Environmental Engineering Reference
In-Depth Information
Key challenges to hydrogen delivery include reducing delivery costs, increasing
energy efficiency, maintaining hydrogen purity, and minimizing hydrogen leakage.
Further research is needed to analyze the trade-offs between the hydrogen produc-
tion options and the hydrogen delivery options taken together as a system. Building
a national hydrogen delivery infrastructure is a big challenge. Such an infrastructure
will take time to develop and will likely include combinations of various technolo-
gies. Delivery infrastructure needs and resources will vary by region and type of
market (e.g., urban, interstate, or rural). Infrastructure options will evolve as the
demand for hydrogen grows and as delivery technologies develop and improve.
HYDROGEN STORAGE
Storing enough hydrogen on board a vehicle to achieve a driving range of greater
than 300 miles is a significant challenge. On a weight basis, hydrogen has nearly
three times the energy content of gasoline (120 MJ/kg for hydrogen vs. 44 MJ/kg
for gasoline); however, on a volume basis, the situation is reversed (8 MJ/L for liquid
hydrogen vs. 32 MJ/L for gasoline). On-board hydrogen storage in the range of 5 to
13 kg is required to encompass the full platform of light-duty vehicles. Hydrogen can
be stored as either a gas or a liquid. Storage as a gas typically requires high-pressure
tanks (5000- to 10,000-psi tank pressure). Storage of hydrogen as a liquid requires
cryogenic temperatures, because the boiling point of hydrogen at 1 atmosphere pres-
sure is -252.8°C. Hydrogen can also be stored on the surfaces of solids (by adsorp-
tion) or within solids (by absorption). In adsorption, hydrogen is attached to the
surface of a material as either hydrogen molecules or hydrogen atoms. In absorption,
hydrogen is dissociated into H atoms, and then the hydrogen atoms are incorporated
into the solid lattice framework. Hydrogen storage in solids may make it possible to
store large quantities of hydrogen in smaller volumes at low pressures and at tem-
peratures close to room temperature. It is also possible to achieve volumetric storage
densities greater than those of liquid hydrogen because the hydrogen molecule is
dissociated into atomic hydrogen within a metal hydride lattice structure. Finally,
hydrogen can be stored through the reaction of hydrogen-containing materials with
water (or other compounds such as alcohols). In this case, the hydrogen is effectively
stored in both the material and the water. The terms
chemical hydrogen storage
and
chemical hydride
are used to describe this form of hydrogen storage. It is also pos-
sible to store hydrogen in the chemical structures of liquids and solids.
HOW A HYDROGEN FUEL CELL WORKS
A fuel cell uses the chemical energy of hydrogen to cleanly and efficiently produce
electricity. Fuel cells have a variety of potential applications; for example, they can
provide energy for systems as large as utility power stations or as small as laptop
computers. Fuel cells offer several benefits over conventional combustion-based
technologies currently used in many power plants and passenger vehicles. They pro-
duce much smaller quantities of greenhouse gases and none of the air pollutants that
create smog and cause health problems. If pure hydrogen is used as a fuel, fuel cells
emit only heat and water as byproducts.
