when such vehicles achieve major market penetration (2050). (See Chapter 6.)

MOLECULAR HYDROGEN AS FUEL

Molecular hydrogen is currently receiving the most attention and financial support as the starting point for fuel cell energy supply. The literature and the many presentations that the committee heard indicate that the manufacture of molecular hydrogen is the consensus approach favored by the majority of leadership within the government, at universities, and in industry. It is favored because it allows the use of a variety of hydrogen sources, ranging from coal and natural gas to biomass, solar, wind, and nuclear energy, as well as a multitude of relatively well understood manufacturing approaches ranging from small to large reformers, water-gas-shift reactors, electrolytic devices, thermal processes, and so on. (See Chapter 8 and Appendix G for a discussion of the various hydrogen production technologies.)

In the early stages of a transformation to a hydrogen economy, molecular hydrogen will probably be obtained from existing sources such as chemical plants and petroleum refineries. Today, about 9 million tons of hydrogen are manufactured annually in the United States2 and transported for chemical and fuel manufacturing as a low- or high-pressure gas via pipelines and trucks or even as a cryogenic liquid (DOE, 2002a). Much experience worldwide has been achieved over many years to make these transportation modes safe and efficient. However, if the volume of hydrogen use grows, new safety and cost issues will surface, requiring major infrastructure changes. The committee found the analysis presented by Joan Ogden, among others, to be reasonable.3 These analysts contend that in the very early stage of transition to the hydrogen economy, supplying of hydrogen for use in fuel-cell-powered vehicles would rely predominantly on over-the-road shipment of cryogenic liquid hydrogen or possibly hydrogen in high-pressure cylinders from existing chemical and petroleum refining plants.4 Because of the high cost of such shipment modes, government subsidies would probably be needed to help fuel-cell-powered vehicles approach cost parity with gasoline-powered cars. It is also possible that pipelines could be used from existing manufacturing facilities, but this would only be possible where location dictated favorable economics as compared with costs for road shipment. The committee believes that as the volume of demand grows, however, this approach will evolve to the use of local distributed hydrogen production based on natural gas reformers and electrolytic units. These alternatives are less capital-intensive than that of building special pipelines coupled to large, dedicated hydrogen manufacturing plants, and are undoubtedly more economic than continued over-the-road shipping.

Whether molecular hydrogen is manufactured centrally or locally, a number of transportation, distribution, and storage requirements pose significant technical, cost, and safety problems. These various requirements could necessitate the use of interim storage facilities at plant sites for inventory or to compensate for demand swings and plant interruptions; the possible use of storage along pipelines and at distribution hubs; storage at the fuel cell vehicle loading stations; and, most critically, storage on board the vehicles themselves. For clarity, on-board vehicle storage is addressed separately from off-board storage, which is associated with distribution from the hydrogen-manufacturing site to the vehicle filling facilities.

The committee notes that resilience to terrorist attack has become a major performance criterion for any infrastructure system. In the case of hydrogen, neither the physical and operating characteristics of future infrastructure systems nor the timing of their construction can be understood in sufficient detail to permit an analysis of their vulnerability. However, the committee does observe that public concerns with terrorism seem likely to influence the choice of any future energy system and that resilience to deliberate attack is best designed in at the beginning.

Centralized Production of Molecular Hydrogen

Table 4-1 underscores key aspects of the costs of moving molecular hydrogen from its place of manufacture to the place where it is used as compared with the same types of costs for today’s conventional fuels such as gasoline and natural gas. The table presents a series of cases that the committee developed for purposes of understanding costs and indicating where research or technology development might play a useful role in reducing them. The increased costs for transportation of molecular hydrogen versus those for conventional fuels are the direct result of the fundamental physical and thermodynamic properties of molecular hydrogen compared with today’s liquid fuels.

Molecular hydrogen is a uniquely difficult commodity to ship on a wide scale, whether by pipeline, as a cryogenic liquid, or as pressurized gas in cylinders. On a weight basis, hydrogen has nearly three times the energy content of gasoline (120 megajoules per kilogram [MJ/kg] versus 44 MJ/kg), but on a volume basis the situation is reversed (3 megajoules per liter [MJ/L] at 5000 pounds per square inch [psi] or 8 MJ/L as a liquid versus 32 MJ/L for gasoline). Furthermore, the electric energy needed to compress hydrogen to 5000 psi is 4 to 8 percent of its energy content, depending on the starting pressure; to liquefy and store it is of the order of 30 to 40 percent of its energy content.5 Pipe-

2  

Jim Hansel, Air Products and Chemicals, Inc., personal communication to Martin Offutt, National Research Council, October 3, 2003.

3  

Joan Ogden, Princeton University, “Design and Economics of Hydrogen Energy Systems,” presentation to the committee, January 23, 2003.

4  

Joan Ogden, Princeton University, “Design and Economics of Hydrogen Energy Systems,” presentation to the committee, January 23, 2003.

5  

Joan Ogden, Princeton University, “Design and Economics of Hydrogen Energy Systems,” presentation to the committee, January 23, 2003.



The National Academies | 500 Fifth St. N.W. | Washington, D.C. 20001
Copyright © National Academy of Sciences. All rights reserved.
Terms of Use and Privacy Statement