A
fuel cell is an
electrochemical energy conversion device. It produces electricity from external supplies of fuel (on the
anode side) and oxidant (on the
cathode side). These react in the presence of an
electrolyte. Generally, the reactants flow in and reaction products flow out while the electrolyte remains in the cell. Fuel cells can operate virtually continuously as long as the necessary flows are maintained.
Fuel cells differ from
batteries in that they consume reactant, which must be replenished, while batteries store electrical energy chemically in a closed system. 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.
[edit] Fuel cell design
In the archetypal example of a hydrogen/oxygen
proton exchange membrane fuel cell (PEMFC), which used to be called solid polymer electrolyte fuel cell (SPEFC) around 1970 and now is polymer electrolyte membrane fuel cell (PEFC or PEMFC, same as the short writing of proton exchange membrane) while the proton exchange mechanism was doubted, 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 later 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.
Construction of a low temperature
PEMFC: Bipolar plate as
electrode with in-milled gas channel structure, fabricated from conductive
plastics (enhanced with
carbon nanotubes for more conductivity);
Porous carbon papers; reactive layer, usually on the
polymer membrane applied; polymer membrane.
A typical PEM fuel cell produces a voltage from 0.6 to 0.7 at full rated load. Voltage decreases as current increases, due to several factors:
- Activation loss
- Ohmic loss (voltage drop due to resistance of the cell components and interconnects)
- Mass transport loss (depletion of reactants at catalyst sites under high loads, causing rapid loss of voltage) [2]
To deliver the desired amount of energy, the fuel cells can be combined in
series and parallel circuits, where series yield higher
voltage, and parallel allows a stronger
current to be drawn, this design is referred to as a
fuel cell stack. Further, the cell surface area can be increased, to allow stronger
current from each cell.
[edit] Fuel cell design issues
- Costs. In 2002, typical cells had a catalyst content of US$1000 per kilowatt of electric power output. The goal is to reduce the cost in order to compete with current market technologies including gasoline internal combustion engines. Many companies are working on techniques to reduce cost in a variety of ways including reducing the amount of platinum needed in each individual cell. 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.[3]
- 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. in 2005 Ballard Power Systems announced that its fuel cell will use Solupor®, a porous polyethylene film patented by DSM,[4] in its fuel cells.[5]
- 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 manage water in cells are being developed by fuel cell companies and academic research labs[6].
- 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. This is particularly challenging as the H2 + O2 -> H20 reaction is highly exothermic, so a large quantity of heat is generated within the fuel cell.
- 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".
[7] 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. In 1955, W. Thomas Grubb, a chemist working for the General Electric Company (GE), further modified the original fuel cell design by using a sulphonated polystyrene ion-exchange membrane as the electrolyte. Three years later another GE chemist, Leonard Niedrach, devised a way of depositing platinum onto the mebrane, which served as catalyst for the necessary hydrogen oxidation and oxygen reduction reactions. This became known as the 'Grubb-Niedrach fuel cell'. GE went on to develop this technology with NASA, leading to it being used on the Gemini space project. This was the first commercial use of a 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.
[8] 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.
[edit] Types of fuel cells
[edit] Efficiency
[edit] Fuel cell efficiency
The efficiency of a fuel is dependent on the amount of power drawn from it. Drawing more power means drawing more current, which increases the losses in the fuel cell. As a general rule, the more power (current) drawn, the lower the efficiency. Most losses manifest themselves as a voltage drop in the cell, so the efficiency of a cell is almost proportional to its voltage. For this reason, it is common to show graphs of voltage versus current (so-called polarization curves) for fuel cells. A typical cell running at 0.7 V has an efficiency of about 50%, meaning that 50% of the energy content of the hydrogen is converted into electrical energy; the remaining 50% will be converted into heat. (Depending on the fuel cell system design, some fuel might leave the system unreacted, constituting an additional loss.)
For a hydrogen cell operating at standard conditions with no reactant leaks, the efficiency is equal to the cell voltage divided by 1.48 V, based on the
enthalpy, or heating value, of the reaction. For the same cell, the
second law efficiency is equal to cell voltage divided by 1.23 V. (This voltage varies with fuel used, and quality and temperature of the cell.) The difference between these number represents the difference between the reaction's
enthalpy and
Gibbs free energy. This difference always appears as heat, along with any losses in electrical conversion efficiency.
[2]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".
[9] 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.
[edit] In practice
For a fuel cell operated on air (rather than bottled oxygen), losses due to the air supply system must also be taken into account. This refers to the pressurization of the air and adding moisture to it. This reduces the efficiency significantly and brings it near to the efficiency of a compression ignition engine. Furthermore fuel cells have lower efficiencies at higher loads.
The tank-to-wheel efficiency of a fuel cell vehicle is about 45% at low loads and shows average values of about 36% when a driving cycle like the NEDC (New European Driving Cycle) is used as test procedure.
[10]. The comparable NEDC value for a Diesel vehicle is 22%.
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.
[11]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.
[12] While a much cheaper
lead-acid battery might return about 90%, the electrolyzer/fuel cell system can store indefinite quantities of hydrogen, and is therefore better suited for long-term storage.
Solid-oxide fuel cells produce exothermic heat from the recombination of the oxygen and hydrogen. The ceramic can run as hot as 800 degrees Celsius. This heat can be captured and used to heat water in a combined heat and power (CHP) application. When the heat is captured, total efficiency can reach 80-90%. CHP units are being developed today for the European home market.
[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.
[13] This equates to less than one minute of down time in a six year period.
A new application is
micro combined heat and power, which is
cogeneration for family homes, 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.
The world's first Fuel Cell operated and certified passenger ship was the "HYDRA" (see picture). Mr. Christian Machens was the founder of the company "etaing GmbH" and realised this project with a small team of young engineers in Leipzig. It was christened in June 2000 in Bonn. The Fuel Cell System (AFC type, 6,5 kWel net output) was built in Wurzen near Leipzig, the hull was built in Hamburg and it was certified by the Germanischer Lloyd (Hamburg). The boat has transported around 2.000 persons without any major technical problems. The big advantage of the AFC technology was that the system could start at freezing temperatures (-10°C) without any problems and that the AFC is not sensitive against salty atmosphere
Not self written but found useful Please do comment & rate on this article 
Back to Community Shelf
2 Nickels awarded!![[Post New]](/templates/default/images/icon_minipost_new.gif){kind=icon}
337
