Fueling U.S. Transportation: the Hydrogen Economy and Its Alternatives
By Heiman, Michael K Solomon, Barry D
TRANSPORTATION is responsible for one-fourth of global greenhouse gas emissions and consumes 75 percent of world oil production. The U.S. transportation sector alone accounts for almost 10 percent of global energy-related greenhouse gas emissions.1 Insofar as it is a carrier and a storage medium for energy (unlike electricity, the other main energy carrier), hydrogen has been promoted as ideal for future transportation, and interest in it has been increasing in the last decade.2 Before a costly and potentially irreversible commitment to a hydrogen energy system is made, it will be critical to first consider criteria for a sustainable transport sector and then determine how hydrogen might measure up and over what time frame. Society’s limited time and resources may make it unfeasible to fully pursue all available options for addressing the problems of climate change and oil dependence, such as energy conservation; diesel, hybrid, and electric vehicles; and biomass fuels. Therefore, a careful comparison of options may be necessary. Interest in alternative-fueled vehicles can be attributed to rising concern over global climate change, peak world oil production, and growing dependence on insecure oil imports by the United States, Europe, and Japan. Hydrogen energy and hydrogen fuel cell vehicles (HFCVs), while not the only alternative for the transportation sector, have received considerable attention from government officials, automakers, and the media.
Although the United States failed to ratify the 1997 Kyoto Protocol, the treaty was implemented by U.S. trading partners, and took effect in February 2005. California’s ambitious carbon- emissions reduction program, announced in 2006, will be linked to it.3 Much of the world seems to be searching for viable (and preferably sustainable) alternatives to petroleum-based transportation to combat climate change. Consequently, a potentially emissions-free, locally available fuel such as hydrogen would be hard to resist, presuming consumer acceptance and cost-effective technologies. However, hydrogen as an energy carrier is only as green as its energy source. If that source is renewable, are there more appropriate uses for the input energy, and more attractive alternatives to power transportation, particularly if energy efficiency and reduced greenhouse gas emissions are primary goals?
As envisioned by Jules Verne in his 1874 book L’Ile Mysterieuse,4 the “hydrogen economy” recently called for by world leaders would not have to start from scratch.5 Hydrogen, one of the most abundant elements on Earth and lightest gas, has been a crucial raw material for many decades. About 85 percent of hydrogen consumed globally is used to produce ammonia for fertilizers or to remove sulfur from gasoline, heating oil, and diesel fuels.6 In addition, the National Aeronautics and Space Administration has been a major user of liquid hydrogen for rocket fuel since 1958.7 Roughly 90 billion normal cubic meters (cubic meters of a gas at standard temperature and pressure, Nm3) of hydrogen are produced annually in the United States and 500 billion Nm3 worldwide- a figure growing by 10 percent per year.8 Economics dictates that 95 percent of current U.S. hydrogen is produced by steam-methane reforming of non-renewable natural gas, so production in this manner is unsustainable. This method also results in more than 10 tons of carbon dioxide (CO2) emissions for each ton of hydrogen produced.9
Fortunately there are many other ways to make hydrogen, such as electrolysis of water (just 2 percent of current U.S. production), petroleum refining (30 percent of world production but negligible in the United States), and gasification of biomass or coal. While electrolysis is potentially the cleanest option, it is usually prohibitively expensive when compared to the cost of using the electricity directly as a primary energy carrier instead of as a hydrogen producer.10 Moreover, electrolysis currently costs 3-4 times as much to generate hydrogen compared to steam reforming of natural gas, while its associated CO2 emissions depend on the fuel source that produces the electricity.11 Reformation of hydrogen from coal (an abundant fossil fuel in the United States) with resulting CO2 sequestration has received much attention from the current Bush administration, with the U.S. Department of Energy (DOE) sponsoring pilot projects. However, while promising, this technology is as yet unproven geologically, economically, or thermodynamically on a scale large enough to support commercial production.
Energy for Transportation in the United States
The United States used 20.6 million barrels of non-renewable petroleum a day in 2006, almost a quarter of the total daily global demand of 84.5 million barrels. About 56 percent of this petroleum demand is used for motor gasoline and on-highway diesel in autos, sport utility vehicles, vans, trucks, buses, and motorcycles. Another 8 percent is used for jet fuel, with smaller amounts for ship fuel, petrochemicals, plastics, and myriad other products made or derived from petroleum. Crude oil and petroleum product imports have been growing and currently account for 66 percent of U.S. oil use. It is difficult to envision this demand changing much in the next decade. As a result, the transport sector is highly dependent on oil, which accounts for 97.8 percent of total transport energy use in the nation.12
Underscoring the importance of the addiction to unsustainable oil use in the United States is recent concern with “peak oil.” While 1970 was the peak year for domestic oil production, substantial debate has occurred over when world oil output will similarly peak. Most of the discussion has centered on the applicability of the model of the visionary geophysicist Marion King Hubbert, who accurately applied statistical physical methods to correctly predict the peak in crude oil production of the United States 14 years beforehand.13 For various reasons, application of his techniques to the world oil situation has been much more vexing, and debate over the ultimate peak date has increased in the last decade. Some analysts claim that world production already peaked in 2005, while others have argued for a date after 2030. More important than the exact date is the immutable geophysical fact that the supply will eventually peak and decline soon thereafter.14
The reality is that while the world will not be running out of oil anytime soon, it will be facing increasingly tight market conditions in the next decade as we run out of economically and politically affordable oil. Unless global oil demand tapers off, petroleum products will face continuous upward price pressures. Higher prices encourage oil companies to explore for and develop supplies in remote areas and in deeper fields. As a result, pressure for development in Alaska and the Outer Continental Shelf will continue. Low-cost fields in the conflict-laden Middle East, location of the world’s largest oil reserves, will reap large economic rents under these conditions, only underscoring how oil security concerns will become even more important than peak oil.
Petroleum markets will increasingly experience supply constraints that are geopolitical and technological in nature (as more nations peak in production). Only a few areas of production growth exist outside the Middle East.15 None of these areas, however, will likely be able to sustain increased production for more than five years. Most surplus oil from Russia, the largest oil producer outside of the Middle East and the United States, is sold in Europe because of closer trade ties. Despite strong trade ties, Mexico- the second- largest supplier of oil to the United States after Canada-is limited by its own needs. Moreover, rapidly rising demand for petroleum products in China and India could make these geopolitical patterns problematic for the United States.
