• Transportation Applications
• Stationary Applications
• Portable Applications
• Endnotes

Fuel cells excelling in transportation applications will require low cost, lightweight, compact configurations, high power densities, good performance, safe operation, flexible configurations, and ease of use.

Size and weight
Fuel cells for transportation, especially automotive applications, need to be lightweight, compact, and offer sufficient performance. Lightweight fuel cells (those with high power density) will minimize the mass of the automobile, both improving fuel economy and decreasing structural materials costs. Fuel cells that are compact allow an automaker to maximize passenger and luggage space for a given vehicle profile. Size and weight restrictions on fuel cells and fuel storage may be less stringent for mass transit applications, allowing a broader range of fuel cell types to be applied.

Performance
Fuel cells for automotive applications require rapid start up times and excellent load following characteristics. Technologies that require warm-up times of many minutes or hours will be unacceptable to automotive users, though may be acceptable for mass transit and heavy transport applications. Fuel cells used for transportation must be able to respond adequately to transient loads such as stop and start driving, sudden acceleration, and hilly roads, and must also be able to begin producing power at or near freezing temperatures. A report for the American Methanol Society found that direct methanol fuel cells have the ability to rapidly respond to stepwise changes in load, and have the ability to start operating at temperatures slightly above freezing. Once operating, the direct methanol fuel cell stack will increase in temperature due to waste heat generation within the stack itself, thus allowing continued operation in low temperatures.¹ Proton exchange membrane fuel cells also have rapid start-up times and good transient response.

Safety
While safety is paramount in all fuel cell applications, it is particularly critical for transport applications. In the event of a malfunction or accident, injury to vehicle occupants and persons in the vehicles vicinity must be minimized. Fuel cells that operate at lower temperatures, with ambient or low pressurization, and constructed from benign materials will be preferred for transportation applications. Fuel cells that operate at very high temperatures, high pressures, and/or contain caustic materials are less desirable for transportation applications, and may require more costly containment and safeguards if they are to be used.

Flexibility
Fuel cells have an advantage over traditional internal combustion engines (ICEs) because both the fuel stack and ancillary components such as the reformer and power conditioning equipment are modular. This allows fuel cell power plant components to be distributed throughout a vehicle, thereby allow for greater vehicle design freedom, and potentially increasing carrying capacity compared to ICEs. Conversely, fuel cells that are less modular may be less attractive for automotive applications though may still be applicable for mass transit.

Ease of Use
Simple fuel filling or recharging procedures will be desirable for transit applications. Regenerative proton exchange membrane fuel cells offer the potential of plugging a vehicle into the electricity grid when not in use to recharge the hydrogen storage. According to Lawrence Livermore National Laboratory (LLNL), "Until a network of commercial hydrogen suppliers is developed, an overnight recharge of a small car at home would generate enough energy for about a 240-kilometer driving range, exceeding the range of recently released electrical vehicles. With the infrastructure in place, a 5-minute fill up of a 35-megapascal hydrogen tank would give a 580-kilometer range.”² In a 1994 study for automotive applications, LLNL and United Technologies Corporation (UTC) found that compared with battery-powered electric cars, the regenerative proton exchange membrane is lighter and provides a driving range comparable to gasoline-powered vehicles. Over the life of a vehicle, they found the regenerative proton exchange membrane would be more cost effective because it does not require replacement.³

A network of commercial hydrogen suppliers is planned under the California Fuel Cell Partnership program (www.fuelcellpartnership.org). The goal is to make as many as 100 refueling stations available “during the pilot stage, which can be several years”. They are considering three different approaches to hydrogen stations: liquid hydrogen, electrolyzers, and anural gas reformers. While the aim of the California project is to make hydrogen refueling commercially available, true commercial availability, i.e. stations that can provide hydrogen under market conditions, is still very dependent upon having commercially available fuel cell powered vehicles and competitively priced hydrogen. While California will have stations that customers can drive into to “fill up with hydrogen”, they will not initially be commercially viable.

Efficiency
High efficiency fuel cells are best for transportation applications because they minimize the need for fuel storage, thus minimizing weight and potentially cost for a given vehicle travel range. International Fuel Cell (IFC) states that "for vehicles lower pressure operation is preferred to operation at elevated pressure. Energy needed for air compression could significantly detract from overall power plant efficiency, the cornerstone benefit to making a transportation fuel cell viable. Pressurized operation also adds to weight, volume, system complexity and maintenance requirements."⁴ However, there can be tradeoffs between maximizing efficiency and optimizing other desirable characteristics. Currently the most efficient fuel cells are high-pressure MOFC and solid oxide fuel cell systems, even without CHP.

Other considerations
Both proton exchange membrane and direct methanol fuel cells have high power densities compared to other major fuel cell types, making them more likely for automotive applications. Also, safety as well as quick start requirements will tend to favor low to medium temperature fuel cells such as proton exchange membrane and direct methanol fuel cell. Larger vehicles, such as buses, trains and long-haul trucks may be able to make use of higher temperature fuel cells such as phosphoric acid, molten carbonate and solid oxide that have longer start up times. If the power of group procurement can be harnessed, commercial vehicle fleets may be one of the most promising near-term applications of fuel cells for transportation. According to UTC: "Buses and fleet vehicles are likely to be the first modes of transportation to use fuel cells. Such vehicles "go home" each night to a central staging point for refueling. That central staging area can then be equipped with a hydrogen filling station.⁵

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Stationary Applications
Large stationary applications, such as utility, industrial, and commercial will favor high-temperature fuel cells due to the efficiency gains afforded by combined heat and power applications. Lower to medium temperature fuel cells will be appropriate for small and medium CHP applications, including residential and commercial uses. Size, weight, and flexibility of fuel cell system configurations will be less important than for transport applications, though remain a factor for smaller application such as residential and light commercial installations.

Utility/ Industrial
The technology with the earliest promise for central station generation, phosphoric acid fuel cells, is the only fuel cell technology commercially available. An 11-megawatt unit was demonstrated in Tokyo, Japan, and several hundred 200-kilowatt units have been installed worldwide. More advanced designs, such as carbonate fuel cells and solid oxide fuel cells, are the focus of major electric utility efforts to bring the technology to the market.

Commercial
Phosphoric acid fuel cells have supplied stationary power for commercial applications for nearly 10 years. All of the higher-temperature fuel cells are desirable for commercial applications because they provide heat, power, and potentially cooling all in one package.

Residential
Residential applications may be more likely to favor lower-temperature fuel cells due to safety concerns. Low-temperature fuel cells can be used for CHP applications such as heating water for household use, though they do not produce high temperature steam such as used in industrial CHP applications.

Portable Applications
Fuel cells for portable power applications, such as laptop computers, will need to be small, lightweight, low-temperature, and easy to refill or recharge. The most likely technology for near-term portable applications is direct methanol fuel cells. NEC Scientists estimate that miniature direct methanol fuel cells can have ten times the energy storage of state of the art lithium batteries, such as those used for laptop computers. According to the Hydrogen Fuel Cell letter, "The energy capacity of the current Li-ion batteries is around 130 Wh/kg. NEC estimates about 1,300 Wh/kg or more for a well-designed portable direct methanol fuel cell based on carbon nanotube technology."⁶ Proton exchange membranes will also likely find portable power applications, such as for portable emergency power generation.

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