NREL Report Finds Operational Problems with CTTRANSIT Prototype Fuel Cell Bus Reduced Availability

Jose Michael

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CTTRANSIT’s fuel-cell hybrid bus. Click to enlarge.

A report on the results of a preliminary evaluation of a prototype plug-in fuel-cell hybrid bus at Connecticut Transit (CTTRANSIT) in Hartford by the National Renewable Energy Laboratory (NREL) found that while the experience for operators and riders with the fuel cell bus has been positive, there have been a number of specific problems and limitations that resulted in bus availability consistently below the 85% target.

These problems include initial quality assurance of the bus manufacturing/integration, the traction batteries, heating and air conditioning, operating in slippery conditions, maximum bus operating speed, and degradation of the fuel cell power system.

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The ISE hybrid drive system for the fuel cell bus. Click to enlarge.

The NREL researchers emphasize that there is no intent to consider this implementation of fuel cell buses as commercial (or full revenue transit service). Their evaluation focused on documenting progress and opportunities for improving the vehicles, infrastructure, and procedures.

The bus. CTTRANSIT introduced its fuel cell bus into service in April 2007. (Earlier post.) The prototype bus uses a series configuration hybrid-electric drive system from ISE Corporation, with a 120 kW fuel cell from UTC Corporation and a 53 kWh Zebra sodium nickel chloride energy storage system. The fuel cell power system and battery pack work together to provide power to two electric drive motors (170 kW continuous total), which are coupled to the driveline through a combining gearbox.

Specific problems reported by CTTRANSIT with the bus have included:

  • Unanticipated Performance Loss for UTC Power Fuel Cell Power System. The cell stack assemblies (CSAs) showed power degradation early in the operation of this bus. When the power degradation of the CSAs falls below 90 kW to 95 kW of the original 120 kW, the system is considered to be at the end of its life and should be replaced.

    This early power degradation was reported with a similar set of California fuel cell buses as well, and UTC Power reports the problem as an issue of contamination within the CSAs causing the premature degradation beyond end of life (at about 800 hours of operation instead of the expected 4,000 hours or more).

    UTC Power made some operations control changes and replaced the CSAs on the CTTRANSIT fuel cell with a new version on 15 January 2008. This issue appears to have been resolved for the CTTRANSIT and California fuel cell buses.
  • Quality Assurance. These issues included loose wiring and incorrect wiring such as that of cooling fans. All issues identified are believed to have been resolved.

  • Traction Battery Failures. The ZEBRA batteries experienced significant issues in this application; three traction batteries on the bus are operated in parallel. A cell in a ZEBRA battery will typically fail during a short circuit. A battery with failed cells has reduced voltage, although it can still be operated. Because the batteries are operated with a direct parallel connection, when the number of failed cells within each of the batteries is too different among the three batteries, it causes an unbalancing of the SOC. This imbalance makes it difficult to keep the batteries in the recommended operating range. The present SOC balancing algorithm will temporarily disconnect a battery to keep the SOC balanced.

    The battery manufacturer (MES-DEA), UTC Power, and ISE have been working on the issue for some time.

  • ISE Drive Motor Failure. The problem started as an indication of an inverter failure. Ultimately, it was a drive motor failure.

  • Insufficient Heating. The powerplant in a hybrid configuration does not develop enough waste heat to warm the passenger compartment in a timely manner. UTC Power and Van Hool continue to work on this issue.

  • Air Conditioning Noise. Awareness of the noise of the air conditioning fans was exacerbated by the quiet operation of the fuel cell bus. UTC Power engineers added baffling and other fixes to the system. UTC Power intends to implement several other changes in hopes of quieting the system further.

  • Operating in Slippery Conditions. The electric propulsion system on the fuel cell bus has some issues with slipping in snowy or icy conditions. ISE continues to study the issue and is considering a software change to the operation of the bus, but is not yet drawing any conclusions.

  • Limited Maximum Speed. The drive axle on the CTTRANSIT fuel cell bus has a maximum speed of 48 mph. For CTTRANSIT, the fuel cell bus cannot be operated on many of the routes in the service area because of the need to travel on the freeway, which requires operation speed of at least 55 mph.

    Although earlier versions of the Van Hool fuel cell bus in operation in California have a maximum speed limit of 65 mph, Van Hool recently (March 2008) decided to require that all of these fuel cell buses be controlled down to 48 mph (via software control for the California buses).

    NREL says that Van Hool is concerned that the center of gravity of the bus is higher than that of a standard bus, because of the height of the fuel cell bus and the weight of the fuel tanks and other equipment on the top of the bus; combined, these issues might allow the bus to tip over when moving at a high speed.

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Availability for the fuel cell bus. Click to enlarge.

The NREL evaluation. The NREL evaluation covers the period from April 2007 through June 2008 and includes results from diesel baseline buses.

During this period, the fuel cell bus accumulated 12,115 miles, and the fuel cell systems accumulated 2,049 hours. These numbers indicate an overall average speed of 5.9 mph, which is significantly slower than the average CTTRANSIT speed of 12 mph. The bus fell significantly short of its target availability (85%), due in large part to the problems noted above. Primary reasons for unavailability were the UTC fuel cell (38%), problems with the batteries (32%), and ISE hybrid propulsion system and fuel system (23%).

The fuel cell bus averaged 4.79 miles/kg of hydrogen, equating to 5.41 miles per diesel gallon equivalent (DGE). The buses are plugged in each night to recharge the batteries; the electric energy added to the fuel cell buses each night currently is not accounted for in the fuel economy calculation.

NREL’s next evaluation report on the CTTRANSIT fuel cell bus will add at least six more months of operation, and most likely will be completed in early 2009. This report will focus on tracking the progress made in maximizing the usage of the fuel cell bus and determine if the new version of the UTC Power fuel cell power system makes significant progress to the expected 4,000 hours of operation before it loses too much power.

NREL reports that CTTRANSIT has secured an FTA grant which will be used for several aspects of its fuel cell bus program including replacing the fuel cell power system to extend operation and testing; purchasing another fuel cell bus for delivery in 2009; and modifying the agency’s chassis dynamometer bay that will allow testing, storage, and maintenance of a fuel cell bus.

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