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Hydrogen (Latin: 'hydrogenium', from Ancient Greek: hydro: "water" and genes: "forming") is a chemical element in the periodic table that has the symbol H and atomic number 1. A hydrogen economy is a vision of the future economy in which energy, for mobile applications (vehicles, aircraft) and electrical grid load balancing (daily peak demand reserve), is stored as hydrogen (H2). A hydrogen economy is a future economy in which the primary form of stored energy for mobile applications and load balancing is hydrogen (H2). In particular hydrogen is proposed as a fuel to replace the gasoline and diesel fuels currently used in automobiles.

Contents

Introduction

Meanings of Hydrogen

The word "hydrogen" has several different meanings that are important to distinguish. Possible uses:

  • Hydrogen is the name of an element.
  • Hydrogen is an atom, sometimes called "H dot" that is abundant in space but essentially absent on earth, because it dimerizes.
  • Hydrogen is a diatomic molecule that occurs naturally in trace amounts in the Earth's atmosphere; chemists increasingly refer to H2 as dihydrogen[2] to distinguish this molecule from atomic hydrogen and hydrogen found in other compounds.
  • Hydrogen is atomic constituent within all organic compounds, water, and many other chemical compounds.

It is especially important not to confuse elemental forms of hydrogen with hydrogen as it appears in chemical compounds.

At standard temperature and pressure it is a colorless, odorless, nonmetallic, univalent, tasteless, highly flammable diatomic gas (H2). With an atomic mass of just 1.00794 g/mol, hydrogen is the lightest element. It is also the most abundant, constituting roughly 75% of the universe's elemental matter. Stars in their main sequence are overwhelmingly composed of hydrogen in its plasma state. Elemental hydrogen is industrially produced from hydrocarbons and is currently used primarily in fossil fuel upgrading but has a variety of other applications in both the energy and other sectors of the world's economy.

The most naturally common isotope of hydrogen contains one electron and an atomic nucleus of one proton. In ionic compounds it can take on either a positive charge (becoming a cation, formally a bare proton) or a negative charge (becoming an anion known as a hydride). Hydrogen can form compounds with most other elements and is present in water and all organic compounds. It plays a particularly important role in acid-base chemistry, in which many reactions involve the exchange of protons between soluble molecules. As the only element for which the Schrödinger equation can be solved analytically, study of the energetics and bonding of the hydrogen atom has played a key historical and theoretical role in the development of quantum mechanics.

Large quantities of H2 are needed in the petroleum and chemical industries. By far the largest application of H2 is for the processing ("upgrading") of fossil fuels. The key consumers of H2 in the petrochemical plant include hydrodealkylation, hydrodesulfurization, and hydrocracking. H2 has several other important uses.

  • used in the hydrogenation of fats and oils (found in items such as margarine), and in the production of methanol.
  • H2 is used in the manufacture of hydrochloric acid
  • H2 is used in certain welding methods
  • H2 is used in the reduction of metallic ores.
  • H2 is an ingredient in some rocket fuels.
  • H2 is used as the rotor coolant in electrical generators at power stations, because it has the highest thermal conductivity of any gas.
  • Liquid H2 is used in cryogenic research, including superconductivity studies.
  • The triple point temperature of equilibrium hydrogen is a defining fixed point on the ITS-90 temperature scale.
  • Since H2 is lighter than air, having a little more than 1/15th of the density of air, it was once widely used as a lifting agent in balloons and airships. However, this use was curtailed after the Hindenburg disaster convinced the public that the gas was too dangerous for this purpose.
  • Deuterium, an isotope of hydrogen (hydrogen-2), is used in nuclear fission applications as a moderator to slow neutrons, and in nuclear fusion reactions. Deuterium compounds have applications in chemistry and biology in studies of reaction isotope effects.
  • Tritium (hydrogen-3), produced in nuclear reactors, is used in the production of hydrogen bombs, as an isotopic label in the biosciences, and as a radiation source in luminous paints.

Hydrogen economy

History of Hydrogen

Hydrogen gas, H2, was first artificially produced and formally described by Theophrastus Bombastus von Hohenheim (1493–1541)—also known as Paracelsus— via the mixing of metals with strong acids. He was unaware that the flammable gas produced by this chemical reaction was a new chemical element. In 1671, Robert Boyle rediscovered and described the reaction between iron filings and dilute acids, which results in the production of hydrogen gas.

In 1766, Henry Cavendish was the first to recognize hydrogen gas as a discrete substance, by identifying the gas from a metal-acid reaction as "flammable," and further finding that the gas produces water when burned in air. Cavendish had stumbled on hydrogen when experimenting with acids and mercury. Although he wrongly assumed that hydrogen was a liberated component of the mercury rather than the acid, he was still able to accurately describe several key properties of hydrogen, including the fact that it produced water when burned. In 1783 Antoine Lavoisier gave the element its name and (with Laplace) reported that pure water is produced by burning hydrogen and oxygen. This was essentially a confirmation of Cavendish's finding (and also some earlier work by Joseph Priestley), but it was Lavoisier's name for the gas that won out.

One of the first uses of H2 was for balloons. The H2 was obtained by reacting sulfuric acid and metallic iron. Because of its relatively simple atomic structure, consisting only of a proton and an electron, the hydrogen atom, together with the spectrum of light produced from it or absorbed by it, has been central to the development of the theory of atomic structure and quantum theory. Furthermore, the corresponding simplicity of the hydrogen molecule and the corresponding cation H2+ allowed fuller understanding of the nature of the chemical bond, which followed shortly after the quantum mechanical treatment of the hydrogen atom had been developed in the mid-1920s.

