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This is the most compact, efficient and easy to control method. A fine mist is injected in the gas stream using a small pump. An additional heat source is usually required to produce complete water evaporation if hot water enthalpy solely does not suffice. Moisture level is directly controlled by varying the amount of vapour injected [4]. 6 Available PEM Fuel Cell Models in the Open Literature 27 • Water exchange through a permeable material. In this approach, gases are circulated on one side of a permeable membrane such as Nafion® .

Embedding the control objectives into the control function that gives rise to such manifold), it is possible to achieve conventional control goals such as global stability, optimisation, tracking, regulation, etc. In the sequel, the basics of the theory of classical sliding-mode control are introduced, focusing on Single-Input Single-Output (SISO) systems. Note that in most sections of this chapter, the possible explicit dependence on time of the dynamical system has been omitted for the sake of clarity and economy of notation.

In this approach, fuel cells are used for low power demands, while battery banks supply additional energy for higher power requirements, when the cell tends to reduce its output voltage. In such cases power References 31 converters can also be used to improve efficiency and extend operating ranges of the stack. For instance, in [36] resonant soft-switching techniques are used to adjust the output voltage of a 250-W PEM fuel cell. Most of the above approaches are based on linear models, providing interesting solutions that efficiently solve different control objectives [10, 27, 39].