Environmental Engineering Reference
In-Depth Information
Hydrogen ICEs (2.0 L, I4 Zetec) exhibit thermal efficiency peaks of 52% (brake
thermal efficiency of 38%), brake specific fuel consumption, BSFC < 230 g/kWh
(gasoline equivalent), whereas gasoline in the same engine has BSFC ~ 260 g/kWh.
Figure 10.41 is a Ragone plot comparing the ICE to fuel cells and batteries.
Fuel cell
1,000
ICE, gas turbine
100
NaS
10
Pb-acid
NiCd, NiMH
1
1
10
100
1,000
Specific power (W/kg)
Figure 10.41 Energy source comparisons
Pressurized hydrogen for personal automobile use is the most practical, even
though the ICE will have 30-50% less power. View this in another way; in a
naturally aspirated ICE the hydrogen gas displaces 30% or more of the induction air
so that power output is similarly reduced. To obtain equal power to gasoline, the
ICE will need higher displacement because stoichiometric mixture consists of 2
parts hydrogen to 1 part oxygen (or 5 parts air). A gasoline-air mixture consists of 1
part heptane vapour to 11 parts oxygen (55 parts air). Because the hydrocarbon
molecules are so much larger than hydrogen, the combustion chamber for hydrogen
will need to be larger. Performance is also not as good as gasoline, so hybridizing a
hydrogen fuelled ICE vehicle makes good sense.
10.3.1 Metal hydride
An alternative to high pressure hydrogen storage, or liquefied hydrogen, is to store
it in a metal hydride. Compounds such as Mg 2 Cu, Mg 2 Ni and Mg will combine
with hydrogen and bind it as Mg 2 NiH 4 and MgH 2 so that as much hydrogen is
stored per unit volume as liquid hydrogen. Metal hydrides are stable at normal
temperatures and pressure but disassociate to hydrogen gas and metal above
260 C. A storage tank of sintered magnesium could be charged with hydrogen and
heat from the ICE exhaust used to liberate it from the metal hydride.
The hydrogen economy, if this comes about, relies on the fact that hydrogen
has a specific energy density (119.6 MJ/kg or 33.22 Wh/kg) three times higher than
gasoline (43.2 MJ/kg or 12 kWh/kg) or any other hydrocarbon fuel.
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