8. Variable speed and fixed speed wind turbines

The difference between the variable speed and fixed speed wind turbines is whether the rotor is designed to run at different speed or constrained to move at a particular speed. Early wind turbine designs generally operated at constant speed. In this type, the rotor speed does not change regardless of wind speed changes. A converter of power electronic frequency is needed in order to link the variablefrequency output of the wind turbine to the constant electrical system frequency. Power electronics required for different speed wind turbines may be more costly, but they make up for the higher costs by spending more time than fixed turbines working at optimum aerodynamic efficiency [33]. A graph of the performance coefficient versus the tip speed ratio shows this difference clearly. Tip speed ratio is known as the ratio between the angular velocity of the blade tips of a turbine and the wind velocity as shown in Eq. (7). In wind turbine with fixed speed, ω is constant, corresponding to a specific wind speed. Hence for any other speed from the wind, the turbine efficiency is reduced.

The aim of the wind turbine with variable speed is to always run at optimal efficiency, with tip speed ratio consistency, corresponding to the maximum performance coefficient, by adapting the velocity of the blades to variations of wind speed. Therefore, wind turbines with variable speed designs are ideal for efficient power generation, regardless of the wind speed. Then again, as a result of the fixed speed operation for constant speed turbines, any variations in the speed of the wind are communicated as instabilities in the mechanical torque and then as instabilities in the electrical power grid [17]. This as well as an increased energy capture capability of the variable speed turbine makes the power electronics cost effective [33]. Therefore, wind turbines with variable-speed are more preferable.

10. Aerodynamic subsystem modeling

Simulation schematic diagram of the wind turbine model.

DOI: http://dx.doi.org/10.5772/intechopen.85064

has kinetic energy in the air given by [17] as:

air can be calculated with (Eq. (4)).

Block diagram of the aerodynamic model.

Figure 5.

53

shown in Figure 5.

Figure 4.

The wind turbine blades and its interaction with the wind make up the aerody-

The blades of a wind turbine rotate due to kinetic energy from the wind which is defined by the wind speed. An object with mass (m) which moves at velocity (v)

The power contained in the moving blades assuming constant velocity is equal to

the differential of this kinetic energy with respect to time as given in (Eq. (3)).

dt <sup>¼</sup> <sup>1</sup> 2

When the air crosses the area "A" brushed by blades of the rotor, the power in

� <sup>m</sup> � <sup>v</sup><sup>2</sup> (2)

� <sup>m</sup> � <sup>v</sup><sup>2</sup> (3)

� <sup>v</sup><sup>3</sup> � <sup>A</sup> � <sup>ρ</sup> (4)

namic subsystem to be modeled. The aerodynamic modeling block diagram is

Modeling and Simulation of a 10 kW Wind Energy in the Coastal Area of Southern Nigeria…

<sup>E</sup> <sup>¼</sup> <sup>1</sup> 2

Pw <sup>¼</sup> dE

Pw <sup>¼</sup> <sup>1</sup> 2

where m represents the mass flow rate per second.
