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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. Because of its low volumetric energy density, hydrogen can be viable as a fuel only in a liquid state or at very high pressures. Yet using liquid hydrogen as a fuel has disadvantages from purely energetic considerations. About 30 percent of the energy content of hydrogen is required just to liquefy it. This energy is not recoverable in a practical sense. For special applications where costs are not a consideration, liquid hydrogen indeed may be viable as a fuel. However, for large-scale energy applications, a requirement for liquefaction would seem to put hydrogen at a great disadvantage.
Compressing hydrogen gas also requires energy, but only a fraction of that required for liquefaction. For example, table 1 shows that hydrogen at 10,000 pounds per square inch (psi) has a volumetric energy density approaching that of liquid hydrogen. Compressing hydrogen from atmospheric pressure to 10,000 psi requires about 11 percent of the hydrogen energy content. This is a more reasonable energy penalty but does involve dealing with fuel at very high pressures. Progress is being made in producing relatively light-weight composite containers for this purpose. Containers are now available to store hydrogen compressed to 5000 psi at about 11 percent hydrogen by weight. (1) The total container volume, however, is about twice the hydrogen storage volume. This exacerbates the hydrogen storage problem and presents DOD with some unique safety problems, especially in combat situations.
The above discussion implies that the best distribution of hydrogen from centralized source to user would be through gas pipelines. Liquefying hydrogen and then transporting the liquid would seem to be viable only for special applications such as space launch. Trucking highly compressed gas over long distances would not be economically viable due to the low volumetric energy content of the compressed hydrogen. A key question regarding pipeline distribution of hydrogen will be how much power must be supplied to overcome pressure drops along the pipeline. For fully turbulent flow, the required power can be shown as
P = (([rho][v.sup.2]/2)(4L/D)f)([pi][D.sup.2] v/4)
where: [rho] is the gas density; v is the gas velocity; L is the pipeline length; D is the pipeline diameter; and f is the friction coefficient. (2)
Since the current pipeline standards have been set by the natural gas distribution system, it is useful to compare the power that must be supplied to hydrogen relative to that which must be supplied to natural gas (mostly methane) for the same energy flux down the pipeline. This ratio is:
[P.sub.H]/[P.sub.M] = ([D.sub.H]/[D.sub.M])[([v.sub.H]/[v.sub.M]).sup.3] ([[rho].sub.H]/[[rho].sub.M])([f.sub.H]/[f.sub.M])
where the subscripts H and M refer to hydrogen and methane respectively. If one were to use the same pipeline diameter ([D.sub.H] = [D.sub.M]) and the same pressure and then increase the velocity to obtain the same energy flux through the pipeline, the pipeline energy loss for hydrogen would be about three times that for methane. This is not a very attractive approach, especially for long pipelines. As another option, if one were to pump at the same pressure and velocity but increase the pipeline diameter to obtain the required energy flux, then the hydrogen energy loss would be about 15 percent of the methane energy loss. Changing the parameters yet again, if one used the same diameter pipe and pumped at the same velocity but increased the pressure, the hydrogen loss would be about 30 percent of the methane loss. Another factor to be considered is Graham's Law, which indicates hydrogen will leak at about three times the rate of natural gas, thereby creating potential safety problems unless special attention is paid.
The conclusion here is that the use of the current natural gas pipeline infrastructure (beyond the existing rights of way) is probably not viable in the long term for a move to a hydrogen economy. A pipeline infrastructure specifically designed for hydrogen will be required. This is not a surprising result, but it will require a substantial long-term investment. For example, a 12-inch diameter pipeline designed for hydrogen costs about $1 million per mile. (3)
In light of the above, table 2 provides an examination of hydrogen as a fuel by rating its properties as an energy source, a coolant, and a medium to be stored and distributed. Table 2 illustrates how many of the features that make hydrogen desirable from one perspective make it undesirable from another perspective.
Despite some of the known, unfavorable qualities inherent in hydrogen, it is important to note that hydrogen is produced and distributed safely in large quantities today and that procedures have been developed to overcome many of its negative attributes at these quantities. However, tables 1 and 2 suggest that, all other things being equal, if hydrogen were readily plentiful, combining it with carbon to make hydrocarbon fuels would be the most desired option because of the logistic simplicity and high-volumetric energy density found in hydrocarbon fuels. In light of this, why even consider moving to a hydrogen fuel economy? At least two reasons support such a move: the ultimate depletion of oil and natural-gas reserves, and environmental considerations such as the production of carbon dioxide as a greenhouse gas. While there is considerable disagreement over when fossil fuel resources will be depleted, there is little disagreement that eventually they will run out.
In the case of oil reserves, the expected time frame of depletion ranges from 20 to perhaps 100 years. The American Petroleum Institute suggests a 95 percent probability that the world's remaining oil reserves will last another 56 years and a 5 percent probability that they will last another 88 years. (4) If this is the case and hydrogen is to be the replacement fuel, then making it viable must be an immediate priority. With regard to the issue of greenhouse gases, the timeframe to watch is set by the time at which the C[O.sub.2] concentration in the atmosphere reaches a level where it produces irreversible climate effects. The predictions in this regard are based upon complex computer models and have considerable uncertainty associated with them. Most project U.S. temperature increases ranging between 3 and 4 [degrees]C over the next 100 years. (5) Such rises in average temperature would have significant climatic impact. Therefore, within the current level of understanding, actions required to address oil reserve depletion and greenhouse gases would …