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A fuel cell is an electrochemical device similar to a battery, but differing from the latter in that it is designed for continuous replenishment of the reactants consumed; i.e. it produces electricity from an external fuel supply as opposed to the limited internal energy storage capacity of a battery.
Typical reactants used in a fuel cell are hydrogen on the anode side and oxygen on the cathode side (a hydrogen cell). In contrast, conventional batteries consume solid reactants and, once these reactants are depleted, must be discarded, recharged with electricity by running the chemical reaction backwards, or, at least in theory, having their electrodes replaced. Typically in fuel cells, reactants flow in and reaction products flow out, and continuous long-term operation is feasible virtually as long as these flows are maintained.
Fuel cells are also attractive in some applications for their high efficiency and low pollution. (The only by-product of a hydrogen fuel cell is water vapor). Some applications that have been suggested include
- baseload utility power plants,
- emergency backup power,
- off-grid power storage,
- portable electronics,
- electrically-powered vehicles, and
- cellular phone power.
Types of Fuel Cells
There are a number of types of fuel cells:
- Proton-exchange fuel cell;
- Reversible Fuel Cell;
- Direct-methanol fuel cell;
- Direct Borohydride Fuel Cells
- Solid-oxide fuel cells;
- Molten-carbonate fuel cells;
- Phosphoric-acid fuel cells;
- Alkaline fuel cells.
Fuel cells are electrochemical devices, so they are not constrained by the maximum thermal (Carnot) efficiency as combustion engines are. Consequently, they can have very high efficiencies in converting chemical energy to electrical energy.
In the archetypal example of a hydrogen/oxygen proton-exchange membrane (or "polymer electrolyte") fuel cell (PEMFC), a proton-conducting polymer membrane separates the anode and cathode sides. Each side has an electrode, typically carbon paper coated with platinum catalyst.
On the anode side, hydrogen diffuses to the anode catalyst where it dissociates into protons and electrons. The protons are conducted through the membrane to the cathode, but the electrons are forced to travel in an external circuit (supplying power) because the membrane is electronically insulating.
On the cathode catalyst, oxygen molecules react with the electrons (which have travelled through the external circuit) and protons to form water.
Fuel cells cannot store energy like a battery, but in some applications, like stand-alone power plants based on discontinuous sources (solar, wind power), they are combined with electrolyzers and storage systems to form an energy storage system. The round-trip efficiency (electricity to hydrogen and back to electricity) of such plants is between 30 and 40%.
The principle of the fuel cell was discovered by Swiss scientist Christian Friedrich Schönbein in 1838 and published in the January 1839 edition of the "Philosophical Magazine" . Based on this work, the first practical fuel cell was developed by Welsh scientist Sir William Grove. A sketch was published in 1843. But fuel cells did not see practical application until the 1960s, where they were used in the U.S. space program to supply electricity and drinking water (hydrogen and oxygen being readily available from the spacecraft tanks). Extremely expensive materials were used and the fuel cells required very pure hydrogen and oxygen. Early fuel cells tended to require inconveniently high operating temperatures that were a problem in many applications. However, fuel cells were seen to be desirable due to the large amounts of fuel available (hydrogen & oxygen).
Further technological advances in the 1980s and 1990s, like the use of Nafion as the electrolyte, and reductions in the quantity of expensive platinum catalyst required, have made the prospect of fuel cells in consumer applications such as automobiles more realistic. (See Hydrogen car)
The fuel cell industry
Ballard Power Systems is a major manufacturer of fuel cells and claims to lead the world in automotive fuel cell technology. Ford Motor Company and DaimlerChrysler are major investors in Ballard. In 2003, most automobile companies were customers of Ballard, with only General Motors and Toyota pursuing internal development of fuel cells for automotive use; in 2004 Nissan and Honda started similar research programs.
Following the two-year 3-bus demonstration projects in Chicago and Vancouver (1998-2000), DaimlerChrysler fuel cell buses went into public use in nine cities across the European Union in 2004. The fuel cells in the buses were manufactured by Ballard Power Systems. The EU's CUTE (Clean Urban Transport for Europe) project is the largest of its type anywhere in the world. These buses reduce pollution and noise, and give a smooth vibration-free ride. London's trial, for example, co-financed by the European Commission Directorate-General for Energy and Transport, runs on Route 25 from Oxford Circus (in the centre of town) out to Ilford in the East End. The oil company BP is providing the hydrogen refuelling facilities in 5 of the 9 trial cities, including London. See the UK Government's Transport for London  for a description of the London trial.
Perth in Western Australia is also participating in the trial with three fuel cell powered buses now operating between Perth and the port city of Fremantle. The trial is to be extended to other Australian cities over the next three years.
