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Author Topic: When will there be the technology for wireless electricity?  (Read 28431 times)

Offline wolfekeeper

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When will there be the technology for wireless electricity?
« Reply #50 on: 14/08/2009 18:21:19 »
Woolfkeeper
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I think that's why they choose >1 Mhz frequency for the witricity system. Metals have a very thin skin depth at those frequencies, and this gives high resistance; so you don't get much eddy current, and not much power is dissipated.
I think you have this the wrong way round, I'm afraid. The higher the resistivity of the metal, the greater the loss - which is what one would expect --ain't it?  No resistance and the skin depth is zero - no loss at all.
For the transmitter and receiver, yes, but you would deal with that in the construction.

But for other things that are just mooching around within the field that you aren't transmitting power to, either a thin skin depth or complete or nearly complete transparency (skin depth >> object diameter) is what you want. With a thin skin depth all the currents occur in a very thin layer and this gives a relatively high resistance which prevents much current from flowing and prevents major power losses. The worse case for losses is when the skin depth is comparable to the thickness of the material in fact.
« Last Edit: 14/08/2009 18:23:24 by wolfekeeper »
 

lyner

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When will there be the technology for wireless electricity?
« Reply #51 on: 14/08/2009 20:28:10 »
Skin depth is a FUNCTION of resistance. You can't have one without the other.
I think we're straying form the point here. DC is best for wired and you can't do wireless in a worthwhile fashion over more than a metre or so except when circumstances absolutely demand it.
 

Offline wolfekeeper

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When will there be the technology for wireless electricity?
« Reply #52 on: 14/08/2009 20:48:48 »
Skin depth is a FUNCTION of resistance. You can't have one without the other.
Well, sort of. Skin depth is a combination of resistivity and relative permeability, which can be substantial for iron, but if something has a small skin depth then that represents very little volume in which losses can occur and so it turns out that losses are small. If the skin depth is very high, then that's usually because the resistance is very high, and then you will get very low losses because resistive losses go as V^2/r.

It's the intermediate case where the skin depth is comparable to the size of the object that is where the worst dissipation occurs.

It's a different case to antennas; where you're trying to put power through them, and there a small skin depth is very problematic. For wireless power transmission, you don't want things around you to act as antennas.
« Last Edit: 14/08/2009 20:50:42 by wolfekeeper »
 

lyner

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When will there be the technology for wireless electricity?
« Reply #53 on: 14/08/2009 21:37:53 »
I'm not sure how anyone thinks they can make a 'near field' device that is effectively screened from far field radiation to a degree where interference is not relevant.
To my mind, it is communication and not power transfer that should get priority and interference is the key factor. A truly low distortion amplifier / transmitter is going to be expensive and inefficient.
But I ask again.Why bother? It won't work at large distances and you may as well use wired connection or batteries. When did any of us have to change the batteries in the TV remote? When did we use a device which couldn't be recharged conveniently overnight by putting it in an appropriate dock?
What is the actual application that would demand a specific Frequency Allocation to allow this to be done? I can guarantee it would tread on someone's toes and for what?
 

Offline wolfekeeper

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When will there be the technology for wireless electricity?
« Reply #54 on: 25/08/2009 17:31:44 »
Batteries are expensive and polluting sources of power; tens or hundreds of times more than wired, whereas wireless is about twice that of wired. And the batteries contain materials that are in limited supply and they are disposable items.

And you only need one monochromatic frequency to transmit power; it's not like you would would be wiping out all communications for thousands of miles; it's a single carrier in an incredibly narrow frequency range.

You know, anyone would think you didn't like the idea and were coming up with spurious 'reasons' why it wouldn't work. ;)
« Last Edit: 25/08/2009 17:33:24 by wolfekeeper »
 

lyner

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When will there be the technology for wireless electricity?
« Reply #55 on: 25/08/2009 23:32:34 »
No. It's just that
1. I was, essentially, a RF Engineer and almost as bad as a Radioastronomer in my feelings about interference. It wouldn't just be you or me, using it - there would be one in each home, presumably, so it actually would be causing interference everywhere and at a high level. The purity of the signal from each transmitter would need to be high and what frequency could you choose? Even a cw signal needs a finite guard band around it (9kHz channels are used for MF) because of the poor selectivity of receivers looking at adjacent channels)
and
2. The actual implementation still seems dodgy. You would need values of effective coupling for a given radius of operation before you could decide on its viability. Then there would be the installation - would you have one in each room? That could (would) produce dead spots between the 'service areas' of each transmitter. Sods Law would have it that a dead spot would be just where you wanted to sit and watch TV or whatever. Do you envisage coils embedded in walls or integrated into pictures or furniture?

