EML4450L16

EML4450L16 - Sustainable Energy Science and Engineering...

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S ustainable E nergy S cience and E ngineering C enter Direct Energy Conversion: Fuel Cells References: Direct Energy Conversion by Stanley W. Angrist, Allyn and Beacon, 1982. Fuel Cell Systems, Explained by James Larminie and Andrew Dicks, Wiley, 2003. Fuel Cell Technology Hand Book, Edited by Gregor Hoogers, CRC Press, 2002
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S ustainable E nergy S cience and E ngineering C enter Hydrogen - Oxygen Fuel Cell 1 2 3 4 5 6 7 9 10 1 11 2 H 2 4 H + + 4 e At the anode the hydrogen gas ionizes releasing electrons and creating H + ions (or protons). This reaction releases energy. O 2 + 4 e + 4 H + 2 H 2 O At the cathode, oxygen reacts with electrons taken from the electrode, and H + ions from the electrolyte, to form water An acid with free H + ions. Certain polymers can also be made to contain mobile H + ions - proton exchange membranes (PEM)
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S ustainable E nergy S cience and E ngineering C enter Fuel Cell Input and Output Hydrogen Energy Fuel Cell Oxygen Energy Electricity Energy = VIt Heat Water Power = VI ; Energy = VIt Gibbs free energy: Energy available to do external work, neglecting any work done by changes in pressure and/or volume. In a fuel cell, the external work involves moving electrons round an external circuit. It is the change in Gibbs free energy Δ G, difference between the Gibbs free energy of the products and the Gibbs free energy of the reactants or inputs is important.
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S ustainable E nergy S cience and E ngineering C enter Hydrogen-oxygen Fuel Cell The basic reaction: O H O H O H O H 2 2 2 2 2 2 2 1 2 2 + + The product is one mole of H 2 O ( 18g = 1 gmole) and the reactants are one mole of H 2 (2g = 1 gmole) and a half a mole of O 2 (32g = 1 gmole) . The molar specific Gibbs free energy, in ‘per mole’ form is commonly used. 2 2 2 ) ( 2 1 ) ( ) ( tan O H H ts reac products g g g g g g g O = Δ = Δ mole kJ g mole kJ g / 6 . 199 / 237 = Δ = Δ Liquid water product at 298K Gaseous water product at 873K Negative sign indicates that the energy is released g
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S ustainable E nergy S cience and E ngineering C enter If there are no losses, then all the Gibbs free energy is converted into electrical energy. Two electrons pass round the external circuit for each water molecule produced and each molecule of hydrogen used. For one mole of hydrogen used, 2N electrons pass round the external circuit- where N is the Avogadro’s number. If -e is the charge of one electron, then the charge that flows is -2Ne = -2F coulombs F is the faraday constant, or the charge on one mole of electrons. The electrical work done moving this charge round the circuit is ( E is the voltage of the fuel cell) Electrical work done = charge × voltage = -2FE joules With no losses, we have 9 Fuel Cell Input and Output H 2 2 H + + 2 e O 2 + 2 e + 2 H + H 2 O Δ g = − 2 FE E = −Δ g 2 F
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S ustainable E nergy S cience and E ngineering C enter Four quantities called "thermodynamic potentials" are useful in the chemical thermodynamics of reactions and non- cyclic processes. They are internal energy, the enthalpy, the Helmholtz free energy and the Gibbs free energy.
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This note was uploaded on 10/22/2011 for the course EML 4450 taught by Professor Greska during the Fall '06 term at FSU.

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EML4450L16 - Sustainable Energy Science and Engineering...

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