Page 190 - Fundamentals of Ocean Renewable Energy Generating Electricity From The Sea

178 Fundamentals of Ocean Renewable Energy

FIG. 7.15 Observed and simulated times series of (A) significant wave height (H s )and (B)zero
upcrossing wave period (T z ) in Galway Bay, Ireland. (Data supplied by the Marine Institute, Ireland,
and the model is described in Section 8.9.1.)

series such as that shown in Fig. 7.15 demonstrates, amongst other things,
two important temporal properties of waves. First, waves are stochastic, and
so time periods of modest wave activity are interspersed with both storms and
periods of relative calm. Although they contain vast amounts of energy, and it
would be a major breakthrough if we could exploit this energy, storm waves can
damage WECs, and hence the majority of WECs would enter survival model
under such conditions. By contrast, periods of relative calm, when electricity
generation would be minimal, represent ‘windows of opportunity’ for device
or array maintenance—particularly prolonged quiescent periods that tend to
occur during summer months. Also, note on Fig. 7.15 the qualitative comparison
of observations against model simulations. Associated with this qualitative
comparison, validation statistics can be derived such as the root-mean-square-
error and bias (Chapter 8), giving confidence in the model when applied to
longer-time series suitable for resource assessment.
Directional characteristics: The directional information obtained from di-
rectional wave buoys discussed in the previous section provide useful infor-
mation for many wave energy projects. Wave direction is usually presented
as a ‘waverose,’ or by statistical properties such as mean wave direction.
Directional information is particularly useful for attenuator-type wave energy