Because of the large number of vehicles on the road and their relatively slow turnover, a change in fuel and/or energy carrier must be transitional—that is, sufficient fuel must be available for the large existing fleet while the new fuel is introduced in parallel. The success of energy-efficient GHEVs is instructive in two ways: (1) it demonstrates the huge challenge in moving beyond the relatively simple gasoline system now in widespread use, and (2) it creates an even greater barrier to newer technologies, such as the hydrogen fuel cell vehicle (FCV), by enhancing the fuel economy of “conventional” vehicles. The major “demand parameters” for a light-duty vehicle are shown in Table 3-1.

Transportation applications of fuel cell technology and hydrogen fuels not discussed in this report include urban buses, heavy-duty truck auxiliary power units (APUs) (Lutsey et al., 2003; Winter and Kelly, 2003), delivery vehicles, forklifts, airport baggage-handling vehicles, mining vehicles, golf carts, scooters, boats, and even airplanes. Of these, the hydrogen-fueled urban bus market segment has received the most attention.

Fuel Cell Vehicle Technology

The success of hydrogen in the transportation sector will be dependent on the development and commercialization of competitive FCVs. The challenge is to develop automotive fuel cell systems that are lightweight and compact (i.e., have high power densities by both mass and volume), tolerant to rapid cycling and on-road vibration, reliable for 4000 to 5000 hours or so of noncontinuous use in cold and hot weather, and able to respond rapidly to transient demands for power (perhaps by being hybridized with a battery or ultracapacitor for electrical storage on the vehicle), and able to use hydrogen of varying purity.

TABLE 3-1 Key “Demand Parameters” for a Light-Duty Vehicle

Demand Category

Parameter

Customer

Initial cost

Operational and maintenance costs

Quality

Range (between refueling) and refueling convenience

Passenger/cargo space

Performance (acceleration, speed, ride quality, acceptably low levels of noise, vibration, and harshness)

Safety

Regulatory

Emissions of pollutants (carbon monoxide [CO], oxides of nitrogen [NOx], hydrocarbons [HC], particulates)

Fuel efficiency

Greenhouse gas emissions

Safety

One of the most important attributes for FCVs is fuel efficiency, since less fuel means lower fuel costs, less expensive and bulky on-board hydrogen storage, and less upstream environmental impact. Wang (2002) summarizes the numerous studies comparing the fuel efficiency and life-cycle impacts of FCVs, hybrid electric vehicles (including GHEVs), and potential “transition vehicles” with baseline gasoline and diesel vehicles. Ignoring life-cycle impacts, fuel cells operating on hydrogen are much more energy-efficient than are internal combustion engine (ICE) systems. It is impossible to specify accurately how much more efficient they are, since fuel cells have very different efficiency characteristics (e.g., they are many times more efficient at low speeds and loads, but are less efficient at higher speeds and loads) and because automotive fuel cell systems are in their technological infancy and so their future performance cannot be accurately predicted.

For the purposes of quantitative comparisons, after extensive deliberation and literature review, the committee selected a fuel-efficiency improvement factor of 2.40 for FCVs versus a baseline gasoline vehicle—that is, today’s gasoline vehicles are assumed to use two-and-a-half times as much energy as a comparable FCV. This comparison, an average for all light-duty vehicles, is based on average U.S. driving conditions. (For detailed assumptions, see Wang [2002].) The committee selected a fuel-efficiency factor of 1.45 for GHEVs versus a baseline gasoline vehicle. (See the discussion of hybrid technology in the following subsection, “Market Acceptance and Demand Trajectories.”) Fuel-efficiency factors for diesel-powered hybrid electric vehicles would fall between 1.45 and 2.40. These assumptions of fuel economy are based on averages from Wang’s (2002) review of other studies. In practice, actual differences in fuel economy may vary considerably. For instance, automakers might take advantage of the on-board electricity capability of FCVs and introduce a range of high-energy-consuming appliances and services, which would dramatically increase fuel consumption. Alternatively, FCVs might have relatively higher fuel economy because they disproportionately replace gasoline vehicles in urban settings or because traffic congestion results in slower driving speeds—in both cases taking advantage of FCVs’ better fuel efficiency at lower speeds.

Given these requirements, hybrid and nonhybrid PEMFC systems are the leading contenders for automotive fuel cell power, with additional attention focusing on the direct-methanol fuel cell (DMFC) version of the technology and the possibility of using solid oxide fuel cell (SOFC) systems as auxiliary power units for cars and trucks.

An important attraction of all of these fuel cell systems, both as main vehicle power systems and as APUs, is their ability to support the new wave of vehicle electronics that is being introduced. New or planned electronic gadgetry on vehicles includes navigation systems; extensive on-board communications; voice-actuated controls; exterior alternating current (ac) power supplies; computer-controlled, power-



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