Page 285 - Fundamentals of Ocean Renewable Energy Generating Electricity From The Sea

272 Fundamentals of Ocean Renewable Energy

perspectives. Further, within the context of global warming, it is appropriate
to consider how ocean renewable energy resources will vary in the future,
particularly the tidal range resource, since the embankments of tidal range power
plants are expected to have a lifespan (>100 years) that will witness evolution in
the Earth’s climate. Due to sea-level rise, tidal dynamics are expected to change
in the future.

10.1.1 Timescales of Multiple Ocean Renewable Energy
Resources
The temporal variability of various proxies for a selection of UK ocean
renewable energy resources (tidal elevation, wind speed, and wave height) is
shown in Fig. 10.1A–C for a typical year—2012. UK demand for electricity over
the same time period is also shown (Fig. 10.1D). Fig 10.1A demonstrates clearly
the spring-neap cycle that is characteristic of many tidal regions throughout the
world, and indeed is one of the main challenges that the tidal energy industry
faces: time periods with high potential for electricity generation (springs),
followed by time periods with minimal potential for generation (neaps). By
contrast, neither winds nor waves suffer from such fortnightly variability.
However, beyond seasonal variability, wind and wave resources are stochastic,
and so not predictable, other than at relatively short (e.g. 24–28 h) timescales
(i.e. based on forecasts generated by well-constrained operational models).
Rarely in Fig. 10.1A, B, or C does the variability of individual ocean renewable
energy resources track the pattern of demand for electricity (Fig. 10.1D).
Widén et al. [1] presented a useful seasonal/diel comparison across a range of
renewable energy resources (solar, wind, wave, tide) at a variety of contrasting
locations (Fig. 10.2). It is important to note, in this figure, that each of the
resources has been normalized by the 98th percentile at each location, and
so the magnitudes between resources and locations should not be compared
directly. At high latitudes (e.g. Sweden in this example), there is strong seasonal
variability in the solar resource—something that is absent at low latitudes. The
solar resource is of course strongly linked to the diel cycle (because incoming
solar radiation peaks at noon), and so can map conveniently to the 24 h demand
for electricity. However, it should be noted that at relatively high latitudes,
the seasonal trend in the solar energy resource (summer peak, Fig. 10.2)
tends to be 180 degrees out of phase with the seasonal demand for electricity
(winter peak, Fig. 10.1D). By contrast, although there is a moderate seasonal
correlation between the wave resource and demand for electricity, the wave
resource is independent of the time of day. The two tidal examples shown are
for semidiurnal and diurnal sites at Pennsylvania and Mississippi, respectively. 1
Although the time of HW (or LW), peak flood (or ebb), etc. does not correspond

1. Example time series for semidiurnal and diurnal sites are plotted in Fig. 3.15.