United Technologies (UTX) was the first company to manufacturer and commercialize a large, stationary fuel cell system for use in co-generation power plants in hospitals and large office buildings, but has since discontinued this product due to the high costs of the phosphoric acid technology used. UTX's UTC Fuel Cells subsidiary  continues to be the sole supplier of fuel cells to NASA for use in space vehicles and is also developing fuel cells for cars and buses.
Pros and Cons of Fuel Cells in Various Applications
Environmental impacts of Hydrogen fuel cells
The use of fuel cells is controversial in some applications. The hydrogen typically used as a fuel is not a primary source of energy: it is only an energy carrier, and must be manufactured using energy from other sources. Some critics of the current stages of this technology argue that the energy needed to create the fuel in the first place may reduce the ultimate energy efficiency of the system to below that of the most efficient gasoline internal-combustion engines; this is especially true if the hydrogen has to be compressed to high pressures, as it does in automobile applications (the electrolysis of water is itself a fairly efficient process). It has to be remarked, though, that even the most efficient internal-combustion engines are not very efficient in absolute terms.
As an alternative to electrolysis, hydrogen can be generated from methane (the primary component of natural gas) with approximately 80% efficiency. The methane conversion method releases greenhouse gases, but, since the production is concentrated in one facility, and not distributed on every single vehicle or utility, it is possible to separate the gases and dispose of them properly, for example by injecting them in an oil or gas reservoir.
Fuel Cell design issues
To make fuel cells economically competitive, there are many practical problems to be overcome as well. Water management remains a key problem in Proton Exchange Membrane Fuel Cells (PEMFCs). Not enough water and the polymer loses its ability to conduct protons across the cell, too much and the electrodes will flood, stopping the reaction. Keeping the water level in balance can be a tricky process.
At the same time many other variables must be juggled, including temperature throughout the cell (which changes and can sometimes destroy a cell through thermal loading), reactant and product levels at various cells. Materials must be chosen to do various tasks which none fill completely. Durability and lifetime of the cells can be serious issues for some cells, low power densities for others. Putting all of these factors together hasn't been accomplished decisively yet, and remains the challenge.
Fuel Cell Applications
Fuel cells are very useful as power sources in remote locations, such as spacecraft, remote weather stations, and in certain military applications. A fuel cell system running on hydrogen can be compact, light weight and have relatively few moving parts.
A future application is combined heat and power for homes and office buildings. This is economically possible in areas where the cost of natural gas is much lower than that of electricity. This type of system would give nearly constant electric power (selling it to the grid when it is not consumed within the building) and at the same time produce hot water from the waste heat. The most likely technique to be used for this is SOFC, but prototypes also exist for PEM-based systems.
Because fuel cells have a high cost per kilowatt, and because their efficiency drops with increasing power density, they are usually not considered for applications with high load variations. In particular, they are not suited for energy storage systems, unless weight is a major consideration. An electrolyzer and fuel cell would return less than 50 percent of the input energy, while a much cheaper lead-acid battery might return about 90 percent.
The first hydrogen refuelling station was built in Reykjavík, Iceland on April 2003. The station has served since summer 2003 three buses built by DaimlerChrysler that are in service in the public transport net of Reykjavík. The station produces the hydrogen it needs itself with an electrolysing unit (produced by Norsk Hydro), and does not need filling. Shell is also a partner in the project. The station has no roof in order to allow any leaked hydrogen to escape to the atmosphere. Currently, a team of college students is planning to take a hydrogen fuel cell powered boat around the world. Their venture is called The Hydrogen Expedition.
- The Fuel Cell Way Comprehensive information about fuel cells and hydrogen in several languages.
- The Hydrogen Expedition, "The first circumnavigation of the globe in a hydrogen fuel cell powered boat"
- Category:Fuel cell vehicles
- Hybrid-Vehicle.org: Information on hybrid vehicle history, hybrid vehicle technology and practical application of hybrid technology in a broad spectrum of vehicles..
- 2004-02-16, ScienceDaily: New Reactor Puts Hydrogen From Renewable Fuels Within Reach Citat: "...The first reactor capable of producing hydrogen from a renewable fuel source - ethanol - efficiently enough to hold economic potential has been invented..."
- Fuel Cells Hydrogen Technology News and Events
- BBC News, 16 June, 2000: Fuel cells get even cooler
- PhysicsWorld: Fuel cells
- PhysicsWorld: Fuel cells: power for the future Quote: "... The high efficiency of fuel cells, which can range from 40% to 70%, and the possibility of improving the overall efficiency of such systems using combined heat and power (CHP) designs would significantly reduce emissions...."
- How Hydrogen Can Save America by Wired Magazine
- Fuel Cells in Laptops
- General Motors´ Fuel Cell
- Plug Power
- Fuel Cell Knowledge Presents an overview of fuel cells and related technologies, plus in depth analysis of solid oxide fuel cells.
- Fuel Cell Today All sorts of information on fuel cells and hydrogen
- How Stuff Works: Fuel Cells
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