Witricity - I'd like some opinions

[ford2go]

witricity -- I'd like some opinions

Witricity is the name of a company that hopes to sell wireless power transfer. I question their basic physics, and I'd appreciate any comments. I have no commercial interest in any of this -- it's purely an academic exercise. Also, I'm not trying to rip this company -- I'm just using the name for a reference. I would be glad to be proven wrong.

One of the main folks at this company is from MIT, and I've seen some of his papers -- more math than I'm comfortable with, but I still disagree. (perhaps foolishly).

The concept that they espouse is resonant coils. The transmitting and receiving coils are resonant, and they are claiming that this boosts the efficiency of the transfer. One of the examples that they sometimes use is the shattering of glass via a specific sound frequency.

I disagree based on the following:
1. The receiver resonance is degraded when power transfer becomes significant. The 'Q' of a resonant circuit is related to the resistance in that circuit. In this case, it would seem to be a parallell LC circuit with the power drawn being represented by a low value resistance -- also in parallel. For any serious power transfer, I believe that the parallel resistance would essentially degrade the resonant effect, until it was negligible.

2. The glass shattering example would not seen to be applicable. This is a short term event, and does not really represent continuous power transfer. If I understand it correctly, sonic waves at the resonant frequency of a glass object will be more easily sustained. There is no power being drawn from this system, the glass is merely vibrating at its natural frequency.
At some point, the vibrations cause enough physical shifting to overcome the structural bonds. There is efficient coupling of the sonic energy to the glass. However, there is no output from the glass, except for the brief amount when the glass shatters. A better example might be one showing how mechanical power could be derived by exciting a tuning fork with the appropriate sonic energy.

Again, I welcome all responses.

 

[Bob S]

This witricity wireless system (conceived by Nickola Tesla?) is almost certainly less efficient than using an extra meter or two of power cord for an appliance. Any individual who wears a chain necklace or a metal wrist watch band should be wary of this wireless unit. Also, is it safe for pets? In the end, witricity is a gimmick that adds a quirky feature without adding any obvious value.

 

[turin]

Also keep in mind that ALL wireless electromagnetic transmission from transmitter to receiver, w/o exception, is wireless power transfer, and a signal cannot be received unless power is received. Another interesting point is that early AM radios, like in the 30's, did not even require their own power supply. Rather, they were literally powered off of the broadcast signal.

I believe that your point about low Q is quite valid in this case. However, there may be some strange near-field effect that they're exploiting. Can you give references to this MIT guy's papers?

 

[ford2go]

If you google witricity, it will get you to the company. From there you can find the main theoretical guy -- His last name starts with sou I think. More googling should eventually get you to a white paper -- I've seen it, but I didn't keep it.

 

[Ouabache]

However, there may be some strange near-field effect that they're exploiting. Can you give references to this MIT guy's papers?

I first read about this a week ago on BBC - Technology News, in article titled: http://news.bbc.co.uk/2/hi/technology/8165928.stm" . This reference also includes a video.

They do make mention of the near field in that reference. Following is a quote:

...at a distance of less than one wavelength - the "near field" - (the field a transmitter would emit) is almost entirely magnetic.

Hence, Witricity uses low frequency electromagnetic waves, whose waves are about 30m (100ft) long. Shorter wavelengths would not work.

There are a couple of scientific http://www.witricity.com/pages/papers.html" on this topic. They were referenced on the WiTricity website.

Wireless Power Transfer via Strongly Coupled Magnetic Resonances, by Adres Kurs, Aristeidis Karalis, Robert Moffatt, J.D. Joannopoulos, Peter Fisher, Marin Soljacic, Science, Vol 317, No 5834, pp 83-86, June 6, 2007

Efficent wireless non-radiative mid-range energy transfer, by Aristeidis Karalis, J.D. Joannopoulos and Marin Soljacic, Annals of Physics, Vol. 323, Issue 1, pp. 34-48, June 8, 2007.

 

[Born2bwire]

A quick glance at the BBC article leads me to believe that this is definitely in the realm of feasibility. He laments the fact that we have all these gadgets and devices that run off of batteries that we usually use not far from a power source. We like the batteries because we are not tethered to an outlet more so than the need for having power when an outlet is inaccessible. Wireless power transmission is feasible on the order of a few feet or meters like they are suggesting. I know a researcher that is trying to design antennas for medical implants. Instead of having to perform surgery to replace an implant (or have a physica jack interface, if that is even possible, I don't know) with an antenna that will receive a low frequency signal to recharge the battery. This allows you to recharge your implants without having any kind of invasive surgery. This does not seem too different from that.

So the idea to wirelessly recharge various low power gadgets without having to plug them into an outlet is a nice idea. I think I have also heard of really near field rechargers where you would place the item onto the pad itself. The nice thing being is that you only need a single recharger (depending on the mood of cooperation of the industry) for your various phones, mp3 players, game devices, wireless mice, etc. These are nice idea of convenience.

 

[turin]

So, basically, they've invented the transformer? Whoopty doo. If you have to be within meters of the power source, then how is it better than an extension cord?

 

[Bob S]

Nikola Tesla invented the wireless transformer in about 1899. See this 1919 article by Tesla:
http://www.tfcbooks.com/tesla/1919-05-00.htm

 

[jtbell]

If you have to be within meters of the power source, then how is it better than an extension cord?

I have ten or so electronic devices sitting on the shelves under the TV in my living room. If I could power them wirelessly, I wouldn't have a clutter of power cords and power strips behind the shelves.

 

[ford2go]

Transmitting electrical power over a few feet is feasible -- I just don't believe that it's very efficient. I did find somewhere on the Witricity site, a reference to transmitting 40 watts @ 60% efficiency. They didn't go into detail about the efficiency, and I'm not going to speculate.

They do seem to be talking about power sources a lot larger than a couple of AA batteries.

My main point of discussion is the claim that the resonant transformer mechanism enables high efficiency over a distance. I maintain that with any significant loading, the resonance of the receiveing coil is of little or no value. I don't see any significant benefit to the resonance. As noted, I will gladly be proven wrong.

The note about the radios is interesting. Crystal radios were used in the early days ( pretty much gone by the 30s), but they were not wonderful. Because the crystal (diode) has no power source, it loads the tuning circuit, reducing the selectivity. They used high impedance headphones to help minimize this loading. I had a couple of toy ones when I was a kid ( just after dirt was invented). Connecting the antenna clip to a good link (e.g. the metal on a telephone dial)would work. However, I could only hear one station -- the most powerful one in my area. Tuning had little effect other than to reduce the signal at the extremes of the tuner. While the signal mechanism was far field rather than near, it does illustrate how the receiving resonance was impacted by the load.

A side note on Tesla (only my opinion) :
I don't think that this is the same concept as a Tesla coil, which uses a resonant
transformer to efficiently produce an extremely high secondary voltage. Power derived from Tesla coils is generally in the form of lightning arcs. Fluorescent bulbs can also be activated in the vicinity of the coil. They are spectacular -- but ultimately probably not practical.