Is a Fresnel Lens a Better Option for Solar Thermal Applications?

[MIC]

I would like to know if anybody has thoughts on this idea, vs using parabolic dishes and a distributed oil heat transfer system for solar thermal applications.

I can find cheap fresnel lens', and I would like to use them to concentrate the light, then columnate it so that many concentrated beams of light could hit one spot. I know it is important to get the columnating lens precisely positioned with exact curvature, but it could be small lens. I think it would be better to deal with light for moving the heat, and transform it to heat at the one end location.

I heard that an extremely large fresnel lens doesn't work well, and I am having problems finding these funded experiments from the 70's online. I would like to find more resources for these results if anyone knows of where to find them. Thank you in advance.

 

[Redbelly98]

I can find cheap fresnel lens', and I would like to use them to concentrate the light, then columnate it so that many concentrated beams of light could hit one spot.

The columnation will require a diverging lens. I doubt you'll find one in the Fresnel type, but you could use a standard lens since it would be smaller than the focussing lens.

But, why not just use the Fresnel lens to focus the sunlight onto the object you are heating?

 

[MIC]

What I would like to do is concentrate the light in many location (lots of surface area), and then reflect the concentrated light from many (1$, light wieight, fresnel) lens's to one spot. The reason I have to columnate the light is once the light moves through a lens, past the focus, it diverges. This allows me to use small diverging lens's and small reflective mirrors to move the concentrated light/heat.

 

[Redbelly98]

What I would like to do is concentrate the light in many location (lots of surface area), ...

Do you have some idea of how many locations, and how much surface area? And, what is the size of the area to receive all the concentrated light? I'm just curious about ballpark figures here. In principle this should work, since Fresnel lenses are suited for "power" applications like this, and not so good at forming quality images (which you don't need to do).

A cheap way to collimate the beams might be to use spherical mirrors, the type that can be stuck on to a car or truck's side-view mirror to give the driver a wider view. I think I've seen them as large as 2 or 3 inches in diameter, but I forget what the price was. [edit: and, I don't know what focal length these mirrors typically are.]

Of course, it would be advisable to experiment with a single pair (Fresnel lens + collimator) to see if it does what you need, before investing in many of them.

 

[MIC]

I would like to start off with just one fresnel, one collimating lens, and one mirror. my Fresnel lens is 18 cm * 26 cm, with a focal length about 27.5 cm. I still need to find the right specs for a collimating lens. With the structure I built to get th focal length measurement, I burned a hole about 1 cm wide.
Right now I have 11 of these fresnel lens's:), and a couple plano concave lens's with the wrong curvature. The plano concave lens' has about 3.5 cm Diam. and cost about $2.50

 

[Redbelly98]

With the focal length about the same size as the lens dimension, there will be a lot of aberration. Not sure how practical this will be, but go ahead and experiment to find out.

The collimation lens is pretty much determined by two parameters: focal length f, and diameter D.

1. Focal length f
The collimation lens needs to have a focal length less than the 27.5 cm of the Fresnel lens, in order to reduce the diameter of the beam. You can use either a negative (concave) lens between the focal point and the Fresnel lens, OR a positive (convex) lens beyond the focal point.

2. Diameter D
The collimator should have the same f/D (focal length divided by diameter) ratio as the Fresnel lens, or larger. If it's diameter is too small, then when you collimate the beam some of it will not "fit" through the collimator.

So, the collimator should have
f/D < 27.5/26 = 1.06
or
D > f/1.06 or 0.95 f

The beam diameter will be reduced by the same factor as the focal length ratio. Eg., a f=5.5 cm collimator will reduct the beam diameter by a factor of 5.5/27.5 or roughly 1/5. And, it must be about 5-6 cm in diameter in order to capture the entire beam.

Hope I haven't rambled on too much ... let me know if there's anything you don't understand. I could try to draw a figure if that would help.

 

[MIC]

No, actually I am hoping to find the right lens, and continue experimenting. I don't understand why I need a lens so much larger than the diameter of the light at the focus. if I place the lens near the focus, I can easily capture all of the light within this lens with is just 3.5 cm in diameter.

 

[MIC]

I am having problems continuing with this experiment. I feel like I would have to experiment by making a custom collimating lens, and then somehow make copies of it. Optics, such as telescopes, are so very exact. Also, with the fresnel lens there is no one diameter, because it is made up of many concentric rings. These rings are 'printed' onto a rectangular cutout. These fresnel lens are exact copies, maybe I can flip one over and use two of these lens exactly lined up, about 27 cm away from each other, to concentrate and then colimate the light.

 

[Redbelly98]

A lens placed at the focus will have very little effect on the beam, and not collimate it. The lens must be placed 1 focal length away from the focus to collimate the beam.

I'll try to post a figure later to show what's going on, but don't have time at this moment. In the meantime, here's an experiment you might try: place two of your Fresnel lenses 2f (2x27 or 54 cm) apart, and see how well you can collimate the beam the beam of sunlight.

If you place the 2nd lens at the focus of the sunlight beam, you might melt the plastic. Be careful and wary of where the focus is at all times.

 

[Redbelly98]

I found appropriate figures online.

Scroll about 1/2 way down this page, to Figures A, B & C:
http://www.lhup.edu/~dsimanek/scenario/lenses.htm

In particular, look at Figure B. The concave lens is 1 focal length away from where the focus would be (if the concave lens were not there).
If the concave lens is placed anywhere else, the emerging beam will not be collimated.

 

[MIC]

I tried what you said, but I used 4 fresnel lens, 2 in each hand to shorten the focal length. The light was very seperated(like a rainbow) before it reached the ground. I did have to hold it at about twice the focal length to get a bit of collimating.

Then I stacked 2 and then 3 of my plano-concave lenses using a hose clamp. I found the best results when I held 2 fixed lens's near the focal point. I think the column can be tuned by stacking lens's. Anyways, this collumn still wasnt perfect, but at the hot point ( about 4 1/2 ft away now ) it caused some smoking on the dry grass.

 

[Redbelly98]

Ah, I forgot about combining 2 lenses to make an effective lens of half the focal length. That suggests the attached setup, where the beam size is reduced to half it's width. This makes for 4 times the intensity of the original beam, neglecting losses due to transmission and aberations.