# Does anyone use Fresnel Lens?



## Evan2468WDWA (Jun 8, 2014)

You can get the from the old rear projection TVs and use them a giant magnifying glasses to focus the sun's rays. I've heard they can reach fairly high temperatures. 
http://www.physicsforums.com/showthread.php?t=315719
This seems like it would be a great fuel saver for small scale refining operations. I've read people on the form saying that copper isn't worth the costs of refining, but with this maybe some of the borderline metals like copper would become worthwhile.


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## GotTheBug (Jun 8, 2014)

I actually have one leaning against the wall waiting for me to get to it. My plan was a solar powered charcoal maker, to start with. I don't think they will generate enough heat to melt metal. Also seems that if it did proximity would be a definite issue.


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## Evan2468WDWA (Jun 8, 2014)

GotTheBug said:


> I actually have one leaning against the wall waiting for me to get to it. My plan was a solar powered charcoal maker, to start with. I don't think they will generate enough heat to melt metal. Also seems that if it did proximity would be a definite issue.


I've heard of them reaching around 2000F. If that's true then looking at this table it seems like it could melt quit a few types of metal. It seems promising to me. I'm going to be on the lookout for one of those tvs. Maybe I can get a broken one for free like in this video.


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## g_axelsson (Jun 8, 2014)

Temperature isn't all there is. To melt something you must put in more energy than it gives off (cooling by heat transmission, air convection, radiation) until the melting temperature is reached. I would be surprised if you could melt anything larger than a few grams of copper at a time. The larger the melt the more heat is lost to the surrounding.

Anyone wanting to direct melt with solar power has to build a sun follower or constantly moving the lens to keep the focused spot on the melt.

Göran


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## patnor1011 (Jun 8, 2014)

Goran is correct. I also experimented with them some time ago. While it was able to burn black IC chips it produced a lot of smoke and lens will get damaged by it and also by proximity of fire/hot metal.


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## Evan2468WDWA (Jun 8, 2014)

g_axelsson said:


> Temperature isn't all there is. To melt something you must put in more energy than it gives off (cooling by heat transmission, air convection, radiation) until the melting temperature is reached. I would be surprised if you could melt anything larger than a few grams of copper at a time. The larger the melt the more heat is lost to the surrounding.
> 
> Anyone wanting to direct melt with solar power has to build a sun follower or constantly moving the lens to keep the focused spot on the melt.
> 
> Göran


What you and patnor1011 makes sense. It seems like utilizing this idea will be more difficult then I thought. Still I haven't given up yet. If I can get one of these things for free it would definitely be worth doing. Even if I can't find a way to make use of it playing around with it would still be a lot of fun. 

I still think I can make some use of it. Even if I can't get copper to work out for me I can use it on gold and silver. Melting a few grams of gold or silver at a time is worthwhile, and besides I don't own a torch. 

Also, I would like to try and use it to depopulate circuit boards. I wouldn't even have to use the focal point to do that. I'm thinking I would get some safety equipment like heavy protective gloves, a Full-Face Respirator and some sunglasses. Then I would put some aluminium foil under where I was working to catch the drops of solder. I bet I could use it to quickly depopulate boards. Afterwards I could collect the solder refine it for lead and tin. Well I'm not going to do any of this right away. I still need to learn a lot more before I would feel comfortable trying any actual refining, but I might as well be on the look out for a free Fresnel Lens while I'm learning. 

I still haven't given up on copper. I'll just have to be a bit more creative. Making an apparatus that move the lens around is to difficult and costly for me. Instead I thing I would find a steel crucible, paint it black and put it inside a clear container in which I would create a vacuum. With such a set up I would probably only need to manually adjust the lens every hour or half hour.


