Showing posts with label fuel cell. Show all posts
Showing posts with label fuel cell. Show all posts
Wednesday, March 26, 2008
Polymer Electrolyte Membrane (PEM) fuel cells
There are several kinds of fuel cells, but Polymer Electrolyte Membrane (PEM) fuel cells—also called Proton Exchange Membrane fuel cells—are the type typically used in automobiles. A PEM fuel cell uses hydrogen fuel and oxygen from the air to produce electricity.
Fuel cell design issues
- Costs. In 2002, typical cells had a catalyst content of US$1000 per kilowatt of electric power output. In 2008 UTC Power has 400kw Fuel cells for $1,000,000 per 400kW installed costs. 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.[4]
- 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 cells will use Solupor®, a porous polyethylene film patented by DSM.[5][6]
- Water and air management[7] (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 like electroosmotic pumps focusing on 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 2H2 + O2 -> 2H20 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.
Fuel cell design
n essence, a fuel cell works by catalysis, separating the component electrons and protons of the reactant fuel, and forcing the electrons to travel through a circuit, hence converting them to electrical power. The catalyst is typically comprised of a platinum group metal or alloy. Another catalytic process takes the electrons back in, combining them with the protons and the oxidant to form waste products (typically simple compounds like water and carbon dioxide). 
Condensation of water produced by a PEMFC on the air channel wall. The gold wire around the cell ensures the collection of electric current.[2]
In the archetypal hydrogen–oxygen proton exchange membrane fuel cell (PEMFC) design, a proton-conducting polymer membrane, (the electrolyte), separates the anode and cathode sides. This was called a "solid polymer electrolyte fuel cell" (SPEFC) in the early 1970s, before the proton exchange mechanism was well-understood. (Notice that "polymer electrolyte membrane" and "proton exchange membrane" result in the same acronym.)
On the anode side, hydrogen diffuses to the anode catalyst where it later dissociates into protons and electrons. These protons often react with oxidants causing them to become what is commonly reffered to as multi-facilitated proton membranes (MFPM). 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, either liquid or vapor.
In addition to this pure hydrogen type, there are hydrocarbon fuels for fuel cells, including diesel, methanol (see: 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 PEM fuel cell produces a voltage from 0.6 V to 0.7 V 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)[3]
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. Such a design is called a fuel cell stack. Further, the cell surface area can be increased, to allow stronger current from each cell.
http://en.wikipedia.org/wiki/Fuel_cellFuel cell
A fuel cell is an electrochemical energy conversion device. It produces electricity from various external quantities 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 are different 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.
Many combinations of fuel and oxidant are possible. A hydrogen cell uses hydrogen as fuel and oxygen as oxidant. Other fuels include hydrocarbons and alcohols. Other oxidants include air, chlorine and chlorine dioxide.
http://en.wikipedia.org/wiki/Fuel_cell
Fuel cell FAQ
- When was the fuel cell invented?
- Fuel cells were initially demonstrated in 1839, by Sir William Grove. However, a truly workable fuel cell was not demonstrated until 1959. After use in NASA's space programme, interest in fuel cells died down somewhat until the 1990s when research and development started to lead towards greater prospects of commercialisation.
- Which fuels can be used in a fuel cell?
- Most fuel cells use hydrogen at the point where the electrochemical reaction takes place. This hydrogen can be chemically generated or reformed from a variety of normal fuels, including gasoline, natural gas or methanol. There is no consensus as to the single best fuel.
- What types of fuel cells are there?
- There are a number of types of fuel cell which are normally distinguished by the electrolyte they contain. The best-known types are alkaline, molten carbonate, phosphoric acid, proton exchange membrane and solid oxide. Direct methanol and regenerative fuel cells are also being extensively researched.
- What is a fuel cell?
- A fuel cell is an electrochemical device that produces electricity and heat from a fuel (often hydrogen) and oxygen. Unlike a conventional engine, it does this without burning the fuel and can therefore be more efficient and cleaner.
- Can I buy a fuel cell?
- In general, fuel cells are in the development phase and are not yet commercially available. Many companies are currently running field trials of alpha and beta development units and hope to be commercialising this technology from as early as this year in some cases. However, some products, such as educational fuel cells, are commercially available now, as listed in our
- Why use a fuel cell?
- Fuel cells have a number of advantages over other technologies for power generation. They have the potential to use less fuel than competing technologies and to emit no pollution when used. There are also many reasons why a fuel cell might be useful in specific environments, such as the high quality of electricity generated or their quiet operation.
- What devices could a fuel cell power?
- In principle, a fuel cell could power any device that requires electrical energy to function. This could range from a mobile phone up to a factory. Presently, the majority of attention is focussed on powering automobiles, houses and medium-sized portable electrical equipment. However, announcements have suggested that portable computers may be an early application.
- How much does a fuel cell cost?
- Since fuel cells are not yet fully commercialised, they are produced in small numbers. Consequently, they tend to be more expensive than they will be when selling in significant quantities. However, as technology improves, cost reduction is proceeding towards meeting challenging cost targets set by the automotive industry. People involved in the industry generally believe that this can be accomplished and that in any case, costs will continue to decrease and will be significantly lower by the end of this decade than they are now.
- What is the difference between a fuel cell and a battery?
- Whilst a battery chemically stores and releases electricity, a fuel cell produces energy by reacting a fuel with air. A battery will therefore run out of power and have to be recharged or disposed of. A fuel cell, however, will continue to function and produce power as long as the fuel and oxygen are supplied to it.
- Is hydrogen safe?
- Like any other fuel, hydrogen is potentially dangerous and is flammable. However, so are gasoline, diesel and natural gas and this has not prevented their use to power cars, alongside the correct safety features. Hydrogen even has some advantages as it is non-toxic, a definite benefit over most fuels. Use of hydrogen would therefore present new but not insurmountable safety challenges.
- Why not burn hydrogen instead of using it in a fuel cell?
- Hydrogen is an extremely clean-burning fuel. However, any combustion process will produce small amounts of pollutants whereas a fuel cell has the potential to emit none. In addition to this, a fuel cell can inherently be more fuel-efficient than an internal combustion engine. However, there may be applications where burning hydrogen makes sense and it is possible to imagine that hydrogen fuel cell powered cars and hydrogen internal combustion engine powered cars could run side-by-side on the roads.
- Are fuel cells a renewable energy source?
- Fuel cells themselves are not a power source: rather they use a fuel to produce power. If this fuel is obtained from renewable sources, then fuel cells can be an important part of the energy chain, perhaps with hydrogen being used to store intermittent energy and fuel cells converting this hydrogen back to power when required.
What is a fuel cell?
A fuel cell is an electrochemical device that combines hydrogen and oxygen to produce electricity, with water and heat as its by-product. As long as fuel is supplied, the fuel cell will continue to generate power. Since the conversion of the fuel to energy takes place via an electrochemical process, not combustion, the process is clean, quiet and highly efficient – two to three times more efficient than fuel burning.
No other energy generation technology offers the combination of benefits that fuel cells do. In addition to low or zero emissions, benefits include high efficiency and reliability, multi-fuel capability, siting flexibility, durability, scalability and ease of maintenance. Fuel cells operate silently, so they reduce noise pollution as well as air pollution and the waste heat from a fuel cell can be used to provide hot water or space heating for a home or office.
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