Page 38 - Wind Energy Handbook
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12                                                      THE WIND RESOURCE


              5.0
                        Synoptic peak
              4.5
              4.0
              3.5                                               Turbulent peak
             Spectrum, f.S(f)  3.0  Diurnal peak


              2.5
              2.0
              1.5
              1.0

              0.5
              0.0
                     10 days  4 days     24 h    10 h              2 h  1 hr 30 min 10 min  3 min    1 min 30 s     10 s    5 s
                                           Frequency, log (f)

            Figure 2.1  Wind Spectrum Farm Brookhaven Based on Work by van der Hoven (1957)

          time of day (diurnal variations) which again are usually fairly predictable. On these
          time-scales, the predictability of the wind is important for integrating large amounts
          of wind power into the electricity network, to allow the other generating plant
          supplying the network to be organized appropriately.
            On still shorter time-scales of minutes down to seconds or less, wind-speed
          variations known as turbulence can have a very significant effect on the design and
          performance of the individual wind turbines, as well as on the quality of power
          delivered to the network and its effect on consumers.
            Van der Hoven (1957) constructed a wind-speed spectrum from long- and short-
          term records at Brookhaven, New York, showing clear peaks corresponding to the
          synoptic, diurnal and turbulent effects referred to above (Figure 2.1). Of particular
          interest is the so-called ‘spectral gap’ occurring between the diurnal and turbulent
          peaks, showing that the synoptic and diurnal variations can be treated as quite
          distinct from the higher-frequency fluctuations of turbulence. There is very little
          energy in the spectrum in the region between 2 h and 10 min.



          2.2 Geographical Variation in the Wind Resource

          Ultimately the winds are driven almost entirely by the sun’s energy, causing differ-
          ential surfaceheating. The heating is most intense on land massescloser tothe equator,
          and obviously the greatest heating occurs in the daytime, which means that the region
          of greatest heating moves around the earth’s surface as it spins on its axis. Warm air
          rises and circulates in the atmosphere to sink back to the surface in cooler areas. The
          resulting large-scale motion of the air is strongly influenced by coriolis forces due to
          the earth’s rotation. The result is a large-scale global circulation pattern. Certain
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