I do grant you that, for an application such as a cordless mouse, it could be perfect. There must be similar applications which I haven't thought of - but very limited. You're talking of a requirement for a mW or so of power (mean) and laying down a field which would cover a whole room / house on a continuous basis. When you think of the fuss people are making about leaving on the little red lights on TVs etc, this would be consuming some real power - several watts, at least. You could almost see that little disc going round. I don't think that's a spurious reason.

As for batteries - there have been recent threads discussing the use of super capacitors as a storage medium. But I am a bit hesitant to go in that direction just yet - not 'till you can get a system from Curry's.
 

Offline wolfekeeper

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When will there be the technology for wireless electricity?
« Reply #56 on: 26/08/2009 00:28:17 »
You could stuff the coil under a circular rug or something, that would cover the whole room. You really want a coil about 6 feet across maybe; perhaps a few coils in a large room. I don't imagine the coils would be on most of the time though; you'd want the coils to switch on only when something is thirsty, otherwise you're just throwing away power.

Then again, transformers are throwing away power anyway.

Done right, this would be less polluting and cheaper than existing systems.

Getting an RF slot for this is the trick of course. I checked out the bandwidth allocation sheets a while back. There's *enormous* amounts of bandwidth allocated for what are probably defence and such like. They've basically just stolen large swathes of bandwidth; I bet they're not using it much either, and the equipment for it, is likely to be a horrible bandwidth hog.
 

lyner

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When will there be the technology for wireless electricity?
« Reply #57 on: 26/08/2009 09:29:58 »
I do like the "on demand" idea. It would require all devices to have energy storage, of course but a tiny amount would do.
Apart from dead spots, the system wouldn't suffer from incoming interference, at least.
I still have severe doubts about efficiency, though. It could be great as a non-contact charging 'dock'; all the remotes could be placed on a particular table but is there an issue with such low power devices. The charge rate needed for hand tools would be too high, I think. What would you actually use the system for?
 

lyner

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When will there be the technology for wireless electricity?
« Reply #58 on: 26/08/2009 12:13:36 »
I just had another thought on the subject of interference and here is a seriously (unspurious) objection.
The ratio of received MF (say) broadcast signal and local 'power' signal in an adjacent channel would be tiny, in most cases :  say -60dB. That would impose unrealistic limits on the receiver selectivity filter which is designed on the assumption that the adjacent channel will be 30dB LOWER (that's 90dB adrift - using ball park figures)). This requirement  would certainly not be met by existing designs. Rather than needing a 9kHz free band I think you would be needing to free-up  spectrum space corresponding to three or four channels on either side of the power channel. That's a huge chunk of the MF band wiped out at a stroke. You always have to be backward compatible in these matters and you couldn't expect to retrofit filters into every existing MF receiver.
 

Offline wolfekeeper

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When will there be the technology for wireless electricity?
« Reply #59 on: 16/09/2009 00:12:42 »
The thing is there are already systems transmitting inductively.

For example induction hobs run at about 26khz I believe. So that band is already being radiated into.

There seems to be standards for inductive loops that run at up to ~167 khz or something already.

That would be slightly low frequency for this kind of thing, but still possibly usable.
 

Offline wolfekeeper

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When will there be the technology for wireless electricity?
« Reply #60 on: 18/09/2009 18:22:56 »
And I'm not saying you wouldn't need to leave space for it. We already do do that for power transfer (Microwave oven frequencies); and then somebody worked out how to use it as a white space (Wifi) anyway.

But you only need one very narrow frequency band for the whole country (plus guard bands).
 

Offline wolfekeeper

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When will there be the technology for wireless electricity?
« Reply #61 on: 26/09/2009 01:48:36 »
I just had another thought on the subject of interference and here is a seriously (unspurious) objection.
The ratio of received MF (say) broadcast signal and local 'power' signal in an adjacent channel would be tiny, in most cases :  say -60dB. That would impose unrealistic limits on the receiver selectivity filter which is designed on the assumption that the adjacent channel will be 30dB LOWER (that's 90dB adrift - using ball park figures)). This requirement  would certainly not be met by existing designs. Rather than needing a 9kHz free band I think you would be needing to free-up  spectrum space corresponding to three or four channels on either side of the power channel. That's a huge chunk of the MF band wiped out at a stroke. You always have to be backward compatible in these matters and you couldn't expect to retrofit filters into every existing MF receiver.
FWIW this technology is basically already out there. The Oyster cards on the London Underground are powered this way.
 

