Page 92 - High Temperature Solid Oxide Fuel Cells Fundamentals, Design and Applications

Thermodynamics 69

the mass flow of the utilised fuel and of the transferred oxygen by the ion
conduction through the electrolyte. The stoichiometric demand on oxygen
related on the inlet fuel mass flow is given by the figure p020. We finally get for
the enthalpy flow at the anode outlet

fiAn0 = hFI . [(I - uf) * (LHv + h:AnO) + uf ' (1 + PO20) ' hiGAnO]* (64)
The enthalpy flow at the cathode outlet can be calculated by the difference of
the enthalpy flow of the non-depleted air and the enthalpy flow of the oxygen
being transferred to the anode both with the thermodynamic state at the cathode
outlet

Equations (5 8) and (63) yield with Eq. (6 5)

fiCu0 = mFI. [A ' PA0 * hicUo - uf ' PO20 ' hE2cuo]

The specific generated heat qFCresults from Eqs. (56)-(66) as

The use of Eq. (6 7) depends on a constant excess air h as probably regulated by
a oxidation control by an O2 measurement after the burner. The necessary excess
air h to cool the cell for a fixed heat extraction qFC can be calculated by
rearranging Eq. (6 7) as

3.5 Thermodynamic Theory of SOFC Hybrid Systems
The process environment of the cell model in Figure 3.1 must be related
reversibly to the ambient state to define the reversible system. As mentioned
above we assume U, ---t 0 and the flows consist of unmixed components to
assure a reversible process. Figure 3.8 shows the reversible fuel cell-heat
engine system that fulfills these requirements. The reactants air and fuel in
the ambient state TO,po are brought to the thermodynamic state of the cell T,p
by the reversible heat pumps HPA (for air) and HPF (for fuel). The necessary