Session 1: Introduction to Wind Power Systems 1.1 Aims and Objectives
The aim of this session is to give a brief introduction and overview to Wind Power Systems, looking at the technology, characteristics and applications. This information will prove useful in your future project work!
1.2 Introduction
Over the last 30 years there has been a significant increase in the use of wind to generate electricity. Wind energy is the conversion of the kinetic energy available from the wind, through the use of wind turbines. The wind turbine converts the kinetic energy into mechanical energy, which is then converted by a generator into electrical energy.
There are two main types of wind turbine, Horizontal axis wind turbines which are the standard conventional version we see throughout the countryside, and Vertical axis wind turbines which have their rotors arranged vertically, although these are not very common. There are two main subtypes of the vertical turbines, Darrieus and Savonius. Throughout the rest of the document only the horizontal version will be discussed. An example of a Horizontal wind turbine can be seen in Fig. 1, while a Vertical axis Darrieus wind turbine is shown in Fig. 2.
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Figure 1: Horizontal Wind Turbine [1]
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Figure 2: Vertical Axis Darrieus Wind Turbine [2]
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1.3 Wind Turbine Components
There are 3 main components to a horizontal wind turbine structure, the Rotor, Nacelle and Tower. The rotor is the portion of the wind turbine that collects the energy from the wind; it usually consists of 2-4 fibreglass or metal blades which rotate at a rate determined by the velocity of the wind and the aerodynamics of the blades. The blades are attached to the hub, which in itself is attached to the main low speed shaft. An overview of blade aerodynamics for wind turbine can be seen in [3]. The nacelle contains the mechanical components connected to the rotor, the low speed shaft, a gearbox, used to increase the rotational speed to that specified for the generator, a high speed shaft, and the generator. There are 3 main types of generator used; doubly fed induction generators (DFIG’s) are the dominant technology for large and medium sized systems, although rare earth permanent magnet synchronous generators (PMSG’s) are starting to break into those areas; while squirrel cage induction generators (SCIG’s) are usually used in lower power systems. Reluctance topologies are another rarer alternative.
The tower is not only used to mount and support the wind turbine; it also ensures that the wind turbine is clear of the ground and allows it to reach stronger winds which are found higher off the ground. The tower also contains a mechanism to alter the yaw of the nacelle so that it is always facing in the wind direction; it also usually contains the power electronic components to allow for the export of the electricity to the grid.
Figure 3 gives an overview of all the major component just described, while Fig. 4 gives an overview of the costs of different sections of a wind turbine.
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Figure 3: Wind Turbine Components [4]
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Figure 4: Costs of components of a wind turbine [5]
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1.4 Wind Turbine Growth
Over the last 30 year there has been a trend towards bigger turbines with a corresponding increase in power, an example of which can be seen in Fig. 5. The turbines energy output increases with the swept area of the blades; however, the increase in blade size increases the volume, mass, and cost of the materials needed. Some information about offshore wind turbines is given in Fig. 6. The theory and principles of the power extraction will be covered in Section 1.5.
Figure 5: Wind Turbine Increasing Size [6]
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1.4.1 Growth of Offshore Wind Turbines
Figure 6: Offshore Wind Turbines Statistics [7] Page 8 of 17
1.5 Theory & Principles
This section looks at the main theory and principles for extracting power from wind turbines.
1.5.1 Wind Power
The theoretical power available from the wind depends on the density of the air (𝜌 in kg/m3), the area swept by the blades (𝐴 in m2) as shown in Fig. 7, and the velocity of the wind (𝑣 in m/s), and is given by (1):
Swept Area “A”
Radius “R ”
Figure 7: Area swept by Turbine Blades
𝑃 = 1𝜌𝐴𝑣3
This equation shows the cubed relationship between the available power and the wind velocity, showing for a small increase in velocity a large increase in power occurs. This can be seen in in Fig. 8.
Figure 8: Cubed relationship between wind velocity and power
𝑤𝑖𝑛𝑑
(1)
2
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1.5.2 Coefficient of Power (𝑪𝒑)
However, in practice the power calculated from (1) cannot be extracted. The flow of air over the wind turbines blades makes the wind turbine spin; in effect the wind turbine extracts energy by slowing the wind down. There is a theoretical limit to how much energy can be extracted by the wind turbine blades, it is 59.26% or 16/27 and is called the Betz Coefficient. 100% of the energy cannot be extracted, as the wind would have given up all of its kinetic energy and stop, and it is the flow of wind which makes a turbine function. In reality the Betz Coefficient also cannot be met, and the power extracted is dependent on the aerodynamics of the blades. The power extracted (𝑃𝑒𝑥𝑡) depends on the power coefficient (𝐶𝑝), and therefore (1) becomes (2).
