Environmental Engineering Reference
In-Depth Information
and will be exothermic over the return path. The second reaction, sort of a bottoming
cycle, releases poisonous carbon monoxide, further contributing to hydrogen produc-
tion as shown schematically in Figure 12.1. This hydrogen gas is then compressed
using additional energy input as electrical power from an energy storage system (ESS)
to high pressure hydrogen. Not shown are the filtering stages that yield high pressure
hydrogen at five-9's purity (e.g. 0.99999 pure). Water must be carried aboard a vehicle
that uses a hydrogen reformer in order to support the hydrogen production reactions
and also as a humidifier source for the fuel cell stack. The point being that a reformer
can be built to supply the on-board needs of a fuel cell stack by including fuel (NG or
alcohol) and water, with some of the water being consumed in hydrogen generation
and the remainder necessary to humidify the fuel cell input gases.
The debate continues on the viability of plug-in hybrid electric vehicles with
some arguing that overall emissions are actually worse than continuing to use
gasoline or diesel ICE propulsion power plants in vehicles. However, recent studies
show this is not the case and that PHEVs can be a viable means to reduce overall
emissions while meeting the transportation needs of a large segment of the popu-
lation [3]. In their work, Stephan and Sullivan [3] show that PHEVs can reduce
CO
2
emissions by 25% in the near term and 50% in the long term, and that if
PHEVs displace the conventional vehicles (CVs), then there is only a small benefit
by introducing PHEVs. However, if PHEVs are displacing HEVs, then a good
policy is to first upgrade a high CO
2
emitting plant to a cleaner coal plant from a
WTW perspective. Table 12.1 is a compilation of Stephan and Sullivan's results
with additional calculations on effective energy use.
Table 12.1 Energy consumption, TTW
Vehicle type
Specific energy
consumption (MJ/km)
Effective energy
consumption (Wh/mi)
WTW emissions
(gCO
2
/km)
CV
5.15
2,300
432
HEV
3.53
1,577
296
PHEV
0.92
411
274*
*Based on electricity generated by a coal-fired plant.
In the following sections, we first review some historical work on personal rapid
transit (PRT) and then move on to automated highway systems (AHS), a topic that is
core to this chapter, and then review in more detail how electric power can be
transmitted to a moving vehicle. It is useful to keep in mind that when one considers
the transportation of people, not cargo, the ratio of vehicle mass to unit capacity
(people) is close to a constant. For example, a mid-size passenger car with a capacity
of four people (using standard mass of 75.5 kg/person) may have a mass of 1,200 kg,
or about 5 kg/unit capacity. A transit bus of 14,000 kg plus 52 passengers then
equates to 4.6 kg/unit capacity, which is close to the same metric. This is why some
in the PRT business advocate small, 2- or even 4-passenger means of conveyance.
