Musk is not an expert on space-based solar power. He ignores the fact that a space-based collector not only gets sunlight at far greater intensity, it wouldn't be occluded half of the time the way a ground collector is.
I'm not Keith, but from what I've read about such systems, this isn't a concern: when they hit the Earth's surface, the microwave beam is very large so it wouldn't have a fatal effect on anyone or anything that happened to cross through it, though it might warm you up a bit. Also, the frequency is pretty important; microwave ovens work because they're tuned to the resonant frequency of water, which most of our food is largely composed of, so it excites the water molecules and makes them heat up. If the beam is tuned to something else, it might have very little effect on lifeforms crossing through it (which is probably the intention, since tuning it for water would make it attenuate too much on cloudy days).
... microwave ovens work because they're tuned to the resonant frequency of water...
Bzzt. Microwave ovens use 2.4GHz because there's an ISM band [wikipedia.org] there. There is no resonance at 2.4 GHz for water [flitetest.com]. If there was your food would explode in the oven.
Boy is that a hard myth to dispel. I have no idea how it started but microwaves work on simple induced eddy currents in the "load", ie, your meal.
Anyone who thinks a microwave oven is tuned is encouraged to show me the stabilized power supply and PLL lock to a stable reference... Guess what? The frequency output of a magnetron is all over the place, because it doesn't matter. It would work just as well at 3GHz too.
Yes, very low energy per given area can ensure safety. But that means absolutely huge receivers are required to get a useful power level.
To receive the kind of power levels we're talking about efficiently, you're going to need pretty damned big receivers. Luckily, they don't have to be large single physical structures - you'll have a rectenna array rather than one big one.
It works fine for every other kind of power plant, except they typically have to be sited on waterways, unlike a rectenna field. You're inventing objections which do not exist.
So do solar panels. The difference is a rectenna array generates 300 W/m^2 for 24 hours a day, while solar panels generate about 200 W/m^2 from 4-6 hours a day in most decent locations. In crappy locations for solar (Seattle, Germany), the energy per land area is *way* higher for space beamed power.
' a rectenna array generates 300 W/m^2 for 24 hours a day
That is possible on the high end, but the topic of this thread was doing so with safe energy levels, and to be safe you need to be a lot closer to the 20 W/M^2 range. You can have a setup with higher levels in the center and lower at the edges of the beam, but when beaming from such a great distance it is difficult to manage such an intensity distribution.
For even 80 W/M^2 average, it would take a receiver surface area of about 3 square miles to produce 600 MW ( the size of a medium fossil plant, just fo
UV radiation is the component of sunlight that burns, it is only a slice of the spectrum. That part of the spectrum is not 200 W/m2.
Microwaves interact differently with the human body than UV radiation, so the comparison would not necessarily be telling. My quick searches showed safe levels need to be in the range I specified, I'll admit I don't know the science behind them.
Standing inside the fence of a rectenna array makes as much sense as going inside the furnace of a coal power plant. In other words, no sense at all.
300W/m^2 is the average intensity on the receiving antenna elements. The intensity outside the fence is much much lower. There's a buffer zone between the edge of the antenna and the fence.
Standing inside the fence of a rectenna array makes as much sense as going inside the furnace of a coal power plant. In other words, no sense at all.
300W/m^2 is the average intensity on the receiving antenna elements. The intensity outside the fence is much much lower. There's a buffer zone between the edge of the antenna and the fence.
That is not the configuration that most articles describe, where the antenna covers the full range down to safe levels. And it makes no sense because if you don't you are wasting a lot of energy falling on what is now a unusable zone outside the receiver, not to mention still taking up all that area which was part of the original point.
These space to earth systems are still a paper exercise. To state that "300W/m^2 is the average intensity on the receiving antenna elements." is not based on anything prov
' a rectenna array generates 300 W/m^2 for 24 hours a day
That is possible on the high end, but
Not even possible on the high end. The diffraction-limited beam spread means that you can't focus the beam to a tight spot, without making the space transmitter correspondingly larger. The Glaser sizing of a kilometer-scale transmitter in space beaming to a ten-kilometer scale spot on the ground wasn't picked at random; those are the sizes you need. And the keep-out zone around the ground receiver is much larger; due to the diffraction wings.
