4. Wave

4.3. Technology Types

There are many designs being pursued by developers to harness the power of waves. Wave devices can be categorised according to the location and depth in which they are designed to operate, i.e. shoreline, near shore or offshore; or by the method used to capture the wave power. Here, the latter method has been used to categorise the devices as follows:

 

Attenuator – This is a long floating device which is aligned perpendicular to the wave front. The device effectively rides the waves and captures the energy as the wave moves past by selectively constraining the movements along its length. A current example for the attenuator is the Pelamis device, earlier concepts were the MCCabe Wave Pump (sea trials) and the Cockerel Raft (concept stage).
 
 
 
(Axisymmetrical) Point Absorber – This is a floating structure that absorbs wave energy in all directions by virtue of its movements at or near the water surface. It has small dimensions compared to the typical wavelength, tending to have diameters of a few meters. The point absorbing characteristic basically means the capacity to absorb energy from the sea area larger than the device dimensions. In reference to the fundamentally same effect in radio (i.e. acoustic) waves, this effect is also called antenna effect. Buoy type designs, for example, act as point absorbers. Typically, but not necessarily, such buoys are axisymmetric. Current examples for this category are the Wavebob, the OPT PowerBuoy and the Aquabuoy. An example for non-axisymmetric point absorber, however with very similar characteristics, is the SeaREV. OWC buoys (OEBuoy, Sperboy, MRC) also have point absorbing characteristics, however they are usually considered under the OWC category.

 
 
Oscillating Wave Surge Converters (OWSC) – This is a near-surface collector, mounted on an arm pivoted near the seabed. The arm oscillates as an inverted pendulum due to the movement of the water particles in the waves. Current examples for this category are the completely submerged Waveroller and the surface-piercing Oyster. An earlier device of this type, the Japanese Pendulum, had the flap hinged near the surface, hanging downwards, inserted into the caisson structure.
 

 
Oscillating Water Column (OWC) – This is a partially submerged, hollow structure, which is open to the sea below the water surface so that it contains air trapped above a column of water. Waves cause the column to rise and fall, acting like a piston, compressing and decompressing the air. This air is channelled through an air turbine to produce power.  When properly designed for the prevailing sea state, OWCs can be tuned to the incident wave period in order to resonate. By this means, OWC can actually be quite efficient and present point absorbing characteristics. A particular case of this category is the OWC buoy, which is a floating OWC. Among the currently proposed devices are the Sperboy, the MRC, and the Backward Bent duct type OE Buoy. Classical OWCs are shoreline devices either built directly into the shoreline (Pico OWC, Limpet OWC) or integrated in breakwaters (Mutriko OWC).
 
 
 
Overtopping Device – This consists of a wall over which the waves wash, collecting the water in a storage reservoir. The incoming waves create a head of water, which is released back to the sea through conventional low-head turbines installed at the bottom of the reservoir. An overtopping device may use collectors to concentrate the wave energy. Overtopping devices are typically large structures due to the space requirement for the reservoir, which needs to have a minimum storage capacity. The devices can be floating like the Wave Dragon, currently largest wave energy converter being developed, or fixed, land-based structures, like the SSG (Sea Wave Slot Cone Generator, integrated into a breakwater). An early example for overtopping devices was the TAPChan device in Toftestallen/Norway, where a tapered channel provoked wave overtopping into a reservoir on land.
 
 
 
Submerged Pressure Differential – This is a submerged device typically located near shore and attached to the seabed. The motion of the waves causes the sea level to rise and fall above the device, inducing a pressure differential which causes the device to rise and fall with the waves. When properly designed for the sea state, this category also has significant point absorbing characteristics. A well-recognised example for the realisation of this concept to date is the AWS (Archimedes Wave Swing), which also has good point absorbing characteristics. Another device that can be considered under this category is the Waverotor.

 

 

 

There are several categories of wave energy devices by power capture mechanisms, and distinctions are rare. Often, there are only three fundamentally different categories considered, namely OWC, overtopping device, and bodies with wave-induced (relative) motion. Typically, all devices except the overtopping type also have point absorbing characteristics. Point absorption is the ability to absorb power from a larger area than the physical dimension of the device – also known as the antenna effect. There is no common categorisation widely accepted within the international research and technology development (RTD) community, due to different aims. The list above was made with the aim of distinguishing the concepts which are currently the most popular, on basis of their operational principle.

The following device list highlights a selection of proposed devices and PTO mechanisms. NB.  This list is a limited selection of proposed devices. It does not reflect the credibility, or the opinion, that other devices have less potential.

Click here for Device list pdf (available in English only).

llp logoThis project has been funded with support from the European Commission (EU Lifelong Learning Programme Agreement no LLP/LdV/TOI/2009/IRL – 515). This publication reflects the views only of the author, and the Commission cannot be held responsible for any use which may be made of the information contained therein.

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