Lightweight Electric/Hybrid Vehicle Design
Lightweight Electric/Hybrid Vehicle Design
Lightweight Electric/Hybrid Vehicle Design
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Process engineering and control of fuel cells, prospects for EV packages 85<br />
fuel and material restrictions limit the practical efficiency to about 50%, which is achieved by<br />
modern, large, low speed diesel engines, but automotive gasoline and diesel engines achieve much<br />
lower efficiencies, particularly when averaged over a standard driving cycle.<br />
A true direct energy conversion device is one which can convert the Gibbs free energy of a<br />
chemical reaction directly into work. A fuel cell converts the Gibbs free energy of a chemical<br />
reaction into a stream of electrons under isothermal conditions. The change in Gibbs free energy<br />
of a reaction is given by:<br />
ΔG r = H r − TS r<br />
Fuel-cell reactions which have negative entropy change (e.g. H 2 + (1/2)O 2 = H 2 O) generate heat<br />
and those with positive entropy change (e.g. C 2 H 6 + 3.5 O 2 = 2CO 2 + 3H 2 O and CH 3 OH + 1.5O 2 =<br />
CO 2 + 2H 2 O) extract heat from the surroundings. For a fuel cell operating at constant temperature<br />
and pressure, the maximum electrical energy is given by the change in Gibbs free energy:<br />
W el = −ΔG = nFE (1)<br />
Where n = the number of electrons in the reaction, F = Faraday’s constant (96 500° C/equivalent)<br />
and E = the reversible potential. If all reactants are at standard conditions of 1 atm and 25 o C:<br />
For the reaction<br />
ΔG o = −nFE o (2)<br />
H 2 (g) + (1/2)O 2 (g) = H 2 O(l)<br />
the Gibbs free energy change 12 is −237 kJ, n = 2, and therefore the maximum reversible potential,<br />
E o = 1.23 V. The maximum reversible potential under actual fuel-cell operating conditions can be<br />
calculated from the Nernst equation. For the general reaction:<br />
aA + bB = cC + dD<br />
The free energy change can be expressed:<br />
ΔG = ΔG o + RT ln([C] c [D] d /[A] a [B] b )<br />
Substituting equations (1) and (2) gives:<br />
E = E o + (RT/nF) ln([A] a [B] b /[C] c [D] d<br />
For the hydrogen/oxygen fuel cell this can be simplified 1 to:<br />
E = Eo 1/2 + (RT/2F) ln[PH2/PH O] + (RT/2F) ln[PO ]<br />
2 2<br />
Normal practice for conventional power generation is to use the thermal efficiency, expressed<br />
as the electrical output as a percentage of the heat of combustion of the fuel. It is common practice<br />
in Europe to use the lower heating value (LHV) or lower calorific value (LCV) (water as gas),<br />
whereas in the United States it is common practice to use the higher heating value (HHV) or Gross<br />
Calorific Value (GCV) (water as liquid). The heat of combustion is equal to −ΔH, the change in<br />
enthalpy. The thermal efficiency of a fuel cell is given by:<br />
Gibbs free energy converted to electricity<br />
Thermal efficiency = Enthalpy change ( −heat of combustion)