Introductory Chapter: General Design Aspects of Horizontal-Axis Wind Turbines

1. Introduction

Numerous research contributions in developing wind industry technologies worldwide have been initiated since the oil crisis in 1973, and various configurations of wind turbines and large-scale wind farms have been installed in many places. These clean energy sources can make a substantial and economically competitive contribution to the future energy needs.

Irrespective of the specific application, a wind turbine system design should be based on the cost-effective production of energy. The main objective should be based on the minimum cost of energy depending on the rotor diameter, rated power as well as the wind characteristics for a given site. The economic feasibility of large-scale wind turbines operated as a part of electrical power systems has been considered by H.M. Bae [1]. In this paper, the design variables were taken to be the rotor diameter, rated power, and number of the installed machines. Maximization of the total net value of the generated power, which is equal to the annual expected fuel cost savings minus the total cost of the system, was taken as the main system objective. Power was considered as constraint rather than design objective. Hansen [2] addressed optimum blade shapes for maximizing the power coefficient of the rotor. He presented a method to obtain the optimum blade chord and twist distributions for better aerodynamic performance. Another important consideration in the design of wind energy generator systems is to reduce vibration without increasing structural weight. This is because the economics require that large wind turbines operate reliably for long periods of time while subject to significant vibratory loads [3, 4].

In this chapter, the wind turbine will be analyzed as a system in order to build a general model for its structural design optimization. The most significant design objectives as well as design environment and constraints are defined and measured. All effective system design variables and parameters are identified and discussed. Several design alternatives will be considered to see how the various design criteria are affected in each case.

2. System definition and main function

A wind turbine can be defined as a device that converts the wind’s kinetic energy into useful mechanical power. This produced power can be exploited in many applications such as:

1. Corn grinding and wood sawing.

2. Battery charging.

3. Water pumping (e.g. irrigation and agricultural purposes).

4. Domestic use (e.g. heating and illumination).

5. Electricity producing (wind/diesel systems, wind farm, and utility operation).

The present chapter focuses on horizontal-axis wind turbines (HAWTs) utilized for electricity generation.

3. System components and subcomponents

To carry out its intended function, a wind turbine system must have the following main subsystems:

1. Momentum exchange device (main wind rotor): This consists of rotating aerodynamical surfaces, called the blades that provide the main driving aerodynamic forces. The blades are mounted on a rotting hub/shaft assembly.

2. Power transmission mechanism (power train unit):

   This is composed of the following subcomponents:

   • Shaft/bearing assembly

   • Speed-up mechanism (gearbox)

   • Braking system

   • Energy utilizer (generator)

3. Control system for changing the blade setting angle to limit the turbine output at high wind speed and for yawing the rotor so that it may face the wind properly.

4. Casing unit (Nacelle) for housing the power train and control units. It is the interfacing device that connects these units together with the rotor and the supporting tower structure.

5. Tower, which supports the above units and elevates the rotor above the earth’s boundary layer.

6. Foundation, which provides firm fixation of the system to the ground.

A typical horizontal-axis wind turbine system is shown in Figure 1.

Figure 1.

Horizontal-axis wind turbine system components.

4. Design variables

The definition of wind turbine variables and parameters is of great importance in formulating a design optimization model. Actually, a wind turbine is a complicated network composed of thousands of interconnected elements. However, a breakdown of the system may help identify the most important design variables for each subsystem.

The following list shows the main design variables of wind turbine subsystems, with emphasis on variables related to the blade and tower structures.

4.1 Rotor variables

1. Main variables

   • Diameter (rotor size)

   • Location with respect to tower (e.g., upwind or downwind rotor)

   • Number of blades (one, two, or three)

2. Blade variables

   • General layout (length – chord and twist distributions – coning angle).

   • Cross section (airfoil type – construction).

   • Blade-Hub connection (semi-articulated – fully articulated – hingeless).

   • Material of construction (aluminum – steel – fiberglass composites, etc.).

3. Rotor support (hub/shaft) variables

   • Type of hub (fixed – teetered)

   • Type of bearings

   • Shaft construction and dimensions

Type of mounting of rotor shaft onto gearbox.

4.2 Power transmission variables

1. Gearbox

   • Type (e.g., parallel shaft – planetary – …).

   • Size and type of casing

   • Gear ratio

2. Generator

   • Type (induction – synchronous – permanent magnet – …..).

   • Mechanical and performance characteristics (torque-speed relation, rated power, …..)

