fresnel lens 3

Video from rimstar.org about Fresnel lenses. He discusses the question of which side of the lens should face the sun.

From about 2:20 to 4:50, he measures the focal length of his lens in both orientations. His is the same 32 inches both ways. He then does a heat gain test with both orientations (to 6:10) and finds that with the grooved side facing the sun, he gets significantly more heat. His test is to heat 150 ml of water by 20F degrees at the focal point and measure the time in minutes. Shorter time indicates better heating.

Grooves facing sun: 1:55 (one minute, 55 seconds)
Flat side facing sun: 4:40

He provides an explanation of why this effect occurs - spherical aberration. He goes on to explain the difference between a linear and a spot lens from 8:00.

A very interesting site is rimstar.org. Have a look around at some of the other impressive work shown there.

I found this interesting at NTKJ Co., Ltd. (Nihon Tokushu Kogaku Jushi) Fresnel lens supplier: an illustration showing the orientation of their Fresnel lens for solar concentrator with the statement: "Our standard fresnel lenses can also be used for the same purposes but with the opposite facing design. Namely its plano side faces focus and the fresnel surface faces parallel light source."

Perhaps this applies only to NTKJ lenses?

Their website is a useful catalog of different types of commercially available Fresnel lenses. I don't know if this company manufactured either of my Fresnel lenses.

The Green Power Science store is a source of Fresnel lenses of different sizes and types.


A video description of my temporary optical test bench used for measuring focal length of Fresnel lenses.

I was able to show myself that both of the lenses I have (your mileage may vary) focus much more precisely if the parallel light enters the lens on the Fresnel side but this not always have to be the case hence you might consider testing your lenses in a similar manner?

Finally, I notice that many who work with Fresnel lenses tend to use stands similar to those in the picture.

It is easy to understand why this is the most common approach. You need a rigid frame around the lens to keep it straight and what better way to add a pivot than at the center point of the frame?

I would imagine that the movement of the sun over time and the wind would make the this type of stand problematic.

This is a sketch of my stand concept. Imagine two cylinders, one inside the other. The Fresnel lens would be in a frame but the frame would have arms which are pivoted at each side of the work surface on the outer cylinder. Each arm would carry an adjustable counterbalance. The length of the arms would be so that the focal point would always land on the work table (the top of the inner cylinder) with the lens pivoting around it in two axes (#1 and #4). These axes could potentially be motorized with tracking added later.

The other degrees of freedom are #2 which allows the work table to rotate so different sides of the work could face the sun and #3 allows the work table to be raised and lowered, moving the work vertically in the focus.

Sorry if that is not more clear but it is only a concept that I hope to refine as I build something.

For now, I need the benches free so I must finish with these focal length measurements.

Thank you for your interest.

George Plhak
Lion's Head, Ontario, Canada

fresnel lens reading list
fresnel lens
fresnel lens 2
fresnel lens 3 (this article)

AND Use of fresnel lens for 3D solar printing.

interesting diy solar concentrators

I have been watching the progress of сергей юрко on YouTube with his very interesting solar trough project.

сергей юрко is Sergey Briskly according to Google Translate. He describes himself as "Sergey, Mirgorod, Poltava region" (Ukraine) and gives his phone number in the video description on YouTube. He has a series of videos in which he describes the construction method and progress of his split trough solar concentrator, intended for heat capture and electricity production. His videos I've watched so far show the progress of what is now a rather large array.

I am using Google Translate to understand what I can.

In this latest video, Sergey walks the viewer through the array and the process of building from ordinary building materials with ordinary tools. He even shows the fasteners he is using.

The reflectors are an interesting split design. He is located at almost 50N latitude so for winter use, his array would point fairly low in the sky.

The array does not have provision for tracking. Sergey has oriented the array east-west with seasonality adjustment on the back legs of the frame (where the yellow back leg joins the blue)? The bottoms of the ribs are hinged where they meet the ground support so the array "tilt" can be adjusted to better match the season.

The construction of the collector assembly, with insulation and multiple copper collector tubes is interesting. Although glass wool insulation decouples the collectors from some wind and thermal loss from the front, it does nothing to decouple the collectors from heat loss to the ambient air. It should be possible although more complex to use insulated glass tubes in the design.

Sergey presents numerous charts in Russian text. I would love to understand more about his work. This latest video is from last week and fairly complete (31 minutes). I encourage you to look at his work. I have not yet had any correspondence with Sergey.

