Fundamentals of wind energy - Wind Power Generation and Wind Turbine Design - page 34

b. Wind velocity is proportional to the height from the earth's surface. With the

continuously increasing blade length of large wind turbines, the differences of
the dynamic wind loads between the rotating blades become signifi cantly large,
resulting in a large resultant unbalanced fatigue load on the turbine blades, and
a resultant unbalanced torsional moment on the main shaft, and in turn, on the
wind tower.
During wind turbine's operation, a minimum clearance must be maintained c.

between the blade tips and the wind tower. Therefore, high blade stiffness is
required to avoid the collision between the blades and the tower. In practice, the
maximum blade length is constrained by required stiffness and stresses of blades.
Large wind turbines become more susceptible to variations in wind speed and

d.

intensity across the swept area.
Transportation and installation of long-length blades remain challenges to e.

the wind power industry. The length of a blade for a 4.5-5 MW wind turbine
ranges 50-70 m. It is very diffi cult to ship such long blades through current
highways and installed on the top of 120-160 m wind towers.
The tower strength is another consideration. For a given survivable wind speed,

f.

the mass of a wind turbine is approximately proportional to the cube of its blade
length and the output power is proportional to the square of it blade length.
Typically, the mass of a 4.5-5 MW is of 200-500 tons. It was reported that
doubling the tower height generally requires doubling the diameter as well,
increasing the amount of material by a factor of 8 [86].

To ensure the sustainability of the increase in power output and turbine size, all
these challenges must be carefully and effectively addressed.

7.2 Offshore wind turbine

With several decades of experience with onshore wind technology, offshore wind
technology has presently become the focus of the wind power industry. Due to the
lower resistance, wind speeds over offshore sea level are typically 20% higher than
those over nearby lands. Thus, according to the wind power law, the offshore wind
power can capture much more power than the onshore one. This indicates that an
offshore wind turbine may gain a higher capacity factor than that of its land-based
counterpart. In addition, because the offshore wind speeds are relatively uniform
with the lower variations and turbulence, it enables the offshore wind turbines to
simplify the control systems and reduces blade and turbine wears.
Sweden installed the fi rst offshore wind turbine in 1990, with the unit capacity
of 220 kW. Denmark built its fi rst demonstration offshore wind turbines in 1991,
which consists of 11 units, with the unit capacity of 450 kW. With the develop-
ments of offshore wind technology in the next several years, offshore wind tur-
bines entered the stage of industrial production in 2001. Today, high capacity wind
turbines focus on the offshore application. In 2009, nearly 600 MW offshore wind
power were added and connected to electric grids, basically by European coun-
tries, bringing the total accumulative installed offshore wind power capacity to
more than 2,000 MW. It is expected that in 2010 ten additional European