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neilzero
2010-May-27, 01:29 AM
On SBSP/SPS, GEO stationary orbit will likely be best when we have a million megawatts of SBSP/SPS. In the meantime, solar synchronous semi polar orbit has numerous advantages. Even far Northern countries can receive the beam during, and a few hours before, the evening peak demand period. With rare exceptions they won’t pay more than 4 cents per kilowatt hour for electricity at other times of day as their own generators are adequate, and not practical to shut down for a few hours. Other advantages are international companies and/or governments can share the start up costs, ship propulsion and the SPS is considerably closer than GEO but still in continuous sunlight. GEO satellite owners will be unhappy about megawatts, being transmitted near by.
Economies of scale likely run out at less than 100 megawatts, making many SPS practical. This is because 100,000 volts dc at 100 amps = 10 megawatts. More volts or more amps creates rapidly rising costs in space, so logically we build many medium size SPS. The Sun delivers about 1300 watts per square meter, but following the numerous conversions, and losses perhaps only 130 watts per square meter is delivered to the grid. Thus one square kilometer of solar panel in space, puts 130,000,000 watts = 130,000 kilowatts = 130 megawatts on the grid. That may be a billion solar cells in series parallel. That many connections presents a reliability problem, especially for open cells which are likely to reach temperatures of thousands of degrees as the very high voltage arcs across the open circuit. Solutions, especially at one million volts, get very complicated, heavy and expensive.
Almost a million laser diodes connected in series to use the million volts DC produce similar potential arcing which will damage nearby diodes.
Laser diodes are superior as they can focus on to existing solar farms (or ships at sea) with dimensions as small as 60 meters by 100 meters = about 5000 square meters = one megawatt if the beam is 200 watts per square meter. Microwaves cannot produce that small a spot except from LEO = low earth orbit, which is shaded by Earth about 40 minutes out of each 100 minute orbit. Neil

korjik
2010-May-27, 01:46 AM
seems like it would be alot simpler to do it on the ground, where installation and mantinence would be alot easier.

Murphy
2010-May-27, 02:18 AM
Personally I'm a big supporter of Space Solar Power, but I would certainly recognize that it won’t provide much power until several major technological advancements are made. Number one is of course more efficient solar cells, current efficiencies are about 20%-25%, which is just not good enough for this application; we're going to need to double that. Luckily, there are experimental cell types that have already done 40% efficiency in labs, and it seems it will not be that hard to get workable versions above 50% in the coming decades. Then there's the question of making them lighter, but that also seems to be developing nicely.

Those hurdles are mainly taken care of by advancements in material science, but a greater problem is how to get these huge satellites up there in the first place. That may not be such a problem if we're just talking about very small units not producing much power, they can probably be launched with existing rockets. But if we're talking long term and we want a fleet of gigawatt sized units then we need some much more capable launch system, something like a Space Elevator.

I think if we start such projects now, we could have small demonstration plants up within 10 years, after that I think it will still likely be a few more decades before the technology matures enough to provide a significant chunk of the Earth's energy needs. But it is something worth investing in, as it's really one of the few practically unlimited energy resources, which also doesn't negatively affect the Earth's environment.

cjameshuff
2010-May-27, 03:37 AM
Laser diode efficiencies are poor. No laser is particularly efficient. High voltage will not be an issue, the voltage dropped across a diode is nowhere near enough to arc across the diode, but removing waste heat from the diodes will be a major problem. Optical ranges also face greater attenuation due to the atmosphere, greater conversion losses on the ground side (especially if you re-use broad spectrum solar panels instead of making photovoltaic collectors tuned for the power beam), and pose an eye hazard on the ground.

Microwave transmission will require a dish or phased array structure far larger than optical components with a similar spot size on the ground, but the microwave transmitter can more easily be assembled out of metal mesh and smaller modules. It's entirely feasible to lock the beam from a phased array to a pilot beam from the ground, avoiding misalignment issues, and rectennas can pass sunlight, allowing them to share land with crops.

Microwave transmission is basically superior to lasers in every way for sending energy from orbit to ground. I doubt optical power transmission will ever be used for transmission to ground. It's far more suitable for transmission to a spacecraft, to supply extra power during an acceleration phase for example, where the smaller spot size can reduce collector mass carried by the spacecraft and there's no concern about damaging someone's eyesight.

