A hydrogen economy is a hypothetical economy in which the energy needed for motive power (for automobiles and other vehicle types) or electricity (for stationary applications) is derived from reacting hydrogen (H₂) with oxygen. While the primary purpose is to eliminate the use of carbon-based fossil fuels and thus reduce carbon dioxide emissions, a secondary goal is to provide an energy carrier to replace dwindling supplies of petroleum as well as to provide energy independence to countries without oil resources.

In the context of a hydrogen economy, hydrogen is an energy storage medium, not a primary energy source (see nuclear fusion for an entirely separate discussion of using hydrogen isotopes as an atomic energy source). Nevertheless, controversy over the usefulness of a hydrogen economy have been confused by issues of energy sourcing, including fossil fuel use, global warming, and sustainable energy generation. These are all separate issues, although the hydrogen economy affects them all (see below).

Proponents of a hydrogen economy suggest that hydrogen is an environmentally cleaner source of energy to end-users, particularly in transportation applications, without release of pollutants (such as particulate matter) or greenhouse gases at the point of end use. Analyses have concluded that "most of the hydrogen supply chain pathways would release significantly less carbon dioxide into the atmosphere than would gasoline used in hybrid electric vehicles" and that significant reductions in carbon dioxide emissions would be possible if carbon capture or carbon sequestration methods are utilized at the site of energy or hydrogen production.^[1]

Critics of a hydrogen economy argue that for many planned applications of hydrogen, direct distribution and use of energy in the form of electricity, or alternate means of storage such as chemical batteries, fuel plus fuel cells, or production of liquid synthetic fuels from CO₂ (see methanol economy), might accomplish many of the same net goals of a hydrogen economy, while requiring only a small fraction of the investment in new infrastructure.^[2] Hydrogen has been called the least efficient and most expensive possible replacement for gasoline (petrol).^[3][4] A comprehensive study of hydrogen in transportation applications has found that "there are major hurdles on the path to achieving the vision of the hydrogen economy; the path will not be simple or straightforward".^[1]

Rationale

Elements of the hydrogen economy

A hydrogen economy is proposed to solve the ill effects of using hydrocarbon fuels in transportation, and other end-use applications where the carbon is released to the atmosphere.

In the current hydrocarbon economy, the transportation of people and goods (so-called mobile applications) is fueled primarily by petroleum, refined into gasoline and diesel, and natural gas. However, the burning of these hydrocarbon fuels causes the emission of greenhouse gases and other pollutants. Furthermore, the supply of hydrocarbon resources in the world is limited, and the demand for hydrocarbon fuels is increasing, particularly in China, India and other developing countries.

Hydrogen has a high energy density by weight. The fuel cell is also more efficient than an internal combustion engine. The internal combustion engine is said to be 20–30% efficient, while the fuel cell is 35–45% efficient (some even higher) (not accounting for losses in the actual production of hydrogen, which would result in an overall efficiency of about 25%) and together with the electric motor and controller, the drive train overall efficiency approaches 24% with low idling losses.^[5]

Current Hydrogen Market (Economy)

Timeline

Hydrogen production is a large and growing industry. Globally, some 50 million metric tons of hydrogen, equal to about 170 million tons of oil equivalent, were produced in 2004. The growth rate is around 10% per year. Within the United States, 2004 production was about 11 million metric tons (MMT), an average power flow of 48 gigawatts. (For comparison, the average electric production in 2003 was some 442 gigawatts.) As of 2005, the economic value of all hydrogen produced worldwide is about $135 billion per year.^[6]

There are two primary uses for hydrogen today. About half is used to produce ammonia (NH₃) via the Haber process, which is then used directly or indirectly as fertilizer. Because both the world population and the intensive agriculture used to support it are growing, ammonia demand is growing. The other half of current hydrogen production is used to convert heavy petroleum sources into lighter fractions suitable for use as fuels. This latter process is known as hydrocracking. Hydrocracking represents an even larger growth area, since rising oil prices encourage oil companies to extract poorer source material, such as tar sands and oil shale. The scale economies inherent in large scale oil refining and fertilizer manufacture make possible on-site production and "captive" use. Smaller quantities of "merchant" hydrogen are manufactured and delivered to end users as well.

