with this application. The ones we are working with the light comes in straight and then is redirected to all intersect at a certain point. There other kind is a defuser, where light comes in straight and is then scattered. the scattering one can be used in reverse though, which is how the rear car things work to provide a larger field of vision. So it will grab light from many angles, and then kick it out down the tube straight... right to our waiting focusing fresnel. Just a thought.

 

Another good idea!

Hopefully I'll have a chance to swing by home depot this weekend and I can check out these light tubes. Do you know how much they run?

[freeztar]

I just did a little experiment to see how hot the 3:00 sun here in Atlanta could get through a fresnel. I focused the beam on my oven thermometer (the type that slips onto the oven grates; it has a pyrex faceplate covering the needle and readout bars and measures 4" x 3" x 2.5"). I focused a single 8" x 10" fresnel lens onto the thermometer and the needle started moving within a few seconds. The needle quickly moved to 350 degrees F and paused there. At that point (about 10 sec. after focusing) I noticed small swirls of black smoke wisping off the faceplate. The needle held at 350, but I suspect this was because the heat had compromised the instrument. About three seconds later the pyrex cracked with a loud 'ping' sound. I decided to stop at that point. :)

 

So my conclusion from this experiment is my test lens can produce temperatures at least in the 350 degree F range, but most likely this is much higher. I will need a better thermometer for further experiments.

 

Something I noticed while experimenting was that the heat is extremely localized, at least with my relatively small lens. I do not think a lens of this size could char material in a dutch oven. With several of these lenses it might be possible, but light tubes or mirrors will probably be necessary. I definitely believe that copper coils should be used around the heating vesicle. Of course, they should be painted black (and perhaps filled with a high-temp liquid ;)).

 

Yesterday evening when I returned from work, I tried to use two lenses in series, but it was impossible to get the focal points to align properly.

I placed the first lens appropriately to obtain a good focus, but when I added the second lens 'above' it, the focal length of the first lens was skewed. I tried moving them into different configurations with no luck (although this was at 6:30, right before the sun hit the treeline).

Turtle (or anyone for that matter), do you know of a formula that would work for calculating the focal points of two fresnel lenses in series?

[Turtle]

...Turtle (or anyone for that matter), do you know of a formula that would work for calculating the focal points of two fresnel lenses in series?

 

By in series do you mean projecting the focused beam of one through the other? If so, I think that at the least there is no advantage in doing this. I think that to get 'more power' (;)) from the Fresnels you need either a larger single lens or an array of multiple lenses reflecting their beams off multiple mirrors onto the receiver. Does that ring true? :)

 

PS paint the back of the oven thermometer black & focus the beam there, rather than on the face perhaps? The one I bought has marks to 500F, but I used it inside the receiver rather than directly in the beam.

[freeztar]

By in series do you mean projecting the focused beam of one through the other?

 

Actually, I was thinking of focusing the unfocused beam of one into the other. Nonetheless, you may still be correct.

I think that to get 'more power' (;)) from the Fresnels you need either a larger single lens or an array of multiple lenses reflecting their beams off multiple mirrors onto the receiver. Does that ring true? :)

 

Those are known ways, correct. I'm experimenting to confirm.

PS paint the back of the oven thermometer black & focus the beam there, rather than on the face perhaps? The one I bought has marks to 500F, but I used it inside the receiver rather than directly in the beam.

 

The thermometer is black actually (except for the face obviously). Unfortunately, from my tests the day before I was unable to get the needle to move and this caused me to aim my trials today directly at the face. I did not think the pyrex would crack, but I should have considered it given the quick temperature increase I witnessed.

[Nitack]

Update:

 

So I got the lens. It is a big sucker and I can only imagine what I am going to be able to do with it. Unfortunately the sun was not showing on Sunday and I was not able to test. But I got just about everything I need. Just going to build a frame using aluminum for easy deconstruction and storage. Using a cast iron dutch oven and for my temp tests going to have an oven thermometer inside. If it gets above 600 (hoping it does) I may need something more suited for higher temperatures. I will post pictures as the project starts to develop.

[freeztar]

I've decided to get one of the 48" lenses to try out.

Nitack, have you built your frame yet? If so, how did you do it?

 

I saw one on youtube that looked pretty nice. It looks like he retrofitted an easel or something so the lens could tilt forwards or backwards. It seems that, ideally, one would want a stand that tilts forward and back and up and down to adjust the focal point.

I'm wondering if it might be possible to use a camera tripod, without it tipping from the weight?

 

Any other ideas?

[Nitack]

Hey Freeze,

 

I actually got the majority of my frame built last night. I framed out the outside using 1x2 on ends so that the frame is deep. Lined the bottom edge with 1x1 all the way around to create a lip for the lens to sit on. The lens fit in great. Tonight I attach more 1x1 on the other side with the lens in place to lock it in.

