Page 37 - Wind Energy Handbook

2

The Wind Resource

2.1 The Nature of the Wind

The energy available in the wind varies as the cube of the wind speed, so an
understanding of the characteristics of the wind resource is critical to all aspects of
wind energy exploitation, from the identification of suitable sites and predictions of
the economic viability of wind farm projects through to the design of wind turbines
themselves, and understanding their effect on electricity distribution networks and
consumers.
From the point of view of wind energy, the most striking characteristic of the
wind resource is its variability. The wind is highly variable, both geographically
and temporally. Furthermore this variability persists over a very wide range of
scales, both in space and time. The importance of this is amplified by the cubic
relationship to available energy.
On a large scale, spatial variability describes the fact that there are many different
climatic regions in the world, some much windier than others. These regions are
largely dictated by the latitude, which affects the amount of insolation. Within any
one climatic region, there is a great deal of variation on a smaller scale, largely
dictated by physical geography – the proportion of land and sea, the size of land
masses, and the presence of mountains or plains for example. The type of vegeta-
tion may also have a significant influence through its effects on the absorption or
reflection of solar radiation, affecting surface temperatures, and on humidity.
More locally, the topography has a major effect on the wind climate. More wind
is experienced on the tops of hills and mountains than in the lee of high ground or
in sheltered valleys, for instance. More locally still, wind velocities are significantly
reduced by obstacles such as trees or buildings.
At a given location, temporal variability on a large scale means that the amount
of wind may vary from one year to the next, with even larger scale variations over
periods of decades or more. These long-term variations are not well understood,
and may make it difficult to make accurate predictions of the economic viability of
particular wind-farm projects, for instance.
On time-scales shorter than a year, seasonal variations are much more predict-
able, although there are large variations on shorter time-scales still, which although
reasonably well understood, are often not very predictable more than a few days
ahead. These ‘synoptic’ variations are associated with the passage of weather
systems. Depending on location, there may also be considerable variations with the