Page 144 - Fundamentals of Ocean Renewable Energy Generating Electricity From The Sea

Wave Energy Chapter | 5 135

As we can see, the previous equation is exactly equivalent to Eq. (5.51). We can
use a similar solution as presented in Eq. (5.59) to find the amplitude and phase
of the motion for the point absorber. In order to find the solution, we just need to
replace mass (m) with total mass (m+m tun +m add ), and the damping coefficient
with c pto + c hd .

5.5 WAVE RESOURCE ASSESSMENT

For tidal resource assessment, it is generally sufficient to measure and analyse 3
the resource over a relatively short timescale (e.g. 30 days) in order to quantify
the resource over a much longer time period (e.g. >1 year). In addition, there
4
is minimal interannual variability in the tidal resource, and so the Annual
Energy Yield (AEY) for 1 year will generally provide a good estimate of the
AEY for any year. By contrast, waves exhibit variability over a wide range
of timescales, from storm (<24 h), to seasonal, and interannual variability. It
is therefore essential that any wave resource assessment captures this range of
timescales.
Gunn and Stock-Williams [13] analysed 6 years of NOAA WaveWatch III
model output to assess the ‘theoretical’ global wave energy resource (Fig. 5.15).
Spatial data of this form, including regional studies at higher resolution [14],
are useful for providing an overview of which areas are energetic (e.g. the
North Atlantic—Fig. 5.15), and so represent a good opportunity for wave energy
conversion, and which regions are less energetic (e.g. the Arabian Sea). Further,
many regions, setting aside any practical constraints, can perhaps be identified
as being too energetic, for example, much of the Southern Ocean, where the
wave climate would present significant challenges to device survivability (see
Section 5.5.2). However, spatial resource assessments of mean wave conditions,
even when based on relatively long-time periods (e.g. 6 years used to compile
Fig. 5.15), do not give potential developers and investors sufficient information
about temporal variability.
Temporal wave resource assessment can be performed directly either from
observations or from examining time series generated by (validated) numerical
models. The problem with relying on instrumentation, for example, directional
wave buoys (Section 7.2.1), is, in addition to cost, potential limitations on the
length of the deployment available for subsequent analysis. For example, in
northwest Europe, it is generally considered that a minimum of around 7 years
of data would be required to capture the interannual variability described by
the North Atlantic Oscillation (NAO)—a large-scale mode of natural climate
variability that has important impacts on the climate of northern Europe [15]
(Fig. 5.16). Further, limited measurements during summer months, for example,

3. For example by harmonic analysis.
4. Provided the site is not strongly influenced by wave-tide interaction.