WPT breaks all connections, Part 1
Wireless power transfer (WPT) has been the subject of intense interest, bordering on folklore, for over a century. The arrival of the modern cell phone with its innumerable “apps” and rising battery consumption, has recently sparked off a huge upsurge in consumer demand for wireless chargers.
WPT is still maturing as we speak. Mysterious signs have started popping up all around us — in airport kiosks, hotels, coffee shops, inside our cars, even on branded furniture. These aren’t teasers placed surreptitiously by Night Shyamalan, but logos of the three dominant, competing standards shown in Figure 1. We also see a high-level diagram of modern WPT technology in the same figure, showing the basic process of wireless power transfer through coupled coils as per these standards. All the three standards, despite some differences, are broadly called “inductive MPT” in this article, because they share the same underlying physical principle: the law of induction, as we will learn shortly.
And we are just getting started! Soon we may start hearing about exotic techniques such as “evanescent wave coupling” and “dielectric resonators”. We may even find ourselves immersed 24/7 in fields of “ubiquitous energy”. Then again, we may not, for health reasons. So, while the debate is still on, and our future speculative, surprisingly, so is our past. Only one thing seems to be certain: WPT as we know it today, didn’t start in its current form, despite appearances. The story has enough twists and turns to keep us on the edges of our seats for a while.
Tracing back the history of WPT is tricky business. We may stumble over some misleading hype strewn along the way. There are potholes of hyperbole too. We may even start to wonder whether a part of us somehow wants to believe in the unbelievable: in the mysterious, the mythical, the magical — in light sabers, wormholes, powerful beams of inter-galactic energy, and fanciful stuff like that. That tendency of ours may indeed have helped to some extent, nominate Nikola Tesla (1856-1943), a brilliant engineer in his own right, as the rockstar of WPT today. In a rather quaint move, a certain company at the forefront of wireless power technology refers to their CTO as the Chief Tesla Officer.
Really? You can almost visualize him driving around in a Model S sedan, dressed in a black shirt emblazoned with bolts of lightning to complete the picture. Tesla must surely be glowing from far beyond upon hearing of all this. But the truth is, we still don’t know the truth. We need to ask: What fraction of the legend is fact? How much of it is just that: a legend? How exactly did we get here? And where are we headed? This article tries to answer that, and also serve as a high-level review of the current state of WPT.
Tesla Burns Rubber
They say it all started in May 1891 when Tesla, a naturalized American of Serbian origin, performed a live demo of wireless power transfer in front of the American Institute of Electrical Engineers in New York City. Though no scientific records of that event exist, there is a news report on Page 2 of the archives of The Daily Nevada State Journal, Reno, Nevada, dated Tuesday, June 30, 1891.
From that mildly sensationalized description, it seems Tesla first used his now-famous “Tesla coils” to generate 250-kV across two large zinc plates facing each other several feet apart, and then waved an entrancing glowing tube between them. But based on what we know today, this entire setup was perhaps just a large capacitor, and the resulting glow was probably just electroluminescence at work within a long phosphor-coated gas-filled or vacuum tube placed between the plates. Tesla seems to have been part of the intervening dielectric. But discounting any nervous breakdowns, there was definitely no incandescent bulb being lit wirelessly as is often claimed. However, Tesla did think about the possibility of wireless power, or “broadcastable electricity” in general, well before we ever watched Star Wars, or complained about our cell phones beeping annoyingly just
before running out of power at 2am.
Figure 1: The basic WPT process and the logos of the competing inductive standards.
The main principle Tesla seems to have exploited in his demo was based on a static electric field (electrostatics). Whereas in modern WPT, we typically use a time-varying magnetic field — which then induces an electric field (voltage) at some distance in a copper coil. We collect the electrical energy from the coil and condition it to charge our phones. This latter principle was discovered by Michael Faraday way back in 1831. It is our familiar law of magnetic induction, used in every single transformer made. It was the same principle Tesla used to create the high initial voltage and resulting steady electric field in the first place, but perhaps not the principle, or reason, behind the observed glow. It’s not for no reason that Albert Einstein kept pictures of Michael Faraday, Isaac Newton and James Maxwell in his study room — no one else’s.
