How Does Wireless Power Transmission Work?

[Hood]

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?

 

[berkeman]

If you look at the "Related Discussions" links at the bottom of this page, does that help to answer your questions? It's a very frequently asked question here at the PF

 

[himakar]

In wireless power transmission via solar power satellite
Is the power is converted in space solar panels or the energy is converted after reaching of microwave signals to earth...?

 

[anorlunda]

In wireless power transmission via solar power satellite
Is the power is converted in space solar panels or the energy is converted after reaching of microwave signals to earth...?

If you mean satellite-to-ground power transmission. There aren't any yet.

 

[sophiecentaur]

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?

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.

 

[nsaspook]

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.

Sadly most of these claims are on the same level as an April Fools Joke.
http://neenya.com/aprilfools/rg/

 

[jim hardy]

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?

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)

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.

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.

 

[sophiecentaur]

Sadly most of these claims are on the same level as an April Fools Joke.
http://neenya.com/aprilfools/rg/

Oh boy. What a fabulous offer. Where do I sign?

 

[jim hardy]

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

Of course, it’s impossible to think about the transfer of power across distance without suggesting some early 20th-century ideas about transporting electrical energy wirelessly instead of through a wired grid. In fact, what we’re talking about here is different. Those grand ideas involved not the transfer of a little bit of power via a magnetic field over a moderately short distance; that was about wireless transmission over long distances (even bounced off the ionosphere, apparently).

Here’s my understanding of the fundamental difference. WiTricity’s technology relies on the fact that the E-field is largely localized between the capacitor plates, and the B-field extends to the items being charged. This holds under “near field” conditions. But at long distances, under “far field” conditions, the E- and B-fields cohabitate. There is potential for… things to go awry if we wander around in the presence of such strong fields. (If cell-phone haters are concerned about cell phone radiation, imagine how they would react to large-scale wireless power transmission…)

So, to put that question to rest, no, this does not begin the realization of Tesla’s grand vision.

My ancestors are all from Missouri so i'll have to be shown...

 

[himakar]

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...

 

[zoki85]

MIT witricity is poor as regards efficiency, transmission distance and other things...

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...

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:

 

[anorlunda]

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...

You are confusing dreams with reality.

It will never happen IMHO, because of two words - death ray.

 

[milesyoung]

It will never happen IMHO, because of two words - death ray.

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.

 

[sophiecentaur]

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?

 

[sophiecentaur]

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:.

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

 

[milesyoung]

So they only work for 8 hours a day?

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.

 

[russ_watters]

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?

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.

 

[russ_watters]

Why only 8? That would be giving up one of the main advantages of space-based solar.

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.

 

[milesyoung]

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.

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.

 

[sophiecentaur]

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.

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?

 

[anorlunda]

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.

The NASA SERT program considered a 1GW plant "full scale". Earth needs about 2TW of electric capacity. To supply only half of that requires 1000 times more than NASA envisioned. We can send up more than one power sat, but 1000 of those would be far too expensive., and ten 100GW sats would make the death ray sound more believable.

Besides, the public will be ready to believe the death ray scenario, even if all the world's engineers say nah. Someone will make a movie where The Pentagon insists on the ability to secretly repurpose those power sats as weapons, then terrorists; yada yada yada. Come to think of it, "Diamonds are Forever" already used that plot 44 years ago.

Even today, many people associate nuclear power with Hiroshima and believe that nuclear reactors can make nuclear explosions.

I still say that it will never happen, but not because of engineering reasons.

 

[zoki85]

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

I agree that overall efficiency was probably lower than the reported one. Anycase, they proved the concept worked. And it is a bit strange there have been so little reports of actual applications of it since 1975. If the military got interested in such technology that could explain some things.

 

[jim hardy]

20mW/cm2

just for some perspective.
hmm
i gigawatt, about one modern steam plant
at 20 mw/cm^2
would be
1E9watts divided by .02 w/cm^2 = 5E10cm^2 = 5E6m^2 = a square √5 km per side ? 1235 acres?
That's a mighty big dish antenna
a thousand of them per terawatt ?It's hard enough getting environmental permits for windmills...

 

[sophiecentaur]

I agree that overall efficiency was probably lower than the reported one. Anycase, they proved the concept worked. And it is a bit strange there have been so little reports of actual applications of it since 1975. If the military got interested in such technology that could explain some things.

It doesn't actually seem strange to me - just that someone has got real about it.
Jim's sums would help people to get real, too!

 

[milesyoung]

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?

Direct insolation tops out at around 2.5 MWh/m² a year in the Sahara. The solar constant is ~1366 W/m², so that's about 12 MWh/m² a year, i.e. the energy you get in the Sahara compared to space is around 20%.

I can't give you a detailed power budget (I think a lot of the technology involved is still hypothetical), but I think we can transmit power from a satellite to a ground station at higher than 20% efficiency, but that's just a guesstimate, of course.

 

[milesyoung]

The NASA SERT program considered a 1GW plant "full scale". Earth needs about 2TW of electric capacity. To supply only half of that requires 1000 times more than NASA envisioned. We can send up more than one power sat, but 1000 of those would be far too expensive., and ten 100GW sats would make the death ray sound more believable.

I doubt anyone envisioned all our power coming from spaced-based solar. I also seriously doubt anyone would ever allow anyone microwave channel to have anything near lethal intensity at ground level.

I still say that it will never happen, but not because of engineering reasons.

Maybe people just need to be a bit more informed, but I see your point about nuclear power.

 

[zoki85]

I also seriously doubt anyone would ever allow anyone microwave channel to have anything near lethal intensity at ground level.

Nobody wouldn't allow... for normal purposes.
:D

 

[Baluncore]

With regard to power transmission from space to Earth. Is there an electromagnetic equivalent to the Vortex Ring of fluid dynamics? Might such a system work at the macro scale or possibly only at a quantum scale? What topology of antenna would be needed and what wavelength would be best for long distance transmissions?

 

[zoki85]

With regard to power transmission from space to Earth. Is there an electromagnetic equivalent to the Vortex Ring of fluid dynamics? Might such a system work at the macro scale or possibly only at a quantum scale? What topology of antenna would be needed and what wavelength would be best for long distance transmissions?

EM waves that propagate in a straight line without reflection and acceptable atenuation are the best candidates. For Earth atmosphere & ionosphere between satellites in orbit and ground station facility frequency range 1...10 GHz is compromise between wave atenuation and antenna size. Suitable antennae are of phased array-rectifying antenna (rectenna) types.

 

[Baluncore]

But what is it that prevents EM energy being transmitted and received like vortex rings in air? We need to get away from the inverse square law so as to increase the efficiency of transmission while also reducing the size of the antennas needed. Maybe it is time for someone to invent the “photon torpedo”.