APPLICATIONS – TRANSPORTATION | Rail Vehicles: Fuel Cells

APPLICATIONS – TRANSPORTATION | Rail Vehicles: Fuel Cells

Rail Vehicles: Fuel Cells AR Miller, Vehicle Projects Inc. and, Supersonic Tube Vehicle LLC, Golden, CO, USA & 2009 Elsevier B.V. All rights reserved...

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Rail Vehicles: Fuel Cells AR Miller, Vehicle Projects Inc. and, Supersonic Tube Vehicle LLC, Golden, CO, USA & 2009 Elsevier B.V. All rights reserved.

Introduction This article concerns the rationale, history, principal issues, and potential of fuel cell-powered rail vehicles. Issues include fuel cell type, hydrogen storage, special factors affecting fuel cell rail, and the question of which rail applications are appropriate for hybrid powertrains. It concludes with a brief discussion of a supersonic concept vehicle, a cross between a train and an airplane that operates in a hydrogen-filled tube and levitates on a gas film, thereby overcoming an inherent efficiency limitation of aircraft.

Why Fuel Cell Rail? Carbon dioxide emissions and energy security are related issues affecting the rail industry and transportation sector as a whole. They are related by the fact that in many nations nearly 100% of the energy for the transport sector is based on oil, and oil is an insecure primary energy and the principal source of carbon dioxide emissions. World oil reserves are diminishing, prices have recently reached unprecedented heights and volatility, and political instability threatens supply disruptions. A consensus has been reached that the burning of fossil fuels and consequent atmospheric release of waste carbon dioxide is a significant factor in global climate change. The greenhouse gas effect is the likely cause of melting of the polar ice caps and the increased severity of storms. Catenary-electric and diesel-electric are the two dominant, conventional types of locomotive, and the former superficially appears to be a solution to both problems. However, a factor potentially affecting both energy security and carbon dioxide emissions is energy efficiency (traction work divided by chemical energy of the fuel), because a more efficient locomotive uses less energy and, for most locomotives, burns less oil. When viewed as only one component of a distributed machine that includes an electricity-generating plant, possibly coal- or oil-fired, catenary-electrics are the least energy-efficient locomotive type. Diesel-electric locomotives, although collectively worse air polluters than an equal number of catenary-electric locomotives driven by coal-fired power plants, are more energy-efficient overall. Moreover, a catenary-electric is much more costly than an equivalent diesel-electric locomotive because of the higher infrastructure costs (US$6–8 million per mile). Relatively low infrastructure cost is the reason that diesel-electrics are

almost universally used on large landmasses with dispersed population centers, such as the USA. The lower energy efficiency of the catenary-electric locomotive is most accurately shown in a ‘well-to-wheels’ analysis. A complete analysis would include the energy consumption of the ‘well’, for example, the energy to pump and refine oil or the energy to mine and process coal. Moreover, the efficiencies depend on the specifics of the application, in particular, the duty cycle. To make a meaningful comparison by using a common primary energy, consider using a diesel engine as the prime mover in the two types of locomotives undergoing the same duty cycle. For a catenary-electric, the following are the midpoints of the typical range of efficiencies for the various processes involved in taking the energy of diesel fuel to traction power in the locomotive: Mitsubishi 8 MW diesel engine-alternator for an electricity-generating plant (43.5%), voltage conversion (97%), copper transmission from power plant to locomotive (80%), and onboard conversion to traction power (85%). The product of these estimates gives the estimated overall efficiency of a catenary-electric locomotive as 29%. Coal-fired steam-generating plants have similar, but probably lower, efficiencies compared to the diesel plant. For a diesel-electric with the prime mover onboard, the midpoint efficiencies are as follows: 3 MW onboard diesel engine (37.5%), engine ancillaries (94%), alternator (96.5%), and onboard conversion to traction work (90%). Estimated overall efficiency for a diesel-electric locomotive is therefore 31%. While the efficiencies of the two conventional types of locomotives are similar, this analysis dispels any misconception that a catenary-electric locomotive is a high-efficiency vehicle. However, compared to other common forms of transport, either type of conventional locomotive pulling a train is much more energy efficient: rail freight is 3–4 times more efficient on a tonne–km basis than rubbertired road trucks and 50 times more efficient than airfreight. The poor efficiency of airfreight is due primarily to the power required to overcome induced drag, the drag caused by the wings diverting the incoming air to downwash and thereby providing lift to hold the vehicle aloft. The equation for induced drag is as follows: Di ¼

