Wind Energy Theory

A wind turbine transforms the kinetic energy of the wind into electrical energy. Wind turbines can range in size from small ones installed on large yachts and buildings, to giant ones like those shown in Figures 1 and 2.

A wind farm is a set of wind turbines that are linked together to supply electricity to a local community or to an electricity grid for a larger population. Wind farms are often located along coastlines where the winds are strong. The Woolnorth wind farm in Tasmania, shown in Figures 1 and 2, is an example of this.

How do wind turbines work?

Wind turbines transform the kinetic energy of the wind into electrical energy.

The main steps are:

Step 1: Moving air pushes against the blades of the turbine, making them rotate. In the process, some of the kinetic energy of the moving air is transformed into the mechanical (rotational kinetic) energy of the spinning blades. (The wind still has some kinetic energy as it flows away from the turbine.)

Step 2: The shafts and the gears inside the gear box transfer the mechanical energy of the turbine to the generator. (The gears make the drive shaft to the generator spin faster than the shaft connected to the blade hub.)

What is the best place to locate a wind farm?

Wind turbines should be located where there are steady strong winds – though not so strong they would damage the turbines.

They work at their greatest possible energy efficiency, however, when they operate in ‘smooth’ air – that is, when the air particles are moving in the same direction and not whirling around and moving in different directions.

Ideal sites for wind turbines therefore are:

Away from obstructions such as forests, towers and rocky outcrops. (These would cause the air to swirl around.)
At the highest point possible.

The best place is on top of a smooth hilltop, where the wind can concentrate and increase in speed.

The power of the wind

The power available from the wind is proportional to the cube of the wind speed! This means:

If the wind speed were 2 times greater, the power available from the wind would be 8 times greater.
If the wind speed were 3 times greater, the power available from the wind would be 27 times greater.
If the wind speed were 10 times greater, the power available from the wind would be 1000 times greater!

This shows how important it is to locate wind farms in places where the winds are strong.

What determines how much power is generated by a wind turbine?

The amount of electrical power produced by a wind turbine doesn’t only depend on the speed of the wind, and how smoothly it flows, however. It also depends on the way the turbine is built:

The number of blades
The length of the blades
The shape of the blades
The mass (weight) of the blades
The pitch (angle) of the blades to the wind
The height of the tower
The gears used
The type of generator used
The computer system that controls the operation of the turbine and its power output (where this is used)

What is the energy efficiency of wind turbines?

Wind turbines do not produce electricity all the time. Although the wind might be available for as much as 70 % of the time, it is often not strong enough to operate the wind turbine at full capacity. The combination of absence of wind and inadequate wind strength means that even in a good location the wind turbine, over the course of a year, will generate only about 30 % of the amount it could generate in a constant strong wind.

A good site might have a 35 % capacity factor. This means that the turbines will produce 35% of their capacity on average over a year.

Apart from problems with the wind itself, some of the kinetic energy of the wind is transformed into heat energy (the gears and shafts get hot) and sound energy (the blades, gears and shafts make some noise as they spin). This is summarised in Figure 6.

Advantages of wind turbines

The main advantages of wind turbines, once they are built*, are:

They are a renewable energy resource.
They do not emit greenhouse gases or any other pollution.
They are more energy efficient than most power stations that burn fossil fuels.
They are less costly to run than many other energy resources.
They can be established in remote areas where other energy resources are not practical, even in places like Antarctica and on ocean-going yachts.

Figure 7 shows one of the wind turbines that were built by an Australian company in Antarctica. The headquarters of this company are in Darwin.

*Greenhouse gases and other pollutants are produced during the manufacture, transport and installation of wind turbines, but once they have operated for a year or so, they will have compensated for this. Overall, wind turbines help reduce the amount of greenhouse gases emitted into the atmosphere.

Disadvantages of wind turbines

Some of the disadvantages of wind turbines are:

The electrical power delivered varies because wind speed and direction vary. Sometimes the wind speed is too low to even start rotation. Therefore they can only be used to provide some of the electrical power people need.
They can be damaged by very strong winds and also corroded by salt in the air when near the sea.
It can be costly to connect them to the electricity grid, due to the distances involved.
Some people think they spoil the landscape.
They need to have flashing lights on the top to warn the pilots of any aircraft that fly overhead of their presence. People who live close by sometimes complain the flashing lights disturb their sleep.
In many countries, most wind turbines are constructed off-shore, usually because people do not want them across the land. This is a much more expensive location, because the turbines are more costly to install and to maintain, because of problems such as corrosion of metal parts by sea water and damage from the constant movement of waves and sand. This means the electricity they generate is more expensive than that generated by coal-fired power stations.
There may be an impact on local bird populations.