**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:

$$\text{Minimize}; \text{Unit energy cost} = \frac{\text{Total annual cost}}{\text{Annual energy produced}} \quad (\\$/\text{Kw.h}) \tag{1}$$

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:


**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:


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.

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.

**Figure 3.** *Wind turbine machine cost as a function of rated power [1].*

*Introductory Chapter: General Design Aspects of Horizontal-Axis Wind Turbines DOI: http://dx.doi.org/10.5772/intechopen.106330*

#### *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:

$$\mathbf{E} = \mathbf{8.760 } \mathbf{P\_r} \left[ \frac{\mathbf{Exp}\left(-\hat{\mathbf{v\_{in}}}\right) - \mathbf{Exp}\left(-\hat{\mathbf{v\_r}}\right)}{\hat{\mathbf{v\_r}} - \hat{\mathbf{v\_{in}}}} - \mathbf{Exp}\left(-\hat{\mathbf{v\_{out}}}\right) \right] \* \mathbf{availability factor (Kwh)} \tag{2}$$

where Pr 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, <sup>V</sup>^ <sup>¼</sup> <sup>V</sup> V ffiffi π 4 p , and are defined as:

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

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

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, Cp. Several authors have studied optimum blade shapes for maximizing Cp, which, for a prescribed value of the design tip-speed ratio (rotational speed\*radius/wind speed), depends on the following design variables:


Optimization results show that:


c. 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]:


**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**.

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

**Figure 5.** *Campbell diagram for a two-bladed wind turbine.*

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:

a. *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.


*Introductory Chapter: General Design Aspects of Horizontal-Axis Wind Turbines DOI: http://dx.doi.org/10.5772/intechopen.106330*

**Figure 6.** *Wind turbine noise spectrum characteristics [10].*

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.

#### *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:


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.


*Introductory Chapter: General Design Aspects of Horizontal-Axis Wind Turbines DOI: http://dx.doi.org/10.5772/intechopen.106330*


#### **Table 1.**

*Some alternatives for wind turbine blade and tower designs.*
