Author Topic: Got a Fresnel lens, help me decide what to build!  (Read 4280 times)

Offline excaliber

  • Survivalist Mentor
  • *****
  • Posts: 504
  • Karma: 26
  • gunning in Iraq
Got a Fresnel lens, help me decide what to build!
« on: October 09, 2014, 04:43:24 PM »
well I found a big screen TV for $10.00, and I've heard about people using these, so I jumped on it, and hauled it home.
but what do I do with it? honestly I dont really see a whole lot of really feasible uses for it other than starting a fire.

one of the issues is having to re-align it every few minutes, maybe use it to evaporate then condense water. but I have read
even that does not guarantee the water is good to drink.

cooking is an option, I can see that working,

Heating water for a shower would work, past that it seems to be more of just a nice novelty item or conversation piece.
melting pennies or turning concrete to lava, really doesnt help anyone. Been through several pages of youtube videos and
haven't been impressed yet.





Offline archer

  • Administrator
  • Ultimate Survival Veteran
  • *******
  • Posts: 17126
  • Karma: 382
  • #ImissAmerica
    • Journey to Greener Pastures
Re: Got a Fresnel lens, help me decide what to build!
« Reply #1 on: October 09, 2014, 05:04:41 PM »
what ever you do, make sure you dont leave it someplace where sun can get to it, you can burn stuff down that way

Offline excaliber

  • Survivalist Mentor
  • *****
  • Posts: 504
  • Karma: 26
  • gunning in Iraq
Re: Got a Fresnel lens, help me decide what to build!
« Reply #2 on: October 09, 2014, 05:07:36 PM »
agreed, and I have seen a few videos with people running sterling engines with them but again it just looks like
a novelty, the motors are never hooked up to a generator or something to produce useable electricity. maybe they dont have enough power.
the one that I did find was actually powering a dental drill conected to a small motor.

Offline bcksknr

  • Survival Demonstrator
  • *******
  • Posts: 2148
  • Karma: 313
  • Child of the Cold War
Re: Got a Fresnel lens, help me decide what to build!
« Reply #3 on: October 09, 2014, 09:25:08 PM »
     You can solve the sun tracking problem by building a frame that pivots at the right angle to allow the lens to focus on an object also attached to the frame. Easier to find a YouTube than to verbally explain. Depending on the size, you could heat a small metal ladle to cast lead bullets. By varying the focus you could probably heat the top of a small cook pot mounted on a swivel. Because the highest heat comes at a small focused spot, there are limited uses. I did some interesting woodburning with a small one. I drew my design in dark graphite pencil on a pine board. The black lines absorbed most of the heat and charred the design into the wood. You still have to carefully aim the "hot spot" on the lines. Perhaps, by heating a blackened metal box, an oven of sorts could be made for baking. It seems to me that reflecting solar energy is a better way to go rather than focusing it through a lens

Offline excaliber

  • Survivalist Mentor
  • *****
  • Posts: 504
  • Karma: 26
  • gunning in Iraq
Re: Got a Fresnel lens, help me decide what to build!
« Reply #4 on: October 10, 2014, 05:20:28 AM »
Thanks,

Offline Carl

  • Mr HamTastic!
  • Forum Veteran
  • *********
  • Posts: 13105
  • Karma: 715
  • COW?...No ,I haven't seen your cow.
Re: Got a Fresnel lens, help me decide what to build!
« Reply #5 on: October 10, 2014, 10:14:45 AM »
I had an ant farm,but the first sunny day discovered the error of the magnifier   :o.

Offline excaliber

  • Survivalist Mentor
  • *****
  • Posts: 504
  • Karma: 26
  • gunning in Iraq
Re: Got a Fresnel lens, help me decide what to build!
« Reply #6 on: October 10, 2014, 11:36:30 AM »
any more ideas???

d3nni5

  • Guest
Re: Got a Fresnel lens, help me decide what to build!
« Reply #7 on: October 10, 2014, 12:27:47 PM »

A quick YouTube search

http://www.youtube.com/results?search_query=fresnel+lens

  • solar water heater/pool heater
  • solar cooker
  • boiler running a steam engine
  • stirling engine

And my personal favorite...

