Page 292 - Fundamentals of Ocean Renewable Energy Generating Electricity From The Sea

Other Aspects of Ocean Renewable Energy Chapter | 10 279

will occur offshore, with typical increases in the median wind speed of around
0.6 m/s, and with significant seasonal trends.

Wave Energy
In a relatively early study, Harrison and Wallace [12] used a simple model (based
on the Pierson-Moskowitz wave energy spectrum) to investigate the sensitivity
of the wave energy resource to the west of Scotland to changes in the annual
mean wind speed (a proxy for climate change). A 20% reduction in annual mean
wind speed lowers available wave power in this region by 67%, whereas an
equivalent increase raises the available power by 146%. Although it is clear that
changes in wind speed will significantly alter the wave energy resource, it is
important to know how the resource is likely to change in the future, and we
can learn something about the future wave resource by examining past trends.
A study of wave height variations to the west of Norway from 1881 to 1999
demonstrated that there is a positive trend in mean wave height from 1960 to
1999 [13], but a similar study shows that the southern California coast witnessed
a negative trend in wave heights, and hence wave energy, towards the end of the
21st century [14].
There have been relatively few studies that examine how wave heights,
and particularly wave power, are likely to vary in the future. In a multimodel
ensemble of wave-climate projections, Hemer et al. [15] found a projected
decrease (2070–2100 time period compared with 1979–2009 baseline) in annual
mean significant wave height (H s ) over 26% of the global ocean area, and a
projected increase in annual mean H s over 7% of the global ocean, but mainly
concentrated in the Southern Ocean (Fig. 10.5). Within the context of wave
energy conversion, Janji´ c et al. [16] found an overall reduction in wave energy
flux along the European coast towards the end of this century, and this is in
agreement with Reeve et al. [17] who found a 2–3% (depending on the IPCC
AR4 climate scenario) reduction in available wave power at the Wave Hub site,
UK, but stress that such changes are relatively small compared with natural
variability of the wave climate at this location. Further, it is noted that variance
of wave climate projections associated with study methodology dominates other
sources of uncertainty (e.g. climate scenario and model uncertainties) [15].
Climate model uncertainty (i.e. intermodel variability) is significant nearly
globally, and its magnitude is comparable or greater than that of the common
signal (the signal simulated by multiple model ensembles of H s simulations) in
all areas, with the exception of the eastern tropical Pacific [18].
Other issues associated with global warming that can influence the wave
energy resource are reduction in sea ice extent and sea-level rise. A reduction
in the extent of sea ice will increase fetch lengths (e.g. Fig. 10.6). In northern
Europe, for example, northerly winds would generate larger waves in this region
due to a reduction in Arctic sea ice extent (and hence a longer fetch), and this
effect needs to be considered in wave models that consider the future climate. In
addition, sea-level rise would lead to increased water depths in coastal waters,