Near or far - siting wave energy converters

27 October 2010

Is the nearshore a sensible place to put wave energy converters? Dr Matt Folley from the Queen’s University of Belfast believes that it would be inappropriate to dismiss nearshore technologies, based solely on information about the gross wave energy resource. He explains how research into the exploitable wave energy resource will level the playing field for wave energy converters

Looking out to the sea on a stormy day it is impossible not to be impressed by the apparently vast amount of energy in the waves and be overcome by the thought that if only this energy could be harnessed it could make a significant contribution to the world’s energy supply. Of course, inventors and engineers have been thinking just that for hundreds of years, with the amount of effort in recent years increasing significantly, initially as a response to the 1970s oil crisis and subsequently in the 2000s due to climate change. As a consequence, a large range of wave energy converters have been proposed and continue to be developed in an attempt to harness this resource. In the last few years this has progressed so that the first prototype wave energy converters, such as LIMPET, Pelamis and Oyster, have been constructed and deployed. However, the industry remains in its infancy.

In such a young industry there necessarily remain a large number of areas where knowledge is limited; the wave energy resource is one such area. Although it may seem obvious that a comprehensive understanding and accurate measure of the wave energy resource is required, effort has traditionally been focused on understanding the fundamental hydrodynamics of wave energy converters and development of novel device concepts.

The first quantitative estimates of the wave energy resource were made in 1974 by Professor Stephen Salter and Dr Dennis Mollison using data collected by the Ocean Weather Ship ‘India’. This weather ship was stationed in deep water, 700km off of the western coast of Scotland. It was estimated that the annual average gross wave energy density, defined as the total wave energy crossing into a circle of one metre diameter in a year, to be approximately 800MWh; although a subsequent analysis that removed systematic calibration errors reduced this to approximately 700MWh/m. That is, if all this wave energy could be converted into electricity, then there would be enough wave energy crossing into a one metre diameter circle to power approximately 100 homes. The Ocean Weather Ship ‘India’ was stationed at a particularly energetic site. However, many potential sites for wave farms have an annual average gross wave energy resource of over 300MWh/m making wave energy a potentially significant source of renewable energy.

By definition the gross wave energy resource includes all of the wave energy. Thus, it includes all the wave energy during extreme events, where survival is often a more pressing requirement than energy production. It also includes wave energy incident from all directions even though wave farm layout is likely to limit the amount of energy available to individual devices from specific directions. However, the average gross wave power density (simply the annual average wave energy density divided by the number of hours in a year) continues to be used as the standard measure of the wave energy resource. This is exemplified by the publication of local and global maps of the wave energy resource that continue to use this measure.

A consequence of using the average gross wave power density as the measure of the wave energy resource is that the nearshore wave energy resource appears much smaller than the offshore wave energy resource. This is often cited as the reason that wave energy converters should be located offshore in deep water and possibly why the vast majority of wave energy converters conceived are designed for deep water.

The Marine Energy Research Group at Queen’s University Belfast (QUB) has been involved in the research and development of wave energy technologies since the 1970s. The group’s focus has principally been on shoreline and nearshore technologies. This includes the development with wavegen of the LIMPET shoreline oscillating water column and more recently the development of Oyster with Aquamarine Power. Although both technologies have been proven to be technically successful it became clear that the historical definition of the wave energy resource was limiting the perceived potential of the technologies. Consequently, in 2008 the research group at QUB endeavoured to define a measure of the wave energy resource more appropriate for the analysis of all wave energy converters. They termed this measure the exploitable wave energy resource.

Exploitable wave energy resource

The exploitable wave energy resource is designed to discount the contributions from the highly energetic sea-states and to account for the directional distribution of the wave climate. This would provide a more appropriate measure of the wave energy resource. To achieve this, the exploitable wave energy resource is calculated by constraining the gross wave power density in two ways.

The first constraint is to only include the wave energy that crosses a straight line orthogonal to the predominant direction of wave propagation (this is sometimes called the net wave power density). Wave energy takes many hundreds of kilometres of open water to develop and any wave energy converter placed in the lee of another wave energy converter will experience a reduced wave energy resource. Thus, to maximise the power output of a wave farm, the wave energy converters will logically be strung-out in a line orthogonal to the predominant direction of wave propagation. In this arrangement it is the wave energy that crosses the line of the wave farm that is the resource that can be effectively exploited.