If we think about it, we do transmit a huge amount of energy between the two adjacent coils of any transformer quite effortlessly. And we do it “wirelessly”, i.e. without galvanic contact between the two distinct coils (before 1831 that too must have sounded like science fiction). The problem however is, the physical distance between the transmitting and receiving coils (windings in this case) is very small, and there is also usually a shared “magnetic core” to channel the magnetic flux more effectively between them. So, physically speaking, this is not a practical configuration to charge our cell phones with. Naturally, we don’t want to be seen walking around with a lump of ferrite core sticking out of our khaki pockets. However, with some modifications, that very “transformer principle” did eventually get extended to charging our cell phones. And it is what constitutes WPT in the form we are most familiar with today, though with resonance principles thrown in. But that’s it, so far.
One thing you couldn’t blame Tesla for, was not thinking big. In 1899, barely a few years after his first demo, he got cracking. He unleashed several spectacular public displays of “wireless power transmission”, a.k.a. giant bolts of lightning — from a metal globe perched on top of a 142-ft long mast constructed over his 80-ft high Colorado Springs lab. On one such occasion as Wikipedia reports:
“He produced artificial lightning (with discharges consisting of millions of volts and up to 135 feet long). Thunder from the released energy was heard 15 miles away in Cripple Creek, Colorado. People walking along the street observed sparks jumping between their feet and the ground. Sparks sprang from water line taps when touched. Light bulbs within 100 feet of the lab glowed even when turned off. Horses in a livery stable bolted from their stalls after receiving shocks through their metal shoes. Butterflies were electrified, swirling in circles with blue halos of St. Elmo’s fire around their wings.”
Underwriters Laboratories (UL) would have remained thoroughly unimpressed were they around. Because, birds and bees be damned, this new demo actually involved gigantic atmospheric discharges driven by extremely high voltages generated by his now-infamous Tesla coils. Tesla also inadvertently knocked out the dynamo of the El Paso Electric Company, plunging the city into darkness for a week. He would definitely have been on the radar after that, had the radar been invented. The popular perception very soon was that Tesla was merging rapidly into the fast lane. To back that hypothesis, you could perhaps have smelled burning rubber if not cordite in the air around Colorado Springs that fateful day.
Tesla was fully charged up by then. Pretty soon he was planning on delivering, or should we say diverting, kilowatts of energy wirelessly across the Atlantic from a brand-new upcoming tower of power. In 1901 he started to build this 187-ft Wardenclyffe tower in Long Island, New York. A fellow tower was planned for the southern coast of England much later — one of 30 planned receiver towers, of which the Wardenclyffe tower would serve as a hub “city”. It would become the Constantinople of the 20th century.
The transmitted transatlantic energy was eventually going to not only include data for broadcasting, wireless telecommunications and other purposes, but electrical energy to power entire industries. The fabled “World Wireless System” was taking shape. Its ultimate goal? To provide energy wirelessly for all of Earth’s industries one day in the near future, free of charge to boot, as ordained by Tesla himself. For the initial project, Tesla had managed only a rather small investment from J.P. Morgan — about $3 million by today’s standards. Mr. Morgan may have had some serious doubts about the business model thereafter. And he did make that clear soon.
Looking up Tesla’s well-known wireless power transfer patent dated January 18, 1902, granted in 1914, it seems that Faraday’s Law was to be used once again, to first generate a very high voltage (millions of volts). That would then be used to initiate (unleash) low-frequency longitudinal waves, i.e. atmospheric discharges. The frequencies involved in Tesla’s presumed transatlantic attempt are said to be 150 kHz for the first step, and 8 Hz for the second (yes Hertz, not kilo, Mega or Giga). Tesla had clearly discovered the underlying “principle” behind sending electricity through air, in other words lightning, and also that the earth was a very good electrical conductor with which to “close the circuit” — on account of its extremely high cross-sectional area and thereby low resistivity.
No surprise therefore that Tesla did eventually get labeled a crackpot by many people, losing even the financial support of J.P. Morgan. The Wardenclyffe tower was scrapped to pay off his debts before it was ever fully completed. Tesla declared bankruptcy in 1916, and died alone and penniless in 1943.
To the persistent claim that Tesla is the person who invented WPT, there are those who still light up with distant disbelief. You might as well leave your cell phone under a tree in a raging thunderstorm, they argue. But it is also true that some of Tesla’s more than 700 US patents have lived on, and continue to shape the world we live in. Tesla is for many such reasons considered by some as the greatest engineer and innovator who ever lived.