Interestingly, one of the first quantum effects to be explicitly noticed (but not understood at the time) was Maxwell's observation, half a century before full quantum mechanical theory arrived, that the specific heat capacity of H2 unaccountably resembles that of a monatomic gas below room temperature. According to quantum theory, this behavior arises from the spacing of the (quantized) rotational energy levels, which are particularly wide-spaced in H2 due to its low mass. These widely spaced levels inhibit equal partition of heat energy into rotational motion in hydrogen at low temperatures. Diatomic gasses composed of heavier atoms do not have such widely spaced levels and do not exhibit the same effect.

Hydrogen, or more specifically H2, is widely discussed in the context of energy. Hydrogen is not an energy source, since it is not an abundant natural resource and more energy is used to produce it than can be ultimately extracted from it. However, it could become useful as a carrier of energy, as elucidated in the United States Department of Energy's 2003 report, “Among the various alternative energy strategies, building an energy infrastructure that uses hydrogen — the third most abundant element on the earth’s surface — as the primary carrier that connects a host of energy sources to diverse end uses may enable a secure and clean energy future for the Nation." One theoretical advantage of using H2 as a carrier, is the localization and concentration of environmentally unwelcome aspects of hydrogen manufacture. For example, CO2 sequestration could be conducted at the point of H2 production. Hydrogen fuel cells could be powering our cars in the not to distant future. A hydrogen economy is desired in order to solve the problems of energy supply and the ill effects of using hydrocarbon fuels. Petroleum, which accounts for most of the hydrocarbons imported by industrialized countries, is refined into gasoline and diesel fuels to be used in automobiles and aircraft. Natural gas, another hydrocarbon fuel, and coal are burned for the generation of electricity. The burning of hydrocarbon fuels causes the emission of greenhouse gases and other pollutants. Furthermore, the remaining supply of hydrocarbon resources in the world is limited, and the demand for hydrocarbon fuels is increasing, particularly in China, India and other developing countries.

In a hydrogen economy, hydrogen fuel would be manufactured from primary energy sources and feedstocks, replacing gasoline. Grid load balancing of electricity is a major issue in energy supply. Currently, this is done by varying the output of generators. However, electricity is hard to store efficiently for future use. The most cost-efficient and widespread system for large-scale grid energy storage is pumped storage, that is, pumping water up to a dam reservoir and generating electricity on demand from that via hydropower. However such systems will not scale down to portable applications. Smaller storage alternatives such as capacitors have very low energy density. Batteries have low energy density and are slow to charge and discharge. Flywheel power storage can be more efficient than batteries with about the same size, but there are safety concerns due to explosive shattering.

Because of the large quantity of energy released, per gallon, during its combustion, (i.e., its high energy density), hydrocarbon fuel is utilized in automobiles and aircraft. Fears that sources of hydrocarbon fuels will run out and concerns over global warming due to carbon dioxide (CO2) tailpipe emissions have given rise to a search for an alternative fuel to hydrocarbon fossils which does not have these problems. Some believe that fuel cells, using hydrogen as a fuel, will be able to replace most internal combustion engines and will be able to solve most grid load balancing needs in the future. Hydrogen is the most abundant element in the universe. It also has an excellent energy density by weight, which leads to it being used for spaceships like the space shuttle. Emissions of a hydrogen-oxygen fuel cell, in theory, consist of pure water. The fuel cell is also more efficient than an internal combustion engine. The internal combustion engine is said to be 20-30% efficient, while the fuel cell is 75-80% efficient (not accounting for losses in the actual production of hydrogen)and together with the electric motor and controller the drivetrain overall efficiency approaches 40% with low idling losses.

Hydrogen production is a large and growing industry. Globally, some 50 million metric tons of hydrogen, equal to about 170 million tons of oil equivalent, were produced in 2004. The growth rate is around 10% per year. Within the United States, 2004 production was about 11 million metric tons (MMT), an average power flow of 48 gigawatts. (For comparison, the average electric production in 2003 was some 442 gigawatts.) Because hydrogen storage and transport is expensive, most hydrogen is currently produced locally, and used immediately, generally by the same company producing it. As of 2005, the economic value of all hydrogen produced worldwide is about $135 billion per year. There are two primary uses for hydrogen today. About half is used to produce ammonia (NH3) via the Haber process, which is then used directly or indirectly as fertilizer. Because the world population and the intensive agriculture used to support it are both growing, ammonia demand is growing. The other half of current hydrogen production is used to convert heavy petroleum sources into lighter fractions suitable for use as fuels. This latter process is known as hydrocracking. Hydrocracking represents an even larger growth area, since rising oil prices encourage oil companies to extract poorer source material, such as tar sands and oil shale.

