1. DESIGN, SYSTEM PERFORMANCE, ECONOMIC ANALYSIS
WIND POWER
DESIGN, SYSTEM PERFORMANCE, ECONOMIC ANALYSIS

2. OVERVIEW Type of wind turbine Power in wind Wind turbine structure
Average power
Wind turbine performance calculation

3. TYPE OF WIND TURBINE
General terminology: wind-driven generator, wind generator, wind turbine, wind-turbine generator (WTG), wind energy conversion system (WECS)
Two types of wind turbine:
Horizontal axis wind turbine (HAWT)
Vertical axis wind turbine (VAWT)

4. TYPE OF WIND TURBINE
HAWT
VAWT

5. TYPE OF WIND TURBINE
Upwind wind turbine equipped with an active yaw system
Upwind wind turbine equipped with an passive yaw system
Downwind wind turbine equipped with a passive yaw system

6. TYPE OF WIND TURBINE

7. TYPE OF WIND TURBINE Advantage: Disadvantage:
Downwind let wind itself control the yaw
Disadvantage:
Wind shadowing effects of the tower
Required complex yaw control system

8. TYPE OF WIND TURBINE Advantage: Disadvantage: No need yaw system
Nacelle located on the ground
Always on pure tension
Disadvantage:
Blades close to the ground
Very little starting torque

9. POWER IN WIND

10. POWER DENSITY (W/m²)

11. OBSERVATION FROM POWER EQUATION
Power in the wind depends on:
Air density
Area that wind flow through (i.e. swept area of the turbine rotor)
Wind speed
Power increases as the cube of wind speed

12. POWER vs WIND SPEED
Power in wind, per square meter of cross section, at 15°C and 1 atm

13. ROTOR SWEPT AREA

14. POWER vs SWEPT AREA
Power increases as proportional to swept area of the rotor
This implies that power is proportional to square of the diameter; the bigger, the better
This explains economic of scale of wind turbines

15. IMPACT OF TOWER HEIGHT
Wind speed near the ground is greatly affected by the friction that air experiences
Smooth surface, such as sea (less friction)
Rough surface, such as city with tall buildings (more friction)
Wind speed as a function of
Height
Earth’s surface

16. WIND SPEED CORRECTION Power law often used in US
Hₒ = reference height of 10 m
vₒ = reference wind speed at Hₒ
α = friction coefficient
Alternative law used in Europe
z = roughness length
These are just approximation, nothing beats actual site measurements!

17. FRICTION COEFFICIENT

18. ROUGHNESS CLASS

19. EXAMPLE 1
A wind turbine with a 30 m rotor diameter is mounted with its hub at 50 m above a ground surface that is characterized by shrubs and hedges. Estimate the ratio of specific power in the wind at the highest point that a rotor blade tip reaches to the lowest point that it falls to.

20. Solutioan 1

21. ALBERT BETZ’S FORMULATION.
Air mass, flowing through the turbine :

22. POWER EXTRACTED

23. Substitute the air mass and to extracted power
The perturbation factor, a is defined as the fractional decrease of wind speed at
the wind turbine

24. Po is power of wind turbine and Cp is power coefficient

25. Maximum power coefficient
This is maximum power coefficient in theory
How should we design a so that we can have better power coefficient?

26. TIP SPEED RATIO

27. ROTOR EFFICIENCY vs TSR
For a given wind speed, rotor efficiency is a function of the rate at which a rotor turn
Rotor turns too slow letting too much wind pass  efficiency drop
Rotor turns too fast causing turbulence  efficiency drop

28. EXAMPLE 2
A 40 m, three bladed wind turbine produces 600 kW at a wind speed of 14 m/s. air density is the standard kg/m³. At what rpm does the rotor turn when it operates with TSR of 4.0?
SOLUTION 2
TSR = (rpm x pi x D)/(60 x v)
rpm = (TSR x 60 x v)/ (pi x D)
= (4 x 60 x 14)/ (3.14 x 40)
= rpm

29. WIND ENERGY CONVERSION SYSTEM (WECS)
The main components:
When wind speed varies, how can optimally extract wind energy while maintaining a constant voltage frequency?

30. VARIABLE ROTOR SPEED
For maximum efficiency, turbine blades should change their speed as wind speed changes
The challenge is to design a machine that can accommodate variable motor speed at a fixed generator speed

31. ENERGY CONVERSION AND CONTROL

32. ENERGY CONVERSION AND CONTROL
Turbine speed control
Goal:
To achieve highest rotor efficiency (extract the highest amount of wind energy) i.e. operate at optimal TSR
To protect the turbine from strong wind
How?
Adjust angle of attack at the turbine blades
Stall or pitch control
Generator speed control
Goal:
To maintain constant voltage frequency i.e. operate at 50/60 Hz
How?
Multiple gearboxes designs for different rotor speed to generator speed ratios
Different generator designs and power converters are used to adjust the voltage frequency to be the same as the grid frequency

33. TYPES OF GENERATORS Synchronous generators
Operate at a constant rotational speed to generate constant voltage frequency
Require separate DC source for magnetic field
Fixed rotor speed. Need gear box to adjust rotor speed

34. TYPES OF GENERATORS Asynchronous generators
Induction generator operating at a rotational speed higher than voltage frequency
Absence of separate DC source, easy to maintain but require reactive power support
Flexible rotor speed. Can design rotor circuit to adjust rotor speed

