Naked Science Forum
General Science => General Science => Topic started by: dentstudent on 06/06/2007 08:19:42
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We still have to plug all our things in - will there soon be a way of getting some sort of signal that creates a current in a remote receiver?
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I'm pretty sure it would be possible already. You could aim an emitter in line with a receiver. Except if you ever walked in between, you would get burned or harmed with the microwaves. So, no, I don't think there will be wireless power available to the general public soon.
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Why microwaves? Why not electromagnetic wavelengths for example that are not so dangerous? It would just need to be some sort of resonant frequency, no?
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It depends on what you mean by 'wireless'. You can have wireless induction loops that work at lower frequencies, but because they are low frequencies, it is not practical to make them directional, so one has to be close to them (you cannot use them to transmit energy a great distance).
Microwaves are not terribly dangerous, but if you are in the path of several kilowatts of power, it doesn't matter how that is delivered, if it is delivered through you, then you are likely to be heated up (if not electrocuted) by that energy.
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I used to work for a company that made radar for the navy. We used to play by frazzling things at the open end of the waveguide. I was quite surprised how quickly green leaves turned brown & shrivelled.
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I guess my vision is where I'm in my office and all my equipment (ie PC, PDA, phone, coffee machine, fan, light etc) all operate without leads and plugs. So, it's a very local scale. I guess that this type of thing outside would be very energy inefficient and problematical.
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I guess my vision is where I'm in my office and all my equipment (ie PC, PDA, phone, coffee machine, fan, light etc) all operate without leads and plugs. So, it's a very local scale. I guess that this type of thing outside would be very energy inefficient and problematical.
The problem is, it does not matter how local, if you are in the middle of an energy field of sufficient strength to run a major electrical appliance, I would like to have it shown that it will not do damage to you before I sit there.
OK, some of the scare stories about the energy from WiFi systems or mobile phones, that are running at about 1W, is silly; but that is not saying that worrying about a signal that is 3KW in strength is silly.
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So, are you saying that it's not the possibility that's the problem, but the amount of energy required to operate these systems, and the human health impact?
(a thought has just come to me - how big would a solar panel have to be to operate a pc? If the side panels of the pc were photo-voltaic (?) cells, you could put it anywhere!)
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A solar panel to power a PC at midday on the equator would have to be about half a square metre, at midday in the UK about 1 square metre, and it would need to cover about 40 rooms inside when it was dark, as it consumes far more power than the lighting in your room use let alone emits.
I think there has been talk of using inductive coupling for low powered devices, by winding the coils right you could make the effect very short range - in fact electric toothbrushes are charged using this principle. However it is never going to be very efficient.
I guess you could build a table with lots of little coils in it, which detects where a device that needs power is and just energises those coils, however it would require you virtually to make a copper table (there would be so many coils), plus a whole lot of electronics, so I don't think it is going to be very cheap...
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Buld a crystal set like my dad did when he was a kid and you will find out that some power is already transmited wirelessly.
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Indeed this tech has existed for a while. If you've ever seen old radio antenna, like they used to have in England, these not only used the radio waves to generate sounds on the radio, but also absorbed enough of the waves to power the radio. This is why the antenna were so large, because the individual waves are very weak energy.
To generate enough electricity flying through the air to act as a short of wi-fi for electric gadgets would require electromagnetic waves with high energy and at frequencies not used for radio, tv, or cell phones.
They would also have to pack enough energy to power many gadgets, which would require either many transmiters in every conceavable location, or a few very powerful ones, which would put out enough juice to electrocute people.
So, based on my own limited knowledge of this issue, I must agree with previous posters that such large scale wireless electric transport is never going to happen.
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Tesla demonstrated "the transmission of electrical energy without wires" that depends upon electrical conductivity as early as 1891.
http://en.wikipedia.org/wiki/Nikola_Tesla
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Hertz had done it before, (in 1886 I think); any advance on 1886?
(Trust me, the story goes back further than Hertz's work.)
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This came up today on BBC webnews.....
http://news.bbc.co.uk/2/hi/technology/6725955.stm
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I was just about to post that [>:(]
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The efficiency tends to be poor. The aerials or coils or whatever tend to be big in relation to the gap you are trying to cover. Microwave transmission would be possible over about 10 metres with 3 metre parabolic dishes which is a bit silly.. Much easier to run a wire.. Even if you could get the transmission loss down the conversions from AC to DC and then to Radio Frequency and then RF - DC (and then maybe DC-AC) will introduce considerable extra losses.
