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ENG 40B DR Mathias P Point

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Information about ENG 40B DR Mathias P Point
Education

Published on February 20, 2008

Author: Arley33

Source: authorstream.com

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Fuel cell technology – bringing the potentially explosive reaction of burning hydrogen and oxygen under control by wisely employing the electrons of hydrogen molecules to do Work in the electric circuit. :  Fuel cell technology – bringing the potentially explosive reaction of burning hydrogen and oxygen under control by wisely employing the electrons of hydrogen molecules to do Work in the electric circuit. In 1839 a British chemist William R. Grove noticed and later demonstrated that during the electrochemical reaction when hydrogen and oxygen create water electric current is created. Sir William Robert Grove developed two electrochemical cells (batteries): the first cell consisted of zinc in dilute sulfuric acid and platinum in concentrated nitric acid, separated by a porous pot, that was practically used for the early American telegraph and the second cell, a "gas voltaic battery" was the forerunner of modern fuel cells. Thus, Grove is known as "Father of the Fuel Cell". Such cell does not have moving parts on the macro scale, works silently, and its only byproduct is water. But fuel cells based on this phenomenon for over a century were only a laboratory curiosity. Only in in the 60s of the XX century NASA started using the light and compact versions of fuel cells to power their space craft with electric energy. Today these technologies are beginning to find applications to power cell phones, lap tops, homes and electric motor powered cars, tanks, submarines etc. Experts estimate that replacing the traditional methods of generating electric energy by combustion of coal with fuel cells should reduce the emission of CO2 by 40 – 60 %, and the emission of NOX by 50-90%. Advantage of fuel cells is also related to their demonstrated fuel flexibility In a fuel cell the reactants are not stored in the cell, like they would be in a battery. Direct conversion of chemical energy into electrical occurs as the reactants are fed to it continuously. The products of the reaction – electric current, water and heat - are continuously withdrawn. It operates as a continuous-flow system as long as fuel and oxygen are supplied, and produces a steady electric current. H2 can be fed to the fuel cell system directly or can be generated within the fuel cell system by reforming hydrogen-rich fuels such as methanol, ethanol, and hydrocarbon fuels. Types of Fuel Cells The heart of the cell is the ion conducting electrolyte and fuel cells are classified primarily by the kind of electrolyte they employ. This determines the kind of chemical reactions that take place in the cell, the kind of catalysts required, the temperature range in which the cell operates, the fuel required, and other factors. These characteristics, in turn, affect the applications for which these cells are most suitable. Polymer Electrolyte Membrane (PEM) Fuel Cells Direct Methanol Fuel Cells Alkaline Fuel Cells Phosphoric Acid Fuel Cells Molten Carbonate Fuel Cells Solid Oxide Fuel Cells Regenerative Fuel Cells:  In a fuel cell the reactants are not stored in the cell, like they would be in a battery. Direct conversion of chemical energy into electrical occurs as the reactants are fed to it continuously. The products of the reaction – electric current, water and heat - are continuously withdrawn. It operates as a continuous-flow system as long as fuel and oxygen are supplied, and produces a steady electric current. H2 can be fed to the fuel cell system directly or can be generated within the fuel cell system by reforming hydrogen-rich fuels such as methanol, ethanol, and hydrocarbon fuels. Types of Fuel Cells The heart of the cell is the ion conducting electrolyte and fuel cells are classified primarily by the kind of electrolyte they employ. This determines the kind of chemical reactions that take place in the cell, the kind of catalysts required, the temperature range in which the cell operates, the fuel required, and other factors. These characteristics, in turn, affect the applications for which these cells are most suitable. Polymer Electrolyte Membrane (PEM) Fuel Cells Direct Methanol Fuel Cells Alkaline Fuel Cells Phosphoric Acid Fuel Cells Molten Carbonate Fuel Cells Solid Oxide Fuel Cells Regenerative Fuel Cells You can make a hydrogen fuel cell in your kitchen in about 10 minutes, and demonstrate how hydrogen and oxygen can combine to produce clean electrical power. 1 To make the fuel cell, we need the following: One foot of platinum coated nickel wire, or pure platinum wire. A piece of wood A 9 volt battery clip. A 9 volt battery. Some transparent sticky tape. A glass of water. A volt meter. 