Fuel Cell Systems Explained - from and for SET students

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3.5 Fuel crossover and internal currentsEven if the electrolyte is designed toconduct only ions, it always occurs that a small amount ofelectrons pass through it. In practical fuel cells the fact that a certain amount of fuel diffuse unusedthrough the electrolyte is more important. The unused fuel will react directly with oxxygen at thecathode and no current is produced. These effects – fuel crossover and internal current – areessentially equivalent. One diffused hydrogen molecule effects in the same way as two electrons ofinternal current. The amount of these losses will be very small in respect to the irreversebilities.However,, in low temoerature fuel cells it causes a very noticeable drop of OCV, about 0.2V (20%)for a PEM. Noticeable in the steep initial falls of the voltage plots.A small change in fuel crossover and/or internal current, caused for example, by a change inhumidity of the electrolyte, can cause a large change in the OCV.Measurements of the internal current are problematic, but can be done by the measurement of thefuel consumption. The consumption rate of hydrogen [moles/s] is related to the current by thefollowing equation.The equation of the cell voltage can be refined, including the internal current density i n .To conclude, the effect of internal current is much less for high temperature cells, because i 0 ismuch higher. For low temperature cells internal current and fuel diffusion is usually not of greatimportance in terms of operating efficiency, but it has a marked effect on the OCV.3.6 Ohmic lossesThe losses occured by the electrical resistance of the electrodes and the interconnection and by theresistance against the flow of ions in the eelctrolyte. The ohmic voltage loss is important in all typesof fuel cells, especially in SOFC, and is mainly caused by the electrolyte. However the electricalresistance of the electrodes and bipolar plates is not negligible.The voltage drop is given by Ohm's law, using the current density and the area-specific resistance.ΔV ohm = i x rwith i in [mA/cm 2 ]r in [kΩcm 2 ]There are three ways of reducing the internal resistance: highly conductiv electrode material design and material of the bipolar plate or interconnection electrolyte as thin as possible(but have to prevent shorting of the electrodes, wide enough to provide a flow of electrolyte,physical robust, due to electrodes are built on it)3.7 Mass transport and concentration lossesDue to the electrochemical reactions at the eelctrodes, hydrogen and oxygen are consumed. Thatleads to a change in concentration, which leads on the other hand to a change in partial pressure ofthe reactants, which results in a voltage drop over the cell. The pressure changes depend on thecurrent drawn from the cell and the physical characteristics of the gas supply system, relating to thefluid resistance, the flow rate and the design of the gas supply along the electrodes.There is no overall satisfactory analytical solution to this problem. One theoretical approach ismentioned, which shows the effect of reducing the partial pressure on the OCV.
Assumption:Introduction a limiting current density i 1 at which the hydrogen consumption rate is equal to themax. supply rate. At i 1 the pressure would have reached zero.P 1 is the pressure when i is zero. If we assume a linearly fall from P 1 to P(i 1 ), tha the pressure at anycurrent density is given bySubstitution in above equation delivers the voltage drop due to mass transport.The negative sign is due to the fact that it is a voltage drop.The term RT/2F will be different for each reactant and is in general substitute by the constant B.This theoretical approach has many limitations, like: supply of air instead of pure oxygen lower temperature cells mixed fuel production and removal of reaction products build-up nitrogen in air systemsAn empirical approach is more favourable, because it provides an equation which delivers muchbetter results and will be used in the rest of the chapter.With m is about 3x10 -5 V and n is about 8x10 -3 cm/mA.The mass transport or concentration losses are important when Hydrogen is supplied by a reformer. The reaction rate on a demand change is too slow. The supply at the air cathode is not well circulated. Build-up nitrogen can block the oxygen supply at high currents. Water is not removed qickly enough in PEMFC.3.8 Combining the irreversebilitiesThe operating voltage of a fuel cell at a current density i is given bywith E: reversible OCVi n : internal and fuel crossover equivalent current densityA: slope of the Tafel linei 0 : exchange current densitym & n: constants in the mass-transfer overvoltager: area-specific resistanceThis equation can be simplified, due to i n is usually very small and has little impact on operatinglosses of fuel cells at working currents, so it will be cancelled. The term of the activationovervoltage can be rewritten as
Page 1 and 2: -Fuel Cell Systems Explained-(Summa
Page 3 and 4: 1.5 Fuel cell typesBesides the prac
Page 5 and 6: Fuel cells can be used successfully
Page 7 and 8: to the HHV or LHV. If the informati
Page 9 and 10: The products are three parts of H 2
Page 11 and 12: Figure 3.2 shows the performance of
Page 13: In order to get the current instead
Page 17 and 18: To summarize the irreversibilities
Page 19 and 20: 4.4 Water management in the PEMFCWt
Page 21 and 22: With Pe: electrical Power of the st
Page 23 and 24: 4.4.5 External humidification - pri
Page 25 and 26: 4.6 PEMFC connection - the bipolare
Page 27 and 28: with T 1 : entry temperatureη c :
Page 29 and 30: thus leads to higher efficiency. He
Page 32 and 33: minimum difference. This so-called
Page 34: converted into hydrogen at operatin

Fuel Cell Systems Explained - from and for SET students

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