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http://news.bbc.co.uk/1/hi/technology/6725955.stm
That demonstrates my point.. The coils are big with respect to the gap they are covering.
I gather the transmission coupling between the coils was 40%.. (60% waste).
AC(mains) - DC - RF ... 50% maybe (if that) ...then the 40% coupling.... RF - DC ...70%
Overall efficiency ...50 x 40 x 70 = 14% AC mains to DC output. Lot of waste.
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It's not necessarily the case that efficiency is poor- there's nothing to stop you just having a big coil set in the floor or ceiling when you build the house. The efficiency tends to only be poor when the distances are several times the coil diameter. Then again cables tend to be quite long as well, and much more cumbersome.
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It's not necessarily the case that efficiency is poor- there's nothing to stop you just having a big coil set in the floor or ceiling when you build the house. The efficiency tends to only be poor when the distances are several times the coil diameter. Then again cables tend to be quite long as well, and much more cumbersome.
The efficiency just has to be poor if you don't have a huge iron / ferrite core around the reeiving appliance. Unless you do this, the field will spread around in many directions and any conductor will have a current induced in it and consume the power. At higher frequencies, the power will be radiated in all directions.
The toothbrush charger is the only example which I can think of with 'non-contact' transfer of power and that is hideously inefficient. Even that system uses very near contact.
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For inductive transfer you don't get significant radiative losses, you mainly get resistive losses in the coil.
With resonant systems you can get maybe ~80% efficiency at distances comparable to the size of the coil and there's little if any coupling to humans because they are non magnetic; or to other conductors if they don't resonate at the right frequency.
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If you needto have a lot of power transfered (the whole point of the exercise) then you would have to match the transmit and receive circuits. I cannot envisage a system in which the losses are as small as 20%.
Which frequency would be used? How would you limit radiation losses or just poor flux linkage? A transformer has very poor efficiency if you don't have a good magnetic circuit - i.e. a good amount of iron coupling the primary and secondary circuits.
I don't see how you can get away with the problem of needing a high rate of change of flux in your secondary circuit if most of the flux is going elsewhere.
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If it is not about increased efficiency, why bother?
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There was the SHARP project:
http://www.friendsofcrc.ca/Projects/SHARP/sharp.html (http://www.friendsofcrc.ca/Projects/SHARP/sharp.html)
This was all about powering an aircraft by microwaves.
Incidentally, the professor who designed the airframe went on to design and build a flying ornithopter.
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The system highlighted by a couple of earlier posters (BBC News website) has a three-page story on it in the recent (Feb 2009) edition of Physics World magazine, pg.23-25. It's not obvious that very much progress has been made in the past 2 years.
http://physicsworld.com/cws/article/print/37532
In the magazine article they state they operate at 10MHz, that the energy is carried almost entirely in the magnetic field, and that the maximum magnetic field strength is only a tiny* 10^-4 Tesla, even very near the coils.
* tiny: the word of the article.
The internationally accepted guidelines for human exposure to EM fields, ICNIRP ( http://www.icnirp.de/documents/emfgdl.pdf ) set an upper limit of 0.1µT for general-public exposure to magnetic fields at 10MHz.
The tiny field of this power-coupling demo is 1000x greater than the currently permissable general-public exposure. This may pose a problem for general deployment [::)]
Although the article is at pains to point out that that energy losses in wood, brick, plastics is rather low, I imagine any sheet metal would absorb a lot of energy. With the kind of source/receiver geometries illustrated, the efficiency will inevitably (I would have thought) decrease as the receive coil is made smaller. In general the coupling efficiency will decrease as the source-receiver distance becomes a larger multiple of the antenna size.
I'm also slightly confused that the article appears to place great emphasis on the Q of the receiver coil being very high (as much as 1000) to maxmise coupling and efficiency but it strikes me (although perhaps I'm being naive?) that as soon as you load the receiver coil with a light bulb its Q will drop to something very small... [???]
Some of the comments following the on-line version of the IoP article I reference sound very valid, particularly the "detuning" of the coils if one brings any metal or large modestly conducting object anywhere near them. High-Q coils are easily detuned (this is one of several principles upon which a 'metal detector' can be constructed).
I think this vision of wireless power transmission is rather driven by sci-fi fantasy (and a good headline / enticing photograph for the media). Low-power coupling (eg for mobile phone / mp3 chargers) of 1-2watts over distances comparable or smaller than the physical size of the transmit coil are probably technically feasible - though whether commercially viable or desireable is another matter.