Sergey: you will have a wider audience if you edit the automatic transcription of YouTube CC (closed caption) text on the video so viewers better understand your narration in other languages, like English. The default transcription (voice recognition) is not very good and gives poor language translation results.

I heard recently from Ebrahim Hashemi in Shiraz, Iran.

Hello
I am Ebrahim Hashemi, from Shiraz, Iran and I have invented the new method of ‘fixed-focus solar concentrators’ which is a unique method of using solar parabolic dishes.
Now, please introduce my website to all individuals interested in the solar energy.
it is my site: http://fixedfocus.ir/viewpage.php?page_id=2
Respectfully yours
E.Hashemi sand20251@gmail.com

I found it interesting that Ebrahim is keeping the center of rotation of the dish the same as the focal point. This is essentially what I have done with my parabolic trough. It makes the tracking mechanism simpler. I did ask him how he accomplishes the bearing on the back of the dish.

Some ideas from the internet for your inspiration.

Thank you for your interest,

George Plhak
Lions Head, Ontario, Canada

fresnel lens 2

I had written in fresnel lens about wanting to determine the focal length of a large Fresnel lens removed from an old rear projection television. I want to know the focal length to design a mount to aim the lens at a fire-proof adjustable work table in a safe manner.

I am using laser pointers to see where the light beam is bent. I have a measurement for each lens but I am confused about which side of the lens to point toward the sun. I am seeing an unusual effect that I wanted to explore further.

The simple test setup I was using showed me that I was on the right track but I realized that I could improve the setup to make the measurement more accurate and repeatable.

This is a picture (click to enlarge) of my temporary wood shop optical bench setup. There is one Fresnel lens lying flat on the left table. You can see the circular rings. The other is hanging vertically between the two tables. I have made some improvements which I'd like to tell you about.

I would like to use one of my lenses to construct a solar concentrator, primarily to fuse materials. An example by Dan Rojas of Green Power Science. Grant Thompson "the King of Random" shows how to remove the lens from the donor set.

A very thorough article on using Fresnel lenses for communications purposes.

I had read an instructable which suggested using two laser pointers resting on chairs. I am trying to use the same concept but using a more stable setup. I had some difficulty following the laser beam when I was manually holding the pointer.

As before I am using two benches with the lens suspended between them. Previously, the benches were about the same height. Now one bench is about 1 inch shorter in height. This difference allows space for a sliding cart to hold the laser pointers. The cart rides in a track so that I can smoothly move the laser pointer across the front of the lens.

There are two laser pointers on the cart. The yellow emits a red dot and the black emits a green dot. The pointers are held to the cart with hot glue. The height of the pointers is adjusted before gluing with a pair of tapered shims under each pointer to bring the dot high enough so that it just skims the out table top. If you enlarge the picture, you will notice the red laser dot just at the tabletop height of the out table (the higher one).

The red pointer has a rocker switch which allows it to stay ON. I must hold the button on the green pointer to keep it on.

A view of the side of the cart and it's track. The track is not fastened to the table and may be lined up parallel with the lines I have marked on the tabletop at 5, 10 and 15 inches back from the lens. The lens is on the left side.

You'll notice I haven't got the fronts of the laser pointers at the same distance but I don't think it matters. I have been using only one of the pointers at a time, not both of them. They are fixed down, the button on the green is over the beam so it is less tipsy and they can be slid easily.

I have the sheet to be tested hanging level with its focus (the center of the Fresnel pattern) at table top height (the height of the out table). I have two short strips of wood with hook eyes in the ends that I clamp to each side of the sheet.

In this way, I can easily unhook the sheet and reverse it so that I can send the beam into either the Fresnel side of the sheet or the smooth side. It is necessary to move the out table away to make some space to flip the lens. Normally I push the tables together to clamp hold flat the Fresnel lens at the desired height.

Here are my results so far (shown also in the first picture above):

Fresnel lens focal length
38x29in (96.5x73.7cm) Lens A	30.5in (77.5cm)
35x27in (88.9x68.6cm) Lens B	28.0in (71.1cm)

Note that my results are with the Fresnel surface of each lens facing the laser pointer (the top of the two illustrations).

I thought the lens performance would be approximately equal in the two orientations. In my observation, the determination of the focal length is less clear when the smooth surface of each lens is facing the pointer. I will do some more work and report shortly.

Thanks for your interest.

George Plhak
Lion's Head, Ontario, Canada

fresnel lens reading list
fresnel lens
fresnel lens 2 (this article)
fresnel lens 3