Shorter wavelengths make things better... but if you go too sho
Keith can give you the accurate and current story on it.
But as I recall it (from the proposals in the early days of the L5 society and some experience with microwave and synthetic aperture techniques):
The powersat has many transmitters. Each single transmitter, even with a very directional antenna, puts out a very wide beam. (It might cover the whole face of the planet (and beyond)). One transmitter would look more like a radar transmitter at least (26,199 - 3,959) = 22,240 miles away - about 9 times the distance from New York to Los Angeles.
The transmitter/antenna devices distributed across the platform are phase-synchronized by a pilot signal transmitted from the ground rectenna site. They compute the complex conjugate of the (equivelnt at their frequency) signal they receive and transmit that. This orchestrates them so they form a beam that retraces the path through space, correcting for flexing of the powersat structure, the turbulence of the atmosphere, clouds, aircraft - metal, wood, or feathered - rainstorms, ionospheric distortions, etc. and focuses on the pilot transmitting antenna(s), like a hologram.
A number of factors defocus the beam somewhat, so you get a spot that covers the rectenna efficiently rather than tightly focussed on the pilot transmitting antenna(s), or leaking all over the surrounding county. The main one is the diffraction limit at the frequency involved, given the size of the transmitting array and distance from it: The bigger the transmitting array, the more tightly focussed the spot on the rectenna site can be. You don't want it TOO tight, to keep the power density reasonable (like a few times sunlight's power density). But the battle is to get it tight enough so your rectenna farm isn't city-sized, not to keep it from being too tight.
If pilot lock is lost by a single transmitter, it no longer stays locked to the rest of them - its signal spreads out like that of any lone transmitter. It stops contributing to the power in the rectenna and "shines" on the whole face of the planet - becoming microwave background noise. If pilot lock fails completely, all the transmitters VERY quickly go out of sync with each other (and can be deliberately given individual drift rates to insure this happens quickly). They ALL shine, incoherently, all over the world. All the power spreads out over the whole face of the planet and beyond. That part of the sky gets loud in microwaves, so you don't want to park a commsat there. But it's not going to toast Tokyo, or cause malfunctions in old-style pacemakers in Cleveland.
Of course you can design the transmitters so any that doesn't have pilot lock just shut down, if the solar array can accept the loss of the load. You can also modulate the pilot with a cryptographic identifier, to keep people from stealing power - or warming Central Park slightly - by setting up a second pilot transmitter at another site and making the "hologram" deliver a second spot.
Meanwhile you're not going to have roast birds falling out of the sky (like you do with the point-focus solar power systems). Microwave ovens cook because they use a frequency that is strongly absorbed by water. Milimeter-wave power systems use a frequency that is chosen to NOT be strongly absorbed by water. This lets it go THROUGH clouds, and birds, rather than being absorbed and heating them. They're not PERFECTLY transparent to it. But at the frequencies and power levels involved at the best focus you can get it's more like having an incandescent lamp in the room than like being in a microwave oven.
Meanwhile the rectenna is spidery enough that most of the sunlight passes through it, and efficient enough that most of the power does not. You can put it up on poles and graze cattle under it, without cooking the cows or the grass.
Microwaving power to Earth from space (Score:1)
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Or just forget about space-solar power altogether, as Elon Musk recommends. [youtube.com]
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The fact that he helps run the U.S.'s largest ground solar installer isn't a conflict of interest?
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Musk is not an expert on space-based solar power. He ignores the fact that a space-based collector not only gets sunlight at far greater intensity, it wouldn't be occluded half of the time the way a ground collector is.
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36% more intensity and 400% more hours of sunlight at only 14,000x more money.