The second constraint is that the maximum wave power density that can be exploited is capped as a multiple of the average wave power density. This constraint accounts for the maximum output of the wave energy converter’s power take-off system and electrical generator. For economic reasons this is typically equal to three or four times the average power output, resulting in a load factor similar to wind turbines. Technically, the appropriate limit to apply to the wave power density depends on the relationship of the system efficiency with incident wave power and thus the particular technology. However, a cap of four-times the average wave power density is considered a reasonable approximation.

Having defined a more appropriate measure of the wave energy resource, the Marine Energy Research Group at QUB investigated the change in this resource as it approaches the shore. The investigations were performed using the third-generation spectral wave model SWAN. SWAN models the propagation of the wave spectral energy density in time and space. In addition, it also models the modification of the wave spectral energy density due to wind growth, refraction, shoaling, bottom friction, whitecapping and surf breaking, together with changes to the spectral shape due to the internal hydrodynamics of the waves. These models are the industry-standard for modelling wave climates and sea-state transformations.

Further investigation

Initial investigations focused on idealised test cases so that the effect of each loss mechanism could be isolated. These investigations showed that in typical sea-states refraction is the principal cause of the reduction in the gross wave power density from offshore to nearshore. In addition, in water with a depth of less than two to three times the significant wave height, surf breaking also becomes a significant loss mechanism. However, few wave energy converters are proposed to be deployed in such shallow water. Bottom friction, which is often quoted as being the main loss mechanism, was found to typically account for less than a 10% reduction in wave power density.

Further investigations used wave hindcast data 20km off of the coast of West Orkney, Scotland. At this site the bathymetry was approximated as a 1:100 slope. These investigations showed that to the 10m depth contour the annual average gross wave energy resource decreased by 30%, whilst the annual average exploitable wave energy resource decreased by only 13%. A similar study off of the coast of South Uist, Scotland, where there is a 1:400 seabed slope, showed a decrease of 44% in the annual average gross wave energy resource, but only a 23% decrease in the annual average exploitable wave energy resource.

Finally, investigations were also performed using nine years of wind/wave hindcast and bathymetry data for the Wave Energy Converter Test Site at the European Marine Energy Centre located in the Orkney Islands, Scotland. This analysis showed that there is less than a 10% difference between the exploitable wave energy resource at the ‘deep water’ test berth (located in 50m water depth) and the nearshore test berth (located in 10m water depth).

All these investigations show that the difference between the gross and exploitable wave energy resources is smaller nearshore than it is offshore. The seabed to the nearshore can be considered as acting as a filter that keeps only the exploitable waves. That is, the seabed refracts the incident waves so that they come from a more concentrated direction and also causes the largest waves to break limiting their power density and destructiveness. Viewed from this perspective the nearshore appears much more inviting.

Inappropriate perceptions

The conclusion from these investigations is that whilst the wave energy resource is smaller nearshore compared to offshore, the difference is much less significant than use of the gross wave energy resource suggests. The difference is sufficiently small to suggest that nearshore wave energy converters cannot be dismissed based solely on the available resource; other factors that determine the cost of energy become relatively more significant.

The different environments in the nearshore and offshore means that in general the technologies developed are distinct. For example, Pelamis requires a minimum of about 50m water depth for deployment because the moorings must have sufficient compliance to minimise loads during storms; it could not be deployed in the nearshore. Indeed, nearshore wave energy converters can be cheaper or have a better performance than their offshore counterparts. For example, wave energy converters that react against the seabed are generally cheaper in shallower water due to a reduction in load-path lengths. In addition, wave energy converters that exploit the surge motion of the waves generally have a better performance in the nearshore as depth-induced shoaling increases the waves’ surge amplitudes. Electrical cable lengths and other shore connectors will also be shorter when the wave energy converters are closer to the shoreline reducing both costs and failure rates.

The economics of wave energy converters remains to be proven. However, the investigations have shown that it would be inappropriate to dismiss nearshore technologies based solely on information about the gross wave energy resource. A new, more suitable, measure for comparing sites for wave energy converters has been identified and this has been termed the exploitable wave energy resource. This has levelled the “playing field” so that the most promising technologies are developed without being hampered by inappropriate perceptions of the wave energy resource.

Dr Matt Folley, Senior Research Fellow, Marine Energy Research Group, School of Planning, Architecture and Civil Engineering, Queen’s University Belfast, Northern Ireland. Email: [email protected]

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