Hydrogen is the most abundant element in the universe. It can combine with oxygen to form water, H2O. This process releases energy, so hydrogen can be used as a fuel. However, natural free hydrogen (hydrogen not already bound up in water or some other molecule) is rare on Earth, so if large quantities are desired, they must be manufactured. This can be done either by reforming methane (CH4), or by using electricity or catalysts to split H2O into H2 and O2 (molecular hydrogen and oxygen, respectively). Hydrogen can be combined with oxygen in a fuel cell, which produces electricity, or in a flame or explosion, which produces heat. In either case, the only chemical byproduct of the reaction is water.
Many people have proposed that the world switch from its present energy economy, which is based mostly on burning the fossil carbon found in coal, oil, and natural gas, to a hydrogen economy. In a hydrogen economy, H2 would be used as a fuel and an energy carrier. It would be produced from methane or water or both. Hydrogen from water would be manufactured using electricity from renewable sources such as windmills or solar cells (or, as some urge, nuclear power plants). This hydrogen would be re-combined with oxygen in fuel cells to produce electricity to run cars, appliances, lights, and other devices.
The hydrogen economy idea has the backing of many governments, including that of the United States, but is years—probably decades—away from realization. There are many technical problems to be overcome before hydrogen can be made widely useful and affordable. The government of Iceland has committed to achieving an all-hydrogen energy economy by 2050.
Historical Background and Scientific Foundations
History of the Hydrogen Economy Concept
British chemist Henry Cavendish (1731-1810) discovered hydrogen in 1766. The fact that it combines with oxygen to form water was described a few years later by French chemist Antoine Lavoisier (1743-1794). Today, hydrogen is used in the manufacture of ammonia (mostly for fertilizer), methyl alcohol, and hydrogenated fats for the stiffening of processed foods. In the nineteenth and early twentieth centuries, it was used as a lifting gas in balloons and airships because it is the least dense of all gases at any given temperature and pressure.
In the nineteenth century, the possibility of a hydrogen economy—an energy economy based on hydrogen rather than on carbon-intensive fuels such as coal, petroleum, or natural gas—occurred to several visionary thinkers. Although ways to make hydrogen in bulk from natural gas were not yet known, electrolysis was understood. Electrolysis (pronounced ee-lek-TROL-ah-sis) is the splitting of water molecules by an electric current into hydrogen and oxygen. This process can be written in chemical notation as follows:
2H2O + electricity → 2H2 + O2
As early as 1874, French novelist Jules Verne (1828-1905) had one of his characters predict a hydrogen economy based on the electrolysis of water in his novel L'archipel en feu (The Mysterious Island). The character proclaims that “water will one day be employed as fuel.” The character continues: “[T]he hydrogen and oxygen which constitute it, used singly or together, will furnish an inexhaustible source of heat and light…. Some day the coalrooms of steamers and the tenders of locomotives will, instead of coal, be stored with these two condensed gases, which will burn in the furnaces with enormous calorific power.”
However, Verne did not say where the electricity was to come from that would electrolyze the water to produce all this hydrogen and oxygen. This is a crucial point, and explains why ships and railroads still do not run on hydrogen 130 years later: electrolysis uses more energy than it produces. Water can never be employed as a fuel. It is, in effect, not fuel but exhaust—a substance with no chemical energy left to give.
But Verne's vision was not necessarily without merit. The energy lost in electrolysis might be an acceptable price to pay for certain gains, such as hydrogen's portability and cleanliness at the point of use. When combined with oxygen in a fuel cell, hydrogen supplies electricity at high efficiency and with zero noise or on-site pollution. The principle of the fuel cell, a sort of chemical battery that allows hydrogen to combine with oxygen slowly and coolly, extracting the energy of the reaction as electricity, has been known since the mid-nineteenth century.
