June 10, 2007
MIT Electromagnetic Energy Beam

Energy beams are possible.

Researchers at MIT have shown that it's possible to wirelessly power a 60-watt lightbulb sitting about two meters away from a power source. Using a remarkably simple setup--basically consisting of two metal coils--they have demonstrated, for the first time, that it is feasible to efficiently send that much power over such a distance. The experiment paves the way for wirelessly charging batteries in laptops, mobile phones, and music players, as well as cutting the electric cords on household appliances, says Marin Soljačić, professor of physics at MIT, who led the team with physics professor John Joannopoulos.

Notice at the bottom of this post that I filed it under "Energy Transportation". Why? They achieved 45% efficiency. But at a shorter distance they achieved 70% efficiency. Suppose they can get this up to 90+% efficiency. That would open up the possibility of recharging an electric car automatically when it parks in the right spot. That would solve a basic problem with shorter range electric cars that are used for daily short range commuting. Drive a 10 or 20 mile daily commute, park in the garage, and next morning the car will be fully recharged by an electric power transmission magnet in the garage. Ditto for, say, special parking spaces at workplaces.

Sounds like this approach has the potential to have little of the energy leak away.

Moffatt, an MIT undergraduate in physics, explains: "The crucial advantage of using the non-radiative field lies in the fact that most of the power not picked up by the receiving coil remains bound to the vicinity of the sending unit, instead of being radiated into the environment and lost." With such a design, power transfer has a limited range, and the range would be shorter for smaller-size receivers.

They call it WiTricity.

In contrast, WiTricity is based on using coupled resonant objects. Two resonant objects of the same resonant frequency tend to exchange energy efficiently, while interacting weakly with extraneous off-resonant objects. A child on a swing is a good example of this. A swing is a type of mechanical resonance, so only when the child pumps her legs at the natural frequency of the swing is she able to impart substantial energy. Another example involves acoustic resonances: Imagine a room with 100 identical wine glasses, each filled with wine up to a different level, so they all have different resonant frequencies. If an opera singer sings a sufficiently loud single note inside the room, a glass of the corresponding frequency might accumulate sufficient energy to even explode, while not influencing the other glasses. In any system of coupled resonators there often exists a so-called “strongly coupled” regime of operation. If one ensures to operate in that regime in a given system, the energy transfer can be very efficient.

While these considerations are universal, applying to all kinds of resonances (e.g., acoustic, mechanical, electromagnetic, etc.), the MIT team focused on one particular type: magnetically coupled resonators. The team explored a system of two electromagnetic resonators coupled mostly through their magnetic fields; they were able to identify the strongly coupled regime in this system, even when the distance between them was several times larger than the sizes of the resonant objects. This way, efficient power transfer was enabled. Magnetic coupling is particularly suitable for everyday applications because most common materials interact only very weakly with magnetic fields, so interactions with extraneous environmental objects are suppressed even further. “The fact that magnetic fields interact so weakly with biological organisms is also important for safety considerations,” Kurs, a graduate student in physics, points out.

I find the ability to use cordless appliances less interesting than charging cars because this approach sounds pretty directional.

The car charging problem has other potential solutions. For example, imagine a garage where a robotic apparatus automatically come down from the ceiling or up from the floor to plug into a receptacle.

What I wonder: Is there any health risk to an electromagnetic energy beam?

Share |      Randall Parker, 2007 June 10 05:11 PM  Energy Transportation

Jerry Martinson said at June 10, 2007 6:35 PM:

I'm not an expert but I think the IEEE limit for Pointing vector in uncontrolled environments is 2 to 4W/m^2 in the RF frequency range. So I think this looks like at least 60W/m^2 so it is over that limit. However, this is likely between 500kHz and 10MHz so I'm not sure exactly what the limit is. The human body absorbs very little in this range and the other limit that is set, which is more important, is the SAR or "specific absorption rate". This is where this technique does very well compared to current state of art as most of the other powercasting schemes are in the RF/Microwave range which gets absorbed by human tissue. Quite a bit of research has been done lately in the RF and Microwave bands on biological effects and mostly they've revealed little other than heating effects, though the hunt continues. At the lower frequencies (but above power line frequencies), it isn't as much recent research. I do know that there are companies testing devices that are mostly electric field in the kHz range for cancer treatment.

