Fuel Cell Technology
Whenever there is an energy crisis or oil shortage, there is always talk about alternative methods of power other than the use of oil. One of the most talked about technologies is fuel cell technology. This is not to say that fuel cells are a new concept fresh from the drawing board, but in fact the idea is a well-proven and well-used method of producing electricity. There are many types of fuel cells, each with benefits, each with limitations. The use of fuel cells to power vehicles, houses, and electronic devices is a considerable field of research and has the potential to change the way we think of power.
Here are the five main areas of fuel cell research with their respective benefits and limitations:
Alkaline fuel cell: Used by NASA to power spacecraft from the beginning of the space program. Is very efficient, and uses a very high quality combination of Hydrogen and Oxygen to achieve this effect. Main limitation is that the cells are very fragile and not very tolerant of any sort of contamination. This makes it unlikely to see such fuel cells for average consumers. Potassium hydroxide is used as the electrolyte for this reaction. Hydroxyl ions (OH-) move from cathode to anode. Here, hydrogen bonds to form water. The extra electron is released and does work before returning to the cathode to form more OH- ions. This cell is up to 70 percent efficient.
Phosphoric-acid fuel cell: This fuel cell has potential backup generators or secondary generating plants for peak usage hours on power grids. This fuel cell requires a long warm up period as the cell operates at over 300 degrees F (150 C). This long warm-up makes it unsuitable for use in devices that need to be turned on and off frequently; cars, electronics, etc. This fuel cell is reliable and produces electricity on a similar efficiency rating as Alkaline Fuel Cells and is more tolerable to impurities. Phosphoric acid is the electrolyte used in this cell. A catalysts of platinum causes hydrogen to become hydrogen ions and migrate to the cathode. The electrons are forced to travel through the wires to migrate to the cathode portion of the cell. Once the electrons reach the cathode, the hydrogen and oxygen bond to form water. The water is then removed from the cell. When used without steam generators, efficiency of about 50 percent, increased to 80 percent when equipped with steam generators.
Solid oxide fuel cell: These fuel cells only operate once warmed up to 1,832 degrees F (1000 C). This limitation means that these cells can only be used in permanent locations with the necessary equipment to warm the cell up to operation temperature. Special materials are also needed to build the cells to withstand the temperatures within the system during operation. Though the high heat effects longevity and reliability of the system, the cells can be used to create steam from the excess heat produced, increasing the efficiency of the units. Zirconium oxide and calcium oxide are used to form crystal lattice structures for the electrolyte. At the high operating temperatures, oxygen moves through the lattice and become negatively charged. The hydrogen at the anode releases electrons to do work. The electrons then move to the cathode to join with oxygen completing the circuit. Efficiencies can range up to about 60 percent.
Molten carbonate fuel cell: These cells are perhaps the most likely to be used on a large scale to produce electricity and replace oil or coal fired plants. They operate at 1,112 degrees F (600 C), meaning they too can produce steam to be used by regular turbines. This double production of electricity combined with the lower cost of the unit, as exotic materials are not needed to withstand operating temperatures makes this fuel cell commercially viable. When operating temperature is reached the carbonate salts used as electrolytes melt and conduct carbonate ions from cathode to anode. At the anode hydrogen and oxygen combine to form water and release electrons. The electrons move through the circuit and return at the cathode. The electrons are used to form new CO3 ions and continue the process. Efficiency is about 60 percent without using the waste heat, 80 percent when steam generators are used as well.
Proton exchange membrane fuel cell: A very simple fuel cell, this unit operates at very low temperatures, 176 degrees F (80 C), and can accept any type of hydrogen and oxygen to power the unit. Can withstand movement and impurities, but is not as efficient when compared to other fuel cell technology. Using a membrane that allows only positive ions to pass through, the cell forces electrons to flow through wires to get to the hydrogen ions that move from anode to cathode. Most likely to be used in consumer products due to flexibility in fuel sources. A reformer is needed to turn fuels into hydrogen for the cell to use, and this reduces fuel cell efficiency. Efficiency reaches about 50 percent under the best conditions.
Bibliography
http://www.howstuffworks.com/fuel-cell.htm
http://fuelcells.si.edu/index.htm