The U.S. National Aeronautics and Space Administration (NASA) devised practical hydrogen-oxygen fuel cells in the 1950s and 1960s, needing a quiet, compact energy source for its manned spacecraft and knowing that ordinary electric batteries would weigh too much. The electrical systems on the Apollo spacecraft that went to the moon were powered by fuel cells. So clean were the waste products of the cells that the astronauts used them as drinking water. However, the NASA cells were too expensive to have any commercial uses.
In the 1960s, the possibility of a global greenhouse effect caused by carbon dioxide (CO2) from fossil fuels first began to be taken seriously by many scientists. The possibility that oil supplies might someday run out was also widely discussed. The idea of a world where energy from windmills and solar cells was stored in hydrogen for convenient, clean use in vehicles and buildings became suddenly attractive. Such a system would free people from oil addiction and abolish a major source of greenhouse-gas emissions at the same time.
The term “hydrogen economy” was coined in 1970 by maverick chemist John Bockris to describe such a system. In the 1990s, as scientific evidence of human-caused global warming became ever more convincing, the concept of the hydrogen economy drew the attention of a wide range of observers, including governments, energy experts, and environmentalists. There is little doubt that a hydrogen economy would be desirable; the debate, which continues today, is over whether it can be practical.
Producing and Using Hydrogen
Free hydrogen is found only in trace amounts in nature, so cannot be extracted as a primary chemical fuel. It is therefore considered not an energy source but an energy carrier, a way of moving energy from a primary source to an end-user, much like electricity. About 40 million tons of hydrogen are manufactured each year from natural gas (methane) using a method called the Haber process. This process—named after its discoverer, German chemist Fritz Haber (1868-1934)—exploits the fact that at high temperatures, methane (CH4, where C stands for carbon) combines with water in the form of steam to release hydrogen and carbon monoxide (CO). In chemical notation, the reaction is written as follows:
CH4 + H2O → CO + 3H2
Energy is lost in this reaction—that is, less energy is obtained by burning the hydrogen than could have been obtained by burning the methane from which it was made. Specifically, 15% of the energy in the methane is lost steam reformation. This energy cost may be worthwhile when there is a task that hydrogen can be used for that methane cannot, or if some other value is obtained. For example, more useful energy can be extracted from a given amount of hydrogen in a fuel cell than by burning it explosively with oxygen, as is done in the cylinders of a standard engine. In fact, by reforming methane into hydrogen and then combining the hydrogen with oxygen in a fuel cell, more energy can be extracted from the original methane than by burning it directly, despite the energy lost in the conversion.
According to the American Physical Society, a fuel-cell-driven vehicle using hydrogen from methane would get slightly higher well-to-wheels efficiency (2,867 British thermal units of energy per mile traveled, i.e. 1.9 million joules per kilometer) than an equivalent-size hybrid car burning methane in an internal combustion engine (2,368 BTUs/mile or 1.6 MJ/km)—a 19% improvement. It is therefore not necessarily true that energy is lost, overall, by converting methane to hydrogen. Depending on the technologies used, there can be a gain.
There are other reasons, in addition to possibly lower efficiency, not to burn natural gas directly in vehicles. Although less polluting than gasoline, methane does release the greenhouse gas carbon dioxide into the atmosphere.
Hydrogen is ultimately no cleaner than the energy sources used to produce it. Carbon monoxide is created during the extraction of hydrogen from methane, which, if released into the atmosphere eventually becomes carbon dioxide. However, it is possible to capture this carbon in the large facilities where hydrogen is produced and inject it deep underground to prevent it from causing climate change. Alternatively, there are industrial processes that capture the carbon as a black powder that can be sold as a useful product (carbon black) or sequestered from the atmosphere with relative ease.
Natural gas (which is mostly methane, and is the source of the methane used to manufacture hydrogen) has the advantage of being fairly cheap. There is a well-established supply system for it. However, natural gas has the disadvantage that it is a fossil fuel and will someday be all used up. It is also, like petroleum, often obtained from politically unstable parts of the world. It must be chilled to extremely low temperatures (–260°F or 160°C) and is transported around the world in gigantic tankers, each one carrying up to 30 million gallons (114 million liters) of super-cold liquid gas that contains as much energy as a small nuclear weapon.