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## pinman (Jun 8, 2014)

This topic reminded me of this video.
http://m.youtube.com/watch?v=8tt7RG3UR4c


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## g_axelsson (Jun 9, 2014)

You can melt small amounts of metal with a fresnel lens, but as this video shows it has a hard time to melt more than a gram, the copper wire conducts heat away too fast.

https://www.youtube.com/watch?v=drE54ctrHBY

Göran


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## Pantherlikher (Jun 9, 2014)

I have a few of them waiting for me to get too.
Craigslist.com is a good place to look for free projection TVs as well as a lot of other stuff.
I have this idea to build a solar pool heater... As well as other ideas to explore.

My additional thought on this would be to use mirrors to direct the focal point to where I need it.

I have found an antenna rotater as well to work on moving things. From your living room you can turn a knob and it rotates the super antenna on top of the house to get old school TV shows in.

When you add mirrors into the equation, ideas are endless. Like building a laser super weapon... :mrgreen: 

B.S.


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## metatp (Jun 9, 2014)

To get the heat you desire, you will probably need to make sure it is a "spot" lense and not a "linear" lense. It really makes a difference when focusing on what you want to melt. Also, make sure you use welder googles. It may be a pain, but it is extremely bright.

"Spot" lenses are harder to find. They are usually in the older (pre-2000) tvs. I have about 10 of them, and only one is a "spot" lense.

Tom


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## Smack (Jun 9, 2014)

Won't work for PM's, just doesn't get hot enough, mid 700 degree range is about it for spot or linear, I have both. They work well for de-soldering or pre-heating.


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## Lou (Jun 10, 2014)

At 1.36 kW/m^2, a couple square meters of focusing lenses...you're melting platinum!


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## Evan2468WDWA (Jun 13, 2014)

g_axelsson said:


> You can melt small amounts of metal with a fresnel lens, but as this video shows it has a hard time to melt more than a gram, the copper wire conducts heat away too fast.
> 
> https://www.youtube.com/watch?v=drE54ctrHBY
> 
> Göran


Interesting video. If I could find the plans building a sun following device might be doable after all. 



metatp said:


> To get the heat you desire, you will probably need to make sure it is a "spot" lense and not a "linear" lense. It really makes a difference when focusing on what you want to melt. Also, make sure you use welder googles. It may be a pain, but it is extremely bright.
> 
> "Spot" lenses are harder to find. They are usually in the older (pre-2000) tvs. I have about 10 of them, and only one is a "spot" lense.
> 
> Tom


Thanks for the advice. I'll definitely use welder goggles. Also, I didn't know that there were different types of fresnel lens. Hopefully I'll get lucky a get a spot lens on my first try. 

I've been thinking about it and I'm starting to feel pretty enthusiastic about this project. There seems to be several different consideration that will determine how well this will work out for me. Amount of energy going into the system (By system I mean the thing I'm trying to melt and my crucible), Amount of energy leaving the system and the total amount of thermal energy I need to reach the desired results. I've been doing some reading and it seems like the total amount of energy needed to heat up and melt most metals isn't that great. The problem is energy over time or in other word power. The maximum power I'm going to get from my Fresnel Lens isn't going to be that great which under normal circumstance would place severely limits on the amounts of metal I can melt. Still I think there might be a way. If I can limit the energy from leaving the system then I can still melt decent amounts of metal. 

Here is how I'm currently envisioning things. I get the largest Fresnel Lens I can in order to maximize the power output. I look for plan on the internet for how to build the sun following device. I get a crucible. I don't know that much about crucibles so this might change, but right now the crucible I'm envisioning would be a small metal box painted black so that it wont reflect back the sun's rays thereby letting energy escape from this system. The metal will have to be fairly thick so that it can conduct heat, and it would have to be made out of a material which wont melt at the temperatures it will encounter. Next I will get a glass display dome big enough to hold my crucible. I'm think of getting a glass display dome because they aren't that expensive, they are made out of very clear glass and the shape they are in should make them fairly good at withstanding pressure. The display dome will rest on a wooden base. I'll use a rubber seal to make it air tight. I'll drill a whole through the base which I'll use to suck the air out creating a vacuum which are know for being good thermal insulates. Lucky there is some very good information on this site about how to make vacuums. I'll also need an insulator to set my crucible on so that not to much heat escapes through the bottom. I've been reading about insulators and so far I'm considering three options. One would be fiber glass. The benefit of this option is that its easily available and reasonably priced. Another option I'm considering is Aerogel. Its a great insulator, but I looked it up on ebay and it's kind of expensive. Lastly I'm considering asbestos. It's a good insulator and after I little effort I found someplace online I could buy it and found it to be reasonably priced. I'll worry about what to choose latter when the project is closer to completion. 

