338
batteries. It is thus the energy density of Li-ion batteries that enabled the latest
reemergence of the BEV, even though they still have smaller driving ranges and
longer charging times than ICV s. It has also been pointed out that EVs are only as
clean as the electricity they use, a somewhat obvious observation for industrial ecologists (Moyer 2010 ).
Relative to the incumbent ICV, alternative/advanced powertrains have higher
tank-to-wheel energy effi ciency but also higher cradle-to-gate production impacts,
due to the nature of their components, such as batteries, fuel cells, and electric
motors (Demirdöven and Deutch 2004 ; ANL 2014 ). In the case of HEVs, it is relatively simple to show that the fuel savings far outweigh the additional production
impacts. Life cycle comparisons of the other alternative powertrains are complicated by the fact that they use electricity and hydrogen as fuel, which can be produced in many different ways (Samaras and Meisterling 2008 ; Notter et al. 2010 ;
Hawkins et al. 2012 ). Currently, most hydrogen is produced through steam reforming of hydrocarbon fuels. To eliminate the need for fossil fuels, it is frequently
stated that the hydrogen for FCVs should ideally come from electrolysis of water
powered by renewable electricity. However, it would be considerably more energy
effi cient to use renewable electricity directly in BEVs rather than convert it into
hydrogen through electrolysis and then back into electricity in a fuel cell. The detour
via hydrogen has the advantage, though, that hydrogen is easier to store than
electricity.
4 Lightweight Materials
In addition to more effi cient powertrains, the PNGV also researched lightweight
materials for vehicle mass reduction. Such a mass reduction increases the fuel economy of the vehicle without reducing its size. The use of lightweight materials is
usually also seen as necessary to compensate for the higher mass of advanced powertrains. A material is regarded as lightweight if it achieves signifi cant mass reduction relative to mild steel without compromising other design parameters, but there
is no precise defi nition. The considered materials are typically aluminum and magnesium alloys, fi ber-reinforced polymers, and advanced high-strength steels (AHSS)
(DOE 2014 ). With the exception of AHSS, the primary production of lightweight
materials has signifi cantly higher environmental impacts than mild steel production.
In fact mass reduction potential appears to be correlated to production impacts
(Geyer 2013 ). Again, LCA is required to quantify the trade-off between the increase
in material production emissions and the decrease in vehicle use phase emissions.
The trade-off needs to be studied on a case-by-case basis, but different studies of
similar cases frequently yield confl icting results. There is signifi cant debate about
the amount of mass reduction lightweight materials can achieve in practice, since
this is not directly observable and has to be either modeled or derived from analysis
of proxy data sets. The same is true of the relationship between vehicle mass reduction and fuel economy improvement. Initial use of simplistic rules of thumb is
R. Geyer
batteries. It is thus the energy density of Li-ion batteries that enabled the latest
reemergence of the BEV, even though they still have smaller driving ranges and
longer charging times than ICV s. It has also been pointed out that EVs are only as
clean as the electricity they use, a somewhat obvious observation for industrial ecologists (Moyer 2010 ).
Relative to the incumbent ICV, alternative/advanced powertrains have higher
tank-to-wheel energy effi ciency but also higher cradle-to-gate production impacts,
due to the nature of their components, such as batteries, fuel cells, and electric
motors (Demirdöven and Deutch 2004 ; ANL 2014 ). In the case of HEVs, it is relatively simple to show that the fuel savings far outweigh the additional production
impacts. Life cycle comparisons of the other alternative powertrains are complicated by the fact that they use electricity and hydrogen as fuel, which can be produced in many different ways (Samaras and Meisterling 2008 ; Notter et al. 2010 ;
Hawkins et al. 2012 ). Currently, most hydrogen is produced through steam reforming of hydrocarbon fuels. To eliminate the need for fossil fuels, it is frequently
stated that the hydrogen for FCVs should ideally come from electrolysis of water
powered by renewable electricity. However, it would be considerably more energy
effi cient to use renewable electricity directly in BEVs rather than convert it into
hydrogen through electrolysis and then back into electricity in a fuel cell. The detour
via hydrogen has the advantage, though, that hydrogen is easier to store than
electricity.
4 Lightweight Materials
In addition to more effi cient powertrains, the PNGV also researched lightweight
materials for vehicle mass reduction. Such a mass reduction increases the fuel economy of the vehicle without reducing its size. The use of lightweight materials is
usually also seen as necessary to compensate for the higher mass of advanced powertrains. A material is regarded as lightweight if it achieves signifi cant mass reduction relative to mild steel without compromising other design parameters, but there
is no precise defi nition. The considered materials are typically aluminum and magnesium alloys, fi ber-reinforced polymers, and advanced high-strength steels (AHSS)
(DOE 2014 ). With the exception of AHSS, the primary production of lightweight
materials has signifi cantly higher environmental impacts than mild steel production.
In fact mass reduction potential appears to be correlated to production impacts
(Geyer 2013 ). Again, LCA is required to quantify the trade-off between the increase
in material production emissions and the decrease in vehicle use phase emissions.
The trade-off needs to be studied on a case-by-case basis, but different studies of
similar cases frequently yield confl icting results. There is signifi cant debate about
the amount of mass reduction lightweight materials can achieve in practice, since
this is not directly observable and has to be either modeled or derived from analysis
of proxy data sets. The same is true of the relationship between vehicle mass reduction and fuel economy improvement. Initial use of simplistic rules of thumb is
R. Geyer