Chalmers University of Technology in Sweden has advanced inductive power transfer technology to enable high-power battery charging without the use of a human or a robotic arm. A new form of silicon carbide semiconductor, as well as a copper wire as thin as a human hair, have been produced. These two variables have suddenly made high-power transmission by air a viable option. The technology is ready for immediate industry presentation.
Inductive power transfer technology is a wireless method of transferring electrical power between two objects using an electromagnetic field. The technology works by creating an oscillating magnetic field between two coils, one in the power transmitter and one in the receiver. When the two coils are in close proximity, the magnetic field induces an electrical current in the receiver coil, which can be used to charge a battery or power an electrical device.
Inductive charging is not a new concept; electric toothbrushes have used the technology for decades, and in recent years, mobile phones and other portable electronics have adopted it. However, until now, it has not been feasible for charging high-power batteries such as those found in electric trucks.
The breakthrough in wireless charging opens up new possibilities, especially for vehicles that require frequent charging in demanding environments. For instance, urban electric ferries that regularly traverse waterways will no longer need human assistance or robotic arms to charge their batteries. City buses, driverless electric trucks used in industry, mining, and agriculture can also benefit from this technology.
Professor Yujing Liu, the Professor of Electric Power Engineering at the Department of Electrical Engineering at Chalmers, explained that the technology could be built into the wharf to charge the ferry when passengers get on and off. Charging can take place 30-40 times per day, which is the most obvious application. Even electric trucks can use the technology in the future, as charging them at sufficiently high power means the charging cable is very thick, heavy and difficult to handle.
The recent technological advances in materials such as high-power semiconductors based on silicon carbide, known as “SiC components,” allow for higher voltages, temperatures and much higher switching frequencies compared to traditional silicon-based components. This is important because the frequency of the magnetic field limits how much power can be transferred between two coils of a given size. Working with frequencies four times higher than previous systems for vehicle wireless charging means induction becomes more attractive.
The copper wires in the coils that send and receive the oscillating magnetic field that forms the bridge for the energy to flow across the air gap are another technological advancement. These new coils are made of braided “copper ropes” comprising up to 10,000 copper fibres, each between 70 and 100 microns thick, similar to a strand of hair. These braids of what is known as Litz wires are optimised for high currents and frequencies and have only been commercially available in the last few years.
Liu emphasises that charging electric vehicles requires several conversion steps between direct current and alternating current and between different voltage levels. However, the efficiency achieved is approximately 98 per cent from direct current in the charging station to the battery. Liu adds that the results published by his research group have attracted a lot of attention, and they are among the best in the world in terms of efficiency in this power class, between 150 and 500 kW.
Liu also notes that induction charging is unlikely to replace charging with a cable. The technology is suitable for vehicles that require frequent charging in demanding environments. Although the technology itself may not be more sustainable than conventional charging, it can make things easier when electrifying large vehicles and speed up the phase-out of diesel ferries.
