EML4450L18

# EML4450L18 - 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 and Sources: 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 Fuel Cell Hand Book, US DOE - available on the web.

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S ustainable E nergy S cience and E ngineering C enter Performance of a single cell operating on hydrogen as the fuel and oxygen in air as the oxidizer. The porous electrodes of either carbon or nickel employed are separated by a 30% solution of KOH (potassium hydroxide). The cell operates at a temperature of 298K and the fuel and air are supplied at one atmosphere. The faradic efficiency (the fraction of the reaction which is occurring electrochemically to give a current is called faradic or current efficiency, η F = 1/(nFN fu ); where N fu is the total number of moles of fuel reacted electrochemically per second) for the cell is estimated to be 95% 20% of the fuel supplied to the cell will escape through the electrolyte unreacted. w =0.25cm l =12cm Separation between electrodes, w = 0.25cm Height of the cell, l = 12 cm Depth of the cell, d = 6cm Average electrolyte velocity, u = 5cm/s (Supplied by an external pump) Fuel Cell Design Calculation
S ustainable E nergy S cience and E ngineering C enter The physical properties of the electrolyte at 298K area as follows: Concentration: 30% KOH (wt) or c b = 6.9 x 10 -3 mole/cm 3 Density: ρ = 1.294 gm/cm 3 Dynamic Viscosity: μ = 2.43 x 10 -2 poise Kinematic Viscosity: ν = 1.887 x 10 -2 cm 2 /s Conductivity: σ = 0.625 (ohm-cm) -1 Diffusion Coefficient for OH - ions: D = 1.5 x 10 -7 cm 2 /s Electrolyte Properties

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S ustainable E nergy S cience and E ngineering C enter We now calculate the open circuit voltage. We assume that each electrode reaction follows the half-cell reactions Anode E o = 0 Cathode E o = 1.23v Yielding a cell reaction of We may now apply the Nernst equation to our cell potential where P t is the total pressure of the mixture and n i /n t is the mole fraction of the i th constituent. H 2 ( g ) 2 H + + 2 e 0.5 O 2 ( g ) + 2 H + + 2 e H 2 O ( l ) H 2 ( g ) + 0.5 O 2 ( g ) H 2 O ( l ) E = E o RT n e F ln P H 2 O P H 2 P O 2 1/2 P i = n i n t P t Open Circuit Voltage
S ustainable E nergy S cience and E ngineering C enter The partial pressures in the Nernst equation are often eliminated in favor of a function that is derived from general equation of state, generally denoted by f , the fugacity , which is a measure of the tendency of a component to escape from a solution. It is equal to partial pressure only when the vapor behaves like an ideal gas. We also define a quantity called the activity a A = f A /f A o . We can then write the Nernst equation as The activity of H 2 O to be used in this equation should be that of water in 30% KOH solution. This value is somewhat less than one, but we may take it as one to make our answer conservative. The hydrogen is supplied at one atm (has an activity of one since the activity is equal to the partial pressure of an ideal gas), the activity of oxygen is 0.21. Then we have

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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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EML4450L18 - Sustainable Energy Science and Engineering...

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