A more attractive way to make hydrogen than steam reformation of methane, in many ways, is the production of hydrogen by electrolysis using electricity derived from truly inexhaustible resources such as wind power and solar cells. An economy based on electricity from the wind and sunlight, which would use hydrogen wherever a portable liquid fuel was needed, could in theory continue for many millions of years—as long as the sun continued to shine. Such a system might even be affordable.
WORDS TO KNOW
CARBON BLACK: A commercial useful form of nearly pure carbon consisting of tiny particles. Used as a black pigment in inks and plastics and in tire manufacture. Over 8 million metric tons are produced each year, mostly from natural gas or heavy oil feedstocks.
ELECTROLYSIS: The process by which an electrical current is used to cause a chemical change, usually the breakdown of some substance.
FOSSIL FUELS: Fuels formed by biological processes and transformed into solid or fluid minerals over geological time. Fossil fuels include coal, petroleum, and natural gas. Fossil fuels are non-renewable on the timescale of human civilization, because their natural replenishment would take many millions of years.
FUEL CELL: Device that combines a fuel (e.g., hydrogen) and oxygen at a low or modest temperature to produce electricity directly. In a fuel cell, fuel is not burned (combined rapidly with oxygen) to produce heat, but combined ionically through a membrane. Fuel cells are more efficient users of fuel than thermal systems, but are also more expensive.
HABER PROCESS: The chemical process by which nitrogen and hydrogen are combined with each other at high temperature and pressure over a catalyst to produce ammonia.
INTERNAL COMBUSTION ENGINE: An engine in which the chemical reaction that supplies energy to the engine takes place within the walls of the engine (usually a cylinder) itself.
PHOTOVOLTAIC ELECTRICITY: Electricity produced by photovoltaic cells, which are semiconductor devices that produce electricity when exposed to light. Depending on the cell design, between 6% and (in laboratory experiments) 42% of the energy falling on a photovoltaic cell is turned into electricity.
SOLAR CELL: A device constructed from specially prepared silicon that converts radiant energy (light) into electrical energy.
In 2003, electricity from wind turbines was already the cheapest type of bulk power available—cheaper than coal, nuclear power, or photovoltaic electricity. Wind power's biggest drawback, namely the irregular, on-off nature of wind, would not matter if windmills were devoted to producing hydrogen rather than feeding their electricity to the grid of wires that supplies homes and businesses with power. The hydrogen would simply
build up whenever the wind blew, storing the windmill's power output for later retrieval by fuel cells. Solar cells, although still much more expensive than wind, are becoming cheaper at a rapid rate, which is not surprising for devices that are manufactured using the same basic methods as computer chips.
A few scientists and corporations have suggested using nuclear power plants to produce hydrogen. This is technically possible, but it seems unlikely that this source could ever compete with the low (and declining) cost of wind power without major government aid. Nuclear power has also been controversial in some countries because of its links to radioactive waste, nuclear-weapons proliferation, and possible terrorist attacks.
Impacts and Issues
Despite the many desirable aspects of a global hydrogen economy, transition to a hydrogen economy may be difficult for a number of reasons. Almost every aspect of the possible hydrogen economy is fiercely debated among experts. A few of the issues raised by critics of the concept, along with some of the counterpoints mentioned by its defenders, are as follows:
- Hydrogen is dangerous. When it escapes and mixes with air, it can explode or burn out of control. Images of the hydrogen-filled Hindenburg airship, which burned and crashed in New Jersey in 1937, are often recalled as terrifying proof of the dangers of hydrogen. Conversely, gasoline and the other liquid fuels, on which we presently rely for most transportation, are also dangerous and explosive, both as liquids and vapors. Hydrogen, unlike gasoline or its fumes, rises rapidly when released, making explosive mixtures hard to obtain at accident scenes. It cannot coat victims with a burning liquid and when it does burn, produces a pale flame that emits only 10% of the infrared radiation (radiant heat) of a gasoline or methane flame. Most of the people killed in the Hindenburg crash were killed by falls or burning gasoline, not by the hydrogen fire. In fact, the fire only occurred initially because the hydrogen was stored in a gigantic, fragile bag.
