Is there a limit to how much power could be transmitted wirelessly?

Question

This question kind of goes back to Nikolas Tesla's ambitious project of transmitting power wirelessly. I know that project failed for multiple reasons, but my question is more straight forward. I know that the power decreases with the reciprocal of the distance square, among many other factors, but let's say there is a hypothetical ratio station transmitting 1 kW of RF power, and let's say there is a receiving antenna very close by, let's say a few feet away with a clear line of sight; will that antenna able to receive hundreds of watts of power? Without getting into environmental conditions, gain, resonance frequency and all that; is it physically possible for an antenna to received hundreds of watts of power wirelessly? It just seems to me that there is a limit to how much power antenna can actually receive, and if so, what is that limit?

Answer 1

Using directional antennas, hundreds of watts have been transmitted to drones. Further, a bit higher in the EM spectrum, lasers have been used to remotely power unmanned aerial vehicles (UAV's).

In fact, there is even a proposal for the converse: powering remote sensors using energy in the 915 MHz band transmitted by UAV to ground-based devices.

That said, while there is no physical limit on the transmission of power by electromagnetic radiation (Sol has been broadcasting 3.8 x 10²⁶ W continuously for a few billion years; a supernova, about 10³⁷ W for days), efficiency drops with distance. [But -- see caveat.]

There has been talk of space-based solar power (SBSP) transmitting RF energy to Earth, but the ramifications are many -- cost of launching photovoltaic arrays, energy density and heating effects on Earth, etc.

Caveat

User71659 mentions that there is a theoretical limit of ~10²⁹ W cm^-2. Given a radiator of the surface area of the visible universe (~10⁷⁴ cm², by a very rough estimation for a sphere ~14 x 10⁹ light years in radius), then the limit is ~10¹⁰³ W (or not -- a radiator could have folds).

_{At last -- an everyday use for a number greater than a googol.}

Answer 2

Many attempts to use microwave beamforming to perform Wireless Power Transfer (WPT) have been successful in theory at low powers but fail to meet safe limits of W/m power field per MHz for human tissue damage at high power levels.

However, with appropriate shielding and ferrite absorption to satisfy FCC & IEC limits, many WPT implementations for RF 75 kHz and up for battery charging vehicles have been successful in the 11 and 22 kW range some with very high efficiency (96%). This has been done overhead for a transit bus and underneath for parked trucks and cars.

Tesla's dream was not a reality partially due to significant transmission losses.

Answer 3

Much of the work on power transmission takes place in the near field. When the transmitting and receiving antennas are coils, this system is better modeled by thinking of it as a weakly coupled transformer rather than as an RF system.

There are companies that have proposed using microwaves for power transmission. This would be a far-field application that can be modeled as RF transmission using antennas. A major obstacle to high power operation is regulations concerning maximum RF energy exposure. The other technical obstacle is inefficiency.

The Paul Reynolds website https://liesandstartuppr.blogspot.com/ contains much discussion of proposed wireless power transmission schemes.

Answer 4

The proponents of space-based solar power stations believe that GW can be transmitted and received by microwave RF. From what I've seen it's more likely that it is possible than it being economical. The idea is to have relatively low energy density, not far off of terrestrial solar energy density (!) so that it can't become a giant death beam microwaving flora and fauna in its path. That's also about all that's possible from geosynchronous orbit. The receiver might be a rectenna array.

It's fairly speculative, and the folks involved have a variety of motivations, but in any case there have been small scale demostrations of power transfer. Next year a Japanese experimental satellite is scheduled to attempt a transfer of about 1kW from orbit to earth. I've seen proposals for much larger demonstrations but not funded ones.

If you don't care about cost, size, efficiency, etc. there are probably no hard limits.