Fuel cell

For other uses, see Fuel cell (disambiguation).

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.

An animated graph that shows the pricipal working of a fuel cell
An animated graph that shows the pricipal working of a fuel cell

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 byproduct 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, and
  • electrically-powered vehicles.
Contents

Types of fuel cells

There are a number of types of fuel cells:

Science

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 cathode side, hydrogen diffuses to the cathode catalyst where it dissociates into protons and electrons. The protons are conducted through the membrane to the anode, but the electrons are forced to travel in an external circuit (supplying power) because the membrane is electronically insulating.

On the anode catalyst, oxygen molecules react with the electrons (which have travelled through the external circuit) and protons to form water.

In this example, the only waste product is water vapor.

Also, there is the possible use of fuel cells at home, to store energy at cheaper off-peak electricity rates and release it during peak-use hours. It may even be profitable to sell back some of the energy to the power grid, like it is done in wind turbines. Peak power production reaches twice the average level, which means that the very expensive power-plant capacity is sized for levels used for a short period of time. Also, power plants are most efficient at one nominal production rate, and their efficiency can drop off significantly at off-peak rates.

History

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" [1] (http://www.efcf.com/media/ep010813.shtml). 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.

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.

DaimlerChrysler fuel cell buses went into public use in nine cities across the European Union in 2004. The EU's CUTE (Clean Urban Transport for Europe) project is the largest of its type anywhere in the world. These busses 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 refulleing facilities in 5 of the 9 trial cities (including London). See the UK Government's Transfort for London [2] (http://www.tfl.gov.uk/buses/downloads/fuel-cell-buses.pdf) for a description of the London trial.

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 [3] (http://www.fossil.energy.gov/programs/powersystems/fuelcells/fuelscells_phosacid.html)this product due to the high costs of the phosphoric acid technology used. UTX's UTC Fuel Cells subsidiary [4] (http://www.utcfuelcells.com/fuelcells/index.shtm)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 is generated from electrolysis of water by electricity. 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.

Other types of fuel cells don't face these problems. For example, biological fuel cells take glucose and methanol from food scraps and convert it into hydrogen and food for the bacteria.

Fuel cell design issues

There are practical problems to be overcome as well. Although the use of fuel cells for consumer products is probable in the near future, most current designs will not work if oriented upside down. They currently cannot easily be scaled to the small size needed by portable devices such as cell phones. although some progress is being made by companies like Toshiba [5] (http://www.toshiba.co.jp/about/press/2004_06/pr2401.htm). Current designs require venting and therefore cannot operate under water. They may not be usable on aircraft because of the risk of fuel leaks through the vents. Technologies for safe refueling of consumer fuel cells are not yet in place.

Hydrogen fuel cell advantages

There are several advantages to hydrogen as well. Clean, renewable energy sources like solar and wind power are non-continuous and unreliable through time: the power from these sources is not always available at the time it is needed. The electricity produced from solar panels or wind generators could be stored in large battery complexes, but this would be expensive, and batteries have a limited storage capacity and lifetime, and are often made of highly polluting materials. If the electricity is used to produce hydrogen however, the energy can be stored more easily.

External links



da:Brćndselscelle de:Brennstoffzelle fr:Pile ŕ combustible nl:Brandstofcel [[ja:燃料電池]] pt:Célula combustível

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