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The Wind

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    The Wind
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          The origin of wind
      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 powers a global
      atmospheric convection system reaching
    from the earth's surface to the stratosphere
            which acts as a virtual ceiling.
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     The Atmosphere (Troposphere)
    The atmosphere around the globe is a very
       thin layer. The globe has a diameter of
    12,000 km. The troposphere, which extends
     to about 11 km altitude, is where all of our
    weather, and the greenhouse effect occurs.


        On the picture you can see a stretch of
    islands 300 km across, and the approximate
      height of the troposphere. To look at it at a
    different scale: If the globe were a ball with a
       diameter of 1.2 meters the atmosphere
              would only be 1 mm thick.


Source: www.windpower.org
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  An estimated 1 to 3 % of energy from the Sun
 that hits the earth is converted into wind energy.
This is about 50 to 100 times more energy than is
converted into biomass by all the plants on earth
through photosynthesis. Most of this wind energy
 can be found at high altitudes where continuous
 wind speeds of over 160 km/h occur. Eventually,
the wind energy is converted through friction into
  diffuse heat all through the earth's surface and
                     atmosphere.
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    The wind resource
    •   The Geostrophic Wind
    •   Geographical variations
    •   Global circulations
    •   Annual and seasonal variations
    •   Synoptic and diurnal variations
    •   The surface boundary layer
    •   Turbulence
    •   The energy in the wind
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                       The Geostrophic Wind
            winds balanced by the Coriolis and Pressure Gradient forces

      An air parcel initially at rest will move from high pressure to low pressure
     because of the pressure gradient force . However, as that air parcel begins
       to move, it is deflected by the Coriolis force to the right in the northern
       hemisphere (to the left on the southern hemisphere). As the wind gains
     speed, the deflection increases until the Coriolis force equals the pressure
     gradient force. At this point, the wind will be blowing parallel to the isobars.
             When this happens, the wind is referred to as geostrophic.
    The geostrophic wind is found at altitudes above 1000 meters above ground
                                        level.




Source: http://ww2010.atmos.uiuc.edu
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    Geographical variations
    • Different surface heating from the sun
       – High surface heating close to equator
       – Day and night differences
    • The non-uniformity of the earth’s surface
       – Land masses and ocean
       – Hills an mountains
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     Source: www.bergey.com
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                                  Wind Resources at 50 (45) m Above Ground Level
                                 Color Sheltered   Open    At a sea   Open sea   Hills and
                                        terrain    plain    coast                 ridges




     Source: www.windpower.org
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               Global circulations
                                                                               http://www.windpower.org/en/tour/wres/globwin.htm




Source: Wind Power Plants, Fundamentals, Design, Construction and Operation.
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     Global circulations
     • Rossby circulation
        – Northern and southern hemisphere
        – Moves warm air to the poles and cold air to the subtropical areas
     • Hadley circulation
        – Equatorial regions (30oS – 30oN)
        – Produces the constant wind systems of the north-east and south-east
          trade winds
     • Monsoons
        – Large scale motion due to differences in temperature
            • Indian ocean
            • Atlantic ocean
            • Africa
     • Tropical cyclones
        – Rising hot humid air at the equatorial regions
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     Annual and seasonal variations
     • Year to year variation in annual wind speeds is hard to predict
     • Characterized by probably distribution
         – Wind speed probability is calculated as a Weibull curve




     Where:
              F    Wind speed probability            [-]
              k    Shape factor                      [-]
              A    Weibull parameter                 [-]
              V    Wind velocity                     [m/s]
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     Synoptic and diurnal variations

     • Synoptic variations
       – Passage of weather systems
           • Catabatic wind


     • Diurnal variations
       – Predictable daily variations
           • Sea land breeze
           • Mountain top – mountain valley wind
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                                            Catabatic wind
                                                          Synoptic variation




Source: Wind Power Plants, Fundamentals, Design, Construction and Operation.
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                                         Sea land breeze
                                                           Diurnal variation




Source: Wind Power Plants, Fundamentals, Design, Construction and Operation.
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     Sea land breeze
         Diurnal variation
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     Surface Boundary Layer
     • The principal effects
       governing the properties of
       the boundary layer:
        –   Strength of the geostrophic wind
        –   Surface roughness
        –   Coriolis force
        –   Thermal effects
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       Vertical wind speed gradient




Where:
 h1 Reference height [m]
 h2 Height                [m]
 z0 Roughness length [m]
 v1 Wind velocity at the reference height [m/s]
 v2 Wind velocity         [m/s]
Source: Wind Power Plants, Fundamentals, Design, Construction and Operation.   And Wind Energy Handbook
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       Obstacles on the ground




Source: Wind Power Plants, Fundamentals, Design, Construction and Operation.
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       The wind stream over hills




       The wind stream over hills with a gradient lower than 10% is accelerated at
       the hill top, but without disturbing stalls and turbulence. This is an excellent
       opportunity for utilizing the power in the wind


Source: Wind Power Plants, Fundamentals, Design, Construction and Operation.
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                               Wind Spectrum




                               • Synoptic variations
                                  – Passage of weather systems
                               • Diurnal variations
                                  – Predictable daily variations

Source: Wind Energy Handbook
                               • Turbulence
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                             Turbulence
     Turbulence refers to fluctuations in wind speed on a relatively fast
        time-scale, typically less than 10 min.




     Where:
        V Wind velocity                [m/s]
        V Mean wind velocity           [m/s]
        v’ Turbulent velocity          [m/s]
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     The power and energy in the wind




     Where:
              r   Density       [kg/m3]
              V   Wind velocity [m/s]
              A   Area          [m2]
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Time [hours]




Energy [MWh]
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     Wind measurements
     • The cup anemometer


     • The vane anemometer


     • The ultrasonic anemometer


     • The hot-wire anemometer
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     The wind Rose
                        To show the information
                        about the distributions of
                          wind speeds, and the
                     frequency of the varying wind
                       directions, one may draw a
                       so-called wind rose on the
                         basis of meteorological
                      observations of wind speeds
                           and wind directions.

				
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posted:5/3/2011
language:English
pages:28