Environmental Engineering Reference
In-Depth Information
Table 1.7 Well to wheels percentage efficiencies for internal combustion vehicles fueled with
hydrogen produced from different primary energy resources
Primary
energy
Energy
carrier
Type of
vehicle
Primary energy to energy
carrier % eff.
a
Vehicle %
eff.
b
Weel-to-
wheels % eff.
Natural gas
Hydrogen
ICV
73 (350 bar)
20
15
28 to electricity
c
Nuclear
Hydrogen
ICV
20
2.6
47 for electrolysis
c
11 to electricity
c
Photovoltaic
Hydrogen
ICV
20
1.0
47 for electrolysis
c
1 for photosynthesis
d
Biomass
Hydrogen
ICV
20
0.1
59 for biomass to H
2
c
a
Includes carrier distribution efficiency (93% for electricity, 86% for compressed hydrogen,
99.8% for oil [
52
])
b
Taken from Ref. [
52
], evaluated on US FTP driving cycle for Diesel vehicle and attributed to H
2
vehicle
c
Taken from Ref. [
54
] and corrected for footnote a
d
Generally considered as comprised between 0.1 and 1% [
55
]
for direct use on BEVs, 28 and 11% from nuclear and photovoltaic, respectively).
The hydrogen produced by electricity is then converted back to electricity by fuel
cells in HFCEVs, leading to the expectable conclusion of well-to-wheels effi-
ciencies lower than BEVs (4.5 and 1.8% starting from nuclear and photovoltaic,
respectively, to be compared with 17 and 6.7%).
The option of using hydrogen in conventional internal combustion engines is
analyzed in Table
1.7
in terms of well-to-wheels efficiency. These data show that
also attributing to hydrogen ICVs the same high vehicle efficiency of Diesel
vehicles (20%, Table
1.5
), only producing hydrogen from natural gas (with carbon
dioxide emissions) it is possible to reach a primary energy efficiency comparable
to that of other conventional fuels for ICVs (19% for Diesel vehicles, Table
1.5
).
On the other hand, if hydrogen is produced from nuclear or renewable resources,
the well-to-wheels efficiency for ICVs results always lower than the corresponding
efficiencies estimated for paths involving fuel cells and electric drivetrains
(HFCEVs, Table
1.6
), with the additional drawback of NO
x
emissions at the
engine exhaust associated with hydrogen combustion.
The well-to-wheels analysis can give useful indications regarding the potential
of a specific technology in terms of primary energy utilization efficiency, but is not
sufficient to point to one technology or energy carrier as the best option. In fact,
the high well-to-wheels efficiency obtained for BEVs, especially starting from a
primary resource at high energy density such as nuclear, has to be weighed in the
light of indispensable requisites which road vehicles must have to be compatible
with their practical usage. Among these characteristics the driving range still
represents, on the base of current technology, the main limit to a widespread
diffusion of battery powered vehicles, essentially due to not yet satisfactory
performance of electric energy storage systems in terms of specific energy per
weight/volume of the storage. In particular, some evaluations based on the current
technologies [
56
] have evidenced that also with the most advanced Li/ion batteries
