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Wireless power transmission

  1. Jan 27, 2014 #1
    Wireless electricity transmission works by passing alternating current through one coil to which another coil corresponds and thus is coupled. At least this is the way WiTricity explains it. They also say that it's very much like mechanical resonance and that's why it's more efficient over greater distances than just ordinary induction.
    What I don't get is how can two coils be coupled like that? Is there some eigenmagnetic frequency that every coil has which allows them to be coupled when the frequency of the magnetic field hits that eigenfrequency?
    If it's so straightforward why the technology isn't widely implemented yet?
     
  2. jcsd
  3. Jan 27, 2014 #2

    berkeman

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    If you look at the "Related Discussions" links at the bottom of this page, does that help to answer your questions? :smile: It's a very frequently asked question here at the PF
     
  4. Feb 7, 2015 #3
    In wireless power transmission via solar power satellite
    Is the power is converted in space solar panels or the energy is coverted after reaching of microwave signals to earth...????
     
  5. Feb 7, 2015 #4

    anorlunda

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    If you mean satellite-to-ground power transmission. There aren't any yet.
     
  6. Feb 7, 2015 #5

    sophiecentaur

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    It just isn't straightforward - and, as you say, it isn't widely implemented. There are claims of all sorts fantastic performance but they don't seem to bear too much scrutiny. It seems that many of the ideas that Tesla originated are treated with undue reverence and some very romantic attitudes. His Patent is full of airy fairy and non specific notions and I couldn't find any actual numbers there.
    There is coupling between any two conductors through space. The mutual coupling between coils can be optimised by tuning /matching but the efficiency drops rapidly with distance. Videos of 'working systems' seem to show equipment working remotely but with no actual Power measurements. I think there are a number of charlatans in the business as there is money burning holes in would-be investors pockets.
     
  7. Feb 7, 2015 #6

    nsaspook

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    Sadly most of these claims are on the same level as an April Fools Joke.
    http://neenya.com/aprilfools/rg/ [Broken]
     
    Last edited by a moderator: May 7, 2017
  8. Feb 7, 2015 #7

    jim hardy

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    Sounds right to me.

    Have you ever lived with a piano ?
    If so you've heard its strings repeat sounds from the radio or a conversation in the room
    each string responding to its particular "eigenfrequency" ( which term i just now learned, http://www.merriam-webster.com/dictionary/eigenfrequency )
    The string is stimulated by all acoustic energy striking it
    but of course only that energy at its resonant frequency adds cycle by cycle, building up in intensity.
    Recall Q = (energy stored) / (energy dissipated per cycle)


    A coil will respond similarly to electromagnetic energy striking it
    and if its Q be >1 then energy at its natural frequency will accumulate
    "Tuned RF Radios" use a variable capacitor to tune the antenna to desired station

    But you'll never get out more energy than was put in,..


    So to me, yes it's very similar to mechanical resonance.
     
  9. Feb 7, 2015 #8

    sophiecentaur

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    Oh boy. What a fabulous offer. Where do I sign?
     
    Last edited by a moderator: May 7, 2017
  10. Feb 7, 2015 #9

    jim hardy

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    goodgrief they're serious !

    http://www.cnn.com/2014/03/14/tech/innovation/wireless-electricity/index.html
    has a nice video

    http://witricity.com/
    sales pitch

    http://www.eejournal.com/archives/articles/20140123-witricity/
    just like the piano, i think

    Figure_red.png

    My ancestors are all from Missouri so i'll have to be shown...
     
  11. Feb 8, 2015 #10
    i think that transmission of power is available from space to earth
    by means of microwave transmission system.....,,,,,
    i think this plan have been started by scientists of japan...
     
  12. Feb 8, 2015 #11
    MIT witricity is poor as regards efficiency, transmission distance and other things...

    As concerns microwave power transmission pioneers google for a name Bill Brown.
    I don't know what is a current world record, but NASA experiment in 1975 transmitted 30 kW+ power at 1 mile distance, at about 85% efficiency:

     
  13. Feb 8, 2015 #12

    anorlunda

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    You are confusing dreams with reality.

    It will never happen IMHO, because of two words - death ray.
     
  14. Feb 8, 2015 #13
    The proposed transmission systems for space-based solar power have very large receiving antennas. The intensity of the microwave beam would be completely insignificant to you, even if you were standing right at its center.

    The NASA SERT program, for instance, limited the "center-of-beam" intensity to around ~20 mW/cm2, which is less than you'd get on a sunny day on earth.
     
  15. Feb 8, 2015 #14

    sophiecentaur

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    One thing we are not short of, on the Earth's surface, is area. For every m2 of solar receiver that's put up in space, you could put probably 100m2 of land based receivers. So they only work for 8 hours a day? Put up three times as many. And, if the 20mW/cm2 that's quoted in the above post, you would need a pretty enormous receiving array on the ground in any case.
    Can someone tell me where my skepticism is wrong?
     
  16. Feb 8, 2015 #15

    sophiecentaur

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    One thing they don't seem to mention is the efficiency of the AC - microwave conversion at the transmitter. It definitely isn't 100%. That was nearly forty years ago and I don't know of any other significant ground based systems since. There must be a good reason for that
     
  17. Feb 8, 2015 #16
    Why only 8? That would be giving up one of the main advantages of space-based solar.

    This article mentions significantly higher "center-of-beam" intensities than 20 mW/cm2, but the rectenna still has to have a diameter of around 3 km.

    Edit:
    I just realized I read your post completely backwards, you're arguing for an Earth-based solution.
     
    Last edited: Feb 8, 2015
  18. Feb 8, 2015 #17

    russ_watters

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    My first thought as well. By that math, such a device would produce less electricity than the solar array on Earth could have done on its own.
     
  19. Feb 8, 2015 #18

    russ_watters

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    You misread: that's the normal earth-based solar that only works for 8 hours. But in 8 hours of collecting sunlight it would collect more electricity than if it was getting 24 hours of space-based reflected sunlight. So it seems like paying more for less, to me.
     
  20. Feb 8, 2015 #19
    I haven't done any estimate, but I suppose the continuous production and increased efficiency makes it worthwhile to consider. My personal take on it is that there are too many problems in its engineering that, each taken by itself, would be incredibly difficult to solve.

    And the thing would have to be huge to make economical sense.
     
  21. Feb 8, 2015 #20

    sophiecentaur

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    And what about the cost of getting it up there and keeping it on station? Three times the output - less the efficiency problems of RF generation and spillage. It seems to me that the enthusiasts are looking for a problem to solve with space travel where the Terrestrial solution already exists at a much lower cost. Obviously, the best sites for Solar power would be near the equator so there could be a significant cable loss for the terrestrial system but that's the only downside I can see for terrestrial solar gen.
    What increase - if you haven't done the sums?
     
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