The United States has been in this situation before. Starting with the oil embargo from the Arab members of the Organization of the Petroleum Exporting Countries (OPEC) in October 1973, the U.S. government spent nearly a decade actively exploring a large range of domestic energy options. Among these were oil shale, tar sands, and coal gasification and liquefaction.16 By the time pilot projects for these synthetic fuels got off the ground, high energy prices suppressed demand growth, and policy support for these projects all but disappeared when gasoline prices tapered off.17 In addition to their increasingly high price tag, synthetic fuels development in the United States has been hampered by serious concerns over water requirements, land cover disruption, waste generation, CO2 emissions, and other environmental problems. Even so, with higher oil prices today there is renewed interest in oil shale development in the western United States, especially through in-situ conversion. While U.S. oil shale reserves exceed those of domestic oil, it is still a non-renewable resource, and significant environmental questions remain. The same can be said for tar sands, which have been successfully developed in Canada, while receiving less interest in the United States. Sustainable Transportation Principles
For transportation to be sustainable, greenhouse gas emissions need to be reduced, as do emissions of conventional air and water pollutants. To do this, it is vital to procure a sustainable physical fuel supply (though the two are linked).18 Thus renewable fuels, such as biomass and biodiesel, need to be managed in an environmentally sound and sustainable manner.
Sustainability, however, encompasses much more, such as investments in poverty alleviation; clean water; transportation accessibility; and adequate diet, health care, and education.19 Social concerns must also be taken into consideration: For example, walking and bicycling are sustainable and generally socially desirable, but people’s opinions about these modes of transportation are colored by their perceptions of social class or economic forces like the supply of fuel. Many people may feel forced to use these options because of their low income or fuel supply shortages and would drop them if their income rose or more fuel became available. Based on these considerations, several principles of sustainable transportation emerge (see the box on page 14).
The U.S. transportation sector today is far from sustainable- note the paucity of renewable energy use in the sector. For example, while the use of corn-based (and environmentally problematic) ethanol in the United States is growing rapidly, in 2006 it accounted for less than 2 percent of total transportation energy consumption, or 5.5 billion gallons (including imports).20 This contrasts with Brazil, which is 40 percent reliant on sugarcanebased ethanol. No other nation approaches Brazil and the United States in their rate of renewable transportation fuel use. Based on the composite measurements of the Environmental Sustainability Index of Nations (2005) and the Pilot Environmental Performance Index (2006), developed by the Yale Center for Environmental Law and Policy, the United States ranks poorly. 21 The indices, however, do not include specific measurements for the transportation sector, only broader measurements of sustainable energy use. Furthermore, the carbon neutrality, total greenhouse gas emissions, and net energy gained from biofuels are contested, with a few lifecycle analyses concluding that the fossil fuel energy and global greenhouse gas emissions saved in ethanol and biodiesel production do not compensate for carbon used or emissions generated by the input, accounting for the crop grown, method employed, inputs, and the energy it takes to produce various useful coproducts such as animal feed, residual fuel sources, or chemical byproducts.22
Many candidate transportation fuels and technologies can reduce reliance on motor gasoline. These include more efficient autos; several renewable energy options; other non-renewable fuels (including diesel, which has a higher energy density than unleaded gasoline) that could be used as transitional fuels, especially if they are coupled with carbon capture and storage; and electric and hybrid vehicles. Although many are competitors with hydrogen energy and hydrogen fuel cell vehicles (HFCVs), others, such as renewable energy and hybrid technology, can be linked with hydrogen. While all these options need to be carefully assessed-it is safer for society to diversify its energy sources-sustainable priorities emerge.
Transportation Fuel Alternatives to Petroleum
Four categories of fuels can be derived from renewable energy sources, and these are only sustainable if they are managed as such (see Table 1 on page 15): grain-based ethanol, biomass (cellulosic) ethanol, biodiesel, and biomass (woody) methanol. At present, 95 percent of U.S. liquid biofuels production consists of ethanol, with the rest being biodiesel. Ethanol has been heavily subsidized with a 40-60 cents per gallon federal tax exemption or credit since 1978. Additional credits and grants are available through many states and the federal government for ethanol production, fuel blending, and equipment purchase, including pump installation. As encompassed in the 2005 Energy Policy Act, the U.S. goal is to produce 7.5 billion gallons of this “renewable fuel” by 2012 and 1 billion gallons of cellulosic ethanol by 2015, thereby reducing U.S. petroleum consumption by 0.8 to 1.6 percent and anticipated CO2-equivalent greenhouse gas emissions by 0.4 to 0.6 percent.23
More than 95 percent of U.S. ethanol production is derived from corn. This fuel is blended with gasoline at levels ranging from 5 to 85 percent. While most analyses show a modest net fossil fuel- derived energy gain and CO2 emissions reduction from ethanol and especially biodiesel, these debates hinge on several assumptions regarding the feedstock used, conversion technology, and value of coproducts. 24 Even so, corn production usually exacerbates problems of soil erosion; loss of biodiversity; dependence on gas and oil- based fertilizers, pesticides, and herbicides; increased nitrous oxide emissions (another greenhouse gas); water loss through irrigation; and water and air pollution.25 Furthermore, current ethanol refineries usually require large fossil fuel inputs. Similar to the situation for oildependent agriculture, ethanol production is thus not sustainable at the present time. Domestic production of cellulosic ethanol, in contrast, has three times the resource potential, can avoid most fossil fuel inputs by burning plant lignin (an organic polymer), and is a more energy-efficient product. According to DOE, cellulosic ethanol has the potential to displace more than one-third of the petroleum used in the United States. Owing to these advantages, cellulosic ethanol could possibly reduce CO2 emissions by more than 90 percent and lessen other environmental effects, if it is made from residues or grasses that are not required for field fertilization.26 The first commercial cellulosic ethanol plants opened in late 2007. Feedstocks include agricultural and forestry residues, short-rotation woody crops such as hybrid poplar and willow, municipal solid waste, and perennial grasses.
Biodiesel production and use, while also highly attractive and common in Germany and France, are limited to a potential of 0.4 billion gallons a year in the United States based on the feedstock supply.27 It is defined as a renewable fuel for diesel engines derived from animal fats or vegetable oils such as soybean and palm, which meets the specification of the American Society for Testing and Materials. The fuel is blended with an oil-based diesel distillate at levels ranging from 2 to 99 percent. Finally, biomass methanol, another alternative that can be made from forest residues (such as tree stumps, branches, and leaves) or wood would be an alternative product for up to a quarter of the biomass ethanol feedstock supply. However, methanol and MTBE (methyl tertiary butyl ether-an increasingly banned gasoline additive made from methanol and isobutylene)-are well known for their toxic effects. In addition, methanol has a lower energy content than ethanol (both are lower than gasoline) and is more corrosive. These problems may limit the pursuit of this fuel.
