Environmental Engineering Reference
In-Depth Information
processes while reducing solids in municipal wastewater treatment facilities at competitive costs
(Ekins, Hawkins, and Hughes 2010, 36).
Photo-Electrochemical
Hydrogen can be produced directly from water using sunlight and a special class of semiconduc-
tor materials. These highly specialized semiconductors absorb sunlight and use light energy to
completely separate water molecules into hydrogen and oxygen (USDOE 2006).
Hydrogen production technologies are in various stages of development. Some technologies,
such as steam methane reforming, are well developed and can be used in the near term. Others,
such as high-temperature thermochemical water-splitting, biological, and photo-electrochemical,
are in early stages of laboratory development and considered potential pathways for the long term,
but will not be available any time soon (USDOE 2006).
Storage
Hydrogen storage will be required onboard vehicles and at hydrogen production sites, hydrogen
refueling stations, and stationary power sites. Developing safe, reliable, compact, and cost-effective
hydrogen storage technologies is one of the most technically challenging barriers to widespread
use of hydrogen as a form of energy. Possible approaches to storing hydrogen include:
s0HYSICALSTORAGEOFCOMPRESSEDHYDROGENGASINHIGHPRESSURETANKS
s0HYSICALSTORAGEOFCRYOGENICHYDROGENININSULATEDTANKS
s3TORAGEINADVANCEDMATERIALSWITHINTHESTRUCTUREORONTHESURFACEOFCERTAINMATERIALS
as well as in the form of chemical compounds that undergo a chemical reaction to release
hydrogen. Hydrogen can be stored on the surfaces of solids by adsorption, in which hydrogen
associates with the surface of a material either as hydrogen molecules or hydrogen atoms.
(USDOE 2011b)
Hydrogen has physical characteristics that make it difficult to store in large quantities without
taking up a significant amount of space. On a weight basis, hydrogen has nearly three times the
energy content of gasoline. On a volume basis, however, the situation is reversed, with gasoline
having nearly four times the energy content of hydrogen. This makes hydrogen a challenge to
store, particularly within the size and weight constraints of a vehicle (USDOE 2011b).
To be competitive with conventional vehicles, hydrogen-powered cars must be able to travel more
than 300 miles between fills, which is generally regarded as the minimum for widespread public
acceptance. A typical light-duty fuel cell vehicle will need to carry 4 to 10 kilograms of hydrogen,
depending on the size and type of vehicle, to allow a driving range of more than 300 miles (~483
kilometers), the DOE performance goal. Drivers must also be able to refuel at a speed comparable
to the rate of refueling today's gasoline vehicles. Using currently available high-pressure tank stor-
age technology, placing a sufficient quantity of hydrogen onboard a vehicle to provide a 300-mile
driving range would require a very large tank—larger than the trunk of a typical automobile. Aside
from loss of cargo space, there would also be the added weight of the tank, which would probably
reduce fuel economy. To acheive the 300-mile driving range, new low-cost materials and compo-
nents for hydrogen storage systems are needed, along with low-cost, high-volume manufacturing
methods for those materials and components. Hydrogen storage research is focused primarily on
technologies and systems used onboard a vehicle. Efforts are being made to improve the weight,
volume, and cost of current hydrogen storage systems and to identify and develop new technologies
 
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