Marx Levels the Playing Field
Not to be outdone in the department of retrospectively pissing off UL, in 1924, a German engineer named Erwin Otto Marx came up with his bigger-is-better version. Whereas Tesla coils worked by inducing extremely high voltages by using a steep turns-ratio in a special transformer, his new generator created a similarly high voltage, but by charging a bank of capacitors in parallel from a low voltage, then suddenly stacking all the capacitors in series (bootstrapping). Colossal bolts of lightning from a massive “Marx generator” were generated befittingly from a now almost defunct Soviet-era complex. This facility is occasionally revived it seems, to test aircraft against lightning strikes.
Smaller Marx generators are still in use worldwide today, to test insulators. But, as in the case of Tesla, though energy was certainly transferred “wirelessly”, we ask: Was it ever capable of being harnessed and used to charge say, lithium-ion batteries? And last but not the least, could it be considered environmentally friendly?
Soviet-era ‘Tesla Tower’ restarted with spectacular lightning bolts (VIDEO)
Brown Noses Ahead (MPT)
WPT, as a means for transferring useful energy, say to charge batteries with or operate appliances with, definitely occurred a few decades later, though still not in the “safe” form we prefer to use in today. It first appeared in the form of microwave power transfer (MPT). The enabler was the resonant-cavity magnetron, invented in 1940. The radar soon followed as a result.
Over the period 1961 to 1984 while working for Raytheon, William C. Brown (1916-99) developed his long-range version of WPT. It was in effect a powerful wireless radio-wave broadcaster. In 1963, he successfully delivered 100 Watts over a few feet, with an efficiency of 13%. By 1975 he had sent 30 kilowatts over a mile, at 54% efficiency using high-power magnetrons and special antenna. That was indeed impressive. More importantly, these events were all scientifically documented. They weren’t just flashy demonstrations using smoke and mirrors. These were therefore very likely the first real, recorded cases of useful WPT.
From a modern engineering standpoint, the key thing to emphasize here is that Brown’s proposed method used powerful beams of microwaves, the same used for communication purposes, and also in the cheap microwave oven standing there in our kitchen. We do transfer energy wirelessly in that appliance too.
Microwave Ovens and Induction Cookers
The first commercial microwave oven came from Raytheon in 1954. The use of microwaves to heat food instead of just using it for radars, was actually purely accidental, but bound to happen.
Today we use our microwave oven regularly to deliver thousands of Watts effortlessly “through air” (yes, wirelessly!) into a succulent breast of (former) chicken. But in the process, we have wisely learned to keep ourselves, and extraneous metals, pointedly out of the active space. And therein lies the basic problem with MPT: certain frequencies can be harmful to living (water-based) tissues. That includes our precious pets. We certainly would not relish a hot dog prepared in this fashion. That is why there are mandatory SAR (Specific Absorption Rate) limits imposed even on the tiny mobile phones in our hands — because these also utilize the same microwave frequencies for communication purposes.
No surprise that today, though a few methods of charging commercial electric vehicles (EVs) use focused microwave energy waves typically operating at 2.45 GHz (MPT), most others seem to be turning to safer methods based on Faraday’s Law of induction (Figure 1). The penalty everyone faces by avoiding MPT, is that the separation between transmitter and receiver coils is necessarily small, typically only a few mm. This will be explained in the next section.
Note: Many molecules, such as water, consist of electric dipoles. These rotate as they try to align themselves with the alternating electric field of microwaves. The rotating molecules hit other molecules and put them into motion, thus dispersing energy. This energy manifests itself as heat. Microwave heating is often erroneously explained as a resonance of water molecules, but this is incorrect because those resonances occur only at above 1 Terahertz (THz). That is 1000 GHz, whereas microwaves are 300 MHz to 300 GHz.
Our kitchen microwave oven works at 2.45 GHz.Induction cookers, another fairly common kitchen item, do not use microwaves, but utilize low-frequency time-varying magnetic fields, i.e. Faraday’s Law. However, instead of
harnessing the transmitted energy in a copper coil and then using that to charge
batteries as we do it today in WPT, the energy is converted directly into heat through “eddy currents” generated naturally in response to the changing magnetic field (from the opposing flux). The resultant heating due to these currents occurs in a flat metal plate on top of the cooker, which also serves as the base of the cooking pan. The earliest patents for induction heating date from the early 1900s. In the mid-1950s, demonstration stoves were revealed by the Frigidaire division of General Motors. This was however never put into production. In the early 1970s, commercial models were released for the very first time, by Westinghouse.
Wikipedia: Induction Cooking
Fields or Waves?
