Would a room-sized coil used for inductive coupling and wireless energy transfer be feasible?

Question

My understanding of inductive coupling is that it isn't feasible over long distances because the spread of the magnetic field lines happens rapidly, so not much flux will go through the receiver coil if it's too far apart (not a physicist, this may not be exactly right).

However, if an entire room was a coil (imagine in the walls), anything inside of the room would be by definition exposed to nearly the maximum flux possible (relative to the size of that coil).

Would this be a feasible way to have wireless power throughout a room?

Reasons I could see this not working that I'm not sophisticated enough to evaluate:

• Too much power required for the transmitter coil (expensive and/or problematic magnetic field strength)
• Too little flux for receiver without also being pretty large
• Interference with other devices/Wi-fi in the room that are worse than the benefits
• Angle of receiver being too dynamic would mitigate power benefits unless stationary to align axes

Thoughts?

Answer 1

The main limitation is, the coupling factor is roughly the ratio of cross-sectional areas of the two coils in question (whole-room transmitter and some local receiver). If the resonant Q factor is as high as the coupling is low, reasonable power can still be transmitted (that is, at more than negligible efficiency!), although at very low bandwidth (due to the high Q). Power delivery into a static load (say for lighting) doesn't care about bandwidth, but a dynamic load could need significant energy storage to cover its intermittent demands.

Where power isn't too important, the technique can be used even over wide bandwidths. Audio-Frequency Inductive Loop (AFIL) is a way to deliver audio into hearing aids, for example.

Frequencies where a whole-room loop will still look like an inductive loop, are well outside WiFi range and have no effect.

High power levels as in wireless power transmission are limited by allowable field strength allowed by various standards (emissions and human exposure).

Angle I suppose isn't a problem for most portable and static uses, mostly being upright, but a pair of loops indeed has a null when crossed. If flexibility is required, a diversity strategy can be employed (e.g. three perpendicular loops feeding rectifiers to obtain DC power).

Answer 2

Having a room sized coil to transmit to a small receiver coil (within the room) is entirely feasible and, was used (and maybe still is) for museums and galleries for several decades. It's simple, effective and, inexpensive as a broadcast method.

No modulation is required; the audio was coupled directly to the room sized coil and, receivers just needed a battery powered audio amplifier to drive headphones.

I even built one in the 70s.

As for power transfer of a significant amount, it's not very feasible due to the extremely small coupling factor but, you could light an LED with a small receive coil providing the excitation was a tuned fixed frequency and, the receiving coil was also tuned.

The bigger the receive coil the more power can be extracted.

Answer 3

You'll get more power than you bargained for!

By turning the entire room into the core of a transformer, you will induce eddy current heating into any metallic object in the room.

Answer 4

Yes, it's possible but pretty inefficient. If the mutual inductance of coils is low in comparison to the self-inductance it is not good. The ohmic losses in the primary coil will be related to the power you put into the total field of this coil, while the energy transfer is related to the mutual inductance.

I would be less worried about the eddy current damping or ferromagnetic losses (eddy current penetration depths are actually not so small for low frequencies).