Environmental Engineering Reference
In-Depth Information
Hydrogen is an energy carrier, and on a per mass basis, liquid hydrogen packs
some three times the energy of gasoline. Gasoline has a specific energy density of
12 kWh/kg, but liquid hydrogen weighs 42 kWh/kg. This is some 1,000 times the
energy storage density in a lead-acid battery. However, liquefaction of hydrogen
may be impractical during the near term, so most manufacturers have resorted to
gaseous storage in composite material canisters for mobile use. General Motors and
BMW
2
have announced joint development of liquid hydrogen refuelling devices
for liquid hydrogen hybrids and have invited other OEMs to join the initiative.
Liquid hydrogen is seen as the most practical means to transport hydrogen before
pipelines are in place. As part of the initiative, the automakers plan to standardize
the liquid hydrogen refuelling devices.
At 5,000 psi (350 atm, 35 MPa), 1 L has a mass of only 31 g but it stores
1.3 kWh. The Honda FCX, for example, transports some 156 L of compressed
hydrogen or just 4.8 kg. In effect, the container(s) weigh more than the gas they
store. But the energy stored is 202 kWh. If the vehicle consumes on average
0.5 kWh/mi, this is sufficient for a range of approximately 200 mi assuming fuel
cell conversion efficiencies of just under 50%.
Fuel cell progress in moving from specialty through stationary power into mass
transportation, market applications is paced by manufacturing technology. First
generation fuel cells, regardless of their technology, are true first generation units.
Second generation fuel cell technology is already finding application as stationary
power supplies. For example, Ballard Power Systems has unveiled the industry's
first hydrogen generator set. The unit is a hydrogen fuelled internal combustion
engine, a Ford Motor Co. 6.8 L V10 modified for hydrogen use, that develops
250 kW continuously [34]. This is sufficient power to supply from 20 to 40
households in North America, a supermarket and perhaps a hotel or light industrial
plant. Such systems are now sought as backup power for office and apartment
buildings to power elevators and lights in case of emergencies. Chung Kong
Infrastructure Holdings Limited in China estimates that such units are needed now
in more than 3,000 buildings.
Smaller proton exchange membrane (PEM) fuel cell stationary power units are
also becoming popular as small stationary and portable power generators. Ballard
Power Systems' NEXA power module is a volume produced PEM fuel cell that
generates up to 1,200 W for 24 V
dc
applications. Figure 4.35 shows the NEXA
module, a 560
250
330 mm package weighing only 13 kg.
This PEM fuel cell power module operates directly off 10-250 psig, 99.99%
dry gaseous hydrogen and air. Emissions are
<
0.87 L/h of liquid water and heat.
Hydrogen as fuel for internal combustion engines has also been demonstrated
by several automotive companies. With hydrogen fuel an ICE will have emissions a
fraction of those when gasoline is the feedstock. For example, hydrogen fuel burns
very cleanly in an ICE and emissions of CO
2
are reduced by 99.7%, HCs and CO
are 1/10th that of SULEV regulations, and nitrogen oxides (NO
x
) are 1/25th that of
gasoline and could be reduced to below SULEV requirements with appropriate
2
SAE Automotive Engineering International, Technical Briefs, p. 40, May 2003.
