Clear Teflon Tube for Lightpipe

It seems an odd concept to try and transport the energy while it is still in the form of radiation.
I can't see why you would want to do this. It is surely easier to collect the energy from a larger area then concentrate it and transport it in a convenient form e.g. Heat or electricity.
Del

It is a little odd, yes.

The lightpipe is for the transportation of the energy from the collector to the application, which would be anything from a heat engine generator to water purifier, pyrolizer, gassifier, cooker, etc.

Being able to carry the solar raw would have a couple of advantages; it would make the applications hot swappable, and allow for feeding in multiple collectors for greater heat ranges.

The way this is usually done is through a heat transfer fluid, but this tends to be inefficient and requires a pump.

It's kiind of similar, I guess.

Basically I have a collector which tracks the sun. I need some way of transporting the energy to the application; let's say the boiler for a steam turbine.

I can build the boiler into the hotspot of the collector, but it adds weight, means the collector is now a dedicated steam generator, and means making a bunch of boilers and somehow feeding them all into one turbine.

Alternatively I can heat a fluid and transport that to a boiler, but as I say, that has it's own problems.

So since solutions already exist to transport light in a fairly simple, efficient and safe way...

This is a really good patent on the issue, it's a bit of a read but goes into all the details and considerations.

http://www.freepatentsonline.com/EP0727054.html

I was thinking to plug the pipe's entry point with glass, but have at the other end a stoppered length of copper tube, with which the liquid comes into direct contact. This would absorb all the energy and conduct it into the application.

This would also give the advantage of picking up a lot of the heat which has been absorbed by the liquid, and reclaim an amount of any lost energy.

All right, I think I am with you. And, if I understand correctly, you are looking for a source of this lightpipe?

I found http://www.lumatec.de/e_home/e_home.htm

Among their offerings: "The Liquid Lightguide is a breakthrough in light guide technology. Replacing glass fibers by liquid it set completely new standards for transmission efficiency. The outstanding properties, especially in the ultraviolet spectrum, opened up a whole new range of applications for the optical industry. Over the years the product line has been diversified and refined to cover a spectrum from 250nm to 2000nm."

Sounds like a place to start. Or, am I misunderstanding; are you looking for other assistance?

They don't give much away in technical details, but the principle is the same, yes.

As I say, my question is twofold; what do you guys think of the approach in general, and what can you tell me about sourcing materials, ie what to use and where to get it.

I'm wanting it make it myself, the brief of my device requires that all materials be as easily sourced as possible, and ultimately I want to put 300 watts through each pipe over about two meters.

Here's the original discussion where this was recommended to me:

http://forums.makezine.com/comments.php?DiscussionID=5183

Tho rereading it just now I think I slightly misunderstood what the guy was saying. What I thought he meant was to use a highly refractive liquid with a significantly lower refraction plastic tube (as the linked patent says).

But he mentions mineral oil with pex, which have very similar i.o.r., so what I think he means is that the liquid and tube act as one solid waveguide, with the total internal reflection taking place between it and the air, rather than between the liquid and the polymer wall.

Hmm.

I suspect they don't give much technical detail because it is a proprietary product, protected by (from what I see) a pretty solid patent. I am sure they wish you to purchase some from them, not make your own!

We at CR4 had a discussion regarding the Earth Sure stuff: http://cr4.globalspec.com/thread/50683#comment527331 Not much specific about the light pipes, but their ideas and products in general. Not too flattering.

Well, Del the Cat (a well respected member of the forum) has voiced some concern regarding the venture, and Doorman (just some 'dude' hanging around here) feels similarly cautious about endorsing the project. To me, it just seems that, for today, the technology just isn't there for a viable method to accomplish your goal by using light pipes. It would be great if your idea would bear fruit with just a few dollars of stuff and a few hours of effort, bringing very inexpensive, clean, reliable and endless electrical energy right into my home or office... no effect on the ozone layer, no pollution, no obnoxious noise, no big footprint...

The guys at Earth Sure are trying to convince prospective customers that they have accomplished this, to a degree. While I have not investigated, I doubt it can be called 'inexpensive'. I believe their claims of efficiency are exaggerated and are unsubstantiated. I also believe if their claims were accurate, we would ALL already have one! I certainly would WANT one!

