Selected Fuel Cells for Cogeneration CHP Processes

Abstract

Before going into the details of cogeneration combined heat and power (CHP) processes, it is necessary to establish the operation principles and performance characteristics of the most common fuel cells for consideration in CHP processes. According to the phenomena governing the performance of fuel cells, it is worth noting that fuel cells are electrochemical energy conversion devices, where redox reactions occur spontaneously and the fuel and oxidant are consumed, and the electrochemical energy is transformed into electricity to produce work.

The thermodynamic reversible potentials and over-potential losses are the principal factors that control the net efficiency to convert chemical energy to electrical energy.

The fuel cell operating conditions depend on the electrochemical nature of the electricity production. Normally, activation over-potentials predominate at low current densities and they are also controlled by mass transport over-potentials at high current densities. Thus, the application of fuel cells in CHP processes implies a more detailed knowledge of the operation of every kind of fuel cell. Therefore, in this chapter, the fundamentals of the four typical fuel cells considered for CHP applications are explained, taking electrochemical operation principles and the consequent heat and electricity production into consideration. Generally, fuel cell classification is according to the type of electrolyte used and the operating temperatures.