INTRODUCTION - Wind Resource Assessment: A Practical Guide to Developing a Wind Project - page 7

Figure 1-2. Utility-scale wind turbines. Source: AWS Truepower.
65 to 100 m in height at the hub, with a rotor 70 - 120 m in diameter, and with a rated
capacity of 1 - 5 MW. The turbine must operate reliably and at peak efficiency under
a wide range of wind conditions. This requires numerous components, from nacelle
anemometers to pitch actuators and yaw drives to power electronics, working together
in an integrated system.
Perhaps, the key characteristic of a wind turbine from the perspective of wind
resource assessment is the turbine power curve (Fig. 1-3). This describes the power
output as a function of wind speed measured at the hub. It is characterized by a cut-in
speed, typically around 3 or 4 m/s, where the turbine begins turning and generating
power; a sloping portion, where the output increases rapidly with speed; a rated speed,
typically around 13 - 15 m/s, where the turbine reaches its rated capacity; and a cut-
out speed, above which the turbine control software shuts the turbine down for its
protection.
Although well-operated turbines are finely tuned machines, it is wrong to assume
that a turbine produces exactly the expected power at every wind speed. For example,
blade wear and soiling, equipment wear, and control software settings can all cause
turbines to deviate from their ideal power curve. In addition, power output depends
on wind conditions, such as turbulence, the variation of wind speed across the rotor,
and the inclination of the wind flow relative to horizontal. Taking account of such
variations is part of the process of estimating energy production, and it starts with a
detailed understanding of the wind resource.
Wind power plants are likewise conceptually simple: they are just arrays of wind
turbines linked through a power collection system to the power grid (Fig. 1-4). How-
ever, designing a wind project often entails delicate trade-offs between, for example,
total plant output and construction cost.