2 Your fuel cell is now complete. To operate the fuel cell, we need to cause bubbles of hydrogen to cling to one electrode, and bubbles of oxygen to cling to the other. There is a very simple way to do this. We touch the 9 volt battery to the battery clip (we don't need to actually clip it on, since it will only be needed for a second or two). Touching the battery to the clip causes the water at the electrodes to split into hydrogen and oxygen, a process called electrolysis. You can see the bubbles form at the electrodes while the battery is attached. 3 Now we remove the battery. If we were not using platinum coated wire, we would expect to see the volt meter read zero volts again, since there is no battery connected. The platinum acts as a catalyst, allowing the hydrogen and oxygen to recombine. The hydrolysis reaction reverses. Instead of putting electricity into the cell to split the water, hydrogen and oxygen combine to make water again, and produce electricity. A great :  You can make a hydrogen fuel cell in your kitchen in about 10 minutes, and demonstrate how hydrogen and oxygen can combine to produce clean electrical power. 1 To make the fuel cell, we need the following: One foot of platinum coated nickel wire, or pure platinum wire. A piece of wood A 9 volt battery clip. A 9 volt battery. Some transparent sticky tape. A glass of water. A volt meter. 2 Your fuel cell is now complete. To operate the fuel cell, we need to cause bubbles of hydrogen to cling to one electrode, and bubbles of oxygen to cling to the other. There is a very simple way to do this. We touch the 9 volt battery to the battery clip (we don't need to actually clip it on, since it will only be needed for a second or two). Touching the battery to the clip causes the water at the electrodes to split into hydrogen and oxygen, a process called electrolysis. You can see the bubbles form at the electrodes while the battery is attached. 3 Now we remove the battery. If we were not using platinum coated wire, we would expect to see the volt meter read zero volts again, since there is no battery connected. The platinum acts as a catalyst, allowing the hydrogen and oxygen to recombine. The hydrolysis reaction reverses. Instead of putting electricity into the cell to split the water, hydrogen and oxygen combine to make water again, and produce electricity. A great If the fuel cell is designed to operate also in reverse as an electrolyzer, then electricity can be used to convert the water back into hydrogen and oxygen. (See Figure 1.) This dual-function system is known as a reversible or unitized regenerative fuel cell (URFC). Lighter than a separate electrolyzer and generator, a URFC is an excellent energy source in situations where weight is a concern. Weight was a critical issue in 1991 when scientists at Lawrence Livermore National Laboratory and AeroVironment of Monrovia, California, began looking at energy storage options for an unmanned, solar-powered aircraft to be used for high-altitude surveillance, communications, and atmospheric sensing as part of the Strategic Defense Initiative. Called Pathfinder, the aircraft set an altitude record for solar-powered flight in 1995, flying to 15,400 meters (50,500 feet) and remaining aloft for about 11 hours. Pathfinder's successor, Helios, will remain aloft for many days and nights. For that aircraft, storage devices were studied that would provide the most energy at the lowest weight, i.e., the highest energy density. The team looked at flywheels, super-capacitors, various chemical batteries, and hydrogen- oxygen regenerative fuel cells. The regenerative fuel cell, coupled with lightweight hydrogen storage, had by far the highest energy density--about 450 watt-hours per kilogram--ten times that of lead-acid batteries and more than twice that forecast for any chemical batteries. :  If the fuel cell is designed to operate also in reverse as an electrolyzer, then electricity can be used to convert the water back into hydrogen and oxygen. (See Figure 1.) This dual-function system is known as a reversible or unitized regenerative fuel cell (URFC). Lighter than a separate electrolyzer and generator, a URFC is an excellent energy source in situations where weight is a concern. Weight was a critical issue in 1991 when scientists at Lawrence Livermore National Laboratory and AeroVironment of Monrovia, California, began looking at energy storage options for an unmanned, solar-powered aircraft to be used for high-altitude surveillance, communications, and atmospheric sensing as part of the Strategic Defense Initiative. Called Pathfinder, the aircraft set an altitude record for solar-powered flight in 1995, flying to 15,400 meters (50,500 feet) and remaining aloft for about 11 hours. Pathfinder's successor, Helios, will remain aloft for many days and nights. For that aircraft, storage devices were studied that would provide the most energy at the lowest weight, i.e., the highest energy density. The team looked at flywheels, super-capacitors, various chemical batteries, and hydrogen- oxygen regenerative fuel cells. The regenerative fuel cell, coupled with lightweight hydrogen storage, had by far the highest energy density--about 450 watt-hours per kilogram--ten times that of lead-acid batteries and more than twice that forecast for any chemical batteries. It’s time to take a look at some equations and numbers ;) :  It’s time to take a look at some equations and numbers ;) The efficiency of the conversion of chemical to electric energy can be calculated using equations above. 