The Conversion of Renewable Energy into Electrical Form

Renewable energy is available in a variety of forms. Uniquely, biomass is available in the form of combustible fuel and thus can play a similar role in generation as conventional fossil fuels. To generate electricity, all other renewables require a number of conversion stages that differ from those found in systems based on traditional fuels.

Wind energy is available in kinetic form. The function of a wind turbine is to extract energy from the intercepted wind by slowing it down and to convert this energy into a mechanical form that suits an electric generator. To improve the efficiency of conversion and for other operational reasons, the generator may be interfaced to the mains through a power electronic converter.

• Water power in the form of tides (potential) or water flow (kinetic) requires a turbine to transform this energy into rotational form for further conversion into electricity, again through a generator.

• The kinetic energy in water currents caused by tidal effects can alternatively be captured through an underwater 'wind turbine' that uses the same technology as those on land.

• Wave energy conversion requires specially designed devices that transform the low frequency energy in the waves into (usually) pressure energy in oil, which in turn drives an electrical generator. In other wave energy concepts the rise and fall of waves drives air through a turbine coupled to an electrical generator.

Solar energy is available as radiation ranging from ultraviolet to infrared. Conversion into electricity can be implemented thermally by solar furnaces that raise steam to drive conventional turbines or through solid state photovoltaic devices that utilize the radiation to separate charges in semiconductor junctions. The efficient operation of PV based systems depends on interfacing the PV array to the grid through a power electronic converter.

Renewable Energy in Power Systems Leon Freris and David Infield © 2008 John Wiley & Sons, Ltd

This summary indicates that all renewables, except PV systems, rely on electromechanical generators for the final stage of conversion from mechanical into electrical energy. This chapter introduces the principles of operation of two classes of electrical generators, the 'synchronous' and the 'asynchronous' types, both used extensively in RE applications. Additionally, this chapter deals briefly with the principle of operation of the transformer, a ubiquitous device in multivoltage level power systems. Understanding the operation of the transformer is a necessary prerequisite for the study of the 'asynchronous' type of generator.

Power electronics plays a vital role in PV and an increasingly important role in the wind power area. A review of power electronic devices and the converters based on them is covered in the penultimate section. Finally, the chapter concludes with a description of how electromechanical and/or power electronic converters are used in PV and wind systems.

In what follows a symbol written in regular type indicates that the parameter is a scalar while bold type is used if it is a vector, phasor or a complex number.

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