Renewable energy – wind energy

Renewable energy – wind energy

Prepared by

Wan, Man Pun Assistant Professor School of Mechanical & Aerospace Engineering MA0100 – Environmental Sustainability

Wind energy Wind energy had been a major source of power, before the invention of mechanical heat engines. The use of wind energy dates back to ancient times. Modern applications are mainly for electricity generation.

2

Wind energy

Total installed capacity by end 2012 (MW) Portugal, Canada, 4,525 6,200 France, 7,196 Italy, 8,144 United Kingdom, 8,445

Wind power global capacity [http://www.evwind.es/wp-content/uploads/ 2013/06/wind-energy-1996-2012.jpg]

Rest of world, 39,852 China, 75,564

India, 19,564 United States, 60,007

Spain, 22,796 Germany, 31,332

[Global Wind Energy Council, 2013] 3

Total installed capacity by end 2012 (MW)

Wind energy Availability of wind energy

Global wind map at 80m, mean wind speed in 2005 [http://news.cnet.com/i/bto/20080304/3t_global_wind_540x420.jpg] 4

m – mass  - air density

Wind power density Kinetic energy of the air parcel 1 1 K . E .  mV 2  ADV 2 2 2

D

If the air parcel takes time T to flow pass the turbine rotor, K .E . 

V

A

1 1 AVT V 2  ATV 3 2 2

Power = Energy per unit time Air parcel

1 1 Wind power  ATV 3 T  AV 3 2 2 1 Wind power density (WPD)  AV 3 A 2 1  V 3 2

WPD  V3 5

Wind power classification United States 10 m Class WPD Mean wind (W/m2) speed (m/s) 0 0.0 1 100 4.4 100 4.4 2 150 5.1 150 5.1 3 200 5.6 200 5.6 4 250 6.0 300 6.4 5 400 7.0 400 7.0 6 1000 9.4 1000 9.4 7

WPD (W/m2) 0 200 200 300 300 400 400 500 600 800 800 2000 2000

50 m Mean wind speed (m/s) 0 5.6 5.6 6.4 6.4 7.0 7.0 7.5 8.0 8.8 8.8 11.9 11.9

Classes of Wind Power Potential at 10 and 50 m Levels [Pacific Northwest Laboratory] 6

Wind power classification European Union Class

Shelter Terrain m/s

W/m2

Open Plain m/s

W/m2

Sea Coast m/s

W/m2

Open Sea m/s

W/m2

Hills and Ridges m/s

W/m2

5 4 3

> 6.0 > 250 5.0 - 6.0 150 - 250

> 7.5 > 500 6.5 - 7.5 300 - 500

> 8.5 > 700 7.0 - 8.5 400 - 700

> 9.0 > 800 8.0 - 9.0 600 - 800

> 11.5 > 1800 10.0 - 11.5 1200 - 1800

4.5 - 5.0 100- 150

5.5 - 6.5 200 - 300

6.0 - 7.0 250 - 400

7.0 - 8.0 400 - 600

8.5 - 10.0 700 - 1200

2

3.5 - 4.5 50 -100

4.5 - 5.5 100 - 200

5.0 - 6.0 150 - 250

5.5 - 7.0 200 - 400

7.0 - 8.5

400 - 700

< 7.0

< 400

1

< 3.5

< 50

< 4.5

< 100

< 5.0

< 150

< 5.5

< 200

[European Wind Atlas]

7

Wind characteristics Wind shear The change of wind speed and direction over some distance.

Wind shear due to difference in wind speed

Wind shear due to difference in wind direction

 H Power law for wind shear V  Vo   Ho Measured wind speed

  



Wind shear exponent  1/7 for stable atmosphere

Height of known wind speed 8

Wind characteristics Wind speed Wind speed changes with time.

Location: Burgar Hill, Orkney; 60 m measurement height; 4 hour averages, 2003 [http://www.scotland.gov.uk/Publications/2006/04/24110728/10]

Since WPD  V3, arithmetic average of wind speed, V , is not particularly useful for wind turbine applications. Instead, the mean cubic wind velocity is more often used.