Ci w 2 ð1=2Þb 2 rV 2

½1

where Di is the induced drag (force), Ci is the coefficient of induced drag specific to an airplane, w is the airplane

313

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Applications – Transportation | Rail Vehicles: Fuel Cells

weight, b is the wingspan (a measure of wing surface area), r is the atmospheric gas density, and V is the aircraft velocity. Whenever any combination of variables b, r, and V is larger, the consequence is lower induced drag. However, these same changes increase another important form of drag, pressure drag. Thus, the dominant factor in eqn [1] is airplane weight w. Because the induced drag increases as the square of w, eqn [1] indicates that airfreight is inherently energy intensive – the energy penalty aircraft pay to fly above the weather is large. One solution to the incompatibility of wings and high-speed efficient transport is the aerostatic gas-film bearings of the supersonic tube vehicle, a concept rail vehicle discussed below. A fuel cell locomotive would provide the environmental advantages at the vehicle of a catenary-electric locomotive but higher overall energy efficiency and lower infrastructure costs, both similar to that of a dieselelectric. Compared to the diesel-electric, the efficiency is expected to be somewhat higher (not more than 10–20%) but the infrastructure cost will be somewhat higher. Elimination of the even higher infrastructure costs of the catenary-electric by fuel cell locomotives is the key to economic viability of electric trains in low population density regions. The natural fuel for a fuel cell is hydrogen, which can be produced from many renewable energies and nuclear energy, and thus a hydrogen fuel cell locomotive will not depend on imported oil for its energy supply. Hydrogen produced from renewable primary energies or nuclear energy would additionally be a totally zero-emission vehicle, that is, with zero carbon in the energy cycle.

History The history of fuel cell-powered rail vehicles is short. Only two have operated and a third is nearing completion and should be operational by the end of 2008. The first functional hydrogen-fueled, fuel cellpowered rail vehicle was a 3.6-tonne (1 tonne ~ 1000 kg) underground mining locomotive (see Figure 1) completed by Vehicle Projects Inc in 2002 in a project funded jointly by the governments of the United States and Canada and by private industry. The project was commenced by the nonprofit Fuelcell Propulsion Institute but managed, completed, and the vehicle demonstrated by an international consortium led by Vehicle Projects Inc. Two liquid-cooled proton-exchange membrane fuel cell (PEMFC) stacks powered the locomotive and a reversible metal hydride storage system, primarily for safety in underground operation, stored 3 kg of hydrogen as fuel. An initial version of the vehicle used stacks rated at 14 kW and the final version used 17 kW (continuous gross). Both the fuel cell balance of plant (BoP) and reversible metal hydride storage system were developed primarily by Sandia National Laboratories (Livermore, CA). The platform vehicle was a commercially available battery mine locomotive, the battery of which was replaced by the fuel cell power plant and hydride storage system. The mine locomotive was a pure fuel cell vehicle, or nonhybrid, and employed no traction battery (though a small battery for start-up of the system was carried onboard). Because tractive effort of a locomotive is limited by wheel adhesion, which is the product of a coefficient of friction for steel wheels on steel rails and the locomotive’s

Figure 1 World’s first fuel cell locomotive, a mine locomotive demonstrated in an operating gold mine in 2002.