1.5.3 Tip Speed Ratio (TSR)
𝑃 =1𝜌𝐴𝑣3𝐶 𝑒𝑥𝑡 2 𝑝
(2)
The power coefficient in itself depends on two other parameters, the Tip Speed Ratio (𝜆) and the pitch angle (𝛽); the Tip Speed Ratio is explained as follows. Tip Speed Ratio (TSR) is the ratio between the rotational speed of the tip of a wind turbine blade and the actual velocity of the wind; and is related to the efficiency of the blades with blade designs changing the optimum value. It can be calculated from (3) where 𝜔𝑇 is the wind turbine rotors rotational speed in rad/s, 𝑅 is the radius of the rotor blades in metres, and 𝑣 is the wind speed in m/s.
𝜆 = 𝜔𝑇𝑅 (3) 𝑣
1.5.4 Generic Equation for Power Coefficient
As previously mentioned the coefficient of power depends on the blade aerodynamics it is highly dependent on the pitch angle of the blades and the Tip Speed Ratio. Equations (4, 5) give an example equation including coefficients, while (6) shows an amended version of (2) taking (4, 5) into account.
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1 −𝑐5 1 (4)
𝐶𝑝(𝜆,𝛽)=𝑐1(𝑐2𝜆𝑖 −𝑐3𝛽−𝑐4)𝑒
1 = 1 − 0.035 𝜆𝑖 𝜆 + 0.08𝛽 𝛽3 + 1
𝑐1 =0.5176,𝑐2 =116,𝑐3 =0.4,𝑐4 =5,𝑐5 =21,𝑐6 =0.0068
𝑃 =1𝜌𝐴𝑣3𝐶 (𝜆,𝛽) 𝑚𝑒𝑐h 2 𝑝
𝜆𝑖 +𝑐6𝜆
(5)
Where:
(6)
Figure 9 shows the Power Coefficient vs. Tip Speed Ratio for the equations and coefficients shown above for different pitch angles, while Fig. 10 shows the mechanical output power vs. rotational speed of the turbine for varying wind velocities, with 𝛽 = 0 and 𝑅 = 10𝑚. This shows the effect of 𝐶𝑝 on the power curves. In comparison to Fig. 9; Fig. 11 shows 𝐶𝑝 vs. TSR for different turbine configurations showing how the high speed two and three blade configuration gives a higher 𝐶𝑝 and power extraction compared to the other designs.
Figure 9: 𝑪𝒑 vs. TSR
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Figure 10: Mechanical Power vs. rotor speed with varying wind velocities
Figure 11: Variation of 𝑪𝒑 vs. TSR for different turbine configurations [8]
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1.6 Generator Types
There are 3 main types of generator used for wind turbines; squirrel cage induction, doubly fed induction, and permanent magnet synchronous generators. Block diagrams showing the main component of each system are shown in Fig. 12 and will briefly be discussed.
Low speed shaft
Low speed shaft
Low speed shaft
Low speed shaft
High speed shaft
High speed shaft
3 phase variable AC
DC link
Wind Turbine
(A)
Wind Turbine
(B)
Wind Turbine
(C)
Wind Turbine
(D)
Drive Train & Gearbox
Drive Train & Gearbox
Squirrel Cage Induction Generator
Doubly Fed Induction Generator
3 Phase AC/DC Converter
3 Phase AC/DC Converter
3 Phase Grid Connected DC/AC Inverter
3 Phase Grid Connected DC/AC Inverter
Grid connected 3 phase AC supply
Grid connected 3 phase AC supply
3 phase variable AC
DC link
DC link
DC link
Low speed shaft
3 phase variable AC
Drive Train
Drive Train
Permanent Magnet Synchronous Generator
3 Phase AC/DC Converter
3 Phase Diode Bridge
3 Phase Grid Connected DC/AC Inverter
Boost Converter
Grid connected 3 phase AC supply
3 Phase Grid Connected DC/AC Inverter
Low speed shaft
3 phase variable AC
DC link
Permanent Magnet Synchronous Generator
Grid connected 3 phase AC supply
Figure 12: Block Diagram of Wind Turbine Systems, A) Squirrel cage induction, B) Doubly fed induction, C) & D) Permanent magnet synchronous
The systems shown in Fig. 12 can be split into 2 main groups and are the most common implementations; squirrel cage induction and permanent magnet use fully rated converters (A, C and D) while doubly fed induction use partially rated converters (B) (usually 25% rated), this leads to significant cost savings, although the control is slightly more complicated. Parts C) and D) show two options for permanent magnet generators; C) uses a fully rated AC/DC converter to control the output of the generator, while D) uses a combination of a diode bridge and boost converter to control the output. Option D) is mainly used for lower powers up to 10’s of kW. A lot more information on these systems will be discovered as you research your project area.
