As transit authorities seek to reduce fleet emissions, they are now evaluating their options. Many are considering the benefits of battery electric buses, and they are looking at the realities of implementing these zero-emission buses (ZEBs) into existing transit routes.
Existing routes are designed around the range, load capacity and hill-climbing abilities of diesel and compressed natural gas (CNG) buses. And the traveling public does not want these familiar routes changed.
Battery-powered electric buses have proven to perform well when routes are shorter, the terrain is flat, and the weather is warm.
But those are not real-world conditions for most urban transit systems.
In real-world transit conditions, fuel-cell electric buses offer the best direct replacement option for diesel and CNG buses.
Fuel Cells Improve the Performance of Battery Electric Buses on Demanding Routes
Driving range for longer routes
Fuel cell electric buses have a range between refueling that is similar to diesel and CNG powered buses, which gives transit planners predictability and flexibility. Many cities are using fuel cell electric buses as direct drop-in replacements on routes that were previously served by diesel or CNG buses.
Climbing power for hills
The effort of going up a hill can be very draining on the battery state-of-charge. The additional power requirements reduce operational range and can lower the top speed.
In comparison, in hydrogen powered buses, the fuel cell “engine” keeps generating power for the electric drive even on routes with long, steep uphill grades.
Cold weather performance
In every northern climate, cold weather is a reality for many months of the year. Cold temperatures slow the electrons in batteries and reduce energy output, which significantly affects the hauling power and range of battery-powered electric buses.
Battery output drops as operating temperature falls. Below 25°C, power output from lithium batteries reduces by:
- 10% at 0°C.
- 20% at -20°C
- 25% at -30°C
Charging time lengthens: At -20°C recharging is 20 times slower than at +20°C.
While some electric buses have thermal management systems to keep the battery warm, this system consumes significant power (up to 20 or 30kW), which drastically lowers the vehicle’s total driving range.
In colder climates, buses must also be able to heat the passenger compartment. These auxiliary heating systems consume additional power, which reduces the total travel range even further. Some bus manufacturers include a diesel power unit for passenger heat, which may be effective, but generates emissions.
And one can ask, is a bus with a diesel-powered heater a true ZEB?
Overcome the limitations with fuel cell/battery hybrid technology
A hybrid drivetrain configuration with both battery and a fuel cell power generator can compensate for the limitations of 100% battery-powered buses. The fuel cell system can directly power the electric drivetrain and also recharge the batteries keeping them in a constant state of charge. (Regenerative braking also charges the batteries.)
Fuel cell electric buses are capable of 18 hours of non-stop service. And refueling takes less than ten minutes.
The fuel cells ensure consistent delivery of electric power, which improves performance under maximum load.
In cold conditions, a fuel cell/battery hybrid bus has an extra advantage. The fuel cell process generates only two by-products: water and heat. So in cold weather, the heat can warm the passenger compartment, which eliminates the need for additional polluting diesel heaters.
For these reasons and more, cities around the world are moving to fuel cell electric buses to fulfill all or part their ZEB mandate.
When you make the move to bring ZEBs into your transit fleet, know that, in the consideration of hydrogen fuel cell vs battery electric buses, an effective solution is a hybrid of the two technologies.
It provides bus operators all the advantages of electric buses with the performance level of diesel or CNG buses.
Join in the conversation. What are your thoughts on hydrogen fuel cell and battery-powered buses? Let us know in the comments.
For more on the subject, read our white paper below on hybrid hydrogen fuel cell buses. The paper weighs the advantages of each type of hybrid drivetrain for standard transit routes and demands.