Fuel cell
From Wikipedia, the free encyclopedia
- For other uses, see Fuel cell (disambiguation).
A fuel cell is an electrochemical energy conversion device that runs on hydrogen gas and air. Fuel cells differ from batteries in that they are designed for continuous replenishment of the reactants consumed; they produce electricity from an external supply of fuel and oxidant (typically oxygen or air, although chlorine and chlorine dioxide have also been used among others<ref>S. G. Meibuhr, Electrochim. Acta, 11, 1301 (1966)</ref>) as opposed to the limited internal energy storage capacity of a battery. Additionally, while the electrodes within a battery react and change as a battery is charged or discharged, a fuel cell's electrodes are catalytic and relatively stable.
Typical reactants used in a fuel cell are hydrogen on the anode side and oxygen on the cathode side (a hydrogen cell). Usually, reactants flow in and reaction products flow out. The electrolyte remains in the cell. Virtually continuous long-term operation is feasible as long as these flows are maintained.
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[edit] Technology
In the archetypal example of a hydrogen/oxygen proton exchange membrane fuel cell (PEMFC), a proton-conducting polymer membrane, (the electrolyte), separates the anode and cathode sides.
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 electrically insulating. On the cathode catalyst, oxygen molecules react with the electrons (which have traveled through the external circuit) and protons to form water. In this example, the only waste product is water vapor and/or liquid water.
In addition to pure hydrogen, there are hydrocarbon fuels for fuel cells, including diesel, methanol (direct-methanol fuel cells) and chemical hydrides. The waste products with these types of fuel are carbon dioxide and water.
The materials used in fuel cells differ by type. The electrode/bipolar plates are usually made of metal, nickel or carbon nanotubes, and are coated with a catalyst (like platinum, nano iron powders or palladium) for higher efficiency. Carbon paper separates them from the electrolyte. The electrolyte could be ceramic or a membrane.
A typical fuel cell produces about 0.86 volts. To create enough voltage, the cells are layered and combined in series and parallel circuits to form a fuel cell stack. The number of cells used is usually greater than 45 but varies with design.
[edit] Fuel cell design issues
- Costs. In 2002, typical cells had a catalyst content of USD $1000 per kW of electric power output. This is expected to be reduced to $30/kW by 2007.<ref>http://www.fuelcellcontrol.com/evs19.html</ref> Ballard Power Systems have experiments with a catalyst enhanced with carbon silk which allows a 30% reduction (1 mg/cm² to 0.7 mg/cm²) in platinum usage without reduction in performance.<ref>http://www.fuelcellsworks.com/Supppage2336.html</ref>
- The production costs of the PEM (proton exchange membrane). The Nafion® membrane currently costs €400/m². This, and the Toyota PEM and 3M PEM membrane can be replaced with the ITM Power membrane (a hydrocarbon polymer), resulting in a price of ~€4/m². One of the bigger companies is using Solupor® (a porous polyethylene film).<ref>http://www.ecn.nl/bct/solupor.en.html</ref>
- Water management (in PEMFCs). In this type of fuel cell, the membrane must be hydrated, requiring water to be evaporated at precisely the same rate that it is produced. If water is evaporated too quickly, the membrane dries, resistance across it increases, and eventually it will crack, creating a gas "short circuit" where hydrogen and oxygen combine directly, generating heat that will damage the fuel cell. If the water is evaporated too slowly, the electrodes will flood, preventing the reactants from reaching the catalyst and stopping the reaction. Methods to dispose of the excess water are being developed by fuel cell companies.
- Flow control. Just as in a combustion engine, a steady ratio between the reactant and oxygen is necessary to keep the fuel cell operating efficiently.
- Temperature management. The same temperature must be maintained throughout the cell in order to prevent destruction of the cell through thermal loading.
