Folley:Outsourcing_book_temp.qxd 20/08/2009 11:33 Page 47
W
ave & T
Gross Wave Energy Resource versus Exploitable Wave Energy Resource –
idal
Towards a Superior Method of Energy Analysis
a report by
Matt Folley
Senior Researcher, Marine Renewables Research Group, Queen’s University Belfast
There is currently a large range of technologies designed to exploit it does effectively limit the contribution of very high wind speeds to
wave energy all at different stages of development, from initial ideas the average wind energy resource.
on the backs of envelopes to full-scale commercial prototypes
deployed in some of the most aggressive and energetic seas in the Returning to wave energy, the question is:  ‘Is there a more suitable
world. However, the exploitation of wave energy remains limited, measure of wave energy resource than one that simply includes all of
and large-scale wave farms are yet to be developed. This is partly the wave energy passing through a particular point?’ Specifically, is
because wave energy technology remains in its infancy and is there a measure that accounts appropriately for the range of parameters
approximately 20 years behind the development of wind turbine that make up the wave energy resource and that is also suitable for the
technology. Wave energy is also hampered by the lack of a clear large range of device concepts that currently exist including offshore,
assessment of wave energy resource, which is required to provide an nearshore and shoreline technologies? In particular, the resource
economic argument for the development of wave farms. measure must not distort selection of the most promising technologies
because the resource measure is either overly generous or overly
It may at first seem a relatively simple task to calculate the wave pessimistic for any one technology or group of technologies.
energy resource. Years of observational data on waves together with
modern computer simulations of the world’s oceans mean that it is It may be questioned why wave energy resource needs to be reduced to
possible to estimate which waves occur at a particular site with a a single value; after all, it is inherently complex and so not very suitable
relatively high level of confidence and accuracy. The hydrodynamics of for such a severe reduction. A multiple-valued or parametric measure of
waves are relatively well understood and so it would seem that the wave energy resource would seem more appropriate. However, a single-
average wave energy resource can be calculated by simply combining valued wave energy resource is extremely convenient for the calculation
these two pieces of knowledge to produce the average gross wave of total potential wave energy resource for regions, countries and
energy resource. Indeed, this has been done and maps showing globally. This is because it will be used to inform investment decisions
estimates of the average gross wave energy resource can be readily based on potential levels of return and can be easily understood by non-
downloaded from the Internet. technical decision-makers. A single-valued wave energy resource is also
useful for the initial assessment of a wave farm site’s potential yield,
Maps of the average gross wave energy resource may be able to where it must be decided whether a site is likely to be economically
indicate on a global scale the more promising locations for wave viable and therefore worthy of further investment. Thus, the  ‘market
farms – for example, not surprisingly they show that the Atlantic pull’ for a single-valued wave energy resource is extremely strong and it
Ocean has a much larger wave energy resource than the is wise to accept that, in reality, it will be used to make strategic
Mediterranean Sea. However, they tell us nothing  about the decisions irrespective of its actual appropriateness.
characteristics of the wave energy resource and how well suited it is
for exploitation. Given that a single-valued measure of wave energy resource will be
used, it is clearly important that this measure provide the most accurate
A brief review of how wind energy resource is typically defined can indication of what the yield of a wave farm will be; however, the gross
help to throw light on the complexity of resource estimation. It is well wave energy resource is inadequate for this in two significant ways.
known that wind power density increases with the cube of the wind First, the gross wave energy resource includes all of the incident wave
speed. Therefore,  a simple calculation of the gross wind energy energy, including all of the energy in highly energetic seas, although in
resource would involve multiplying the probability of occurrence of these seas the majority of wave energy cannot be exploited. Second, the
each wind speed with the cube of that wind speed to determine its gross wave energy resource includes the wave energy passing through
contribution to the gross wind energy resource.
Matt Folley is Senior Researcher in the Marine
However, this method of calculation is rarely used because the higher
Renewables Research Group at Queen’s University
wind speeds make an overly large contribution to the wind energy
Belfast (QUB). He is the author of over 30 scientific
resource. The contribution from high wind speeds is overly large
publications in the field of wave energy and has
worked with key industrial developers, including
because wind turbines generally become less efficient at high wind
Wavegen Ltd on the development of LIMPET and
speeds and are limited by their generator rating so that, although the
Aquamarine Power Ltd on the development of Oyster.
Dr Folley completed his PhD on the design of wave
wind energy resource physically exists, it is not generally exploitable. In
energy converters at Lancaster University in 1991
actuality, average wind speed is commonly used as an effective before joining the QUB group in 2000.
measure of wind energy resource, which is clearly at variance with the
E: m.folley@qub.ac.uk
cubic relationship between wind speed and wind power density, but
© TOUCH BRIEFINGS 2009
47