I think that this idea has real promise. The main concerns I have right now is price. I don't know how much all this will end up costing me, but I'm guessing it will be somewhere in the hundreds by time I'm done. I wonder how much metal I would need to melt to make the investment in time and money worthwhile. Another concern I have is weather or not a glass display dome can withstand the pressure. If not I'll have to figure out another alternative.


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## Auggie (Jun 13, 2014)

I have personally melted zinc pennies with a Fresnel lens. I simply held the lens in my hands and focused it manually until I got a good beam and held it for about maybe 30 seconds to a minute before any action but the pennies did melt (slowly).

I imagine mounted in a frame with a swivel mechanism to allow it to be easily re-positioned you could melt zinc pennies all day!  But as has been pointed out, the maximum heat output limits the usefulness of such a solar furnace.

There is a solar furnace in France that uses mirrors to reflect enough light into its lens to make some serious heat.

A fun project to experiment with.


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## Lou (Jun 13, 2014)

I'd wager one could melt several grams of gold with nothing more than a fresnel lens out of an old projection TV.


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## GotTheBug (Jun 13, 2014)

If you surround your crucible with ceramic blanket it will hold the heat a lot better.


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## g_axelsson (Jun 15, 2014)

Evan2468WDWA said:


> I don't know that much about crucibles so this might change, but right now the crucible I'm envisioning would be a small metal box painted black so that it wont reflect back the sun's rays thereby letting energy escape from this system. The metal will have to be fairly thick so that it can conduct heat, and it would have to be made out of a material which wont melt at the temperatures it will encounter.


Not a good idea, metals dissolve other metals when molten. It can also stick to the surface (brazing is an example of that process). What paint would you use that could survive at the temperature of the molten metal.


Evan2468WDWA said:


> Next I will get a glass display dome big enough to hold my crucible. I'm think of getting a glass display dome because they aren't that expensive, they are made out of very clear glass and the shape they are in should make them fairly good at withstanding pressure. The display dome will rest on a wooden base. I'll use a rubber seal to make it air tight. I'll drill a whole through the base which I'll use to suck the air out creating a vacuum which are know for being good thermal insulates. Lucky there is some very good information on this site about how to make vacuums.


Another bad idea, display domes are not designed to withstand the pressure from the atmosphere and would shatter in a spray of glass shards long before reaching a good vacuum. I actually have a vacuum evaporator (used to put down carbon or metals on the surface of objects) and the glass dome is a cm thick. Any melting inside the dome would emit fumes that would make the inside foggy quite soon.


Evan2468WDWA said:


> I'll also need an insulator to set my crucible on so that not to much heat escapes through the bottom. I've been reading about insulators and so far I'm considering three options. One would be fiber glass. The benefit of this option is that its easily available and reasonably priced. Another option I'm considering is Aerogel. Its a great insulator, but I looked it up on ebay and it's kind of expensive. Lastly I'm considering asbestos. It's a good insulator and after I little effort I found someplace online I could buy it and found it to be reasonably priced. I'll worry about what to choose latter when the project is closer to completion.


Aerogel is called "frozen smoke", it is really fragile and expensive and asbestos is carcinogenic. 