If energy for hydrogen production were available (from wind, solar or nuclear power), use of the substance for hydrocarbon fuel production has the potential for expansion by a factor of 5 to 10. Present U.S. use of hydrogen for hydrocracking is roughly 4 MMT/year. It is estimated that 37.7 MMT/year of hydrogen would be sufficient to convert enough domestic coal to liquid fuels to end U.S. dependence on foreign oil importation, and less than half this figure to end dependence on Middle East oil. Coal liquification would present exactly the same greenhouse problems as burning foreign oil, but it would eliminate the political and economic vulnerabilities inherent in oil importatation. Hydrogen production is a large and growing industry. Currently, hydrogen production is 48% from natural gas, 30% from oil, and 18% from coal; water electrolysis accounts for only 4%. The large market and sharply rising prices have also stimulated great interest in alternate, cheaper means of hydrogen production. For an analysis of possible future hydrogen markets. Globally, about 50 million metric tons of hydrogen were produced in 2004; the growth rate is about 10% per year. The energy in the current flow corresponds to about 200 gigawatts. Within the U.S., production was about 11 million metric tons, or 48 GW (10.8% of the average U.S. total electric production of 442 GW in 2003). Because hydrogen storage and transport are so expensive, most hydrogen is currently produced locally, and used immediately, generally by the same company producing it. As of 2005, the economic value of all hydrogen produced is about $135 billion per year. 48% of current hydrogen production is from natural gas, 30% is from oil, 18% is from coal, and electrolysis accounts for about 4%.

There are two primary uses for hydrogen today. About half is used to produce ammonia (NH3) via the Haber process, which is then primarily used directly or indirectly as fertilizer. The other half of current hydrogen production is used to convert heavy petroleum sources into lighter fractions suitable for use as fuels. This latter process is known as hydrocracking. Because the world population and the intensive agriculture used to support it are both growing, ammonia demand is growing. Hydrocracking represents an even larger growth area, as rising oil prices encourage oil companies to extract poorer source material, such as tar sands and oil shale.

The short-term future

The large market and sharply rising prices have also stimulated great interest in alternate, cheaper means of hydrogen production. One particular method that has gained considerable commercial interest and U.S. government funding is high-temperature thermochemical electrolysis of water (H2O). Some prototype nuclear reactors operate at 850 to 1000 degrees Celsius, considerably hotter than existing commercial plants. Thermochemical electrolysis of water at these temperatures converts more of the initial heat energy into chemical energy (hydrogen), potentially doubling efficiency, to about 50%. Such electrolysis has been demonstrated in a laboratory, but not at a commercial scale.

The potential savings, just for the existing hydrogen market, could be substantial. General Atomics predicts that hydrogen produced in a High Temperature Gas Cooled Reactor (HTGR) would cost $1.53/kg. In 2003, steam reforming of natural gas yielded hydrogen at $1.40/kg, making the new scheme unattractive. At 2005 gas prices, hydrogen cost $2.70/kg, so a savings of tens of billions of dollars per year is possible with the nuclear-powered supply. Much of this savings would translate into reduced oil and natural gas imports.

One side benefit of a nuclear reactor that produces both electricity and hydrogen is that it can shift production between the two. For instance, the plant might produce electricity during the day and hydrogen at night, matching its electrical generation profile to the daily variation in demand. If the hydrogen can be produced economically, this scheme would compete favorably with existing grid energy storage schemes. What is more, there is sufficient hydrogen demand in the United States that all daily peak generation could be handled by such plants.

Rationale

Electricity has revolutionized the quality of human life since the late 19th century by enabling easier use of available energy sources. Inventions such as the dynamo and electric lighting sparked its growth on direct current. Later the alternator and alternating current enabled electric power transmission over long distances in a grand scale. Currently, grid load balancing is done by varying the output of generators. However, electricity is hard to store efficiently for future use. The most cost-efficient and widespread system for large-scale grid energy storage is pumped storage, which consists of pumping water up to a dam reservoir and generating electricity on demand from that via hydropower. However such systems will not scale down to portable applications. Smaller storage alternatives such as capacitors have very low energy density. Batteries have low energy density and are slow to charge and discharge.

Around the time electricity started to come in use, another portable energy source was born. With internal combustion engines burning hydrocarbon fuels automobiles came into use. Internal combustion engines beat the competition at the time, such as compressed air, or electric automobiles powered by batteries, because they provided better range, by virtue of the efficiency of the internal combustion engine and high energy density of the hydrocarbon fuel. The high power-to-weight ratio of internal combustion engines also made it possible to build aircraft that have a higher density than air. Present concerns regarding the long term availability of hydrocarbon fuels and global warming due to carbon dioxide (CO2) tailpipe emissions have given rise to a search for an alternative to hydrocarbon fossil fuels which does not have these problems. Some think that fuel cells, using hydrogen as a fuel, are today's equivalent to the internal combustion engines of old. Hydrogen is the most abundant element in the universe. It also has an excellent energy density by weight, which leads to it being used for spaceships like the space shuttle. Emissions of a hydrogen-oxygen fuel cell, in theory, consist of pure water. The fuel cell is also more efficient than an internal combustion engine. High efficiency generators or fuel cells that run on hydrogen could replace electrical distribution systems. Similar systems are currently used with natural gas to produce electricity.

A system that produced hydrogen from other energy sources would centralize carbon emissions at the production site. This could be an advantage in that the emission control system may be better maintained and easier to inspect than systems on automobiles owned by individuals. Unfortunately, pure hydrogen is not widely available on our planet. Most of it is locked in water or hydrocarbon fuels. Pollution reduction at the production site may be offset by energy losses when converting to hydrogen. This is called the production problem. Hydrogen also has a poor energy density per volume. This means you need a large tank to store it. The large tank reduces the fuel efficiency of the vehicle. Because it is a small energetic molecule, hydrogen tends to diffuse through any liner material intended to contain it leading to the embrittlement, or weakening of its container. This is called the storage problem. Fuel cells are still expensive. Some require expensive platinum group metals. Many have a low service life. They also used to be pretty bulky, but this is improving. Some think improved knowledge of nanotechnology and mass production will eventually solve this problem.

