Wireless power delivery has been on the research agenda for well over a century and the first patent was issued in 1894 in the wake of experiments conducted in the early 1890s by Serbian-born Nikola Tesla, in whose eminent memory the famous electric car company was named. His work led quickly to successful demonstration of power transfer via near field inductive or capacitive coupling, now well established in various commercial systems from induction hobs for cooking to mobile phone charging cradles, as well as medical implants such as artificial cardiac pacemakers.
But the dream of eliminating power cables can only be achieved through far field transmission of electricity, and that has proved much more elusive. Now it turns out that development of dense small cell architectures for 5G millimeter wave spectrum, at frequencies above 6 GHz, could help overcome this longstanding challenge, at least initially for low power applications such as charging IoT sensors.
The potential to deliver electric power over the air via high frequency electromagnetic waves has been known some years, but we had to wait for 5G for the investment in infrastructure to yield candidate components. Before 5G, high frequency power transmission had proved rather a graveyard for start-ups as a number tried and failed – some, such as uBeam, also trying ultrasound waves rather than RF as carriers. But then Guru Wireless came along and in 2017 succeeded in raising venture capital to develop the technology of RF lensing that had been under research for 20 years in academia, operating in 24 GHz spectrum.
This was demonstrated at the Consumer Electronics Show in early 2020, transmitting enough power over a few meters to illuminate a light bulb. This was rather reminiscent of the first demonstration of nuclear fusion delivering power to a kettle, but wireless power delivery now looks set for more rapid progress on the back of mmWave with beamforming and dense antenna arrays.
While Guru might be an isolated example of a start-up in the field just now, a paper just published in the journal ‘Nature’ by researchers at Georgia Institute of Technology sets the stage for more widespread deployment of power over mmWave, exploiting 5G infrastructure.
The starting point is that high frequency waves do not just provide more spectrum but the wavelengths also allow smaller antenna and therefore denser transmission arrays. This in turn encourages multiple beamforming, concentrating the waves such that information transmission via line of sight is possible over large distances before the signal has attenuated too far. Such beams also increase the amount of electromagnetic radiation and therefore power that can be concentrated on a given surface area at the receiving end.
However, the requirements of power transmission are slightly different, dependent even more on harvesting as much radiation as possible. For this the Georgia Tech team adopted the Rotman lens system already developed for 5G beamforming arrays, working logically like an optical lens even though the physical mechanism is subtly different. An optical lens relies on insertion of a medium such as glass that slows down the waves, so that with lens curvature a given wave front emerges at a different angle from how it entered.
The Rotman system instead exploits the slightly different times taken for beams to reach each part of the receiving system, enabling them to be identified and processed accordingly. This can eliminate interference, or focus power. The lens is then adopted as an intermediate component between the receiving antennas and the rectifiers for 5G energy harvesting. The structure can then map a set of selected radiation directions to an associated set of beam ports.
Insertion of this Rotman lens increased the amount of power that could be harvested from beams by 21 times according to Georgia Tech, transmitting at this stage 6μW (6mths of a watt) over up to 180 meters. This would be enough only to power a single typical IoT device while asleep, at around 5μW, but might be sufficient for intermittent operation. This is then still very much work in progress and yet there are hopes that rapid strides will be taken so that excess capacity over future 5G networks could be used to deliver power alongside data to IoT devices, although charging more energy intensive devices such as smartphones would more challenging.