Is it worth experimenting? Sure, why not. From what I can find, there are only seven patents for liquid-filled light pipes... and nearly all describe reducing UV radiation transmission, with little heat transfer. You may just stumble upon the one thing the others have not considered.

Ok. What other ways are there of doing this? The light pipe isn't really the crux of the project, that would be the tracking system. I just need some easy way of getting the energy out of the collector.

The tracking system? Solar tracking system? Well, no sense reinventing the wheel:

http://en.wikipedia.org/wiki/Solar_tracking

http://cr4.globalspec.com/thread/47737

"...some easy way of getting the energy out of the collector."

There are many, many members here who can address this comment better than I. With some luck they will find this thread and join in.

The tracking system is non electrical and can be made very easily from common recycled and salvaged materials, using basic tools and skills. The idea is that pretty much anyone, anywhere in the world will be able to make the thing, produce all their electricity, water, etc, then ideally start selling them, royalty free.

Most of the work has been not so much in the design itself, but in making it as absolutely basic as possible to source and construct.

If it works out I'll hopefully be spending the next couple years travelling round, working with communities and NGOs, showing people how to make them and develop it to their needs.

http://www.lumatec.de/en Tap on the light tube image. Power transmission is limited to about 5 Watts. The IR device has temperature limitations (may require cooling).

Even 2500 nanometer is a fairly short distance.

Are you sure you need a light pipe with its attendant losses?

I handled one of these liquid light pipes at an exhibition a couple of weeks ago, not the German make, this was from a Japanese company. What was noticeable was the amount of light 'leaking' along the length of the pipe. In my work we use various types of optical coupling all of which are matched to the index of the parts being connected but all of which cause losses.

We work with very low light levels, down to counting single photons, so perhaps these losses are more significant than in a large scale operation such as you describe but I think I would still worry about the losses you will get with a long light pipe.

Sorry, but I fail to see the logic in this. You envisage a solar energy collector feeding a receiver via a light pipe. The collector has to have a large effective incident area to acquire the energy, which in practice means turning numerous panels to face the sun. The panels may be photovoltaic for electricity generation, but in your case it looks like mirrors angled towards a funnel on your light pipe. It would be far simpler to dispense with the light pipe and angle the collected beams from all the mirrors to the final collector directly. You can still choose whatever device you want to receive the energy from the collector, and you would not lose any energy in the light pipe.

> What was noticeable was the amount of light 'leaking' along the length of the pipe.

That is an issue, yes. My plan is to wrap the pipe in tinfoil or similar, then a layer of thermal insulation. This should mean that any energy escaping the pipe (I guess mostly due to impurities in the liquid) will be reflected back initially as radiation, and after a couple of bounces as conducted heat. Most of this heat will then be drawn out of the liquid through the final copper pipe.

Let's say I use mineral oil as my core liquid, which boils at about 300º C, dumping heat into water which boils at 100. The water should take out enough of the heat to prevent the oil boiling. Under ideal conditions (ha!) this should increase the overall efficiency close to 100%, as the energy would have no where to go other than into the water.
Under ideal conditions.

> but in your case it looks like mirrors angled towards a funnel on your light pipe.

Basically, yes. I think two short parabolic troughs and a fresnel reading lens is the simplest setup. Otherwise a Cassegrain double reflector, but that's a hassle.

> It would be far simpler to dispense with the light pipe and angle the collected beams > from all the mirrors to the final collector directly.

Yes, it would be, and this was my original idea. The main issue though is safety. It's not a fresnel reflector -lots of mirrors pointing up- kind of situation, it's a single collector on a equatorial mount, sending energy to something on the ground and probably a couple meters away. Sending a few hundred watts through the air like that is just begging for fried children. I guess I could have some kind of protective housing, but it would need to accommodate the turning of the collector.
Also I'd need to have a heliostat to keep the beam targeted, as well as lenses to collimate the light. This is all doable, but I'm trying to keep the thing simple.

Doing some more reading up on liquid light pipes, there's a bunch of stuff out there, and a lot of them keep talking about very high efficiencies, but I can't seem to find too much in the way of actual numbers. The patent I linked talks about 80% per meter, which I think I can improve on with what I mentioned earlier.

What would be really good here would be a heat pipe:

http://www.cheresources.com/htpipes.shtml

But they look kinda tricky to make, especially as this would need to operate against gravity, so would have to include some kind of internal wick to draw up the working fluid. Super efficient tho.

