Energy in the Future Series: Where can Wireless Charging Help us Efficiently Deliver Power to Rail Assets?
What is wireless charging?
Wireless charging is a method to charge electronic devices without a wire connection to a power source. This can be done through techniques including inductive charging and resonant inductive coupling.
In induction charging, an alternating current is generated in the primary coil (located in the charging plate) which produces an alternating magnetic field. This in turn stimulates an alternating current in the secondary coil (located in the device). By attaching a load to this coil, an AC current is induced which can be used for heating cooking pans on an induction hob, conversion of higher/lower voltages in AC transformers, or charging batteries. Resonant inductive coupling, however, uses the method of transferring power between coils operating at the same resonant frequencies. It is still “inductive” as the primary coil induces a current in the secondary coil but takes advantage of the strong coupling between the resonant coils even when tens of centimetres apart. In the future, this could lead to the charging of electric vehicles on the move.
The above diagram demonstrates how power from the grid charges batteries on trains via inductive charging.
What industries use wireless charging?
In the transport sector, wireless charging was first explored to power electric buses at the University of California, Berkeley in the 1980s. Although the project demonstrated possible wireless charging solutions for electric vehicles (EVs), it was not commercialised since the maximal measured efficiency of power transfer was only 60%.
Since then, routes have been created with charging coils under the road which charge vehicles as they travel along the road above. An example of this is the light tramway system of Seoul zoo has a 2.2 km loop, 370m of which is fitted with a charging system which transmits 62kW of power to trams when their presence is detected. After the success of the zoo tramway, a bus route in Gumi (South Korea) was similarly equipped, to be powered through wireless charging as it was running.
Wireless charging is common in the electronics industry. Wireless charger pads charge devices placed on top of them. They are widely commercially available, following the release of the first wireless charging smartphone in 2012 by Nokia. These wireless charger pads, also known as Powermats, were rolled out in Starbucks stores in the US. According to a survey, 80% of consumers want wireless charging in public areas.
How will wireless charging impact the rail industry?
Current rail technologies rely on a pantograph or third rail system to deliver a power source. Pantographs are prone to wear and cause reduced aerodynamic performance along with possible arcing when the cables are frozen. Overhead cables are difficult to install when there is limited gauging such as under a bridge. Moreover, third rail systems tend to be inefficient since power is lost to the ground. Wireless charging may be a solution to these issues. Combined with batteries in the train, regenerative braking can be exploited – a recovery mechanism that slows the train by converting its kinetic energy into a form that can be stored. This allow for charging while in service and enable lighter and cheaper batteries to be used.
Wireless charging for EVs could increase their convenience and appeal. Bombardier’s PRIMOVE wireless charging system has been implemented in e-buses saving 527 tons of CO2 over a distance of 800,000 kilometres. A survey found ‘more than two thirds of consumers in Germany planning to buy a car are more willing to purchase an electric vehicle if they could charge it wirelessly’. Hence such advantages of EVs can reduce the demand for rail transport.
The implementation of inductive charging methods will enable the charging of customer laptops, tablets and phones and possibly increase the appeal of travelling by rail for commuters and leisure-travellers. Currently, wireless charging is being trialled on board South West Trains, where Powermats are being incorporated into the tables in First Class.
What uncertainties remain?
Installing wireless induction plates would require retrofitting trains to have matching induction coils. However, if train companies are looking to acquire new rolling stock this may not be an issue. Installing wireless charging points at stations and on lines could require the removal of current technology (third rail or overhead lines).
To use wireless charging, trains may need to be a hybrid or have a battery system. This could be expensive which may discourage rail from switching to wireless charging. These costs can be minimised for a commuter service with regular stops. This would be due to the opportunities for regular short charging, which require smaller batteries. For any service, the time at charging points will need to be assessed to establish time needed charge the batteries.
Wireless charging requires train and track to be closely aligned within the range of millimetres for optimum energy transfer. This is problematic for “on-the-go” charging, as high-speed trains are prone to sway. The plates may also cause drag, reducing aerodynamic flow around the train.
Implementing the inductive coils along railway tracks would involve planting them under the track beds. This would involve removing sleepers, rail and ballast which could be a costly process.
What is the current state of R&D?
The European Green Vehicles Initiative is developing EV wireless charging capabilities through a series of projects. FABRIC has been able to deliver 20kW of power to a vehicle moving at 100km/h. UNPLUGGED has developed automatic sensing, communication and activation of wireless charging plates on the roads of Zaragoza in Spain.
Disney research has created a room in which a circulating loop of current charges devices within it. This research could lead to the development of wireless 3D spaces such as offices, where devices can be charged by simply walking into the room.
The University of Washington has also demonstrated running a camera on the trickle of power collected from ambient Wi-Fi signals.
Inductive plates, developed by WiTricity, placed beneath parking spaces are being tested by Furukawa Electric (a Japanese automotive company). This could reduce carbon emissions by increasing appeal of EVs.
What should the rail industry do?
As the rail industry researches the possibility of discontinuous electrification with on board batteries, consideration should be given on the implications of wireless charging. It could be a solution to losses incurred from overhead lines or third rail systems. This could also be useful if a train broke down in a tunnel where there is no electrification. Wireless charging could provide the power needed to charge onboard batteries.
The practicalities of charging a train wirelessly at stations, and/or whilst the train is in motion, should be investigated. Benefits of controlled lateral alignment could be researched to support the design of charging plates which can optimise charging on the basis for rail applications.
Wireless charging pads could be installed in future rolling stock. The inductive charging method could be used in tables to increase the efficiency of charging. Magnetic resonance coupling could be implemented in handrails and window frames for standing passengers.