Page 99 - Fundamentals of Ocean Renewable Energy Generating Electricity From The Sea

92 Fundamentals of Ocean Renewable Energy

maximizing the power output from the wind at a selected location, and
choosing/designing the best wind turbine that can produce maximum power at
minimum cost, it is necessary to understand and examine variations in the wind
speed.
There are large variations in wind speed, both geographically and tempo-
rally. The spatial and temporal variations of wind can be studied at a number
of scales. The spatial variations of the wind resource can be examined for
various climates (e.g. across the globe), and also within a climatic region. The
spatial variations within a region (e.g. northeast of the United States), in general,
depend on the climate of the region, Coriolis force (due to rotation of the Earth),
and physical geography. For instance, surface roughness (land vs. sea, type of
vegetation, size of buildings and structures), and the presence of mountains
greatly influences the variation of wind speed in a region. Fortunately, climate
models and wind data collected over the past decades give us sufficient data
to understand these variations. On a smaller scale, wind energy varies within a
wind farm, as the wind speed significantly reduces in the wake of wind turbines.
This is the subject of micrositing, and will be discussed in Chapter 9.

4.3.1 Atmospheric Boundary Layer
Above certain heights, it can be assumed that the wind speed is not influenced
by the Earth’s surface (ocean or land). At these high altitudes, the wind is mainly
driven by synoptic air pressure gradient and Coriolis (Earth’s rotation) forces.
Synoptic air pressure gradients are associated with weather systems that have
the scale of days (e.g. high- and low-pressure systems), and can be predicted by
numerical weather models. The wind (air flow) that is free from surface effects
is called ‘geostrophic wind’. At lower altitudes, the roughness of the surface
and thermal effects such as air-sea interactions significantly affect the wind
speed. This zone is called the atmospheric boundary layer. Assuming that the
wind is strong enough to sufficiently mix the boundary layer, the wind speed is
mainly controlled by surface roughness; in so-called neutral stability the surface
heating/cooling does not affect the wind profile. The distribution of the wind
near the surface of the ocean or land can be simply represented by a logarithmic
profile:

u * * τ o
u(z) = [ln(z/z o ) + ψ] ; u = (4.6)
κ ρ
*
in which u is the shear velocity, τ o is surface shear stress, κ is von Karman’s
constant (about 0.41), and z o is the surface roughness. ψ is a stability term and is
considered zero in neutral stability condition. Typical values for roughness range
from 0.7 m for forests to 0.0001 m in the ocean. For more accurate estimation
of surface roughness in the ocean, waves should be included. Assuming that the