Aerodynamics of Wind Turbines

It is some sort of tragedy that many get caught up in the idea of generating power from the wind and attempt to build machines before they have mastered the disciplines required.

D. M. Eggleston & F. S. Stoddard (1987)

13.1 Introduction

This chapter reviews some basic concepts associated with wind turbines and de­scribes their general operation and performance, focusing primarily on the aerodynamic issues. The overlap of fundamental problem areas between the helicopter rotor and the wind turbine is considerable. Clearly, the extraction of power from the wind using a tur­bine depends on its efficient performance and aerodynamic design, and in many ways the underlying principles of efficient design parallel those for helicopter rotors. In this respect, specialist engineers in both helicopters and wind energy can benefit from an appreciation of the particular technical issues found on both types of machines. This chapter, however, is not meant to be an all-encompassing treatise on wind energy or the aerodynamic principles of wind turbines. For this, the reader is referred to specialist books such as Walker & Jenkins

(1997) , Spera (1994), Burton et al. (2001), and Manwell et al. (2002).

While complicated in detail, the basic principle of a wind turbine is fairly simple. The wind blows through the turbine and turns the blades, converting the kinetic energy of the wind into rotational energy of the turbine. This rotational energy can be harnessed to produce useful work, usually in the form of electric power from a generator connected to the turbine shaft. The classic aerodynamic analysis of a rotor operating as a wind turbine was developed by Lock et al. (1925) [see review by Glauert (1935, 1983)]. Problems of the wind turbine that are common to the helicopter include understanding and predicting the unsteady blade airloads and turbine performance in both attached and stalled flows, as well as predicting the resulting structural loads and aeroelastic response of the rotating blades. However, there are unique differences as well. Wind turbines are subjected to complicated environmental effects that are not found on helicopter rotors. This includes ground boundary layer effects, atmospheric turbulence and large turbulent eddies, temporal and spatial variations in wind shear, thermal convection or stratifications, the possible effects of an upstream unsteady wake from a support structure (tower shadow) or even the effects of another wind turbine if situated in groups. The net effect is that wind turbines operate in an adverse, 3-D, unsteady (aperiodic) aerodynamic environment that is both hard to define using measurements and also to predict using mathematical models. This has led to difficulties in designing wind turbines that can operate reliably and economically over long periods of time.

The aerodynamic analysis of a wind turbine can be approached using classical momentum theory, the principles of which have been previously introduced in Chapters 2 and 3 for the helicopter. Although the helicopter rotor thrust can be defined uniquely a priori and the

momentum theory then used directly to determine the induced velocity, it will be shown that the classical momentum theory allows only the maximum (ideal) performance limits to be defined for wind turbines. This is rectified to a large extent by the use of blade element momentum theories and blade element methods coupled with inflow models. These types of models have formed the mainstay of the predictive methods used by the wind energy community. The blade element momentum theory allows us to examine the effects of the primary design variables (blade twist, blade planform, number of blades, etc.) as a function of wind speed and blade pitch on energy extraction. However, there are several important nonideal and nonlinear aerodynamic effects to consider that have important effects on wind turbine performance, including the effects of airfoil section. The blade loads and the performance of a wind turbine is directly determined by aerodynamic forces generated on the airfoils. Therefore, a better understanding of the underlying flow physics on the blades is essential if accurate modeling of the turbine aerodynamics and acceptable predictions of the loads and power output are to be made. In this regard, airfoil design philosophy used for wind turbines can be quite different to that used to design helicopter rotors.

The second part of this chapter focuses on more advanced topics associated with wind turbines, including vortex wake theories and unsteady aerodynamic effects. While these top­ics are also common to helicopter rotors (see Chapter 8, 9, and 10), there are unique aspects of the aerodynamic problems associated with wind turbines that require special attention. These include the presence of unsteady variations in wind speed, yawed flow operation and tower wake or “shadow” effects. While all of these problems have been addressed by wind energy analysts to a lesser or greater extent, it seems that many of the resulting models are rooted deeply in empiricism and often do not offer today’s analysts the flexibility needed to embark confidently on new and improved wind turbine designs. This problem can be rectified only by further research at a fundamental level using both experimental and analyti­cal approaches and also by careful and systematic validation against measurements of the mathematical models used to predict wind turbine loads and performance. In this regard, additional high-quality measurements in both the wind tunnel and in the field are sorely required.