Consider though that, after much wrangling, many of the worlds mobile phone manufacturers have just agreed to standardise on a mini-USB connector as a universal charger input.
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I think I agree with about every one of your comments techmind.
The wavelength for 10MHz is 30m. That means a metal structure of 15m would resonate in free space. With dielectric, you'd need, say, 10m.That's about house size. So you wouldn't be allowed to have any mains wiring in the building because that would almost certainly resonate at some stage.
As for mobile phone leads. It's a real scandal, at the moment . Every time you buy a new phone, you need a whole new set of accessories; sound commercial sense (the bastards).
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If you needto have a lot of power transfered (the whole point of the exercise) then you would have to match the transmit and receive circuits. I cannot envisage a system in which the losses are as small as 20%.
That's about what has been demonstrated though.
Which frequency would be used? How would you limit radiation losses or just poor flux linkage?
They don't radiate much power- you're trying to generate near-fields, not far field. And the secondary coil is resonating with a counter field (basically Lenz's law).
A transformer has very poor efficiency if you don't have a good magnetic circuit - i.e. a good amount of iron coupling the primary and secondary circuits.
True, but that's not tuned.
I don't see how you can get away with the problem of needing a high rate of change of flux in your secondary circuit if most of the flux is going elsewhere.
It only works when you get adequate coupling coefficient of course. That's why you're limited to a few radii; and rate of change of flux is why they generally use higher frequencies for this, mains frequency is too low.
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The system highlighted by a couple of earlier posters (BBC News website) has a three-page story on it in the recent (Feb 2009) edition of Physics World magazine, pg.23-25. It's not obvious that very much progress has been made in the past 2 years.
No, probably not, but the technology does work. And you could at one point buy a wireless christmas tree that used Tesla's scheme.
In the magazine article they state they operate at 10MHz, that the energy is carried almost entirely in the magnetic field, and that the maximum magnetic field strength is only a tiny* 10^-4 Tesla, even very near the coils.
* tiny: the word of the article.
The internationally accepted guidelines for human exposure to EM fields, ICNIRP ( http://www.icnirp.de/documents/emfgdl.pdf ) set an upper limit of 0.1µT for general-public exposure to magnetic fields at 10MHz.
The tiny field of this power-coupling demo is 1000x greater than the currently permissable general-public exposure. This may pose a problem for general deployment [::)]
A lot of this stuff seems very conservative. I've got a double halbach array sitting on my desk in front of me that is about 3x over the static field limit ;-), and I think the individual magnets are slightly over as well.
With the kind of source/receiver geometries illustrated, the efficiency will inevitably (I would have thought) decrease as the receive coil is made smaller.
Perhaps; it's not clear that this is so in general.
I'm also slightly confused that the article appears to place great emphasis on the Q of the receiver coil being very high (as much as 1000) to maxmise coupling and efficiency but it strikes me (although perhaps I'm being naive?) that as soon as you load the receiver coil with a light bulb its Q will drop to something very small... [???]
It will effect it slightly, but you use an inductive pickup, the load impedance seen by the coil would be very low, because it's divided down by transformer effect.
I think this vision of wireless power transmission is rather driven by sci-fi fantasy (and a good headline / enticing photograph for the media). Low-power coupling (eg for mobile phone / mp3 chargers) of 1-2watts over distances comparable or smaller than the physical size of the transmit coil are probably technically feasible - though whether commercially viable or desireable is another matter.
Well, it clearly does work. I've seen vids of people levitating LEDs using maglev, and then using this technique to light them on youtube.
Consider though that, after much wrangling, many of the worlds mobile phone manufacturers have just agreed to standardise on a mini-USB connector as a universal charger input.
I think there's some scope for these kinds of systems if they can be developed more and the issues ironed out. You wouldn't want to use them for high power applications like heating, but lighting perhaps and laptops and cell phones and similar don't use very much power at all and efficiency perhaps isn't so very critical.
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I'm also slightly confused that the article appears to place great emphasis on the Q of the receiver coil being very high (as much as 1000) to maxmise coupling and efficiency but it strikes me (although perhaps I'm being naive?) that as soon as you load the receiver coil with a light bulb its Q will drop to something very small... Huh?
It will effect it slightly, but you use an inductive pickup, the load impedance seen by the coil would be very low, because it's divided down by transformer effect.