 

For a stand I am going to build a very simple A-frame. Probably use a couple 2x4 and lay them so they meet at the corners, and attach them using an angle bracket on the inside angle. That way it will give me a nice V on the top. Drop some bolts in center mass on either side of the frame and they can sit in the V made by the 2x4s. The weight is not that much so it should be able to hold easy enough. As long as it is balanced I don't need a locking or tightening mechanism, but I am going to use some fender washers and wing-nuts just for the sake of keeping it in place with out supervision. The design is simple, I might improve upon it later, but for now it will allow me to tilt both forwards and back and keep it at what ever angle I need. It is also relatively portable so I can move it to wherever I am working very easily.

 

The weather is killing me here though. This whole week is hazy or rainy here. According to the weather man I will not get any charcoal weather till next Monday and I will be working during the peak hours. BLAH! :(

[Turtle]

I've decided to get one of the 48" lenses to try out.

Nitack, have you built your frame yet? If so, how did you do it?

 

I saw one on youtube that looked pretty nice. It looks like he retrofitted an easel or something so the lens could tilt forwards or backwards. It seems that, ideally, one would want a stand that tilts forward and back and up and down to adjust the focal point.

I'm wondering if it might be possible to use a camera tripod, without it tipping from the weight?

 

Any other ideas?

 

I think a camera tripod might be tippy. If you fasten the feet down it should stay in place and has the advantage of built in movements and locking in position.

 

My bias to a frame to hold the lens flat is to use wood. The general form of the old school blackboards as drawn below, and then affixed to whatever rotating support you care to make. Camera tripod, rotating spindel on a broad base, rotating disk platform, etcetera. I think if you fix the frame solidly to a base that carries the receiver, and then rotate the whole setup to track? :D (this smilie needs a pencil behind its ear.) :(

[Nitack]

rough sketch of what I am doing, although not to scale. when turned completely horizontal the focal point of the lens will be six inches above the ground.

[Turtle]

rough sketch of what I am doing, although not to scale. when turned completely horizontal the focal point of the lens will be six inches above the ground.

 

Nice! I think your design would be more stable than mine in a wind. Put your setup on a rotating base and you're in business. The 'rotating base' could just be a piece of plywood on the ground, as long as you have a base/connection to all the leg ends.

 

:D Do I smell smoke?? :D:cap: :(

[Nitack]

I don't know about more stable, but it is very portable as it is essentially three separate pieces that kind of all lean on eachother... Perhaps I will add some footers to the A-frames to increase stability though... perhaps nail a 1x1 piece of plywood to each foot. I don't want to mount the whole thing to one big piece of plywood on the ground given that the firing spot is the ground and it is just begging the whole rig to go up in flames...

 

Perhaps I can attach wheels on the bottom of each footer for ease of movement. That is all down the line, for now I still have to produce charcoal.

[freeztar]

For a stand I am going to build a very simple A-frame. Probably use a couple 2x4 and lay them so they meet at the corners, and attach them using an angle bracket on the inside angle. That way it will give me a nice V on the top. Drop some bolts in center mass on either side of the frame and they can sit in the V made by the 2x4s. The weight is not that much so it should be able to hold easy enough. As long as it is balanced I don't need a locking or tightening mechanism, but I am going to use some fender washers and wing-nuts just for the sake of keeping it in place with out supervision. The design is simple, I might improve upon it later, but for now it will allow me to tilt both forwards and back and keep it at what ever angle I need. It is also relatively portable so I can move it to wherever I am working very easily.

 

I'm having trouble picturing the stand.

An A-frame is just 2 pieces of wood correct? How will it stand up?

Also, how will you be able to tilt the lens? :(

 

EDIT: nevermind, missed your pic while typing, makes sense now

[Nitack]

two separate A-Frames, one on each site. The bolts (long bolts) would rest in the "V" the the two board ends make. Like I said, not the most stable, but very portable.

[freeztar]

Got it.

 

So what about height to achieve focal point?

I would imagine we do not want to be sticking our hands in front of the lens adjusting the height of the stand where the dutch oven will sit. It seems a lot safer to be able to adjust the height of the lens.

 

What were your plans for this and do you have any ideas on how your rig can be built to allow height adjustments?

[Nitack]

Well the dutch oven is about six inches tall itself, so my idea was for starters to place it on a couple bricks to get some hight. We don't want the focus directly on the oven, but with a little high the light should be spread enough to be in the right range. Extensive testing will be needed to get exact hight and temperature ratios.

 

As for adjusting the frame, I considered that. At the moment this is just a cheap way to get a proof of concept using the two by fours. For a more adjustable frame I was thinking of using steel angle iron because it already has evenly graduated holes and it would just be a matter of where you inserted the screws. You could adjust that according to your application.

[freeztar]

Or what about a springed-hinge instead of a brace?

Or you could put peg holes along the sides of the 2x4's and use dowels and a notched rod to hold the A-frame?

 

Just brainstorming....gotta go now, I'll check back later tonight...

[Nitack]

New A-frame design. Still very simple/cheap to construct, but gives more ability to control the hight of the focal point. Still using two 2x4s because I want the stability that the four inches provides, but now they will be joined up in a flush manner with one overlapping the other. Then I will be attaching an angle iron to each A-frame in the manner of my drawing. Since those already have holes drilled in it gives me perfect adjustment heights. Just move the lens to a higher or lower hole.

 

Credit goes to Freeztar because his dowel and hole concept gave me the idea.