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MirceaKitsune

Some questions about wireless charging

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I recently saw a documentary about wireless charging devices. It didn't go into quite a lot of technical detail, but it explained how it uses magnetic forces to transfer electricity over the air. The whole discussion made me curious about a bunch of things, regarding the idea of wireless charging:

 

1 - Rather than using a magnetic field, I often wondered whether this could be done over short distances with an electric arc. The question of course is if it's possible to have electric arcs over distances of even a meter, but which are not dangerous to any living being touching it. From what I know, this is sadly not possible... as the voltage requited to get an arc over such a distance should be very big. After all, even the arc in an electric lighter where the two metals are milimiters away from each other stings badly if you touch it. But maybe there's still a balance which can create a harmless arc? Like filling the environment with a special gas, or use smart materials?

 

2 - Getting back to wireless charging via magnetism; From what was explained, it sounds like this is a rather simple technology, and something that could have been done in the past. Instead, the system was said to only become available later this year or next year. Why didn't wireless charging reach the market during the late 00's as worst?

 

3 - How far will wireless charging via magnetic field be able to go? It's nice that it will allow you to charge your phone by putting it in a pot... and I saw that parking lots are getting such chargers under the pavement for compatibility with electric cars even. But will it be possible to power entire houses with it? And is there a chance that in the future, such magnetic fields could encompass whole cities... meaning that laptops and mobile phones will permanently have their battery charged wherever you go?

 

4 - I know that realistically this is very unlikely, but I'm still curious: If someone were to build a huge generator or battery... say a few hundred meters tall. Could it convert the Earth's megnetic field into any useful amount of electric current? In an actual scenario however, how much electricity could be extracted from the Earth's magnetic field? I'm estimating that one would need an enormous device to even power a little led with it, since the field is probably not that strong. Still, I do believe that small satellites and space stations obribing Earth might be able to do this in order to regenerate power... in case none are already.

Edited by MirceaKitsune

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The 00's? I don't think modern electromagnetism was even developed until the 1800's, and that's when changing magnetic fields began being used to cause electrical flow. Even late that century Tesla developed a system that could power lamps remotely.

 

Wireless power transfer can also be done via lasers and receivers in a similar manner to solar radiation / panels. Though the solar power hitting the ground isn't strong enough (esp. under the atmosphere), a directed laser could power anything from far away, even charging cars considering that the power is being transferred at c. But, the power density required for such things would probably yield ionizing radiation which can burn things and cause cancer and the conversion from electricity to laser back to electricity is very inefficient where losses can be <40%.

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2 - Getting back to wireless charging via magnetism; From what was explained, it sounds like this is a rather simple technology, and something that could have been done in the past. Instead, the system was said to only become available later this year or next year. Why didn't wireless charging reach the market during the late 00's as worst?

 

 

I've had an inductively-charged electric toothbrush for years. The technology has been available for quite a while. The wireless charging for e.g. phones people are working on is probably not the same thing.

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Small correction: By 00's I meant between 2000 and 2010.

 

And yeah, I can see how a very strong magnetic field could even lead to cancer. Cell phones in the 90's were believed to be cancerous from what I remember, which is why IIRC there is a limited range of frequencies which are considered safe to use.

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1) You can do this with IR, as was demonstrated a few years back. Arc? Well you can make an arc, but are you shooting for the least efficient and most dangerous wireless charger on the planet, i mean gas is a horribly poor conductor? You can make really long arcs, have you heard of lightning? Unfortunately, the longer you make an arc, the lesser the safety of the said arc is, and no arc is really all that safe...

 

2) Tesla

 

3) How far do you need it to go, and how much power are you willing to provide?

 

4) Earth's magnetic field already has a useful amount of power, northern lights are a proof there-of. Problem is that you need a really long antenna to harvest it, superior conductive properties wouldn't hurt either... Think something the length of east to west coast of the US for example, or if you want to work in extreme environments, something a bit shorter, but closer to the magnetic poles...

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Of course, I know about lightning. My question there was if any technology can create an electric arch (lightning blast) over distances of even < 1m, but which is safe for people to touch and it not lethal or painful. I do however imagine that no amount of technology can achieve such a thing.

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You can create an arc that is < 1m long, you need a lot of voltage to do this. Following Paschens law, the breakdown voltage necessary to jump 1m of air is 3.5x10^6V.

 

There are many problems with your idea however, and no, it can not be made safe. First problem is the current of an arc has to be stabilized, because an arc will only stop at the power supply level, this is a problem. Arc looses a lot of energy to photon and heat emission as the current travels through the plasma. And then there is the safety of the arc itself, the moment you speak of a an arc, you are talking about plasma heated so much that it emits photons; you are talking temperatures in 18-19K degrees C. This much heat may cook the proteins in humans, considering that Medium Rare steaks are cooked to 54 degrees C, and an exposure to a water at that temperature is painful at any time over about a second (from many, many personal experiences).

