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Space based power?

  1. Jul 23, 2008 #1
    The New York Times today has an opinion piece suggesting that orbital solar collectors could send power to Earth facilities by means of radio waves. (http://www.nytimes.com/2008/07/23/opinion/23smith.html?th&emc=th) Is this really practical? What are the safety concerns?

    If such a collection system could be built, would it be possible to place it (them) between the Earth and the sun, thus helping to cool Earth's surface?

    Any opinions welcome, thanks.
     
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  3. Jul 23, 2008 #2

    Mech_Engineer

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    I can think of a few problems with such a solution-

    - Some of the energy beaing "beamed down" would get diffracted in the atmosphere. How much I don't know, but this could cause some localized atmospheric heating.

    - What if a plane or wildlife flys through the energy beam? Cooked?

    - Is it really more cost-efficient (watts per dollar) to build a space-based solar array that beams down power than just building a bigger one on the ground, especially taking into account maintinence issues?
     
  4. Jul 23, 2008 #3
    The opinion piece author seems to think the cost per kilowatt would be in the range we are now paying for energy. I have no idea if the cost estimate is realistic, or if maintenance was considered.

    I suppose the barbeque effect (birds or planes) could be made less problematic if the energy were sent in small packets via a large number of sending stations, so that the energy would be distributed over a huge volume of atmosphere, and only concentrated at the focus point. Or, perhaps the beam could be be made large in cross section so as to be sub-lethal, and focused by parabolics near the collector.
     
  5. Jul 23, 2008 #4
    IIRC there was a post in some other forum on here about the Japanese actually planning on doing this. No idea where it went and can't find it, so take the numbers that I remember with a grain of salt, but they did address some of the points Mech_Engineer brought up. They were using a particular frequency to transmit the energy that would have less than 10% loss through the atmosphere. The energy would be received on the ground over a large area at ~5x the rate of the standard solar output.
     
  6. Jul 24, 2008 #5
    http://www.treehugger.com/files/2008/07/japan-envisions-orbitting-solar-power.php

    https://www.physicsforums.com/archive/index.php/t-69482.html

    Based on the above, the idea is possible but dangerous and expensive.

    Maybe the dangers could be mitigated. Maybe the huge amounts of solar energy available in orbit could someday overcome the inefficiencies.

    It appears the Japanese have a research program, involving distributing the incomming energy over a large area of solar collectors, so the beam is not anywhere strong enough to cook birds and airplanes.

    But what about the second part of my question? Could orbital collectors or reflectors be put in place to reduce solar radiation reaching the surface, so controlling atmospheric warming and climate change? Could stationary satalites be placed so they would stay between the earth and the sun?
     
    Last edited: Jul 24, 2008
  7. Jul 24, 2008 #6

    Redbelly98

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    Is there some current technology that efficiently converts the solar spectrum (near u.v. to near i.r.) into a beam of radio waves?
     
    Last edited: Jul 24, 2008
  8. Jul 24, 2008 #7

    Mech_Engineer

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    I don't think there is an orbit that exists that would put a satellite between the Sun and the Earth permanently. If the satellite were orbiting the Earth in a geosynchronous orbit for example, it would rotate with the Earth; and if it was orbiting slightly closer to the sun than the Earth, its orbital period would be shorter than that of the Earth's.

    To block a significant portion of the incident solar radiation on the planet, the combined surface area of the satellites would have to be huge, and something would have to be done with all of that energy they were intercepting. More important than that, would it be scientifcally and morally justifiable to do so? Do we have accurate models of the global climate system to know how much heat to take away or add? Answer: no.
     
  9. Jul 24, 2008 #8
    good points, thanks.
     
  10. Jul 24, 2008 #9

    chroot

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    Well, the satellites could orbit at one of the Lagrange points (like the SOHO spacecraft at L1) and never be eclipsed by the Earth. You'd have to use a bit of fuel for station-keeping, though, since such orbits are not stable.

    - Warren
     
    Last edited: Jul 24, 2008
  11. Jul 24, 2008 #10

    Redbelly98

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    How could such a satellite be useful for reducing the solar radiation reaching Earth???
     
  12. Jul 24, 2008 #11

    chroot

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    Since this is about collecting solar power from space, I'd say the point is to increase the solar power reaching the Earth; the power would just be transmitted as radio waves instead of light.

    - Warren
     
  13. Jul 24, 2008 #12

    Redbelly98

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    The post you're replying to was a reply to this statement:

     
  14. Jul 24, 2008 #13

    chroot

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    Redbelly98,

    The L1 point (though not stable) would also be an ideal place to put some kind of giant reflecting surface, which would cast a shadow on the Earth, reducing its solar input. Theoretically possible? Sure. Worth doing? Probably not. Even if you got past all the technological hurdles of putting a giant reflector at L1, the resulting shadow would play havoc with local weather.

