Environmental Engineering Reference
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introduces such methods, so as to estimate the performance of a wind turbine. The
procedure applies to single wind turbines, while ongoing developments lead
towards integration of entire wind parks, and possibly large networks.
The approach is restricted to large-scale horizontal-axis wind turbines here. It
is also assumed that the produced electrical energy is directly fed into the grid.
This is not an essential restriction but facilitates putting the presented work in a
relevant context. For a detailed overview of different types of wind turbines and
corresponding modes of operation see, e.g. [2].
2 Power performance of wind turbines
2.1 Introduction to power performance
In the past 30 years, recommendations and standards were defi ned to determine
the power performance of a wind turbine. Permanently developed, the International
Electrotechnical Commission (IEC) set the international standard IEC 61400-12 and
its revised version IEC 61400-12-1 in 2005 [3]. These common guidelines defi ned
the power performance characteristic of a wind turbine by the so-called power curve
and its corresponding estimated annual energy production (AEP). The IEC standard
gives a good estimation of long-time power production (through the AEP), which is
of primary importance for an economical approach to wind energy.
Regarding actual power performance, the power curve is a powerful tool to
estimate the power extraction process, as it quantifi es the relation between incom-
ing wind and power output of a wind turbine. Simultaneous measurements of wind
speed u ( t ) and power output P ( t ) must be performed for the wind turbine con-
cerned. Here the power output P ( t ) is the net power released by the wind turbine
into the electrical grid. From this data collection, a functional relation P ( u ) can be
defi ned, and a two-dimensional curve of power output vs. incoming wind speed can
be derived. This is what power performance refers to. Such procedure can then be
applied on single wind turbines in order to characterize their power performance,
monitor their behavior over time as well as predict their power production. While
this prediction is well described by the IEC defi nition of AEP, monitoring methods
can be defi ned based on a dynamical approach to wind energy conversion.
In order to test power performance, a measurement of the wind velocity must be
performed. As a wind turbine distorts the incoming wind fi eld, a measurement in
the rotor plane or closeby is not useful, at least not without further corrections.
Instead the incoming upstream velocity is generally chosen as representative of the
wind fi eld, and measured from a meteorological mast at turbine hub height, a cer-
tain distance in front of the turbine. Based on these considerations, it becomes
possible to quantify the power performance of a wind turbine in simple ways.
2.2 Theoretical considerations
The purpose of this section is to give a simple understanding of fl uid mechanics
applied to wind turbines. A detailed description of the formulas and derivations
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