Four sources of fossil fuels could also be substituted for petroleum-based gasoline and diesel-compressed natural gas (CNG), oil shale, tar sands, and coal liquefaction. Two combined-cycle coal gasification electric power plants have been built in the United States, but neither produces transportation fuels. A fifth option, methanol derived from natural gas, is usable today in many flex- fuel vehicles but has been limited by concerns over its toxicity and thus is used mainly to make MTBE, formaldehyde, and other industrial chemical products. CNG is a low-cost, clean-burning fuel, emitting about half the CO2 of gasoline or diesel per gallon equivalent. It could be seen as a transitional fuel or one that could be coupled with CO2 sequestration. Use of CNG requires vehicles to have a large gas storage tank and a much more developed refueling network than the one currently in place, since these vehicles have a maximum range of 200-250 miles and only about 1,200 fueling stations exist in the United States. In addition, most of the gas supply is given a higher priority for applications such as heating, industrial use, and electricity generation than for transportation.
The other nonrenewable fuels have high CO2 emissions, and thus carbon sequestration would have to be considered. Among these, while oil shale and coal gasification/liquefaction have only been assessed in the United States in recent years, commercial projects have existed in Brazil, Estonia, China, and elsewhere. Shell Exploration and Production Co. anticipates that a commercial oil shale project in the Green River Formation of western Colorado may be viable after 2015.28 Meanwhile, the first coal gasification- liquefaction plant in the United States is planned for eastern Pennsylvania. Employing the Fischer-Tropsch process first used by Nazi Germany and South Africa when their access to global oil was blocked, this facility will use waste coal from more than 150 years of anthracite mining to produce 40 million gallons per year of clean-burning diesel fuel and 41 megawatts (MW) of electricity. In the process, 2.5 to 3.5 times more CO2 is generated than if the coal was burned directly-a major release not being controlled through carbon sequestration.29
Finally, potential low- or no-carbon options to gasoline vehicles include more efficient autos, increased use of diesel fuel and vehicles, gasoline (or diesel)-electric hybrids, all-electric vehicles, “plug-in” hybrids, HFCVs, and direct combustion of hydrogen. Diesel autos can travel roughly 30 percent farther on a gallon of fuel than a comparable gasoline model. Diesel models, such as the Volkswagen Jetta, can get more than 40 miles per gallon. While many more options are available in Europe, where diesel sales may soon overtake those of gasoline cars, most energy-efficient diesels do not yet meet U.S. air quality standards. Thus, despite their better mileage, diesels account for less than 5 percent of U.S. motor vehicle sales and could be much better promoted. The most popular of the growing options for hybrid cars is the Toyota Prius; almost 400,000 have been sold in the United States since 2000.30 The Prius gets 50 or more miles per gallon under certain driving conditions, and new models that get 100 miles per gallon are possible with plug-in hybrids. Further development of all-electric vehicles, while stagnant over most of the last decade, can be expected to take off again with increased interest in plug-in hybrids, improved battery management, and electricity storage. The CO2 emissions of electric vehicles, as is the case with hydrogen, are a function of the energy source, ranging from zero or low (hydro, biomass, wind, geothermal, and nuclear) to high or higher (natural gas, oil, and coal). Nuclear power, while an emissions- free source of electricity, raises concerns of weapons proliferation, plant safety, waste disposal, and high cost. These concerns temper any prospects of a nuclear power revival. Thus, limited funds might be better spent on hybrid and diesel fleet conversion, energy efficiency, and truly renewable sources of energy. According to the carbon stabilization wedge theory,31 nuclear power does not have to play a major role in reducing global CO2 emissions, given the alternatives.
The Bush Administration’s Hydrogen Economy
As with electricity, the sustainability of hydrogen fuel, whether directly burned in an engine or used to feed an HFCV, depends on the feedstock used, the technical ability to sequester CO2 emissions, overall conversion efficiency, infrastructure, cost, and other factors.
In his 2003 State of the Union Address, President Bush announced $1.2 billion in research funding intended to ensure that the United States will lead the world with development of clean hydrogenpowered vehicles. Developed over the previous year as a public-private partnership between DOE and the three major U.S. automakers, Bush’s “FreedomCar and Fuel [Cell] Initiative” was promoted by DOE as allowing freedom from petroleum dependence while also enabling Americans to “drive where they want, when they want, and in the vehicle of their choice” through use of affordable and convenient fuel. An anticipated reduction of more than 500 million metric tons of carbon equivalent greenhouse gases per year by 2040-an amount equivalent to what the U.S. released from transportation in 2001- was portrayed as a convenient, yet secondary, benefit to energy security and affordability.32
Not to be outdone, then-EC President Romano Prodi announced that he wanted his presidency to be remembered for only two things: expansion of the European Union eastward and hydrogen energy.33 The EC’s High Level Group for hydrogen and fuel cells released a 2003 report calling for a public-private partnership with increased funding for research and development and installation of basic hydrogen transport and delivery infrastructure. 34 Bush and Prodi then issued a joint statement proclaiming that Europe and the United States would work together, as hydrogen was the key to sustained economic growth35-a goal somewhat short of sustainable development. Back in the United States, California Governor Arnold Schwarzenegger allocated funds for two dozen hydrogen fueling stations along state highways-the first step in what he envisioned as a future statewide system-and pledged to convert one of his five Hummers to hydrogen.36
Optimistic projections notwithstanding, the U.S and European policy directives calling for a “hydrogen economy” (a term first coined by General Motors) were noticeably vague on the source of the hydrogen. Acknowledging that hydrogen is merely an energy carrier and not a primary fuel, the directives accepted the current practice of hydrogen extraction directly from fossil fuels-principally from natural gas but eventually from coal-and through nuclear-driven electrolysis. Renewable sources of hydrogen through electrolysis were portrayed as desirable, more so in Europe than in the United States, but in both cases, they were considered relatively insignificant for another 30 or 40 years.37
In September 2003, the collaborative research and development effort between DOE and its private partners was renamed the FreedomCar and Fuel Partnership.38 With the addition of the major oil companies, the mission was defined as assisting development of a “clean and sustainable energy future,” with hydrogen technology far enough advanced by 2015 to allow the private sector to make an informed decision on whether to move forward with large-scale commercialization. While new funding was earmarked for development of reliable hybrid electric vehicles and the batteries they depend upon, the central focus of the partnership remains development of HFCVs with supporting infrastructure. A primary objective is for hydrogen to be cost equivalent with gasoline per gallon delivered to the consumer, independent of generation pathway. Auxiliary goals address improvements in overall fuel efficiency as, for example, through a 50 percent reduction in vehicle weight, improvements in vehicle life expectancy, and development of on-board hydrogen storage systems of a high enough energy density to compete with gasoline-driven internal combustion engines (ICEs).