Evan2468WDWA said:


> I think that this idea has real promise. The main concerns I have right now is price. I don't know how much all this will end up costing me, but I'm guessing it will be somewhere in the hundreds by time I'm done. I wonder how much metal I would need to melt to make the investment in time and money worthwhile. Another concern I have is weather or not a glass display dome can withstand the pressure. If not I'll have to figure out another alternative.


That depends on how high you price your time. If it is worth any at all I think this is a loosing concept.

Göran


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## bmgold2 (Jun 15, 2014)

Evan2468WDWA said:


> What you and patnor1011 makes sense. It seems like utilizing this idea will be more difficult then I thought. *Still I haven't given up yet.* If I can get one of these things for free it would definitely be worth doing. Even if I can't find a way to make use of it *playing around with it would still be a lot of fun.*



Having fun has to count for something. I hope you figure something out with this idea even if it is just melting very small amounts of whatever metal you have be it gold, silver, copper, or even just solder. Solar power has always interested me.

Here is a link to a site selling solar sheet magnifiers that claims it can reach 3000 degrees F. 



From the link above said:


> Simple Small solar furnaces can be made for experimentation and learning.
> This sheet will provide a sufficient quality of heat to melt, Zinc, Aluminum, Copper and Brass.
> Heat up some very small iron nails or tacks in a small glass tube and add a little bit of steam
> and you'll produce hydrogen.
> ...



I notice it doesn't mention HOW MUCH metal it would melt but it does give some encouragement that it _might_ be possible. I wonder if you could build a frame to hold several of these page size magnifiers each aimed at something like a melting dish? Check around online and locally and you might find this size fresnel lens that you could experiment with pretty cheap (if you haven't found the big one yet).

Remember, if you consider this a hobby, it doesn't HAVE to be profitable. HAVE FUN!!!

Google search for fresnel lens solar furnace


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## Smack (Jun 15, 2014)

That's the thing with a fresnel lens, you can't use multiple lenses and focus them in one spot without using mirrors. And you have to have clear sky and be able to keep everything moving with the sun. A parabolic mirror would be better in my opinion having used fresnel lenses for a few years now. Even if you get the proper temp out of one, the volume of metal that you can melt is very small. You can create a make shift solar lens using clear plastic sheeting and put water in it and the water becomes a magnifying glass. There are many videos on the tube about all this.


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## pgms4me (Jul 2, 2014)

Lots of good comments in this thread. You can do some small sample melting with 12 inch square ones like the ones from overhead projectors. the tv ones work much better,however you have to make sure when you remove it from the tv that you remove the front linear lens attached to the spot lens.they separate easily. They can melt anything in small amounts,platinum,tungsten,and titanium. I had to find a type of highly insulated firebrick to make a hole in to hold the metal.It melts the firebrick too if your not careful. i found 36 inch diagonal lens easiest to use but did try a couple of 50 inch ones. focus point size is critical to melt the metal your dealing with. You can usually heat things up quick enough in small amounts(20-30 grams) that the moving isnt much of a problem to keep the focus spot centered while melting.I used a small swivel frame . the focus spot on a larger lens keeps enough distance from the lens so fumes are not a problem,especially if your using a small fan. do make sure the fan does not create any draft near your melting spot.It would rob it of too much heat and possibly cause things to not melt. You will know when its working right when you can melt an older nickel easily.WELDING GLASSES A MUST!
You can use it to boil liquids if you put your pot on a stand and focus the spot to about 6 inch diameter aimed at a mirror below the pot. adjust the mirror to aim the spot at the pot bottom and your good to go. the whole thing should be mounted so the lens ,stand and mirror can all be turned as a unit to keep the proper position of the sun on it. BTW Temperatures have been measured to over 9000 degrees f with these under ideal conditions..as far as making the coffee goes,a 36 inch lens puts out about 2700 btu's with good sun in the middle of the day. About the same as a one burner stove.


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## butcher (Jul 3, 2014)

I can see it now, looking out the window, and my woods are on fire, darn I left that lens out there.


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