Environmental Engineering Reference
In-Depth Information
world's coastlines is estimated at between 2 and 3 million megawatts. In optimal
wave areas, more than 65 megawatts of electricity could be produced along a single
mile of shoreline (EERE, 2010a). Because the wind is originally derived from the
sun, we can consider the energy in ocean waves to be a stored, moderately high-den-
sity form of solar energy. According to certain estimates, wave technologies could
feasibly fulfill 10% of the global electricity supply if fully developed (WEC, 2010).
The west coasts of the United States and Europe and the coasts of Japan and New
Zealand are good sites for harnessing wave energy.
W
ave
e
nergy
: F
aCTs
, p
araMeTers
,
and
e
quaTions
Three main processes create waves: (1) air flowing over the sea exerts a tangen-
tial stress on the water surface, resulting in the formation and growth of waves; (2)
turbulent air flow close to the water surface creates rapidly varying shear stresses
and pressure fluctuations (when these oscillations are in phase with existing waves,
further wave development occurs); and (3) when waves have reached a certain size,
the wind can exert a stronger force on the upwind face of the wave, resulting in addi-
tional wave growth.
Waves located within or close to the areas where they are generated are called
storm waves
; waves known as
swell waves
can develop at great distances from their
point of origin. The distance over which wind energy is transferred into the ocean to
form waves is called the
fetch
. Sea state is the general condition of the free surface
of a large body of water, with respect to wind waves and swell, at a certain location
and moment (see Table 7.1).
The shape of a typical wave is described as
sinusoidal
(i.e., it has the form of
a mathematical sine function) (see
Fig u r e 7.2
). The difference in height between
the peaks and troughs is height
H
, and the distance between successive peaks (or
troughs) of the wave is wavelength λ. The time in seconds required for successive
TABLE 7.1
World Meteorological Organization
(WMO) Sea State Codes
WMO Sea
State Code
Wave Height
(meters)
Characteristics
0
0
Calm (glassy)
1
0-0.1
Calm (rippled)
2
0.1-0.5
Smooth (wavelets)
3
0.5-1.25
Slight
4
1.25-2.5
Moderate
5
2.5-4
Rough
6
4-6
Very rough
7
6-9
High
8
9-14
Very high
9
Over 14
Phenomenal
