A hydrogen economy is a hypothetical future economy in which the primary form of stored energy for mobile applications and load balancing is hydrogen. In particular it is discussed as a method for replacing the petroleum based hydrocarbon fuels currently used in automobiles.
Rationale
Electricity has revolutionized our lives 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 with less losses via power lines.
Currently, the load balancing is done by varying the output of generators. However, electricity is hard to store efficiently. 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. Such systems are obviously bulky, expensive and non-portable. Capacitors are expensive and have low energy density, and 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.
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 per weight, which leads to it being used for spaceships like the Space Shuttle. The tailpipe emissions of a hydrogen fuel cell powered automobile consist of water (H2O) and are carbon dioxide (CO2) free. The fuel cell also is 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 (H2) is not widely available on our planet. Most of it is locked in water (H2O) 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.
The production problem
Hydrogen production is a large industry. Globally, about 50 million metric tons of hydrogen are produced each year.
However current hydrogen production is too low to satisfy transportation requirements. About half of worldwide hydrogen production is used by the chemical industry to produce ammonia used for fertilizer, most of the remaining production is used for hydrocracking low grade hydrocarbon fuels into higher grade fuel. The low grade fuel may be conventional oil or non-conventional oil.
There are several processes which can yield hydrogen from several sources at different efficiencies and costs. 48% of current hydrogen production is from natural gas, 30% is from oil, 18% is from coal, electrolysis accounts for about 4%.
Commercial bulk hydrogen is usually produced by the steam reforming of natural gas. At high temperatures (700-1100 °C), steam reacts with methane to yield syngas.
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CH4 + H2O → CO + 3 H2
Additional hydrogen can be recovered from the carbon monoxide (CO) through the water-gas shift reaction:
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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 can be converted into syngas and methane, also known as town gas, via coal gasification.
Another alternative is electrolysis. This only requires electricity and water to generate hydrogen. However electrolysis is low efficiency and hence anti-economic 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. The only additional requirement, besides water, is a high temperature heat source. The heat source may be provided by a thermal power plant.
There has also been research in other high-efficiency thermochemical processes such as the sulfur-iodine process. This process only requires water and a high temperature heat source.
An example of an almost pollution free system, possible with near term technology, would be where concentrated solar thermal power is used to produce hydrogen from water, using either the sulfur-iodine process or high-temperature electrolysis, and then a hydrogen fuel cell is used to produce electricity for mobile applications. Nuclear power is carbon dioxide free, controversy aside, and could also provide as a heat source.
For the longer term nanotechnology research on photosynthesis may lead to more efficient direct solar production of hydrogen or carbon dioxide neutral synthetic hydrocarbon fuels.
The storage problem
Some attention has been given to the role of hydrogen to provide grid energy storage for unpredictable energy sources, like wind power. The most obvious competitor is pumped storage. The primary difficulty, with using hydrogen for grid energy storage, is that such use assumes that converting power to hydrogen and back is cheap, which it is not. Water turbines and electric wires are much cheaper than electrolysis plants, fuel cells, and hydrogen pipelines. Pumped storage is much cheaper and has much less energy conversion losses than hydrogen storage.
Although it is generally not referred to as load balancing, the varying rates of supply, refining, and consumption of hydrocarbons are balanced by storing liquid hydrocarbons, generally in the familiar tank farms around refineries. Such storage is practical because the economic value of the hydrocarbons stored is very large compared to the cost of the tank.
Natural gas is also stored in tanks, but these are much less common because natural gas is much more expensive to store, as it is a low energy density per volume gas which requires a large expensive container. Typically, if the gas would be stored for longer than a few months and a gas pipeline connection is available, it is cheaper to flare it off and buy more when needed. As a result, the primary form of storage for natural gas is within the distribution pipelines themselves.
Hydrocarbons are also 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 as in the Space Shuttle. However liquid hydrogen is cryogenic and boils around 20.268 K (–253 °C or -423 °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.
Another problem, 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 (possibly as much as 20%). If significant amounts escape, this may eventually cause an abundance of oxygen and lack of water. Hydrogen gas (H2) may also form free radicals (H) in the stratosphere due to ultraviolet radiation, that can then act as a catalyst for ozone depletion. An increase in stratospheric hydrogen from leaked H2 could exacerbate the depletion process.
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 several orders of magnitude.
There are proposals to use hydrides as the carrier for hydrogen instead of pure hydrogen. Hydrides can be coherced, 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.
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 gallons of space and 600 pounds versus 15 gallons and 150 pounds. A standard gasoline tank weighs a few dozen pounds and is made of steel costing less than a dollar a pound. Lithium, the primary constituent by weight of a hydride storage vessel, currently costs over $40 a pound.
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.
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.
More exotic hydrogen carriers have been proposed, like carbon buckyballs and nanotubes, but these are still in the early research stage.
Storage is the main technological problem of a viable hydrogen economy.
Transmission
By far the cheapest way to move energy around the planet is in the form of oil in a pipeline or supertanker, or coal on a barge or rail car. (Uranium in a high-security armored rail car is even better, but unpopular.) Natural gas pipelines (and LNG 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 cost so much for the energy moved that power stations are generally located within a hundred miles of the loads they serve, so that energy can be moved as coal, oil or gas rather than as electricity. 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 than even long-distance electric lines. They are more expensive not just because the electrolyzers and fuel cells cost so much, but because a pipeline carrying hydrogen is much more expensive than a wire carrying electricity. Hydrogen is about three times bulkier 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 only likely to expand with newer technology, as wires suspended in air can scale up voltage with marginal material costs, but high pressure pipes have material costs directly proportional to the amount of gas enclosed.
End use
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 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 had, they may make sense in an advanced hybrid automobile.
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.
See also
Further reading
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Jeremy Rifkin, The Hydrogen Economy, Penguin Putnam Inc, 2002, ISBN 1585421936
- Joseph J. Romm, The Hype about Hydrogen, Fact and Fiction in the Race to Save the Climate This book is very skeptical of the feasibility and economics of using either hydrogen or fuel cells for transportation.
External links
Last updated: 05-07-2005 11:49:18
Last updated: 05-13-2005 07:56:04
