Page 313 - Renewable Energy Devices and System with Simulations in MATLAB and ANSYS

300 Renewable Energy Devices and Systems with Simulations in MATLAB and ANSYS ®
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TABLE 12.2
Enthalpy H, Entropy S, and Gibbs Free Energy G of
Various Substances at 1 atm and 25 °C
Substance State H (kJ/mol) S (kJ/mol K) G (kJ/mol)
Gas 0 0.13 0
H 2
Gas 0 0.205 0
O 2
Gas −74.9 0.186 −50.8
CH 4
CH 3 OH Liquid −238.7 0.1268 −166.4
H 2 O Gas −241.8 0.1888 −228.6
H 2 O Liquid −285.8 0.0699 −237.2
Gas −393.5 0.213 −394.4
CO 2

Fuel cell efficiency η can be calculated by evaluating the ratio of the Gibbs free energy ΔG to the
enthalpy change ΔH in the chemical reaction:

∆G
η= (12.10)
∆H
The change in enthalpy in a chemical reaction can be calculated by

∆H = ΣH reactants − (12.11)
ΣH products

where
∑H products is the sum of the enthalpies of the products as a result of chemical reaction
∑H reactants is the sum of the enthalpies of the reactants participating in the chemical reaction

Table 12.2 shows the enthalpy, entropy, and Gibbs free energy at 1 atm and 25 °C for a few sub-
stances. The values in Table 12.2 can be utilized based on the state of the reactants and the products
to calculate Gibbs free energy or electrical energy output ΔG, enthalpy change due to chemical reac-
tion ΔH, net entropy ΔS, heat liberated Q at an absolute temperature T, and theoretical efficiency η
of the fuel cells.

12.5 FUEL CELLS FOR TRANSPORTATION
Transportation electrification is growing to meet the challenges of reducing emissions and meet-
ing the increasing power demand of electrical loads in automobiles. Fuel cell vehicles (FCVs) are
promising owing to their merits of high efficiency and low or zero emissions as compared to con-
ventional internal combustion engine vehicles. FCVs offer the benefits of zero emission without the
charging time and driving range limitations of electric vehicles (EVs). Automotive industries like
Honda, Toyota, GM, Ford, and Kia are all developing their FCVs. Fuel cells are being considered
for providing the propulsion power to the vehicle and also for onboard power generation to power
the electrical loads in the vehicle.
Because of a short start-up time, relatively low operating temperature, and faster response,
PEMFCs are considered mainly for vehicle propulsion applications. In the past, fuel cells were
mainly considered for the propulsion applications. But recently, fuel cells are being investi-
gated as APU for onboard power generation to supply power to the accessory loads for either
engine-on or engine-off conditions. For APU, either the PEMFC or the high-temperature SOFC
could be used.