 

Containing this kind of power takes a special kind of a power unit, which is not undoable, but it is not for the faint of heart, nor is it light or portable...

 

In short, this is not easy, this is not safe, this is not practical, this is not efficient; in short, this is not a solution to wireless charging.

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That makes sense, thanks for clearing it up. I probably should have researched how electric arcs work before asking, since the temperatures and necessary condisions seem to speak for themselves... and what you said clearly sounds like nothing even close to safe. So yeah, wireless charging via electric arc (which people and animals could touch without risk) is surely not an option.

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1a - The house of Ampère, at Poleymieux-au-mont-d'or, now a museum for electricity, displays an X-ray generator whose voltage is stabilized by a parallel arc. The operator adjusted with a screw the arc length, like half a metre, to set the X-ray generator voltage. As you guessed, this must have been uneasy: scary noise, electrocution, production of X-rays there. We're speaking about 100kV+.

 

1b - Capacitive coupling would be possible. It's more sensitive to conductors than magnetic coupling, for instance a limb must disturb it.

 

2 - I made the RFID in 1989. Induction supplies power to the chips on the tag or card. Charging GSM phones could have been made then if only GSM had existed. And ol' Nikola had done it long before.

 

3 - The range equals essentially the diameter of the biggest coil, usually the base station. Then, you may also decide how strong fields you accept in your home, and how much power you want to waste. A near-field doesn't radiate power and ideally only takes the power delivered to the receiver, but in real life you'll use copper to create the field, and the resistivity creates losses.

 

3b - In 1989, only the electric field was limited by law and science considered that electromagnetic fields had only thermal effects. As well, an RFID card uses to serve for few seconds in a day. I wouldn't cover a house nor a city with permanent strong magnetic fields, as an elementary precaution, and because science and law can evolve.

 

3c - The induction created by the reader, and by the card itself, lets electronics go crazy. It's like 0.2V/cm2 and is a difficulty when designing an RFID card. Other electronic devices work only because they're far from the RFID, which should hence be small.

 

3d - At the RFID demo I wasted 7W to provide 100mW to the card.

3e - Chargers for hand-held phones are hopefully more efficient. This requires big coils close to an other.

3f - I wouldn't like to waste even 20% when charging a car. Efficient coupling by 2m2 over 0.5m isn't obvious. If a connector saves 5% or even 2% power over the induction method, the connector likely wins.

 

4 - Only changing magnetic fields can provide power. Earth's field changes too little for that.

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4 - Not if you have a wire long enough, which for cars would not be achievable. And another thing, you can deliver power via other em spectrum frequencies, microwaves or IR for example. I'd be interested to know how much if any efficiency would be gained by that...

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Wireless charging - or the equivalent of - is what goes on inside a transformer.

 

As for charging over distance (or the equivalent of) I can recall someone living near a radio transmission station installing a large coil in his loft to draw power for some of his domestic alliances! Unfortunately this caused a shadow area in the radio transmission, which enabled the authorities to eventually trace the cause. I understand he was fined under some sort of legislation.

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Wireless charging - or the equivalent of - is what goes on inside a transformer.

 

As for charging over distance (or the equivalent of) I can recall someone living near a radio transmission station installing a large coil in his loft to draw power for some of his domestic alliances! Unfortunately this caused a shadow area in the radio transmission, which enabled the authorities to eventually trace the cause. I understand he was fined under some sort of legislation.

 

Now that's interesting. You can actually draw power from a radio transmission, by sucking in the signal and converting it to electricity? How does that work exactly?

 

This makes some sense actially. Since at the end, radio transimissions are just infrared light. And we can already convert photons to electricity, solar panels being the most common example. So theoretically at least, a solar panel sould be able to generate a current from any radio transmission too, right? Practically of course, most radio signals are way too weak for that... unless you're right near an antenna like this guy.

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This makes some sense actially. Since at the end, radio transimissions are just infrared light. And we can already convert photons to electricity, solar panels being the most common example. So theoretically at least, a solar panel sould be able to generate a current from any radio transmission too, right? Practically of course, most radio signals are way too weak for that... unless you're right near an antenna like this guy.

 

Radio transmissions are not infrared light, hence the different names. They are both electromagnetic radiation, but at different frequencies/wavelengths. (IR is around a micron to around a millimeter, radio is the longer wavelengths)

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Electric and magnetic fields in EM radiation cause electrons in conductors to move around.

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Well wireless charging doesnt have to mean floating electricity. It could be manually generating electricity, like how some of those flashlights work, in which you spin the handle and that charges the flashlight.

 

Sources: I have one of those flashlights

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Now that's interesting. You can actually draw power from a radio transmission, by sucking in the signal and converting it to electricity? How does that work exactly?

In simple terms, in exactly the same was as any induction method. Perhaps I should say he was quite close to the transmitter - like adjacent to the field containing the transmission aerials.

