Energy issues have been at the center of the national security debate for some time, and the current situation in the Persian Gulf underscores the strategic importance of sound energy policy. Activities or developments--geopolitical, environmental, technological, or regulatory--that materially change the energy security equation are, naturally, of great interest to the Department of Defense (DOD). The announcement by President George Bush in his State of the Union address that he intends to accelerate research and development (R&D) for hydrogen-powered vehicles toward the objective of total U.S. energy independence has great potential impact on DOD. This paper examines a number of technical issues connected with energy independence through hydrogen and how they might affect DOD. We conclude that the move to a hydrogen economy will be a massive undertaking, requiring large investments and decades to accomplish. We will show that, with few exceptions, pure hydrogen is not a viable fuel for DOD missions, primarily because of the DOD requirement for compact, high-volumetric energy density power sources. As a result, to meet its unique needs, DOD likely will have to increase its dependence on nuclear power and support R&D that investigates ways to use hydrogen to synthesize hydrocarbon fuels in an environmentally compliant fashion. Several suggestions and recommendations will be made in this regard.
Hydrogen as a Fuel
Hydrogen is a much-studied element, large quantities of which are produced today for industrial applications. Most of this hydrogen is a chemical commodity rather than an energy commodity. There are, of course, some specialized uses of hydrogen as a fuel, such as in rocket propulsion. Given the high-energy content of hydrogen and its intrinsic non-pollutant properties, it is reasonable to ask: why has it not been used widely as a fuel? Table 1 indicates some of the reasons.
In this table, various fuels have been normalized to a typical gasoline. The comparison is done on a mass and volume basis. The second column of table 1 shows that, on a pound-for-pound basis, hydrogen has a higher heating value than the other fuels shown. This well-known fact is often touted as one of the great advantages of hydrogen. However, the volume comparison (third column) shows that hydrogen has the lowest heating value per unit volume among the fuels listed (excluding the lithium ion battery). Even liquid hydrogen at -253 [degrees]C has only one-fourth the volumetric energy content of gasoline. Furthermore, liquid hydrogen requires complex cryogenics, while gasoline is liquid at room temperature and can be stored and transported easily in inexpensive containers.
The volumes required for the storage and transportation of fuels and the costs of the fuel storage containers are big issues and significant factors in why hydrogen has not emerged as a general-purpose fuel to date.
There are important fuel properties other than energy density. Among these are the limits of flammability, flame speed, minimum ignition energy, auto ignition temperature, ignition properties in the presence of catalysts, and environmental impact. Hydrogen has very wide limits of flammability (4-75 percent hydrogen concentration) and a very high flame speed. It also has a low spark ignition energy (0.0182 millijoules [mJ]) and an auto-ignition temperature somewhat higher than hydrocarbon fuels. However, unlike hydrocarbon fuels, hydrogen can ignite at low temperatures in the presence of catalysts such as rust, and the ensuing flame is nearly impossible to detect with the naked eye. On the positive side, hydrogen is an environmentally benign fuel, producing mainly water when combusted or used in fuel cells. All of the fuel properties of hydrogen impact both positively and negatively on its viability as a fuel.
Another important consideration in evaluating a fuel is the ease of storage and distribution. …