6 - 2/12/2011 MEM 415– FUEL CELL ENGINES -- Lecture # 6...

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Unformatted text preview: 2/12/2011 MEM 415– FUEL CELL ENGINES -- Lecture # 6 -THERMODYNAMICS AND POLARIZATION (Region V) – Additional Notes Dr. E. Caglan Kumbur Assistant Professor of Mechanical Engineering Office: Curtis 160 Ph: 215.895.5871 e-mail: [email protected] -- Feb 14, 2010 -MEM 415– Lecture #6 Sample Problems…. Problem on Thermodynamics Effect of Oxygen Enhancement Given an H2-air fuel cell operating at 100 C with vapor phase water as the product. Determine the approximate expected change in voltage if air is replaced with oxygen, with all these remaining the same. Solution: Nernst Equation E (T , P) * ) (G o (T ) RuT & ( PA / P o )+ A ( PB / P o )+ B # ln $ ' ! nF nF % ( Pc / P o )+C ( PD / P o )+ D " E (T , P ) * 1 0.5 ) (G o (T ) RuT & ( aH 2 ) ( aO2 ) # ' ln $ ! 1 nF nF $ ( aH 2O ) ! % " Nernst Equation for H2 PEM fuel cell So the only difference is the mole fractions for oxygen between these two cases. Air Different Oxygen MEM 415– Lecture #6 1 2/12/2011 Sample Problems…. Problem on Thermodynamics Effect of Oxygen Enhancement Given an H2-air fuel cell operating at 100 C with vapor phase water as the product. Determine the approximate expected change in voltage if air is replaced with oxygen, with all these remaining the same. Solution: Solution: The increase to switching oxygen (~12 mV) is small, considering the fact that air is free. However, use of pure oxygen brings increased cost, oxidizer storage requirement, and safety concerns. MEM 415– Lecture #6 Sample Problems…. Problem on Thermodynamics Effect of Pressure Increase Given an H2-air fuel cell operating at 60 C with vapor phase water as the product. Determine the approximate expected change in thermodynamic maximum OCV if cathode pressure is doubled and all the other things remain the same. Solution: Solution: What do you think about the increase? MEM 415– Lecture #6 2 2/12/2011 Sample Problems…. Sample Problem on Expected OCV Solution: MEM 415– Lecture #6 Sample Problems…. Sample Problem on Expected OCV Solution: Assuming constant specific heats evaluated at the desired temperature, we can solve for entropy and enthalpy change. MEM 415– Lecture #6 3 2/12/2011 Sample Problems…. Sample Problem on Expected OCV Solution: MEM 415– Lecture #6 Sample Problems…. Sample Problem on Expected OCV Solution: MEM 415– Lecture #6 4 2/12/2011 Sample Problems…. Sample Problem on Expected OCV Solution: MEM 415– Lecture #6 Sample Problems…. Sample Problem on Expected OCV Solution: Using Nernst Equation Difference between Case 2 and 1 MEM 415– Lecture #6 5 2/12/2011 Sample Problems…. Sample Problem on Expected OCV Solution: MEM 415– Lecture #6 Sample Problems…. Sample Problem on Expected OCV Solution: MEM 415– Lecture #6 6 2/12/2011 Sample Problems…. Sample Problem on Expected OCV Solution: MEM 415– Lecture #6 Sample Problems…. Sample Problem on Expected OCV Important Notes on Nernst Equation: -Appropriate pressure and mole fractions are chosen based on the global oxidation or reduction equation. -If the species is not a reactant in the primary electrode reaction (e.g. the oxygen in the anode in the example), it does not appear directly in the Nernst Equation. It only indirectly affects the voltage by changing the mole fractions It th th for the active species. -Nernst equation accounts only for the thermodynamic equilibrium effect of concentration change, and in practice: a change in the mole fraction has a greater effect than predicted due to the kinetic effects not yet accounted for. MEM 415– Lecture #6 7 2/12/2011 Region Region V – Finished… We are now done with Region V. Now, we will start to focus on Region 1, 2, 3 and 4. MEM 415– Lecture #6 Summary… ! We are done with Thermo for now. ! Read Chapter # 3 in your book. ! Look at your thermo notes for complete understanding of what we have done. ! We will start to analyze the other regions of the polarization curve… MEM 415– Lecture #6 8 ...
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This note was uploaded on 04/26/2011 for the course MEM 415 taught by Professor Kumbur,emin during the Spring '11 term at Drexel.

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