- Durability, service life, and special requirements for some type of cells. Stationary applications typically require more than 40,000 hours of reliable operation at a temperature of -35 °C to 40 °C, while automotive fuel cells require a 5,000 hour lifespan (the equivalent of 150,000 miles) under extreme temperatures. Automotive engines must also be able to start reliably at -30 °C and have a high power to volume ratio (typically 2.5 kW per liter).
- Limited carbon monoxide tolerance of the anode.
[edit] History
The principle of the fuel cell was discovered by German scientist Christian Friedrich Schönbein in 1838 and published in the January 1839 edition of the "Philosophical Magazine".<ref>http://www.efcf.com/media/ep010813.shtml</ref> Based on this work, the first fuel cell was developed by Welsh scientist Sir William Robert Grove in 1843. The fuel cell he made used similar materials to today's phosphoric-acid fuel cell. It wasn't until 1959 that British engineer Francis Thomas Bacon successfully developed a 5 kW stationary fuel cell. In 1959, a team led by Harry Ihrig built a 15 kW fuel cell tractor for Allis-Chalmers which was demonstrated across the US at state fairs. This system used potassium hydroxide as the electrolyte and compressed hydrogen and oxygen as the reactants. Later in 1959, Bacon and his colleagues demonstrated a practical five-kilowatt unit capable of powering a welding machine. In the 1960s, Pratt and Whitney licensed Bacon's U.S. patents for use in the U.S. space program to supply electricity and drinking water (hydrogen and oxygen being readily available from the spacecraft tanks).
UTC's Power subsidiary was the first company to manufacture and commercialize a large, stationary fuel cell system for use as a co-generation power plant in hospitals, universities and large office buildings. UTC Power continues to market this fuel cell as the PureCell 200, a 200 kW system.<ref>http://www.utcpower.com/fs/com/bin/fs_com_Page/0,5433,03100,00.html</ref> UTC Power continues to be the sole supplier of fuel cells to NASA for use in space vehicles, having supplied the Apollo missions and currently the Space Shuttle program, and is developing fuel cells for automobiles, buses, and cell phone towers; the company has demonstrated the first fuel cell capable of starting under freezing conditions with its proton exchange membrane automotive fuel cell.
In 2006 Staxon introduced an inexpensive OEM fuel cell module for system integration. In 2006 Angstrom Power, a British Columbia based company, began commercial sales of portable devices using proprietary hydrogen fuel cell technology, trademarked as "micro hydrogen."[citation needed]
[edit] Types of fuel cells
| Fuel Cell Name | Electrolyte | Qualified Power (W) | Working Temperature (°C) | Electrical efficiency | Status |
|---|---|---|---|---|---|
| Metal hydride fuel cell | Aqueous alkaline solution (e.g.potassium hydroxide) | ? | above -20 50%Ppeak @ 0 | ? | Commercial/Research |
| Electro-galvanic fuel cell | Aqueous alkaline solution (e.g., potassium hydroxide) | ? | under 40 | ? | Commercial/Research |
| Zinc-air battery | Aqueous alkaline solution (e.g., potassium hydroxide) | ? | under 40 | ? | Mass production |
| Microbial Fuel Cell | Polymer membrane or humic acid | ? | under 40 | ? | Research |
| Reversible fuel cell | Polymer membrane (ionomer) | ? | under 50 | ? | Commercial/Research |
| Direct borohydride fuel cell | Aqueous alkaline solution (e.g., sodium hydroxide) | ? | 70 | ? | Research |
| Alkaline fuel cell | Aqueous alkaline solution (e.g., potassium hydroxide) | 10 kW to 100 kW | under 80 | Cell: 60–70% System: 62% | Commercial/Research |
| Direct-methanol fuel cell | Polymer membrane (ionomer) | 1mW to 100 kW | 90–120 | Cell: 20–30% System: 10–20% | Commercial/Research |
| Reformed-methanol fuel cell | Polymer membrane (ionomer) | 5W to 100 kW | (Reformer)250–300 (PBI)125–200 | Cell: 50–60% System: 25–40% | Commercial/Research |
| Direct-ethanol fuel cell | Polymer membrane (ionomer) | up to 140 mW/cm² | above 25 ? 90–120 | ? | Research |
