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Gross Wave Energy Resource versus Exploitable Wave Energy Resource
a point irrespective of the direction of propagation, but a wave farm will The second constraint can be achieved by resolving the incident
typically consist of a string of wave energy converters orientated to face wave power to only include that which is travelling the same way as
the predominant direction of wave propagation. Thus, waves the principal direction of wave energy propagation, since this is likely
approaching from the sides will not be fully available to any one to be the direction with which a wave farm will be aligned.
converter in the wave farm because its neighbours will have already
extracted some of the wave energy. It remains to be seen how use of the exploitable wave energy
resource affects global maps of wave energy, but it will clearly help
That a wave farm will have a specific orientation is a consequence of the to promote locations with more consistent and directionally
characteristics of wave energy resource, which is concentrated near the concentrated wave energy resources, which are highly desirable
surface of the ocean and has a dominant direction of propagation characteristics for wave farm sites. However, at a local level  this
that requires many hundreds of kilometres of open water to grow to re-definition of wave energy resource also changes the relationship
exploitable levels. This is why the wave energy resource is large in the between offshore and nearshore resources in a significant way.
North Atlantic, which has thousands of kilometres of open water, and
small in the Mediterranean Sea, where the fetch length is much shorter. Using the traditional measure of gross wave energy resource, the
Thus, any wave energy converter placed in the lee of another will have offshore wave energy resource is often quoted as being about twice
a smaller resource to exploit due to the energy extracted by the first as large as the nearshore wave energy resource, where the
wave energy converter and so is likely to be less productive. The natural nearshore is defined as having a water depth of between 10 and 15
consequence of this is that a wave farm will undoubtedly consist of a metres. This has led to the marginalisation of the nearshore as a
string of wave energy converters orientated to face the predominant potential location for wave energy converters, even though it may
direction of wave propagation. have a number of advantages such as a reduced distance to shore
and a smaller maximum wave height due to the larger waves
This is unlike the layout of a wind farm, which can be much more breaking further offshore.
flexible and 2D.  Its  layout is primarily determined by the local
topography and the distance downstream of a wind turbine required for Using the exploitable wave energy resource, however, the difference
the wind energy resource to recover. Incidentally, this is an important between offshore and nearshore resources is generally significantly
distinction between the layouts of wave and wind farms that has not reduced; the nearshore resource is often only 10–20% less than the
always been appreciated when comparing the seascape usage of offshore resource. This is because as waves approach the shore they
offshore wind farms and wave farms. are influenced by the seabed, which filters out the most energetic
waves while leaving the smaller, more exploitable waves largely
In place of gross wave energy resource, a superior measure of wave unaffected. It also refracts the waves so that they all approach from a
energy resource would be one that includes only the wave energy that more concentrated directional sector. Thus, a much larger proportion
is reasonably exploitable. Thus, the exploitable wave energy resource of the nearshore wave energy resource is exploitable compared
with the offshore wave energy resource. This means that  although
there is a large difference between gross wave energy resources,
Given that a single-valued
the difference between the exploitable wave energy resources is
measure of wave energy
much smaller.
resource will be used, it is clearly
Ultimately, modification of the definition of the wave energy
important that this measure resource will not directly change the cost of generating electricity by
provide the most accurate
waves, but at this early stage in the development of wave energy
technology perceptions are important both within the industry and
indication of what the yield of
in society in general.
a wave farm will be.
While it is perhaps not actually dishonest, it is highly misleading to use
a measure of resource that includes energy that is technically
must be able to discount the contribution of highly energetic sea-states unexploitable. Thus, the historical use of gross wave energy resource is
as well as the contribution from waves that are not travelling in a misleading although it is likely that this has been due to a lack of rigour
direction orthogonal to the alignment of the wave farm. rather than any attempt to increase the apparent resource artificially.
The first of these constraints can be achieved by simply defining a More significantly, the use of the gross wave energy resource has
maximum wave power that a sea-state can contribute to the inadvertently marginalised the development of nearshore wave
resource. Arguments can be made for a number of different values of energy technologies as offshore wave energy technologies have
maximum wave power, but four times the average incident wave chased the unexploitable wave energy resource offshore.
power is suggested as reasonable. This is  because the typical load Consequently, use of the exploitable wave energy resource will help
factor of wave energy converters – about 30% – means electricity to level the playing field so that all technologies are assessed on
generation from sea-states with higher powers than about four times similar grounds and effort focused on genuinely promising wave
the average will most likely be limited by the generator rating and energy technologies unhindered by inappropriate measures of
should be discounted. wave energy resource. n
MODERN ENERGY REVIEW VOLUME 1
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