Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: David Skytte on 15/05/2010 22:30:03
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David Skytte asked the Naked Scientists:
Hi
In your most recent show (Beer and Brain cells) (http://www.thenakedscientists.com/HTML/podcasts/show/2010.05.09/) there was a segment on future homes which mentioned various envorinment friendly trends.
But they also talked about hot spots around the house for charging, amongs other things, mobiles and laptops.
My question then is: Is wireless energy transfer also more energy efficient due to less power lost in cables, or is it actually less efficient as extra energy is needed to "beam" out electricity? In other words, would my electricity bill go up or down if I were to change all my outlets to wireless?
Thank you for a great programme (http://www.thenakedscientists.com/HTML/podcasts/).
David Skytte
What do you think?
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If the technology is working properly, it should not make much difference. "Hot spots" are not working properly if they are coupling energy into things other than the intended load. If they are "leaking" energy, they'll be heating up nearby metallic objects.
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Transferring power via a wireless system will always be a relatively inefficient means but the gains in convenience may well justify it, if you can place your mobile phone or lap top computer on the shelf and know that it will be fully charged when you pick it up a may be worth while paying a bit more for the power.
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The idea of "hotspots" is to allow charging, often by "nearfield" em radiation, conveniently and inadvertently rather than efficiently. Contactless charging can be reasonably efficient if the em radiation is guided (more or less what wires do) or by inductive coupling (what transformers do efficiently), but this is not the point in this case.
Before worrying about power wastage it is worth looking at the power used in everyday activities. I have not calculated this, but I would expect making a cup of tea uses much more power than fully charging your mobile from flat for example.
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It's not inconceivable that hotspots could be more efficient. If they are smart enough to detect the presense of a real load, they might dissipate almost no energy when they have no load.
This might result in less wasted energy than having a lot of charging devices and power supplies plugged into the line that are not connected to any load, which, I suspect, is what a fair number of people actually do.
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I await the GFETD (Geezer focussed energy tranfer device) with interest :-)
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I await the GFETD (Geezer focussed energy tranfer device) with interest :-)
My toothbrush already has one [;D]
We probably would not want to call it the "Geezer Energy Transfer Focusing Device".
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That's "GETFocD" - OOooo, eh, I see your point!
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Ahem! Anyway, isn't it just a question of how "lossy" (no - not lousy) these things are. They may not be all that effective at transferring energy, but that might be OK as long as they don't go coupling a lot of energy into places where energy should not go.
The current (npi) situation may be worse in the US than it is in the UK. Power outlets in the UK have switches, so people may be inclined to switch inactive power supplies and chargers off. In the US, there are no switches, so you have to pull the thing out of the socket to turn it off, and people being what they are, tend not to do that, so at any point in time there are lots of transformers and PSU's idling and wasting energy.
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Ahem! Anyway, isn't it just a question of how "lossy" (no - not lousy) these things are. They may not be all that effective at transferring energy, but that might be OK as long as they don't go coupling a lot of energy into places where energy should not go.
The current (npi) situation may be worse in the US than it is in the UK. Power outlets in the UK have switches, so people may be inclined to switch inactive power supplies and chargers off. In the US, there are no switches, so you have to pull the thing out of the socket to turn it off, and people being what they are, tend not to do that, so at any point in time there are lots of transformers and PSU's idling and wasting energy.
That brings up an interesting question. If these things don't dissipate energy when it's not needed, what keeps current transformers from having a load-detector in them and cutting power down to a trickle when there's no load? Is there some technical reason you can't do this, or is it just not worth the cost vs. the savings to consumers?
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That brings up an interesting question. If these things don't dissipate energy when it's not needed, what keeps current transformers from having a load-detector in them and cutting power down to a trickle when there's no load? Is there some technical reason you can't do this, or is it just not worth the cost vs. the savings to consumers?
Do you mean current transformers or current transformers? (Sorry, it's an EE joke. EEs don't have a lot of jokes btw.)
Er well, I think it's largely a cost thing, and that might be the advantage of having a centralized, easy to use, charging centre. If all the stuff is in one place, it can be made a bit more sophisticated by amortizing the increased cost over a lot of different devices. It can also create a standard interface for a lot of different devices so you don't end up with a drawer full of incompatible chargers that end up going into landfills.
