Wireless Electricity

Some magazine* ran a small article claiming we’re not far from receiving electricity by wireless. Uh, excuse me, but didn’t Tesla want to go this route ages ago? Has anyone heard what may be new in this field?

*I’ll post the source later today. I have to ask my daughter what she was reading. It was some family-friendly type magazine. Nothing heavy like Popular Science, and nothing too shady like Slutz Monthly. :wink:

Howstuffworks article
The leading current product, as far as I know, is powermat.
They apparently sell custom battery covers for a variety of devices which can be charged on the mat.

The Master has spoken (though rather long ago).

In addition, there are an assload of relevant threads in CCC. The most applicable ones (in that they discuss specific, recent wireless power concepts) are probably uh oh…Cecil done gots PWNED!. "What’s up with ‘broadcast power’?, and Broadcast power, the last of which has links to even more threads.

Thumbnail summary: of course you can transmit wireless power, if you’re willing to accept miniscule amounts and horrible efficiency.

I think I read somewhere that there has been an advancement. Part of the problem with wireless power is that it wastes a lot. IIRC, this is gotten around by resonant coupling the transmitter and receiver.

I still don’t know why this works. I’ll bet it is still pretty inefficient, and I don’t think that power companies will be beaming their power to you anytime soon.

Tesla’s system would have made modern radio, cell, TV, etc impossible from all the interference. It also was horribly inefficient. If youre powering homes wirelessly then you’re going to need something like 10x the power plants to keep up with demand because of the inverse square law. So, its not exactly a plug in replacement for the current system. Its an economic and environmental nightmare.

Wireless power has been here as long as Tesla’s time. Its just not feasible for many applications. The only two successful consumer applications I know of are charging a Sonicare and charging a Palm Pre via inductive charging. I know a guy who worked in AV installations that worked with wireless power. He said it was a pain. He had to point an antenna at a receiver to power a projector. So the implementation matters.

Right now there’s a push to standardize on some wireless power scheme for phones, mp3 players, etc. In a few years we will probably just lay our toys on a power mat overnight that will charge them. That’s probably what you read about. Its still inefficient but the distance are so short between the surfaces (a few millimeters) that its not a big deal for portable devices that dont use all that much power to begin with. You wouldnt do this with a hair dryer or space heater.

[my bolding]
THANK YOU! I’ve had one of those for several years and have always wondered how the battery charges without a direct metal contact between the base and the handle. So, am I using a ridiculous amount of electricity just to recharge an electric toothbrush?

There’s an inverse square law in place, so when the transmitter and receiver practically touch like with the powermat or in a toothbrush charger, it’s not that inefficient.
Very small devices (not cellphones or the like) can also be powered by Wifi signals within a few meters of the router.

I have a Seiko Kinetic watch that I have an inductive charger for, but you have to orient the watch just-so on the pedestal. Do these other types of chargers, like the mats, require the device to be in a particular orientation?

I have heard that the palm pre cradle holds the palm in a particular orientation. The tooth brush chargers also hold the brushes in a specific way. The engineering demos of the mats I have seen only require the device to be placed on the mat. But I have not seen a charging mat outside the lab.

Id love to see a credible and tested by 3rd party cite for this. The last I heard someone was selling snake-oil about doing this, but when you consider a 500milliwatt transmission and the inverse square law its not possible. It would take years to charge a cell phone battery.

Resonant coupling gets around the inverse square law. Like I said, I’m not entirely sure why, but apparently it does work. I expect that it still is quite inefficient, but a group at MIT was able to power a 60 Watt lightbulb from almost 6 ft with 40% efficiency. I’m pretty sure that 40% efficiency is more efficient than a straight up inverse square would give you.

Sorry, I don’t have a good cite. But as I said, it’s not for cellphones, only very undemanding things, a small sensor or switch or something.

The inverse square law only applies to power being radiated equally in all directions, like light from the Sun. It doesn’t apply to a collimated laser beam, say. Focusing an electric field will increase coupling efficiency, as would making the transmitter and receiver self-resonant at the same frequency, but neither of this tweaks would be easy to implement.

That said, research into this area is still one of the biggest wastes of time I can think of - the last thing an energy-hungry society needs is a monstrously inefficient means of transporting electricity just for the sake of it. I could easily demonstrate this to the researchers involved by slapping them with an ostrich feather, and then my fist, and let them decide which method is more effective at delivering energy.

Despite my vitriol, I’d expect this technology to become popular in making mobile phone charging mats, though this would involve the major phone OEMs deciding a common standard between them, which they probably will. They’ve already been collaborating to produce a universal (wired) phone charger, which is a good thing.

CookingWithGas may not be pleased to know that a similar principle is at work with induction hobs. These are an outstandingly bad idea that should have died a death on the marketplace, but they’re still available to buy if one favours novelty over a cooker that actually works well.

It depends on the size of the antennas. Basically, for the transmission to be efficient, the size of the antenna needs to be comparable to the distance you’re transmitting the power. So a charging mat several centimeters on a side, with a several-centimeter phone sitting on it a millimeter away? High efficiency. But for something that can fit on a house, getting power from a plant that services an entire city? Dream on.

Actually, it does apply to laser beams, too, just with a much smaller coefficient. A laser beam that didn’t spread out would violate the Heisenberg uncertainty principle.

No, you’re using a method which uses more electricity than a direct connection but the trade off is its easier to clean, use, charge and maintain the brush.

A typical transformer transfers power via magnetic fields between two coils that don’t touch each other.

Your brush and it’s base are like a transformer except the distance between the non-touching coils makes it a little less efficient than a typical transformer but not so bad that it’s a huge waste of power.

I posted something like this on another forum recently:

It would be possible for the banks to all get rid of ATM machines, and instead dispense cash by putting powerful blowers at centralized locations, and blowing hundred-dollar bills in every possible direction - you just grab the cash you need.

This is about as practical as broadcast power.

CurtC, I just showed that to one of my office-mates, and he pointed out that if a stack of hundreds misses your bill-catcher and hits you instead, it won’t kill you. So the bill-blower is actually more practical than Tesla’s scheme.

The OP might have seen an article going around the last couple weeks about a company “WiTricity” (www.witricity.com). They’re getting some buzz about providing wirelsss power to laptops and other small appliances. It involves hanging what looks like picture frame on the wall. To me it sounds like just another approach based on inductors. And the appliances have to be witricity-ready. I paid attention mostly because they’re in Watertown MA, right next door to my town.

There has also been work done in South Korea to power cars and buses using resonant transfer technology. I described it in my blog.


IIRC Bombardier is also using similar technology for some of their new light rail. The lack of physical contact makes the system easier and since the energy transfer is magnetic instead of electrical it is also safer.


It is also being used for wireless charging for conventional battery powered electrical cars.