President Bush released his Advanced Energy Initiative in February 2006 as part of the Energy Policy Act of 2005. Designed to increase U.S. energy security through greater reliance on coal, nuclear, natural gas, and renewable energy sources, the initiative was justified by the president as a reaction to increased demand by China and India for oil supplies in a more competitive global economy while maintaining “reasonable” consumer energy prices. Among other things, the initiative implements Title VI of the act authorizing DOE to construct a prototype nextgeneration nuclear power plant at its Idaho National Laboratory for the specific purpose of producing hydrogen.39
Only as Green as Its Source
With more than 4 million “flexiblefuel” vehicles already on the road capable of running on ethanol, gasoline, CNG, or a gasoline- ethanol blend, hydrogen is promoted by the current Bush administration as the fuel of the future.40 The Energy Policy Act of 2005 sets a goal of 100,000 hydrogen-fueled vehicles by 2010 complete with a viable supporting infrastructure and mass-market penetration through two-and-a-half million vehicles by 2020.41 Increased support for renewable energy notwithstanding, it appears that the vast majority of hydrogen produced today and envisioned for the immediate future will be “brown” insofar as hydrogen from fossil fuel sources will generate CO2 as a byproduct. Electrolysis will largely come through conventional fossil fuel-derived electricity and especially nuclear power. These U.S.-based hydrogen sources are promoted as the key to energy independence due to abundant supply- 240 years of proven reserves at current consumption rates for coal and no foreseeable near-term limits for uranium reserves.42 They are also presented as “green,” anticipating mass application of carbon sequestration for coal sources and the assumption that nuclear power results in no net CO2 emissions. The latter point ignores fuel mining, fuel enrichment, fabrication, and other components of the nuclear fuel cycle that generate 5-10 percent of the carbon output of coal per unit of energy output.43
At present, more than 95 percent of U.S. hydrogen is generated through steam reformation of natural gas, a high-temperature, energy- intensive, nickel-catalyzed process with a net thermal efficiency of only 60-70 percent compared to 70-85 percent yield for electrolysis. However, limited supply, volatile market prices, and significant CO2 emissions render natural gas an unreliable and unsustainable hydrogen source. With 27 percent of the world’s coal supply, the United States may ultimately designate coal as its preferred source for hydrogen despite a net thermal efficiency of only 48 percent, even less than that of natural gas. The result is that more CO2 is released per unit of energy generated from the reformation of fossil fuel to generate hydrogen than is derived from burning the fossil fuel directly for energy. This leaves electrolysis derived from renewable resources as the only sustainable way to generate hydrogen without significant carbon emissions and/or radioactive wastes. However, as explained, even though electrolysis can attain a higher thermal energy conversion efficiency for hydrogen production, not considering the cost of controlling carbon dioxide release and other pollution control measures, the least expensive renewable electrolysis of hydrogen is almost four times as expensive as natural gas when used to create hydrogen, twice that of coal per unit of hydrogen produced, and more than five times the cost of gasoline as a fuel per unit of energy produced.44
The programs in the United States and Europe optimistically predict the problem of carbon emissions from fossil fuel reformation will be solved through sequestration, likely in depleted underground oil and natural gas fields or through deep-sea injection.45 According to DOE, hydrogen produced through the nation’s abundant coal reserves, together with coal gasification and carbon capture and storage, holds the promise of “near-zero greenhouse gas emissions.”46 Carbon sequestration, however, is a very expensive proposition, as the carbon oxides must be separated from other flue gases, compressed, and transported long distances if not injected on site. Although coal gasification is a promising way to produce hydrogen from a fossil fuel (because the flue gas is already purified), carbon capture and sequestration remains a potential dealbreaking expense. To achieve commercial viability, it will have to significantly come down in price, encouraged through targeted research, development, and demonstration projects, and perhaps ultimately through the as-yet politically unpopular imposition of carbon taxes. With steam reforming of natural gas, more than 10 kilograms (kg) of CO2 are released per kg of hydrogen generated. This figure doubles if the source is coal. Again with electrolysis, hydrogen is only as green as its source: Coal-based electrolysis of hydrogen generates about 7.4 times the CO2 per net unit of useful energy than if an energy-equivalent unit of gasoline were used directly as the energy source for automobiles.47
As a result, if coal and natural gas are to be viable sources for hydrogen production, CO2 must be sequestered. The two main methods entail biological sequestration-a process that may be enhanced through forest expansion, improved fertilization, and other measures designed to promote a net shortterm gain in plant photosynthesis- and more permanent geological sequestration. Long-term geologic sequestration projects under development envision injection of liquid CO2 into secure formations-perhaps linked with enhanced oil and natural gas recovery, as is currently practiced, or in deep saline aquifers-as well as deepsea (deeper than 3,000 meters below sea level) placement. Here the high pressure and cold temperatures are expected to keep the CO2 liquid and secure.
At present more than 20 small-scale federally subsidized carbon sequestration demonstration projects are under development in the United States. Three larger projects already in progress are associated with gas recovery in the North Sea and Algeria and coal gasification with enhanced oil recovery along the Saskatchewan- North Dakota border. Moreover, in May 2007, General Electric and BP announced a global joint venture to develop at least five 500-MW coaland/ or petroleum coke-based48 hydrogen electric power plants, each with carbon capture and geologic storage. While the goal of this partnership and the smaller carbon sequestration projects is to achieve competitive market acceptance within a decade, the challenges, when extended to the generation of transportation fuels, are significant. For example, carbon capture and sequestration is only feasible at a large-scale hydrogen production facility, thereby penalizing distributed generation that may consume less energy for fuel transportation and result in less fuel loss.49 In addition, a sudden release, as occurred in 1986 at Lake Nyos in Cameroon when the super-saturated, cold, deep lake released a massive quantity of the gas, may not only undo the desired sequestration, but it may also cause deaths among nearby populations. Furthermore, as with any underground storage of a gas or liquid under pressure, permanence is contextual with geological sequestration. Routine escape is anticipated depending on the formation in question (for example, from fractured oil and gas fields), the initial pressure achieved, and chemical interaction with the surrounding material. However, while permanence cannot be assured, some scientists contend that leakage might be slow enough-particularly if sequestration occurs below the ocean floor-to allow deep oceanic absorption and re- equalization of atmospheric carbon almost back to pre-industrial levels over the ensuing centuries.50
The permanence of sequestration notwithstanding, CO2 liquefaction is a costly and inefficient process, significantly reducing the net yield obtained. So while carbon sequestration and coal-based hydrogen production remain promising technologies that deserve further research funding, they still support non-sustainable use of fossil fuels. However, given the reality of abundant coal supply and use, large-scale sequestration, once proven, will likely serve as an unavoidable bridge to a more sustainable system based on renewable sources of energy.
Providing Infrastructure for the Hydrogen Economy
The United States uses more than 1 million metric tons of gasoline per day. It would take the equivalent of the nation’s entire existing electric output to generate through electrolysis the 3.77 billion cubic meters of hydrogen necessary for replacement. This figure does not even cover what it would take to replace with hydrogen the 230,000 metric tons of jet fuel and 497,500 metric tons of petroleum distillates, including diesel, consumed daily.51 Whatever the source, serious energy inefficiencies and logistical barriers are associated with the production, compression or liquefaction, transport, and storage of hydrogen before conversion back to electrical and then kinetic energy in an HFCV.
