Sunday, 12 September 2010

A litany of failures, part one: liquid core light pipe.

The original idea I had for transporting the energy once collected, and we're going back more than a year here, was a liquid core light pipe. Essentially a fat, squishy fibre optic cable.

This works through the principle of total internal reflection, whereby light bounces without loss off the barrier between two materials of reasonably different levels of refraction. Such as glass and plastic, or water and air, or in this case: silicon and mineral oil.

The latter being pretty ideal, as it's easy to get, very refractive, has a high boiling point, and optically clear.

The former is what killed it. Silicone isn't particularly ubiquitous, cheap or clear, but the thing that really makes it inappropriate for this is that due to it's micro-surface texture, it scatters the light too much. What you really want here is something nice and smooth, like Teflon FEP or AF.

Unfortunately, tho those would be perfect for the application in every way (AF 2400 has the lowest refractive index of any polymer, to the point that you can even use water as your core (trust me, that's impressive)), it's almost impossible to get your hands on. And expensive when you do.

As you can see the silicone pipe did basically function, but comprehensive transmissivity analysis (digital camera, laser pointer, piece of paper and photoshop's histogram function) showed that even over only a meter and without bends, only about 10% of light was making it out the other end. The rest being scattered through the pipe wall.

And I wanted to put hundreds of watts through the thing. I guess this is why most everyone told me this was not a likely idea.
But it's quite pretty. I capped it with glass beads and hose clamps, attached a small l.e.d. torch with reflective tape, and am now using it as a rather dim light for my room.

So, since this was not going to do what I wanted I went on to investigate heat pipes, which are actually very promising and worth future further inspection, but ultimately for simplicity's sake have opted for a heat transfer fluid thermosyphon. Like most everyone else.
More on this later.