| Formic acid fuel cell | Polymer membrane (ionomer) | ? | 90–120 | ? | Research |
| Proton exchange membrane fuel cell | Polymer membrane (ionomer) (e.g., Nafion® or Polybenzimidazole fiber) | 100W to 500 kW | (Nafion)70–120 (PBI)125–200 | Cell: 50–70% System: 30–50% | Commercial/Research |
| RFC - Redox | Liquid electrolytes with redox shuttle & polymer membrane (Ionomer) | 1 kW to 10MW | ? | ? | Research |
| Phosphoric-acid fuel cell | Molten phosphoric acid (H3PO4) | up to 10MW | 150-200 | Cell: 55% System: 40% | Commercial/Research |
| Molten-carbonate fuel cell | Molten alkaline carbonate (e.g., sodium bicarbonate NaHCO3) | 100MW | 600-650 | Cell: 55% System: 47% | Commercial/Research |
| Proton-conducting ceramic fuel cell | H+-conducting ceramic oxide | ? | 700 | ? | Research |
| Direct carbon fuel cell | Several different | ? | 700-850 | Cell: 80% System: 70% | Commercial/Research |
| Solid-oxide fuel cell | O2--conducting ceramic oxide (e.g., zirconium dioxide, ZrO2) | up to 100MW | 700–1000 | Cell: 60–65% System: 55–60% | Commercial/Research |
[edit] Efficiency
Fuel cells are not constrained by the maximum Carnot cycle efficiency as combustion engines are, because they do not operate with a thermal cycle. At times, this is misrepresented when fuel cells are stated to be exempt from the laws of thermodynamics. Instead, it can be described that the "limitations imposed by the second law of thermodynamics on the operation of fuel cells are much less severe than the limitations imposed on conventional energy conversion systems"<ref>http://web.mit.edu/afs/athena.mit.edu/org/m/mecheng/fcp/about%20f%20cells.html</ref>. Consequently, they can have very high efficiencies in converting chemical energy to electrical energy, especially when they are operated at low power density, and using pure hydrogen and oxygen as reactants.
The efficiency of a fuel is very dependent on the current through the fuel cell: as a general rule, the more current drawn, the lower the efficiency. A cell running at 0.6V has an efficiency of about 50%, meaning that 50% of the available energy content of the hydrogen is converted into electrical energy; the remaining 50% will be converted into heat. For a hydrogen cell the second law efficiency is equal to cell voltage divided by 1.23, when operating at standard conditions. This voltage varies with fuel used, and quality and temperature of the cell. The difference between enthalpy and Gibbs free energy (that cannot be recovered) will also appear as heat.
For a fuel cell operated on air (rather than bottled oxygen), losses due to the air supply system must also be taken into account.
It is also important to take losses due to production, transportation, and storage into account. Fuel cell vehicles running on compressed hydrogen may have a power-plant-to-wheel efficiency of 22% if the hydrogen is stored as high-pressure gas, and 17% if it is stored as liquid hydrogen.<ref>http://www.efcf.com/reports/E04.pdf</ref>
Fuel cells cannot store energy like a battery, but in some applications, such as stand-alone power plants based on discontinuous sources such as solar or wind power, they are combined with electrolyzers and storage systems to form an energy storage system. The overall efficiency (electricity to hydrogen and back to electricity) of such plants (known as round-trip efficiency) is between 30 and 50%, depending on conditions.<ref>http://gltrs.grc.nasa.gov/reports/2006/TM-2006-214054.pdf</ref> While a much cheaper lead-acid battery might return about 90%, the electrolyser/fuel cell system can store indefinite quantities of hydrogen, and is therefore better suited for long-term storage.
[edit] Fuel cell applications
Fuel cells are very useful as power sources in remote locations, such as spacecraft, remote weather stations, large parks, rural locations, and in certain military applications. A fuel cell system running on hydrogen can be compact, lightweight and has no major moving parts.