1 law takes the form: ΔH=Q+WEL because fuel-cell operation is a steady- flow process If the cell operates reversibly and isothermally , Q=TΔ S and ΔH=T Δ S+ WEL The greatest electrical work for a reversible cell is equal to the change of the thermodynamic potential WEL=ΔH-TΔS=ΔG where ΔH is the heat of the chemical reaction process The heat transfer to the surroundings required for isothermal operation is: Q=ΔH-ΔG For each molecule of a diatomic hydrogen consumed, two electrons pass to the external circuit. On the basis of 1 mol of H2 the charge q transferred between the electrodes is: -e=charge on each electron q=2 NA(-e) coulomb NA= Avogadro’s number because NA * e is Faraday’s constant F q=2 F The electrical work is then the product of the charge transferred and the emf of the cell (E volt) of the cell: WEL = -2 F E joule The electromotive force of a fuel cell is the difference in electric potential between the electrodes without any work load The emf of a reversible cell is therefore E = - WEL / 2 F = ΔG / 2 F :  The efficiency of the conversion of chemical to electric energy can be calculated using equations above. 1 law takes the form: ΔH=Q+WEL because fuel-cell operation is a steady- flow process If the cell operates reversibly and isothermally , Q=TΔ S and ΔH=T Δ S+ WEL The greatest electrical work for a reversible cell is equal to the change of the thermodynamic potential WEL=ΔH-TΔS=ΔG where ΔH is the heat of the chemical reaction process The heat transfer to the surroundings required for isothermal operation is: Q=ΔH-ΔG For each molecule of a diatomic hydrogen consumed, two electrons pass to the external circuit. On the basis of 1 mol of H2 the charge q transferred between the electrodes is: -e=charge on each electron q=2 NA(-e) coulomb NA= Avogadro’s number because NA * e is Faraday’s constant F q=2 F The electrical work is then the product of the charge transferred and the emf of the cell (E volt) of the cell: WEL = -2 F E joule The electromotive force of a fuel cell is the difference in electric potential between the electrodes without any work load The emf of a reversible cell is therefore E = - WEL / 2 F = ΔG / 2 F Slide7:  These equations may be applied to hydrogen/oxygen fuel cells at STP, with pure hydrogen and oxygen as reactants and pure H2O vapor as product. If these species are assumed ideal gases, then the reaction occurring is the standard formation reaction for H2O (g) at 298.15 K, for which values from Table C.4 are: ΔH = ΔH°f 298 = -241,818 J /mol and ΔG = ΔG°f 298 = - 228,572 J/mol in such a case we have WEL=ΔH-TΔS=ΔG=- 228,572 J/mol Q=ΔH-ΔG = -13,246 J/mol E = - WEL / 2 F = ΔG / 2 F = 1,184 volts If, as is more commonly the case, air is the source of oxygen, the cell receives O2 at its partial pressure in air. Because the enthalpy of the ideal gases is independent of pressure, the enthalpy change of reaction for the cell is unchanged. However, the GIBBS ENERGY is affected and we must use a different equation: Gi - Ḡi = - RT ln yi The Nerst equation can be expressed in terms of the mole fractions of the gases, (G = E in some older books) Applying this equation when air is used rather than pure oxygen does not significantly reduce the emf and work of a reversible cell, as demonstrated on p.531 of our course textbook. If we calculate enthalpy and Gibbs-energy changes of reaction using equations relating the values to temperature and calculate ΔH , ΔG and WEL , Q and E 60 deg C, rather than 25 deg C, the voltage and work output of a reversible cell reduce only by a small amount. The higher temperature fuel cells (i.e., MCFC and SOFC) make a high temperature heat available at the exhaust that can be recovered to obtain higher efficiency or for cogeneration purposes. In pressurized fuel cell systems, the addition of a gas expander before the steam generation may be energetically advantageous :  The higher temperature fuel cells (i.e., MCFC and SOFC) make a high temperature heat available at the exhaust that can be recovered to obtain higher efficiency or for cogeneration purposes. In pressurized fuel cell systems, the addition of a gas expander before the steam generation