3.3.2. Transportation
Transportation accounts for almost 20% of anthropogenic GHG forcing due to the combined effects of carbon dioxide and nitrous oxide emissions, leaking refrigerant gases and ozone formed as a by product of hydrocarbon emissions from nearly 700 million motor vehicles. Passenger vehicles and cargo trucks contribute 75% of these emissions (58). Although chlorofluorocarbon (CFC) emissions are declining due to their phase-out, the replacement gases will result in nearly the same future forcing as today.
Removal of carbon dioxide from vehicle exhausts, either conventional, electric motor hybrids or fuel cell vehicles is not practical due to the logistics and cost of removing and recycling spent liquid sorbents that would have to be used (58). Replacing internal combustion and diesel engines with hydrogen fuel cells would eliminate the sources of ozone and nitrous oxide, but might only reduce carbon dioxide emissions, since capture and disposal of carbon dioxide produced from the generation of hydrogen for transportation may not be practical.
Fuel cells can use hydrogen from either fossil fuels or plant biomass in an electrochemical reaction to generate electricity that is then used to power an electric motor that turns the wheels. The hydrogen then reacts with atmospheric oxygen to produce water, a harmless byproduct (59). Fuel cells could increase fuel efficiency by 2-3 times present day (80-100 mpg), so that even if all of the carbon in the fuel source were emitted to the air, this would delay for several decades the return to some baseline level, since miles driven in the U.S. double every 20 years (60) and can be expected to follow a similar pattern as the rest of the world catches up. However, present day fuel cells use a rare expensive metal, platinum, which has no substitute, as the catalyst for key reactions. The fuel cells also do not produce enough heat energy to prevent freezing up of the system in cold weather (61).
The production, transportation and storage of hydrogen for fuel cells are also all daunting problems (62). There is insufficient crop or waste plant biomass to produce all the hydrogen needed (or even the ethanol) for the number of present day vehicles (in the U.S., 120 billion gals. of gasoline are consumed annually), let alone for the much larger fleets of the 21st century and the processes used to obtain hydrogen from these sources are too expensive (63).
Hydrogen can also be obtained from fossil fuels (natural gas and coal), but it cannot be shipped or pipelined as a liquid or gas economically in the quantities needed for use due to the lack of a sufficient pipeline distribution network and in the case of liquid hydrogen, problems with excessive losses regardless of how it is transported (59).
Instead, until such a pipeline network is in place, hydrogen will probably have to be produced on-site at a refueling station by a reforming reactor, probably from natural gas pipelined in. Since it is too expensive to capture the carbon dioxide from this process, it will be emitted to the air, limiting the effectiveness of fuel cells in mitigating global warming. On-board reformers are too heavy and expensive and require long start up times (59, 61).
On-board storage of hydrogen is also a problem with only gas or metal hydride forms now being considered (64). The driving range using available 2000-5000 psi tanks is too limited, about 100-170 miles (65), but use of 10,000 psi tanks to extend the driving range as some now envisage (66), seems improbable to us due to safety concerns. Hydride storage systems are also potentially dangerous due to water reactivity and require excessive energy to release the hydrogen (59).