However in the application that you suggest, charging a EV or PHEV while parked or stopped at a light, even if the Pointing vector magnitude is high, with some smart electronics and field containment all the exposure could be under a parked/stopped car where no reasonable person would be.

A concern I'd have with a device like this is that it might not couple to ordinary materials but a circuit in many common products is a synchronous buck rectifier also uses magnetic fields (albeit mostly tightly contained in a transformer) but they sometimes run at 1MHz - roughly where this scheme is proposed. There might be odd effects in some of these circuits if they are in sync if the field strength caused by this approach is within 1% of the field strength of a power conversion circuit in the transformer. It might also produce some interesting patterns on CRT screens.

K said at June 10, 2007 8:41 PM:

Ordinarily I would dismiss a proclaimation on radiated electricity very quickly. We all have some bias and I scoff at powercasting - perhaps unreasonably.

But these people are at a top school so they are not playing parlor tricks. It still sounds iffy.

I don't see this at home for recharging EVs or PHEVs. That takes a lot of power. And it it isn't needed anyway.

The EV-1 by GM had a very simple charging scheme and no connection hassles. Better yet, let a robot do it.

A garage robot should be able to spot the car, hook on, charge, disconnect and store itself. When a driver got in the car it would disconnect. Or we we could program charge times. The robot could speak with the car via Bluetooth protocol.

That isn't rocket science. If we can't create a robot to do that for a modest cost then what in hell are robots likely to prove for?

Randall Parker said at June 10, 2007 9:52 PM:


I agree about the robots for charging cars. The car should be able to say with Bluetooth "Here I am and I need electricity.". Then the robot says "Where is your connector" and the car says where it is with some sort of homing beep sound or radio wave transmission. Image processing capability in the robot could help it move its connector into the car receptacle.

Jerry Martinson said at June 10, 2007 10:20 PM:

So I actually read their papers and photos and stuff. They need to get better media relations for this as the experiment setup is visually impressive but I haven't seen it in any of the articles. After reading the article, they are under the Pointing vector limits for the frequency ranges that they are operating on so safety isn't an issue unless unknown new hazards are uncovered (and existing technology that this would replace already has many hazards so the risk doesn't have to be zero).

Few things impress me but this is a real breakthrough that will certainly have industrial applications. Some technical refinements look easily possible that could increase the range, efficiency, and power to make it suitable for parked car charging. The EV-1 systems out here in NorCal that I've seen weren't all that cumbersome to handle and there would be other ways to automate this so I'm not sure that it would be terribly earth shattering.

A running car needs about 20kW to move at highway speeds. Looking at the contraption and their scientific papers, it certainly looks possible to develop a contraption that could continuously couple 20kW transfered to a moving vehicle that has the surface area of a car and is embedded a reasonable depth into a standard highway lane, if newly constructed. However, I'm not sure that one could be made durable enough to be embedded in a resurfacing project.

Considering amortized financing and maintenance it costs about $200,000 per year for a conventional urban freeway lane mile that handles 40000 cars per day. That's about $0.14/mile/car. Gas costs about $0.12/mile for average nickel-per-watt ICE engines.

Say one were to make already proposed "platooning" lanes in congested areas that had one of these contraptions in it (so the cost could be amortized across more cards), you could get about 300,000 cards per day per lane on a platoon lane all powered with electricity. Even assuming a meager 60% efficiency with this scheme, electricity costs only about $0.04/mile. So a lane mile of this would save about $10M a year in energy. If the average car spends 15 miles on the system per day, it'd save about $5k per vehicle life in energy. Clearly this looks plausible economically. It seems that this is something that DoE/DoT could look into.

