Wind Electric

A Brief History of Wind | How Do Wind Turbines Work? | Components of a Wind Turbine | Wind Capacity Worldwide and in Australia | Large Scale Wind Turbines in Australia | Small Scale Wind Turbines | Managing Variability in Wind Turbine Systems | The Future | Further Information | References

A Brief History of Wind

After the 1930s many isolated rural areas used small wind turbines for direct current electrical power. The extension of the Western Australian grid since the 1940s has greatly reduced their use. Isolated farms have caused an upsurge of interest in small-scale wind powered electricity generation in recent times. They are sometimes used in conjunction with small diesel generators and photovoltaic cells in remote off grid areas.

 

How Do Wind Turbines Work?

Wind energy conversion systems ('wind turbines') are designed to convert the energy of wind movement (kinetic energy) into mechanical power, that is the movement of a machine. In wind turbine generators, this mechanical energy is converted into electricity and in windmills this energy is used to do work, such as pumping water, mill grains or drive machinery. Electricity generated can be either stored in batteries, or used directly. There are three basic physical laws governing the amount of energy available from the wind.

The first law states that the power generated by the turbine is proportional to the wind speed cubed. For example if the wind speed doubles, the power available increases by a factor of eight; if the wind speed triples then twenty seven times more power is available! Conversely, there is very little power in the wind at low speed. This law means that accurate and detailed local wind speed data is necessary to determine the likely energy yield from a given site, and generators should be chosen for that particular site. Average wind speed information alone is often of limited value.

The second law states that the power available is directly proportional to the swept area of the blades. That is the power is proportional to the square of the blade length. For example, doubling the blade length will increase the power by four times, and tripling the blade length will increase the power by nine times.

The third law states that there is a maximum theoretical efficiency of wind generators of 59%. In practice, wind turbines are less efficient than this, due to system losses. The best wind generators have efficiencies of about 53%. Practical wind turbines are designed to work between certain wind speeds. The lower speed, called the 'cut in speed' is generally 3 - 4 ms-1, as there is too little energy below this speed to overcome system losses. The 'cut out speed' is determined by the ability of the particular machine to withstand high wind. The 'rated speed' is the wind speed at which the particular machine achieves its maximum rated output. Above this speed, it may have mechanisms that maintain the output at a constant value with increasing wind speed (see Figure 1).

Figure 1 Power Output from a Wind Turbine as a function of Wind Speed.


Figure 1 shows an ideal power curve for a small wind turbine with a furling mechanism. Vc is the cut-in speed at which the turbine starts to produce power, Vr is the rated speed at which the turbine reaches it's rated power and Vf is the furling speed, which is the wind speed at which the machine shuts down to avoid damage. Pr is the rated output of the turbine. This curve would be typical of a horizontal-axis tow or three bladed machine. The curve is ideal as the machine follows the peak power available from the wind until it reaches the generator capacity and then regulates to maintain a steady output until shut down.

 

Components of a Wind Turbine

A wind turbine usually comprises the following parts (see Figure 2):

Rotor: The blades of the rotor are designed to spin in the wind, driving the turbine generator. Sometimes gearing is used to increase the frequency for electricity generation.

Generator: This generates the electricity when there is sufficient wind to rotate the blades. There are now many designs of generator, including some with new powerful permanent magnets. Electricity is transferred to the next stage (either for storage, exporting to the grid or for direct use) using cabling.

Directional system: Horizontal axis machines require a mechanism to swing them into line with the wind. Small machines usually have a tail assembly for furling. Large machines usually have a 'servo mechanism' that orients them into the direction of maximum output.

Protection system: Modern wind turbines are usually equipped with mechanisms to prevent damage in excessively high winds. Large machines may use active methods involving aerodynamic and mechanical brakes to shut down generation at high wind speeds. Smaller systems may use passive methods such as furling or changing the blades' pitch so that they present a smaller surface to the wind and thereby reduce the speed of rotation.
 
Tower: The tower raises the turbines assembly well above the turbulent air currents close to the ground and captures higher wind speeds, as described earlier in this fact file. Tower design is particularly critical, as it must be as tall as economically possible, robust, enable access to the turbine for maintenance, and yet not add unnecessarily to the cost of the system. A particularly important aspect of tower design is elimination of resonance between the frequency range of rotating blades and the resonant frequency of the tower.

Figure 2 Major sub-components of a large-scale wind generator
(courtesy of CREST and the Renewable Energy Policy Project – Wind Turbine Development : Location of Wind Manufacturing (PDF)).

Wind Capacity Worldwide and in Australia

Globally there are many countries that have invested heavily in wind technologies. Figure 3 below is a pie chart of distribution via the worlds continents.

Figure 3 Percentage of world installed capacity by continent in 2007 (World Wind Energy Association (WWEA) world statistics 2007 publication (PDF)).

Even though Australia is a small player on the world stage the domestic wind installed capacity is making inroads to the energy mix. Figure 4 shows the cumulative MW installed capacity from the year 2000 to 2006.

