Publications

Survey of Energy Resources 2007

Wave Energy Technologies

There are several significant reviews of wave energy (Thorpe, 1999; Clément, et al., 2002; Brooke, 2003; IEA, 2003; Wavenet, 2003; Previsic, et al., 2004). These show that many wave energy devices are at the R&D stage, with only a small range of devices having been tested at large scale or deployed in the oceans. This slow rate of progress is because wave energy devices face a number of design challenges:

Design Waves. To operate its mechanical and electrical plant efficiently, a wave energy device must be rated for wave power levels that occur much of the time (e.g. in the UK this would be 30-70 kW/m). However, the device also has to withstand extreme waves that occur only rarely and these could have power levels in excess of 2 000 kW/m. This poses a significant challenge, because it is the lower power levels of the commonly-occurring waves that produce the normal output of the device (and hence the revenue), while the capital cost is driven by the civil structure that is designed to withstand the high power levels of the extreme waves.
Variability of Wave Power Levels. Waves vary in height and period from one wave to the next and from storm to calm conditions. While the gross average wave power levels can be predicted in advance, this inherent variability has to be converted to a smooth electrical output if it is to be accepted by the local electrical utility. This usually necessitates some form of energy storage.
Variability in Wave Direction. Normally, offshore waves travel towards a wave energy device from a range of directions, so a wave energy device has to be able to cope with this variability either by having compliant moorings (which allow the device to point into the waves) or by being symmetrical. Another approach is to place the wave energy device close to the shore, because waves are refracted as they approach a coastline, so that most end up travelling at right angles to the shoreline.
Wave Movement. The relatively slow oscillation of waves (typically at ~ 0.1 Hz) has to be transformed into a unidirectional output that can turn electrical generators at hundreds of rpm, which requires a gearing mechanism or the use of an intermediate energy transfer medium.

Different devices have different solutions to these challenges, as exemplified by just four of the main types of device deployed at large scale over the past few years:

Oscillating Water Column. The Oscillating Water Column (OWC) comprises a partially submerged structure forming an air chamber, with an underwater aperture. This chamber encloses a volume of air, which is compressed as the incident wave makes the free surface of the water rise inside the chamber. The compressed air can escape through an aperture above the water column which leads to a turbine and generator. As the water inside falls, the air pressure is reduced and air is drawn back through the turbine. Both conventional (i.e. unidirectional) and self-rectifying air turbines have been proposed. Even with this commonality of operating principles, the examples of oscillating water column actually deployed vary considerably from the bottom-standing, shoreline-based concrete device developed by Wavegen (2007) in Scotland (Fig. 14-2) to the tethered, nearshore steel device deployed by Energetech (2007) in Australia (Fig 14-3).
The Pelamis. The Pelamis is a floating device comprised of a series of cylindrical hollow steel segments that are connected to each other by hinged joints. The device is approximately 120 m long, 3.5 m in diameter and is loosely moored in water depths of ~ 50 m so that it points into the waves (Fig. 14-4). As waves run down the length of the device, the segments move with respect to each other and actuate hydraulic cylinders incorporated in the joints to pump oil to drive a hydraulic motor/generator via an energy-smoothing system. The device has been deployed in Scotland and a small scheme of three devices is currently being deployed in Portugal (OPD, 2007).
" The Wave Dragon. This device uses a pair of large curved reflectors to gather waves into the central receiving part, where they flow up a ramp and over the top into a raised reservoir, from which the water is allowed to return to the sea via a number of low-head turbines. A quarter-scale prototype (58 m wide x 33 m long) rated at 20 kW has been deployed in a Danish inlet (Fig. 14.5) and a full-size device (estimated to have a generation capacity of ~ 4 MW) is being constructed for a site in Wales (Wave Dragon, 2007).
The Archimedes Wave Swing. This consists of a buoyant cylindrical, air-filled chamber (the 'Floater') that can move vertically with respect to the cylindrical 'Basement', which is fixed to the sea bed. As a wave passes over the top of the device, it alternatively pressurises and depressurises the air within the Floater, changing its buoyancy, which causes the Floater to move up and down with respect to the Basement (AWS, 2007). This relative motion is used to produce energy, using a linear electrical generator. A 2 MW Pilot scheme has been deployed and tested in Portugal (Fig. 14-6).

This range of devices, plus the many others that are currently being developed, indicate that wave energy is currently an immature technology, without a clear consensus on which are eventually likely to prove the successful devices.

This state of affairs is compounded by a significant non-technical challenge faced by wave energy developers, namely that the technologies are being developed by small companies, with a total investment of US$ 5-10 million in each company (one or two companies have exceeded this, many are below this range). This is a small amount on which to research, develop and deploy a completely new technology, thereby increasing the chances of failures in early prototypes, which could lead to a loss in confidence in this sector.

Additionally, in many countries there is a high cost associated with obtaining licences, gaining permits and carrying out environmental impact assessments, which small companies find difficult to meet. Moreover, once deployed in free energy markets, wave energy has to compete with established renewable energy technologies that have benefited from billions of dollars of cumulative investment.

It is promising to note that several common themes are starting to emerge from different developers, e.g.

overtopping devices (e.g. the Wave Dragon, Seawave Slot-Cone Generator and Wave Plane use this capture mechanism);
bottom-mounted hinged plates moving back and forth and operating hydraulic pumps (e.g. the BioWAVE, Oyster and WaveRoller);
Oscillating Water Columns (e.g. Energetech OWC, Superbuoy and Wavegen's LIMPET).

However, it is disheartening to see several new developers needlessly 'reinventing the wheel' and repeating mistakes in device design that were first made decades ago.