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Geezer, the reason most portable bits of kit are not simply pluggable into the mains is that they don't want the weight of the charger added to the kit. It is a separate item and can be reasonably universal (see Apple's designs). An half efficient Near-field RF power harvester would also be a bit bulky if it needs to gather a similar amount of power. I think the trade off is convenience (and no wires) against efficiency, but only on very low power items. A laptop charger is big/heavy because it charges fast. It would not be desirable to have it built in. A very low power RF charger would be good if you could just throw the laptop into a corner and let it get on with it overnight.
Generally an RF charging "space" is a good idea but it won't be efficient, just convenient, and it won't charge things fast.
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The efficiency for wireless power can be up to about 85-90% at close range, but it drops off to about 50% at 1-2 coil diameters, or a bit further than that with care and if you're not bothered about safety (i.e. if you're not going to be holding the equipment while it charges; it's going to be completely safe for short exposure to the field as long as you don't sit in it all day or something).
When I did the calculations a while back it looked like the weight of the charging system should be very, very roughly the same as the weight of a similar power of battery, so for low charge rates that's not a problem.
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wolfekeeper, I guess you are speaking of highly directed near-field power to get 85%-90% efficiency and (obviously) rather less than 1 coil diameter away. This is more what I would call a transformer :-) It is really the magnetic field coupling factor between two coils. I think the point is that this is an almost contact system that just avoids the wires (some kettles use such a system) and it can be quite efficient. I got the impression that what was being suggested was a more generic "region" to harvest RF power.
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It's not directed energy, it's what's called 'near field'. In this case it's an oscillating magnetic field. Oscillating magnetic fields on their own don't radiate, they hang around close to the coil, just like an electromagnet. If you have a varying electric AND magnetic field then they form a radio wave and radiate off, which you don't want, because they tend to go off in all directions and that would be very inefficient.
It's not a conventional transformer, because that needs a magnetic core. It's sometimes called a resonant transformer.
The Witricity system's hotspots work as resonant transformers.
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Hmm, oscillating magnetic fields do radiate, it's just that the field falls off as roughly the inverse cube so quite rapidly. Transformers don't have to have a core although the one's we are used to seeing most of certainly do. RF transformers can be just two co-wound air-coils for example.
However, I remember seeing something that you describe demonstrated once a while ago. They dramatically transferred several kilowatts of power whilst walking about between the two coils. It is about resonantly coupling the two coils. I never followed it up in detail, but you are right that quite high efficiency can be attained this way. I must take another look into it all when time permits. If you have any references...
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Hmm, oscillating magnetic fields do radiate, it's just that the field falls off as roughly the inverse cube so quite rapidly.
To some extent it's a question of proportions, here the conditions don't radiate to infinity very significantly, nearly all the power is near field. Also the receiver tends to create a counter field which helps.
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Wolfekeeper:
When you mentioned "efficiency" above, was that a measure of the effectiveness of the coupling? Am I right in thinking that at 50%, the total power consumed has also been significantly reduced?
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It's complicated, but basically power and efficiency and distance are interrelated, but if you're getting 50% losses you'll probably get about 50% less power at the receiver or perhaps a bit less; most of the rest goes as resistive losses.
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It's complicated, but basically power and efficiency and distance are interrelated, but if you're getting 50% losses you'll probably get about 50% less power at the receiver or perhaps a bit less; most of the rest goes as resistive losses.
Ah! So I think you were referring to real energy losses. Thanks.
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Yes, efficiency is always power out/power in. Maximum power transfer goes down with distance also.
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Overall efficiency tends to be low with wireless sysytems ...not only poor coupling efficient but conversions between AC and DC and RF and back again. By comparison the losses in a few metres of wire are pretty neglible.
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Well, less, but energy efficiency isn't really everything. I've got wireless headphones, and there's a charging system but it's quite fiddly- it has to plug in. Using wired headphones often when I got up I would forget and trip over the cable, which is dangerous and broke the headphones. The power they use is very low anyway, so efficiency isn't really much of an issue, and within reason, isn't much of an issue for most portable equipment; it's a couple of watt-hours of power. If it was only 10% efficient that would only be 20 watt hours per day (and actually it doesn't need recharging every day anyway), a fiftieth of a kilowatt hour. The average person is using 2-4 kilowatts or so continuously.