Q is not, primarily, a matter of resistance or impedance, however you transform them. Q represents the fraction of Energy loss compared with the stored (reactive) energy. If you are to have a high Q (tuning) and transfer significant Power, then you would need an enormous level of reactive power. That would involve high voltages or high currents - either of which would be impractical / unsafe.
The problem, here is that we haven't decided what sort of power this imagined system is supposed to be transferring and over what distance. It doesn't surprise me that a small set of Christmas tree lights could be supplied and that you could possibly boil some water out in the middle of a hayfield but we need electrical and biological compatibility for a home distribution system. I don't want to wear a Faraday suit when I sit down to watch my remotely powered TV set.
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Well, the public limits seem to be 0.92 Tesla hertz. So if the field is uniform, then you can in principle get up to 0.92 volts per meter squared of coil area per turn of the coil irrespective of frequency. That seems to represent a usable voltage.
A copper wire half a millimeter wide has a resistance (if I have my calculations right) of about 64 mOhms per meter; so for a square coil that's up to 3.1 watts per turn; it's in the ballpark of a useful field, and you can always add more turns, or use a larger diameter conductor (probably thin copper pipe is best at these frequencies). If I've got my calculations right, that wire weighs about 9 grams per turn.
The occupational limits give 4.7 times higher power though.
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wolfekeeper
Are you treating this as a transformer?
I don't see where the resistance of the wire comes into it. The resistance of the secondary winding is not usually a consideration. If you get about 1v per turn then this is an emf which, in series with a source impedance, can be used to feed a load from. The problem is that, unless you get good flux linkage (as in a normally designed transformer) the leakage inductance (? is that the term?) severely limits the current you can draw. How can you limit this leakage inductance if there is no core?
Do you have an answer about the Q issue and the actual amount of Power which could be transfered? You see, I think you are leaving out some important considerations.
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The resistive losses are important due to the maximum power theorem. You can get up to half that 3+ watts delivered to the load.
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That is not the case. No one builds a transmitter with resistance deliberately added into the output stage. You match your load to the generator by presenting the conjugate impedance to the source. The max power theorem, of course, applies in some cases but the impedance you match to is the one you're stuck with - the actual generator. In fact, very few power supply systems use a matched system, rather using a low impedance source. This wastes much less power. Consider the internal resistance of a car battery (or even the mains); you would never match your load to that or it would blow up.
But I can read between the lines of your argument. You are really envisaging a very low power system where total power consumption is not an issue but cordless connection is necessary.
I still wonder about compatibility with local electronic equipment.ω
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I was just trying to give an example of what you could achieve, while keeping within the standards. Adding more turns gives much more power.
As an example of what is easily achieved, there's a short range example here (about 4-6 cm):
http://hackedgadgets.com/2008/12/05/magnetic-levitation-light-bulb-uses-wireless-energy-transfer/
They don't state efficiency, but it's not always the most important thing.
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Actually, adding more turns to a transformer secondary increases the emf. The power will depend upon the impedances of the load and generator and the matching circuits etc. etc..
As you say, efficiency may not be important - but that will be true only for very low power devices.
I can't really imagine doing away with 13A sockets around a room and replacing them with this system of yours.
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That is not the case. No one builds a transmitter with resistance deliberately added into the output stage. You match your load to the generator by presenting the conjugate impedance to the source. The max power theorem, of course, applies in some cases but the impedance you match to is the one you're stuck with
My point was that the receiver coil has mass. Clearly if you make the receiver coil big and heavy, then you will have a very low-loss system; but you get a heavy coil to carry around. The number I got from the example is in the same ballpark as batteries, so it looks like it would be practical from that angle.
The other side of the coin is the losses from the transmitter coil. Clearly the bigger and beefier you make that, the better- it's generating a field, and most of it misses the receiver.
That's not inherently harmful (you're just shuttling energy into and out of a magnetic field) unless:
a) you get too much resistive losses in the transmitter (I think that the sending loop should really be a *big* flat sheet of aluminium).
b) it's absorbed by some wiring loops (basically, earth or live loops). In the UK a lot of power uses a ring mains arrangement, that could be a bit of a pain, although it might be possible to put a tuned filter around the wire to kill if it resonates.
c) your next-door neighbour decides to go for a free-ride off your 'leccy. [;)] (Presumably you would check energy sent and compare it with how much energy your own appliances are using.)
d) the relevant authorities come to demand to know why you're radiating like crazy [;)]
But I can read between the lines of your argument. You are really envisaging a very low power system where total power consumption is not an issue but cordless connection is necessary.