Applications – Transportation | Rail Vehicles: Fuel Cells

weight, a locomotive has a fixed operating weight. The combined fuel cell and reversible metal hydride storage systems were lighter by about 1100 kg than the lead–acid traction battery they replaced. Accordingly, a ballast of the same weight was added to the hydride fuel cell locomotive. The locomotive was demonstrated in a working gold mine, the Campbell Mine of Placer Dome Inc., in Red Lake, ON, Canada, and it was a complete success. In operation alongside conventional battery locomotives, the fuel cell locomotive was superior in every measure of

Table 1 locomotives

Comparison of battery and fuel cell mine

Parameter

Battery

Fuel cell

Power, rated continuous Current, rated continuous Voltage at continuous rating Energy capacity, electrical Operating time Recharge time Vehicle weight

7.1 kW (gross)

17 kW (gross)

76 A

135 A

94 V (estimated)

128 V

43 kWh

48 kWh

6 h (available) 8 h (min) 3600 kg

8h 1 h (max) 2500 kg (without ballast)

Source: Reproduced from Miller AR and Barnes DL (2002) Fuel cell locomotives. In: Proceedings of Fuel Cell World, Lucerne, Switzerland. Copyright by Vehicle Projects Inc.

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performance outlined in Table 1. Demonstrations were completed in late 2002. The Railway Technical Research Institute of Tokyo, Japan, developed a rolling test lab that was powered by a low power density 120 kW (continuous net) PEMFC power plant and fueled by compressed hydrogen. This vehicle was also nonhybrid. Although the rail car was of the passenger type conventionally carrying the power units under the floor, the fuel cell power plant and its associated test equipment filled most of the car interior and onboard ‘passengers’ were limited to technicians. The vehicle did make a few short runs in 2006, but its fate is unknown and no known published papers on its operation have been published, although a poster on its energy efficiency was presented at the 8th World Congress on Railway Research in May 2008. A North American public–private partnership funded by the Burlington Northern and Santa Fe (BNSF) Railway Company and the US Department of Defense is developing a prototype fuel cell–battery hybrid switch (shunt) locomotive (see Figure 2) for urban rail applications. Switch locomotives are used in railyards for assembling and disassembling trains and moving trains from one point to another. This prototype is intended to lead to commercial locomotives that will (1) reduce air pollution in urban railyards, particularly yards associated with seaports, (2) increase energy security of the rail transport system by using a fuel independent of imported oil, (3) reduce atmospheric greenhouse gas emissions, and (4) serve as a mobile backup power source (vehicle-to-

Figure 2 Fuel cell hybrid switch (shunt) locomotive under construction at the Burlington Northern and Santa Fe (BNSF) Topeka System Maintenance Terminal on 9 January 2008. Engineering design of the fuel cell power plant and project management are the work of Vehicle Projects Inc.

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Applications – Transportation | Rail Vehicles: Fuel Cells

grid) for critical infrastructure on military bases and for civilian disaster relief efforts. Vehicle Projects Inc executed engineering design of the fuel cell power plant, based on Ballard stacks, and leads a consortium executing other project tasks. The railyard demonstration is planned for the Los Angeles Basin and the Inc-to-grid demonstration at Hill Air Force Base, Utah. This fast-paced project commenced in May 2006, and the locomotive will be completed in late 2008. Contributing to the fast pace are (1) the platform of the fuel cell hybrid locomotive is based on a 127-tonne commercially available diesel–battery hybrid switcher, (2) both the fuel cell power plant and the compressed-hydrogen storage system are derived from the CitaroTM fuel cell transit bus, and (3) private funding (through BNSF Railway) supported project start-up. Citaro fuel cell buses, widely used in European cities, have a combined operating experience of more than 1.5 million kilometers. At 127 tonne (280 000 lb; 1 lb ¼ 0.45359 kg), continuous net power of approximately 250 kW from its PEMFC power plant, and transient power well in excess of 1 MW when power is supplemented by the traction battery, the hybrid locomotive will be the heaviest and most powerful fuel cell land vehicle yet built. For energy storage, 14 lightweight carbon-fiber composite tanks are located above the traction battery (see Figure 3). The Citaro buses use the same type of tank, also located at the roofline. The light weight of the carbon-fiber tanks allows the roof location without adversely affecting the vehicle’s center of gravity. Hydrogen fuel storage uses readily available hardware and proven safety design measures. The 14 tanks have a combined storage of 70 kg of compressed hydrogen at 350 bar (5100 psi; 1 bar ¼ 105 Pa). This storage system provides fuel for a rigorous 8–10 h switcher duty cycle. Similar to the mine locomotive, the fuel cell power plant and hydrogen storage are substantially lighter than the diesel genset and diesel fuel tank they replace, and a ballast of approximately 9000 kg must be added to bring the operating weight up to 127 tonne. This is equivalent to a steel cube slightly more than 1 m3 in volume. The fuel cell power plant under construction is shown in Figure 4.

