Environmental Engineering Reference
In-Depth Information
Kaplan, the blades should move at twice the water velocity, whereas the skirted Francis is
most effi cient when the two speeds are roughly equal. The Pelton theoretically performs best
when its blades are moving at half the water velocity.
Reaction turbines use guide vanes at the rotor inlet to adjust the direction of the inlet fl ow
and the fl ow rate in order to vary the shaft power in response to the electrical load. The turbine
runner has a series of vanes whose complicated geometry is designed to extract maximum
power under design conditions. These vanes cannot be moved instantaneously, which has
implications for the power quality achievable from such turbines. For standalone applications
where fast control is essential, electronic control of the loads is used to maintain turbine
speed.
From Figure 2.2 it is clear that effi ciency depends critically on specifi c speed, and this in
turn depends on the effective head. Thus if the effective head varies signifi cantly, the speed
of the rotor needs to be adjusted to maintain high effi ciency. Most turbines, however, operate
at a fi xed speed and effi ciencies may thus vary to some extent.
Small hydro systems can be grid-connected by driving an appropriately sized induction
generator (see Chapter 4) provided that there is suffi cient fl ow control. Flows should be con-
trolled by the guide vanes to limit the output of the generator to the rated value.
2.4 Wind Power
2.4.1 The Resource
Winds result from the large scale movements of air masses in the atmosphere. These move-
ments of air are created on a global scale primarily by differential solar heating of the earth's
atmosphere. Therefore, wind energy, like hydro, is also an indirect form of solar energy. Air
in the equatorial regions is heated more strongly than at other latitudes, causing it to become
lighter and less dense. This warm air rises to high altitudes and then fl ows northward and
southward towards the poles where the air near the surface is cooler. This movement ceases
at about 30 °N and 30 °S, where the air begins to cool and sink and a return fl ow of this cooler
air takes place in the lowest layers of the atmosphere.
The areas of the globe where air is descending are zones of high pressure. Conversely
where air is ascending, low pressure zones are formed. This horizontal pressure gradient
drives the fl ow of air from high to low pressure, which determines the speed and initial direc-
tion of wind motion. The greater the pressure gradient, the greater is the force on the air and
the higher is the wind speed. Since the direction of the force is from higher to lower pressure,
the initial tendency of the wind is to fl ow perpendicular to the isobars (lines of equal pres-
sure). However, as soon as wind motion is established, a defl ective force is produced due to
the rotation of the earth, which alters the direction of motion. This force is known as the
Coriolis force. It is important in many of the world's windy areas, but plays little role near
to the equator.
In addition to the main global wind systems there is also a variety of local effects. Differ-
ential heating of the sea and land also causes changes to the general fl ow. The nature of the
terrain, ranging from mountains and valleys to more local obstacles such as buildings and
trees, also has an important effect.
The boundary layer refers to the lower region of the atmosphere where the wind speed is
retarded by frictional forces on the earth's surface. As a result wind speed increases with