4.3 Control system variables

1. Rotor yaw control

2. Blade pitch control

4.4 Nacelle variables

Configuration – dimensions – construction – mounting – material

4.5 Tower variables

1. Type (truss – tubular – guyed pole)

2. Layout parameters (height – width)

3. Type of cross section

4. Material of construction (steel – concrete – …)

5. System objectives

A successful wind turbine design should ensure efficient, safe, and economic operation of the machine. It should provide easy access for maintenance and easy transportation and erection of the system components and subcomponents. Good designs should incorporate esthetic features of the overall machine shape. In fact, there are no simple criteria for measuring the above set of objectives. However, it should be recognized that the success of structural design ought to be judged by the extent to which the wind turbine main function is achieved.

5.1 Cost of energy production

The effectiveness of the design should be based on the end-product economics; i.e., the cost of energy produced. This may be expressed on an annual basis as:

$$Minimize;\text{Unit energy cost}=\frac{\text{Total annual cost}}{\text{Annual energy produced}}\left(\text{\$}/\mathrm{Kw}.\mathrm{h}\right)$$E1

In Ref. [5], it was demonstrated that designs of large wind turbines are projected to be cost competitive for utility applications when produced in quantity. The cost of electricity produced can be decreased when operated at sites with a mean annual wind speed of about 6.5 m/s at 10 m height.

5.1.1 Annual cost

The main cost items of a wind turbine are incurred in the following major stages:

1. Initial capital cost

2. Operation, maintenance, and repair costs

3. Other cost items

A breakdown of cost components of each stage is shown in Figure 2.

Figure 2.

Main cost items of a wind turbine.

Capital cost analysis depends on the development of statistical cost estimates, which relate the various design parameters and variables of the turbine to its total capital cost or its subsystem costs. The most significant design variables that have a bearing on wind turbine system costs are:

1. Rotor size

2. Rated power

3. Rated wind speed

4. Expected service life of the machine

5. Quantity of production

6. Type and material of construction of the various components

7. Degree of utilization of the machine

8. Type of generator and power transmission systems

For large-scale machines, Figure 3, taken from Ref. [1], shows typical machine cost as a function of rated power for different rated wind speeds and rotor sizes. The curves were determined by interpolating statistical cost estimates and shown on a logarithmic scale.

Figure 3.

Wind turbine machine cost as a function of rated power [1].

The initial capital is transferred to annual rates by multiplying with annualization factor (charge rate), which depends on the interest rates and machine life. Operation and maintenance costs are usually given as a fraction of the total capital. They are greatly influenced by how easy it is to exchange components for maintenance and repair.

5.1.2 Annual energy productivity

The annual energy yield of a wind turbine is readily defined as the total number of kilowatt hours (Kwh) actually produced by the machine installation in a year (8760 hours). It depends very much on the site wind characteristics and machine performance characteristics. W. R. Powell [6] derived the following expression for the annual energy, E:

$$\mathrm{E}=8.760{\mathrm{P}}_{\mathrm{r}}\left[\frac{\mathrm{Exp}\left(-{\widehat{\mathrm{v}}}_{\text{in}}^2\right)-\mathrm{Exp}\left(-{\widehat{\mathrm{v}}}_{\mathrm{r}}^2\right)}{{\widehat{\mathrm{v}}}_{\mathrm{r}}^2-{\widehat{\mathrm{v}}}_{\text{in}}^2}-\mathrm{Exp}\left(-{\widehat{\mathrm{v}}}_{\mathrm{ou}}^2\right)\right]\ast \text{availability factor}\left(\mathrm{Kwh}\right)$$E2

where P_r is the rated power, and the term between brackets is called the capacity factor, which is given by the ratio of the average output power to the maximum rated power. All wind speed terms are described in a non-dimensional form, $\widehat{\mathrm{V}}=\frac{\mathrm{V}}{\overline{\mathrm{V}}}\sqrt{\frac{\mathrm{\pi }}{4}},$ and are defined as:

V_in: Cut-in speed at which the machine starts to develop power

V_out: Cut-out speed at which the machine shuts down in high winds

$\overline{\mathrm{V}}$: Mean wind speed in a year

The availability factor accounts for the availability of the wind turbine for service in the period in which the wind speed is in its operating range. Powell’s expression was based on a Rayleigh wind distribution and a quadratic power-speed curve.