may be energetically advantageous There is a new innovative hydrogen electrode from the Ovonic Metal Hydride Fuel Cell, a fundamentally new type of fuel cell that can store electrical energy inside the fuel cell stack. This new technology from Ovonic Fuel Cell Company has the potential to revolutionize power systems due to its unique performance advantages, including onboard energy storage, instant start, and good lowtemperature performance. Lower cost is a key feature of the metal hydride fuel cell since it does not use expensive noble metals or exotic components. In addition, it requires no battery; thus, reducing weight, cost, and complexity. The metal hydride fuel cell has key features and advantages that make it a practical power device. Slide9:  Combining a mole of hydrogen gas and a half-mole of oxygen gas from their normal diatomic forms produces a mole of water. A detailed analysis of the process makes use of the thermodynamic potentials. This process is presumed to be at 298K and one atmosphere pressure, and the relevant values are taken from a table of thermodynamic properties. QuantityH20.5 O2H2OChangeEnthalpy00-285.83 kJΔH = -285.83 kJEntropy130.68 J/K0.5 x 205.14 J/K69.91 J/KTΔS = -48.7 kJEnergy is provided by the combining of the atoms and from the decrease of the volume of the gases. Both of those are included in the change in enthalpy included in the table above. At temperature 298K and one atmosphere pressure, the system work is W = PΔV = (101.3 x 103 Pa)(1.5 moles)(-22.4 x 10-3 m3/mol)(298K/273K) = -3715 J Since the enthalpy H= U+PV, the change in internal energy U is then ΔU = ΔH - PΔV = -285.83 kJ - 3.72 kJ = -282.1 kJ The entropy of the gases decreases by 48.7 kJ in the process of combination since the number of water molecules is less than the number of hydrogen and oxygen molecules combining. Since the total entropy will not decrease in the reaction, the excess entropy in the amount TΔS must be expelled to the environment as heat at temperature T. The amount of energy per mole of hydrogen which can be provided as electrical energy is the change in the Gibbs free energy: ΔG = ΔH - TΔS = -285.83 kJ + 48.7 kJ = -237.1 kJ For this ideal case, the fuel energy is converted to electrical energy at an efficiency of 237.1/285.8 x100% = 83%! This is far greater than the ideal efficiency of a generating facility which burned the hydrogen and used the heat to power a generator! Although real fuel cells do not approach that ideal efficiency, they are still much more efficient than any electric power plant which burns a fuel. Comparison of electrolysis and the fuel cell process In comparing the fuel cell process to its reverse reaction, electrolysis of water, it is useful treat the enthalpy change as the overall energy change. The Gibbs free energy is that which you actually have to supply if you want to drive a reaction, or the amount that you can actually get out if the reaction is working for you. So in the electrolysis/fuel cell pair where the enthalpy change is 285.8 kJ, you have to put in 237 kJ of energy to drive electrolysis and the heat from the environment will contribute TΔS=48.7 kJ to help you. Going the other way in the fuel cell, you can get out the 237 kJ as electric energy, but have to dump TΔS = 48.7 kJ to the environment. School Bus Exhaust May Be Cancer Source POSTED: 3:00 p.m. PDT October 16, 2003 SAN FRANCISCO -- Children breathing diesel exhaust from school buses can face a cancer risk 30 times greater than usual, according to a study released by the California Air Resources Board. The study was modeled on similar research by the Natural Resources Defense Council, a San Francisco non-profit environmental advocacy group, and found that children who ride the buses for many years run the highest risk. Jerry Martin, air resource board spokesman, said the study was based on kids who rode the bus twice a day for 13 years. "It's confirmed: dirty diesel buses are bad for kids," said NRDC scientist Dr. Gina Solomon. "Now we need to act to protect our children from diesel pollution that can cause cancer and contribute to asthma." To remedy the risk, both CARB and NRDC recommend replacing the old diesel buses with cleaner ones that run on compressed natural gas, and retrofitting the old buses with diesel particulate traps. Additional recommendations include minimizing bus idling time at schools, discouraging caravans of multiple buses, and opening bus windows whenever possible. We have alerted state school district representatives to the results of this report, and hope they will follow these recommendations," Martin said. Martin said Proposition 40, approved by state voters in 2000, allocated roughly $12.5 million for improving state school buses. This money has already been used to make some of the improvements suggested in the study, Martin said. West Contra Costa Unified School