35. WIND TURBINE GENERATORS CLASSIFICATION BY SPEED CONTROL
Wind turbine generators can be divided into 5 types:
Type 1: Fixed speed (1-2% variation)
Type 2: Limited variable speed (10% variation)
Type 3: Variable speed with partial power electronic conversion (30%)
Type 4: Variable speed with full power electronic conversion (full variation)
Type 5: Variable speed with mechanical torque converter to control synchronous speed (full variation)

36. TYPE 1: FIXED SPEED SYSTEMS
Turbine speed is fixed (or nearly fixed with 1-2% variations) to electrical grid’s frequency. This implies that the turbine may not operate at optimal TSR
Use aerodynamic to control turbine blades by stall or pitch control
Simple and reliable construction of electrical parts while cause higher stress in mechanical parts
Use either synchronous or induction generators and connect then directly to the grid. Induction generators are preferred to due their low maintenance and cost

37. TYPE 2-4: VARIABLE SPEED SYSTEMS
Decouple the electrical grid frequency and mechanical rotor frequency using power electronics systems to interface the grid, allowing variable speed operation to achieve optimal TSR
Use synchronous or asynchronous (induction) generators with power electronics

38. TYPE 2: VARIABLE SLIP IGs
Induction generators with variable resistors added in the rotor circuit to control rotor the rotor speed
Use slip as an adjustable resistance to vary speed thus vary the power output

39. TYPE 3: DFIG Known as Doubly Fed Induction Generator
Instead of variable resistors in Type 2, this Type 3 design adds AC-DC-AC converters to the rotor circuit
Rotor frequency is decoupled from grid frequency. The machine can still be synchronized with the grid while the wind speed varies

40. TYPE 4: INDIRECT GRID CONNECTION
Allow the turbine to rotate at its optimal speed
AC output from machine contain different frequency. Its frequency is decoupled with grid frequency
AC-DC-AC converter is used to connect the AC output to the grid
Full control and flexibility in design and operation of wind turbine
The rating of power electronics are higher than Type 3

41. TYPE 5: SPEED/TORQUE CONVERTER
Mechanical control
Speed/torque converter:
To achieve maximum power, P = τ x ω
To adjust the rotor speed according to grid frequency
Operate as typical synchronous generators

42. SPEED CONTROL FOR WIND TURBINE
Speed must be controlled for 2 purposes:
To achieve high energy conversion efficiency while producing constant voltage frequency during normal operation
To protect wind turbine during turbulent weather
Turbine speed can be controlled through:
Electrical parts: Generator, Power converters
Mechanical parts: Gearbox, Yaw control, Turbine blades (aerodynamic design)

43. POLE CHANGING INDUCTION GENERATOR
External connections that switch number of poles, by changing the circuit at the stator winding. There is no change to the rotor

44. MULTIPLE GEARBOXES
Gearboxes in wind turbine do not typically change gear
Two gearboxes with separate generators for low wind speed and high wind speed gear

45. PASSIVE YAW CONTROL

46. PITCH CONTROLLED TURBINE
Active control
Decrease ‘angle of attack’ to decrease lift-to-drag ratio
Control the blade by monitoring generator output power using electronic system
Use hydraulic system to slowly rotate the blades

47. STALL CONTROLLED TURBINE
Passive control
No moving parts
Aerodynamic design of the blade
Blade twisted slightly from hub to tip
Create turbulence on the blade during high wind speed
Avoid moving parts and complex control systems

48. ACTIVE STALL CONTROLLED TURBINE
Resemble pitch controlled turbine during low wind speed to optimize lift-to-drag ratio
During high wind speed, increase ‘angle of attack’ to decrease lift-to-drag ratio i.e. turn blades toward stall during high wind speed

49. PITCH vs STALL CONTROL
Stall control only relies on the aerodynamic design while pitch control can actively regulate the output power better during high wind speed. Hence, the pitch control can have a constant power output during high wind speed

50. Average power in the wind (W/m²)
OVERALL EFFICIENCY
Power extracted from the wind (W/m²), depending turbine blade (rotor) efficiency
Electrical power output (W/m²), depending on the gearbox and generator efficiency
Average power in the wind (W/m²)

51. EXTRACTED POWER FROM WIND

52. IDEAL POWER CURVE Rated wind speed, start to shed some of wind power
Rated power
Shedding the wind
Power delivered (kW) 
Furling or cut out wind speed
Rated wind speed
Cut in wind speed
Wind speed (m/s) 
Machine must be shut down. Above this speed, output power is zero
Need to overcome friction in the drive train and some electrical losses

53. REAL POWER CURVE
It is difficult to find rates wind speed for a large wind turbine.
Discrepancy from inability to precisely shading the wind
Rated wind speed is used less often these days
How to shed wind power?

54. HOW TO SHADE WIND POWER Pitch-controlled turbines
Active control by reducing ‘angle of attack’
Stall-controlled turbines
Passive control using pure aerodynamic design
Active stall control
Induce stall for large wind turbine by increasing ‘angle of attack’
Passive yaw control
Small kW size turbine, causing axis of turbine to move off the wind

55. ROTOR DIAMETER vs GENERATOR RATED POWER