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Energies 2012 , 5 , 3425-3449; doi:10.3390/en5093425 energies ISSN 1996-1073 www.mdpi.com/journal/energies Review Wind Turbine Blade Design Peter J. Schubel * and Richard J. Crossley Faculty of Engineering, Division of Materials, Mechanics and Structures, University of Nottingham, University Park, Nottingham NG7 2RD, UK * Author to whom correspondence should be addressed; E-Mail: peter.schubel@nottingham.ac.uk; Tel.: +44-(0)-115-95-13979 . Received: 23 April 2012; in revised form: 21 June 2012 / Accepted: 30 August 2012 / Published: 6 September 2012 Abstract: A detailed review of the current state-of-art for wind turbine blade design is presented, including theoretical maximum efficiency, propulsion, practical efficiency, HAWT blade design, and blade loads. The review provides a complete picture of wind turbine blade design and shows the dominance of modern turbines almost exclusive use of horizontal axis rotors. The aerodynamic design principles for a modern wind turbine blade are detailed, including blade plan shape/quantity, aerofoil selection and optimal attack angles. A detailed review of design loads on wind turbine blades is offered, describing aerodynamic, gravitational, centrifugal, gyroscopic and operational conditions. Keywords: wind turbine; blade design; Betz limit; blade loads; aerodynamic 1. Introduction Power has been extracted from the wind over hundreds of years with historic designs, known as windmills, constructed from wood, cloth and stone for the purpose of pumping water or grinding corn. Historic designs, typically large, heavy and inefficient, were replaced in the 19th century by fossil fuel engines and the implementation of a nationally distributed power network. A greater understanding of aerodynamics and advances in materials, particularly polymers, has led to the return of wind energy extraction in the latter half of the 20th century. Wind power devices are now used to produce electricity, and commonly termed wind turbines. The orientation of the shaft and rotational axis determines the first classification of the wind turbine. A turbine with a shaft mounted horizontally parallel to the ground is known as a horizontal OPEN ACCESS Energies 2012 , 5 3426 axis wind turbine or (HAWT). A vertical axis wind turbine (VAWT) has its shaft normal to the ground (Figure 1). Figure 1. Alternative configurations for shaft and rotor orientation. Wind Direction Wind Direction VAWTHAWT The two configurations have instantly distinguishable rotor designs, each with its own favourable characteristics [1]. The discontinued mainstream development of the VAWT can be attributed to a low tip speed ratio and difficulty in controlling rotor speed. Difficulties in the starting of vertical turbines have also hampered development, believed until recently to be incapable of self-starting [2]. However, the VAWT requires no additional mechanism to face the wind and heavy generator equipment can be mounted on the ground, thus reducing tower loads. Therefore, the VAWT is not completely disregarded for future development. A novel V-shaped VAWT rotor design is currently under investigation which exploits these favourable attributes [3]. This design is currently unproven on a megawatt scale, requiring several years of development before it can be considered competitive. In addition to the problems associated with alternative designs, the popularity of the HAWT can be attributed to increased rotor control through pitch and yaw control. The HAWT has therefore emerged as the dominant design configuration, capitalised by all of today's leading large scale turbine manufacturers. 2. Theoretical Maximum Efficiency High rotor efficiency is desirable for increased wind energy extraction and should be maximised within the limits of affordable production. Energy ( P ) carried by moving air is expressed as a sum of its kinetic energy [Equation (1)]: ³21 AV P   VelocityAir VareaSwept A DensityAir   (1) A physical limit exists to the quantity of energy that can be extracted, which is independent of design. The energy extraction is maintained in a flow process through the reduction of kinetic energy and subsequent velocity of the wind. The magnitude of energy harnessed is a function of the reduction in air speed over the turbine. 