Yes, basically, you're looking for a system that isn't too heavy to carry but can supply power in the say 20-80% efficiency sweet spot for up to maybe 100 watts per item with range up to a few metres.
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I could ask why you would actually need cordless communication devices in the home in the future. (I can't think what else you would need such low remote power) If this technology were possible then your friendly home computer could recognize your voice and do anything you wanted. They only use tricorders on USStsrship Enterprise when they are off the ship.
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Well, I already do use cordless communications devices, such as cell phones, wireless headphones, computer mice, remote controls, laptops.
With wired headphones in particular I find that a cord is a huge pain. I keep forgetting about it and end up tripping over the cable. It's dangerous and I get through a lot of headphones. Wireless headphones work pretty well, but then I have to recharge the batteries relatively frequently. Today for example, I changed the batteries in both my headphones and my mouse.
Laptops are even worse.
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Fair comments about the cordless devices you (and I) use but keyboards and mice are only passing technology.
I think we should restate the problem and then decide which solution is best. We need personal inputs of sound and video and we need to make our needs / questions known. I think we can ignore the situation with power tools, which need a lot more power than we could consider supplying wirelessly. We are only dealing with the requirement to carry information.
Batteries are very small and light - the ultimate case would be the battery in a watch which you fit and forget (literally) for years. My ipod battery lasts for hours. A laptop battery could supply the screen for several times longer if it didn't have to run the disc and whizzbang processors. In any location where this wireless power supply could operate, there could be a wireless information network with a much greater range which could handle all the data to feed a laptop 'dumb terminal' from a central server. The battery need only last for a day - everyone sleeps.
Input devices can all communicate passively - in fact we are surely aiming at using our hands and voices, only. But wireless communication requires tiny levels of power in the devices - could even be done with passive transponders.
If you really want to use wireless power transmission, then it can be achieved, as you suggest, but is it really the best solution in all the cases you quote? The best engineering matches the solution to the need and not the other way round.
I do find wireless energy transfer very useful in one application - my rechargeable electric toothbrush- a perfect solution.
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I think we can ignore the situation with power tools, which need a lot more power than we could consider supplying wirelessly.
Not necessarily. Peak power is a lot higher, but average power is very low. So you just stuff a battery in them so that they work for say, half an hour continuous and then recharge wirelessly.
Power drill batteries are often only about ~35 watt hours or so. That's perfectly doable wirelessly.
We are only dealing with the requirement to carry information.
Batteries are very small and light
About the same power/weight as a receiver coil in fact.
My ipod battery lasts for hours. A laptop battery could supply the screen for several times longer if it didn't have to run the disc and whizzbang processors.
Actually, I think the screen is the biggest single drain; you can turn the rest off most of the time, the screen has to stay on and has a back-light.
If you really want to use wireless power transmission, then it can be achieved, as you suggest, but is it really the best solution in all the cases you quote? The best engineering matches the solution to the need and not the other way round.
For things like headphones and ipods it may actually be lighter than the batteries.
I do find wireless energy transfer very useful in one application - my rechargeable electric toothbrush- a perfect solution.
It's OK. I used to have a non wireless electric toothbrush, it worked just as well.
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And what of efficiency of these systems? just how much power would you need to transmit over 100 feet to light a 100 watt bulb at the far end of your garden? how big would the transmitter and reciever be? a receiver for each appliance? and remember the transmitter would need to be plugged in, it would need to transmit energy for the receiver to power up.
At the end of the day a single piece of copper wire across the floor must be a better deal than a transmission system the size of the empire state building.
This is another one of those ideas from "The raving lunatics and Tesla society"
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This is another of the things that 'could be' done if there were no alternative and under some very special circumstances. Wireless transfer of information is proven to be worthwhile. The reason that power transmission hasn't take off is because there are several fundamental limitations. The limitations are not imposed by technology per se.
If you have a source of wireless power in your home then how can you be sure that there is nothing around which will be dissipating that power - either by chance or by design, by your thieving neighbour? We are supposed to be preaching efficiency, these days.
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Yeah, but still, be realistic- we're talking about things that burn a few watts-hours per day here; whereas the average house burns ~kilowatt hours per person per day, and the average net footprint of people in the UK is... i forget, maybe 50x higher still.
If you're the kind of person that turns off their cell phone to 'save power' rather than extend battery life, then I'm forced to laugh at you.
