The Hydrogen Economy
Over 95% of the hydrogen generated today derives from fossil fuels and is used at the site of production for synthesizing ammonia fertilizer, converting heavier hydrocarbons in crude oil into lighter fractions that are more suitable for fuels, or producing methanol from carbon monoxide.
Several methods are available for generating hydrogen from fossil fuels. All require high temperatures (over 700°C) and pressures (more than 3 times atmospheric pressure). They all finish with the “water-gas shift reaction,” in which CO reacts with steam to produce CO2 and H2. Their conversion efficiencies (energy content of products divided by energy content of ingredients) are around 70%.
The only common method for producing hydrogen that does not directly require fossil fuels is electrolysis, in which an electric current passes through water and releases hydrogen and water. In practice, electrolysis achieves conversion efficiencies of 40% to 63%. [1]
Many other methods of hydrogen production are under investigation. One is the direct splitting of water into hydrogen and oxygen. This occurs spontaneously at temperatures above 2500°C. The sulfur–iodine cycle lowers the temperature needed for direct water splitting to around 900°C. In a pilot study, this process achieved efficiencies of about 43%. [2] The eventual goal is to couple direct water splitting with heat production from nuclear reactors or solar collectors, but existing nuclear and solar facilities are not designed to operate at 900°C and would require major modifications.
Biological production of hydrogen through photosynthesis by microorganisms is yet another method. Efficiency of this hydrogen production, however, is less than 2%. [3]
General deployment hydrogen powered vehicles (which will require refueling at least as often as those running on traditional fuels) also thus awaits a functioning network of hydrogen refueling stations, and vice-versa. There are two strategies for supplying hydrogen to a network of refueling stations: A few large facilities might produce hydrogen and then ship it long distances, or many small facilities might produce hydrogen locally.
Large hydrogen production facilities would benefit from economies of scale and should be able to produce hydrogen at much less cost than small facilities. The disadvantage of central facilities is distributing hydrogen, a low energy content fuel, over long distances.
One immediate solution to long-distance hydrogen transport is hydrogen gas pipelines. [4] Unfortunately most of the 3 million kilometers of existing natural gas pipelines are unsuitable for hydrogen because they are usually composed of materials that leak the smaller hydrogen molecules or become brittle by reactions with hydrogen. Moreover, hydrogen has less than a quarter of the energy content of natural gas; to deliver the same amount of energy, the hydrogen pipelines must operate at higher pressures or be constructed of tubes of larger diameter than natural gas pipelines. Constructing a hydrogen pipeline network would cost over $600 billion for the United States and over $2.5 trillion worldwide. [5] Adequately pressurizing the network would also be require considerable energy at considerable cost itself.
Other solutions for distributing hydrogen involve truck or sea transport. A large load of hydrogen gas, if compressed at high pressures to achieve a practical energy density, is probably too dangerous for the nation’s highways or seaways. More likely is that trucks or ships with cryogenic storage capabilities would carry liquid hydrogen. The high cost of first liquefying the hydrogen and then losing some through transport, however, favor the development of more decentralized hydrogen production.
The number of hydrogen refueling stations has grown exponentially, to a total of 182 worldwide in 2007. Only in certain locations, such as around San Francisco or Los Angeles, can a hydrogen vehicle travel any distance from home and expect to find a compatible refueling station. Nearly all existing hydrogen vehicles use compressed gaseous hydrogen (5000 psi to 10,000 psi, or 34 MPa to 69 MPa).
Despite steady progress, not one piece of this economy—from fuel cell vehicles to facilities for production and distribution of hydrogen—is ready for general adoption.
[1] International Energy Agency (2005) Prospects for Hydrogen and Fuel Cells, Organization for Economic Cooperation and Development, Paris, http://www.iea.org/textbase/nppdf/free/2005/hydrogen2005.pdf.
[2] International Energy Agency (2005) Prospects for Hydrogen and Fuel Cells, Organization for Economic Cooperation and Development, Paris, http://www.iea.org/textbase/nppdf/free/2005/hydrogen2005.pdf.
[3] Rupprecht, J., B. Hankamer, J. H. Mussgnug, G. Ananyev, C. Dismukes, and O. Kruse (2006) Perspectives and advances of biological H-2 production in microorganisms. Applied Microbiology and Biotechnology 72:442-449.
[4] International Energy Agency (2005) Prospects for Hydrogen and Fuel Cells, Organization for Economic Cooperation and Development, Paris, http://www.iea.org/textbase/nppdf/free/2005/hydrogen2005.pdf.
[5] International Energy Agency (2005) Prospects for Hydrogen and Fuel Cells, Organization for Economic Cooperation and Development, Paris, http://www.iea.org/textbase/nppdf/free/2005/hydrogen2005.pdf.
This is an excerpt from the book Global Climate Change: Convergence of Disciplines by Dr. Arnold J. Bloom and taken from UCVerse of the University of California.
©2010 Sinauer Associates and UC Regents
