Features of Conventional and Renewable Generation - Renewable Energy in Power Systems - page 47

Figure 2.20 Artist ' s impression of axial fl ow tidal current turbines. (Reproduced with permission of
Marine Current Turbines Limited)
from this resource, the magnitude of this variability is small compared to demand
variations.
2.7 Wave Power
2.7.1 The Resource
The passage of wind over the surface of the sea results in the gradual transfer of energy into
the water to produce waves, so wave energy is also an indirect form of solar energy. Wind
power typically has densities in the range 1.2-1.8 kW/m 2 . Waves with a typical power density
of 50 kW per metre of wave front or crest length are in effect a highly concentrated form of
solar energy. The distance over which this process of wave generation occurs is called the
'fetch' and longer fetches produce larger, more powerful waves as do stronger winds and
extended periods of wind. Sea waves are characterized by their wave height ( H ), period ( T )
and crest length. The power per metre of crest length is proportional to the period and to the
square of the wave height.
Wave records indicate that the heights and periods of ocean waves vary continuously over
time, which results in wave power varying almost continuously. However, the conversion of
the energy of a wave face into electricity output tends to partly smooth this variability.
Because of these irregularities the calculation of the power density requires an averaging
process. The signifi cant wave height H S is approximately equal to the average of the highest