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Power Mad

So I had an idea this morning, while driving to work, that I felt I should float out in public. Maybe someone, somewhere who actually has more knowledge of the topics in question, could be infected with the concept and look into it.

This has probably been thought of by someone else before, but I don't recall having heard of it anywhere else.

One problem with Solar Power is that sometimes it gets dark out. Night-time and clouds reduce the reliability and efficiency of power production.

One long standing Sci-fi solution to the issue is Solar Power Satellites. Up in space, they can generate power all the time. Also, as solar cells generate more than twice as much power with twice the light, and as the intensity of the light in space is far greater than down at the bottom of the atmosphere well, they can be much more efficient. But hauling all that stuff into space is expensive! Also, getting the power back to earth is a dodgy thing, using microwave transmission, with unknown environmental impact and power loss.

As for taking on the cost issue, the development of carbon nano-tube fibers is recently approaching the tensile strengths needed to build the classic BeanStalk space elevator into Geosynchronous orbit. These would reduce the cost of putting materials into orbit by a hundred-fold or so.

And this is where I had my idea...

Carbon nano-tubes also have another useful ability or two… With the right molecular orientation, they can be conductive. But the energy densities needed by a power sat array may be a bit much for a nano-tube beanstalk. But If you have something conductive, I think you can make a Faraday cage from it. Or a wave-guide.

Imagine a hollow beanstalk acting as a microwave wave-guide to channel the power down from the satellite, perhaps preventing unwanted side effects and power loss that might otherwise be seen with the microwave beam approach.

And that's my grandiose mad science scheme for the day.


( 14 comments — Leave a comment )
Feb. 11th, 2006 12:49 am (UTC)
Good idea in principle. Waveguides would work a treat. The only big glitch is orbit. The only feasible orbit to run a beanstalk down from is a near-circular equatorial orbit, and that puts the satellite in shadow on a regular basis. There's also a small glitch in having the beanstalk accomodate small (less than a couple of miles) variations in orbital height, but I think those can be solved.
Feb. 11th, 2006 01:06 am (UTC)
With the rotational tilt of the planet at 23˚, Geosync orbit only really has shadow issues for two short periods a year. And even during those periods, the daily shadowed time is fairly short. on the order of an hour.

Beanstalk base dynamics is already a topic of much discussion. A mobile platform base is pretty much assumed to be needed, to move the base a couple miles, to wiggle it out of lower satellite orbital paths.
Feb. 11th, 2006 01:33 am (UTC)
Okay, quick and dirty calculations has a beanstalk top at Geo orbiting about at about 5800 MPH. Earth diameter is about 8000 miles, so I 'd guestimate that the satellite will be about an hour and a third in total dark, in the penumbral shadow cone, then another half hour in varying degrees of twilight. Just as a rough calculation.

As far as percentage of year when the satellite can enter shadow, Quick calculation seems to indicate it'd be that part of the year when the vertical component of the orbital plane is less than about 10.5˚ above/below the earth orbital plane. Or roughly 70% of the year with no shadow at all.
Feb. 11th, 2006 04:17 am (UTC)
Oh, I realized a third glitch: orbital slots. You'll need spots in the Clarke belt for the tops of the beanstalks, and that's already pretty crowded with communications satellites. Logistically that's easy to solve, politically might be a bit more problematic.
Feb. 11th, 2006 06:43 am (UTC)
Easy solution to that...

Hey, if you let us take down your bird at this slot, we'll give you space on our station up here that will replace it so you can continue transmitting, and we'll take care of keeping it up there and running.
Feb. 11th, 2006 08:57 am (UTC)
In fact, it'd be easier to put up more bandwidth up on the beanstalk top, as you don't have to put so much space between them, if they are attached to a fixed structure. Also, you have the possibility of putting transponders lower down on the beanstalk, where you can have the advantage of fairly fixed position in the sky, and lower light-speed latency.
Feb. 11th, 2006 06:24 am (UTC)
The second-biggest problem with beaming power down from the heavens is that you really wouldn't want to have the power density at Earth's surface much higher than what you'd get from sunlight, as that would do unpleasant things to birds flying through the beam, cattle grazing under it, and would create a big enough thermal to to very strange things to local weather. This makes the tradeoff of a receiver array vs. a ground-based solar power array iffy, for areas with lots of sun.