With respect, I think that "Sending a few hundred watts through the air like that is just begging for fried children" is overstating the case. That full power would be concentrated only immediately in front of the receiver. There is no need to protect the collector (except from vandalism). All you need is a finger-proof grid extending well in front of the receiver.

Ah, I see what you mean. Rather than sending it concentrated, have a much longer focus mirror concentrating on the app.

That basically tho would be a heliostat, which as I understand it are pretty finicky things to get right, I'm not sure my tracking system would be up to the trigonometry.

But I'll give it some thought.

I assume that you don't find appealing the techniques that are used in solar observatory telescopes, and reflector telescopes (both optical and microwave). They use rigid waveguides. This is not as flexible as a curving light guide, but you can use off-the-shelf polished-and-coated aluminum tubing and mirrors; and it is hard to beat the transmission efficiency of filtered air. There is room to improve these products with graduated refractive index coatings, which would make subsequent reflections within the tubing more shallow and therefore more efficient. However, if the lenses and mirrors are accurate enough, the tubing becomes redundant.

This was my second idea. I'd need to do a very good job of collimating and aiming a pretty tight beam; once the light started bouncing off the walls I'd lose it quick.

It's basically a heliostat with a housing, and like I say, keeping mirrors accurately oriented as not only the sun, but the whole collector assembly turns = tricky.

Hard to design, hard to build, and potentially not too efficient after it's gone through a couple of mirrors, lenses, and tubing.

However if the lightpipe refuses to function, I may well have no other option.

Teflon, especially FEP, is proving tricky to find.

However it looks like silicone rubber tube might be an option. It's not as optically clear, but at 1.40 the refractive index is low enough (5% lower than mineral oil's, give or take) and it should be easier and cheaper to source.

I'm reading promising things regarding efficiency, one paper cites 50% loss over 30 meters (!), though that was using a specialised inner lining.
Still...

If you want to light an area which would otherwise be dark in your house, I have seen 40 cm tubes installed in some UK house roofs (usually flat), with what looked like silver foil over the inside, with a clear glass/plastic cap, that worked brilliantly in bringing light down 2 meters.....One every meter or so is really fantastic....

If you want energy for heating or similar, I think that you are going the wrong way personally. You should collect the energy on the roof into say water and circulate the water to warm whatever you want to warm up......

Water is bad enough if you get a leak in a house, but mineral oil could REALLY damage your house, the local environment AND not forgetting, your wallet!!!

In an extreme case, you might need to dig out the foundations to get rid of the oil in an environmentally correct manner....It is not something that you want to get into the underground water table for example!!! In Germany, correcting this would cost you a fortune.....

Ok so that's put together now. I couldn't get my hands on any teflon so have gone with silicone tube filled with mineral oil, capped with decorative glass beads and hose clamps.

It does give total internal reflection, but there is quite a bit of emission through the tube wall. I need to get my hands on a digital camera to test the efficiency, but it doesn't look incredibly good. Pretty good, but not incredibly.

I think it's going to come down to a combination of the light itself carrying the heat + drawing the absorbed heat out of the oil. These two together may well give something interesting.

The whole thing has so far cost less than 10€ for a meter and is definitely good enough already for lighting purposes.

Try spray painting the exterior of the optical tubing with metallic paint. I would also recommend using the thinnest tubing wall available (more like a film), as the wall material may end up absorbing most of the light. You can inflate this within a more sturdy tube.

You may also want to try painting the smooth interior of a tube/hose with (highly reflective) metallic paint.

The larger the diameter, the fewer the reflections.

Well that didn't go well.

Using a sheet of paper and my laptop's webcam as a light sensor I figure only about 10-12% of the light is making it through the pipe. The mineral oil seems to be doing a good job, no doubt I'm losing a bit to my less than ideal glass caps, but I think the main problem is the silicone scattering the light.

This can be solved with teflon FEP or AF 1800/2400, but they are still proving just about impossible to source, and if I'm having this much trouble with them it kind of removes this as an easy to construct by anyone solution.

But if you could get teflon tube, I think this would totally work.

Switching focus to heat pipes.

Teflon (PTFE) tube here & here & here & here.

Cheers for the research. Unfortunately PTFE is generally too opaque. Also I wanted to find something local to Spain.

Ta tho.

You may only need the wicking action at the evaperator. A small pump and narrow tubing can move the condensate fluid from the condenser back to the evaperator.