5. Types of wind turbine

Considering how the turbine spins, two kinds of wind turbines can be defined. The mechanism is the same only the direction of the spin differs. Wind turbines that rotates along its vertical axis is the vertical axis wind turbines (VAWT), while the ones that spins about a horizontal axis is the horizontal axis wind turbines (HAWT).

#### 5.1 Horizontal axis wind turbines

The horizontal axis wind turbine (HAWT) is a turbine whose rotor rotational axis is parallel to the ground and wind stream [13]. Its primary rotor shaft and electrical generator are at the pinnacle of the tower and must be faced directly to the wind. Micro turbines are directed by a wind vane, with larger turbines utilizing a wind sensor coupled with a servomotor. The gear box is located in the drive train and is used to convert the slow blade movement into much quicker rotation capable Modeling and Simulation of a 10 kW Wind Energy in the Coastal Area of Southern Nigeria… DOI: http://dx.doi.org/10.5772/intechopen.85064

Figure 1. A horizontal axis wind turbines [30].

Ref. [10, 11] conducted a research and reported on the wind energy reserve in Nigeria at 10 m (or 40 m) height based on data analysis on 10 wind stations across the North West, North East, North Central, South East and South West geopolitical zones. The research showed some promise, with some sites having wind regime between 3.6 and 5.1 m/s, therefore confirming that Nigeria falls into the moderate wind regime according to the Beaufort scale. Along these lines it can be inferred that the sites are potential wind farm areas. This is because most wind turbines start generating electricity at wind speeds of around 3–4 m/s, known in wind generation as the cut in speed. The report also suggested that Nigerian shoreline areas from Lagos State through Ondo, Delta, Rivers, Bayelsa to Akwa-Ibom States also showed promising potentials for harvesting moderate wind energy throughout the year. Coastal regions constitute majority of oil and gas activities in the country, with these activities causing environmental degradation while some of these communities are also cut off from the electricity grid hence leading to a quest for alternate

Arrays of large turbines, called wind farms, are utilized to generate power as a means of reducing fossil power generation in developed countries. By the start of the twentieth century in Denmark, there were already in subsistence some 2500 windmills used to drive mechanical loads like grinding mills and water pumps with an estimated total peak power in the region of 30 MW. By 1910 there were electric generators ranging in power from 5 to 25 kW driven by wind and in use in the United States. During World War I, windmill engineers in the United States were manufacturing 100,000 small-scale farm windmills yearly, mostly used as water pumps [12]. One of the very first modern design horizontal-axis wind generators was used in the Soviet Union by 1931. It was a 100 kW generator placed on a 30-m tall tower and connected to the Nation's 6.3 kV electricity distribution system. It was accounted for to have had a yearly capacity factor of about 32%, which shares close similarity to the efficiency exhibited by current wind machines [12]. As stated earlier, turbine blades can spin about a horizontal or a vertical axis, with horizontal axis rotation being older and more popular. They can also come with blades or be bladeless. Vertical axis wind turbines are not used as much because they produce

Considering how the turbine spins, two kinds of wind turbines can be defined. The mechanism is the same only the direction of the spin differs. Wind turbines that rotates along its vertical axis is the vertical axis wind turbines (VAWT), while the ones that spins about a horizontal axis is the horizontal axis wind turbines

The horizontal axis wind turbine (HAWT) is a turbine whose rotor rotational axis is parallel to the ground and wind stream [13]. Its primary rotor shaft and electrical generator are at the pinnacle of the tower and must be faced directly to the wind. Micro turbines are directed by a wind vane, with larger turbines utilizing a wind sensor coupled with a servomotor. The gear box is located in the drive train and is used to convert the slow blade movement into much quicker rotation capable

energy sources.

less power [3].

(HAWT).

48

5. Types of wind turbine

5.1 Horizontal axis wind turbines

4.2 History of wind energy for wind farms

Wind Solar Hybrid Renewable Energy System

of enough energy to drive an electrical generator [14]. Most HAWTs are either two or three blades, but the number of blades has no limit, it depends solely on the designer. HAWT could also be classified as upwind and downwind turbine. In Ref. [15] it is stated that a gear system is used for stepping up the speed of the generator, although designs may likewise utilize an annular generator. Some designs operate at fixed speed, but variable speed turbines have better efficiency and employ a power converter to communicate with the transmission system. All turbines come with protective lineaments for damage limitation during turbulence. In such turbulence the system is also controlled by feathering the blades into the wind hence stalling them, and brought to a halt with the aid of brakes (Figure 1).

#### 5.2 Vertical axis wind turbine (VAWT)

The main rotor shaft of this type of turbines are arranged vertically, hence the name. The major advantage of this arrangement is the turbine does not need to follow the direction of the wind to exhibit high efficiency, which is advantageous in sites with highly variable wind directions. Also advantageous is its ability to be mounted on a building because it is much less steerable. The drivetrain and electrical machine can also be positioned close to the ground with the aid of a direct drive from the rotor arrangement to the ground-based gearbox, enhancing availability for repairs. Energy efficiency over time is still poor, a severe drawback. Key disadvantages also include the relatively low rotational speed with the consequence being increased torque with a proportional increase in cost of the drive train, reduced power coefficient, pulsating mechanical torque, and modeling difficulties for accurate wind flow studies leading to issues of rotors design analyses prior to fabrication [16].

