Contactless charging for Electric Buses

A new technology for charging electric buses has been introduced in Europe by Conductix-Wampfler. Called Opportunity Charging, this economical option helps in contactless charging of electric buses where the driver needn’t leave the bus for recharging. The technology also helps in doing away with the staff having to undergo training in electrical engineering.

For local public transport in particular, the electric bus is the mode of transport of the future. With respect to the total cost of ownership over its entire service life, it is far more economical to run than the CNG-powered buses, hybrids or pure diesel buses. The electric drive-train is a compelling solution first and foremost due to its excellent energy balance: a recent comparison from the USA has revealed that the electric buses selected consume about $9,000 worth of energy in a year, whereas a comparable diesel bus burns fuel worth around $50,000. The acquisition costs of an electric bus which are still higher at present, pay off no later than in the fourth year of operation. In China, this insight is no longer being called into question, but instead is being put into practice on a large scale – for example in rapidly growing cities such as Shenzhen.

To boost the profitability of a bus fleet, however, it is not just about having the right drive-train: you also need the right charging technology. Due to regular opportunity charging with IPT® Charge, a wireless battery charging system of Conductix-Wampfler, fleet operators can buy cheaper buses with smaller batteries in which the cell chemistry is kept healthy by more-frequent, but shorter charging cycles, meaning that the cells have a longer service life . With the operational concept termed opportunity charging, much less weight needs to be carried around and no special journeys to battery exchange stations are needed. This has a positive effect on the overall cost and on system efficiency. And that’s without even factoring in the tax benefits, or the fact that there are no emissions penalties to pay and that the maintenance costs are lower.

Conductix-Wampfler has been demonstrating for many years how well inductive charging of batteries can work in the context of local public transport: there are about 30 electric buses in Genoa and Turin that have been using the company’s IPT technology since 2002. Ten years on, the Italian public transport companies AMT and GTT are in no doubt about the success of the application of this technology under everyday conditions. The buses in Turin reliably travel 200km a day without needing to stop anywhere for a prolonged period or having to return to the depot for charging.

However, the bus routes with the IPT charging systems haven’t only proven themselves from the economical point of view, but also because they are very quiet and produce zero emissions. As the chargers can be integrated almost invisibly, they neither compromise the cityscape nor detract from the tourist attractions. There’s nothing for people to get hurt by tripping over, and there are no secondary costs due to vandalism or rodent damage.

Technology and vehicles

Inductive Power Transfer – or IPT – is an energy transfer system for electric vehicles that works by magnetic resonance coupling. The system consists of two main components: a primary coil which is connected to the electricity grid via a power feed converter, and a secondary coil integrated in the bus chassis. This technology permits an efficient, automatic, contactless transfer of electricity.

IPT focuses on short but regular charging for city buses. The battery is fully charged overnight and then topped up as the need be and as possible over the course of the day at suitably equipped stops, generally by about 10–15% of its capacity, depending on how long the bus stays at the stop.

The topping-up of the batteries at bus stops, e.g. at terminals, railways stations or hubs, ensures that the buses have sufficient range to reach the next charging point, so that the energy they need to store can be kept to a minimum. This allows the capacity of the batteries to be reduced by as much as 75%, greatly reducing the purchase price and weight of the vehicles and not impinging on the size of the passenger compartment.

The number of charging stations is individually adjusted to suit the size and the operating situation of the bus fleet: the more buses there are in a fleet and the more charging stations there are, the more flexible the system and the shorter the charging cycles can be. This also has a very beneficial effect on the acquisition and operating costs for the infrastructure (TCO), as the cost is divided over several buses. Today, buses can already be charged at sufficiently high charging capacities (60kW or 120kW) at bus stops or at route end stations while the passengers embark and disembark.

While charging, the pickup coil on the bus takes up a position about 40mm from the primary coil in the ground, which has two positive effects:

– The magnetic field is concentrated in such a way that 95% of the energy taken from the electricity grid is transferred to the battery during normal operation. This makes the IPT technology, in a direct comparison, virtually as efficient as charging via a charging cable with high level battery chargers available today, and in many cases superior to low-priced plug-in battery chargers.

– The stray magnetic fields remain restricted to the immediate vicinity of the coil. Beyond the bus contours, the field values are significantly below the values permitted by the ICNIRP’s recommendations.

If you also take into account the fact that with the operating concept of opportunity charging the weight that needs to be carried around is also significantly reduced and that no journeys need to be made especially to battery charging stations, the efficiency analysis very quickly tips in favour of inductive charging. The fact that the IPT charging stations are not subject to any mechanical wear, as a result of doing away with plugs and cables, boosts the cost advantage even further.

Conductix-Wampfler has now launched the second generation of its tried-and-tested charging technology on the market: the system “Urban Solutions” is even easier to integrate in existing infrastructures. Thanks to its enhanced diagnostic functions and improved network connectivity, it now guarantees even greater operational transparency for the user.

Increased comfort and safety

During short stops at bus stops, the electric bus is automatically recharged inductively. This does away not only with the need for handling heavy charger cables and plugs, but also the danger of forgetting to recharge the batteries on time – which can result in deep discharging that damages the bus battery. A conveniently positioned monitor in the vehicle cockpit allows the driver to keep an eye on the charging process from his seat: at no time is he required to leave the vehicle or to leave his seat unattended. The contactless charging of the buses is not only more comfortable but also safer for staff, especially when it is raining or snowing. As the bus driver doesn’t need to leave the bus for recharging or never comes in any kind of contact with the charging accessories, there is no need for staff with electrical engineering training (as called for by local regulations in some countries when dealing with a connected load of over 30kW).

Further, pilot and test projects have been or will be equipped with the same technology and operating approach as the one used in Turin, Genoa and industrial projects. Among these are local public transport scenarios in Japan, Lucerne (Switzerland), Lörrach (Germany), Rotorua (New Zealand), Utrecht (Holland) as well as Los Angeles and Chattanooga (USA).

Even if the high initial investment in electric propulsion is still a hurdle for fleet operators at the moment, the switch to hybrid or natural-gas buses is no real alternative for city buses in the medium term. It’s only a matter of time before we are hit by the next acute oil price rises to which the price of natural gas is tied. Even now, the higher cost of investment in the electric buses breaks even within at most four years due to the lower energy consumption. The zero-emission buses are also highly recommended in the light of international legislation: in California it is already mandatory for 15% of all urban transportation to be zero-emission, and there are also restrictions on diesel in a number of megacities in Asia already. Based on the assumption that the price of the vehicles and batteries will continue to fall, the TCO models will come out in favour of electric buses with opportunity charging much sooner than one might expect. And once the increasing emissions offset costs of diesel and hybrid buses included in the equation, the result of any comparison will be very conclusive.

Mathias Wechlin

Product Manager Inductive Power Transfer,IPT

Conductix-Wampfler GmbH, Germany