Because fuel cells have no moving parts, and do not involve combustion, in ideal conditions they can achieve up to 99.9999% reliability<ref>http://www.fuelcells.org/basics/benefits.html</ref>. This equates to less than one minute of down time in a six year period.
A new application is combined heat and power (CHP) for family home, office buildings and factories. This type of system generates constant electric power (selling excess power back to the grid when it is not consumed), and at the same time produce hot air and water from the waste heat. A lower fuel-to-electricity conversion efficiency is tolerated (typically 15-20%), because most of the energy not converted into electricity is utilized as heat. Some heat is lost with the exhaust gas just as in a normal furnace, so the combined heat and power efficiency is still lower than 100%, typically around 80%. In terms of exergy however, the process is inefficient, and one could do better by maximizing the electricity generated and then using the electricity to drive a heat pump. Phosphoric-acid fuel cells (PAFC) comprise the largest segment of existing CHP products worldwide and can provide combined efficiencies close to 80% (45-50% electric + remainder as thermal). UTC Power is currently the world's largest manufacturer of PAFC fuel cells. Molten-carbonate fuel cells have also been installed in these applications, and Solid-oxide fuel cell prototypes exist.
However, since electrolyzer systems do not store fuel in themselves, but rather rely on external storage units, they can be successfully applied in large-scale energy storage, rural areas being one example. In this application, batteries would have to be largely oversized to meet the storage demand, but fuel cells only need a larger storage unit (typically cheaper than an electrochemical device).
One such pilot program is operating on Stuart Island in Washington State. There the Stuart Island Energy Initiative [1]has built a complete, closed-loop system: Solar panels power an electrolyzer which makes hydrogen. The hydrogen is stored in a 500 gallon tank at 200 PSI,and runs a ReliOn fuel cell to provide full electric back-up to the off-the-grid residence. The SIEI website gives extensive technical details.
Protium, a rock band originating at Ponaganset High School in Glocester, RI was the world's first "hydrogen fuel cell powered band". The band was powered by a 1 kW Airgen Fuelcell from Ballard Power systems. The band has played at a number of fuel cell advocacy events inluding the Connecticut CEP, and the 2003 Fuel Cell Seminar in Miami beach, FL.[citation needed]
[edit] Suggested applications
- Base load power plants
- Electric and hybrid vehicles.
- Auxiliary power
- Off-grid power supply
- Notebook computers for applications where AC charging may not be available for weeks at a time.
- Portable charging docks for small electronics (e.g. a belt clip that charges your cell phone or PDA).
[edit] Hydrogen vehicles, boats and refuelling
- For more details on this topic, see Hydrogen station.
The first public hydrogen refueling station was opened in Reykjavík, Iceland in April 2003. This station serves three buses built by DaimlerChrysler that are in service in the public transport net of Reykjavík. The station produces the hydrogen it needs by itself, with an electrolyzing unit (produced by Norsk Hydro), and does not need refilling: all that enters is electricity and water. Royal Dutch 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.
- For more details on this topic, see Hydrogen highway.
The GM 1966 Electrovan, is the automotive industry's first attempt at making an automobile powered by a hydrogen fuel cell. Dr. Craig Marks states that the Electrovan weighed more than twice as much as a normal van. It could travel up to 70 miles an hour.
There are numerous prototype or production cars and buses based on fuel cell technology being researched or manufactured. Research is ongoing at a variety of motor car manufacturers. Honda has announced the release of a hydrogen vehicle in 2008[[2]].
Currently, a team of college students called Energy-Quest is planning to take a hydrogen fuel cell powered boat around the world (as well as other projects using efficient or renewable fuels). Their venture is called the Triton.[citation needed]Type 212 submarines use fuel cells to remain submerged for weeks without the need to surface.
