How much electricity can I expect a turbine to produce?

How much electricity can I expect a turbine to produce?

The output of the turbine depends on key factors such as:

• Swept area of the blades;
• Availability of wind at the site;
• The characteristics of the available wind;
• Availability and efficiency of the turbine; and
• Real power curve of the turbine including cut-in/out wind speeds.

It should be a reasonably obvious assumption that turbines with longer blades have the potential to produce more electricity than turbines with smaller ones. However it can also be the case that two similar looking turbines with blades of the same length can produce different amounts of electricity due to variances such as generator efficiency. Furthermore, placing the same model turbine in different sites will produce different amounts of electricity. So it is impossible to predict exactly the output of a wind turbine. However, with a thorough resource assessment, site assessment and accurate turbine performance parameters it is possible to estimate a range within which the turbine should perform.

It is in your own interest to install a high quality turbine. A broken down turbine will not produce any electricity. The turbine should be maintained to keep producing as much electricity as it can for as long as it can. A turbine may be seen to be turning in light winds but the generator may not be producing electricity. A turbine which experiences turbulent flows of air will produce less power than one experiencing steady, laminar flows of air.

Swept area is an important indicator of the likely output of a turbine. The manufacturer's rating must be considered in conjunction with the swept area. It is possible to quickly use the swept area to check if the claims of a manufacturer or the rated power of a turbine is unrealistic or optimistic. If you compare two turbines of equal rating and one has a smaller swept area than the other, it is more likely that the one with the larger area will be closer to the rating. 

All turbines have power curves which are graphical representations of key specifications such as:

• The wind speed at which the turbine starts to generate ('cut-in');
• The wind speed at which the turbine produces its rated output ('rated wind speed'); And
• The wind speed at which the turbine shuts down for safety reasons ('cut-out').

The power curve will give an indication of the power production at a range of wind speeds. Manufacturers provide power curves but these are often somewhat optimistic or not based on real certified data. Only power curves drafted by an independent third party laboratory can be given any credence. It is best to use swept area in conjunction with the turbine rating as the specifications to help decide what size turbine you require.

Figure 1 below is a sample power curve. It shows that for this sample turbine the cut-in wind speed is between 2 metres per second (m/s) and 3 m/s, the rated wind speed is 10 m/s (22 mph) and the cut-out wind speed is 15 m/s. Between 10 m/s and 15 m/s control systems are used to prevent the turbine from over speeding while still availing of high energy winds to produce electricity. Once an upper limit is reached most turbines must deploy some means of avoiding excessive rotational speeds in high winds or storms. This is necessary to prevent damage and the control can include turning the turbine away from the wind (yawing), applying a brake to the rotating shaft or feathering/pitching the blades out of the wind.

Figure 1: Sample power curve for micro-scale wind turbine

The peak output of a turbine can exceed the rated power during times of high winds or gusts. This peak output must be known so that electronics such as the inverter can be sized correctly to prevent damage to their components. There may be a lag between the electrical spike occurring and the application of mechanical control mechanisms to prevent the damage so the system must be able to cope with such instances. Numerous inverters and controllers have been burned out in the past due to inadequate sizing for these events.

If you are making a decision between two turbines based on rated power compare two power curves of both turbines at 5 m/s and 10 m/s so that a direct comparison can be made (assuming both power curves are accurate and third party certified). 5 m/s and 10 m/s wind speeds occur more often than 15 m/s speeds.

A typical cut-in wind speed for a domestic or small scale turbine would be 3m/s. A low cut-in speed is desirable because the turbine will be producing electricity for longer periods and in periods of less energetic winds. Better to be producing something rather than nothing.

Typical rated wind speeds for small turbines are between 10 m/s and 15 m/s. As with cut-in speeds the lower a rated wind speed is the more often the turbine will be producing its rated output. This is one reason why turbines should not be selected on name plate rating alone. It may be the case that a turbine will only reach its rated output infrequently if the wind speed required occurs infrequently.

The key performance indicator for a turbine system is the amount of units it produces over a year or over its life-time, not the amount it produces during the 3 windiest nights or months of the year.