a system based on central station production will have a much smaller number of sites but with larger emissions. In summary, the extent to which criteria pollutants would be an issue in a hydrogen energy system would depend on the specific production technologies deployed, the extent of their deployment, and the pollution control equipment and regulatory regimes that are implemented.
End use of hydrogen in fuel-cell-powered vehicles will result in a much different mix of types of emissions compared with those from today’s gasoline vehicles, and also a much different profile of where in the life cycle (i.e., at resource extraction, production, distribution, and/or end use) the emissions will occur. Today’s gasoline or diesel-powered car is a major source of criteria pollutant emissions in the United States, whereas the hydrogen-powered fuel cell vehicle will not emit any criteria pollutants. The only significant emission will be water in the form of vapor or liquid. Small amounts of hydrogen and nitrogen dioxide may be emitted from combusting the tail gas that passes through the fuel cell unreacted. The widespread use of hydrogen-powered fuel cell vehicles will have a positive impact on air quality in many urban areas of the United States, where cars currently are responsible for large amounts of emissions. However, as noted above, it is during the production phase of the fuel cycle of hydrogen that the potential for the emission of criteria pollutants or greenhouse gas emissions exists.
In addition to regulated environmental toxicants, the requirements of a hydrogen energy system with respect to resources such as water and land should be considered. In all of the production processes mentioned in this report, water is used as a source for at least a portion of the hydrogen production—one-third by mass of the hydrogen from biomass gasification comes from water.6 In the hydrocarbon and coal-based processes, a significant portion of the hydrogen comes from water used in the water-gas-shift reaction. In the electrolytic processes, water is split using electricity. In the nuclear processes, water is split using high temperatures. Water is also used as a coolant in many of the processes, and large amounts are needed to grow biomass efficiently. The fuel cell, however, produces water from hydrogen and oxygen. The net balance and also the location of water needs were not reviewed by this committee.
Similarly, study is needed of the impact of large-scale biomass growth for feedstock for its impact on land use and any effect on nutrient runoff and eutrophication secondary to fertilizer demand (NRC, 2000).
Molecular hydrogen is a short-lived trace atmospheric gas (with approximately a 2-year lifetime), having tropospheric concentrations of approximately 0.5 part per million. Its global distribution favors slightly higher concentrations in the Northern Hemisphere (about +5 percent), and well-defined seasonal cycles are observed (Novelli et al., 1999). A percentage of today’s atmospheric burden of molecular hydrogen is believed to be secondary to biomass burning and technological processes such as motor vehicle use (Novelli et al., 1999). Hydrogen is removed from the troposphere by surface deposition and by chemical destruction via oxidation with hydroxyl (OH) (Novelli et al., 1999). Various authors have noted that hydrogen is one of many gases that are removed from the troposphere by OH, and that, furthermore, the resulting decreases in concentrations of OH could lead to higher concentrations of methane and tropospheric ozone (Derwent et al., 2001), both of which are established climate forcing agents (NRC, 2001b).
Finding 9-6. Any future hydrogen energy system, if based on coal, natural gas, or uranium, will likely imply some of the same environmental consequences that the use of those same resources has caused in today’s energy system. The scope and magnitude of these consequences will depend on the nature of the hydrogen technologies deployed, on the portfolio mix of primary resources on which these technologies are based, and on the pollution control equipment and regulatory regimes that are implemented.
Recommendation 9-7. The committee recommends that the Department of Energy initiate a comprehensive assessment of the suite of environmental issues anticipated to arise secondary to deployment of a hydrogen energy system, and that the DOE develop a quantitative understanding of the trade-offs and impacts.
As part of its effort, the committee reviewed the June 3, 2003, draft of “Hydrogen, Fuel Cells and Infrastructure Technologies Program: Multi-Year Research, Development and Demonstration Plan” (DOE, 2003b). Although very impressed by the plan and its thoroughness, the committee believes that several general aspects of the plan need to be addressed in greater detail. Comments on the individual technology sections of the plan are contained in Chapter 8.
First, the plan is focused primarily on the activities within the Office of Hydrogen, Fuel Cells and Infrastructure Technologies in the Office of Energy Efficiency and Renewable Energy, and it only casually mentions activities in the Office of Fossil Energy; the Office of Nuclear Energy, Science and Technology; the Office of Science; and activities related to CO2 management. Development of an RD&D plan for the totality of the DOE’s hydrogen program will require a plan with better balance and integration.
Second, it is very difficult to identify priorities within the myriad of activities that are proposed. A general budget is