Commencing in June 2003, an industry-government consortium began a program to develop fuelcell-powered locomotives as prototypes of a new generation of locomotive that will (1) reduce air pollution in urban rail yards, particularly yards associated with seaports, (2) increase energy security of the rail transportation system by using a fuel independent of imported oil, (3) reduce atmospheric greenhouse-gas emissions, and (4) serve as a mobile backup power source for critical infrastructure (“power-to-grid”) on military bases and for civilian disaster relief efforts.

Led by Vehicle Projects LLC of Denver, Colorado, USA, the consortium is presently developing two fuelcell-powered locomotives. The consortium includes potential end-users (e.g., BNSF Railway Company), a locomotive manufacturer, a university, federal laboratories, and technology companies. Under development are (1) a 127-tonne fuelcell-battery hybrid switcher (shunt) locomotive for use at the Port of Long Beach and (2) a 109-tonne pure fuelcell (non-hybrid) road-switcher locomotive for line-haul and military power-to-grid applications. Together, these locomotives will address all of the aforementioned objectives.

As part of the program's feasibility study, we have considered five technologies, listed in Table 1 [click on the image below], for storing fuel onboard a fuelcell locomotive. Although weight is generally not an issue for locomotives, volumetric hydrogen density of the storage system is critical because locomotives must obey strict regulations governing the shape and volume of their vehicle envelope. Locomotives must pass through tunnels, under bridges, and alongside trains and buildings; accordingly, volume of the storage and powerplant are limited and volumetric density is a critical design parameter.

Table 1 provides the theoretical limits of volumetric hydrogen density for the five storage technologies. This limit ignores the volume of the storage tank, piping, valves, heat exchangers, and other ancillaries. Under such assumptions, the limit of volumetric density of an AB₅ reversible metal hydride is five times that of compressed gas storage at 340 bar.

While the limits of hydrogen volumetric density provide a valuable baseline of comparison, of more interest to the vehicle developer is the practical volumetric density; that is, the density after the high-pressure tank, vacuum insulation, heat exchanger, chemical reactor, or water reactant is included in the volume of the system. Figure 1 illustrates a practical comparison of storage using metal hydrides at 10 bar and compressed hydrogen at 340 bar. After including the ancillaries, the volumetric density of the metal-hydride system is not five times greater than the compressed-gas system but only 2.2.

This paper will use similar methods to compare the practical hydrogen volumetric densities for all five storage technologies in Table 1: compressed hydrogen, liquid hydrogen, reformed methanol, dissociated ammonia, and reversible metal hydrides.