Page 33 - Wind Energy Handbook

SCOPE OF THE BOOK 7

values are achieved in practice (see Chapter 3). The power coefficient of a rotor
varies with the tip speed ratio (the ratio of rotor tip speed to free wind speed) and is
only a maximum for a unique tip speed ratio. Incremental improvements in the
power coefficient are continually being sought by detailed design changes of the
rotor and, by operating at variable speed, it is possible to maintain the maximum
power coefficient over a range of wind speeds. However, these measures will give
only a modest increase in the power output. Major increases in the output power
can only be achieved by increasing the swept area of the rotor or by locating the
wind turbines on sites with higher wind speeds.
Hence over the last 10 years there has been a continuous increase in the rotor
diameter of commercially available wind turbines from around 30 m to more than
60 m. A doubling of the rotor diameter leads to a four-times increase in power
output. The influence of the wind speed is, of course, more pronounced with a
doubling of wind speed leading to an eight-fold increase in power. Thus there have
been considerable efforts to ensure that wind farms are developed in areas of the
highest wind speeds and the turbines optimally located within wind farms. In
certain countries very high towers are being used (more than 60–80 m) to take
advantage of the increase of wind speed with height.
In the past a number of studies were undertaken to determine the ‘optimum’ size
of a wind turbine by balancing the complete costs of manufacture, installation and
operation of various sizes of wind turbines against the revenue generated (Molly
et al., 1993). The results indicated a minimum cost of energy would be obtained with
wind turbine diameters in the range of 35–60 m, depending on the assumptions
made. However, these estimates would now appear to be rather low and there is no
obvious point at which rotor diameters, and hence output power, will be limited
particularly for offshore wind turbines.
All modern electricity-generating wind turbines use the lift force derived from the
blades to drive the rotor. A high rotational speed of the rotor is desirable in order to
reduce the gearbox ratio required and this leads to low solidity rotors (the ratio of
blade area/rotor swept area). The low solidity rotor acts as an effective energy
concentrator and as a result the energy recovery period of a wind turbine, on a good
site, is less than 1 year, i.e., the energy used to manufacture and install the wind
turbine is recovered within its first year of operation (Musgrove in Freris, 1990).

1.3 Scope of the Book

The use of wind energy to generate electricity is now well accepted with a large
industry manufacturing and installing thousands of MWs of new capacity each
year. Although there are exciting new developments, particularly in very large
wind turbines, and many challenges remain, there is a considerable body of estab-
lished knowledge concerning the science and technology of wind turbines. This
book is intended to record some of this knowledge and to present it in a form
suitable for use by students (at final year undergraduate or post-graduate level) and
by those involved in the design, manufacture or operation of wind turbines. The
overwhelming majority of wind turbines presently in use are horizontal-axis, land-