Offline excaliber

  • Survivalist Mentor
  • *****
  • Posts: 504
  • Karma: 26
  • gunning in Iraq
Re: Got a Fresnel lens, help me decide what to build!
« Reply #8 on: October 10, 2014, 05:54:26 PM »
Yeah, I've probably watched a 100 youtube videos, but like I said most are just novelty items, a very small sterling or steam engine running but without any kind of a load on it, I did find one big sterling engine operating (again with no load on it) but it offered no info, and the owner says someone came and paid him a lot of money for it so it is gone, and he now doesn't have the resources to build another right now??? also I don't thing those engines will have much longevity due to high heat and no oiling or cooling system.

Honestly a cooker or hot water heater is about all I think it is good for, would have been nice to have when I was younger and had to go break the ice for the cows in the winter. just sit it by the pond and let it work.

Offline bcksknr

  • Survival Demonstrator
  • *******
  • Posts: 2148
  • Karma: 313
  • Child of the Cold War
Re: Got a Fresnel lens, help me decide what to build!
« Reply #9 on: October 12, 2014, 08:43:54 AM »
     A good thing to remember is that a dark, preferably black surface will absorb more energy from sunlight. A light colored surface will reflect more energy. Trying to start a fire with a magnifier (glass or Fresnel) is easier if the tinder is dark or black. Trying to melt ice or snow will be very difficult. If you cover the ice with dark ashes, it will absorb more energy, convert it into heat and speed melting. Back in the day, ashes used to be spread on icy roads, not so much for traction, but to speed melting when the sun came out.
     This is another reason to carry some charred cotton cloth, charred wood or charred fungus (dried shelf mushroom) in your fire starting kit, especially if you also carry a magnifying glass. Remember, a lens from a camera, binocular or even some eyeglasses can focus enough light to start a fire. Each of my larger kits have a 5x7 inch Fresnel lens of the lightweight plastic sheet type used to magnify written material. I guarantee that it is plenty large enough to start a fire with even "questionable" tinder and doesn't weigh as much as a decent size magnifying glass.
     If your going to heat water or cook with solar energy, the container should always be blackened to maximize absorption. Just a little physics; sunlight itself is not "hot". It carries energy in the form of photons that have the potential to be transformed into heat. For this to happen, the photons must strike and be absorbed by a material surface, where their energy is transferred to atoms in the surface of the material, which then in turn give off energy in the form of "heat". Light is transformed into heat. A surface that is black will absorb more energy than a white surface. This is why a white shirt, worn on a sunny day feels cooler that if a black shirt is worn. 

Offline jb59

  • Survivor
  • ***
  • Posts: 111
  • Karma: 3
  • New TSP Forum member
Re: Got a Fresnel lens, help me decide what to build!
« Reply #10 on: October 12, 2014, 06:05:08 PM »
I obtained three fresnel lenses from old tv's this past summer. All were different concave lenses but I couldn't get either one to produce a good flame. I watched videos on how it's supposed to work. I guess I got the wrong lenses. Good luck with yours.

I'm in S.E. Pa btw.

Offline Perfesser

  • Survivalist Mentor
  • *****
  • Posts: 728
  • Karma: 24
Re: Got a Fresnel lens, help me decide what to build!
« Reply #11 on: October 13, 2014, 04:12:02 PM »
Though it will concentrate light to a point, if you put something halfway to the focal point it will still get a lot more heat than just regular sunlight.
Maybe a super fast water heater? Solar cooker?

I made one and after some initial fun and experimenting it's been sitting in the yard for "UV stability testing".
Lots of time and effort needed to make a frame, build a stand that pivots on 2 axis for something cool but not that useful.
On top of that I used a pair of lenses and between the size and the 2 focal points and it's too big to fit in any door.
While adjusting this I put some plywood halfway to the focal point to block the light (after burning my hand in 1/2 a second). Wasn't long before the entire plywood was smoldering.