The hydrogen engine is a spark ignition engine running on hydrogen fuel.
Conventional gasoline engines used in cars can be converted into hydrogen
engines by some modifications.
Main exhaust emission is water. As far as energy efficiency and applicability
in ground transportation are concerned, the main challenges lie in hydrogen
generation and on-board storage.
Hydrogen storage
The following table shows the heat value of hydrogen compared to gasoline.
|
Heat value per
weight |
Heat value per
volume |
Hydrogen |
33
kWh/kg |
3
Wh/l |
Gasoline |
10 kWh/kg |
9000
Wh/l |
Source: Buchner 2000
Due to the low volume-specific heat value of H2 (about 3000 times smaller
than that of gasoline), on-board storage in both cars and railways is a
challenge.
The following methods exist:
- as compressed gas
- in liquid form
- in a hydrogen-absorbing metal alloy (metal hydride).
The benefits of metal hydride storage include the fact that a relatively
large volume of hydrogen can be stored per tank, and the fact that it is almost
impossible for large volumes of hydrogen to be released into the air, which
gives a safety advantage. The disadvantage of this method are tank weights. In
order to combat this, R&D (e.g. at Mazda) is putting effort into the
development of metal alloys with higher absorption capacity.
Hydrogen generation
The most common way of obtaining hydrogen is water electrolysis according to
the reaction:
2 H2O -> 2 H2 O2
Another promising approach lies in direct conversion from fossil sources.
Options discussed are:
- Reforming from carbon gas
- Partial oxidation of heavy hydrocarbons
- Steam reforming from natural gas
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