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Hydrogen
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When used as a fuel in an internal combustion engine, hydrogen burns "more efficiently than gasoline, and burning hydrogen creates less air pollution. Hydrogen has a higher flame speed, wider flammability limits, higher detonation temperature, burns hotter, and takes less energy to ignite than gasoline. This means that hydrogen burns faster, but carries the danger of pre-ignition and flashback" (w1). Hydrogen is the main candidate to be used in fuel cells. When burned, hydrogen produces relatively large amounts of water which may be detrimental to combustion engines. In cold climate, the water mist clouds generated may cause visibility problems.

Hydrogen gas "has the highest energy content per unit of weight of any known fuel" (w1), which makes it, in principle, an ideal energy carrier. Hydrogen does not occur in large quantities on the Earth's surface, however, in its gaseous form. Due to its extremely low boiling point storage other than as compressed gas is difficult.

There are two main methods for larger scale industrial production of producing hydrogen gas (1).

  1. Splitting water into hydrogen and oxygen through electrolysis.
  2. Synthesis gas production from steam reforming or partial oxidation of hydrogen-containing fuels.

Both methods require large amounts of energy. The resulting hydrogen gas is dried, purified, compressed, and sent to storage (1).

Hydrogen can also be produced by reforming conventional gasoline and diesel fuel.

The overall environmental performance of hydrogen as a fuel depends largely on the primary energy source, i.e. whether fossil fuels are used or regenerative ones. In addition, further processing and distribution steps determine the total environmental impacts of hydrogen production (2).

Hydrogen's high flammability creates a risk of explosion in enclosed spaces. Since it is lighter than air, any fuel leak rapidly disperses with no pooling of vapors. It is non-toxic, but since it displaces air, any release in an enclosed space could cause asphyxiation.

At normal temperatures and pressures hydrogen exists as a gas, making it more difficult to address transportation and storage issues in comparison to liquid fuels. As with natural gas, hydrogen is normally stored as a compressed gas (CH2) or a liquefied gas (LH2).

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Components of a hydrogen fueling system include the following (3):

  • A chemical infrastructure for producing hydrogen
  • A hydrogen processing module and - in the case of CH2 - a hydrogen compressor
  • Hydrogen storage tanks
  • Fueling technology

According to (w1), "motor vehicles and furnaces can easily be converted to use hydrogen as a fuel". This may be a gross oversimplification, however. Still, the use of hydrogen as a fuel necessitates the creation of a new storage and fuel distribution infrastructure, as well as adequate on-board storage systems (w1).

According to (4), "breakthroughs in storage technology would have the biggest impact in accelerating the acceptance and commercialization of fuel cell vehicles". "Hydrogen storage is constrained by container weight and volume. Depending upon whether hydrogen is stored as liquid or a gas, it requires six to eight and six to ten times more storage space than gasoline, respectively" . Storage systems being developed include compressed hydrogen, liquid hydrogen, and chemical bonding between hydrogen and a storage material (for example, metal hydrides).

It may further be anticipated that safety issues will have to be addressed with regard to the infrastructure of hydrogen fueling. Hydrogen being a light and highly flammable gas, similarities with natural gas may in this respect emerge.

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Emissions of concern from combustion engines are NOx and, in cold climates, water vapor. Particulate emissions can be produced from the engine oil but should be very low. Emissions of toxic compounds and other hydrocarbons will come from used engine oil only.

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According to (w1), "currently the most cost-effective way to produce hydrogen is steam reforming. According to the U.S. Department of Energy, in 1995 the cost was US$ 7.00 per gigajoule (GJ) in large plant production. This assumes a cost for natural gas of US$ 2.30 per gigajoule. The production of hydrogen by electrolysis using hydroelectricity at off peak rates costs between US$ 10.00 to US$ 20.00 per gigajoule."
Estimates of 1998 pump prices in Germany (excluding taxes) (5) were as follows:

Source Euro Cent/kWh US$/GJ
Natural gas 5-6

13-16

Wood 14-20 36-52
Electrolysis (solar energy) LH2/GH2

26/21

67/54
Electrolysis (hydropower) LH2/GH2

12/10

31/25


According to (1), the cost of developing a pipeline distribution infrastructure for hydrogen gas "could be enormous". To avoid this expense, some researchers have proposed a decentralized production of hydrogen at refueling stations (1). In an infrastructure cost assessment of different hydrogen fueling systems for fuel cell vehicles it is estimated that "maintaining the existing natural gas infrastructure and producing and installing small-scale steam methane reformers to produce hydrogen at the local fueling station requires annual capital investments of between US$ 600 to US$ 800 for each new direct hydrogen fuel cell vehicle sold" (6).

See (6) and (7) for cost assessments of hydrogen fueling systems and of storage and transport respectively.

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Hydrogen's use as a vehicular fuel is still in the research and development stage. However, most automobile manufacturers now have prototype or demonstrator developed hydrogen-powered cars based on internal combustion engines as well as fuel cells. In (w3) a number of hydrogen-fueled cars are listed as well as filling stations that have been installed around the world in connection with various projects. Compared to other alternative fuels, hydrogen-powered vehicles are probably the furthest from commercialization . Most manufacturers have shifted attention away from hydrogen fueled internal combustion engines toward hydrogen fueled fuel cell powered vehicles. Fuel cell buses and fuel cell cars currently exist as prototypes. A small scale fleet is tested in a number of European cities as a joint project starting 2003/4.

Regarding the potential for hydrogen production in Latin America, "in Brazil, 92% of electricity generation is from hydro-power. There is enough surplus overnight electric power capacity in the Sao Paulo Metropolitan Region (SPMR) to fuel 12,000 buses. There is already substantial experience of re-fuelling with high pressure gaseous fuel, through the fleet of over 300 CNG (compressed natural gas) buses operating on a daily basis in the SPMR" (w1). However, it is generally agreed that hydrogen fueled, fuel cell technology is many years away from commercialization.

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Hydrogen is an attractive energy carrier only if it is generated using renewable resources, and thus does not lead to significant fossil CO2 emissions. It could for example be an interesting option to store and distribute the electricity from large wind, water or solar power stations in the form of hydrogen through electrolysis. When hydrogen produced by steam reforming of natural gas is used, approximately 10% more CO2 is emitted per energy unit compared to gasoline, while generating greater costs (2).

In order to use hydrogen on a wider scale, researchers must develop more practical and economical ways to store and process hydrogen (w1). A number of fuel cell experts interviewed in (4) agree that hydrogen storage technology should be the focus of major research and development activities in the future. Breakthroughs in storage technology would have a large impact in accelerating the acceptance and commercialization of fuel cell vehicles.

Furthermore, codes and standards related to hydrogen storage and transportation need significant work in the near future before there can be any significant market share for hydrogen vehicles (4).

High production costs and low density have prevented the use of hydrogen as a transportation fuel in all but test programs so far (w4). According to the Alternative Fuels Data Center (AFDC) (w4), "it may be 20 to 30 years or more before hydrogen is a viable transportation fuel and then perhaps only in fuel-cell-powered vehicles."

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