## 6. Efficiency of the wind turbine system

The conservation of mass demands that the measure of air in and out of a turbine must be equivalent. Consequently, Betz's Law defines maximum achievable wind power drawn by a wind turbine as 16/27 (59.3%) of the aggregate kinetic energy of the air entering the turbine. The best hypothetical power yield of a wind turbine is therefore 16/27 times the kinetic energy of the air entering the turbine effective area (Figure 2).

$$P = \frac{16}{27} \times \frac{1}{2} \times \rho \times v^3 \times A = \frac{8}{27} \times \rho \times v^3 \times A \tag{1}$$

Pitch angle control was utilized in controlling the mechanical power and the system was simulated using SIMULINK software. From the simulation results, the response of the suggested system offers quick recovery faced with different dynamic system disturbances with the controller boosting power thereby improving system efficiency.

Modeling and Simulation of a 10 kW Wind Energy in the Coastal Area of Southern Nigeria…

A Research by the Czech Technical University studied the need for gearless wind turbines is looked into due to positives such as reliability, and reduction in downtime due to less moving parts. The design simulation was carried out using

MATLAB SIMULINK. Results showed the wind turbine had the ability to sustain an electric-power scheme. The system allowed for the independent control of both reactive and active power, suggesting that the gearless design is suitable for turbines

In a paper on the transient response of Doubly Fed Induction Generator using an accurate model [22], the transient execution of various models of DFIG considering

overcurrent due to voltage sag was simulated. The findings from the paper include the importance of consideration of saturation effect on transients but less so for steady state analysis. Also, the rotor speed of saturated model reaching steady state

A detailed model of fixed speed wind turbine (FSWT) stability studies with stator transient was addressed in a PhD work by [23]. The addition of the stator current transient permit a precise speed divergence forecast. A model for stability of power system analysis like Doubly-Fed Induction Generator (DFIG) wind turbine was also suggested in the same work including the stator flux transient. By doing so, the analysis of Fault Ride-Through (FRT) is done. However, such representation gives rise to difficulties when looking into the implementation of the positive sequence fundamental frequency simulation tools, as a result of small time-step

A model of DFIG wind turbine was introduced in [24], the stator transient was not considered at normal operation. However, the use of a current controller still

A basic model of a DFIG wind turbine, compatible with the natural frequency representation was projected by [25]. Both stator and rotor flux dynamics were neglected in the model. This model is comparable to a steady state representation, while the controller of the rotor current is assumed to be instantaneous. Therefore, iteration process which is not favorable in the implementation model is required to

With the introduction of time lags representing current control delays, algebraic loops can be avoided [26]. Nevertheless, it is assumed that the maximum power tracking (MPT) in this model is directly proportional to the arriving wind speed, although in common practice, the generator speed or the generator output power drives the MPT. Miller et al. [27] presented another DFIG simplified model. According to this model, the generator is simply modeled as a current source that is controlled; hence the rotor parameters are omitted. This proposed simplified model did not take into account the limiters of rotor current and the FRT schemes are not clearly modeled. Demonstrations of detailed FSWT models for power system are presented in [28]. In this paper, the generators are modeled thoroughly. They need very small time-step therefore complicating the execution in a standardized fundamental frequency simulator. Ref. [29] proposed a simplified model of an FSWT model.

In a power system network comprising different generation unit, there is bound to be frequency stabilization and control issues. In the work of [31] the frequency responses of the grid power system network and other variables of the grid

connected wind during the period of grid dynamics show improved performance as

saturation impact was looked at and a few parameters that influence rotor

prerequisite and inconsistency with normal power system parts.

solve algebraic loops between the grid model and the generator model.

with variable speed [21].

value quicker than unsaturated model.

DOI: http://dx.doi.org/10.5772/intechopen.85064

demands high simulation resolution.

shown in the simulation results.

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Figure 2. The air flow through area A [36].

where ρ is the density of air, A is effective area of disk, v is wind velocity, and P is power.

The power accessible from the wind is directly proportional to the cube of the speed of the wind. Meaning if the speed of the wind is doubled then the output power from the turbine is given eight times. Therefore, wind turbine designs have to take this into account by ensuring designs can support higher wind loads than those from which they can generate electricity, in order to prevent them from damage. Wind turbines approach maximum efficiency at wind speeds between 12 and 15 m/s. Over this wind speed, the power yield of the rotor must be controlled to diminish main thrusts on the rotor blades and in addition the load on the general wind turbine system [17].

As wind energy is free, wind-to-rotor efficiency, losses in the generator and power electronics are the major factors that affect the final cost of wind power generation. To keep parts from corroding, extracted power is fixed above rated operating speed as theoretical power increments at the cube of wind speed, which reduces the efficiency. Turbine efficiency can diminish somewhat after some time because of wear. Examination of 3128 wind turbines 10 years or older in Denmark demonstrated that half of them did not diminish in efficiency, while the other observed a decrease of 1.2% per year [18]. Vertical turbine efficiency is lower than their horizontal counterparts.