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1.7 Wind Resources
An important consideration for Wind Power Systems is the amount of available energy from the wind. Figure 13 shows the United Kingdom (UK) annual mean wind speed at 45m with a 1km resolution, while Fig. 14 shows the global wind speed at 80m with a 5km resolution. These both show that the UK is very windy and a good choice to exploit wind energy.
Figure 13: UK mean wind speed at 45m [9]
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Figure 14: Global wind speed at 80m [10]
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1.8 Operating Characteristics
For wind turbine operation there is a relationship between the wind velocity, the output power, and the control of the wind turbine. In terminology wind speed and wind velocity are used interchangeably, here speed will be used instead of velocity for consistency with many sources of data, therefore the operating characteristics can be explained with the power curve shown in Fig. 15.
Figure 15: Wind turbine operating characteristics
As shown in Fig. 15 there are 3 operating conditions, cut in, rated and cut out wind speed and 3 main regions.
Cut in speed: At very low wind speeds the wind exerts insufficient torque on the turbine blades to make them rotate. However, as the wind speed increases the turbine will begin to rotate and power will start to be generated. The speed when the rotation and power generation occurs is called the cut-in speed, and is usually between 3-4 m/s. (On Fig. 15, 4m/s is shown).
Rated speed: Once the cut in speed has been reached, with increasing wind speed the output power rises quickly. However, between a speed of 12-17m/s depending on the design of the turbine the output power reached the limit of the electrical generator. This limit is called the rated output power and the wind speed is the rated speed or rated wind speed. At higher wind speeds the turbine must be controlled to limit the power to this maximum level. Typically with large turbines this is achieved with pitch control.
Cut out speed: As the wind speed increases after the rated value is reached pitch control is used to limit the power. However, there is a limit to the forces that the turbine blades and structure can
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withstand and once this is reached a brake is applied to stop rotation occurring. This speed is called the cut-out speed and is usually around 25m/s as shown in Fig. 15.
The three regions shown on Fig. 15 can be discussed as follows:
Region 1 covers the low wind speed range and the power is below the rated value; therefore in this region the turbine is operated in its most efficient condition to extract the most power. In Region 2 the rated power has not been reached, this is a transition region where noise and torque are controlled. While in Region 3 the rated output power has been reached, and this region is concerned with above rated wind speed operation, power is still being extracted but with the use of pitch control to limit the power to the rated value until the cut out speed is reached.
1.9 References
[1] Vestas Horizontal Wind Turbine. Available: http://www.vestas.com/en/media/images.aspx
[2] Vertical Axis Darrieus Wind Turbine Available:
http://www.johnsonsysteminc.com/renewable-energy-products/vertical-windmill-pole-
turbine/
[3] Wind Turbine Blade Aerodynamics. Available:
http://www.gurit.com/files/documents/2_aerodynamics.pdf
[4] Wind Turbine Components. Available: http://www.cleanlineenergy.com/technology/wind-
and-solar
[5] Costs of components of a wind turbine. Available:
http://www.mentorconsult.net/energy/wind-energy/
[6] Wind Turbine Increasing Size. Available:
http://commons.wikimedia.org/wiki/File:Wind_turbine_size_increase_1980-2011.png
[7] Infographic on the 2014 offshore statistics:. Available:
http://www.ewea.org/statistics/offshore-statistics/
[8] Variation of Cp vs. TSR for different turbine configurations Available:
http://buckville.com/drupal/misc/turbine_efficiency.jpg
[9] UK mean wind speed at 45m Available: http://www.ms1energy.co.uk/uk-wind-map/
[10] Global wind speed at 80m. Available: http://www.3tier.com/en/support/resource-maps/
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