## 5 HAWT Blade Design
### Aerodynamics  
Aerodynamic performance is fundamental for efficient rotor design [19]. Aerodynamic lift is the force responsible for the power yield generated by the turbine and it is therefore essential to maximise this force using appropriate design. A resistant drag force which opposes the motion of the blade is also generated by friction which must be minimised. It is then apparent that an aerofoil section with a high lift to drag ratio [Equation (4)], typically greater than 30 [20], be chosen for rotor blade design [19]:  
\[\mathit{Lift~{}to~{}Drag~{}Ratio}=\frac{\mathit{Coefficient~{}of~{}lift}}{ \mathit{Coefficient~{}of~{}drag}}=\frac{C_{{}_{L}}}{C_{{}_{D}}} \tag{4}\]  
The co-efficient for the lift and drag of aerofoils is difficult to predict mathematically, although freely available software, such as XFOIL [21] model results accurately with the exception of post stall, excessive angles of attack and aerofoil thickness conditions [22, 23]. Traditionally aerofoils are tested experimentally with tables correlating lift and drag at given angles of attack and Reynolds numbers [24]. Historically wind turbine aerofoil designs have been borrowed from aircraft technologies with similar Reynolds numbers and section thicknesses suitable for conditions at the blade tip. However, special considerations should be made for the design of wind turbine specific aerofoil profiles due to the differences in operating conditions and mechanical loads. \begin{table}
\begin{tabular}{l l} \hline
**2.0 MW Generator** & \\ \hline _Type_ & Doubly fed machine \\ _Voltage_ & 690 V ac \\ _Frequency_ & 50 Hz \\ _Rotational speed_ & 900–1900 \\ _Stator current_ & 1500A _@_ 690v \\ \hline
**Mechanical Design** & \\ \hline Drive train with main shaft supported by two spherical bearings that transmit the side loads directly onto the frame by means of the bearing housing. This prevents the gearbox from receiving additional loads. Reducing and facilitating its service. \\ \hline
**Brake** & \\ \hline Full feathering aerodynamic braking with a secondary hydraulic disc brake for emergency use. \\ \hline
**Lightening Protection** & \\ \hline In accordance with IEC 61024-1. Conductors direct lightening from both sides of the blade tip down to the root joint and from there across the nacelle and tower structure to the grounding system located in the foundations. As a result, the blade and sensitive electrical components are protected. \\ \hline
**Control System** & \\ \hline The generator is a doubly fed machine (DFM), whose speed and power is controlled through IGBT converters and pulse width modulation (PWM) electronic control. Real time operation and remote control of turbines, meteorological mast and substation is facilitated via satellite-terrestrial network. TCP/IP architecture with a web interface. A predictive maintenance system is in place for the early detection of potential deterioration or malfunctions in the wind turbines main components. \\ \hline \end{tabular}
\end{table} 
Table 5: \(Cont.\)The effects of soiling have not been considered by aircraft aerofoils as they generally fly at altitudes where insects and other particulates are negligible. Turbines operate for long periods at ground level where insect and dust particulate build up is problematic. This build up known as fouling can have detrimental effects on the lift generated. Provision is therefore made for the reduced sensitivity to fouling of wind turbine specific aerofoil designs [25]. The structural requirements of turbine blades signify that aerofoils with a high thickness to chord ratio be used in the root region. Such aerofoils are rarely used in the aerospace industry. Thick aerofoil sections generally have a lower lift to drag ratio. Special consideration is therefore made for increasing the lift of thick aerofoil sections for use in wind turbine blade designs [25, 26]. National Advisory Committee for Aeronautics (NACA) four and five digit designs have been used for early modern wind turbines [1].