Page 116 - Fundamentals of Ocean Renewable Energy Generating Electricity From The Sea

Chapter 5

Wave Energy

Similar to wind energy, and in contrast to tidal energy, wave energy is a
stochastic form of electricity generation. Although modern forecasts make it
possible to predict waves with certainty over relatively short timescales (e.g. 24–
48 h), any longer-term planning must rely on statistical trends such as seasonal
variability. This is one of the challenges of wave energy conversion, another
being the extreme nature of the locations where, by their very nature, the wave
climate could be suitable for electricity generation. However, wave energy has
huge global potential, and its geographical distribution is generally more diverse
than tidal energy, which tends to be confined to a relatively low number of ‘hot
spots’ such as flow around headlands or through straits. Therefore, there is much
global interest, R&D, and investment in wave energy projects and technologies.
In this chapter, we investigate the nature of wind waves, through a con-
sideration of linear wave theory, examining fundamental properties of waves
such as dispersion wave power, and wave transformation in shoaling water. We
introduce the various wave energy converter (WEC) technologies and examine
the theory of heaving point absorbers in some detail. Finally, we consider wave
resource assessment and characterization, examining timescales of variability,
and the theoretical versus technical resource.

5.1 WAVE PROCESSES
Looking back at Fig. 1.14, ‘waves’ in the ocean occur over a vast range of scales,
from long-period (tidal) waves, with a period of several hours, to capillary waves
that have periods of less than 0.1 s. However, the waves that are suitable for
electricity generation within the context of ‘wave energy’ are wind waves and
swell waves, which generally have periods in the range 2–25 s. Wind waves
are generated due to transfer of wind energy and momentum into the wave
field. Initially, when a sea surface is calm, small pressure fluctuations associated
with turbulence in the airflow above the water surface are sufficient to induce
small ripples, or dimples, on the sea surface. Once these ripples have formed,
the small slopes provide a mechanism for horizontal winds to act further upon
the sea surface, leading to the development of sizeable waves. These locally
generated waves are known as ‘wind waves’, and waves that propagate far from
their source of generation are known as ‘swell waves’.
Fundamentals of Ocean Renewable Energy. https://doi.org/10.1016/B978-0-12-810448-4.00005-7
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