Principal Issues Fuel Cell Type Presently, five fuel cell types are potentially appropriate for fuel cell rail, namely, (1) solid oxide (SOFC), (2) molten carbonate (MCFC), (3) phosphoric acid (PAFC), (4)PEMFC, and (5) alkaline (AFC). For a locomotive, with a fixed-weight requirement, volumetric power density is more important than gravimetric power density. All of the fuel cell rail vehicles described above

used the PEMFC type. Comparison of the PEMFC type and others is summarized below. Solid oxide fuel cell is not commercialized to the extent that a system large enough for a rail vehicle can presently be purchased. Additionally, the SOFC type has at present a very short cycle life, and precommercial products have a thermal cycle life as short as five cycles (i.e., the stack can be brought up to operating temperature and then allowed to cool down to ambient temperature a total of five times before it fails). Diesel locomotives are generally not turned off but idle for long periods of time, whereas railway operators would prefer to minimize idle periods, and there is also strong environmental pressure to minimize idle time. Thus, the present long-idle characteristic of diesel locomotives is not a good rationale for an SOFC system that cannot be turned off. The SOFC can be thought of as a ‘glass’ fuel cell, and its ruggedness for rail applications is questionable. It does have a reasonable power density. Molten carbonate fuel cell systems are precommercialized in sizes as high as megawatt power ratings, but the volumetric power density is about one-tenth that of the PEMFC system (see below). For typical power requirements of locomotives, an MCFC stack alone, ignoring BOP, would occupy nearly the entire engine compartment. Phosphoric acid fuel cell systems are well developed and have a long commercialization history, with at least two-hundred 200-kW stationary power systems sold since the early 1990s. Its commercial development seems to be coming to an end, primarily because it has resisted cost reduction. Phosphoric acid fuel cell–battery hybridpowered transit buses using reformed methanol as fuel have been developed, in particular, by Georgetown University. Although an advantage of the PAFC type is long life, and some have achieved 40 000 h lives, the power density is low compared to PEMFC. Accordingly, the transit buses were fuel cell–battery hybrids. They were capable of operating at a steady state in an urban driving (duty) cycle but were not capable of long-haul operation because the traction battery would soon be depleted. The PEMFC type is the type preferred by and actively being developed by automakers. Its principal advantages are high volumetric power density, approaching 3 kW L1 for prototypes, short start-up times, and high tolerance of gas-pressure differential between cathode and anode. Its disadvantages include high cost and relatively short (but increasing) stack life. Stack life is a decreasing function of current density. Depending on current density, which is modulated in a fuel cell–battery hybrid system, state-of-the-art commercial PEMFC stacks are capable of about 12 000 h lives. A successful urban transit bus program, based on a Citaro chassis and using nonhybrid 300 kW (continuous gross) Ballard fuel

Applications – Transportation | Rail Vehicles: Fuel Cells Fuel cell power plant

317

Radiators

Traction battery

Hydrogen tanks

Ballast

Figure 3 Computer-aided design model of completed fuel cell hybrid switch (shunt) locomotive. Top view includes the hood, with the ventilation system, covering the compressed-hydrogen storage tanks. Bottom view, with the hood removed, shows the 14 carbon-fiber composite tanks at the roofline. The fuel cell power plant resides in the right half of the rear compartment behind the traction battery.

cells, has logged over 1.5 million kilometers of road experience worldwide. More than 10 cities cooperated in the program, denoted by the acronym CUTE (Clean Urban Transit for Europe). Alkaline fuel cells, vis-a`-vis acid systems such as PAFC and PEMFC, employ a different chemical mechanism of oxygen reduction at the cathode, the rate-limiting process in a hydrogen–oxygen (air) fuel cell. Potentially, a substantial increase of current and power density is possible by using an alkaline electrolyte. Non-

noble catalysts are also adequate, and at the typically high potassium hydroxide concentration of 54%, the boiling point of the electrolyte is very high. High operating temperature translates into high current and power density, and ultimately low cost. A disadvantage is that the alkaline electrolyte absorbs carbon dioxide from the cathodic air and forms insoluble carbonate. This challenge may be overcome by scrubbing the air of carbon dioxide, but the volume and additional hardware of the scrubber tend to nullify the inherent high current density