It's unclear why you think larger SPSs are more expensive than multiple smaller ones, or why you think extreme voltages are necessary. The main problem with large SPSs is initial cost. You also assume photovoltaics. Solar thermal power could achieve efficiencies higher than the minimal-mass thin film photovoltaics that would have to be supplied from the ground, and there's the additional benefit in that it could be built largely out of materials derived from orbital sources. Also, if you ship up equipment for manufacturing components out of asteroid or lunar materials, you can use that equipment for a variety of purposes. This seems like a better bargain than shipping up photovoltaics that will just need replacement a few years down the line.

Jens
2010-May-27, 04:05 AM
Something I've wondered: if you were beaming electricity to the ground through microwaves, and an airplane happened to pass through the beam, would it be an unpleasant experience?

cjameshuff
2010-May-27, 04:22 AM
Something I've wondered: if you were beaming electricity to the ground through microwaves, and an airplane happened to pass through the beam, would it be an unpleasant experience?

Probably nobody would notice. The intensity is generally less than that of sunlight, and exposure will be brief. Can't rule out some electronics coupling due to some accident of circuit layout and malfunctioning.

A laser beam could be worse. It's generally best to avoid blinding the pilots of an aircraft in flight, even if it's just temporarily. And if you use infrared as has been suggested before (more efficient and cheaper laser diodes, better penetration of haze), you don't have much warning that you're being blinded until you start getting holes in your field of vision. Low power visible lasers are "eye safe", meaning that it hurts enough to look at them that the aversion reflex prevents damage...but when your entire environment is illuminated with such, you've got a problem. The narrower beam doesn't really help much, because the beam's illuminating a field full of shiny things that could cause specular reflections back up to a nearby plane...you'd want to keep your distance.

Of course, the simple solution to both is to make the area above the collectors a no-fly zone.

Van Rijn
2010-May-27, 05:19 AM
The concepts I remember back in the '70s had fairly low intensity. The advantage to the microwaves on the ground side, despite the low intensity, is that they can be converted quite efficiently to DC with a fairly simple rectenna (rectifying antenna), essentially a wire mesh and diodes. The problem is that you need to build the solar power satellites for the rectennas.

neilzero
2010-May-30, 02:06 AM
Will a wire mesh really work? Are you thinking a scaled up Arecibo radio telescope? Otherwise I think we need a billion plus dipoles for a 3 kilometer radius rectenna receiving ten gigahertz microwaves = 3 centimeters wavelength. 30 million square meters at 3 gigawatts = 100 watts per square meter = 1/100 watt per square centimeter = 1/10 the allowable microwave oven leakage = most humans would be OK long term unless they were very close to heat exhaustion for some other reason. Neil

cjameshuff
2010-May-30, 04:31 PM
Will a wire mesh really work? Are you thinking a scaled up Arecibo radio telescope? Otherwise I think we need a billion plus dipoles for a 3 kilometer radius rectenna receiving ten gigahertz microwaves = 3 centimeters wavelength. 30 million square meters at 3 gigawatts = 100 watts per square meter = 1/100 watt per square centimeter = 1/10 the allowable microwave oven leakage = most humans would be OK long term unless they were very close to heat exhaustion for some other reason. Neil

No, I'm not thinking of a scaled up Arecibo, I'm not sure what made you think I was...all the serious proposals I've seen involve a field of rectenna arrays. I'm not sure what your objection is there. The number of diodes needed as rectifiers? These are tiny low-power things, far easier to manufacture than large-area photovoltaics. The dipoles themselves? They should be about as difficult and expensive to manufacture as paper clips.

neilzero
2010-May-30, 09:10 PM
Arecibo and most other radio telescopes use a wire mesh shaped as a segment of a parabola. Such could serve as a rectenna, except the focal point would be kilometers above Earth's surface for a rectenna with a radius of 3 or more kilometers. I think you agree billions of paper clips for a very large rectenna. The installation of the dipoles likely costs more than the dipoles, and the many acres is also pricey. If mounted several meters above the surface some other uses of the land are likely practical. Neil