Thread: Principle of parabolic refraction?

I am working on a project to heat water for my uncles ranch. Its in the mountains and i figure light could be cost effective. i am not sure my design is even feasible. I know that the bottom two reflectors are sound. but i am not sure if the parabolic refractor on top is possible. please input would be amazing.

I don't understand the purple and red rays, could you explain how they achieve those angles?

I don't understand the drawing at all; looks like a small convex parabolic mirror inside a huge glass lens. This can't be what you mean, so please tell us what you do mean.

And anyway, the end result, apparently, is lots of rays in a narrow parallel bundle. You don't need them to be parallel, as long as they all hit whatever the water is in.

Good luck, and looking forward to discussing your project.

Well i suppose i should explained better ha-ha. The top parabolic shape labeled parabolic refractor would be some sort of clear material which would refract incoming light downward. the other two labeled parabolic reflectors 1 and 2 would serve only to focus the light. each individual colored line represents a ray of light. i showed several to show that if my idea is sound no matter the angle of the sun the light would be refracted downward. again i am unsure as to whether or not that is possible. The blue lines merely show the angles.

Originally Posted by Leszek Luchowski

And anyway, the end result, apparently, is lots of rays in a narrow parallel bundle. You don't need them to be parallel, as long as they all hit whatever the water is in.

i suppose you are right i could remove two reflectors. that would suffice i think. I just need light to be hitting the water all day even when its not high noon. If the refractor idea is sound i could achieve this.

I am still not sure what shape your "refractor" is. Or what it is to begin with, is it something different from a lens?

I'm getting the impression that it's a kind of dome made of uniformly thin transparent material. A bit like the front half of a blister packaging. In this case it won't achieve much because its two surfaces are parallel (and very close together, but that is less important here).

If you want to concentrate light by refraction you need a lens that is thin at the edges and thick in the middle. For the size we are talking about, the middle part would be quite thick indeed (on the order of half a meter or more), heavy and difficult to make (= expensive to buy or order).

You can cut some cost and weight by using a Fresnel lens. But I still have a gut feeling that mirrors would be more practical for your purpose.

Good luck, and keep us posted.

ok so this would be a better design i think.. also what do you mean by using mirrors? do you mean like the solar generator in the mojave desert? please explain. also i would very much rather avoid solar trackers.

At the concept level, I would drop the lens and the smaller mirror (or reflector as you call it) and just leave the big mirror (reflector) with the water container in its focus.

That's just an idea. Before you start building, whatever the design, you will need to work out the optics in some geometric/mathematical detail. Which is more than I have time for.

Some other ideas:

• Instead of putting the big water tank inside the optical setup, you might replace it with a piece or pieces of tubing, with the tank placed at a higher point so heated water rises to it and colder water comes down to be heated. The tank would have to stand somewhere to the north(*) of your optics so as not to overshadow it.
  You might also use multiple small mirrors instead of a big one, with a tube at the focus of each of them.
  Whatever container or tube receives the light, paint it black so it absorbs solar radiation better.
  Last not least, learn from what others have already done. Note: this Wikipedia article is just a tiny part of what you can find on the Web on this subject.

Good luck, and keep us posted.
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(*) I assume that you are in the northern hemisphere, and outside of the tropical belt between the tropics.

Originally Posted by Leszek Luchowski

just leave the big mirror (reflector) with the water container in its focus.

well you see the its a parabolic reflector and would only direct light to the focus if its directly above it. like at noon, but not the rest of the day. thats why the lens was necissary.

I am still worried about the lens. Even if it's Fresnel, it's going to be expensive, heavy, and unwieldy if you make it of glass. If it's made of plastic, its efficiency will be dubious - it will absorb or dissipate much of the energy.

Please look at the devices in the Wikipedia article I linked to. Some are very simple yet they work. I believe some of them use black cylindrical tubes placed parallel to the axis of the Earth and inside semi-cylindrical mirrors, which helps avoid building a solar tracker.

Last thing: in your drawing, all the rays start at one point just above the lens. This is not correct. The Sun is so big and so far away that you can easily assume its beams are parallel (although they come from different directions depending on time of day and year).

I'm afraid this is about as much as I can do for your project. Once again, good luck.