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Applications – Transportation | Rail Vehicles: Fuel Cells

Figure 4 The fuel cell power plant under construction at the Burlington Northern and Santa Fe (BNSF) Topeka Rail Shop, Topeka, KS, USA, on 9 January 2008.

Table 2

Theoretical hydrogen volumetric densities

Fuel system

Conditions of storage

H2 density (kg m3)

Gaseous H2 Liquid H2 Methanol Liquid ammonia Reversible metal hydride

350 bar (5100 psi) (T ¼ 298 K) r ¼ 0.070 g mL1 (P ¼ 1 bar, T ¼ 20 K) r ¼ 0.79 g mL1 (T ¼ 298 K) r ¼ 0.62 g mL1 (P ¼ 7.2 bar, T ¼ 288 K) AB5 alloy (LaNi5), r ¼ 8.3 g mL1, wt% ¼ 1.5, P ¼ 10 bar

25 70 99 110 125

Source: Reproduced from Miller AR, Hess KS, and Barnes DL (2007) Comparison of practical hydrogen-storage volumetric densities. In: Proceeding of the National Hydrogen Association Annual Hydrogen Conference, San Antonio, TX, 91–22 March. Copyright by Vehicle Projects Inc.

and low cost. The AFC type, operating at nearly 2001C, has been successfully used as the cabin power source in aerospace applications, starting with the Apollo program and continuing with the Orbiter. This notwithstanding, the AFC type has not been commercialized in terrestrial applications at higher power rating than a few kilowatts. Hydrogen Storage Storage of hydrogen onboard the vehicle is a greater technical challenge than producing power from a fuel cell. Methods of storage appropriate for locomotives include (1) direct storage of hydrogen as a compressed gas, (2) direct storage as a liquid, (3) direct storage as a reversible metal hydride, (4) onboard chemical transformation to hydrogen of a carbon-based feedstock such as a hydrocarbon or alcohol, and (5) physical dissociation of liquid ammonia to hydrogen. For industrial vehicles in general, and especially for locomotives, minimum volume of the fuel storage system or power plant is more important than minimum weight. That

is, a high hydrogen volumetric density is more important than a high gravimetric density. Table 2 displays the limits or theoretical values of hydrogen volumetric density, as kg m3, for the five fuels mentioned above. These limits are a theoretical construct – they provide a measure of the best possible volumetric density that a given fuel can attain. They omit the volume of the container, associated hardware, and chemical processor. For example, if one had a cubic meter of hydrogen at a pressure of 350 bar, but stored in tank, with piping, etc., of infinitesimal volume, the system would store 25 kg of hydrogen, corresponding to a volumetric density of 25 kg m3. In the case of methanol, which requires reacting the alcohol with water at high temperature over a catalyst to produce hydrogen according to the equation CH3 OH þ H2 O-3H2 þ CO2

½I

the limiting volumetric density also omits the volume of the reactant water (in principle, water can be obtained from the

Applications – Transportation | Rail Vehicles: Fuel Cells

fuel cell). The results show that, in the limiting case, the reversible metal hydride is capable of the highest hydrogen volumetric density, namely, 125 kg m3, and compressed hydrogen, the lowest. Real systems require volume for their hardware (e.g., tank, piping, and valves, as well as chemical reactors for methanol and ammonia), and thus the practical hydrogen volumetric densities shown in Table 3 are smaller than the theoretical values of Table 2. The practical densities were computed from the known volumes of actual systems. For example, based on scale-up of the storage system of the commercially available BMW Hydrogen 7TM automobile, which operates on liquid hydrogen, the hydrogen volumetric density of a real liquid hydrogen system is 26 kg m3 rather than the 70 kg m3 for the theoretical system. The density of the practical methanol system includes the reactant water, as well as the reformer hardware. ‘Storage efficiency’ is defined as the practical density/ theoretical density  100%. For example, liquid H2 has a storage efficiency of 26 kg m3/70 kg m3  100% ¼ 37%. Storage efficiency is a measure of how closely a storage system approaches its volumetric density limit or Table 3