Fuel cells

See also: Fuel cell

The underlying premise of a hydrogen economy is that fuel cells will replace internal combustion engines and turbines as the primary way to convert chemical power into motive and electrical power. The reason to expect this changeover is that fuel cells, being electrochemical, can be more efficient than heat engines. Currently, fuel cells are very expensive, but there is active research to bring down fuel cell prices. Some types of fuel cells work with hydrocarbon fuels while all can be operated on pure hydrogen. If and when fuel cells become cost-competitive with internal combustion engines and turbines, one of the first adopters will be large gas-fired power plants. These are currently being built in large numbers by a highly competitive industry, their owners can work with operational constraints (tight temperature ranges, low shock, slow power ramps, etc), power to weight is not an issue, and even small efficiency gains are worth quite a lot. If reforming natural gas into hydrogen and then using that hydrogen in a fuel cell is somehow more efficient than burning the natural gas, gas-fired powerplants will do that instead.

Much of the popular interest in hydrogen seems to attach to the idea of using fuel cells in automobiles. The cells can have a good power-to-weight ratio, are more efficient than internal combustion engines, and produce no damaging emissions. If safe hydrogen storage can be found, and cheap fuel cells can be manufactured, they may be economically viable in an advanced hybrid automobile (hybrid in the sense of fuel-cell/battery combination). Hydrogen fuel cells have the benefit of zero-point of use emissions (though the production of hydrogen can produce upstream emissions), low noise, and high efficiency. Many well-to-wheels studies show that a hydrogen fuel cell vehicle will be about twice as efficient as a similarly sized conventional gasoline vehicle, and perhaps 50% more efficient than a hybrid vehicle (Wang, 2002).

An accounting of the energy utilized during a thermodynamic process, known as an energy balance, can be applied to automotive fuels. With today's technology, the manufacture of hydrogen via steam reforming can be accomplished with a thermal efficiency of 75 to 80 percent. Additional energy will be required to liquefy or compress the hydrogen, and to transport it to the filling station via truck or pipeline. The energy that must be utilized per kilogram to produce, transport and deliver hyrogen (i.e., is well-to-tank energy use) is approximately 50 megajoules. Subtracting this energy from the enthalpy of one kilogram of hydrogen, which is 141 megajoules, and dividing by the enthalpy, yields a thermal energy efficiency of roughly sixty percent (Kreith, 2004). Gasoline, by comparison, requires less energy input, per gallon, at the refinery, and comparatively little energy is required to transport it and store it owing to its high energy density per gallon at ambient temperatures. Well-to-tank, the supply chain for gasoline is roughly 80 percent efficient (Wang, 2002).

Another pathway proposed for hydrogen production, that of distributed electrolysis, would take advantage of existing infrastructure to transport electricity to small, on-site electrolyzers located at filling stations. Hydrogen can be produced through electrolysis of water, which is roughly 70 percent efficient (using the lower heating value for hydrogen). However, accounting for the energy used to produce the electricity (i.e., enlarging the system boundary) and accounting as well for transmission losses will reduce this efficiency. Natural gas combined cycle power plants, which account for almost all builds of new electricity plants in the United States, generate electricity at efficiencies of 60 percent or greater. Increased demand for electricity, whether due to hydrogen cars or other demand, would have the marginal impact of adding new combined cycle power plants.

On this basis, distributed production of hydrogen would be roughly 40 percent efficient. However, if the marginal impact is referred to today's power grid, with an efficiency of roughly 40 percent owing to its mix of fuels and conversion methods, the efficiency of distributed hydrogen production would be roughly 25 percent. (Note that, analogous to hydrogen production from a fossil fuel, gasoline must be refined from crude oil, the "primary energy resource" (Nakicenovic, 1998).) The distributed production of hydrogen in this fashion will generate air emissions of pollutants and carbon dioxide at various points in the supply chain, e.g., electrolysis, transportation and storage. Such externalities as pollution must be weighed against the potential advantages of a hydrogen economy. Other fuel cell technologies based on the exchange of metal ions (i.e. zinc-air fuel cells) are typically more efficient at energy conversion than hydrogen fuel cells, but the widespread use of any electrical energy -> chemical energy->electrical energy systems would necessitate the production of electricity. In summary, the so-called production problem is seen to be a combination of two different problems: one of producing hydrogen efficiently from energy sources, and the other of locating suitable (renewable or at least less polluting) energy sources to do it.

Production

See also: Hydrogen production

The production and distribution of hydrogen for the purpose of transportation is being tested in limited markets around the world, particularly in Iceland, Germany, California, Japan and Canada. There are several processes which can yield hydrogen via water splitting using various energy sources at different efficiencies and costs. As of 2005, 48% of hydrogen production (for industrial processes) is from natural gas, 30% is from oil, 18% is from coal, and 4% is from electrolysis.

Molecular hydrogen is not available in convenient reservoirs, although hydrogen exists in large quantities as an atmospheric trace gas and is produced by microbes and consumed by methanogens in a rapid biological hydrogen cycle. Most hydrogen on earth is locked in water. Hydrogen can be produced using fossil fuels via steam reforming or partial oxidation of natural gas and by coal gasification. Produced in this fashion, hydrogen will generate less CO2 than conventional internal combustion engines (including the emissions during fuel production, delivery, and use in the vehicle) and thus contribute less to global warming (Wang, 2002; Kreith, 2004). It can also be produced via electrolysis using electricity and water, consuming approximately 50 kilowatt hours of electricity per kilogram. Nuclear power can provide the energy for hydrogen production by a variety of means[1], but has other disadvantages which may or may not be decisive. Solar power has also been considered, but is location-dependant.

