Environmental Engineering Reference
In-Depth Information
as natural gas, oil, or coal, or from another synthetic fuel like methanol or ethanol. Whether this
reforming is done at the fuel depot, where the hydrogen is then loaded and stored on the vehicle,
or is accomplished on board the vehicle in a portable reformer, the reforming operation preserves
at best only 80% of the parent fuel's heating value (see Table 3.3). This additional loss lowers the
maximum power fuel efficiency of hydrogen to 33.2%. Furthermore, if hydrogen is liquified for
storage on the vehicle, rather than being stored as a compressed gas in tanks, an additional energy
penalty of about 30% is incurred because energy is needed to liquify hydrogen at the very low
temperature of
8 C. Altogether, these synthetic fuel transformation penalties diminish the
fuel efficiency advantage of fuel cells compared to conventional internal combustion engines in
vehicles fueled by conventional hydrocarbon fuels. 19
The hydrogen fuel cell used in vehicles utilizes a solid polymer electrolyte, called a proton
exchange membrane (PEM). Only a fraction of a millimeter in thickness, the membrane is coated
on both sides with a very thin layer of platinum catalyst material that is required to promote the
electrode reactions producing the flow of electric current through the cell and its external circuit,
as described in Section 3.12. Carbon electrodes, provided with grooves that ensure the gaseous
reactants [hydrogen at the anode and oxygen (in air) at the cathode], are distributed uniformly
across the electrode surfaces and are sandwiched on either side of the PEM, forming a single cell.
As many as a hundred or more cells are stacked in series, mechanically and electrically. The fuel
and oxidant are supplied under a pressure of several bar, so as to increase the maximum power
output per unit of electrode area, which is usually of the order of 1 W/cm 2 . Water, formed at the
cathode, must be removed, but the PEM must remain moist to function properly. In addition, heat
is released in the cell reaction, so that the cell must be cooled, maintaining a temperature less
than 100 C.
Table 8.5 lists the salient features of several fuel cell vehicles developed during the 1990s in
Europe and the United States: two versions of a compact car (NECAR 4 and NECAR 3), a U.S.
compact car (P2000), a van (NECAR 2), and a municipal bus (NEBUS). They all are based upon a
conventional vehicle with the replacement of the engine and fuel tank by a fuel cell, electric motor,
and fuel storage system, while maintaining the original vehicle's passenger capacity and range.
The arrangement of the replacement fuel cell system in the vehicle is shown in Figure 8.8 for the
NECAR 4 and NEBUS vehicles.
NECAR 4 is the most recent version of the Daimler compact car. Its hydrogen fuel is stored in
liquid form in a cryogenic insulated fuel tank. The previous version of this car, NECAR 3, utilizes
liquid methanol as the fuel, which is converted to hydrogen in an onboard reformer. NECAR 2 and
NEBUS store hydrogen gas in tanks pressurized to 300 bar and weighing about 75 times more than
the fuel they contain, adding to the vehicle mass. In general, these vehicles weigh more than their
conventional counterparts.
The values for fuel economy of these vehicles, expressed as kilometers per megajoule of fuel
heating value (km/MJ) or kilometers per liter (km/L) of gasoline equivalent (in terms of fuel heating
value), are listed in the table as well. For comparison, Table 8.5 also lists the fuel economies of the
conventional vehicles of the comparable class, both CI- and SI-powered. The fuel cell vehicles have
comparable fuel economies, but are clearly not distinctly superior to their conventional cousins.
252
.
19 Fuel cells used in electric power systems do not suffer such disadvantages. They do not require synthetic
fuel and operate at high temperatures where coolant streams can be used to generate steam for extra electric
power output.
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