Practical hydrogen volumetric densities

Fuel system

Practical H2 density (kg m3)

Storage efficiency (%)

Compressed H2 Reversible metal hydride Methanol reformer Liquid H2 Ammonia dissociator

10 20 23 26 44

40 16 23 37 40

Source: Reproduced from Miller AR, Hess KS, and Barnes DL (2007) Comparison of practical hydrogen-storage volumetric densities. In: Proceeding of the National Hydrogen Association Annual Hydrogen Conference, San Antonio, TX, 19–22 March. Copyright by Vehicle Projects Inc.

theoretical density; it is a measure of how well a storage system lives up to its potential, the limits of Table 2. In conclusion, with today’s technology, liquid ammonia, at 44 kg m3, has the highest practical hydrogen volumetric density. Compressed hydrogen, at 10 kg m3, has the lowest. Compressed hydrogen and liquid ammonia, at 40% each, have the highest storage efficiency, and reversible metal hydride storage, at 16%, has the lowest. In choosing a hydrogen storage system for a vehicle, factors other than volume may be important. Four examples are weight, safety, cost, and thermodynamic efficiency. Hybrid Power Potential benefits of a hybrid powertrain (see Figure 5) are (1) enhancement of transient power and hence tractive effort, (2) regenerative braking, and (3) reduction of capital cost. However, these potential benefits may be weak in locomotives and other forms of railway motive power because of the characteristics of steel wheels on steel rails and the large kinetic energy of trains. The effectiveness of a hybrid powertrain in exploiting these benefits depends heavily on the duty cycle and the characteristics of the operating route. There are also a number of fixed operating requirements, propulsion system design issues, and fundamental operating constraints that will determine the feasibility of applying hybrid power systems. For example, the time required for a train to negotiate a long grade may require a hybrid system with an impractically large auxiliary storage capacity. The rational starting point for engineering design of a fuel cell-hybrid vehicle is the duty cycle. Figure 6 shows a typical duty cycle – that is, function p(t), where p is the vehicle power and t is the time – recorded from an in-service yard-switching locomotive. The vehicle’s

Fuel cell DC/DC converter

Fuel cell prime mover

Aux storage (battery or flywheel)

319

Traction motors

Bidirectional DC/DC converter

Figure 5 Main components of a fuel cell hybrid powertrain. ‘Aux storage’ represents either a battery or flywheel auxiliary energy/power device. Arrows point in the direction of power flow. The traction motors are used as generators during braking. A system using alternating current (AC) traction motors would be analogous to the direct current (DC) system shown.

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Applications – Transportation | Rail Vehicles: Fuel Cells 1400

800

1200

700

Power (kW)

500 800 400 600 300

Energy (kWh)

600

1000

400 200 200

0 0.0

100

1.0

2.0

3.0

4.0

6.0

5.0

7.0

8.0

9.0

0 10.0

Time (h) Tractive power (kW)

Mean power (kW)

Energy (kWh)

Figure 6 Example of a duty cycle, power as a function of time, for a switch locomotive operating in a rigorous environment, the Port of Long Beach, CA, USA. Data courtesy of BNSF Railway Company.

