Thermodynamic of Fuel Cells

The first law of thermodynamics
“Energy may neither be created nor be destroyed,
but may be converted from one form to another.”
dE = ?Q - ?W (1)
Upon Integration
?E = Q – W (2)
E - system energy
Q and W - heat input into the system and the work
done by the system respectively

Types of systems
Open system:
Permits both mass and energy transfer
Closed system:
Permits only energy transfer
Isolated system:
Permits neither mass nor energy transfer
through its boundaries.

A fuel cell is an open system,
which permits the flow of mass
and energy through its boundaries.

Open systems
Energy change equation for an open system
?E = ?U + ?KE + ?PE + ? (PV) (3)
U - internal energy of the system
KE, PE - kinetic & potential energies
PV - pressure-volume work

Enthalpy
Enthalpy (H) combines the internal energy
and the PV work terms
H = U + PV (4)
Enthalpy – measure of total system energy

Energy change in an open system
Combining equations 2, 3 and 4:
?H = Q – W (5)
valid only for steady flow conditions, where
?KE and ?PE = 0