Re:Microwaving power to Earth from space (Score:4, Informative)
I'm not Keith, but from what I've read about such systems, this isn't a concern: when they hit the Earth's surface, the microwave beam is very large so it wouldn't have a fatal effect on anyone or anything that happened to cross through it, though it might warm you up a bit. Also, the frequency is pretty important; microwave ovens work because they're tuned to the resonant frequency of water, which most of our food is largely composed of, so it excites the water molecules and makes them heat up. If the beam is tuned to something else, it might have very little effect on lifeforms crossing through it (which is probably the intention, since tuning it for water would make it attenuate too much on cloudy days).
Re: Microwaving power to Earth from space (Score:0)
There has been a long history of sailors 'cooked' in the radar's path. RF burns are deep and penetrating.
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Of course radar emits all frequency ranges, and therefore applies equally to all forms of RF emission...
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no, there is documented higher incidence of testicular and eye tumors though
the power per unit area is much HIGHER being near ship antenna than these proposed collection systems
Re:Microwaving power to Earth from space (Score:4, Informative)
... microwave ovens work because they're tuned to the resonant frequency of water ...
Bzzt. Microwave ovens use 2.4GHz because there's an ISM band [wikipedia.org] there. There is no resonance at 2.4 GHz for water [flitetest.com]. If there was your food would explode in the oven.
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Boy is that a hard myth to dispel. I have no idea how it started but microwaves work on simple induced eddy currents in the "load", ie, your meal.
Anyone who thinks a microwave oven is tuned is encouraged to show me the stabilized power supply and PLL lock to a stable reference... Guess what? The frequency output of a magnetron is all over the place, because it doesn't matter. It would work just as well at 3GHz too.
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Yes, very low energy per given area can ensure safety. But that means absolutely huge receivers are required to get a useful power level.
To receive the kind of power levels we're talking about efficiently, you're going to need pretty damned big receivers. Luckily, they don't have to be large single physical structures - you'll have a rectenna array rather than one big one.
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A bunch of little ones all together, a smaller number of bigger ones, what does it matter? It still must cover a tremendous area.
It does take up a lot of space, but you can put it someplace crappy that nobody goes anyway and it doesn't look like much of anything.
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Good luck with that.
It works fine for every other kind of power plant, except they typically have to be sited on waterways, unlike a rectenna field. You're inventing objections which do not exist.
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Oh, by the way, are you aware of the scale of area coverage we are talking about?
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Oklahoma?
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So do solar panels. The difference is a rectenna array generates 300 W/m^2 for 24 hours a day, while solar panels generate about 200 W/m^2 from 4-6 hours a day in most decent locations. In crappy locations for solar (Seattle, Germany), the energy per land area is *way* higher for space beamed power.
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' a rectenna array generates 300 W/m^2 for 24 hours a day
That is possible on the high end, but the topic of this thread was doing so with safe energy levels, and to be safe you need to be a lot closer to the 20 W/M^2 range. You can have a setup with higher levels in the center and lower at the edges of the beam, but when beaming from such a great distance it is difficult to manage such an intensity distribution.
For even 80 W/M^2 average, it would take a receiver surface area of about 3 square miles to produce 600 MW ( the size of a medium fossil plant, just fo
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Why do you need 20W/m^2 for safety? The sun beams 200W/m^2 on sunny days and the worst damage it does is sunburn and deterioration of plastics.
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Microwaves interact differently with the human body than UV radiation, so the comparison would not necessarily be telling. My quick searches showed safe levels need to be in the range I specified, I'll admit I don't know the science behind them.
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Standing inside the fence of a rectenna array makes as much sense as going inside the furnace of a coal power plant. In other words, no sense at all.
300W/m^2 is the average intensity on the receiving antenna elements. The intensity outside the fence is much much lower. There's a buffer zone between the edge of the antenna and the fence.
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Standing inside the fence of a rectenna array makes as much sense as going inside the furnace of a coal power plant. In other words, no sense at all.
300W/m^2 is the average intensity on the receiving antenna elements. The intensity outside the fence is much much lower. There's a buffer zone between the edge of the antenna and the fence.