In general, wind machines with higher rated to cut-in speed ratios can both produce more energy and have higher capacity factors, but, unfortunately, they cost more. The selection of optimum rated to mean wind speed ratio is also a compromise. High capacity factors are available at low rated speeds, but less energy will be produced. The rated speed depends on the specific load application and rotor size, while the cut-in speed depends on the mechanical and power transmission system design. Variable pitch machines can adjust the blade angles to the wind in order to capture more energy over a wide range of wind speeds. However, cost will be incurred in the needed control systems.

The maximization of the annual energy production may be attained by maximization of the rotor power coefficient, C_p. Several authors have studied optimum blade shapes for maximizing C_p, which, for a prescribed value of the design tip-speed ratio (rotational speed*radius/wind speed), depends on the following design variables:

• Type of airfoil section (C_L/C_D ratio)

• Blade configuration and chord distribution

• Blade twist variation

• Number of blades (i.e., rotor solidity)

Optimization results show that:

1. The higher the lift-to-drag ratio, the better the aerodynamic performance of the machine.

2. The greater the blade number, the better the performance.

3. There is an optimum value for the power coefficient at a certain tip-speed ratio, called the design tip-speed ratio.

5.2 Weight considerations

An improved technology would result in a lightweight design, which performs the intended function efficiently. Lightweight also furthers some other objectives such as lower cost and better performance characteristics. Therefore, minimization of structural weight can be taken as a useful criterion for measuring the success of a wind turbine design. This would include both the tower and rotating blades as they are the main structural components of the machine. The component’s weight depends on the material of construction, dimensions, and configuration.

5.3 Fatigue life

The fatigue life of the major structural components must be adequate to allow the production of enough energy to balance the initial investment. Approximately half the failures caused by fatigue occurred in the rotor assembly. This is expected because the rotor is the primary structure, which transfers wind loads to other structural components.

The design variables necessary for predicting fatigue life may be classified as follows [7]:

1. Wind speed characteristics of the site

2. Material strength levels and safety factors

3. Statistical distribution of stress levels

4. Choice and definition of applied loads, which are stratified in Figure 4.

Figure 4.

Definition of wind turbine loads.

5.4 Design for minimum vibration

The reduction or control of the vibration of wind turbine structural components is an important design consideration. Vibration can greatly influence the commercial acceptance of the machine because of its adverse effects on performance, cost, stability, fatigue life, and noise. Such undesired effects become more pronounced in the case of large horizontal-axis wind turbines [8], which have the unique feature of slender rotating blades mounted on flexible tall towers.

When the machine is operating, the rotating blades of the main rotor are the prime source of vibration, which is then transmitted to the supporting tower structure primarily through a time-dependent shearing force at the hub. The forcing frequencies are integer multiples of the rotation rate. A common way to present natural frequency data and look for possible resonances is to plot the Campbell diagram as shown in Figure 5.

Figure 5.

Campbell diagram for a two-bladed wind turbine.

The intersection of one of the radial lines with one of the system natural frequency curves indicates a potential for resonant vibration near the rotor speed at the intersection point.

A good design philosophy for vibration reduction is to separate the natural frequencies of the structure from the harmonics of air loads or other excitation. This would avoid resonance where large amplitudes of vibration could severely damage the structure. Frequency placement is one of the techniques that have been used for reducing helicopter rotor blade vibrations [9]. The mass and stiffness distributions of the blades are to be tailored in such a way to give a predetermined placement of blade natural frequencies. Frequency placement can also help in controlling the forced response of the blade. Another way of vibration reduction is to minimize the induced shearing forces transmitted to the supporting structure by the rotating blades.

5.5 Noise reduction

Wind turbine design may also be judged by its noise annoyance potential perceived by the nearby residents in both indoor and outdoor environments. The main sources of sound radiated from a wind machine are summarized in the following subsections.

5.5.1 Mechanical noise sources

These are mainly associated with the power-transmission system operation. This noise depends on the types and sizes of the gear box, generator, and bearings, and their mechanical and performance characteristics.

5.5.2 Aerodynamic noise sources

These are mainly associated with rotation of the blades in the surrounding air. It comprises three major components:

1. Rotational noise produced by the steady thrust and in plane torque loads acting on the blades. This noise is characterized by a large number of discrete frequency bands [10], which are harmonically related to the blade passage frequency. As a result of the low rotational speed of wind turbines, the associated acoustic energy resides in the low-frequency and sub-audible ranges (≤ 20 HZ). It was shown that the acoustic pressure depends on the following design variables:

   Position of the receiver – wind velocity – R.P.M – diameter – number of blades – airfoil type – plan form geometry of the blades – coning and pitch angles of the blades.

