Sizing the drive system - Propulsion Systems for Hybrid Vehicles - page 203

comparison, a compressed natural gas alternative fuel vehicle has storage tanks
rated at 3,600 psi. An economical issue with higher pressure storage is that the
electricity needed to pressurize the vehicle's fuel tanks will alter its overall effi-
ciency. For that reason, it is unlikely that a business case can be made for increasing
the pressure to 10,000 psi. Liquefied hydrogen is a more likely option. The
Daimler-Chrysler NeCar 4 incorporates a cryogenic liquefied hydrogen storage
system. Other than concerns with safety and distribution of liquid hydrogen is the
issue of the energy required by the liquefaction process. As with very high pressure
storage, the benefits of liquid hydrogen may be lost in terms of its total energy
picture. Storage in metal hydrides, which are metal alloys, in a loose, dry, powder
form is also viable. Certainly, the concerns with safety and containment are miti-
gated by hydrides. Unfortunately, the mass of metal hydride systems is unfavour-
able since they are six to ten times as massive as liquid hydrogen storage.
Alternatives to gaseous, liquid or metal hydride storage of hydrogen would be to
simply generate the hydrogen gas on-board by reformation of methanol, gasoline or
other hydrocarbon fuel stock. Reformers are generally quite complex and very costly
and currently not economical solutions. There are three reformer technologies now
available: (1) partial oxidation (PO x ), (2) steam reformation and (3) autothermal
reformers (ATR) [35]. The three share many common features and each is composed
of a primary reformer, followed by additional processing to convert CO to CO 2 using
water or oxygen. As noted later in Chapter 10, the use of steam reformation is seen by
many as being more economical to use with methanol fuel. PO x and ATR may be
more suited to reformation of gasoline, methane and other hydrocarbons. However,
one serious disadvantage of methanol is that it is tasteless, extremely poisonous and
corrosive.
Table 4.18 illustrates the US Department of Energy technical targets for fuel
cell development. In this table power refers to net power, or fuel cell stack power

Table 4.18 DOE fuel cell technical targets (50 kW peak power)

System characteristic Units CY2000 CY2004

Power density of stack W/L 350 500

Specific power of stack W/kg 350 500

Stack system efficiency at 25% rated power % 55 60

Precious metal loading g/kW pk 0.9 0.2

Cost in high volume mass production $/kW 100 35

Durability (to less than 5% degradation in
performance)

h > 2,000 > 5,000

Transient performance (10-90% power) s 3 1

Cold start-up to maximum power at 40 C

min 5 2

Cold start-up to maximum power at 20 C

1 0.5

Emissions < Tier 2 < Tier2

CO tolerance in steady state ppm 100 1,000

CO tolerance in transient state ppm 500 5,000