Fuel Cell Buses Show Significant Increase in Durability and Reliability
July 28, 2010
The U.S. Department of Energy (DOE) National Renewable Energy Laboratory (NREL) first began evaluating hydrogen-fueled transit buses in 2000 as part of its extensive technology validation efforts. These evaluations are funded by DOE and the U.S. Department of Transportation's Federal Transit Administration. Over the years, NREL has collected and analyzed data on nine early generation fuel cell buses operated by four transit agencies in the United States.
"The transit industry provides an excellent test-bed for developing and optimizing advanced transportation technologies," said Leslie Eudy of NREL's Hydrogen Technologies and Systems Center. "These vehicles log thousands of miles every month, generating lots of data very quickly."
In 2007, one of the manufacturers replaced the early generation fuel cell power systems in five of the buses with newer systems that featured improvements based on lessons learned during prior operation. According to NREL's evaluation, these current generation systems show significant improvements in durability and reliability.
"Reliability increased by 21% after the installation of the new fuel cell systems," Eudy said.
One measure of reliability and durability for the transit industry is "miles between roadcalls." A roadcall is the failure of an in-service bus that requires it to be replaced on route or causes a significant scheduling delay. NREL data show a substantial increase in fuel cell-related "miles between roadcalls" after the installation of the new fuel cell systems.
As of June 2010, two of the fuel cell systems have accumulated a record number of hours without requiring repair or replacement of single fuel cells or cell stacks—one bus accrued more than 7,000 hours, and another more than 6,000. And, the fuel cells continue to operate at rated power.
"These impressive results demonstrate that fuel cell technologies are indeed proving themselves in real-world applications, offering great promise for our nation's portfolio of advanced transportation options," Eudy said.
How does a fuel cell work?
A fuel cell uses the chemical energy of hydrogen to cleanly and efficiently produce electricity with water and heat as byproducts. To see this process in action, view the fuel cell animation.
Fuel cells are unique in terms of the variety of their potential applications; they can provide energy for systems as large as a utility power station and as small as a laptop computer. A single fuel cell produces approximately 1 volt or less—barely enough electricity for even the smallest applications. To increase the amount of electricity generated, individual fuel cells are combined in series to form a stack. Depending on the application, a fuel cell stack may contain only a few or as many as hundreds of individual cells layered together. This "scalability" makes fuel cells ideal for a wide variety of applications, from laptop computers (50-100 Watts) to homes (1-5 kW), vehicles (50-125 kW), and central power generation (1-200 MW or more).