Could a giant radiometer generate electricity?

December 18, 2005 10:10 AM

PhysicsFilter: Is there any reason why a giant radiometer couldn't be used as a way to generate electricity?
posted by spilon to Science & Nature (12 answers total)

I'd suggest it's because the rotation is of extremely low torque and any attempts to tap the energy would kill the effect. Traditional solar power collection methods are significantly more efficient.
posted by wackybrit at 10:18 AM on December 18, 2005

Because the amount of power generated would be dramatically tiny.
posted by kickingtheground at 10:19 AM on December 18, 2005

Response by poster: A tiny amount of power, sure, but the toy is tiny. What if it was the size of a hot air balloon? And then what if you had a thousand of them all wired together the way they do with windmill farms? Couldn't someone, at least theoretically, figure out a way to get these things spinning just as fast as windmills do? Just curious...
posted by spilon at 10:36 AM on December 18, 2005

The (fiction) movie Pitch Black has devices that are powered by something like just a radiometer. There's not really much focus on them, though, except that they require the sun for power.

As for why you wouldn't actually be able to use them for powering things, I think it's what wackybrit and kicking the ground said. The rotation depends on low air pressure in the container ("Higher gas pressure in the bulb does not work because there is more air resistance to the motion of the vanes." from the Wikipedia article) and adding a load would inhibit motion. You'd only get a tiny bit with a light-enough load to allow the vanes to keep moving.
posted by Godbert at 10:37 AM on December 18, 2005

If the amount of torque generated is so small that it must be in a vacuum lest the air resistance be too great to overcome, then doesn't that tell you something about the magnitude of the effect? Just making it bigger isn't going to change that.
posted by Rhomboid at 10:41 AM on December 18, 2005

It's theoretically possible, but the efficiency would be incredibly low. It would make a lot more sense to use that land with either poor-efficiency photovoltaics (solar cells).

(for anyone confused- power generated does depend on scale, but efficiency does not; making a device bigger doesn't* make it more efficient.)

*some devices have efficiencies which are weakly dependent on scale, but it is a reasonable simplification to neglect this dependence.
posted by JMOZ at 11:27 AM on December 18, 2005

Several comments lead me to understand that a Crookes radiometer (light mill) would be inefficient according to the Carnot cycle principles governing engines, but I couldn't find anything I was confident with. Still, people have tried to do more with the idea.

A Swiss inventor made a clock out of a light mill.

Apparently Tesla patented a much larger device using the same principle.
posted by dhartung at 12:09 PM on December 18, 2005

Creating a nearly perfect vacuum on a large scale would require a sphere with very thick steel or concrete walls. Maybe you could do it in space, but then you have to beam down the power. For a start, you should learn how a radiometer works. According to this article, even the Encyclopaedia Britannica *cough* got it wrong.
posted by weapons-grade pandemonium at 1:11 PM on December 18, 2005

weapons-grade pandemonium- a radiometer does not use a "nearly perfect vacuum." In fact, it uses a fairly mediocre vacuum which would not be particularly hard to maintain, even using glass. In a very good vacuum, a radiometer does not work- read the wikipedia entry in the question (which gets the effect right) for more information. (In science, Wikipedia is often better than Britannica. This FPP talks about scientific accuracy in Wikipedia, while personal experience tells me that Wikipedia has a much wider range of topics covered than a conventional encyclopedia.)

Anyhow, two rather important figures of merit of this scheme for power generation would be: 1) power generated vs. area required and 2) power generated vs. cost, both of which would be exceedingly unfavorable.

dhartung is correct to point out the lack of a true thermodynamic (Carnot) treatment for the efficiency of this type of device (at least- I've never seen one), but Tesla's patent and other work using this effect are not truly relevant to the wider alternative energy discussion because of their very low efficiency of energy conversion. (much much less than 1%)

In contrast, modern solar cell efficiency ranges from 5% (for extremely inexpensive polymer-based solar cells) to 10-20% (for common silicon based solar cells that people have on their roof) to 40% (for extremely high-efficiency and expensive multijunction solar cells under concentration.) A lot of these technologies did not exist even 25 years ago, and they have rendered a lot of competing power conversion ideas obsolete.
posted by JMOZ at 1:49 PM on December 18, 2005

Response by poster: Wow -- thanks for all the interesting info and links, folks. Now, what the heck am I gonna do with the giant glass ball that I just had mounted on top of my house.....
posted by spilon at 7:19 PM on December 18, 2005

I wonder if making the light side of the radiometer photvolataic would increase the rotation speed as well as generating a secondary power source.
posted by parallax7d at 7:32 PM on December 18, 2005

spilon- wouldn't that hypothetical giant glass ball make a fantastic aquarium?

Or better yet- go with a true alternative energy approach and add a light pipe, drill through your roof and ceiling, and get some natural light into your kitchen.

Also, thanks for asking this question; it's an interesting conversation and it's always nice to think about alternative energy conversion techniques (or so says the guy who works on a few problems in energy conversion professionally). You might find this FPP about energy conversion in a backpack suspension (and the comments within) interesting if you hadn't already seen it.

parallax7d- I think you would want to make the dark side a photovoltaic rather than the light side, because photovoltaics are designed to absorb light (and heat). I don't think it would spin as fast as a radiometer with a simple matte black side, which will absorb more heat than a PV. I'm not sure how you'd couple out the power generated in a rotating photovoltaic within a vacuum environment, but I'm pretty sure it would not be a very robust or economical system.
posted by JMOZ at 10:15 PM on December 18, 2005