Turbulence is a combination of natural turbulence and the wake created by wind the turbines themselves. On the one hand, spacing is important: The higher the level of turbulence, the further apart the wind turbines need to be. Spacing not only contributes to the stability of the wind turbine but also helps to avoid wake turbulence.
However, on the other hand the WECs should not be too far apart, as this could significantly reduce the potential yield of the site. Enercon Sales’ Planning department is in charge of optimising wind farm layouts in terms of the turbines’ stability, sound power levels, shadow casting and yield – key factors for feasibility studies.
To carry out feasibility studies, Enercon experts not only rely on standard programs and special wake model software but also on an exclusive profitability calculation program developed by their company.
Testing structural safety
“Wake” is the swirl of air which occurs behind the WEC once the air has passed around the rotor. Two parameters are necessary to calculate the minimum permissible distance between the wind turbines at which structural safety is ensured: the natural surrounding turbulence and the average wind speed.
Natural turbulence can be determined by looking at wind statics with respect to the characteristics of the terrain. The average wind speed can be taken from a wind expertise or can be determined by comparing with neighbouring wind farms.
The result of calculations made with the “wake” model is recognised as a valid expertise for Enercon wind power farms with the same wind turbine types. For wind farms with different types of wind turbines, an additional tool is used to determine the permissible spacing and stability of the WECs based on results of the wake assessment.
This analysis requires the installation coordinates and the wind distribution function. A pre-calculation such as this, is a valuable instrument for determining the wind farm layout. However, external turbulence observations and stability certifications are still necessary for wind farms with mixed turbine types.
“Once pre-calculations have been done, an external expertise is generally just a formality,” says Stephan Kettler, wind farm planner at
Enercon GmbH in Aurich. “Standard configurations with distances of 5 times the rotor diameter in the main wind direction and 3 times the rotor diameter in the secondary wind direction are a thing of the past, as they do not make sufficient use of the area’s yield potential.”
For the initial site configuration, Enercon Site Assessment compiles a yield estimation and noise forecast. Then Sales carries out a feasibility study including energy yield estimation, investment cost calculations, operating costs and financing.
In order to consider all the aspects, Enercon has developed a program which compares various farm layouts and their potential profitability. The Planning, Site Assessment and Sales departments then use a multiple feedback process to first determine the ideal configuration in terms of structural safety.
Next, yield and farm efficiency are calculated for this configuration. And finally, Sales takes look at profitability from another angle by comparing different turbine sizes, tower heights and number of wind turbines.
Profitability versus efficiency
Wolfgang Lübbe from Enercon Sales department in Aurich, points out the advantages of this process based on the Löningen wind farm near Cloppenburg/Lower Saxony.
According to the “conventional” planning method, five wind turbines could fit on the site. But turbulence observations and structural safety calculations carried out by Enercon showed that it was possible to install seven turbines: Energy yield estimations and profitability forecasts indicated that a configuration with seven wind turbines would be more profitable forwind the investor than five.
Although the wind energy farm’s efficiency was lower with seven wind turbines (only 86.8 % vs. 93 % with five wind turbines), the pre-tax profit with seven turbines was roughly three million Euros higher than with a standard layout plan.
Whereas investment was the same for both models, return of investment was considerably improved with seven machines.
On flat terrain efficiency is not the crucial criterion
Owners and planners often consider wind farm efficiency as being the main criterion. But on a flat terrain with good wind conditions it is not the decisive factor, explains Lübbe. A relatively low gain in efficiency often comes with a higher price. For example, the efficiency of the Scharrel wind farm (near Cloppenburg) would have been approximately 4 % higher with only four wind turbines instead of the final number of seven wind turbines.
The expected pre-tax profits would have been a third less – under the same economic conditions. Of course, this does not mean that farm efficiency does not play an important role in configuring a farm. Wolfgang Lübbe: “Wind farm efficiency is a vital criterion when the farm is on exposed or rough terrain. But here again, it is definitely worth using Enercon’s farm layout optimisation plan.”
Wind energy capacity installed by ENERCON
Worldwide. Number of installed wind turbines: more than 16.000. Total wind power: more than 20 GW.
Germany. Number of installed wind turbines: more than 8,000. Total wind power: more than 10 GW.
Wind energy market shares (2009, BTM):
1. Vestas, 12.5%.
2. GE, 12.4%.
3. Sinovel, 9.2%.
4. Enercon, 8.5%.
5. Goldwind, 7.2%.
6. Gamesa, 6.7%.
7. Dongfang, 6.5%.
8. Suzlon, 6.4%.
9. Siemens, 5.9%.
10. RePower, 3.4%.