Two days ago, the local newspaper Karjalainen of my birth region ran an April-fools’ piece about wireless electricity transfer. According to the “article”, the entire country would soon be switching to the new technology, permanently ridding us of blackouts and transmission fees.

Obviously, this is not the case, and wireless transfer on that scale is pretty much infeasible on that scale. However, across smaller distances, this is indeed no longer the case. Very strictly speaking, gigawatts of electricity are being wirelessly transmitted all the time – over transformers. Think about it: there are no wires running from the primary to the secondary; the coupling is purely magnetic. Hurr hurr.

But there are also some applications that are both wireless and contactless. For instance, the so-called resonant inductive coupling is currently being utilized in many small-power systems. The idea is simple: we basically have a transformer with the iron core removed – so only two coils. Since only a small portion of the flux of each coil is linked with the another, the system would be very inefficient as such. Thus, a capacitor is connected in parallel with both of the coils. The capacitances are selected so that both the primary and the secondary have the same resonance frequency.

The equivalent circuit of the coupling is shown below. As you can see, it’s almost equal to a typical transformer. The most obvious difference are the capacitors in parallel with the primary and secondary branches. Furthermore, due to the weak magnetic coupling between the coils we have the relationship L_\mathrm{m} << L_{\sigma1}, L_{\sigma2} between the leakage and magnetizing inductances, i.e. the exact opposite situation compared to an iron-cored transformer.

Equivalent circuit for the coupling.
Equivalent circuit for the coupling.

Both the primary and secondary form a resonant circuit, where energy is being continuously transferred between the magnetic field and the capacitor back and forth at the resonance frequency. If the circuits have a large Q factor, these oscillations decay very slowly. Thus, despite the weak magnetic coupling between the coils, enough energy can be extracted from the secondary to reach a very reasonable efficiency of 80 % or so.

And indeed, this technology is commonly used in RFID cards for example, as well as some mobile phones and tablets. And of course, the iconical Tesla coil also works on pretty much the same principle.


 

Some more exotic examples also exist. One idea is to use a metamaterial superlens to concentrate the magnetic fields created by the primary coil, on the secondary coil located a significant distance away.

Now, metamaterials have been surrounded by an overgrown hype in the recent years. Indeed, a simple google search yields quite a few headlines about “invisibility cloaks”. Never mind the fact that pretty much all of the research only works in two dimensions, and usually for microwaves instead of visible light. Admittedly, a 3D “cloak” seems to be under construction – for sound.

However, things are much simpler when wireless transmission systems are concerned. Firstly, operation is typically desired in the transfer direction only – unlike 360 degree invisibility – with little regard to what happens elsewhere. Furthermore, the frequencies used are usually so much lower that the materials are much simpler to manufacture. Indeed, the “meta”material used for the superlens consisted of lots of small coils of 19 mm or so in the diameter, stacked inside a rectangular honeycomb.

The structure of the superlens.
The structure of the superlens. Figure source here.

So far, the results don’t seem to have been exactly stellar. Nevertheless, the system allegedly works better than it would without the meta-lens. On top of that, it looks like some scifi weapons-array, so the technology definitely is worth keeping tabs on!


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Wireless transfer of electricity

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