[freeztar]

Originally Posted by Nitack

Sadly enough Freez, your artistic skills are better than mine. You have the right idea, but it would be difficult to impliment I think. First, the track is the hardest part for obvious reason.

The track could be constructed fairly easily and cheaply using pvc pipe. It can be bolted to the ground, bent along the sun arc, and a slit cut into the top. The stand would sit in the slit and be attached to a ball bearing that would move inside the pipe. Two legs would touch the ground on either side of the pipe with wheels, for support.

Also, we would need a tracker hooked up to the lens to change it's angle as well.

Yes, a major challenge...

I have been thinking on this a lot over the past few weeks... wife says I have been talking about solar power and lenses in my sleep! The fresnel will be viable if it is attended, but for something more passive or less labor intensive Turtles parabolic design will end up as the way to go I think. I am still going to construct my Fresnel setup, but after that is demonstrated to be effective I think I am going to attempt to make the turtle setup work as well.

The trough certainly requires a little less maintenance, but you would need a rather large trough to get the temperatures necessary. If a 4 foot trough gets to around 200 degrees, then how big of a trough would we need to get to 700 degrees?

[Nitack]

Originally Posted by freeztar

In other news, I got my package of fresnel lenses the other day and hope to do some experimenting this weekend. This morning I took one out (8"x10") in the sun and burnt a hole through a green leaf in about 15 seconds. Not too bad, but I had hoped for more. I plan on using several at a time to turn up the heat. I bought an oven thermometer so I'll post some temperatures over the weekend.

What is your method for focusing several at once onto the same spot? Just wondering if you are going to use mirrors?

!! Just had a thought... What happens when you take light tubes, Light tube - Wikipedia, the free encyclopedia, funnel the light all to point at your point of heat, and at the end are Fresnel lenses? I wonder if you could make a number of these into an actual viable solar furnace... imagine the application potential if you could get a number of these all channelling light to the same spot?

[freeztar]

Originally Posted by Nitack

What is your method for focusing several at once onto the same spot? Just wondering if you are going to use mirrors?

I might have to use mirrors, but I'm going to try and angle them slightly to point to the same spot. I will lose some solar energy on the fresnel surface by angling them, but it should still provide an ample amount of additional heat. We will see...

!! Just had a thought... What happens when you take light tubes, Light tube - Wikipedia, the free encyclopedia, funnel the light all to point at your point of heat, and at the end are Fresnel lenses? I wonder if you could make a number of these into an actual viable solar furnace... imagine the application potential if you could get a number of these all channelling light to the same spot?

You get the award for the coolest idea of the day.
Upon first thought, it seems that this would work. The question is, how much solar energy actually makes it through the tube?
If the tubes were flexible, then their inputs could track the sun and no mirrors would be needed.

Now, where does one find a light tube?

[Nitack]

Originally Posted by freeztar

You get the award for the coolest idea of the day.
Upon first thought, it seems that this would work. The question is, how much solar energy actually makes it through the tube?
If the tubes were flexible, then their inputs could track the sun and no mirrors would be needed.

Now, where does one find a light tube?

Light tubes can be found at Home Depot, sold as indoor skylight type options.

I do believe they lose some of their energy in the funneling process. However, considering you could funnel the light from a 3'x3' Fresnel into here, and then refocus it at the end by a smaller and closer fresnel, and could do this with MANY tubes, means the energy output could be huge. The only concern at that point would be melting the fresnel lense at the very end. It would not be at the focal point of the larger fresnell, but condensing 3'x3' of light down to 1'x1' or smaller could still be above the tolerance of the acrylic.

[freeztar]

Good points. I plan on testing several lenses in series this weekend, so I'll let you know of any melting problems I encounter. I'm hoping that the unfocused beam from the first lens will not be enough to melt the second in series, but some experimentation will prove it for sure. I have 22 lenses, so I'm not afraid to melt a few in the name of science.

[Nitack]

Originally Posted by freeztar

I have 22 lenses, so I'm not afraid to melt a few in the name of science.

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

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.

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.

[Turtle]

Originally Posted by freeztar

...
The trough certainly requires a little less maintenance, but you would need a rather large trough to get the temperatures necessary. If a 4 foot trough gets to around 200 degrees, then how big of a trough would we need to get to 700 degrees?

I'm thinking in the 12 to 16 foot range using the same uninsulated 4" galvanized pipe receiver as the 4 foot experimental model I built. For the reflector material I would use thin aluminum sheets and polish them by hand with an auto polisher and either rubbing compound or jeweler's rouge. I would make the supports as I did for the experimental model but use plywood and paint it. I would orient the axis of the receiver pipe along an East/West line, and adjust the axis of the parabola to a few degrees below the Sun's daily zenith. If I wanted more capacity, I would make identical modules and hook them end to end. For charcoal production, it would be a load-it-n'-leave-it operation.

[freeztar]

Originally Posted by Nitack

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]

Originally Posted by freeztar

...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.