It is important to understand the wind behaviour because wind is the “fuel” of the wind turbines. When there is no wind there is no power.

## Q: How much power is available from the wind?

*A:This of course depends on how fast the wind is blowing. The figures used in the examples below are the theoretical power available and not what is possible to achieve from a turbine.*

At wind speed of 5 m/s theoretical power available is 77 W/m²

At wind speed of 10 m/s theoretical power available is 613 W/m²

The above two figures are used to illustrate the exponential nature of power available in the wind with relation to wind speed.

*
In the first example the wind speed is 5 m/s and in the second 10 m/s. This shows a two-fold increase in wind speed. In the first example the power available is 77 W/m² and in the second 613 W/m². This shows an eight-fold increase in power available.
This is very important to remember as wind speeds are often *much closer to the lower figure of 5 m/s than they are to the higher figure of 10 m/s.

## Q: What is constant wind speed?

*A: Constant wind speed is wind speed that is always the same and does not change. This does not exist in the real world but only in laboratory wind tunnels. Therefore, production figures given at constant wind speed are no use when thinking about actual production of the turbine at the installation site.*

## Q: What is average wind speed?

*A: Average wind speed is made up of measurements of wind speed over given intervals of time. If readings are taken every five minutes for a period of one hour then twelve sets of readings bould be taken. If these readings are added together and then divided by twelve then the average is found.*

Example 1

Reading | Wind Speed m/s |
Theoretical Power Available (Watt) |

1 | 5 | 77 |

2 | 3 | 17 |

3 | 7 | 210 |

4 | 2 | 5 |

5 | 8 | 314 |

6 | 1 | 0 |

7 | 9 | 447 |

8 | 5 | 77 |

9 | 4 | 39 |

10 | 6 | 132 |

11 | 3 | 17 |

12 | 7 | 210 |

Average = 5 m/s 1545 W

Why average wind speed can be misleading?

The above example shows 12 readings that give an average of 5 m/s. If this average is used to calculate the power in one hour then we have 77 Wh. However if we use actual power available in the wind at the time of measurement and use this a a figure we get 129 Wh.

Example 2

Reading |
Wind Speed m/s |
Theoretical Power Available (Watt) |

1 |
0 |
0 |

2 |
0 |
0 |

3 |
0 |
0 |

4 |
0 |
0 |

5 |
0 |
0 |

6 |
0 |
0 |

7 |
10 |
613 |

8 |
10 |
613 |

9 |
10 |
613 |

10 |
10 |
613 |

11 |
10 |
613 |

12 |
10 |
613 |

*Average = 5 m/s 3678 W*

Example 2 also shows 12 readings that give an average of 5 m/s. If this average is used to calculate the power in one hour then we have 77 Wh. However if we use the actual power available in the wind at the time of measurement and use this a a figure we get 306 Wh. Theses two examples are simple examples to illustrate that average wind speeds can be misleading and that exactly how averages are made up is important on what is actually generated.