Once isolated, hydrogen is transported, stored, and made ready for use in a vehicle as a compressed gas, chilled liquid, or through a combination of the two. Metal hydrides can also be used to carry hydrogen, but their substantial weight per volume carried and the high heat required to release the gas make them currently impractical for transportation applications. As is the case with natural gas, compressed hydrogen gas through pipeline delivery for urban areas and nearby concentrated use (as at a refinery) is the least expensive method but is prohibitively expensive for dispersed rural application. In addition, hydrogen can embrittle metal pipes, which are already prone to leaks due to the high pressure required for hydrogen transport given its low energy density and atomic weight. Securing the right-of-way for new pipelines presents additional problems.
Preparation of hydrogen for transport and storage consumes a large amount of energy. Here the gaseous pathway comes out ahead, though still well behind conventional storage and transport of natural gas and gasoline. A temperature of -253[degrees]C is needed to liquefy hydrogen. Roughly 30 percent of the energy available through the process is needed just for initial liquefaction, and an additional 10 percent is necessary for maintenance of the liquid or to achieve the volumetric equivalent energy density (at 800 atmospheres or 11,760 pounds per square inch (psi)) in a compressed gas. Additional energy is required to maintain pressures and/or the liquid state, especially over long distances and time periods. Yet liquefied hydrogen has only 7 percent the density of water and just 2.5 percent the energy of gasoline per unit of volume at an ambient pressure of 1,000 psi, while as a compressed gas, hydrogen contains only 30 percent the energy of methane at an equivalent pressure of 11,600 psi.52
With both liquefaction and compression, very heavy steel-walled tanks, trucks, and pipelines are required to store and transport the hydrogen. This further cuts down on end-use efficiency. According to an expert panel assembled by the American Physical Society, storage may well be the largest roadblock for President Bush’s hydrogen initiative.53 While transport inefficiencies can be reduced through direct piping from source of generation to dispersed fueling stations or by on-site hydrolysis or reformation of natural gas, and vehicle weight can be reduced through a new generation of lightweight filament-wound carbon fiber tanks, the situation remains that due to its low weight per unit of energy delivered, the process of generation, compression or liquefaction, and transport of hydrogen requires a substantial amount of energy. To carry the equivalent amount of energy transported by a single, light gasoline delivery truck, liquid hydrogen would require 5 trucks, and compressed hydrogen would require 19.
Switching to pipelines to avoid the safety problems, traffic congestion, and inefficiencies associated with trucking, major barriers still arise from the physical nature of hydrogen. For a 3,000-km pipeline-conceivable for a hydrogen plant at the mouth of a well or mine with requisite on-site carbon sequestration- due to the higher compression and lower volumetric energy density for hydrogen, the mass fraction (compressor) consumption for hydrogen is 50 percent of the energy transferred versus only 20 percent for natural gas. In addition, we can expect much more leakage for hydrogen compared to methane due to its smaller molecular size.54
Finally, physical laws also limit delivery of hydrogen to end- use vehicles. For example, liquid fuels can be transferred by gravity alone, while hydrogen gas requires additional pumping pressure to empty a supply tank and account for the fact that the pressurized gas, as it is released from a holding tank, cools and thus condenses in the supply tank from which it was transferred. On- board reforming of hydrogen, an alternative to off-site hydrogen formation and delivery, is no longer considered a viable option due to unacceptable start-up requirements, the lack of carbon sequestration if the source were natural gas, and even higher energy loss than with off-site formation. 55 An often-cited European report concluded that the total energy needed to generate, compress, transport, and store hydrogen at a distributed filling station- thereby reducing energy loss associated with long-distance transport- would still consume 40-80 percent of the original fuel’s energy, and even more for liquid hydrogen, depending on the size of the filling station and the source of the electricity employed. By comparison, the current well-to-tank loss for gasoline is only 12 percent, and for natural gas, it is a mere 5 percent.56 Even if the hydrogen were derived from less-expensive and more energy-efficient natural gas instead of through electrolysis, it makes more environmental and economic sense to use methane directly as a hybrid vehicular fuel than to convert it first to hydrogen for that purpose.57
Net Energy Yield and CO2 Emissions
Analysts speak of “well-to-wheels” analysis when calculating the lifecycle energy demand and environmental costs of different transportation fuels. While results vary widely according to input assumptions, geographically specific fuel pathways, and vehicle employment; sophisticated modeling is being conducted through an ongoing collaborative effort headed by the Joint Research Center of the European Commission (JRCEC).58 Research suggests that energy and greenhouse gas emissions savings are only achieved through more efficient HFCVs (30-40 percent), as natural gas-derived hydrogen, burned directly, consumes more net energy than a conventional internal combustion engine and offers only minimal greenhouse gas savings. Preliminary well-to-wheels analysis from energy source procurement to final use of the fuel in an HFCV or similar-sized gasolinedriven internal combustion engine suggests that the CO2- equivalent greenhouse gas emissions generated per distance driven are roughly half for compressed hydrogen made from natural gas compared to gasoline, and the energy consumed per 100 km driven is just 68 percent (see Table 2 on page 22). The differences are less, however, if long-distance hydrogen delivery, gasoline hybrid and diesel vehicles, liquefied hydrogen, and hydrogen from electrolysis are considered. So, while the net greenhouse gas emissions are lower when natural gas displaces petroleum as a fuel in any powertrain, the net wellto- wheels efficiency of an HFCV using natural gas- derived hydrogen is roughly 21 percent, close to that of present- day high-performance hybrid ICEs, and the total fossil fuel energy consumed is just 7 percent less.59 With coal-derived hydrogen, CO2 emissions and fossil energy consumption are greater in an HFCV than with the conventional gasoline-driven or hybrid internal combustion engine (Table 2). Although carbon sequestration, if practical and affordable, would lower the CO2 emissions, it would also raise the energy consumed. On the other hand, wind-derived hydrogen through centralized electrolysis has almost no carbon emissions as long as the energy used for hydrogen compression or liquefaction and delivery is also wind derived. Working with the JRCEC data and projecting the current rate of technological advance to 2050, researchers estimated that the final CO2 emission from HFCVs when using methane-derived hydrogen would be slightly lower (8 percent) than that from ICEs, much higher if the hydrogen comes from coal, and a moot comparison if natural gas is no longer an available or affordable hydrogen source.60
Thus, the available data suggest that hydrogen derived from natural gas and used in an HFCV offers a modest (30 percent) advantage with regard to greenhouse gas emissions compared to an advanced hybrid gasoline internal combustion engine. Moreover, there is an energy consumption advantage for the HFCV over the internal combustion engine when we consider the entire fuel lifecycle. Should nuclear or coal energy through electrolysis be used for the hydrogen, the HFCV pathway is approximately half as energy efficient as the internal combustion engine and consumes significantly more energy per distance driven.61
This comparison also sets aside the fact that HFCVs are currently nowhere near commercial application, costing more than 10 times more per rated unit of power compared to the internal combustion engine.62 Nonetheless, current costs, efficiencies, and affiliated CO2 emissions notwithstanding, a sustainable transportation system ultimately must be based on renewable energy. Although achieving a sustainable transportation system requires renewable energy and much lower CO2 emissions, if slowing climate change is the primary justification for the transition, it does not necessarily follow that renewable energy at present should be diverted to hydrogen production.