This causes a lot of confusion, so we need to touch upon this briefly here before
carrying on. It will also help distinguish between MPT and inductive WPT. The
electromagnetic spectrum is shown in Figure 2. We can observe the frequencies used for MPT (2.45GHz) and also for modern inductive WPT (~100-350 kHz and 6.78 MHz).
The first thing we realize is the frequency for MPT is much higher than inductive WPT. But the high-frequency aspect of MPT itself is not the real problem per se. After all, visible light has even higher frequencies and it is safe. The effect of MPT is more related to certain biological effects at certain frequencies. For that reason, mobile devices are routinely tested for compliance to mandatory SAR emission limits over the range 30 MHz to 6 GHz as shown. Not against visible light obviously!
There is another concept we need to understand: far-fields and near-fields. Time-varying magnetic fields induce electric fields, and similarly, as Maxwell showed in 1865 by extending Faraday’s Law, time-varying electric fields generate magnetic fields. There is a certain duality between them. Further, as we go further from the magnetic or electric source, the electric and magnetic fields tend to become more and more tangential to each other, and also become self-sustaining as a result since the resident energy can now keep sloshing constantly between the linked E (electric) and H (magnetic ) field vector components. We thus land up with an electromagnetic wave, such as light, which can travel great distances without attenuation. Through vacuum — forever! And it takes significant energy along with it too. See Figure 3. That is how we get heat and sunshine on our planet from the sun miles away, and how the term light-years came about in the first place. It is also why MPT can manage wireless power transfer over so many miles quite easily, provided we learn to focus it, as we would a powerful light beam.
So, as we move away from either a time-varying electric source or a time-varying magnetic source, we start getting electromagnetic waves. How far away? That’s the question. From Figure 3 we see that at distances less than 1/6th the wavelength, we will be in the “near-field” region. The attenuation will then be significant, requiring us to come “closer” to the source of the field if we want to be able to deliver any significant amount of energy wirelessly. However, at distances greater than 1/6th the wavelength, we will be more in the “far-field” region, and we will then be able to send energy over far greater distances using wave effects (distances being relative to the wavelength of course).
Let’s do the “distance math” for MPT. Its selected frequency of 2.45 GHz corresponds to a wavelength of 300/fMHz = 300/2450 = 0.122 meters, or 12.2 cm. For this frequency, “far-field” would refer to distances greater than 1/2π of 12.2 cm, i.e. 19.4 mm. In other words, even at distances barely greater than about 2 cm, we will start to get a beam of propagating electromagnetic energy (microwaves). And in fact, that is exactly what we depend on to heat up the coffee even within the relatively small confines of our kitchen microwave oven — we use waves inside it, not bare electric or magnetic fields. The latter would give us an induction cooker for example, not a microwave oven.
The moment we talk only in terms of pure electric or magnetic fields, we are implying near-field effects, and depending on the physical distances involved, we are indirectly making a statement about the rate of the time-variation involved, i.e. its frequency.
Let’s check the numbers for inductive WPT too. These methods use frequencies of about 200 kHz. At such a low frequency, the “near-field” extends up to 23,870 cm, or a quarter kilometer away from the source. The other modern inductive standard works at a higher frequency, of 6.78 MHz. For ~7 MHz, we see that its near-field region extends up to 682 cm, i.e. almost 7 meters (its wavelength being six times that). In other words, considering the frequencies we use in modern inductive WPT (with a maximum of 6.78 MHz), we need to look upon all separations up to a minimum of 7 meters as near-fields.
Specifically, we are dealing with rapidly attenuating and varying magnetic fields within that distance, not electromagnetic waves. And that is Faraday’s Law in effect. So, at these small distances, if we wanted to create beams of energy, or waves, instead of just fields, we would need to go to much higher frequencies, as we do in MPT. Though at significant risk to our personal health and safety as mentioned. That is why modern methods intended for homes have defaulted to safe (lower) frequencies of inductive WPT, and the consequent smaller distances for energy transfer, since fields attenuate rapidly. Remember: only waves propagate. Fields die out.
Lastly, keep in mind that our bodies accept powerful magnetic fields 24/7 with no ill-effects. After all, we live on a planet with a strong magnetic field. For all these
reasons, inductive WPT has become so popular.
Figure 3: Near- and Far-Fields.
Part two will cover such topics as Wireless chargers in Automotive, Modern WPT, Early phone wireless chargers, Witricity and more.