Brett Bellmore said at June 11, 2007 4:22 AM:

"Beaming" is exactly what this isn't; Essentially they're running an antenna specifically designed to be bad at radiating, and coupling into the magnetic field rather than electric, because it interacts less with most materials. The field stays local, and doesn't lose (much) energy by radiation, because the ratio of frequency to field size is exactly wrong to generate EM waves.

I can see this being used to charge low power consumer electronics, such as cell phones. Maybe children's toys, too. I doubt it would ever be used to charge electric vehicles, because you're never, IMO, going to get the effiency of this to the point where you'd want to be dumping that much power across such a link. Think of this: You've got to charge an electric car at, say, 50-100kw. Assume 95% efficiency. (Unattainable under realistic circumstances, I think.) Your link is going to be radiating into the enviroment 2.5-5Kw of EM energy. Even ignoring the waste, I suspect the FCC would have something to say about that...

No, robotic plug insertion strikes me as a much more realistic option, especially with the cost of robotics dropping, and the high efficiency available from wires.

Nifty application of EM theory, though.

Videokarma said at June 11, 2007 8:23 PM:

There was a recent article in "Wired" magazine that talked of this technology.. If I find the article I will post here.. The implications and applications are great and many.

rsilvetz said at June 12, 2007 9:19 AM:

Well, Tesla managed to do this at least once. Reportedly he had a 5-order of magnitude power-range. I look forward to the development of this tech.

Although I'm still terribly impressed by Tesla's use of telluric currents. It must have been something to see fire hydrants sparking.

Aime Watts, Jr. said at June 13, 2007 7:38 PM:

Tesla was finally stopped in his transmission of power to everybody when JP Morgan realized that there was no way to 'meter' the electricity captured by the user. This will, undoubtedly, inhibit this technology, too. I can't say as that I blame Morgan for pulling Tesla's financing for the Long Island transmission station, but it is regrettable. Although I do not usually see a use for the government to get involved with the free market, perhaps they might be the very outlet for running broadcast power. Have the Army Corp of Engineers build the transmission stations and have the government finance it all through a flat rate tax based on utilization. That is, the number of vehicles using the grid, square footage of homes on the grid, etc. (The grid being the users of transmitted power.)
There is little doubt that Tesla was transmitting huge amounts of power, the only question was where it all really came from. Oddly enough, he seemed to be harnessing the differential between the ionosphere and the ground using harmonics and resonating frequencies. If that wasn't it then it remains a mystery what was actually being transmitted, more power was coming off his transmitter than could be accounted for coming into it. It seemed like his transmitter was only powered by the incoming electricity but it synchronized, something, and made it available in a series of waves emanating from the transmiter... or the transmitter was resonating something and thereby making it usable. What is clear was that he wasn't simply transmitting what was coming in. Damn, if only he could have fooled Morgan for a few months more...

Brett Bellmore said at June 16, 2007 3:39 PM:

"This will, undoubtedly, inhibit this technology, too."

No, not really; The range on this technique is so short, it's only marginally harder to meter than wall sockets. Tesla's power transmission scheme was continental in range.

donald wilkins said at June 17, 2007 11:53 AM:

Fascinating. If the system could power moving vehicles you could start small with say, Israel or Hawaii. Make it work for an "island" to prove concept then expand into Jordan or Lebanon. I suspect that you would still need an ICE for times when power is not available or for off-road travels. Getting rid of batteries in the hybrid would reduce significantly costs and improve reliability.

How about powering robots? Elimination of power systems on the robot would reduce weight and cost.

Third thought was powering surgerical mechanisms within human body - real internal medicine. The magnetic field should not harm living tissue. One could imagine a piezoelectric snake eating its way through clogged arteries or one with appropriate proteins patching up damaged areas

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