Environmental Engineering Reference
In-Depth Information
Chapter 1
Introduction to Wind Turbine Technology
1.1 How Much Energy is in the Wind?
Since the primary purpose of a wind turbine is to convert the kinetic energy (KE)
of the wind into (usually) electrical energy, it is useful to begin by considering the
amount of energy and power available, and reviewing the difference between those
two concepts. This simple analysis is a gentle introduction to the control volume
(CV) analyses that will be used extensively in later chapters.
Suppose the wind is blowing from left to right in Fig.
1.1
with a wind speed of
U
0
m/s. For simplicity assume that the wind is steady (i.e. not varying in time) and
uniform (i.e. not varying in position). Some effects of unsteadiness (in the form of
turbulence) and non-uniformity will be considered later. The air has constant
density, q, meaning that the flow, as are all flows considered in this topic, is
incompressible. At 20C the density of air at sea level is nearly 1.2 kg/m
3
; this
value can be used in most situations. Most modern turbines are ''horizontal-axis''
wind turbines, designated as HAWTs, for which the axis of rotation of the blades is
parallel or nearly parallel to the wind. Vertical axis wind turbines are not con-
sidered in this text.
In Fig.
1.1
the turbine is represented by a circular blade disk whose area
A = pR
2
where R is the blade radius in m. The following analysis determines the
kinetic energy in the air that passes the rotor disk per unit time, where the term
''rotor'' refers to the blades as a set. The analysis is done in the absence of the
blades, for reasons that will be explained shortly. The unit of energy is the Joule, J,
so the energy that passes will be in J/s, which gives Watts, the unit of power. It is
usually power output that concerns the designer and user of wind turbines.
However, it is usually electrical energy in the form of kilowatt-hours, kWhs, that is
measured and paid for by, say, the electricity utility connected to the turbine.
The right side of Fig.
1.1
shows an elemental volume of the airflow. Its exact
shape is not critical. The volume is about to cross the imaginary line (when viewed
side-on) in the wind that represents the blade disk. The volume of the element is
the product of its area, DA, and length normal to the disk, dx, so its mass is qDAdx
