Fuel Cell Efficiency

In contrast to heat engines (gasoline and diesel engines), the fuel cell does not involve conversion of heat to mechanical energy and the overall thermodynamic efficiencies can be very high.

The thermodynamic derivation of the Carnot cycle of a heat engine states that all the heat supplied to it cannot be converted to mechanical energy, and that some of the heat is rejected. The heat is accepted from a source at higher temperature (TH in Kelvin), part of it is converted to mechanical energy, and remainder is rejected into a heat sink at lower temperature (TSin Kelvin). The greater the temperature difference between the source and the sink, the greater the efficiency. The Carnot efficiency of a heat engine is given by Eq. (1). On the other hand, the fuel cell efficiency is related to the ratio of two thermodynamic properties, Gibbs free energy (AG0) and the total heat energy or Enthalpy (AH0) (Eq. (2)):

Maximum efficiency (Carnot), rCarnot = (TH - TS)/TH Fuel cell efficiency, r|Fuei ceii =

The theoretical thermodynamic efficiency of a hydrogen-oxygen fuel cell is —93% at ambient temperature. To achieve acceptable efficiencies, an internal combustion engine under ideal conditions must operate at a very high temperature. The variation of a hydrogen fuel cell theoretical efficiency versus the corresponding Carnot efficiency of a heat engine is shown in Figure 9.11.

The ambient temperature maximum thermodynamic intrinsic fuel cell efficiencies of different fuels can be very high. These data, along with reversible cell potentials of selected fuels, are listed in Table 9.2.

Fuel cells, therefore, are considered as very efficient electrical energy-producing devices with high power densities at relatively low temperatures. The possible applications of fuel cells are numerous, from micro fuel cells producing only a few

Figure 9.10 Fuel to energy conversions.
Maximum Theoretical Efficiency Pemfc
Figure 9.11 Theoretical efficiency change with temperature of a hydrogen fuel cell and

0 200 400 600 800 1000 1200 1400 a heat engine. °C

watts needed in cell phones, to on-board fuel cells for the automobile sector, and large units able to produce several MW to provide buildings with electricity. Major drawbacks to the widespread commercialization of fuel cells are mainly technological (reliability issues, material durability, catalyst utilization, mass transport,

Table 9.2 Theoretical reversible cell potentials (E rev) and maximum intrinsic efficiencies for candidate fuel cell reactions under standard condition at 25 °C.

Fuel

Reaction

n

-AH° (a)

rev

E [%]

Hydrogen

H2 + 0.5 O2 p H2O (l)

2

286.0

237.3

1.229

82.97

Methane

CH4 + 2 O2 p CO2 + H2O (l)

8

890.8

818.4

1.060

91.87

Methanol

CH3OH + 1.5 O2 p CO2 + 2H2O (l)

6

726.6

702.5

1.214

96.68

Formic acid

HCOOH + 0.5 O2 p CO2 + H2O (l)

2

270.3

285.5

1.480

105.62

Ammonia

NH3 + 0.75 O2 p 0.5 N2 + 1.5 H2O (l)

6

382.8

338.2

1.170

88.36

Proton Exchange Membrane Fuel Cell
Figure 9.12 The proton exchange membrane (PEM) hydrogen fuel cell.

etc.) and cost-related. Different types of fuel cells design exist, with some being more suited to certain applications than others. However, they all function on the same electrochemical principle. Fuel cells, in principle, can be built based on any exothermic chemical reaction.

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