Power and energy from wind turbines
The power output of a wind turbine varies with wind speed. Every turbine has a characteristic wind speed–power curve, often simply called the power curve, shown in Figure 9.
Figure 9 Typical wind turbine wind speed–power curve example
The energy a wind turbine will produce depends on both its wind speed–power curve shown in Figure 9 and the wind speed frequency distribution shown in figure 10 at the site.
The cut-in wind speed, shown in figure 9, is the wind speed below which the turbine does not rotate and generate power. Above the cut-in wind speed, the torque generated by air flow overcomes the frictional torques inherent in the mounting assembly of the turbine blades.
The shut-down wind speed is the wind speed at which the turbine has to be locked down to avoid damage due to excessive centrifugal forces and other mechanical stresses.
The range of wind speeds between cut-in and shut-down is called the operating range of the turbine.
Figure 10 A wind speed frequency distribution example for a typical site, showing the number of hours the wind blows at different speeds during a year
5.1 Calculating wind energy distribution
For each incremental wind speed between the cut-in wind speed and the shut-down wind speed the energy produced can be obtained by using this equation:
Figure 11 Calculating the energy produced at each given wind speed
This data can then be used to plot a wind speed – annual output energy distribution graph, such as that shown in Figure 12 .
Figure 12 Example of a wind speed – annual energy output distribution curve, showing annual electricity production at each wind speed
The total energy produced in a year is then calculated by summing the all the energies produced at each wind speed within the operating range of the turbine.
The wind speed distribution at a site is provided by measuring equipment that records the number of hours for which the wind speed lies within each one metre per second (1 m s−1) wide speed ‘band’, e.g:
- 0–1 m s−1
- 1–2 m s−1
- 2–3 m s−1, etc.
The longer the period over which measurements are taken, the more accurate is the estimate of the wind speed frequency distribution.
Activity 1 Factors on total energy generated
What other factors could affect the total energy generated?
Current commercial wind turbines typically have annual availabilities in excess of 90%, many have operated at over 95% and some are achieving 98%.
5.2 Estimating annual energy production
If the mean annual wind speed at a site is known, or can be estimated, the following formula (Beurskens and Jensen, 2001) can be used to make a rough initial estimate of the electricity production (in kilowatt-hours per year) from a number of wind turbines:
Figure 13 Annual energy production equation
This formula should be used with caution because it is based on an average of the characteristics of the medium- to large-scale wind turbines currently available. It also assumes an approximate relationship between annual mean wind speed (ideally, the mean speed at turbine hub height) and the a frequency distribution of wind speeds that may not be accurate for an individual site. It also does not allow for the different power curves of wind turbines that have been optimized either for low or high wind speed sites.
5.3 Wind speed maps, atlases and computer models
Maps and atlases specifically for wind energy purposes have been developed for many countries. Using long-term wind measurements and the WAsP computer model, a European Wind Atlas (Troen and Petersen, 1989) has been produced by the Risø Laboratory in Denmark. It includes maps of various areas within the European Union (Troen and Petersen, 1989) has been produced by the Risø Laboratory in Denmark. It includes maps of various areas within the European Union (for example, Figure 14), showing the annual mean wind speed at 50 m above ground level for five different topographic conditions: sheltered terrain, open plain, sea coast, open sea, hills and ridges.
Figure 14: Annual mean wind speeds and wind energy resources over Europe (EU Countries) combining land-based and offshore wind atlases (source: Troen and Peterson, 1989)
The atlas includes a series of procedures for taking account of site characteristics to estimate the wind energy likely to be available. These procedures work quite well on sites with a gentle topography but are not so good for very hilly terrain or urban areas. A similar atlas (included in Figure 14) has also been produced to cover the offshore wind energy resource in the European Union (Risø, 2009).
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