The biggest problem, of course, is that most beamed power schemes will have around a 10% end to end efficiency, making generation via satellite even more expensive.

A conducting tether from orbit to ground does neatly solve this problem. Current densities aren't the problem. The problem is that the conducting cable is moving through an uneven magnetic environment (due to the interaction between the solar wind and the Earth's magnetic field), which does extremely un-nice things to the tether (mostly sets up vibration modes that are hard to damp in something that much longer than it is wide, leading to mechanical failure). A realistic scheme would probably involve wide rings with Hall effect thrusters for active stabilization at frequent intervals, with elastic strands coupling cable vibrations to the rings to damp them. As making a nanotube composite conductive is even more of a pain than making a nanotube composite in the first place, you'd probably just run a sun-shaded superconducting cable down the stalk instead of using the stalk itself for conduction.

I have grave doubts about this being cheaper than ground-based energy production schemes, though. Weather and night give you about a 10% effective duty cycle on the ground, requiring a bigger array, but lifting all that mass into space and replacing it every decade or so isn't cheap either.

For efficient solar power generation, my money is on either mass-produced thin-film arrays on the ground, or mirror arrays for running a conventional steam turbine based plant. Both have their problems. Until mass manufacturing gets a lot cheaper, I'd place money on fission power in the medium term. But that's just my guess.

The show-stopper for any ground-based solar scheme is that you either have to have a flexible enough distribution network or enough power storage at the plant to give the illusion of an unvarying supply. The first could be done, but isn't going to be without a good reason, and the second is extremely difficult.

If I remember correctly, Pournelle really liked the solar power satellite idea, for sci-fi author advocates.
Feb. 11th, 2006 09:41 am (UTC)
Why even need a satellite at all? A carbon fiber tube with a conductive fillament within and a conductive ball at the top sweeping through the Van Allen belt would generate a good charge that would drive to ground and could be intercepted to fill batteries or a superconducting ring underground.
Feb. 11th, 2006 10:12 am (UTC)
Yeah, but that only works while the Earth's magnetic field isn't in the middle of flipping again. Which it looks like it's starting to, recently. :)
Feb. 11th, 2006 11:37 am (UTC)
As long as it doesn't turn off completely, it'll still work. If North is North, or North is South, charge will still go to ground.
Feb. 11th, 2006 04:22 pm (UTC)
The problem is that this de-orbits your power-generating tether (a tether like this is changing orbital energy into electrical energy, which is why they've been proposed as a way to perform course corrections). Also, a space elevator wouldn't be moving with respect to the VAB (stationary with respect to the Earth). It could only generate power via field geometry changes near the magnetopause (due to earth/sun field interactions).

Feb. 11th, 2006 04:44 pm (UTC)
If a significant percentage of mass (over %50) of the tether is beyond geosynchronous orbit altitude, the extra velocity should keep the tether from dropping out of orbit. The strength of the tether, however, would have to go beyond anything even remotely possible today.

And there's always the possibility of someone running into it with a plane or helicopter or satellite or something.
Feb. 11th, 2006 05:07 pm (UTC)
As current flows radially along the tether, the Lorentz force acting on it moves it laterally. This will degrade its orbit if it's orbiting faster than Earth rotates (think of this as being akin to eddy current losses). If it's farther out and orbiting more slowly than the Earth rotates, it'll cause it to drift outwards. You'll also get precession and stranger things happening due to the way the Lorentz force works and the fact that the magnetic field isn't a monopole. In addition to removing the tether, this will make a fixed anchor point impossible, which kind of defeats the purpose of the thether in the first place.

Feb. 24th, 2006 12:05 am (UTC)
As for the need for a moving anchor point, why not use an ocean platform like the SeaLaunch people?

David Brin wrote a short story called "Tank Farm Dynamo," available at his site, proposing to use electrified cables to push against the Earth's magnetic field and keep a station in orbit.

This is an odd variation on the power-cable idea, but: what if a dumbwaiter-sized space elevator (good test: lift a bottle of champaigne to orbit!) were used to carry energy not in the form of light or electricity, but as fuel? With lifting costs greatly reduced by the elevator and further reduced by a counterweight system, would it make any economic sense to bring buckets of hydrogen back from orbit? Synthetic gasoline or another energy-dense chemical made with solar energy? Antimatter, even?
( 14 comments — Leave a comment )