lyner

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When will there be the technology for wireless electricity?
« Reply #62 on: 27/09/2009 02:54:48 »
Aren't the oyster cards placed right next to the pad, though? I don't think they are powered and operated from several metres away. That difference in distance makes all the difference in dB of power needed and interference generated. There is a similar system used for the passive key system used in many cars too. The 'transponders' operate when the blade is right in the ignition lock.
The situations where the system is used seem to imply that a wireless mouse system might also work. But aren't we some way away from the idea of powering / charging devices remotely?
My reservations, all along, have been related to the actual numbers involved with your original idea which seemed to suggest much more substantial powers and large distances.
 

Offline wolfekeeper

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When will there be the technology for wireless electricity?
« Reply #63 on: 27/09/2009 17:42:38 »
For any given efficiency, distance is nothing whatsoever to do with power.

The efficiency and the Q factor and distance are interlinked, but power is not.

Increasing power can obviously affect the chances of EMI issues though, but this only affect bands either side of the transmitter's frequency.
 

lyner

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When will there be the technology for wireless electricity?
« Reply #64 on: 28/09/2009 16:59:51 »
That statement just has to be wrong.
A transformer will be highly inefficient if you have an air gap but a small one still allows some useful coupling. That's the obvious argument where 50 Hz coupling is concerned- as with toothbrushes.
Then for thr RF  situation: The radiation resistance of a small structure (e.g. Oyster pad) is very low and it will only radiate if tuned very heavily. The near field is very near. A structure which will give near field coverage for a 'room' will have a radiation resistance which is more or less proportional to its size (for long wavelengths) and will need to lay down a proportionally larger total flux if the receiver is to work anywhere within / near it. That indicates considerably more power is needed. (Remember- you can't have a huge 'receive' structure on your hand held device which could mitigate this, to some degree.)

I have made the point before but you really have to include the actual numbers in this proposed system. The microwatts needed for a nearby and essentially signalling system rapidly become tens or hundreds of mW(minimum) when you are looking for remote 'power' applications.
I don't know what coupling factor you are assuming but it would be several tend of dB, I believe - unless you can show me some sums to the contrary.    
« Last Edit: 28/09/2009 17:02:50 by sophiecentaur »
 

Offline Bored chemist

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When will there be the technology for wireless electricity?
« Reply #65 on: 28/09/2009 18:16:05 »
I think it's right.
Provided that I don't mind squandering power on a horribly inefficient transfer I can transmit as much power as I want down an arbitrarily bad link (until something melts)
 

Offline wolfekeeper

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When will there be the technology for wireless electricity?
« Reply #66 on: 28/09/2009 18:39:48 »
This doesn't work at all like a normal transformer.

It works by making the primary ring, with *very* high Q; about a thousand.

In each cycle, provided the receiver coil subtends enough of the magnetic field that it receives more energy than is lost in that cycle then most of the energy is transferred across. Because the Q can be so high, you only need ~1/1000 of the field to get 50% transfer efficiency, and it's *much* better than that when you get more field coupling than that.

The radiative losses come out of the Q of course; for a coil to ring the radiative losses can't be too high; but a lot of the losses are resistive anyway.

So it's all magnetic field stuff- it's near field, not radiative far field. Near field doesn't propagate.

FWIW you can get arbitrary distances with arbitrarily high efficiency and arbitrarily low radiation this way; but you may not like the weight and size of the coil to do that ;-)
 

Offline wolfekeeper

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When will there be the technology for wireless electricity?
« Reply #67 on: 28/09/2009 19:20:30 »
That statement just has to be wrong.
Nope. Many Tesla coils work this way, you can build a very large Tesla coil if you want, and the energy will go from the bottom to the top coil with good efficiency, even tens of feet. You'll need a proportionately big coil either side though.
Quote
A transformer will be highly inefficient if you have an air gap but a small one still allows some useful coupling. That's the obvious argument where 50 Hz coupling is concerned- as with toothbrushes.
I believe that toothbrushes are capacitively tuned to mains frequency, but I haven't checked.
Quote
Then for thr RF  situation: The radiation resistance of a small structure (e.g. Oyster pad) is very low and it will only radiate if tuned very heavily.
Guess what? They're heavily tuned. That's how it works.