required mean power, maximum power, power response time, and power duration are calculated from function p; its energy storage requirements are calculated from the integral of p. As shown, peak power commonly reaches 600–1000 kW for durations of no more than several minutes, usually corresponding to acceleration of train cars or uphill movement. Between the peaks, however, the power requirements are minimal when coasting a load, or zero when idling between move operations. The idle time, varying from minutes to hours between operations, usually accounts for 50–90% of the overall operation schedule. Analysis of multiple duty-cycle data sets from various railyards shows that the short duration of peak power and long periods of idle time result in mean power usage in the range of only 40–100 kW. For a hybrid vehicle to be self-sustaining, the prime mover, a hydrogen PEMFC in this case, must provide continuously at least the mean power of the duty cycle. The auxiliary power/energy storage devices, which are lead–acid batteries in this hybrid (Figures 2 and 3), must store sufficient energy to provide power in excess of the continuous power rating up to the peak power requirement of the vehicle, which is around 1100 kW in this application. This power and energy must be available while not exceeding a rather shallow depth of discharge, which significantly increases the size of the battery. Allowable depth of discharge is a function of acceptable battery cycle life and recharge rate. With lead–acid batteries, depth of discharge is limited to approximately

80% of full capacity. The relatively large energy capacity of the batteries allows satisfactory load followed by the fuel cell power plant and moreover increases stack life. With regard to the design of a hybrid switch locomotive, the only current practical auxiliary storage devices are batteries such as lead–acid, nickel–metal hydride, or lithium ion. Owing to wheel-adhesion limitations of locomotives operating at low speeds, there are no likely performance benefits to be derived from potentially increased tractive effort available from hybrid power. Brake energy recovery is not practical for switch locomotives because of the lack of available kinetic energy and the relatively poor performance of traction motors in generation mode at low operating speeds. For high-speed heavy applications such as freight, the ability of the auxiliary power device to absorb a significant portion of the available kinetic energy is low. Moreover, the hybrid power plant suffers a double efficiency penalty: losses occur in both absorbing and then releasing energy from the auxiliary device, which, depending on the rate of the process, result in a net storage efficiency of possibly no more than 50% for current battery technology. Based on current cost data, significant cost benefits should be available from the use of a fuel cell hybrid configuration in a switch locomotive. Weight and space limitations constrain available hybrid configurations and prevent the use of the cost-optimized solution. Because of the double efficiency penalty, a hybrid locomotive will require either a 20–40% increase in fuel capacity or a

Applications – Transportation | Rail Vehicles: Fuel Cells

20–40% reduction in operating time for the same duty cycle. As future fuel cell production and operating costs are reduced, the cost advantage of a hybrid will dissipate. Regarding the practicality of hybrid rail vehicles, although they cannot fully reach their potential for enhanced performance, the most likely applications to benefit from enhanced tractive effort and regeneration are commuter rail and long-distance intercity passenger trains. Packaging of the bulky hybrid power plant is relatively easy because it is in a separate locomotive. In contrast, for light rail and mass transit, packaging is difficult because power equipment is distributed over a multiple-unit vehicle and must be mounted under the floor. Because maximum power is required for extended periods and because only a fraction of the large kinetic energy of train can be absorbed in today’s auxiliary storage devices, cases least likely to benefit from enhanced transient power or regeneration are high-speed rail and heavy freight. Yard switchers, and possibly other rail vehicles, may benefit from reduced capital cost (or first cost) of a hybrid power plant. However, this benefit comes at the price of increased complexity and reduced thermodynamic efficiency.

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hybrid switch locomotive is nearing completion. Rail transport is inherently more energy efficient than almost all alternatives, and it is 3–4 times more efficient than trucks and more than 50 times more efficient than air transport. Fuel cell rail combines the environmental advantages, at the locomotive, of the catenary-electric locomotive with the higher efficiency and lower infrastructure cost of the diesel-electric. Of several fuel cell types available, PEMFC has been chosen, primarily because of high volumetric power density and ruggedness, for all the fuel cell rail vehicles discussed in this article. Several options exist for hydrogen storage, but hydrogen storage is more problematic than fuel cell power, and storage remains an open problem. A concept vehicle, a supersonic tube vehicle, has been proposed that would solve the hydrogen storage problem for long-range transport, as well as the induced-drag problem that severely limits the energy efficiency of aircraft. With the exception of the switch (shunt) locomotive, hybrid powertrains are generally not practical for fuel cell rail vehicles.

