Energy%20Materials%20-%20fuel%20cells%201st%20print - Last...

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Unformatted text preview: Last class - Li ion batteries This class - Introduction of fuel cells 1 What is a Fuel Cell? A fuel cell is a device that converts chemical energy into electrical energy, water, and heat through electrochemical reactions. • Fuel and air react when they come into contact through a porous membrane (electrolyte) which separates them. • This reaction results in a transfer of electrons and ions across the electrolyte from the anode to the cathode. • If an external load is attached to this arrangement, a complete circuit is formed and a voltage is generated from the flow of electrical current. • The voltage generated by a single cell is typically rather small (< 1 volt), so many cells are connected in series to create a useful voltage. 2 Anode : H2 : H2 +1/2O2 → 2H2 + 2e→ H2O → H2O + electricity + heat Reaction Cathode : 1/2O2 + 2H+ +2eOverall DC Electricity Oxygen e Hydrogen Hydrogen e e Electrons Protons e e Water Heat e H2 2H+ + 2e0 Volts Membrane 1/2O2 + 2e~1.23 Volts 1/2O2 - - H2 + 1/2O2 --> H2O Approx. 1 volt or less/cell, therefore add cells together 3 Single cell components 4 Fuel Cell Vs. Battery Basic operating principles of both are very similar, but there are several intrinsic differences. • Open system • Anode and cathode are gases in contact with a platinum catalyst. • Reactants are externally supplied, no recharging required. • Closed system • Anode and cathode are solids. • Reactants are internally consumed, need periodic recharging. 5 Fuel Cell Vs. Internal Combustion Engine Similarities: • Both use hydrogen-rich fuel. • Both use compressed air as the oxidant. • Both require cooling. Differences: Fuel cell: • Output is electrical work. • Fuel and oxidant react electrochemically. • Little to no pollution produced. I.C. Engine: • Output is mechanical work. • Fuel and oxidant react combustively. • Use of fossil fuels can produce significant pollution. 6 History of fuel cells - Christian Friedrich Schonbein discovered principle of the fuel cell in 1838. - First fuel cell demonstrated by William Grove in 1839. Grove used four large cells, each containing hydrogen and oxygen, to produce electric power which was then used to split the water in the smaller upper cell. - Commercial potential first demonstrated by NASA in the 1960’s with the usage of fuel cells on the Gemini and Apollo space flights. However, these fuel cells were very expensive. - Fuel cell research and development has been actively taking place since the 1970’s, resulting in many commercial applications ranging from low cost portable systems for cell phones and laptops to large power systems for buildings. 7 Various fuel cells : classified by species of electrolytes 8 Glossary of Terms Used in Describing Fuel Cell Technology Electrochemical reaction: A reaction involving the transfer of electrons from one chemical substance to another. Electrode: An electrical terminal that conducts an electric current into or out of a fuel cell (where the electrochemical reaction occurs). Anode: Electrode where oxidation reaction happens (electrons are released). Cathode: Electrode where reduction reaction occurs (electrons are acquired). In a fuel cell, hydrogen is oxidized at the anode and oxygen reduction occurs at the cathode. Electrolyte: A chemical compound that conducts ions from one electrode to the other. 9 Ion: An atom that carries a positive or negative charge due to the loss or gain of an electron. Anion is a negative ion, cation is a positive ion. An electrochemical cell consists of 2 electrodes + 1 electrolyte Catalyst: A substance that participates in a reaction, increasing its rate, but is not consumed in the reaction. Polymer: A natural or synthetic compound made of giant molecules which are composed of repeated links of simple molecules (monomers). Inverter: A device used to convert direct current electricity produced by a fuel cell to alternating current. Reformer: A device that extracts pure hydrogen from hydrocarbons. Stack: Individual fuel cells connected in series within a generating assembly. 