Wind resource and site assessment - Wind Power Generation and Wind Turbine Design - page 85

has a starting diameter equal to the rotor diameter and is linearly expanding as a
function of the downwind distance. The Ainslie model is based on a numerical
solution of the Navier-Stokes equations with an eddy viscosity closure in cylindri-
cal coordinates. The eddy viscosity is described by the turbulent mixing due to the
induced turbulence, generated within the shear layer of the wake, and the ambient
turbulence [ 33 ].
Other sources of losses could be related to availability, electrical losses, high
wind hysteresis, environmental conditions like icing and blade degradation and
curtailments, of which wind sector management is the most common one. As
explained earlier turbine loading is infl uenced by the wake effects from nearby
machines. For some wind farms with particularly close spacing, it may be neces-
sary to shut down certain wind turbines for certain wind directions. Typically this
is done in a wind climate with a very dominant and narrow main wind direction.

7.3 Uncertainty

Uncertainty analysis is an important part of any assessment of the long-term energy
production of a wind farm. The most important contributors to the uncertainty are:

• on-site wind speed measurements

• long-term correction

• fl ow modelling

• performance of the wind turbine

• park losses

Behind each point a vast number of aspects have to be considered and quantifi ed
including anemometer calibration, mounting effects, inter-annual variability, qual-
ity of the long-term data, vertical and horizontal extrapolation, quality of the topo-
graphic input, power curve uncertainty, wake model, etc. Some inspiration can be
found in [ 34 ].

R eferences

[1] European Wind Energy Association. Wind Energy - The Facts. ISBN-978-1-
84407-710-6, 2009.
[2] IEC 61400-12-1. Wind Turbines Part 12-1: Power performance measure-
ments of electricity producing wind turbines , 2005.
[3] IEA. Recommended practices for wind turbine testing, Part 11: Wind speed
measurement and use of cup anemometry , 1999.
[4] Lindelöw, P., Courtney, M., Mortensen, N.G. & Wagner, R., Are Lidars good
enough? Accuracy of AEP predictions in fl at terrain generated from measure-
ments by conically scanning wind sensing Lidars. Proc. of European Wind
Energy Conf. , Marseille, France, 2009.
[5] Brock, F.V. & Richardson, S.J., Meteorological Measurement Systems,
Oxford University Press, 2001.