1 mean cubic wind velocity V   T



T

0

 V 3 dt  

1 3

9

Wind characteristics Wind speed Wind situation 1: wind blows constantly at 10 m/s for 10 hours. For the 10-hr period, V = 10 m/s = 10 m/s Wind situation 2: wind blows constantly at 50 m/s for 2 hours and remains calm for another 8 hours. For the 10-hr period, V = 10 m/s = 29.2 m/s

10

Wind characteristics

Wind speed histogram and distribution

Wind speed measured at 50 m for White Deer, Texas [Nelson, Wind Energy, CRC Press, 2009] 11

Wind characteristics

Wind speed histogram and distribution

Histogram of wind speed and energy for White Deer, Texas [Nelson, Wind Energy, CRC Press, 2009] 12

Wind characteristics

Wind speed histogram and distribution Rayleigh distribution

1

   V 2  p V   exp      2 V2  4  V  

 V

p(V) – probability of having wind speed V

p(V)

F(V)

Cumulative Rayleigh distribution    V 2  F V   1  exp       4  V  

0 V

F(V) – Frequency of having wind speed V or below

13

Wind characteristics e.g., Measurements in a location reveals an average wind speed of 7 m/s. If the wind speed distribution of the location can be approximated by Rayleigh distribution, what is the probability of having a 10-m/s wind? How many hours per year does the wind blow faster than 10 m/s?    10  2  p 10   exp       0.065 or 6.5% 2 27  4  7  

 10

Frequency of wind blows faster than 10 m/s    10  2  1  F 10   exp       0.20  4  7  

In a year, there are 8760 hours, no. of hours having V  10 = 0.20 x 8760 = 1763.6 hours

14

Wind pressure and Betz Limit

V

Consider a plate put in a wind flow stream, each air molecule transfers 2mV of momentum to the plate when it hits the plate. # of molecules per unit volume

The flux of air molecules per unit area = nV. The momentum transfer per unit area (the pressure), p = 2mV x nV = 2V2

1 2 From fluid mechanics, the pressure on the plate p  V C D 2

Coefficient of drag

If the plate is extracting power from the wind, it should be drifting downwind at a speed of W where 0 < W < V. The pressure on the plate p 

1 2 C D V  W  2

The power density extracted from the wind P = p x W



1 2 C D V  W  W 2

15

Wind pressure and Betz Limit To find out the W that has the max. power transfer to the plate, P 0 W



1 0  C D V 2  4VW  3W 2 2



Max. P occurs at W = V/3 2

Pmax 

1 V V 2  C D V    C DV 3 2 3  3 27 

Pmax  WPD

2

C V  4 27  C 1 V  27 2 3

D

3

D

2 If 2 V 

1  V 2C D 2

Largest possible CD = 4

Pmax 16  WPD 27 Pmax

16  V 3 54

Betz limit WPDA 16

Types of wind turbine

Wind

Drag type wind turbines For a drag wind turbine, the wind pushes on the blades (or sail), forcing the rotor to turn on its axis. Drag type wind turbines are inherently limited in efficiency since the speed of the device or blades cannot be greater than the wind speed.

Rotation

Drag type wind turbine

Lift type wind turbines Lift-type wind turbines make use of the lift force that generates by the wind blowing through the rotor blades, which are shaped as airfoils. Propeller type wind turbine is a popular example of lift-type wind turbines. The mechanism is analogues to sailboats sailing across wind. The rotor blades can move faster than the wind itself.

Blade

Life force Wind Rotation

Life type wind turbine

17

Types of wind turbine

Drag type

Combined drag and lift type

Lift type

18

Major components in a large wind turbine

[Nelson, Wind Energy, CRC Press, 2009] 19

Typical power curve of modern wind turbine

[Nelson, Wind Energy, CRC Press, 2009] 20

End

21