Nomenclature Potential A concept rail vehicle utilizing hydrogen fuel cells and capable of operating at supersonic speed has recently been proposed, analyzed, and, to a limited extent, demonstrated (see Further Reading). Study of the vehicle’s feasibility is based on mathematical analysis of vehicle aerodynamics in a hydrogen atmosphere, and a working model has been built to demonstrate the most problematic aspects of the technology. The central concept is that operation of a vehicle in a hydrogen atmosphere, because of the high speed of sound in hydrogen and low density of hydrogen, would move the transonic region to much higher values and pressure drag would be reduced relative to air. A hydrogen atmosphere requires that the vehicle operates in a hydrogen-filled tube or pipeline. The proposed vehicle, a cross between a train and an airplane, is multiarticulated, runs on a guideway, is propelled by propfans, and flies on a hydrogen aerostatic fluid film. The system solves the open problem of onboard vehicular hydrogen storage by breathing its fuel from the tube itself and, unlike airplanes, does not suffer the energy-efficiency limitation of induced drag (see eqn [1]). While maintaining supersonic speed, the tube vehicle is calculated to require one-tenth the energy of a comparable subsonic airplane.

Conclusions A nonhybrid fuel cell-powered underground mine locomotive was successfully demonstrated in 2002, and a

Symbols and Units b Ci Di p P t T V w q

wingspan induced drag coefficient induced drag power pressure time temperature velocity airplane weight density

Abbreviations and Acronyms AFC BNSF BoP CUTE DC LLC MCFC PAFC PEMFC SOFC

alkaline fuel cell Burlington Northern Santa Fe balance of plant Clean Urban Transit for Europe direct current limited liability company molten carbonate fuel cell phosphoric acid fuel cell proton exchange membrane fuel cell solid oxide fuel cell

See also: Applications – Stationary: Uninterruptible and Back-up Power: Fuel Cells; Applications – Transportation: Buses: Fuel Cells; Hybrid Electric Vehicles: Batteries; Fuels – Safety: Hydrogen: Transportation; Fuel Cells – Overview: Introduction;

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Fuel Cells – Proton-Exchange Membrane Fuel Cells: Cells.

Further Reading Gavalas GR, Voecks GE, Moore NR, Ferrall JF, and Prokopius PR (1995) Fuel Cell Locomotive Development and Demonstration Program. Phase 1: Systems Definition. Los Angeles, CA: Jet Propulsion Laboratory. Hirst E (1973) Energy-intensiveness of transportation. Journal of the Transportation Engineering Division, American Society of Civil Engineers 99(1): 111--122. Miller AR (2001) Least-cost hybridity analysis of industrial vehicles. European Fuel Cell News 7: 15--17. Miller AR (2005) Fuel cell locomotives. In: Proceedings of Locomotive Maintenance Officers Association Conference, Chicago, IL. Miller AR (2008) Hydrogen tube vehicle for supersonic transport: Analysis of the concept. International Journal of Hydrogen Energy 33(8): 1995--2006 (doi:10.1016/j.ijhydene.2008.01.030).

Miller AR and Barnes DL (2002) Fuel cell locomotives. In: Proceedings of Fuel Cell World, Lucerne, Switzerland. Miller AR, Hess KS, and Barnes DL (2007) Comparison of practical hydrogen-storage volumetric densities. In: Proceedings of the National Hydrogen Association Annual Hydrogen Conference, San Antonio, TX, 19–22 March. Miller AR, Hess KS, Barnes DL, and Erickson TL (2007) System design of a large fuel cell hybrid locomotive. Journal of Power Sources 173: 935--942. Miller AR, Peters J, Smith BE, and Velev OA (2006) Analysis of fuel cell hybrid locomotives. Journal of Power Sources 157: 855--861. Ogawa K, Yamamoto T, and Yoneyama T (2008) Energy efficiency and fuel consumption of fuel cells powered test railway vehicle, poster session 2.26. In: Proceedings of the 8th World Congress on Railway Research, Seoul, Korea. Smith HC (1992) The Illustrated Guide to Aerodynamics, 2nd edn. Blue Ridge Summit, PA: TAB Books. Vehicle Projects LLC (2004) Fuel Cell Surface Locomotive Project: Phase 1. Project Reports and Deliverables. Department of Defense Contract F42620-22-D-0036, Task Order 0023.