The Best Use for a Precious Resource?
Large-scale hydropower is a less expensive source for hydrogen, yet most U.S. hydropower resources are already committed for direct electricity production, and additional sites are limited. Another option, reformed biomass, is at present the lowest-cost renewable source of hydrogen after hydropower and figures prominently in many proposals for a hydrogen economy. However, the amount of land required to replace current fossil fuels with biomass is enormous and not without major environmental concerns. This leaves wind and solar power as the only viable renewable energy sources for hydrogen. Yet the cost of solar power rivals the prohibitive expense of hydrogen derived from nuclear-powered electrolysis, while wind power is best used directly to substitute for coal-based electricity from the standpoint of energy efficiency, cost, and CO2 reduction.63
If the primary goal is to reduce greenhouse gas emissions, it is more effective to employ renewable energy to replace conventional coal-fired electricity, currently responsible for 32 percent of total U.S. CO2 emissions. Using renewable energy directly to replace coal-fired electricity displaces approximately 2.7 times as much CO2 when used for the new generation of “clean” coal plants-described as such because they involve some combination of coal washing, gasification, flue gas desulphurization, carbon dioxide purification with sequestration, and other measures designed to enhance efficiency, reduce environmental impact, and possibly recover hydrogen-and 3.4 times as much when used to replace electricity from conventional power plants, compared with using renewable energy to replace gasoline with hydrogen. Even substituting natural gas directly for coal-based electricity displaces 2.7 times more CO2 than if the gas were converted to hydrogen as a first step.64
Up to 90 percent of the electricity generated by wind turbines enters the grid and is delivered to consumers, with transmission and distribution averaging a 2-8 percent loss.65 With the second law of thermodynamics working against it, hydrogen as an energy carrier from wind power cannot match this efficiency because it depends on additional energy conversions-from electrical to chemical, and back to electrical. Taking into account energy loss for compression or liquefaction and for transport, the net energy delivered to the consumer falls to 30 percent or less.
Due solely to conversion inefficiencies and not taking into account the cost of fuel cells or other infrastructure expenses, the actual cost of hydrogen-generated electricity at the point of end use through a fuel cell is roughly four times that for conventional sources of electricity from the grid.66 Hydrogen’s low energy density limits its potential as a fuel for long-distance travel, such as by trains, planes, and boats. Alternatively, synthetic hydrocarbons derived from biomass and organic waste could be more sustainably used for long-distance transport, with local driving powered through electricity based on other renewable resources. This combination is brought together through a “plug-in” hybrid electric vehicle.
Hybrid Vehicles: A Hydrogen Economy Alternative?
Building the infrastructure necessary for a U.S. system of hydrogen generation, distribution, and fueling requires resources comparable to those currently employed for natural gas and petroleum systems-in short, a very ambitious and expensive undertaking. Moreover, were the current fleet of petroleum-based vehicles replaced by HFCVs, the latter would have to come down in price by a factor of 10 or more with cost and maintenance to bring them within the cost range of today’s motor vehicles.67 Optimistic projections in Europe for scale economies with mass production suggest that it will take 30-40 years of continued research and development focusing on lowering the cost and improving the efficiency of HFCVs and an aggressive, heavily subsidized program to introduce the requisite infrastructure before an active consumer market emerges.68
If we achieve a transportation system where the majority of vehicles are fueled by hydrogen, the problem of CO2 emissions will remain until the hydrogen is derived from renewable energy sources and/or the challenge of secure and permanent carbon sequestration is addressed. In the interim, alternatives to achieve the goals of energy independence and security, while also reducing the CO2 emissions and radioactive wastes associated with the hydrogen economy envisioned by the Bush administration and reluctantly accepted as an unavoidable transitional state by the EC, should be considered. Chief among these are the already-proven and consumer- tested highly efficient diesel automobiles and, most recently, the hybrid gas-electric vehicles (HEVs) pioneered by Toyota, Honda, and Ford.
Many hydrogen advocates point to reported HFCV efficiencies 3-4 times that of conventional internal combustion engines (50-60 percent as opposed to 15 percent for a typical car) to bolster the claim that despite higher fuel prices and lower conversion efficiencies for hydrogen production than for gasoline derived from oil, HFCVs are already competitive with gasoline in terms of net useful energy output. Therefore, one can drive several times farther on a gallonequivalent of hydrogen in an HFCV than on a gallon of gasoline. Actual efficiencies notwithstanding, this comparison fails to consider that light HEVs with large battery packs are already increasing gasoline mileage by more than 50 percent and that one can go even farther on a gallon in a diesel hybrid vehicle.69
With renewable energy used to replace fossil fuel-derived electricity, a more effective way to reduce CO2 emissions than conversion to hydrogen, the main rationale for hydrogen production comes as a transportation fuel to replace insecure and unsustainable oil. Yet as an energy carrier, hydrogen has a net yield of only 51 percent when derived from renewable energy through electrolysis, compared to 75-85 percent for electricity stored in a battery. Moreover, the CO2 emissions from an HEV are at least equivalent to those from an HFCV when the hydrogen is derived from natural gas.70 Moving beyond HEVs that rely on fossil fuels, the plug-in hybrid electric vehicle (PHEV) provides a viable alternative. With larger battery packs, PHEVs are designed to run primarily on electricity for a 20-60 mile daily commute, only switching over to gasoline, ethanol, or biodiesel for longer trips. Charging at night with off- peak electricity that costs the equivalent of 50 cents per gallon for gasoline, existing nickel-metal hydride HEVs have a 20-mile electric charge range (lithium ion PHEVs should perform better). Experimental PHEVs have reached 100-180 mpg for the typical car used primarily for short-range commuting, while costing only one-fourth per mile of a conventional car to operate. PHEVs should also be able to travel 3-4 times farther than an HFCV on a kilowatthour of renewable electricity. This is due to the cost for, and more efficient use of, electricity and does not consider vehicle costs or future innovations in HFCVs or PHEVs. If an HFCV were employed, it would still be more efficient in terms of economics, energy, and the environment to couple HFCV technology with a regenerative battery, saving the more costly hydrogen for power backup-in short, a plug- in hybrid electric HFCV.71
Building upon the concept of stabilization wedges,72 modelers working with the U.S. Environmental Protection Agency estimate that it would take 4.3 wedges of approximately 5 billion metric tons of CO2-equavalient gas reduction each to flatten U.S. passenger vehicle emissions back to 2007 levels from those projected for 2050. Assuming 30 percent market penetration each, 2.7 wedges could be had from existing technology through cellulosic ethanol, 1.1-to-2.2 wedges from PHEVs, and a 0.8-to-1.0 wedge from hybrid diesel and gasoline vehicles. Combining the low greenhouse gas fuels with anticipated advances in vehicle technology could generate the additional wedges necessary for stabilization and even generate a surplus to assist with stabilization from trucking, aviation, and rail transportation or to offset passenger vehicle emissions below current levels. With smarter urban planning, car-pooling, and other measures designed to reduce carbon-emitting travel 15 percent, another four wedges could come through travel demand management.73
Hydrogen: Bridge to the Future?