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Yebbut, in the same breath (keystrokes) you are talking about power tools and a lot more.
And WHY? - in nearly every case. Just because you can and because you fancy doing it that way. That is not the Engineer's Way.
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If you have a source of wireless power in your home then how can you be sure that there is nothing around which will be dissipating that power - either by chance or by design, by your thieving neighbour?
You do it the normal way. You measure the current drawn, and compare it with the amount used. If there's a significant difference you investigate.
Any leech would have to have a circuit that resonates at a particular frequency; it's not hard to track that down. TV license vans do much the same thing.
The only show stopper that I'm aware for this kind of thing is the frequency- you'd have to operate it at a frequency that wasn't in use.
It's not particularly expensive, it's not outrageously inefficient, it seems to be safe (you can design it within current occupational limits) and it's more convenient. [O8)]
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I've just remembered a novel (by Robert Forward) where the (space)ship AI system was a sort of semi-distributed robot of sorts, comprised entirely of nano-mechanisms that were powered by laser light.
See: http://en.wikipedia.org/wiki/Rocheworld#James_.28The_Christmas_Bush.29 (http://en.wikipedia.org/wiki/Rocheworld#James_.28The_Christmas_Bush.29)
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If you have a source of wireless power in your home then how can you be sure that there is nothing around which will be dissipating that power - either by chance or by design, by your thieving neighbour?
You do it the normal way. You measure the current drawn, and compare it with the amount used. If there's a significant difference you investigate.
Any leech would have to have a circuit that resonates at a particular frequency; it's not hard to track that down. TV license vans do much the same thing.
The only show stopper that I'm aware for this kind of thing is the frequency- you'd have to operate it at a frequency that wasn't in use.
It's not particularly expensive, it's not outrageously inefficient, it seems to be safe (you can design it within current occupational limits) and it's more convenient. [O8)]
Only to find that your neighbors ornate ring fence is glowing white hot....
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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. The other way to go is to have low frequency so that the skin depth very much deeper than the thickness of conductors; that way the magnetic field just goes through the item and little percentage loss of the field occurs, but that's probably not as good for the power transmission.
The worst case is actually when the skin depth and the item thickness is comparable.
So, what I'm saying is that the fence is unlikely to soak up all the energy; and the powers we're talking about are only 50 watts or so anyway.
But not every site would be suitable for this kind of system, anymore than wifi works everywhere; cos it doesn't.
FWIW there's a reasonably good example on instructables of the basic technology (but his is by no means state of the art):
http://www.instructables.com/id/Wireless-Power-Transmission-Over-Short-Distances-U/
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assuming nikola tesla was'nt lying, and was crazy enough to accomplish this, his claim was that he lit lightbulbs, through the air 40km away from the power source, how would he do this? i can see transferring electro-static energy in the air by using something to focus the energy stream, like electromagnets, or some type of oppositely charged energy.
hmmmmmmm.......
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I think that the current position is that he didn't manage to wirelessly transmit power over such a long distance; the 40km thing seems to have been him using AC current down wires, and the two projects seem to have been confused at some point.
He certainly did transmit stuff over much shorter distances though; a lot of Tesla coils have no direct electrical connection between the top and bottom of the unit and the power is transmitted over several feet through air.
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Woolfkeeper
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.
Litz wire gets over the skin depth problem, to some extent - it uses many strands of thin wire, insulated from each other. It's a real sod to solder! Plus, it costs.
The capacitative / radiative losses are ridiculous at those sorts of frequencies. You would launch a lovely ground wave with a long wire fed with 1MHz.
I think Nicola T would be turning in his grave if he could hear all the misunderstandings about is work. (Unless he really was just a showman - which I don't think he can have been). There can be nothing that he could have achieved that wouldn't be done much easier and better nowadays. He would probably be chuffed to death at how many people charge their toothbrushes with "witricity", these days!
Fearlessmoto
What can you possible mean by focusing a beam of "electrostatic energy"? To transfer energy, the fields need to be changing- i.e. electromagnetic waves.
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Woolfkeeper
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.
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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.
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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.
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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?
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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. [;)]
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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.
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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.
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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?
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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.
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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.
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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).
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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.
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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.
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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.
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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.
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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)
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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 ;-)
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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.
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.
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.
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).
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.
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.
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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.
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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.
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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!
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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.
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I can see that we're only disagreeing about max power and distance. We shall just have to see what develops.