The actual environmental impacts associated with hydrogen production can be compared with alternatives, taking into account not only the emissions and efficiency of the hydrogen production process but also the efficiency of the hydrogen conversion to electricity in a fuel cell. Moreover, most 'green' sources produce rather low-intensity energy (which can be scaled up, albeit at a slight efficiency cost), not the prodigious amounts of energy required for extracting significant amounts of hydrogen, like high-temperature electrolysis (could use solar concentrators for heat).

There is concern about the energy-consuming process of manufacturing the hydrogen. Manufacturing hydrogen requires a hydrogen carrier such as a fossil fuel or water. The former consumes the fossil resource and produces carbon dioxide, while electrolyzing water requires electricity, which is mostly generated at present using conventional fuels (fossil fuel or nuclear power). While alternative energy sources like wind and solar power could also be used, they are still more expensive given current prices of fossil fuels and nuclear energy. In this regard, hydrogen fuel itself cannot be called truly independent of fossil fuels (or completely non-polluting), unless a totally nuclear or renewable energy option were considered.

Fossil fuels

Steam reforming

Commercial bulk hydrogen is usually produced by the steam reforming of natural gas. At high temperatures (700–1100 °C), steam (H2O) reacts with methane (CH4) to yield syngas.

CH4 + H2OCO + 3 H2
Carbon monoxide

Additional hydrogen can be recovered from the carbon monoxide (CO) through the lower-temperature water gas shift reaction, performed at about 130 °C:

CO + H2O → CO2 + H2

Essentially, the oxygen (O) atom is stripped from the water (steam) to oxidize the carbon (C), liberating the hydrogen formerly bound to the carbon and oxygen. The byproduct carbon dioxide (CO2), which is a greenhouse gas, is usually released into the atmosphere, but there is some research into interning it underground or undersea.

Coal

Coal can be converted into syngas and methane, also known as town gas, via coal gasification. [edit] Electrolysis

Electrolysis

Electrolysis is an alternative to using fossil fuels directly to create hydrogen. When the energy supply is chemical, it will always be more efficient to produce hydrogen through a direct chemical path. But when the energy supply is mechanical (hydropower or wind turbines), hydrogen can be made via electrolysis of water. Usually, the electricity consumed is more valuable than the hydrogen produced, which is why only a tiny fraction of hydrogen is currently produced this way. The only requirements are electricity and water, and electrolysis produces just hydrogen and oxygen. However, electricity is much more expensive per unit of energy than methane, and hence the process is uneconomic for large scale production.

Research into high-temperature electrolysis may eventually lead to a viable process that is cost-competitive with natural gas steam reforming. In the high temperature electrolysis process, some of the energy is supplied in the form of heat, which is cheaper than electricity, and can be cheaper than natural gas. If a large enough fraction of the input energy is supplied in the form of heat, and if it is cheap enough, high-temperature electrolysis could be cheaper than steam reforming of natural gas. An example of a CO2 emission-free system, possible with near-term technology, would be if renewable energy sources such as concentrated solar thermal power collectors and wind turbines were used to produce hydrogen from water, using high-temperature electrolysis.

High-temperature electrolysis (HTE)

When the energy supply is in the form of heat (solar thermal or nuclear), the only existing path to hydrogen is currently through high-temperature electrolysis. In contrast with low-temperature electrolysis, high-temperature electrolysis (HTE) of water converts more of the initial heat energy into chemical energy (hydrogen), potentially doubling efficiency, to about 50%. Because some of the energy in HTE is supplied in the form of heat, less of the energy must be converted twice (from heat to electricity, and then to chemical form), and so less energy is lost. HTE has been demonstrated in a laboratory, but not at a commercial scale. HTE processes are generally only considered in combination with a nuclear heat source, because the other non-chemical form of high-temperature heat (concentrating solar thermal) is not consistent enough to bring down the capital costs of the HTE equipment. Research into HTE and high-temperature nuclear reactors may eventually lead to a hydrogen supply that is cost-competitive with natural gas steam reforming.

Some prototype Generation IV reactors have coolant exit temperatures of 850 to 1000 degrees Celsius, considerably hotter than existing commercial nuclear power plants. General Atomics predicts that hydrogen produced in a High Temperature Gas Cooled Reactor (HTGR) would cost $1.53/kg. In 2003, steam reforming of natural gas yielded hydrogen at $1.40/kg. At 2005 gas prices, hydrogen cost $2.70/kg. Hence, just within the United States, a savings of tens of billions of dollars per year is possible with a nuclear-powered supply. Much of this savings would translate into reduced oil and natural gas imports.

One side benefit of a nuclear reactor that produces both electricity and hydrogen is that it can shift production between the two. For instance, the plant might produce electricity during the day and hydrogen at night, matching its electrical generation profile to the daily variation in demand. If the hydrogen can be produced economically, this scheme would compete favorably with existing grid energy storage schemes. What is more, there is sufficient hydrogen demand in the United States that all daily peak generation could be handled by such plants.

