Because they travel a set route and schedule, range anxiety need not be an issue for electric buses. Far more, the question is: How fast, how and where can the battery be recharged? Efficient recharging reduces electric bus operating costs. What, then, are the core elements of an electric bus charging solution?
Alexandre McCraw ABB Ltd. Baden, Switzerland, firstname.lastname@example.org
How far can an electric bus travel before it needs to recharge its batteries? The question comes naturally when thinking about electric cars, especially as the essence of having a car is the freedom to go anywhere, at any time. A bus, however, does not go anywhere at any time, it goes to a specific place, along a specific route and at a specific time, and repeats that day-in and day-out according to a published schedule. The real question, therefore, is not how far it can go but how fast can it recharge before going there again. This article looks at why flash recharging reduces the overall cost of operating an electric bus line and explains the core elements of a solution.
Achieving a low total cost of ownership (TCO) is about operating an electric bus line with only as many vehicles as are needed and for that, having a one-to-one correspondence with the diesel bus equivalent solution is the key. This means having the same schedule, the same layover time, the same numbers of buses and drivers, and the same passenger capacity as a diesel fleet.
Buses, whatever their energy source, are costly to maintain. Conversely, infrastructure, once installed, has significantly lower maintenance costs and longer operational life. Bus operators can, therefore, lower the TCO of their electric fleet with a solution that optimizes the number of vehicles by maximizing the versatility of their infrastructure and by introducing common battery sizes on their bus fleet.
Battery charging strategies
Current charging strategies, whether overnight at the depot or opportunity charging at terminals, are governed by the time needed to charge the vehicle and the constraints imposed by the schedule.
In the case of depot charging, bus operators rely on the battery having sufficient capacity to complete the route. Battery size, however, negatively impacts passenger capacity and increases the cost and weight of the vehicle. Up to 45 percent of the cost of an electric bus with a depot charging strategy comes from the battery.
For opportunity charging strategies, with charging stations at the line terminals, the vehicle must charge in a period shorter than the scheduled layover time. Layover times exist to provide drivers with a break but also to provide delay buffers. During peak periods when layovers are shorter, line operators often face a dilemma: Only partially recharge the battery to keep to schedule and push the battery into deep discharge, or abandon the schedule to enable a full charge? Opportunity charging strategies only allow sufficient charging time by having more buses and drivers and more terminal and depot charging stations. These additional resources increase space requirements, energy consumption and cost. In other words, if charging is not fast enough, schedules suffer and/or costs are high.
Understanding the trade-offs
The cities of Geneva in Switzerland and Nantes in France understand the trade-offs involved in electric bus charging. Both municipalities have implemented an electric bus system that optimizes the number of vehicles and makes the most of charging opportunities. As both cities have different operating realities and different electrical grids systems, the systems they chose are likewise different to leverage those local opportunities. Regardless of these differences, there are core elements that both share.
For instance, the two cities rely not only on charging opportunities at the terminals and depot but also on others along the route, while passengers embark and disembark →1 – 2. Charging along the route at selected stops, so-called flash charging:
• Ensures that batteries are kept in a high state of charge.
• Reduces the need for long charging periods.
• Extends battery life by avoiding deep discharges.
In periods of congested traffic, the vehicle gets most of its energy at stops along the route, while during off-peak operation, the vehicle recovers mostly at the terminus.
The benefits of onboard charging
The chargers at the terminals and along the routes in Geneva and Nantes provide 400 to 600 kW to maximize energy level recovery. Such an infrastructure is only effective when combined with a battery capable of absorbing such a high-power charge. That is why the vehicles are equipped with lithium titanate oxide (LTO) batteries that charge quickly and can operate at up to “10C.” 10C refers to the C-rate, a standard measure of how fast a battery can be charged or discharged. The higher the C-rate, the faster the charge or discharge. This measure is a key differentiator for ABB: A large battery is only helpful if it charges and discharges without impact on the schedule.
The Nantes and Geneva buses also both use onboard charging →3. In such systems, the overhead rail provides a constant DC voltage and the equipment mounted in the vehicle transforms this to a form useable by the auxiliaries and motors. By reversing the flow of power between battery and motor, the DC fed from the overhead rail can be used to charge the batteries.
This avoids breaking the mission-critical communication between the battery and the onboard charger at charging points. On-board charging thus keeps interfaces between the vehicle and charger to a simple physical contact check with no further communication needed.
Another element the Geneva and Nantes buses have in common is a fast-operating pantograph that enables the bus to connect to the charger in under 1 s →4 ,5. Predeployment of the pantograph on nearing a charging point and automatic alignment with the charging hood allows drivers to approach the charging stop as they would any other and speeds the process. Each charging second gained helps reduce the number of flash charging points required along the route as well as the time needed to recharge at the terminal. These savings give the line operator more flexibility, improved schedule recovery times during peak periods and, thanks to common vehicle battery sizes, the ability to re-deploy any bus to any line and easily handle new demand.
Keeping to the schedule
Choosing the right battery charging philosophy keeps electric bus costs low →6. Depot charging strategies require larger, costlier batteries that reduce passenger capacity; opportunity charging strategies quickly impact layover times and require more vehicles and more infrastructure if schedules are to be maintained. The best chance of fulfilling timetables and keeping the commitment to passengers during peak periods, challenging weather, etc. →7, while keeping TCO down, is to charge faster than the layover time and as often as possible, as is done in Geneva and Nantes.
When it comes to electric buses, the focus should not be on the range but on keeping to the schedule. If the recharging is done wisely, an electric bus network can be run not only with a low TCO but also with environmentally friendly credentials, which is of benefit to all.