Distributed hydrogen fuel cells may be a useful substitute for expensive coal- or natural gas-driven peaking power plants used to stabilize the electricity grid and appear ideal to store intermittent wind and solar energy. As such, continued funding for research and development on fuel cells and associated advances in vehicle and fuel tank construction and carbon sequestration applicable across a wider variety of fuel pathways is warranted. The potential of fuel cells to power the transportation sector is limited, however, by the laws of physics as well as economic and environmental considerations. The so-called “hydrogen economy” may well prove feasible for isolated regions, far from existing energy infrastructure and blessed with abundant renewable resources, such as Iceland. Yet even Iceland’s once-vaunted dream of a hydrogen economy may be stalled.74 In the meantime, it is imperative that the world, led by the industrialized nations, commits to sustainably provided energy sources and technologies to stem global climate change. For a hydrogen economy to work, it would have to compete with electricity, a well-established energy carrier supported by an infrastructure already in place. For the transportation sector, the growing market today for hybrid gasoline-electric vehicles is a logical first step, along with more efficient gasoline and diesel- powered automobiles and, very soon, all-electric vehicles and PHEVs.
Coupling renewable fuel sources necessary for a sustainable transportation system (such as cellulosic ethanol) with electricity used through PHEVs, rather than through conversion first to hydrogen for HFCV use, may be the next step. Thus PHEVs would rely on synthetic liquid hydrocarbon fuels derived from atmospheric carbon in the form of biomass ethanol, wood-based methanol, or biodiesel fuel. Biofuels at this reduced level of use are feasible given the much lower backup requirements for PHEVs that are powered by renewable electric energy as compared to biofuels that are used to produce hydrogen for an HFCV.
Renewable energy is a necessary precondition for a sustainable hydrogen future, and not the other way around. A focus on hydrogen first, at least as envisioned by the Bush administration-with reliance on carbon sequestration to permit fossil fuels as the primary sources-threatens to detract attention from energy efficiency and renewable energy. The latter are necessary measures that are already proven as the fastest way to achieve energy security and reduced greenhouse gas emissions.75
The vast majority of U.S. funding already allocated for the hydrogen economy does not sufficiently differentiate between green or brown sources of hydrogen and thus makes a questionable contribution to sustainability. Public and private funds might be better spent on research and development for energy efficiency and conservation, backed by a federal renewable portfolio standard of 20 percent or more for electricity, a parallel renewable fuels standard for hydrogen from other sources with accompanying support limited to appropriate pathways, and a national carbon tax. Already adopted or under consideration in Europe, these measures are desirable, though the carbon tax is considered politically unacceptable for the United States at the present time.
Proponents of the hydrogen economy tend to focus on the lack of political will, or technical and socioeconomic barriers that must be overcome as the main obstacles to this transformation.76 However, the hydrogen economy is not close to being ready. It should be noted that the United States experienced this level of dislocation during the 1970s in response to the OPEC oil embargo, when the national speed limit was lowered to 55 mph, fuelefficiency standards were established, and major state and federal tax credits were extended for energy conservation and renewable energy supply. These measures worked quickly, and energy demand dropped dramatically before picking up again in the 1980s when energy prices fell and the restrictions were removed.
Ultimately, the U.S. public may also have to accept global climate change rather than energy prices as the driving force behind national energy policy, a shift that is currently under way in Europe. While pursuing a variety of long-term energy options, including hydrogen, primary attention should be given to appropriate transportation pathways and technologies that are already within our grasp.77
PRINCIPLES OF SUSTAINABLE TRANSPORTATION
Renewable fuel supply and increased energy efficiency
This principle requires transportation fuels that are non- depletable and sustainably managed, with vehicles or other modes of transport that are significantly more efficient when compared to conventional practices. Only use of non-depletable fuel is sustainable over the long term. A more efficient transport system allows for a smoother transition to sustainable fuel use patterns and eases the social and management burden.
Minimal environmental impact
This requires transport modes that have much lower levels of air and water pollution, lifecycle emissions of greenhouse gases, lower rates of soil erosion, etc., than at present. Air pollution emissions from motor vehicles are harmful to human health, and motor vehicles are a major source of greenhouse gas emissions; corn-based ethanol worsens soil erosion and water pollution; petroleum fuel production and refining are also major sources of environmental problems.
Carbon neutrality
Neutrality will only come with a transportation system that emphasizes greater use of energy sources or modes with lower carbon dioxide (CO2) emissions, or balances those emissions with commensurate carbon sequestration. The transportation sector accounts for one-fourth of global greenhouse gas emissions.
Socially acceptable cost
Fuel and vehicle alternatives that are no more expensive than existing ones (and preferably less costly) when including all environmental costs in prices are necessary for widespread adoption by the public. This principle allows the new vehicles and fuels to compete with existing ones without creating economic hardships for people with lower incomes.
Equitably available fuels and vehicles
Any successful system would require alternative fuels and vehicles that are widely distributed and available for purchase and use. If sustainable transport fuels and modes are not available to a significant segment of the population, many people will continue to engage in unsustainable transportation practices.
Vehicles and fuels that do not compound other major social problems
New fuels and vehicles when widely used should not lead to increased environmental, health, or safety problems, other social risks, nor should they create major new ones. If other major social problems are made worse by the alternative fuels and vehicles, or new ones are created, significant societal resources will need to be diverted to address them.
Rapeseed can be used to make biodiesel, which is sustainable and currently commercial in the United States.
NOTES
1. J. Ogden, R. Williams, and E. Larson, “Societal Lifecycle Costs of Cars with Alternative Fuels/Engines,” Energy Policy 32, no. 1 (2004): 7-27; J. Ogden, “High Hopes for Hydrogen,” Scientific American 295, no. 3 (2006): 94-101; and U.S. Environmental Protection Agency (EPA), A Wedge Analysis of the U.S. Transportation Sector, EPA 420-R-07-007 (Washington, DC: EPA, April 2007).