 

 

This makes some sense actially. Since at the end, radio transimissions are just infrared light. And we can already convert photons to electricity, solar panels being the most common example. So theoretically at least, a solar panel sould be able to generate a current from any radio transmission too, right? Practically of course, most radio signals are way too weak for that... unless you're right near an antenna like this guy.

They are not infrared light. Infrared light is electromagnetic radiation of a completely different frequency. If you're characterising radio waves as infrared light, then on the same premise, your microwave cooker would cook food with infrared light.

 

And as for solar panels, they work with the much higher frequency of light. I think it is stretching imagination to say the particular mechanism by which photovoltaic cells work could be scaled up to operate at radio frequencies - since as far as I'm aware they work directly at the molecular level.

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Aha, I understand now. Yes I was thinking about the fact that both radio and light are are at their core electromagnetic radiation. I didn't know they're so different though, due to simply being different frequencies... hence why for a moment I thought a solar panel might work for both theoretically.

Edited by MirceaKitsune

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I didn't know they're so different though, due to simply being different frequencies... hence why for a moment I thought a solar panel might work for both theoretically.

I'm sorry, but I don't know why you are apparently surprised. They infrared and radio waves are both electromagnetic radiation. Electromagnetic radiation is an oscillation of magnetic and electric fields. Now, oscillation presupposes that there is a rate or frequency of said oscillation. If they were both the same frequency of oscillation they wouldn't be different. We give them different names because their frequency of oscillation is their distinguishing feature.

 

With radio waves the wavelength (another way of expressing frequency) is long enough for us to construct tuned circuits out of bits of metal and the like - for example a TV aerial is a tuned circuit together with coils and capacitors, or lecher lines for higher frequencies, in the associated receiving and transmitting devices. But with infrared and the like, the frequency is so high (very short wavelength) that turned circuits become molecular in size. In other words atoms etc become the active devices in reception and transmission.

 

Now the bizarre aspect of electromagnetic radiation is the wave particle duality. Reduce the intensity of a source of radiation and it appears to be particular in nature. These particles we call photons. I understand physicists have discussed this apparent contradiction for some time, but I further understand the answer lies in quantum mechanics.

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For some reason I had the (potentially stupid) idea that some types of solar panels could work like radio receivers; Tune them in to a given frequency, and they respond to electromagnetic fields on that frequency... in this case by charging something. Because the underlying physics are the same... electromagnetic fields, for both sunlight and satellite light if you will. I understand now that's probably further from reality than I thought.

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Radio waves used for telecomms aren't concentrated enough to be interesting - by far. It's like some kW spread over many km2. Radio beams can and do transmit power if purposely concentrated; a demonstrator was built at La Réunion. Far less efficient than wires, but sometimes you don't want wires.

 

Photovoltaics can't work with radio waves and isn't done, because the photon energy is less than the mean temperature energy (26meV at 300K). Visible light around 2eV fits, but radio waves have 104 to 1010 times less. The proper setup is an antenna or coil coupled with a rectifier like diodes.

 

Antennas do work a light frequency. Kraus proposed it in his book "Antennas" very long ago when the technology was completely out of reach, and few yeas ago it has been done. Diodes are still extremely bad at these frequencies so the combination is worse than photovoltaics, but antennas that concentrate light's field to smaller locations of a photovoltaic cell are claimed to have advantages.

 

Presently, wireless power is made by magnetic fields mainly, as (1) they radiate less far hence are more acceptable (2) they show a decent efficiency (3) they are less sensitive to the surroundings (4) at least in 1989, law limited only the electric radiation. Electric field, or electromagnetic, would be possible, but to my feeling less convenient.

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Enthalpy, please reread his comment (post #20).. He didn't want solar panels to work with radiowaves frequencies..

Edited by Sensei

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Enthalpy, please reread his comment (post #20).. He didn't want solar panels to work with radiowaves frequencies..

I think he did: "For some reason I had the (potentially stupid) idea that some types of solar panels could work like radio receivers".

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It must be language barrier..

 

For me "work like radio receiver", is not the same as "work as radio receiver".

 

"Because the underlying physics are the same... electromagnetic fields, for both sunlight"

Sunlight is visible light.


The "problem" with "working solar panel like radio receiver" (but for visible spectrum) is such that we WANT to get energy from photons with as wide as possible range.

Limiting to just f.e. 650 nm +-10 nm would decrease output power from such device..

 

Radio receiver is receiving at wide range of wavelength - but it's electronics inside fine tunning to specific frequency (they overlap with all others).

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Sunlight is visible light.

 

Yes, but that's because it's the frequency the eye is tuned to.

 

Otherwise I agree with the other comments about using radio waves for charging: Normal wireless transmitters simply spread a weak signal in all directions, since the purpose is simply having enough strength for it to travel hundreds / thousands of kilometers. Even if it could be harnessed by a special solar panel theoretically, it should only produce a silly amount of electricity.

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