Thermochemical production

Some thermochemical processes, such as the sulfur-iodine cycle, can produce hydrogen and oxygen from water and heat without using electricity. Since all the input energy for such processes is heat, they can be much more efficient than high-temperature electrolysis. Many sources of high-temperature heat have been proposed. The most promising is a high temperature nuclear reactor. Concentrating solar collectors might also be used. Coal is not generally considered, because the syngas route is already reasonably efficient. Thermochemical production of hydrogen using chemical energy from coal or natural gas is generally not considered, because the direct chemical path is more efficient. None of the thermochemical hydrogen production processes have been demonstrated at production levels, although several have been demonstrated in laboratories.

Other methods

Nanotechnology research on photosynthesis may lead to more efficient direct solar production of hydrogen, or perhaps carbon dioxide neutral synthetic hydrocarbon fuels.

Storage

Storage is the main technological problem of a viable hydrogen economy. Some attention has been given to the role of hydrogen to provide grid energy storage for unpredictable energy sources, like wind power. The primary difficulty, with using hydrogen for grid energy storage, is that converting power to hydrogen and back is not cheap. An alternative to using this method is pumped storage. Water turbines and dam infrastructure are currently more economical than electrolysis plants, fuel cells, and hydrogen pipelines. Pumped storage is presently more efficient and cost-effective than hydrogen storage.

Hydrocarbons are stored extensively at the point of use, be it in the gasoline tanks of automobiles or propane tanks hung on the side of barbecue grills. Hydrogen, in comparison, is quite expensive to store or transport with current technology. Hydrogen gas has good energy density per weight, but poor energy density per volume versus hydrocarbons, hence it requires a larger tank to store. A large hydrogen tank will be heavier than the small hydrocarbon tank used to store the same amount of energy, all other factors remaining equal. Increasing gas pressure would improve the energy density per volume, making for smaller, but not lighter container tanks (see pressure vessel). Compressing a gas will require energy to power the compressor. Higher compression will mean more energy lost to the compression step. Alternatively, higher volumetric energy density liquid hydrogen may be used (like the Space Shuttle). However liquid hydrogen is cryogenic and boils around 20.268 K (–252.882 °C or -423.188 °F). Hence its liquefaction imposes a large energy loss, used to cool it down to that temperature. The tanks must also be well insulated to prevent boil off. Ice may form around the tank and help corrode it further if the insulation fails. Insulation for liquid hydrogen tanks is usually expensive and delicate. Assuming all of that is solvable, the density problem remains. Even liquid hydrogen has worse energy density per volume than hydrocarbon fuels such as gasoline by approximately a factor of four.

Ammonia storage

Ammonia (NH3) can be used to store hydrogen chemically and then release it in a catalytic reformer. Ammonia provides exceptionally high hydrogen storage densities as a liquid with mild pressurization and cryogenic constraints. It can also be stored as a liquid at room temperature and pressure when mixed with water. Ammonia is the second most commonly produced chemical in the world and a large infrastructure for making, transporting and distributing ammonia already exists. Ammonia can be reformed to produce hydrogen with no harmful waste, or can mix with existing fuels and burned efficiently. Pure ammonia burns poorly and is not a suitable fuel for most combustion engines. Ammonia is very energy expensive to make. Existing infrastructure would have to be greatly enlarged to handle replacing transportation energy needs. Ammonia is a toxic gas at normal temperature and pressure and has a potent odor.

Metal hydrides

There are proposals to use metal hydrides as the carrier for hydrogen instead of pure hydrogen. Hydrides can be coerced, in varying degrees of ease, into releasing and absorbing hydrogen. Some are easy to fuel liquids at ambient temperature and pressure, others are solids which could be turned into pellets. Proposed hydrides for use in a hydrogen economy include boron and lithium hydrides. These have good energy density per volume, although their energy density per weight is often worse than the leading hydrocarbon fuels.

Solid hydride storage is a leading contender for automotive storage. A hydride tank is about three times larger and four times heavier than a gasoline tank holding the same energy. For a standard car, that's about 45 US gallons (0.17 m³) of space and 600 pounds (270 kg) versus 15 US gallons (0.057 m³) and 150 pounds (70 kg). A standard gasoline tank weighs a few dozen pounds (tens of kilograms) and is made of steel costing less than a dollar a pound ($2.20/kg). Lithium, the primary constituent by weight of a hydride storage vessel, currently costs over $40 a pound ($90/kg). Any hydride will need to be recycled or recharged with hydrogen, either on board the automobile or at a recycling plant.

Often hydrides react by combusting rather violently upon exposure to moist air, and are quite toxic to humans in contact with the skin or eyes, hence cumbersome to handle (see borane, lithium aluminium hydride). This is why such fuels, despite being proposed and vigorously researched by the space launch industry, have never been used in any actual launch vehicle.

Few hydrides provide low reactivity (high safety) and high hydrogen storage densities (above 10% per weight). Leading candidates are sodium borohydride, lithium hydride and ammonia borane. Sodium borohydride and ammonia borane can be stored as a liquid when mixed with water, but must be stored at very high concentrations to produce desirable hydrogen densities, thus requiring complicated water recycling systems in a fuel cell. As a liquid, sodium borohydride provides the advantage of being able to react directly in a fuel cell, allowing the production of cheaper, more efficient and more powerful fuels cells that do not need platinum catalysts. Recycling sodium borohydride is energy expensive and would require recycling plants. More energy efficient means of recycling sodium borohydride are still experimental. Recycling ammonia borane by any means is still experimental.