If energy for hydrogen production were available (from wind, solar or nuclear power), use of the substance for hydrocarbon synfuel production could expand captive use of hydrogen by a factor of 5 to 10. Present U.S. use of hydrogen for hydrocracking is roughly 4 million metric tons per year (4 MMT/yr). It is estimated that 37.7 MMT/yr of hydrogen would be sufficient to convert enough domestic coal to liquid fuels to end U.S. dependence on foreign oil importation,^[7] and less than half this figure to end dependence on Middle East oil. Coal liquefaction would present significantly worse emissions of carbon dioxide than does the current system of burning fossil petroleum, but it would eliminate the political and economic vulnerabilities inherent in oil importation.

Currently, global hydrogen production is 48% from natural gas, 30% from oil, and 18% from coal; water electrolysis accounts for only 4%.^[8] The distribution of production reflects the effects of thermodynamic constraints on economic choices: of the four methods for obtaining hydrogen, partial combustion of natural gas in a NGCC (natural gas combined cycle) power plant offers the most efficient chemical pathway and the greatest off-take of usable heat energy.

The large market and sharply rising prices in fossil fuels have also stimulated great interest in alternate, cheaper means of hydrogen production.^[9]

Environmental concerns

Hydrogen gas can be created through the natural gas steam reforming/water gas shift reaction method, outlined above. This creates carbon dioxide (CO₂), a greenhouse gas, as a byproduct. This is usually released into the atmosphere, although there has also been some research into interring it underground or undersea. The steam reformers in methane-based fuel cells convert hydrocarbons into either carbon dioxide or carbon monoxide (CO).^[27]

Recently, there have also been some concerns over possible problems related to hydrogen gas leakage, (this has been pointed out in a paper published in Science magazine by a group of Caltech scientists). Molecular hydrogen leaks slowly from most containment vessels. It has been hypothesized that if significant amounts of hydrogen gas (H₂) escape, hydrogen gas may, due to ultraviolet radiation, form free radicals (H) in the stratosphere. These free radicals would then be able to act as catalysts for ozone depletion. A large enough increase in stratospheric hydrogen from leaked H₂ could exacerbate the depletion process. However, the effect of these leakage problems may not be significant. The amount of hydrogen that leaks today is much lower (by a factor of 10–100) than the estimated 10–20% figure conjectured by some researchers; for example, in Germany, the leakage rate is only 0.1% (less than the natural gas leak rate of 0.7%). At most, such leakage would likely be no more than 1–2% even with widespread hydrogen use, using present technology.^[28]

Safety

The Hindenburg a few seconds after catching fire.

Hydrogen has been feared in the popular press as a relatively more dangerous fuel, and hydrogen in fact has the widest explosive/ignition mix range with air of all the gases except acetylene. Hydrogen also usually escapes rapidly after containment breach. Additionally, hydrogen flames are difficult to see, so may be difficult to fight. An experiment performed at the University of Miami attempted to counter this by showing that hydrogen escapes while gasoline remains by setting alight hydrogen- and petrol-fuelled vehicles.^[29]

In the LZ 129 Hindenburg disaster, two thirds of the passengers and crew survived, though the skin of the Hindenburg may have contributed to the accident. It was concluded at the time by the board of enquiry that the fire was cause by electrostatic discharge of hydrogen leaking from the rear of the craft. Recent research by Addison Bain indicates that the outer fabric was highly inflammable, and that electrostatic sparks ignited the fabric first, which then spread to the hydrogen within.

In a more recent event, an explosion of compressed hydrogen during delivery at the AEP Muskingum River Coal Plant caused significant damage and killed one person.^[30][31]

One of the measures on the roadmap is to implement higher safety standards like early leak detection with hydrogen microsensors.^[32]

The Canadian Hydrogen Safety Program concluded that hydrogen fueling is as safe as, or safer than, CNG fueling.^[33].

References