2. Ground shear noise produced by the unsteady blade loads that vary with the position around the rotor disk as a consequence of wind shear effect. The developed sound is characterized by low-frequency patterns and is largely dependent upon the shape of the wind shear profile.

3. Impulsive noise due to passage of the blades through the tower wake. It is identified with short, transient fluctuations in the radiated acoustic field. This is the most annoying source of noise because of its high degree of coherence and radiation efficiency. It depends, to a great extent, on the structural configuration of the tower.

4. Impulsive noise caused by cross-flow unsteadiness, which is identified by low-frequency sound radiation.

5. Vortex noise generated by vortices in the rotor wake, which are shed by the blades during rotation. This noise is characterized by largely incoherent radiation over a wide frequency range.

It has been demonstrated that noise due to steady and unsteady aerodynamic loading arising from wind shear does not substantially contribute to the acoustic signal from wind machines. On the other hand, it was shown that community annoyance associated with turbine operations was related to coherent impulsive noise and the subsequent coupling of acoustic energy with residential structures. Figure 6, taken from Ref. [10], summarizes the acoustic pressure spectrum associated with large wind turbines for dominate noise sources as a function of frequency.

Figure 6.

Wind turbine noise spectrum characteristics [10].

5.5.3 Noise caused by vibrations of structural components

Sound can be radiated from a wind turbine as a consequence of tower and blade vibrations. The efforts aiming at bringing structural vibrations to a minimum decrease this noise source automatically.

6. System environment and constraints

There are many limitations that restrict wind turbine design, manufacturing, and operation. The most significant among these are given below:

1. Type of application (e.g. electricity generation)

2. Site conditions (location, wind speed characteristics, wind shear, turbulence level, available area, transportation, soil conditions, local electricity system, etc.)

3. Project budget and financial limitations

4. Technological and manufacturing limitations

5. Manpower skills and design experience

6. Availability of certain material types

7. Safety, strength, and stiffness requirements

8. Dynamic and stability requirements of the whole structure

9. Performance requirements

The problem of wind turbine system optimization is that of finding values of the design variables, which best achieve the system objectives and, in the meantime, satisfy all design constraints.

7. Design alternatives and solutions

There are tremendous differences among horizontal axis wind turbines, depending on the size of the rotor and the specific energy application. However, the differences become contained in general design categories for turbines operating in the same environment and for the same application. Based on the selected design objectives, it is possible to identify a number of design solutions that are governed by the choice of the main design variables. Table 1 gives some of the alternatives that concern the blades and tower designs.

Design solution		Effects on system objectives
(1) Rotor Size	(a) Small	Low energy productivity, light weight, low vibration levels, long machine life.
	(b) Large	High energy, heavy weight, high cost, high vibrations and fatigue loads.
(2) Rotor Position	(a) Upwind	Low noise and long fatigue life (No tower shadow effects), cost of control increases (requires yaw drive), has tower clearance problem.
	(b) Downwind	Noisy (tower shadow), reduce cost of control (free yaw), No tower-blade clearance problems.
(3) No. of Blades	(a) One	Less popular, noisy (unbalanced forces), less energy capture, cheaper, easy to erect.
	(b) Two	Reduce cost of transmission, long fatigue life and low vibrations (with teetered hub), cost of hub construction is high.
	(c) Three	More energy capture, more blade weight, low cost of hub construction, balanced gyroscopic forces, more esthetic.
(4) Blade Coning	(a) Without Precone	More energy, high cyclic loads (shorter fatigue life).
	(b) With Precone	Light weight, long fatigue life (reduce bending loads at blade root).
(5) Blade Material	(a) Wood Epoxy	Light weight, long life, high tooling cost, low stiffness.
	(b) Glass-Reinforced Polyester (GRP)	Lightweight, high tooling costs, possible to form complex shapes.
	(c) Steel	Low tooling costs-heavier weight, high stiffness, established technology.
	(d) Aluminum	Light weight, high stiffness.
(6) Tower type	(a) Lattice (Truss)	Cheap-easy transportation and erection – high strength and stiffness.; Poor visual appeal-external access to nacelle – high tower shadow effects.
	(b) Tubular	Expensive – more esthetic – less tower shadow effects – internal access to nacelle.
	(c) Guyed tower	Low weight – low cost – high vibration (very soft) – less popular.
	(d) Concrete	Can be economical for large wind turbines.

Table 1.

Some alternatives for wind turbine blade and tower designs.