10 Fuel Cell Applications Stationary Power Stations – Over 2,500 fuel cell systems have been installed all over the world in hospitals, nursing homes, hotels, office buildings, schools and utility power plants – Most of these systems are either connected to the electric grid to provide supplemental power and backup assurance or as a grid-independent generator for locations that are inaccessible by power lines 11 Fuel cell system for submarine 12 Fuel Cells in Use: Transportation Systems - All major automakers are working to commercialize a fuel cell car Automakers and experts speculate that a fuel cell vehicle will be commercialized by 2010 - Many fuel cell buses are currently in use in North and South America, Europe, Asia and Australia - Trains, planes, boats, scooters, forklifts and even bicycles are utilizing fuel cell technology as well 13 Fuel Cells in Use: Space Systems - For electricity and water 14 Fuel Cells in Use: Portable Systems – Consumer electronics could gain drastically longer battery power with Fuel Cell technology – Cell phones can be powered for 30 days without recharging – Laptops can be powered for 20 hours without recharging 15 Reasons to Pursue Fuel Cell Technologies The following are the primary factors that make fuel cells an attractive alternative to the combustion-based burning of fossil fuels: • Reduced dependence on foreign oil. Because hydrogen can be produced from a variety of energy sources, including renewables, fuel cells would permit the use of non-petroleum sources for transportation and other applications. • No emissions. (environmental benefit) Another big advantage is low or zero emissions: a fuel-cell vehicle running on pure hydrogen produces only water vapor. Fuel cells are “fuel flexible,” so even when other types of fuels are used for stationary power or other applications, the fuel cell systems produce only trace emissions of regulated pollutants. 16 • Performance-related advantages. (efficiency) The internal combustion engines in today’s cars convert less than 20% of the energy in gasoline in on-the-road driving — and that’s after more than a century’s worth of innovations to make them run more cleanly and efficiently! On the other hand, fuel cell cars operating on hydrogen routinely achieve efficiencies of nearly 60%. Similarly, stationary combined heat and power systems have shown electrical efficiencies of 40% or higher, and combined heat and electric efficiencies of higher than 80%. 17 Challenges of Fuel Cell Technology 1. Cost Scientists are working to sort out some major issues that remain. For example, to compete with gasoline-powered vehicles, the cost of an automotive fuel cell system must be reduced to about $30 per kilowatt (kW) Current projected costs are $61 per kW with today’s technology being manufactured at high volumes. Other challenges involving fuel cell durability, hydrogen delivery, infrastructure, and on-vehicle storage still need to be resolved. 18 100,000 Innovators / Early Adopters System Cost ($/kW) 10,000 Backup & Standby Initial Mass Markets Remote / Premium Automotive 1,000 100 10 2000 2005 2010 2015 2020 19 2. Hydrogen Safety Like all good fuels, hydrogen contains a lot of energy. Hydrogen can be handled safely when guidelines for its safe storage, handling and use are observed. Hydrogen’s combustion properties warrant the same caution required when using any fuel, as well as care to address the properties unique to hydrogen. Some of hydrogen’s special properties actually may provide safety benefits compared to gasoline or other fuels. The hydrogen industry makes, distributes, stores and handles hydrogen nationwide and has compiled an exemplary safety record. 20 But Isn’t Hydrogen Explosive? Many have blamed the disaster of Hindenburg on a hydrogen explosion. However, hydrogen burns invisibly, and no evidence of leaks were ever found Using infrared spectrographs, NASA scientists found that the skin of the Hindenburg was treated with compounds which are found in gunpowder and rocket fuel (nitrates and aluminum powder). This, combined with a wooden frame coated with lacquer resulted in a highly flammable ship. 21 3. Hydrogen Storage Hydrogen can be stored in many ways, but commonly it is compressed in steel or composite tanks and held at pressures up to 10,000 pounds per square inch (psi), or liquified at -423 degrees Fahrenheit. These tanks are comparatively heavy. Liquefying is energy intensive, but liquid hydrogen has three times the amount of energy as an equal weight of gasoline. Hydrogen can also be stored in metal hydrides – granular metal that absorbs hydrogen. Similar, but lighter, are carbon nanotubes, and other carbon absorption techniques still in the experimental stage. Hydrogen can also be stored in chemical hydrides by way of chemical bonds. Chemical hydrides typically allow hydrogen to be stored in conventional tanks that only release hydrogen when a certain catalyst is present, making them very safe for transportation. 22 This class - Introduction of Fuel cells Next class - Thermodynamics of Fuel cells 23 ...
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This note was uploaded on 11/29/2010 for the course ENERGY 12314142 taught by Professor Song during the Spring '10 term at 카이스트, 한국과학기술원.

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