    If you've ever experienced a total solar eclipse, you've noticed the quite dramatic temperature drop and resulting winds. The shadow of this reflector could potentially cause pretty dramatic weather disturbances, particularly as it wandered around the Earth's surface due to slight position inaccuracies.

    - Warren
     
    Last edited: Jul 25, 2008
  15. Jul 24, 2008 #14

    chroot

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    And, I should mention, the technical hurdles would be pretty enormous! If you wanted to reduce the Earth's solar input by 10%, you'd need to create a shadow about 4,000 km in diameter, or roughly the size of the United States. The L1 point is only about 1% of the way from the Earth to the Sun, which means your reflector would have to be about the same size as the shadow you wish it to cast. A reflector the size of the United States would certainly be out of the realm of our technological capability.

    If it were made out of a thin aluminum foil, it would weigh approximately as much as 7 million space shuttles.

    - Warren
     
  16. Jul 24, 2008 #15

    Redbelly98

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    Ah, I hadn't realized there is a Lagrange point between the Earth and Sun.

    At least we agree that this would be highly impractical for reducing the sun's radiation.
     
  17. Jul 25, 2008 #16
    This is very enlightening. I was thinking of a cloud made of bits of mylar, which would, I suppose, have less drastic effect, since it would be lighter than aluminum, and would not have sharp shadow edges to cause local anisotropic effects at the Earth's surface. It would not present the engineering problems necessary to build an umbrella over North America, but how could it be made to stay in position?

    This line of thought started when I read that during the three days in which commercial jets were grounded, the absense of contrails caused surface temperatures to rise uniformly (three degrees F. IIRC) across North America, in excess of forcast model predictions.

    I guess I don't see much hope of saving ourselves from climate change by means of a voluntary reduction of greenhouse gasses. Carbon credits look to me like a ponzi scheme.

    I guess it is true that using orbital solar energy arrays to shade the surface wouldn't help if the energy were beamed to the surface by radio wave or whatever anyway. Maybe future generations would use the energy to power lasers to drive solar sail ships or something, instead of beaming it to Earth.

    Thanks for the ideas. Guess I'll go back to studying math.

    R.
     
  18. Jul 25, 2008 #17

    chroot

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    The "aluminum foil" I considered was basically 0.1 mm metalized mylar. The entire structure would weigh as much as seven million space shuttles... and you don't think that's technologically infeasible?

    - Warren
     
  19. Jul 25, 2008 #18

    chroot

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    This sounds like an exercise in correlation versus causation. Please provide a reference for this, if you intend to present it as causal.

    - Warren
     
  20. Jul 25, 2008 #19
    http://www.geotimes.org/june04/geophen.html

    "The empty skies also allowed David Travis of the University of Wisconsin-Whitewater and his co-workers to determine how contrails might be changing local temperature. Their results, first published in Nature in August 2002 and further elucidated last March in the Journal of Climate, showed that during the days when jets were grounded in September 2001, sunshine increased the temperature at ground level instead of being reflected by contrails. At night, the researchers found, the absence of an insulating layer of contrails meant greater ground level cooling."

    http://www.podtech.net/home/3951/dr-david-travis-at-hybridfest

    "Have you ever looked up and watched a jet high in the sky leaving a long contrail? Have you also wondered what effect that contrail might have on global warming and our environment? Dr. David Travis, from the University of Wisconsin-Whitewater, has, and during Hybridfest in Madison, Wisc., he gave a talk entitled “Climate Change: Indications of Global Warming & Threats from Aviation"
     
    Last edited: Jul 25, 2008
  21. Jul 25, 2008 #20
    Thank you for your attention, Warren.

    I suppose we could achieve cooling of the atmosphere to some extent by increasing daytime flights and limiting night time flights...that is, the ones that produce contrails. Maybe someone will build a jet or rocket device specifically designed to maximize contrails. I would not change the atmosphere, personally, if I had the option of using the L1 Lagrange point somehow to allow some control of incoming solar radiation. Atmospheric change is not easily reversible, but building an L1 variable light shield is an engineering problem, one which I presume could be solved, at least in principle.

    I would also point out that moving large masses of material is expensive in the gravity well, but would not be, so much, in the mostly flat solar space. The energy needed in flat space is much less, and is affected more by the desired time of arrival and ones ability to deliver precision thrust to the object than it is affected by the energy needed to start the object moving.

    I don't know about aluminum, but the asteroid belt is said to be rich in iron and nickle.

    The moon, Mars, the asteroid belt. We'd be a lot closer if we'd started thirty years ago.
     
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