Synthesized hydrocarbons

An alternative to hydrides is to use regular hydrocarbon fuels as the hydrogen carrier. Then a small hydrogen reformer would extract the hydrogen as needed by the fuel cell. The problem is reformers are slow and given the energy losses involved plus the extra cost of the fuel cell you were probably better off burning it in a cheap internal combustion engine to begin with. Direct methanol fuel cells do not require a reformer, but provide lower efficiencies and power densities compared to conventional fuel cells, although this could be counter balanced with the much better energy densities of ethanol and methanol over hydrogen. Alcohol fuel is a renewable resource. Solid-oxide fuel cells can run on light hydrocarbons such as propane and methane with out a reformer, or can run on higher hydrocarbons with only partial reforming, but the high temperature and slow startup time of these fuel cells makes then prohibitive for automobiles.

Other methods

More exotic hydrogen carriers based on nanotechnology have been proposed, such as carbon buckyballs and nanotubes, but these are still in the early research stage.

Transportation

Hydrogen seems unlikely to be the cheapest carrier of energy over long distances in the near future. Advances in electrolysis and fuel cell technology have not addressed the underlying cost problem yet. As of 2005, the cheapest method to move energy around the planet is in uranium by rail, but nuclear power has received negative responses. The next cheapest and currently most widely used is in the form of oil in a pipeline or supertanker, or coal by rail or bulk carrier vessel. Natural gas pipelines and liquified natural gas tankers are much more expensive in comparison, which explains why natural gas from Alaska's North Slope is currently reinjected into the ground rather than shipped to the lower 48 states where it would be worth a fortune. Electric power lines move energy at even higher cost than natural gas pipelines; therefore, power stations are generally located within 100 miles (160 km) of the loads they serve. Long-distance power lines are used to average out imbalances between local electrical supply and demand, by moving a small portion of the total electricity generated. For example, California burns an average of about 30 gigawatts of electricity, and has a north-south transmission capacity bottleneck (the 500 kV Path 15) of 5.4 gigawatts. Hydrogen pipelines are unfortunately more expensive (http://www.ef.org/documents/NDakotaWindPower.pdf) than even long-distance electric lines. Hydrogen is about three times bulkier in volume than natural gas for the same energy delivered, and hydrogen accelerates the cracking of steel (hydrogen embrittlement), which increases maintenance costs, leakage rates, and material costs. The difference in cost is likely to expand with newer technology: wires suspended in air can utilize higher voltage with only marginally increased material costs, but higher pressure pipes require proportionally more material.

Environmental concerns

Recently there have been some concerns over possible problems related to hydrogen gas leakage. One issue, that may present itself with widespread hydrogen usage, is permanent hydrogen loss. Molecular hydrogen is light enough to escape into space. With a continuous cycle of hydrogen being liberated and then combined with oxygen, some will leak from containment. If significant amounts escape, it has been hypothesized that this may eventually cause an abundance of oxygen and lack of water. However, it would take a lot of leakage to engender an appreciable and permanent loss-related effect. Another issue is that hydrogen gas (H2) may form water vapor as it reacts with oxygen and cool, or form free radicals (H) due to ultraviolet radiation, in the stratosphere. These free radicals can then act as a catalyst for ozone depletion. A large enough increase in stratospheric hydrogen from leaked H2 could exacerbate the depletion process. However, the issues associated with hydrogen leakage may not really be as much of a problem for various reasons. The amount of hydrogen which leaks today is much lower (by a factor of 10-100) than the estimated 10%-20% figure conjectured by some researchers; in Germany, for example, the leakage rate is only 0.1% (less than the natural gas leak rate of 0.7%). At most, such leakage would likely be no more than 1-2% even with widespread hydrogen use, using present technology. Furthermore, it would take at least 50 years for a mature hydrogen economy to develop, and new technology could likely reduce the leakage rate even more. Also, less NOx, a major smog contributor and ozone depletor, will be produced than today, if fuel cells are used in the future instead of internal combustion engines.

Consumption

The underlying premise of a hydrogen economy is that fuel cells will replace internal combustion engines and turbines as the primary way to convert chemical power into motive and electrical power. The reason to expect this changeover is that fuel cells, being electrochemical, can be more efficient than heat engines. Currently, fuel cells are very expensive, but there is active research to bring down fuel cell prices. Fuel cells work with hydrocarbon fuels as well as pure hydrogen. If and when fuel cells become cost-competitive with internal combustion engines and turbines, one of the first adopters will be large gas-fired powerplants. These are currently being built in large numbers by a highly competitive industry, their owners can work with operational constraints (tight temperature ranges, low shock, slow power ramps, etc), power to weight is not an issue, and even small efficiency gains are worth quite a lot. If reforming natural gas into hydrogen and then using that hydrogen in a fuel cell is somehow more efficient than burning the natural gas, gas-fired powerplants will do that instead. But there is no known "serious" discussion of fuel-cell powerplants.

Much of the popular interest in hydrogen seems to attach to the idea of using fuel cells in automobiles. The cells can have a good power-to-weight ratio, are more efficient than internal combustion engines, and produce no damaging emissions. If cheap fuel cells can be manufactured, they may be economically viable in an advanced hybrid automobile (hybrid in the sense of fuel-cell/battery combination). So long as methane is the primary source of hydrogen, it will make more sense to fill specialized car tanks with compressed methane and run the fuel cells directly off that. The resulting system uses the methane energy more efficiently, produces less total CO2, and requires less new infrastructure. A further advantage is that methane is much easier to transport and handle than hydrogen. Methane used for fuel cells cannot have traces of methanethiol or ethanethiol, which are smelly chemicals injected into natural gas distributions to help users find leaks. The sulfur component of the odorant will destroy the membranes of the fuel cell. Since the technology for running internal combustion engines directly from methane is well developed, low polluting, and leads to long engine life, it is more likely that compressed natural gas (CNG) will be used for transportation in this way rather than in fuel cells for the near future.