Quote
The near field is very near. A structure which will give near field coverage for a 'room' will have a radiation resistance which is more or less proportional to its size (for long wavelengths) and will need to lay down a proportionally larger total flux if the receiver is to work anywhere within / near it.
For low frequencies, near field is a few diameters. For large areas you need low resistance coils. There's no limit on how far you can send, it's mostly just a question of how big the sender coil is, and how much power you put through it (or not).
Quote
That indicates considerably more power is needed. (Remember- you can't have a huge 'receive' structure on your hand held device which could mitigate this, to some degree.)
You don't need high power on a cell phone, and so you don't need a big coil, and you wouldn't put as much power into the larger sender coil either. The losses are strictly proportional to power sent, and efficiency is only a function of geometries (including geometry of the coil, diameters of wires etc). Getting 50% efficiency is not difficult.
Quote
I don't know what coupling factor you are assuming but it would be several tend of dB, I believe - unless you can show me some sums to the contrary.    
Roughly 50% effciency down to about -60 db coupling.
« Last Edit: 28/09/2009 19:22:31 by wolfekeeper »
 

lyner

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When will there be the technology for wireless electricity?
« Reply #68 on: 30/09/2009 18:10:16 »
I am not sure that you have adressed the main query I have. What you have described is a good system for signalling to a limited area. If you have a coupling of 60dB then, for 1mW of power, delivered power(and would that be worth having?) you would need a 1kW transmitter. I'm sure you can't mean that would not radiate significant interference, whatever you might do to localise the fields.
What would be 1. Your spec for power delivered to the device,  2. The field needed within the service area and 3. The level of power radiated by the room / house?

On toothbrushes- what values of L  and C would you expect for a tuned system at 50Hz? My toothbrush and unit are very small and very light so I suspect that they are both very low values. But you don't need high efficiency in that application. The losses are all resistive in the wires.
« Last Edit: 30/09/2009 18:12:05 by sophiecentaur »
 

Offline wolfekeeper

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When will there be the technology for wireless electricity?
« Reply #69 on: 30/09/2009 18:52:45 »
Actually, it's only -30 dB, I had my power/amplitude decibels mixed up.

That aside, if the receiver needs to get 1 W, then at 50% efficiency the transmitter needs to put in 2 W to the transmitter coil.

The reason you *don't* need 1kW is because the transmitter coil is ringing. That means you feed in 2 W (average), but the coil is oscillating with ~kW power (with a Q factor of 1000), but is only losing 1W in resistance and radiative losses and the successful inductive transfer is 1W. (Actually it's not quite that bad, there's a 2Pi factor in the q, so it's actually about 300W power in the resonance of the oscillator).

The radiative losses come out of the system losses, and cannot be more than 1W in that case.
« Last Edit: 30/09/2009 19:02:04 by wolfekeeper »
 

lyner

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When will there be the technology for wireless electricity?
« Reply #70 on: 01/10/2009 02:24:52 »
I see what you mean about the Q.
But I can't imagine building a structure in a house in the presence of wires, pipes and steels which could not radiate a lot more than 0.1% of its reactive power. Any unbalanced impedances will produce unbalanced earth currents and how could that be eliminated without bespoke design for each installation and constant tiffling?
That's my problem, now I have identified the reason for my misgivings.
I think the bottom line is that the technology is all very old so why hasn't it been done already? I can't accept that the Oyster card system is a near enough parallel to prove that a larger system could work,
if I'm proved wrong and someone does it, I am more than prepared to be totally gobsmackef!
 

Offline wolfekeeper

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When will there be the technology for wireless electricity?
« Reply #71 on: 01/10/2009 03:23:50 »
You will get some losses from such things, but they're not going to be resonating, so their impedance will be high, and the losses low. The other thing is that the magnetic field dies away very quickly from the main coil, you can always move the coils around a bit. It won't always work, wireless stuff doesn't always work anyway.

It has been used in some cases; a lot of RFIDs use it to power themselves, some pacemakers recharge that way, there was some experimental use in recharging electric buses, Oyster cards (actually most non-contact cards, including e-passports use this resonant transfer tech), Tesla coils etc. etc.

The main thing would be cell phones, that's a relatively new technology, and if the tech can work for those with good range then we'll start to see wide deployment. That would be the killer app. Also possibly laptops- power cords are real pains for those, you trip over them and break your laptop.
 

lyner

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When will there be the technology for wireless electricity?
« Reply #72 on: 01/10/2009 15:37:42 »
I can see that we're only disagreeing about max power and distance. We shall just have to see what develops.
 

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When will there be the technology for wireless electricity?
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