No ordinary buses
The buses are wirelessly charged electric vehicles (EVs) that operate on routes in the German cities of Berlin, Braunschweig and Mannheim, as well as in Bruges (Belgium) and Södertälje (Sweden). They use PRIMOVE technology from the manufacturer Bombardier and their combined operation has resulted in reducing CO2 emissions by more than 527 tonnes.
As with other wireless charging technologies, PRIMOVE is based on high-power inductive energy transfer. The transfer takes place between sending components that are buried beneath the road surface and receiving equipment that is installed beneath the vehicle. Devices installed along the roadside begin the contactless charging process as soon as the vehicle covers the charging segment.
Wireless charging stations are located in the depot, at end stops and along the route at selected stops where recharging can occur even as passengers alight.
The technologies behind wireless
At its heart, wireless power transfer (WPT) uses magnetic resonance coupling of air core transformers. This technology can provide a convenient, safe and flexible way to charge electric vehicles either while they are stopped or are in motion.
The power transfer system consists of a transmitter coil and a receiver coil. These coils form a system of magnetically coupled inductors. In operation, an alternating current in the transmitter coil generates a magnetic field. This in turn induces a voltage in the receiver coil. The voltage can be used to power a mobile device or charge a battery in an car, bus, truck or tram.
Another EV charging technology, more commonly encountered for now, is that of plug-in EVs (PEVs). Disadvantages of PEVs are their need for bulky and expensive cable-and-plug chargers as well as large and heavy batteries. The sheer bulk of the batteries can limit how far an EV reliant on this charging technology can travel, which may lead to some motorists suffering from the condition known as “range anxiety”.
The most common EV or hybrid EV power transfer systems usually charge at between 3 and 50 kW while the vehicle is turned off. This technology works well when charging at home or in parking garages.
By contrast, inductive power transfer – also known as dynamic wireless power transfer (DWPT) – both addresses range anxiety and allows for recharging while a vehicle is moving along a street.
The concept behind wireless charging is well known. In the late 19th century, Nikola Tesla was granted a patent for a resonant inductive coupling to supply electric current to streetcar motors from a stationary source.
More recently, in 2016, a 20 kW wireless charging system for vehicles was demonstrated at the US Department of Energy Oak Ridge National Laboratory (ORNL) in Tennessee where it achieved 90% efficiency while charging at three times the rate of a common plug-in system. Industry partners from Toyota, Cisco Systems, Evatran and Clemson University International Center for Automotive Research contributed to the technology development.
The power electronics team from ORNL developed the charging system for passenger cars. It included an inverter, isolation transformer, vehicle-side electronics and coupling technologies. For the demonstration, researchers integrated the single-converter system into an electric Toyota RAV4 equipped with an additional 10 kWh battery.
IEC International Standards central to the adoption of EV WPT
Charging electric vehicles without using wires has the potential to replace conductive chargers because of its flexibility and convenience. The use of private and secure radio communications and standardization means that any vehicle would be able to charge at any location.
A number of IEC Technical Committees (TCs), Subcommittees (SCs) and Working Groups (WGs) are involved in the development of the International Standards necessary to the introduction of WPT. The International Organization for Standardization (ISO) is also involved in the development of WPT through one of its TCs, which liaises with the corresponding IEC TC.
IEC TC 69: Electric road vehicles and electric industrial trucks, is responsible for preparing International Standards for “road vehicles, totally or partly electrically propelled from self-contained power sources and for electric industrial trucks”.
IEC TC 69 has four Working Groups (WGs). One of these, IEC TC 69/WG 7, works specifically on “Electric vehicle wireless power transfer (WPT) systems”.
IEC TC 69/WG 7 is working on IEC 61980, a three-part series of International Standards that applies to equipment used in WPT “from the supply network to electric road vehicles”. This series also applies to WPT equipment supplied from on-site storage systems (such as buffer batteries and so on).
IEC 61980-1:2015, the first Standard published in the series, covers general requirements for EV WPT systems including general background and definitions – for example: efficiency, electrical safety, Electromagnetic Compatibility (EMC), protection from electromagnetic field (EMF) and so on.
IEC 61980-2, part 2 of the series to be published later, will cover specific requirements for communication between electric road vehicles and WPT systems including general background and definitions.
As for the third part in the series, IEC 61980-3, also scheduled to be published later, it will cover specific requirements for EV magnetic field wireless power transfer (MF-WPT) systems.
In addition, an SC of the International special committee on radio interference (CISPR) works on Standards related to WPT. CISPR/B (CIS/B): Interference relating to industrial, scientific and medical radio-frequency apparatus, to other (heavy) industrial equipment, to overhead power lines, to high voltage equipment and to electric traction, is involved in work on Amendment 2 Fragment 1 to CISPR 11:2015 Industrial, scientific and medical equipment – Radio-frequency disturbance characteristics – Limits and methods of measurement – Requirements for air-gap WPT.
The following IEC Technical Committees also maintain liaisons with IEC TC 69/WG7:
WPT expanding worldwide
Wireless charging technologies are in use in several places around the world.
In South Korea, the Korea Advanced Institute of Science and Technology (KAIST) has developed a WPT technology called OLEV, short for On-Line Electric Vehicles.
In the town of Gumi, a route has been built that allows buses to recharge while in motion. The technology supplies 60 kHz and 180 kW of power wirelessly to the transport vehicles. The route runs for a total of 35 km and the length of the DWPT section is 144 m made up of four DWPT sections.
Initially, two buses were equipped to recharge while driving over this roadway; the OLEV buses have coils on their underside to pick up power through the electromagnetic field on the road. The DWPT system enables the buses to reduce the size of the reserve battery used to one-fifth that of the battery onboard a typical electric car.
The UK government agency, Highways England, is carrying out test track trials of a wireless road-embedded EV charging technology. Analysis has shown that under different traffic conditions average demand could be as high as 500 kVA (0.5 MVA) per mile.
Highways England considered three types of road construction, including trench-based constructions, full-lane reconstruction and full-lane pre-fabricated construction. The first two methods were found to be viable, but analysis concluded that the full lane pre-fabricated method would probably be prohibitively expensive.
Whichever system will be used, International Standards developed by a number of IEC TCs and SCs will be central to the introduction of WPT to charge electric vehicles.