100% extraction would imply zero final velocity and therefore zero flow. The zero flow scenario cannot be achieved hence all the winds kinetic energy may not be utilised. This principle is widely accepted [4,5] and indicates that wind turbine efficiency cannot exceed 59.3%. This Energies 2012 , 5 3427 parameter is commonly known as the power coefficient Cp , where max Cp = 0.593 referred to as the Betz limit [6]. The Betz theory assumes constant linear velocity. Therefore, any rotational forces such as wake rotation, turbulence caused by drag or vortex shedding (tip losses) will further reduce the maximum efficiency. Efficiency losses are generally reduced by:  Avoiding low tip speed ratios which increase wake rotation  Selecting aerofoils which have a high lift to drag ratio  Specialised tip geometries In depth explanation and analysis can be found in the literature [4,6]. 3. Propulsion The method of propulsion critically affects the maximum achievable efficiency of the rotor. Historically, the most commonly utilised method was drag, by utilising a sail faced normal to the wind, relying on the drag factor ( C d ) to produce a force in the direction of the prevailing wind. This method proved inefficient as the force and rotation of the sail correspond to the wind direction; therefore, the relative velocity of the wind is reduced as rotor speed increases (Table 1). Table 1. The two mechanisms of propulsion compared. Propulsion Drag Lift Diagram Relative Wind Velocity WindvelocityBladevelocity   2 2 ()?3 WindvelocityBladevelocitydr   Maximum Theoretical Efficiency 16% [4] 50% [6] Energies 2012 , 5 3428 Reducing efficiency further is the drag of the returning sail into the wind, which was often shielded from the oncoming wind. Unshielded designs rely on curved blade shapes which have a lower drag coefficient when returning into the wind and are advantageous as they work in any wind direction. These differential drag rotors can be seen in use today on cup anemometers and ventilation cowls. However, they are inefficient power producers as their tip speed ratio cannot exceed one [4]. An alternative method of propulsion is the use of aerodynamic lift (Table 1), which was utilised without precise theoretical explanation for over 700 years in windmills then later in vintage aircraft. Today, due to its difficult mathematical analysis, aerodynamics has become a subject of its own. Multiple theories have emerged of increasing complexity explaining how lift force is generated and predicted. Aerodynamic force is the integrated effect of the pressure and skin friction caused by the flow of air over the aerofoil surface [7]. Attributed to the resultant force caused by the redirection of air over the aerofoil known as downwash [8]. Most importantly for wind turbine rotors, aerodynamic lift can be generated at a narrow corridor of varying angles normal to the wind direction. This indicates no decrease in relative wind velocity at any rotor speed (Table 1). For a lift driven rotor (Table 1) the relative velocity at which air strikes the blade (W) is a function of the blade velocity at the radius under consideration and approximately two thirds of the wind velocity (Betz theory Section 2) [4]. The relative airflow arrives at the blade with an angle of incidence ( β ) dependant on these velocities. The angle between the blade and the incidence angle is known as the angle of attack ( α ). 4. Practical Efficiency In practice rotor designs suffer from the accumulation of minor losses resulting from:  Tip losses  Wake effects  Drive train efficiency losses  Blade shape simplification losses Therefore, the maximum theoretical efficiency has yet to be achieved [9]. Over the centuries many types of design have emerged, and some of the more distinguishable are listed in Table 2. The earliest designs, Persian windmills, utilised drag by means of sails made from wood and cloth. These Persian windmills were principally similar to their modern counterpart the Savonius rotor (No. 1) which can be seen in use today in ventilation cowls and rotating advertising signs. Similar in principle is the cup type differential drag rotor (No. 2), utilised today by anemometers for calculating airspeed due to their ease of calibration and multidirectional operation. The American farm windmill (No. 3) is an early example of a high torque lift driven rotor with a high degree of solidity, still in use today for water pumping applications. The Dutch windmill (No. 4) is another example of an early lift type device utilised for grinding corn which has now disappeared from mainstream use, yet a small number still survive as tourist attractions. The Darrieus VAWT (No. 5) is a modern aerodynamic aerofoil blade design which despite extensive research and development has so far been unable to compete with the modern HAWT design, although recent developments [2,3] could see a resurgence of this rotor type. Due to its efficiency and ease of control, the aerofoil three bladed HAWT (No. 6) has become the wind
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