Page 76 - High Temperature Solid Oxide Fuel Cells Fundamentals, Design and Applications

Chapter 3

Thermodynamics

Wolfgang Winkler

3.1 Introduction

A solid oxide fuel cell (SOFC) is an electrochemical device that converts chemical
energy of a fuel and an oxidant gas (air) directly into electricity without
irreversible oxidation. It can be treated thermodynamically in terms of the free
enthalpy of the reaction of the fuel with oxidant. Hydrogen and oxygen are used
to illustrate the simplest case in the early part (Section 3.2) of this chapter. This
treatment allows the calculation of the reversible work at equilibrium for the
reversible reaction. Heat must also be transferred reversibly to the surrounding
environment in this instance.
During operation of a SOFC, described in Section 3.3, two effects intervene to
reduce the electrical power available from an ideal cell; the first is ohmic
resistance which generates heat; the second is the irreversible mixing of gases
which causes the voltage to fall as progressively more fuel is used in the reaction.
Essentially, this means that a SOFC cannot realistically use all the fuel. Some
fuel, typically about lo%, must be left in the spent fuel stream which exits from
the cell.
The losses in SOFC appear as heat, so it is necessary to consider a
SOFC system as a heat generator as well as an electricity source. In effect,
the whole SOFC system can be treated as a power generating burner, as in
Section 3.4.
In a real engineering device, heat is exchanged within the SOFC in several
ways including fuel processing, air preheating, flue gas cooling, etc. Excess air is
normally required to prevent overheating, while the conversion of hydrocarbons
into hydrogen and carbon monoxide often absorbs heat. The complex heat
pathways are described in Section 3,s.
Ultimately, the heat output from the SOFC can be used to drive a heat engine
such as a piston engine or gas turbine. These combined SOFC/heat engine cycles
are analysed in Section 3.6.
First it is essential to list the symbols and concepts used in this chapter.