Fuel Cell Exercises

Exercise II

Fuel Cell Exercises

Consider a PEMFC producing 7 kW power at 0.7 V.For a pressure of 1 bar, at 298 K (G = - 237.2 kJ/mol) for the above fuel cell running on pure hydrogen and air (21mol% O2). Also consider the = 1.0 (fuel utilization) and = 0.2 (Oxygen utilization).

Calculate:a) Oxygen required for the reactionb) Hydrogen consumedc) Water producedd) Outlet oxygen flow and flow composition.e) Equivalent cell resistance for a current density of 2500 A/m2f) N of cells required if each cell has an equivalent area of 400 cm2

Exercise II

Fuel Cell Exercises

Exercise II

Mass of reacting species

=

= 2for, = 4

Fuel Cell Exercises

Exercise II

Solution:Calculate the current,

! = " = !

"=7000

0.7= 10%&

And then the molar flow,

)(),* +,- =./0

%12%34= 0.026/ (this flow is only the oxygen that reacts with

hydrogen to form water steam)

7)8),* +,- =10%

2 96485= 2) = 0.052/

=10%

2 96485= 8) = 0.052/

Fuel Cell Exercises

Exercise II

Solution:

) = 0. 2, that means 80% of the oxygen exits the without reacting,

(),?-! - = 0.8 (),AB! - = 0.8

(),CDEFG

0.2= 0.104/

Flow composition in outlet of cathode: Mix of water, nitrogen and unreacted oxygen.Water steam flow calculated in c) and oxygen calculated above.Nytrogen (assuming air composed by 79% N2 and 21% O2) :

H) = 0.79 +A*,AB! - = 0.79

(),AB! -

0.21= 0.79

(),* +,-

0.2 0.21= 0.49/

Outlet composition: 75.85% N2; 16.1% O2; 8.05% H2O

Fuel Cell Exercises

Exercise II

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Nernst voltage and Gibbs free energy

"B *BI- = L/

+N O

P8 P(/.4

P8(

Q

Q/

/.4

Where,

P8)

Fuel Cell Exercises

Exercise II

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Voltage dependence

" = "-d *e "deA, "+,-Af+-AB ",B, B-*+-AB

Where," VcR

Fuel Cell Exercises

Exercise II

Solution:e) First we need to determined (if needed) the concentration of the species at inlet of the fuel cell

P8)( = 0.05 (good approximation if no water is suppied)P() = 0.21

P8) = 1

Calculation of the nernst voltage"* f =

jkl//

12%34+

3.k.%13

12%34

./..m.n

/./4

../.k

../.k

/.4

= 1.257"

(note: in our calculations if no information about the OCV is given, the nernst potential should be used instead)

And then the equivalent resistance,

N i?Af ="* f "

U=1.257 0.7

2500= 2.23 10j%`

Fuel Cell Exercises

Exercise II

Solution:

f) Calculation of the total area of the fuel cell

&--+! =OS

Fuel Cell Exercises

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! = " = "

Maximum work that can be obtained is Gibbs free energy

s = "

" = s/

Exercise II