[edit] Economy and the environment
Fuel cells are often promoted as being potentially emission-free if they burn hydrogen, in contrast to currently more common fuels such as methane or natural gas that generate carbon dioxide. However, hydrogen is an energy carrier, not an energy source. Electrolysis, which requires electricity, is used to extract hydrogen from water. As of 2004, 50% of the electricity produced in the United States comes from coal, 20% comes from nuclear, 18% from natural gas, 7% from hydroelectricity, 3% from petroleum and the remaining 3% mostly coming from geothermal, solar and biomass. [3] When hydrogen is produced through electrolysis, the energy comes from these sources. Though the fuel cell itself will only emit heat and water as waste, pollution is produced to make the hydrogen that it runs on. Hydrogen production is only as clean as the energy sources used to produce it.
A holistic approach has to take into consideration the impacts of an extended hydrogen scenario. This refers to the production, the use and the disposal of infrastructure and energy converters.
Nowadays low temperature fuel cell stacks (PEM, DMFC and PAFC) consist of catalysts to a very high amount. This is caused by the fact that poisoning reduces activity and thus the catalyst has to be over-dimensioned.<ref>http://www.fuelcell-magazine.com/eprints/free/johnsonmattheyapril03.pdf</ref> Limited reserves of platinum quicken the synthesis of an inorganic complex very similar to the catalytic iron-sulfur core of bacterial hydrogenase to step in.<ref>http://pubs.acs.org/cen/news/83/i07/8307notw8.html</ref> Although platinum is seen by some as one of the major "showstoppers" to mass market fuel cell commercialisation companies most predictions of platinum running out, and or platinum prices soaring do not take into account effects of thrifting (reduction in catalyst loading) and recycling. Current targets for a transport PEM fuel cells are 0.2 g/kW Pt - which is a factor of 5 decrease over current loadings - and recent comments from major OEMs indicate that this is possible. Also it is fully anticipated that recycling of fuel cells components, including platinum, will kick-in. One company, NedStack, is already stating that its units are 98% recyclable.[citation needed].
[edit] Research & Development
- August 2005: Georgia Institute of Technology researchers use triazole to raise the operating temperature of PEM fuel cells from below 100 °C to over 120 °C, claiming this will require less carbon-monoxide purification of the hydrogen fuel.<ref>http://www.gatech.edu/news-room/release.php?id=618</ref>
- September 2005: Technical University of Denmark (DTU) scientists announced in September 2005 a method of storing hydrogen in the form of ammonia saturated into a salt tablet. They claim it will be an inexpensive and safe storage method.<ref>http://www.fuelcelltoday.com/FuelCellToday/IndustryInformation/IndustryInformationExternal/NewsDisplayArticle/0,1602,6487,00.html</ref>
- January 2006: Virent Energy Systems is working on developing a low cost method for producing hydrogen on demand - from certain sugar/water mixtures (using one of glycerol, sorbitol, or hydrogenated glucose derivatives). Such a technology, if successful would solve many of the infrastructure (hydrogen storage) issues associated with the hydrogen economy<ref>http://wistechnology.com/article.php?id=902</ref>.
- October 1966: General Motors futuristic engineering project GM Electrovan: The world's first operational fuel cell-powered vehicle developed by a car manufacturer. "An Electrovan, Not an Edsel" by Danny Hakim. New York Times. New York, N.Y.: Nov 17, 2002. pg. 3.2
[edit] Related
- Hydrogen storage
- Hydrogen technologies
- Distributed generation
- Electrolysis
- Future energy development
- Grid energy storage
- High-temperature electrolysis
- Hydrogen reformer
- Renewable energy
- Water splitting
[edit] References
<references />
[edit] External links
- BIGS: Fuel Cell Animation
- EERE: Fuel Cell Types
- EERE: Hydrogen, Fuel Cells and Infrastructure Technologies Program
- Arizona State University Fuel Cell Lab Manages Water using Electroosmotic Pumps
- How Stuff Works: Fuel Cells
- LLNL: Direct Carbon Fuel Cell
- LLNL: The Carbon/Air Fuel Cell Conversion of Coal-Derived Carbons
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