Figure 4 Installed capacity in Australia from 2000 to 2006.(courtesy of the Australian Wind Energy Association’s (Auswind) Trade Winds 2004-05 publication (PDF) and the International Energy Agency).

 

Large Scale Wind Turbines in Australia

Wind turbines are rated according to the power output they produce, and range from a few kW up to a few MW. Table 1 below shows a list of the existing wind projects in Australia of installed capacity greater than 10 kW.


PROJECT & LOCATION

OWNER/ DEVELOPER

CONNECT.

YEAR

MAKE, TURBINE

No.

TOTAL SIZE

Breamlea
VIC

Barwon Water

Grid

1987

Westwind
60kW

1

0.06

Flinders Island 1
TAS

Hydro Tasmania

Wind Diesel

1988

na
55kW

1

0.055

Salmon Beach*
WA (Decommissioned)

SECWA, now Verve

Wind Diesel

1988

Westwind
60kW

6

0.36

Cooper Pedy
SA

na

Wind Diesel

1991

Nordex
150kW

1

0.15

Coconut Island (decommissioned)
QLD

Ergon Energy

Wind Diesel

1992

na
10kW

1

0.01

Ten Mile Lagoon
WA

Western Power

Wind Diesel

1992

Vestas
225kW

9

2.025

Aurora (Brunswick)
VIC

Citipower

Grid

1993

na
10kW

1

0.01

Flinders Island 2
TAS

Hydro Tasmania

Wind Diesel

1996

na
25kW

1

0.025

Armadale
WA

na

Grid

1997

Westwind
30kW

1

0.03

Kooragang Island, Newcastle
NSW

Energy Australia

Grid

1997

Vestas
600kW

1

0.6

Thursday Island
QLD

Ergon Energy

Wind Diesel

1997

Vestas
225kW

2

0.45

Crookwell
NSW

Eraring Energy

Grid

1998

Vestas
600kW

8

4.8

Huxley Hill, King Island
TAS

Hydro Tasmania

Wind Diesel

1998

Nordex
250kW

3

0.75

Denham
WA

Verve

Wind Diesel

1999

Enercon
230kW

3

0.69

Epenarra
NT

na

Wind Diesel

1999

Lagerway
80kW

1

0.08

Blayney
NSW

Eraring Energy

Grid

2000

Vestas
660kW

15

9.9

Murdoch
WA

RISE

Research

2000

Westwind
20kW

1

0.02

Windy Hill
QLD

Stanwell

Grid

2000

Enercon
600kW

20

12

Albany
WA

Verve

Grid

2001

Enercon
1.8MW

12

21.6

Codrington
VIC

Pacific Hydro

Wind Diesel

2001

Bonus
1.3MW

14

18.2

Hampton
NSW

Wind Corporation Australia

Grid

2001

Vestas
660kW

2

1.32

Exmouth Advanced
WA

Verve

Research

2002

Westwind
20kW

3

0.06

Toora
VIC

Stanwell

Grid

2002

Vestas
1.75MW

12

21

Woolnorth Stage 1
TAS

Hydro Tasmania

Grid

2002

Vestas
1.75MW

6

10.5

9 Mile Beach
WA

Verve

Wind Diesel

2003

Enercon
600kW

6

3.6

Challicum Hills
VIC

Pacific Hydro

Grid

2003

NEG Micon
1.5MW

35

52.5

Huxley Hill stage 3
TAS

Hydro Tasmania

Wind Diesel

2003

Vestas
850kW

2

1.7

Mawson Base
AAT

Australian Antartic Division

Wind Diesel

2003

Enercon
300kW

2

0.6

Starfish Hill
SA

Tarong Energy

Grid

2003

NEG Micon
1.5MW

23

34.5

Bluff Point (Woolnorth Stage 2)
TAS

Hydro Tasmania

Grid

2004

Vestas
1.75MW

31

54.25

Canunda
SA

International Power/ Wind Prospect

Grid

2004

Vestas
2MW

23

46

Hopetoun
WA

Verve

Wind Diesel

2004

Enercon
600kW

1

0.6

Lake Bonney Stage 1
SA

Babcock & Brown National Power

Grid

2004

Vestas
1.75MW

46

80.5

Rottnest Island
WA

Rottnest Island Board

Wind Diesel

2004

Enercon
600kW

1

0.6

Bremer Bay
WA

Verve

Wind Diesel

2005

Enercon
600kW

1

0.6

Cathedral Rocks SA

Hydro Tasmania & Acciona Energy

Grid

2005

Vestas 2MW

33

66

Cocos (Keeling) Island
WA

PowerCorp/Diesel & Wind Systems

Wind Diesel

2005

Westwind
20kW

4

0.08

Mount Millar (Yabmana)