- Hydrogen is difficult to handle. The H atom is the smallest and lightest of all atoms, and the H2 molecule is the smallest and lightest of all molecules. A given volume of hydrogen contains only 30% as much energy as the same volume of natural gas at the same temperature and pressure; at 170 bar (170 times sea-level atmospheric pressure), a given volume of H2 contains only 6% as much energy as an equal volume of gasoline. Hydrogen is therefore bulky and must be transported under pressure, making it difficult to fit enough hydrogen compactly into a vehicle to give it the range of a gasoline-burning conventional or hybrid car. However, this technical problem can be solved, according to some experts. Carbon-fiber tanks tested by the United States, Germany, and other governments for crash-worthiness can hold hydrogen atpressures of hundreds of atmospheres. Given the greater efficiency of fuel cells at extracting energy from fuel, compared to internal-combustion engines, the problem of hydrogen's bulkiness might not be insurmountable. This would be especially true if cars were designed to weigh significantly less while remaining safe in crashes, exploiting carbon-fiber and other materials: weighing less, they would use less fuel, driving for an acceptable distance with a smaller hydrogen tank than a heavier car.
- There is no infrastructure for hydrogen. The system of manufacturing facilities, trucks, tanks, dispensers, and other devices needed to deliver hydrogen to hundreds of millions of consumers does not yet exist, and creating it will cost many billions of dollars. Defenders of the hydrogen concept reply that a mature hydrogen industry already exists, producing tens of millions of tons of hydrogen a year, and that this industry would only have to increase in size by a factor of three or four to supply enough hydrogen for an all-hydrogen vehicle fleet. Critics of the hydrogen economy claim, in response, that a much larger expansion of the hydrogen industry would be needed.
- If a global hydrogen economy leaked enough hydrogen, it could endanger the ozone layer. In 2003, Tracey K. Tromp and colleagues argued in the journal Science that a global hydrogen economy would leak 10-20% of its hydrogen. Energy expert Amory Lovins, who favors a rapid transition to hydrogen, answered that leakage from the existing hydrogen industry is less than a tenth of this amount, and that there is no reason to assume that so much hydrogen would be lost even from a more extensive hydrogen economy. Lovins also argued that hydrogen's true impact on the environment must be considered as its net effect, that is, the difference between its benefits and its harms. Any harms from hydrogen have to be weighed against the reduced harm from replacing fossil fuel.
Several government commitments to an eventual hydrogen economy have been made. In January 2003, U.S. President George W. Bush announced a $1.3 billion federal Hydrogen Fuel Initiative (about $250 million a year over five years) to reduce U.S. dependence on foreign oil and improve air quality. In June 2003, the Bush administration also announced a partnership with the European Union directed toward the development of a hydrogen economy. In Iceland, which has large hydroelectric geothermal energy resources, the government has committed to an all-hydrogen transport economy by 2050. (Ninety-five percent of Iceland's heat and electricity already come from hydroelectric and geothermal energy.) The world's first hydrogen fueling station, used to fuel city buses running on hydrogen, opened in Iceland's capital city of Reykjavik in 2003.
A rapid change to a hydrogen economy seems unlikely, if only because the amount of technology that would have to be replaced is very large. Technical obstacles are either insurmountable, about to be surmounted, or already surmounted, depending on which expert is talking and which particular obstacle is being described. So far, government and corporate efforts to achieve hydrogen technologies are actually quite small (except in Iceland) compared to the size of the changes that will have to be made if the hydrogen dream is ever to be realized.
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