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Large Fresnel lens

Large Fresnel lens

Large Fresnel lens

(OP)
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?

RE: Large Fresnel lens

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

RE: Large Fresnel lens

(OP)
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?

RE: Large Fresnel lens

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

The problem is that you need to have many "Fresnel-prisms"
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>

RE: Large Fresnel lens

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

RE: Large Fresnel lens

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>

RE: Large Fresnel lens

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

RE: Large Fresnel lens

(OP)
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?

RE: Large Fresnel lens

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

RE: Large Fresnel lens

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...

RE: Large Fresnel lens

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

TTFN

RE: Large Fresnel lens

To do home-made optics...

RE: Large Fresnel lens

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>

RE: Large Fresnel lens

(OP)
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.

RE: Large Fresnel lens

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>

RE: Large Fresnel lens

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>

RE: Large Fresnel lens

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

RE: Large Fresnel lens

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

<nbucska@pcperipherals.com>

RE: Large Fresnel lens

(OP)
To Solar Dave - is that outdoor stability measurable in months or years, is it comparable to mirrors?
To nbucska (Electrical)- to my view mirrors are expensive and unlikely to get less expensive, if compared with Acryl the cost factor for the same collection surface seems to be 1:2 in favour of acryl. Also - consumption of area can be a critical issue and I would assume that while mirrors need thorough constant polishing it should be easier to keep fresnel lenses clean?
 

RE: Large Fresnel lens

I don't see that as a rational assumption.  Dust is dust, whether on a mirror or on a lens.  In either case, dust prevents the optics from doing their job.

TTFN

RE: Large Fresnel lens

Architectural acrylic suffers very little loss of clarity (2%?) over 20 years.  It is a standard material for skylights and windows.

RE: Large Fresnel lens

Dust acts as a light scatterer.  It alters the direction of the light rays striking the fresnel lens, and thus can interfere with the lens' focusing.  Lenses must be washed.  for this reason also, the grooved side of the Fresnel lens should be isolated from the ambient, because it is more difficult to clean the grooved side than the flat side.

David

RE: Large Fresnel lens

(OP)
Dust needs to be removed - true. As an ongoing operation - what is easier - cleaning a acrylic dome or a field of mirrors plus (if required) the mirrors of the concentrator?

To SolarDave - can you point to a source for mechanical property data on acryl?

RE: Large Fresnel lens

IR:
Sorry 4 delay...
On mirror the dust is worst for it effects the light twice.

<nbucska@pcperipherals DOT com> subj: eng-tips

RE: Large Fresnel lens

Most dust are effectively opaque.  

Semi-transparent particles tend to diffuse the energy anyway, so they are essentially opaque as well.

TTFN

RE: Large Fresnel lens

IR:
Yes but even in the case of front surface mirrors a particle due to its thickness may obstruct more light
on the second pass, especially at more acute angles
of incidence.

<nbucska@pcperipherals DOT com> subj: eng-tips

RE: Large Fresnel lens

www.matweb.com has data on acrylic mechanical properties.  Also, www.cyro.com has quite a bit of online information on acrylic properties.

David

RE: Large Fresnel lens

(OP)
Thanks everybody. Need to do some work with the info you gave me and will be back whenever there is news. Please feel free to continue the discussion.

RE: Large Fresnel lens

I think the notion of a liquid lens is very interesting...

The water can be replaced with alcohol, to avoid algae and has lower density than water, Lavoisier used a giant alcohol lens in the 18th century to melt platinum.

People have mentioned infavourable span to weight ratio for giant fresnel lenses, however i wonder if one could overcome this by using a pressurized plastic dome or balloon like structure..?





RE: Large Fresnel lens

About the steering issue of large Fresnel structures, The old lighthouses, bearing Fresnel lenses weighing up to 3 metric tons were able to be rotate by the push of a finger(!) because they where floating in a mercury pond.

Likewise one could envision pressurized greenhouse like Fresnel structure floating in lakes ?

RE: Large Fresnel lens

Just immagine a 150 ft lens mounted on a similar size fixture following the Sum. It has to be heavy enough to maintain its shape in spite of the gravity, you have to clean it regularly,

Now compare it with a number of flat mirrors on the ground independently servoed -- the only disadvantage is than
it occupies larger area.
   

<nbucska@pcperipherals DOT com> subj: eng-tips
read FAQ240-1032

RE: Large Fresnel lens

Alternatively, all the heliostats could be placed on a floating island as well with any concentrators and the whole setup could be rotated as a whole...

One possible advantage I am having in mind putting your solar furnace on water is that you could put some stirling engine in the focal point of the furnace, wich will get heated and make helium expand, now the the second chamber of the stirling engine actually runs under water providing active cooling allowing the expanded helium to be compressed agin.

RE: Large Fresnel lens

I'm concerned about wind loading.

Steve

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