2. J. J. Romm, The Hype about Hydrogen: Fact and Fiction in the Race to Save the Climate (Washington, DC: Island Press, 2004); J. Rifkin, The Hydrogen Economy (New York: Tarcher, 2003); S. Dunn, “The Hydrogen Experiment,” Worldwatch 13, no. 6 (November/December 2000): 14-25; B. D. Solomon and A. Banerjee, “A Global Survey of Hydrogen Energy Research, Development and Policy,” Energy Policy 34, no. 7 (2006): 781-92; U.S. Department of Energy (DOE), National Hydrogen Energy Roadmap (Washington, DC: DOE, 2002); Japan Hydrogen and Fuel Cell Demonstration Project, http://www.jhfc. jp/e/ index.html (accessed 15 September 2006); and European Commission (EC), EC High Level Working Group on Hydrogen and Fuel Cells, Hydrogen Energy and Fuel Cells: A Vision of Our Future: Final Report (Brussels: EC, 2003), http://ec.europa.eu/research/energy/pdf/ hydrogenreport_ en.pdf (accessed 26 June 2007). 3. United Nations Kyoto Protocol to the Framework Convention on Climate Change, http:/ /unfccc .int/resource/docs/convkp/kpeng.html (accessed 28 June 2007); and Office of the Governor (California), “California, New York Agree to Explore Linking Greenhouse Gas Emission Credit Trading Markets,” press release (Sacramento: 16 October 2006).
4. J. Verne, L’Ile Mysterieuse (Paris: Hetzel, 1874).
5. President George W. Bush and former European Commission (EC) President Romano Prodi are strong supporters of increased research and development to develop hydrogen as a transportation fuel substitute. For example, see G. W. Bush, “State of the Union Address,” 28 January 2003, Washington, DC: The White House, Office of the Press Secretary; and R. Prodi, “European Hydrogen and Fuel Cell Technology Platform General Assembly,” speech before the European Hydrogen and Fuel Cell Technology General Assembly, Brussels, 20 January 2004, http://europa.eu.int/ rapid/ pressReleasesAction.do?reference=SPEECH/04/ 26&format=HTML&aged=0&language=EN&gui Language=en (accessed 26 June 2007).
6. DOE, Office of Power Delivery, Office of Power Technologies, Energy Efficiency and Renewable Energy, A Multiyear Plan for the Hydrogen R&D Program, Rationale, Structure and Technology Roadmaps (Washington, DC: DOE, 1999).
7. P. Hoffmann, The Forever Fuel-The Story of Hydrogen (Boulder, CO: Westview Press, 1981).
8. Romm, note 2 above, pages 71-72; C. J. Winter, “Into the Hydrogen Economy-Milestones” (editorial), International Journal of Hydrogen Energy 30, no. 7 (2005): 681-85; and A. B. Lovins, Twenty Hydrogen Myths (Snowmass, CO: Rocky Mountain Institute #E-03- 05, 2003), http://www.rmi.org/images/other/Energy/E03- 05_20HydrogenMyths.pdf (accessed 26 June 2007).
9. N. Z. Muradov and T. N. Veziroglu, “From Hydrocarbon to Hydrogen-Carbon to Hydrogen Economy,” International Journal of Hydrogen Research 30, no. 3 (2005): 225.
10. B. E. Logan, “Extracting Hydrogen and Electricity from Renewable Resources,” Environmental Science & Technology 38, no. 9 (May 2004): 162A-63A.
11. T. E. Lipman, What Will Power the Hydrogen Economy? Present and Future Sources of Hydrogen Energy, report prepared for the Natural Resources Defense Council (Davis, CA: Institute of Transportation Studies, University of California, 2004).
12. Energy Information Administration, DOE, Annual Energy Review 2006, http://www.eia.doe.gov/emeu/aer/ contents.html (accessed 29 June 2007); Energy Information Administration, DOE, International Energy Annual 2004, http://www.eia.doe.gov/iea/contents.html (accessed 26 June 2007).
13. R. Heinberg, The Party’s Over: Oil, War and the Fate of Industrial Societies, revised edition (Gabriola Island, BC, Canada: New Society Publishers, 2005), 95-136.
14. R. K. Kaufmann and L. D. Shiers, “The Effect of Resource Uncertainty on the Peak in Global Oil Production and the Production of Alternatives,” submitted to the Proceedings of the National Academy of Sciences, 2007.
15. S. Dees, P. Karadeloglou, R. K. Kaufmann, and M. Sanchez, “Modeling the World Oil Market,” Energy Policy, 35 (2007): 178-91; Heinberg, note 13 above, pages 116-17.
16. Oil shale and tar sands are low-grade geologic repositories of hydrocarbons that yield usable oil through high-temperature pyrolysis and/or distillation. Coal gasification and liquefaction convert coal into a synthetic gas under high temperature and pressure for further processing into commercial gas or oil fuels.
17. W. D. Constain, “Energy Policy and Boom and Bust Cycles: Government Action and Instability in the Development of Oil Shale,” Policy Studies Journal 13, no. 2 (1984): 401-11; and H. R. Linden, “The Evolution of an Energy Contrarian,” Annual Review of Energy and the Environment 21 (1996): 31-67.
18. Projecting global emissions to 2054, and accounting for expected increase in electricity and fuel use, Princeton University professor of mechanical and aerospace engineering Robert Socolow and his colleagues estimate that we would need to eliminate 7 billion tons of carbon dioxide (CO2) per year just to stabilize at the 2001 emissions level. See R. Socolow, R. Hotinski, J. B. Greenblatt, and S. Pacala, “Solving the Climate Problem: Technologies Available to Curb CO2 Emissions,” Environment, 46 no. 10 (December 2004): 8-19.
19. J. D. Marshall and M. W. Toeffel, “Framing the Elusive Concept of Sustainability,” Environmental Science & Technology 39, no. 3 (2005): 673-82; J. D. Sachs and W. D. Reid, “Investments Toward Sustainable Development,” Science, 19 May 2006, 1002-03; and M. K. Heiman, “Community Attempts at Sustainable Development through Corporate Accountability,” Journal of Environmental Planning and Management 40, no. 5 (1997): 631-43.
20. Ethanol production statistics by nation are reported by the Renewable Fuels Association: http://www .ethanolrfa.org/industry/ statistics/ (accessed 9 April 2007).
21. D. Esty, M. Levy, T. Srebotnjak, and A. de Sherbinin, 2005 Environmental Sustainability Index (New Haven: Yale Center for Environmental Law & Policy, 2005); and D. Esty et al., Pilot 2006 Environmental Performance Index (New Haven: Yale Center for Environmental Law & Policy, 2006).
22. D. Pimentel and T. W. Patzek, “Ethanol Production Using Corn, Switchgrass, and Wood: Biodiesel Production Using Soybean and Sunflower,” Natural Resources Research 14, no. 1 (March 2005): 65- 76; and M. Giampietro, S. Ulgiati, and D. Pimentel, “Feasibility of Large-Scale Biofuel Production,” Bio