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?

 

[Heatseeker]

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?

 

[IRstuff]

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

 

[CalculatorDave]

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

 

[CalculatorDave]

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

 

[Heatseeker]

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?

 

[nbucska]

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

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

 

[IRstuff]

Most dust are effectively opaque.

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

TTFN

 

[nbucska]

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

 

[CalculatorDave]

http://www.matweb.com

has data on acrylic mechanical properties. Also,

http://www.cyro.com

has quite a bit of online information on acrylic properties.

David

 

[Heatseeker]

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.

 

[Vortexx]

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..?

 

[Vortexx]

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 ?

 

[Vortexx]

Here is some good old fashioned awe inspiring engineering:

http://members.aol.com/stiffcrust/pharos/lens.jpg

 

[nbucska]

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

 

[Vortexx]

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.

 

[Vortexx]

Some considerations for inflatable Fresnel structures...

http://www.nasatech.com/Briefs/Mar99/LEW16662.html

 

[Vortexx]

More considerations (very good in-depth paper)

http://sleekfreak.ath.cx:81/3wdev/VITAHTML/SUBLEV/EN1/SOLRCONC.HTM

 

[SteveLepp]

I'm concerned about wind loading.

Steve