Many people believe wireless power transmission (WPT) to be a recent breakthrough but its discovery goes back more than a hundred years.
French physicist Ampère and British scientist Faraday, with their respective laws, contributed in the 18th and 19th century to the development of modern-day electrical engineering, which set the basis for WPT.
In 1893, Serbian-American engineer and inventor Nikola Tesla delivered a lecture where he presented his experiments with magnetic resonance, making him one of the forefathers of WPT.
While more modern experiments have been going on since the 1980s, specialists noted a resurgence of interest for WPT over the last five to ten years, especially in the mobile phone sector – where wireless phone chargers are increasingly becoming the norm.
In 2018, wireless chargers compatible with the latest models of mobile phones from Apple, Samsung and Huawei – amongst others – started to appear, hitting the headlines and opening up new market opportunities.
Around the same time, technology development companies including Ossia and Energous, claimed to be working on larger projects, where electricity could be transmitted over longer distances.
What is WPT and how does it work?
David Schatz, business development vice-president at wireless charging company WiTricity defines WPT as “using a magnetic field to transfer power between a source and a device, without using a wire to connect the two.”
WPT is not a single method and there are a few techniques to obtain wireless energy transmission, including induction, resonant coupling and microwave energy transfer.
Induction is obtained via the transfer of energy between two coils at a very close distance, while resonant induction coupling is based on resonant tunnelling.
Within a five-metre range, electromagnetic waves that have a high angular waveguide are called evanescent waves, as they carry no energy. When the waveguide connects with the transmitter a tunnel is formed: this energy can be converted into electricity.
For longer distances, WPT is obtained through radio and microwave energy transfers, which function through a rectenna, a rectifying antenna which converts microwave energy into electricity.
While the first two methods are considered to be safe for our health, radio and microwaves can have negative effects.
Professors Frank Barnes of the University of Colorado-Boulder and Ben Greenebaum have published a book on the health hazards of electromagnetic fields, discovering that seven megahertz is a problematic frequency for humans, enhancing cancerous cells’ proliferation.
The QI, KI and medium standards
Founded in 2008, the Wireless Power Consortium (WPC) is a standard development group made up of more than 500 companies aiming for the compatibility of wireless chargers and power sources.
In 2010, the consortium launched the world’s first wireless standard, the QI standard.The QI standard has a 5-15 watt range, operating at a frequency between 105kHz and 205kHz, useful for mobile phones.
The WPC has two other standards currently in development, the KI standard, which powers up to 2200 watts and the Medium standard, with a 30-65 watt range.
The KI standard will be used to power kitchen appliances including rice cookers and toasters while the medium standard will charge e-bikes, drones and vacuum cleaners.
WPT and larger applications: from robots to power grids
Home appliances is not the only sector that will increasingly rely on wireless power transmission, as more companies are already using wireless power chargers for robotic applications like drones and fin factories.
In order to produce charging solutions for the aero, industrial and mobile sectors, Seattle-based wireless producer WiBotic has patented a wireless energy mechanism that mixes induction and magnetic resonance.
WiBotic’s Adaptive Matching system consists of a transmitter unit that uses the power source to generate a wireless power signal, which travels to a transmit antenna coil.
The antenna generates both an electrical and a magnetic field, which, when recognised by the transmitter unit, ramps up to deliver the right amount of energy, which is later collected by an antenna on the robot and conveyed to the charger.
Another WPT method is the ball-joint, a wireless charging system that can be turned in any direction. University of Hong Kong and Imperial College London electrical engineering professor Ron Hui is currently working on the project, specifically made for manufacturing facilities.
Professor Hui explained that the structure consists of a ball structure with a mechanical rod attached to it and a socket that accommodates the structure. The wireless energy is transmitted via magnetic resonance, from the transmitter to the receiver, achieving up to 81% of energy efficiency.
“Imagine robots in a manufacturing facility: their arms have to move all the time, and if they are endowed with cable, by continuously twisting it, the cable will break,” said Hui.
“With the use of a ball joint, we can transmit power through the joints of a robotic arm, without using any cable”.
Hui is also working on the world’s first independent harvesting and power supply for smart grids. The project involves harvesting energy around the high-voltage cables and transmitting it wirelessly to an online monitoring system in the transmission tower, reducing the reliance on batteries.
In order to transmit power between the transmission towers, professor Hui and his colleagues have patented a wireless domino power system.
“It’s like a domino arrangement: we have all these resonators which will allow us to pass power one by one – and that will significantly improve energy efficiency. With this technology we can get energy efficiency over 60%,” said Hui.
The future of electric vehicles
The debut of the world’s first electric vehicle wireless charger – produced by BMW in 2018 – kickstarted a succession of companies developing wireless chargers for electric vehicles and mobile robots.
WiTricity – an MIT spinout founded in 2007 – patented a form of wireless energy transfer called highly resonant power transfer, which has the ability to work over slightly longer distances, ranging in the tens of centimetres.
The highly resonant power transfer consists of two devices with matching resonant frequencies coupling into a single magnetic field, transferring power from one device to the other.
Thirteen years later, WiTricity’s systems have a charging rate varying from 3.6 to 11kW and can power different types of car, including sports cars and SUVs.
Schatz says that: “Carmakers are trying to make the electric vehicle to be superior to the gasoline-powered vehicle in every possible way. And wireless charging is seen to be a very, very important way to make electric cars even better than gasoline-powered vehicles.
“The idea is that you simply park your car in your garage or your parking and you never have to actually do anything, the car should always have a relatively full battery. You should never have to visit a gas station. You can just get in and drive in the car should charge itself.
“That’s the user experience that the carmakers are striving for. And wireless charging is the best way to provide that kind of user experience,” says Schatz.
The only challenges, says Schatz, right now are the standardisation of charging systems and the cost of batteries. “The process of standardisation is set to be resolved within 2020 as the organisations that define standardisation are ready to publish these standards, removing one of the obstacles,” he added.
Despite the challenges ahead, wireless power has a great future and is likely to become the norm. As Schatz says: “When systems go wireless, they don’t go back to being wired – nobody is going to propose to go back to the conventional plugin.”