INTRODUCTION - Wind Resource Assessment: A Practical Guide to Developing a Wind Project - page 9

Figure 1-5. Rare visual evidence of turbine wakes in an offshore wind farm. The increased
turbulence behind each turbine causes the water vapor in the air to condense as droplets, forming
a visible contrail. The wind speed in each wake is also reduced. Source: Horns Rev 1 owned
by Vattenfall. Photographer Christian Steiness.
The process begins by producing a detailed picture of how the wind resource is
distributed across the site, supported by measurements and spatial modeling of some
kind. In sites with complex terrain and wide variations in land cover, this can be a
significant technical challenge. A further complication is wake (or array) interference
between turbines. When a turbine extracts energy from the wind, a zone of reduced
wind speed and increased turbulence is created behind it (Fig. 1-5). Any turbines
that happen to be within this wake will generally produce less power than if the
upwind turbines were not there. Fortunately, wakes tend to expand and dissipate with
distance downwind as turbulence exchanges energy with the surrounding, undisturbed
wind flow. How the wakes from all the turbines impact plant production is usually
estimated with a specialized wake model.
1.4 PURPOSE AND ORGANIZATION OF THIS TOPIC
As we have seen, designing a wind project and estimating its energy production depend
on a detailed and accurate assessment of the wind resource, which is where this topic
comes in. The topic is primarily intended to give guidance to practitioners and students
on the accepted methods of wind resource assessment for utility-scale wind farms. The
goal is not to impose conformity in every respect. On the contrary, the topic often
highlights areas where there is room for reasonable variation, even disagreement, on
the approaches that can be used. Nonetheless, the range of variation has limits. It may