Fuel Cell Systems Explained - from and for SET students

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Note:The mole is a measure of the amount of a substance in respect to its molar mass.H 2 molar mass: 2amu1 mole = 2gH 2 O molar mass: 18amu1 mole = 18gOne mole of a substance always has the same number of molecules, given by the Avogardro'snumber N = 6.022 x 10 23 .The Faraday's constant gives the charge of one mole of electrons.F = N x e = 6.022 x 10 23 x 1.602 x 10 -19 C = 96485 CUsing the basic reaction of the hydrogen/oxygen fuel cell, it can be seen that one mole of H 2 andhalf a mole O 2 deliver one mole of H 2 O. However, note that the Gibbs free energy of formation isnot constant. It depends on the temperature and state (liquid or gas). When the values are negative itmeans that energy is released.Electromotive force EMF = reversible open circuit voltage OCFIn a reversible process in the fuel cell, all Gibbs free energy would be converted into electricalenergy. The basic reaction provides two electrons to the electric circuit for each molecule of H 2used. So for one mole of H 2 , 2N of electrons pass through the circuit.Flowing charge: -2Ne = -2F [C]The electric work done equals the product of charge times voltage E. In a reversible process, theelectric work also equals the Gibbs free energy released.Given is the electromotive force EMF or reversible open circuit voltage of the hydrogen fuelcell.In practice the voltage would be lower, due to the fact that some irreversibilities also apply evenwhen no current is drawn from the cell.In generalwith z is the number of electrons per molecule of fuel.2.3 Efficiency and efficiency limitsThe Gibbs free energy is the part of chemical energy which is converted into electrical energy.However, it is insufficient to calculate the efficiency of a fuel cell only with this part of thechemical energy of the reactants by taken the irreversibilities into account, due to the fact that theGibbs free energy depends on the temperature, the pressure and other factors.In order to compare a fuel cell with other fuel burning technologies, is the maximum possbileefficiency derived by the ratio of the electrical energy produced per mole of fuel and the change ofenthaply of formation, that means the energy amount released as heat by burning the fuel.The maximum possible efficiency level, also named thermodynamic efficiency, is given byη max = Δg̅ f / Δh̅ f x 100%Note that Δh̅ f depends on the state of the substance, liquid or gas. For gas it is called the higherheating value HHV, for liquid it it called the lower heating value LHV. The difference betweeenboth is the molar enthalpy of vaporisation. Any statement of efficiency should say whether it relates
to the HHV or LHV. If the information is not given, probably the the LHV is used.Table 2.2 shows the theoretical efficiency limit of a hydrogen fuel cell in respect of the operatingtemperature. However in practice there are some differences. Voltage losses are less at higher temperatures, so the fuel cell voltage is higher at hightemperatures. Waste heat of higher temperature cells can be used. Fuel cells do not always have a higher efficiency limit than heat engines.The mentioned decline in maximum possible efficiency with temperature for the hydrogen fuel cellis not exactly occur for other cell types.2.4 Efficiency and the fuel cell voltageThe operation voltage of a fuel cell can be related to its efficiency.For 100% efficient hydrogen fuel cell, the enthalpy of formation is converted into electric energy.Considering the fact that not all fuel is used during operation, the fuel utilisation coefficient isintroduced.This is equivalent to the ratio of fuel cell current and the current that would be obtained if all thefuel were reacted. The fuel cell efficiency in respect to the HHV is therefore given byA good estimation for µ f is 0.95.
Page 1 and 2: -Fuel Cell Systems Explained-(Summa
Page 3 and 4: 1.5 Fuel cell typesBesides the prac
Page 5: Fuel cells can be used successfully
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 and 14: In order to get the current instead
Page 15 and 16: Assumption:Introduction a limiting
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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