Tarong Energy

Grid

2005

Enercon 2MW

35

70

Walkaway
WA

B&B/National Power Partners/Carbon Solutions

Grid

2005

Vestas V82
1.65MW

54

89.1

Wattle Point
SA

Southern Hydro & Wind Farm Developments

Grid

2005

Vestas
1.65MW

55

90.75

Wonthaggi
VIC

Wind Power Pty Ltd

Grid

2005

REpower
2MW

6

12

Emu Downs Stanwell Corporation/Griffin Energy Grid 2006 Vestas 1.65MW

48

79.2
Yambuk VIC Pacific Hydro Grid 2007 NEG Micon 1.5MW

20

30

Studland Bay (Woolnorth Stage 3)
TAS

Hydro Tasmania/Roaring 40's

Grid

2007

Vestas V90
3MW

25

75

Studland Bay (Woolnorth Stage 3)
TAS

Hydro Tasmania/Roaring 40's

Grid

2007

Vestas V90
3MW

25

75

Snowtown (Stage 1)

TrustPower Limited

Grid

2008

Suzlon S88 turbines
2MW

47

98.7

         

Total

990.975 MW

Table 1 Installed capacity in Australia.(derived from Clean Energy Council).

For a some interesting facts about wind installations in Australia, visit Clean Energy Council.

 

Small Scale Wind Turbines

Wind turbines that have a rated capacity of less than 10kW are usually classified as small-scale turbines. These turbines can also be connected to the grid, but more commonly are used to generate electricity as part of a Remote Area Power Supply (RAPS) or Stand-alone Power Supply (SPS) system in regions where the grid is unavailable. Figure 5 shows four 20 kW Westwind turbines used on Home Island in the Cocos (Keeling) Islands.


Figure 5 This small wind turbine generator is part of a hybrid RAPS system
(Image courtesy of Westwind Turbines).

Windmills/Wind Pumps

Windmills have been used in Australia to pump water from underground bores for nearly a century, and are a common sight in rural Australia. They are designed to operate at lower wind speeds than wind turbines for electricity generation. Windmills 'store' the energy they produce in water tanks so that water is available for feeding livestock, or irrigation in times where there is no wind. These windmill water pumping designs have also been used to provide electricity for rural electrification by using a battery system and low voltage systems. Today the more modern and efficient triple bladed rotor is commonly used in such stand-alone power systems.

Figure 6 A traditional style multiblded wind pump, known as a windmill.
(Image © 2005 Oklahoma Farm Bureau).

 

Managing Variability in Wind Turbine Systems

The greatest challenge to the economic use of wind power is its variability. There are very few areas on the Earth where wind is fairly constant throughout the day and throughout the year. Energy storage, or a backup system, is therefore required for windless or extremely windy periods and to supply energy even when the wind is blowing in a suitable range.

For small systems (up to a few kW) storage systems similar to those used for photovoltaic systems are used. This generally comprises a bank of deep-cycle lead-acid batteries. In hybrid generation systems, wind turbine generators are often coupled with a traditional diesel generator and an array of photovoltaic cells.

For larger systems, the problem of variability is more complex. One possibility is to build an interconnected grid with wind turbines at different locations, thereby reducing the probability of windless conditions. Proposals have also been made to couple wind generators to pumped hydroelectric storage. Suitable sites for economic storage are required for this option. The present strategy is to not consider storage in large grid connected systems, where wind generators are used primarily to replace conventional fuels. Some studies have shown that large grids can absorb about 30% wind penetration without effecting the management of the grid system, although Western Power Corporation's Denham wind project aimed at 70% penetration. If none of these options are economically viable, then large systems must be seen to be fuel savers for conventional systems, with full backup required. However, depending on the wind regime, the backup system may be less capital intensive than baseload systems. For example, gas turbines, which are cheaper to install than a coal-fired system, may be viable in some circumstances, or low load diesel gen sets. New technologies are playing a part in this area. Supercapacitors are being installed by some major wind turbine manufacturers to improve some of the electrical quality issues that are caused by some wind technologies. Flywheels are also a technology that has been applied to this task.

As with other forms of solar energy, another form of storage is to convert the energy directly into its end use. For example, it is normal to use water storage tanks together with wind powered water pumps. In special cases, the energy may be stored directly in the form of heat for water or space heating, as purified water (with reverse osmosis machines), or even in the form of ice for refrigeration.

 

 

The Future

Although the generation of electricity from wind turbines has been economically marginal for many years, the future looks quite optimistic following the development of large-scale systems in both Europe and the United States. Improved subsystem technologies, the move towards mass production and installation experience are all serving to reduce costs significantly and wind turbines have become competitive with conventional-fueled systems in areas with a good wind resource.

For small-scale power generation in remote areas, the main competitor until recently was liquid-fueled systems. Many remote area power supplies (RAPS) combining wind and photovoltaics with diesel backup are now being installed. The outlook for large grid-connected wind farms is promising, with several utilities operating wind monitoring stations.

 

 

Further Information

Information regarding renewable energy resources, technologies, applications, systems designs and case studies.

National Renewable Energy Laboratory (USA)

Verve Energy

Energy Australia

Clean Energy Council

World Wind Energy Association

 

References