That is not the configuration that most articles describe, where the antenna covers the full range down to safe levels. And it makes no sense because if you don't you are wasting a lot of energy falling on what is now a unusable zone outside the receiver, not to mention still taking up all that area which was part of the original point.
These space to earth systems are still a paper exercise. To state that "300W/m^2 is the average intensity on the receiving antenna elements." is not based on anything prov
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' a rectenna array generates 300 W/m^2 for 24 hours a day
That is possible on the high end, but
Not even possible on the high end. The diffraction-limited beam spread means that you can't focus the beam to a tight spot, without making the space transmitter correspondingly larger. The Glaser sizing of a kilometer-scale transmitter in space beaming to a ten-kilometer scale spot on the ground wasn't picked at random; those are the sizes you need. And the keep-out zone around the ground receiver is much larger; due to the diffraction wings.
Shorter wavelengths make things better... but if you go too sho
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It's not a concern because such "systems", or rather, fantasies, make no sense whatsoever and have precisely zero chance of ever being built.
As I understand this (Score:4, Informative)
Keith can give you the accurate and current story on it.
But as I recall it (from the proposals in the early days of the L5 society and some experience with microwave and synthetic aperture techniques):
The powersat has many transmitters. Each single transmitter, even with a very directional antenna, puts out a very wide beam. (It might cover the whole face of the planet (and beyond)). One transmitter would look more like a radar transmitter at least (26,199 - 3,959) = 22,240 miles away - about 9 times the distance from New York to Los Angeles.
The transmitter/antenna devices distributed across the platform are phase-synchronized by a pilot signal transmitted from the ground rectenna site. They compute the complex conjugate of the (equivelnt at their frequency) signal they receive and transmit that. This orchestrates them so they form a beam that retraces the path through space, correcting for flexing of the powersat structure, the turbulence of the atmosphere, clouds, aircraft - metal, wood, or feathered - rainstorms, ionospheric distortions, etc. and focuses on the pilot transmitting antenna(s), like a hologram.
A number of factors defocus the beam somewhat, so you get a spot that covers the rectenna efficiently rather than tightly focussed on the pilot transmitting antenna(s), or leaking all over the surrounding county. The main one is the diffraction limit at the frequency involved, given the size of the transmitting array and distance from it: The bigger the transmitting array, the more tightly focussed the spot on the rectenna site can be. You don't want it TOO tight, to keep the power density reasonable (like a few times sunlight's power density). But the battle is to get it tight enough so your rectenna farm isn't city-sized, not to keep it from being too tight.
If pilot lock is lost by a single transmitter, it no longer stays locked to the rest of them - its signal spreads out like that of any lone transmitter. It stops contributing to the power in the rectenna and "shines" on the whole face of the planet - becoming microwave background noise. If pilot lock fails completely, all the transmitters VERY quickly go out of sync with each other (and can be deliberately given individual drift rates to insure this happens quickly). They ALL shine, incoherently, all over the world. All the power spreads out over the whole face of the planet and beyond. That part of the sky gets loud in microwaves, so you don't want to park a commsat there. But it's not going to toast Tokyo, or cause malfunctions in old-style pacemakers in Cleveland.
Of course you can design the transmitters so any that doesn't have pilot lock just shut down, if the solar array can accept the loss of the load. You can also modulate the pilot with a cryptographic identifier, to keep people from stealing power - or warming Central Park slightly - by setting up a second pilot transmitter at another site and making the "hologram" deliver a second spot.
Meanwhile you're not going to have roast birds falling out of the sky (like you do with the point-focus solar power systems). Microwave ovens cook because they use a frequency that is strongly absorbed by water. Milimeter-wave power systems use a frequency that is chosen to NOT be strongly absorbed by water. This lets it go THROUGH clouds, and birds, rather than being absorbed and heating them. They're not PERFECTLY transparent to it. But at the frequencies and power levels involved at the best focus you can get it's more like having an incandescent lamp in the room than like being in a microwave oven.
Meanwhile the rectenna is spidery enough that most of the sunlight passes through it, and efficient enough that most of the power does not. You can put it up on poles and graze cattle under it, without cooking the cows or the grass.