Large Fresnel lens 1

[Heatseeker]

Is it possible to build a Fresnel lense with an diameter of 50 - 60 metres, maybe assembled in a frame/patchwork style construction? Maybe somebody already did this - are there any references?

 

[nbucska]

Possible but not cheap.

<nbucska@pcperipherals.com>

 

[IRstuff]

The weight to span ratio is too large, there would be no way to adequately control the gravity sag of the structure, which affects the apparent EFL in the middle of the structure.

What is the end purpose?

TTFN

 

[Heatseeker]

Brainstorming - for an solarthermal application. If it is possible, who knows more about that.
I agree it would be a statics nightmare if one would make this a monolithic structure. But if one uses a structure like those made by Sprung Instant Structures and replaces the stressed membrane between the load carrying profiles with an optical material?

 

[nbucska]

Of course I meant reasonable size xparent plastic panels
in a steel grid.

The problem is that you need to have many &quot;Fresnel-prisms&quot;
and each need separate tooling.

I think it would be cheaper and easier to use a large #
of large flat mirrors with separate positioning systems

<nbucska@pcperipherals.com>

 

[Heatseeker]

nbucska thank you for the advice. questions is what is really cheaper?

 

[nbucska]

A molding tool is VERY expensive -- it pays only if you
need thousands of pieces. Do you have any idea, how
many different ones you would need ?




<nbucska@pcperipherals.com>

 

[jeanpaul]

You could use water between sheets of rigid plastic like polycarbonate. It should be economic.

 

[Heatseeker]

nbucska - no I have no idea. It would depend on the support construction, focal length etc one chooses I think.

jeanpaul - thank you for the idea. Is this, on a smaller scale perhaps, being done already?

 

[IRstuff]

For solar-thermal applications, the efficiency requirements of the energy converter usually requires thousands of square meters to get adequate efficiency.

Any optical design faster than f/1 is usually impractical, particularly for simple optics. So if you want an f/2 design, you're talking about a focal length of at least 200 meters, with a clear aperture of 100 meters. This structure would be too big to steer and would have extremely poor efficiency.

Additionally, the plastics and polycarbonates used in most fresnel lenses usually cannot tolerate continual UV exposure to the sun.

A fresnel lens is composed of an array of prismatic features, which is what allows it to bend light. These prismatic surfaces have to change angle as a function of distance from the center of the optics, which is how normal optics works. A sheet of water will bend light at the same angle across its area, unless you vary the thickness.

TTFN

 

[jeanpaul]

For the UV problem, I agree of course
But why not use a cylindrical fresnel lens with a bunch of glass or PC cases in prism shape.
You could in this way obtain a sort of greenhouse...

 

[IRstuff]

Why complicate it with water; you could simply use bulk glass? The gain in index is not necessarily worth ti additional complexity.

TTFN

 

[jeanpaul]

To do home-made optics...;-)

 

[nbucska]

Hi Heat:
I wouldn't even consider water: it can freeze, it can spoil
(algae, bacteria, etc.) Do you even know if it is transparent to IR ?

Do you want to build one unit or mass produce it ? The two
approaches are 100% different !

<nbucska@pcperipherals.com>

 

[Heatseeker]

Thank you all for providing thoughts.
To answer questions and post some final ones:
+ The application requires solar energy from about 5000 squm
+ It could, if built inexpensively enough be replicated several hundred times

to IRstuff
What is your experience with sunlight exposure of PC or Metaacrylate? Does it last days, months or years?

I will be travelling the next 6-8 weeks and not able to participate - please feel free to continue, it has been teaching to me so far. Thank you.

 

[nbucska]

Hi HS:
Honeywell made a solar system with many large flat mirrors
individually servoed but they abandonned it -- I don't know
why.


<nbucska@pcperipherals.com>

 

[jimhuang]

Hi:
Maybe you can find Large Fresnel Lens from

http://www.3dlens.com

 

[nbucska]

One other reason not to use lens but mirror:
The lens must be between the Sun and the target -- i.e.
elevated. The mirror can be located at ground level.

<nbucska@pcperipherals.com>

 

[CalculatorDave]

In my experience, acrylic fresnel lenses have excellent long-term UV stability, great for outdoor use.

 

[nbucska]

I challenge anyone to find any advantage of Fresnel lens
vr. plane mirrors.

<nbucska@pcperipherals.com>