Environmental Engineering Reference
In-Depth Information
Because so much power is generated by higher wind speed, much of the
energy comes in short bursts. The consequence is that wind energy from a
particular turbine or wind farm does not have as consistent an output as fuel-
fired power plants; utilities that use wind power provide power from starting
existing generation for times when the wind is weak thus wind power is
primarily a fuel saver rather than a capacity saver. Making wind power more
consistent requires that various existing technologies and methods be extended
in particular the use of stronger inter regional transmission to link widely
distributed wind farms since the average variability is much less; the use of
hydro storage and demand-side energy management [4]. The Earth is unevenly
heated by the sun resulting in the poles receiving less energy from the sun than
the equator does. Also, the dry land heats up (and cools down) more quickly
than the seas do. The differential heating drives a global atmospheric
convection system reaching from the Earth's surface to the stratosphere which
acts as a virtual ceiling. Most of the energy stored in these wind movements
can be found at high altitudes where continuous wind speeds of over 160 km/h
(100 mph) occur. Eventually, the wind energy is converted through friction
into diffuse heat throughout the Earth's surface and the atmosphere.
The total amount of economically extractable power available from the
wind is considerably more than present human power use from all sources.[5]
An estimated 72 TW of wind power on the Earth potentially can be
commercially viable,[6] compared to about 15 TW average global power
consumption from all sources in 2005.
The strength of wind varies, and an average value for a given location
does not alone indicate the amount of energy a wind turbine could produce
there. To assess the frequency of wind speeds at a particular location, a
probability distribution function is often fit to the observed data. Different
locations will have different wind speed distributions. The Rayleigh mode
closely mirrors the actual distribution of hourly wind speeds at many locations.
Rayleigh flow refers to
diabetic
flow through a constant area duct where the
effect of heat addition or rejection is considered.
Compressibility
effects often
come into consideration, although the Rayleigh flow model certainly also
applies to
incompressible flow.
For this model, the duct area remains constant
and no mass is added within the duct.
Wind power density (WPD) is a calculation relating to the effective force
of the wind at a particular location, frequently expressed in terms of the
elevation above ground level over a period of time. It further takes into
account wind velocity and mass.
