Spray-On Solar-Power Cells Are True Breakthrough

National Geographic ^ | January 14, 2005 | Stefan Lovgren

[djf]

I love these articles. The only time I hear the words "could" and "might" is standing in line to buy a Powerball ticket!

So let me ask some Freepers a question. You know how when they build a bridge, they always have to put a gap in it, or the bridge will expand and buckle?

What would happen if we took a piece of steel, say 100 yards long, painted it black, securely anchored one end and geared up the other end to a generator? Then, when it got good and hot and expanded, we throw cold water on it and generate during the shrinking phase as well?

Sounds pretty low tech, I know. But when that steel wants to get bigger, there ain't much that's gonna stop it. I'm just wondering how much energy it could yield and what the tensile properties would be, how many cycles we could expect.

[chimera]

You are correct in your understanding of thermal expansion and contraction. It is a material-dependent property of matter. But for such a system to be practical you need to do an energy balance. Remember the three laws of thermodynamics: 1) You Can't Win, 2) You Can't Even Break Even, and 3) Things Are Going To Get Worse Before They Get Better (and a nod to you, Prof. Kelly of my undergraduate days). So if it is a man-made process you're contemplating, you need to total up the energy involved in heating a fluid, what it takes to pour the fluid (e.g., is there transport of the fluid, involved, etc.?), and, likewise, for the cooler fluid, the energy it takes to cool it, how much work is involved in cooling the steel rod/slab, etc. No matter how hard you work at it, you're going to have losses somewhere. For example, in heating the steel rod, not all of the heat is going to go into thermal expansion. Some is going to be conducted/convected/radiated away to the environment, and that amount of energy is lost to the conversion process in generating useful work. You're going to come out with a net deficit.

That's not to say the process isn't workable. It depends on how and where you get the input energy. Are you extracting it from a form that, in it's "natural" state, is unavailable to us, that is, "locked up", in a form like chemical or nuclear energy? Is it "free", as in sunlight falling on the ground, or cool water at deep depths in the ocean? If it is "free", how much work do you need to harness it? Generally, the more diffuse the energy source (e.g., solar, wind, hydrothermal, ocean waves, etc.), the more work you have to do to gather it up in sufficient quantities to make it useful. If you have to do a lot of work, like covering thousands of square miles of ground with solar panels or windmills, that is going to impact the economics of the process, plus have significant environmental impacts (which likely drives up the cost, or makes it unworkable). The more intense energy sources (burning of carbon-based fuels, or use of nuclear energy), generally require smaller-scale systems, but extract and processing of fuel, control of the process, and waste products, enter into the picture.

Bottom line is these all have their advantages and disadvantages, it is often a matter of human preference (in the form of economics, politics, lifestyle choices, etc.) that results in some being chosen over others.

[djf]

Once you forge a large steel beam, the work is almost done. It may not even be the best material, because different materials have different coefficients.

But if you make it, and lay it down in the deserts of say Arizona, it could run all day. Expanding and contracting. And unless I'm wrong, the amount of energy it could yield would be fantastic, because each cycle would release the amount it would take to COMPRESS a steel beams length by say, 3 inches.

It's like trying to fit a quart of water into a pint jar!


PS: ALL RIGHTS RESERVED

[chimera]

You'd still have to do work on the contraction phase. Assume the sun does the heating. Okay, that gets you to Tmax. But you're going to have to do work to get you to Tmin to start the cycle over again. That will cut down on the net energy output.

The expansion will generate force, to be sure, but in power generation a key factor is how many "lines of force" you're cutting through with the moving coil. You'd have to have a tremendously powerful magnet of very long length to have the field density necessary to generate significant power over a three inch stroke. There are powerful magnets out there, things like rare-earth magnets and the like, but those are tremendously expensive, and of relatively small size and I haven't seen any that solve the scale-up problem necessary to make a reasonably-sized generator out of them. You've also got a problem with heat removal for such a system. Some of the energy generated is "wasted" as heat in the magnet and coil. If you heat certain rare-earth alloy magnets to a high temperature, they lose their magnetism.

If you don't have a strong magnet, the other option is to wind your coil tighter. But there are practical limits to how many turns of wire you can have without sacrificing the current load it can carry. Winding a lot of small, skinny wires on a coil generally leads to shortened life and burnout. Having heavier wire gives you longer lifetime but cuts down on how much current you produce as you move through the field.

It isn't just a question of how much energy (force) is produced, but the form it presents itself in, and does that lend itself to being readily transformed into useful energy. There is a tremendous amount of energy incident upon the surface of the Earth every minute of every hour of every day. Trouble is, it takes a lot of effort to gather it up in sufficient quantities to be really useful. It's either in a hard to get to place, or not available when you want it in the quantities that you need, and is spread out all over the place in any case.

Thermal expansion and contraction occurs with a relatively long time constant. What you're proposing is essentially linear induction. That means a very long waveform, almost like DC. You're looking at significant investment (in energy as well as dollars) for conversion equipment to get the current into a form suitable for transmission at relatively low losses.

But, I don't know, you may want to do some research on it and see where it leads. I'm just outlining some of the things that come to mind off the top of my head. I have a feeling that if something like this is to be practical, it will require a lot of work on the materials side of things to make it worthwhile. But just because nobody has done it doesn't mean it won't work.

[djf]

With that much force, you just gear it way, way down.

And to get it back to TMIN, you just pump some water through the channels that you conveniently had the forethought to build into it.

I don't think the question is if it would work, the question is how much energy you could get, and how many cycles before the material fatigued.

[Cvengr]

Think man!,...this method allows you to design the sail as a photovoltaic.

[chimera]

Both of those steps require energy, so again the net output is derated by that much. Any kind of gearing is a lossy process because of friction and inertia. Pumping water requires energy. The gearing required to go from a three-inch stroke with a lot of force (the example postulated) to something on the scale of what modern generators use for moving coils through magnetic fields would certainly present both a lossy system and formidable strength-of-materials issues. My guess is you're going to be stripping a lot of gears if you use conventional materials.

[djf]

Could be. Just seems to me it's pretty low tech, but could have a high yield if done right.

And it's better than waiting until 2070 when they have spray on voltaic cells..

Someday, I'll tell ya about my large, rocking, semicircilar arc plan that you set at the equator and it rocks forever...

[RightWhale]

one day become five times more efficient

Oil companies dominate solar cell manufacture. Solar cells are still around $5 per watt. This big dog has been snoozing under the shade tree for 20 years and hasn't moved except to scratch its ribs with a hind foot.

[chimera]

Well, far be it from me to discourage outside the box thinking. Stranger things, for their time, have been proposed, and borne fruit. I was just reading today in Invention and Technology about Eli Janney, a dry-goods store clerk in post-Civil War Alexandria, VA, who had this idea for a railcar coupler to replace the dangerous and crude link-and-pin system used up until then. "The Strongest Handshake In The World", as the new coupler was called, and it became essentially a universal standard...

[steelcurtain]

Dittos. We're hoping to build a house soon, and I'd love to be able to use such solar technology, assuming it were cost-effective.