Power curves for wind turbines - Wind Power Generation and Wind Turbine Design - page 611

The current industry standard IEC 61400-12-1 [3] defi nes, among others, a
uniform procedure for the measurement of power curves. This defi nition relies on
temporal averaging of wind speed and power output. Due to the turbulent nature of
the wind and the non-linear dependency of power on wind speed, this power curve
combines the characteristic of the turbine together with the statistical features of
the wind at the special site under investigation. This combination makes the esti-
mation of the annual energy yield at a certain site especially easy. On the other
hand, systematic averaging errors are introduced through the mentioned non-
linearity, and the power characteristic of the turbine cannot be separated from the
site effects. These weaknesses are well known, and several corrections have been
proposed, e.g. [19].
As an alternative, recently a different approach has been proposed to obtain the
power characteristic of wind turbines [16, 17], the Langevin power curve, which
relies on high frequency measurement data (approximately 1 Hz). Inspired from
dynamical systems theory, the power conversion process is regarded as a relax-
ation process, driven by the turbulently fl uctuating wind speed. The power charac-
teristic can then be obtained for every wind speed as the stable fi xed points of this
process. Averaging errors and infl uence of turbulence are thus avoided. Possible
multiple stable states are also captured, allowing deeper insight in the dynamics of
the power conversion. These features make the dynamical power characteristic
especially interesting as a monitoring tool for wind turbines.
As a work in progress, the simulation of high frequency power output signals
based on eqn (8) is currently developed. One application of this procedure will be
the prediction of energy yields for specifi c wind turbines under specifi c wind
conditions.
References
[1] Gottschall, J. & Peinke, J., Stochastic modelling of a wind turbine's power
output with special respect to turbulent dynamics. J. Phys: Conf Ser , 75 ,
pp. 012045, 2007.
[2] Burton, T., Sharpe, D., Jenkins, N. & Bossanyi, E., Wind Energy Handbook ,
Wiley: New York, 2001.
[3] IEC. Wind turbine generator systems, Part 12: Wind turbine power performance
testing, International Standard 61400-12-1, International Electrotechnical
Commission, 2005.
[4] Böttcher, F., Barth, S. & Peinke, J., Small and large fl uctuations in atmospheric
wind speeds. Stochastic Environmental Research and Risk Assessment , 21 ,
pp. 299-308, 2007.
[5] Betz, A., Die Windmühlen im Lichte neuerer Forschung. Die Naturwissen-
schaften , 15 , pp. 46, 1927.
[6] Rauh, A. & Seelert, W., The Betz optimum effi ciency for windmills. Applied
Energy , 17 , pp. 15-23, 1984.
[7] Rauh, A., On the relevance of basic hydrodynamics to wind energy technology.
Nonlinear Phenomena in Complex Systems , 11 (2) , pp. 158-163, 2008.