Types of fuel cells
Fuel cells are a clean and quiet way to convert chemical-energy of fuels directly into electricity. Specifically, they transform hydrogen and oxygen into electric power, emitting water as their only waste product.
A fuel cell consists of two electrodes, an anode and a cathode, sandwiched around an electrolyte. (An electrolyte is a substance, usually liquid, capable of conducting electricity by means of moving ions [charged atoms or molecules]). The fuel—usually hydrogen—enters at the anode of the fuel cell while oxygen enters at the cathode. The hydrogen is split by a catalyst into hydrogen ions and electrons. Both move toward the cathode, but by different paths. The electrons pass through an external circuit, where they constitute electricity, while the hydrogen ions pass through the electrolyte. When the electrons return to the cathode, they are reunited with the hydrogen and the oxygen to form a molecule of water.
Fuel cells have several advantages: they are quiet, produce only water as a waste product, extract electricity from fuel more efficiently than combustion-boiler-generator systems. They can run on pure hydrogen—usually derived from methane by combining methane with steam at high temperature—or, in one recently developed design, on methane itself. Biomass, wind, solar power, or other renewable sources can supply energy to make hydrogen or other fuels for use in fuel cells, which could be installed in buildings (e.g., schools, hospitals, homes), in vehicles, or in small devices such as mobile phones or laptop computers. Fuel cells today are running on many different fuels, even gas from landfills and wastewater treatment plants.
The principles of the fuel cell were developed by Welsh chemist William Grove (1811–1896) in 1839. As early as 1900, scientists and engineers were predicting that fuel cells would be the primary source of electric power within a few years. It wasn't until the 1960s, however, when the U.S. National Aeronautics and Space Administration (NASA) chose the fuel cell to furnish power to its Gemini and Apollo spacecraft, that fuel cells received serious attention. Today, NASA still uses fuel cells to provide electricity and water (as a byproduct) for the space shuttle.
For years, experts predicted that fuel cells would eventually replace less-efficient gasoline engines and other clumsy, dirty devices for extracting energy from fuel. These predictions have yet to be fully realized, even though fuel cells are becoming more widely used. Automobile manufacturers are developing ways to extract hydrogen from hydrocarbon fuels in on-board devices, allowing a fuel-cell vehicle to run on methanol (as with Mercedes-Benz's and Toyota's prototypes) or even on gasoline, as Chrysler is proposing. DaimlerChrysler expects to produce a fuel-cell bus for the European market by 2003. The proposed 70-passenger bus will cost approximately $1.2 million and have a range of about 186 mi (300 km) and a top speed of 50 MPH (80 km/h).
There are five basic types of fuel cells, differentiated by the type of electrolyte separating the hydrogen from the oxygen. The cells types now in use or under development are alkaline, phosphoric acid, proton exchange membrane, molten carbonate, and solid oxide.
Long used by NASA on space missions, alkaline cells can achieve power-generating efficiencies of up to 70%. NASA's fuel cells use alkaline potassium hydroxide as the electrolyte and the electrodes of porous carbon. At the anode, hydrogen gas combines with hydroxide ions to produce water vapor. This reaction results in extra electrons that are forced out of the anode to produce the electric current. At the cathode, oxygen and water plus returning electrons from the circuit form hydroxide ions that are again recycled back to the anode. The basic core of the fuel cell, consisting of the manifolds, anode, cathode, and electrolyte, is generally called the stack. Until recently, such cells were too costly for commercial applications, but several companies are examining ways to reduce costs and improve operating flexibility.
The fuel-cell type most commercially developed today is the phosphoric acid, now being used in such diverse settings as hospitals, nursing homes, hotels, office buildings, schools, utility power plants, and airport terminals. They can also be used in large vehicles such as buses and locomotives. Phosphoric-acid fuel cells generate electricity at more than 40% efficiency. If the steam produced is captured and used for heating, the efficiency jumps to nearly 85%. This compares to only 30% efficiency for the most advanced internal combustion engines. Phosphoric-acid cells operate at around 400°F (205°C).
Proton exchange membrane cells operate at relatively low temperatures (about 200°F [93°C]) and have high power density. They can vary their output quickly to meet shifts in power demand, and are suited for small-device applications. Experts say they are perhaps the most promising fuel cell for light-duty vehicles where quick startup is required.
Molten carbonate fuel cells promise high fuel-toelectricity efficiencies and the ability to consume coal-based fuels such as carbon monoxide. These cells, however operate at very high temperatures (1,200°F [650°C]) and therefore cannot be used in small-scale applications.
The solid oxide fuel cell could be used in big, high-power applications including industrial and large-scale central electricity generating stations. Some developers also see a potential for solid oxide use in motor vehicles. A solid oxide system usually uses a hard ceramic electrolyte instead of a liquid electrolyte, allowing operating temperatures to reach 1,800°F (980°C). Power generating efficiencies could reach 60%.
Direct methanol fuel cells (DMFC), relatively new members of the fuel cell family, are similar to the proton exchange membrane cells in that they both use a polymer membrane as the electrolyte. However, in the DMFC, the anode catalyst itself draws the hydrogen from the liquid methanol, eliminating the need for a fuel reformer. Efficiencies of about 40% are expected with this type of fuel cell, which would typically operate at a temperature between 120–190°F (50–90°C). Higher efficiencies are achieved at higher temperatures.
Regenerative fuel cells use sunlight as their energy source and water as a working medium. These cells would be attractive as a closed-loop form of power generation. Water is separated into hydrogen and oxygen by a solar-powered electrolyser. The hydrogen and oxygen are fed into the fuel cell, which generates electricity, heat, and water. The water is then recycled back into the system to be reused.
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