Wind energy - General
concepts
Index
What are wind, wind stress,
wind power and wind power class?
Wind
Wind Stress
Wind Power – Wind Power
density
How Can I Calculate the Amount
of Power at a Given Wind Speed?
Wind Power class
Types of wind
 What are wind, wind stress, wind power and wind power class?
Each of these forcing quantities has been used for the ocean and
atmospheric characterization. Wind velocity, and stress are all vector
quantities with two horizontal components. Typically these components are
given as east-wind (positive eastward), and north-south (positive northward).
On the other hand, wind power and wind class are scalar that represent only
the magnitude of the wind. Despite these similarities, the units, magnitude,
and meaning differ greatly.
Wind
Wind is the velocity at which the air
moves. Winds are commonly classified by their spatial scale, their speed, the types of forces that cause them, the geographic
regions in which they occur, or their effect. Forces which drive wind or
affect it are the pressure gradient force, the Coriolis force, buoyancy
forces, and friction forces. When a difference in pressure exists between two
adjacent air masses, the air tends to flow from the region of high pressure
to the region of low pressure. On a rotating planet, flows will be acted upon
by the Coriolis force, in regions sufficiently far from the equator and
sufficiently high above the surface. Since the wind speed changes as a
function of height, the height to which the winds correspond must be known.
For example, the anemometers and scatterometer wind
products are referenced to a height of 10m.
Wind Stress
Surface stress is the momentum flux
through the air-sea interface. One common interpretation is the vertical
transfer of horizontal momentum, per unit area. Stress is approximately
constant with height near the surface; unlike wind speed there is no need to
specify a reference height.
Wind Power
Wind power is the conversion of wind
energy into more useful forms, such as electricity, using wind turbines.
The Wind Energy and Power (Pwr) per unit area (A) is called the Wind Power Density (WPD) and has units
of watts/m2.
WPD=
Pwr / A = ½ * ρa * W3
where ρa is the air
density (1.225 kg/m3) and W
is the wind speed in m/s.
Wind Power Density is a useful way to
evaluate the wind resource available at a potential site. The wind power
density, measured in watts per square meter, indicates how much energy is
available at the site for conversion by a wind turbine.
Wind Power Class
Classes of wind power density for two
standard wind measurement heights are listed in the table below. Wind speed
generally increases with height above ground. In general, sites with a Wind
Power Class rating of 4 or higher are now preferred for large scale wind
plants.
How Can I Calculate the Amount of Power at a
Given Wind Speed?
Wind is a mass of air in
movement, so has kinetic energy.
From physics class:
kinetic energy (joules) = ½ * m * V2
where:
m = mass (kg) (1 kg = 2.2
pounds)
V = velocity (meters/second)
(meter = 3.281 feet = 39.37 inches)
Since energy = power x time
and density is a more convenient way to express the mass of
flowing air, the kinetic energy equation can be converted into a flow
equation:
Power in the
area swept by the wind turbine rotor:
Pwr = ½ * ρa * W3 * A
where:
Pwr =
power in watts (746 watts = 1 hp) (1,000 watts = 1 kilowatt)
ρa = air density
(about 1.225 kg/m3 at sea level, less higher up)
A = rotor swept area, exposed to the wind (m2)
W = wind speed at the hub in meters/sec (20 mph = 9 m/s) (mph/2.24 = m/s)
This equation determines the
power in a free flowing stream of wind. In a real system, it is impossible to
extract all the power from the wind because some flow must be maintained
through the rotor. So, some additional terms to get a practical equation for
a wind turbine are necessary.
Wind Turbine Power:
Pwr = ½ * ρa * W3 * A * Cp *
Ng * Nb
where:
Pwr =
power in watts (746 watts = 1 hp) (1,000 watts = 1 kilowatt)
ρa = air density
(about 1.225 kg/m3 at sea level, less higher up)
A = rotor swept area, exposed to the wind (m2)
Cp = Coefficient of performance (.59 {Betz limit} is the maximum
theoretically possible, .35 for a good design)
W = wind speed in meters/sec (20 mph = 9 m/s)
Ng = generator efficiency (50% for car alternator, 80% or possibly more for a
permanent magnet generator or grid-connected induction generator)
Nb = gearbox/bearings efficiency (depends, could be
as high as 95% if good)
Types of Wind
Seasonal winds
Seasonal winds are winds that only exist during
specific seasons, for example, the Indian monsoon.
Synoptic winds
Synoptic winds are winds associated with
large-scale events such as warm and cold fronts, and are part of what makes
up everyday weather. These include the geotropic wind, the gradient wind, and
the cyclostrophic wind.
As a result of the Coriolis force, winds in the
northern hemisphere always flow clockwise (when seen from above) around a
high pressure area and counterclockwise around a low pressure area (the
reverse occurs in the southern hemisphere). At the same time, winds always
flow from areas of high pressure to areas of low pressure. These two forces
are opposite but not equal, and the path that results when the two forces
cancel each other runs parallel to the isobars. Wind following this path is
known as geostrophic wind. Winds are said to be
truly geotropic only when other forces (e.g. friction) acting on the air are
negligible, a situation which is often a good approximation to the
large-scale flow away from the tropics.
In certain circumstances, the Coriolis force
acting on moving air may be almost or entirely overwhelmed by the centripetal
force. Such a wind is said to be cyclostrophic, and
is characterized by rapid rotation over a relatively small area. Hurricanes,
tornadoes, and typhoons are examples of this type of wind.
Mesoscale winds
Synoptic winds occupy the lower boundary of what
is considered to be "forecastable" wind.
Winds at the next lowest level of magnitude typically arise and fade over
time periods too short and over geographic regions too narrow to predict with
any long-range accuracy. These mesoscale winds
include such phenomena as the cold wind outflow from thunderstorms. This wind
frequently advances ahead of more intense thunderstorms and may be
sufficiently energetic to generate local weather of its own. Many of the
"special" winds, addressed in the last section of this article, are
mesoscale winds.
Microscale winds
Microscale winds take
place over very short durations of time - seconds to minutes - and spatially
over only tens to hundreds of meters. The turbulence following the passage of
an active front is composed of microscale winds,
and it is microscale wind which produces convective
events such as dust devils. Though small in scope, microscale
winds can play a major role in human affairs.
Sea/land breeze
Differential heating is the motive force behind
land breezes and sea breezes (or, in the case of larger lakes, lake breezes),
also known as on- or off-shore winds. Land absorbs and radiates heat faster
than water, but water releases heat over a longer period of time. The result
is that, in locations where sea and land meet, heat absorbed over the day
will be radiated more quickly by the land at night, cooling the air. Over the
sea, heat is still being released into the air at night, which rises. This
convective motion draws the cool land air in to replace the rising air,
resulting in a land breeze in the late night and early morning. During the
day, the roles are reversed. Warm air over the land rises, pulling cool air
in from the sea to replace it, giving a sea breeze during the afternoon and
evening.
|