District spokesman Paul Ehara said he hasn't heard about the CARB study. However, he said busing in his district is only provided for special education students and the rest take public transportation. Ehara said he did not know whether the buses used for the special education students were diesel buses. Marin County School Board Superintendent Mary Jane Burke said she also had not heard about the study. Novato Unified School District bus driver Rosalie, who did not wish to give her last name, said her district does use diesel buses to transport students. Rosalie also said she did not know whether there are any plans to switch to cleaner buses or retrofit existing ones. Representatives of the San Francisco and San Jose unified school districts did not immediately return phone calls made to inquire about the issue. :  School Bus Exhaust May Be Cancer Source POSTED: 3:00 p.m. PDT October 16, 2003 SAN FRANCISCO -- Children breathing diesel exhaust from school buses can face a cancer risk 30 times greater than usual, according to a study released by the California Air Resources Board. The study was modeled on similar research by the Natural Resources Defense Council, a San Francisco non-profit environmental advocacy group, and found that children who ride the buses for many years run the highest risk. Jerry Martin, air resource board spokesman, said the study was based on kids who rode the bus twice a day for 13 years. "It's confirmed: dirty diesel buses are bad for kids," said NRDC scientist Dr. Gina Solomon. "Now we need to act to protect our children from diesel pollution that can cause cancer and contribute to asthma." To remedy the risk, both CARB and NRDC recommend replacing the old diesel buses with cleaner ones that run on compressed natural gas, and retrofitting the old buses with diesel particulate traps. Additional recommendations include minimizing bus idling time at schools, discouraging caravans of multiple buses, and opening bus windows whenever possible. We have alerted state school district representatives to the results of this report, and hope they will follow these recommendations," Martin said. Martin said Proposition 40, approved by state voters in 2000, allocated roughly $12.5 million for improving state school buses. This money has already been used to make some of the improvements suggested in the study, Martin said. West Contra Costa Unified School District spokesman Paul Ehara said he hasn't heard about the CARB study. However, he said busing in his district is only provided for special education students and the rest take public transportation. Ehara said he did not know whether the buses used for the special education students were diesel buses. Marin County School Board Superintendent Mary Jane Burke said she also had not heard about the study. Novato Unified School District bus driver Rosalie, who did not wish to give her last name, said her district does use diesel buses to transport students. Rosalie also said she did not know whether there are any plans to switch to cleaner buses or retrofit existing ones. Representatives of the San Francisco and San Jose unified school districts did not immediately return phone calls made to inquire about the issue. In the USA 600 000 diesel school buses without any catalityc converters deliver kids to school with devastating effects for their pre-teen and teenage growing lungs. One would think that in a country where every politician, including the presidents, declare that he/she would do everything for the children we could use these kinds of buses, equipped with hydrogen fuel cells… http://www.fuelcells.org/info/library/fchandbook.pdf#search=%22Thermodynamics%20of%20fuel%20cells%2 Good reason to study chemical engineering and management ;):  http://www.fuelcells.org/info/library/fchandbook.pdf#search=%22Thermodynamics%20of%20fuel%20cells%2 Good reason to study chemical engineering and management ;) Ovonics is my favorite fuel cell company.:  Ovonics is my favorite fuel cell company. http://ovonics.com/oc_core_all_chart.cfm http://ovonics.com/eb_fc_resource_links.cfm Fuel cell science and technology cuts across multiple disciplines, including materials science, interfacial science, transport phenomena, electrochemistry, and catalysis. Because of the diversity and complexity of electrochemical and transport phenom- ena involved in a fuel cell and occurring at disparate length and time scales, fuel cell modeling and simulation requires a systematic framework parallel to computational fluid dynamics (CFD), here termed computational fuel cell dynamics (CFCD). CFCD deals with the coupling of multidimensional transport phenomena with electrochemical kinetics and the transport of charge (electrons and ions) to provide a comprehensive understanding of fuel cell dynamics. SO STUDY HARD AND ENJOY IT ;)

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