Examples

Several domestic US automobile manufactures have committed to develop vehicles using hydrogen. (They had previously committed to producing electric vehicles in California, a program now defunct at their behest.) Critics argue this "commitment" is merely a ploy to sidestep current calls for increased efficiency in gasoline and diesel fuel powered vehicles. Some hospitals have installed combined electrolyzer-storage-fuel cell units for local emergency power. These are advantageous for emergency use due to their low maintenance requirement and ease of location compared to internal combustion driven generators. The North Atlantic island country of Iceland has committed to becoming the world's first hydrogen economy by the year 2050. Iceland is in a unique position: at present, it imports all the petroleum products necessary to power its automobiles and fishing fleet. But Iceland has large geothermal and hydroelectric resources, so much so that the local price of electricity actually is lower than the price of the hydrocarbons that could be used to produce that electricity.

Iceland already converts its surplus electricity into exportable goods and hydrocarbon replacements. In 2002, it produced 2000 tons of hydrogen gas by electrolysis, primarily for the production of ammonia (NH3) for fertilizer. Ammonia is produced, transported, and used throughout the world, and 90% of the cost of ammonia is the cost of the energy to produce it. Iceland is also developing an aluminum-smelting industry - aluminum costs are primarily driven by the cost of the electricity to run the smelters. Either of these industries could effectively export all of Iceland's potential geothermal electricity. But neither directly replaces hydrocarbons. Plans call for Reykjavik's 80 busses to run on compressed hydrogen by 2005. Research on powering the nation's fishing fleet with hydrogen is underway. For practicality, Iceland may end up processing imported oil with hydrogen to extend it, rather than to replace it altogether.

A pilot project demonstrating a hydrogen economy is operational on the Norwegian island of Utsira. The installation combines wind power and hydrogen power. In periods when there is surplus wind energy, the excess power is used for generating hydrogen by electrolysis. The hydrogen is stored, and is available for power generation in periods where there is little wind. The Hydrogen Expedition is currently working on creating a hydrogen fuel cell-powered ship and using it to circumnavigate the globe, as a way to demonstrate the capability of hydrogen fuel cells.

External articles and references

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General
Books
  • Jeremy Rifkin (2002) The Hydrogen Economy, Penguin Putnam Inc. ISBN 1585421936
  • Joseph J. Romm (2004) The Hype about Hydrogen, Fact and Fiction in the Race to Save the Climate, Island Press. ISBN 155963703X
  • "Chart of the Nuclides". Fourteenth Edition. General Electric Company, 1989.
  • Ferreira-Aparicio, P, M. J. Benito, J. L. Sanz (2005). "New Trends in Reforming Technologies: from Hydrogen Industrial Plants to Multifuel Microreformers". Catalysis Reviews 47: 491-588.
  • Krebs, Robert E. (1998). The History and Use of Our Earth's Chemical Elements : A Reference Guide. Westport, Conn.: Greenwood Press. ISBN 0313301239.
  • Newton, David E. (1994). The Chemical Elements. New York, NY: Franklin Watts. ISBN 0531125017.
  • Rigden, John S. (2002). Hydrogen : The Essential Element. Cambridge, MA: Harvard University Press. ISBN 0531125017.
  • Romm, Joseph, J. (2004). The Hype about Hydrogen, Fact and Fiction in the Race to Save the Climate. Island Press. ISBN 155963703X. Author interview at Global Public Media.
  • Stwertka, Albert (2002). A Guide to the Elements. New York, NY: Oxford University Press. ISBN 0195150279.
  • Kubas, G. J., Metal Dihydrogen and σ-Bond Complexes, Kluwer Academic/Plenum Publishers: New York, 2001
  • Universal Industrial Gases, Inc. – Hydrogen (H2) Applications and Uses.
  • Tikhonov VI, Volkov AA. (2002). Separation of water into its ortho and para isomers. Science 296(5577):2363.
  • Webelements – Hydrogen historical information.
  • Oxtoby DW, Gillis HP, Nachtrieb NH. (2002). Principles of Modern Chemistry 5th ed. Thomson Brooks/Cole
  • Cammack, R.; Frey, M.; Robson, R. Hydrogen as a Fuel: Learning from Nature; Taylor & Francis: London, 2001
  • Kruse O, Rupprecht J, Bader KP, Thomas-Hall S, Schenk PM, Finazzi G, Hankamer B. (2005). Improved photobiological H2 production in engineered green algal cells. J Biol Chem 280(40):34170-7.
  • United States Department of Energy FY2005 Progress Report. IV.E.6 Hydrogen from Water in a Novel Recombinant Oxygen-Tolerant Cyanobacteria System. HO Smith, Xu Q.

Directories

  • Hydrogen and You - H2 & You is a project of the Hydrogen Education Foundation, a 501c3 organization. The Foundation is managed by the National Hydrogen Association. This site was established to answer your questions about hydrogen as an energy source, both now and in the future.
  • Hydrogen - A visual directory of hydrogen fuel energy websites. (EnergyPlanet.info)

See also

HYDROGEN, GENERAL

HYDROGEN PRODUCTION AND STORAGE

HYDROGEN APPLICATIONS

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