The development of wave energy converters (WEC’s) goes far back in time—the first attempts are recorded to have taken place in the 1800s, see Fig. 2.4 and . Actually, the first patent for a wave energy converter dates back the year 1799. In modern time it was not until the energy crisis in the beginning of the 1970s that the field had renewed interest, greatly boosted by an article by Prof. Stephen Salter in the scientific journal Nature in 1974 . However, in spite of very significant research efforts, not the least in the UK, activities were reduced again up through the 1980s and the beginning of the 1990s. By the end of the past millennium activities were picking up in speed again, and this now in a number of countries around the world, but with most efforts seen in the coastal European countries. Over the past decade UK has again put enormous efforts into development of marine renewable energies, including wave energy, and must today be seen as the world leader in the field.
Categorization of WEC’s
The development of WEC’s is characterized by the fact that there is a large number of different ideas and concepts for how to utilize the wave energy resource. The different concepts can be categorized in a number of different ways.
Often a basic categorization using the terms terminator, attenuator and point absorber is used . Terminators are devices with large horizontal extensions parallel to the direction of wave propagation, while attenuators have large horizontal extensions orthogonal to the direction of wave propagation. In contrast point absorbers with extensions small compared to the predominant wavelength of the prevailing waves.
WEC’s can also be categorized by their location—onshore, near shore and offshore. Onshore, or shore-mounted, devices are by nature terminators, and rigidly connected to land. Typical examples hereof are oscillating waver columns and overtopping devices, see further explanation below. Near shore devices are situated at water depths where the available waves are influenced by the water depth, and devices deployed in this region will often be bottom mounted. And thus, at last, devices placed offshore will generally be floating and have access to the waves unaltered by the presence of the seabed.
Classification of WEC’s is also seen by their main working principles . The European Marine Energy Center at the Orkney Islands is using 8 main types, plus one (‘other’—in acceptance of the fact that some WEC’s cannot be put into the existing boxes).
However, in the following, the approach to categorization used by IEA—Ocean Energy Systems  will be used. This approach is illustrated in Fig. 2.5.
Here, all WEC’s consisting of oscillating bodies are put into one category. This category is termed Wave Activated Bodies (WAB’s).
In an attempt to detail the categorization of WEC’s a level further, guidelines have been provided by the EU-FP7 funded EquiMar project on how to categorize WEC’s by subsystems . The WEC is in this case broken into the following subsystems, which can then be individually categorized:
The concept for detailed categorization and breakdown of a WEC is developed further by DNV-GL in , which provides a generic system breakdown useful as base of a generic risk ranking and failure mode analysis.
Examples of Various WEC Types
In the following a wide range of examples of WECs, however only a fraction of the technologies that are currently being developed around the world, are presented, here categorized according to the categories defined by IEA—Ocean Energy Systems (following Fig. 2.5).
Oscillating Water Column
There is a number of shore-based (fixed) oscillating water columns (OWC) WECs that has been operating, on Islay in Scotland (operated by WaveGen), the Pico plant on the Azores in Portugal (Fig. 2.7), at the port Mutriku breakwater in Spain (Fig. 2.6), Sagata port Japan and OceanLinx Australia (Fig. 2.7). The unidirectional rotation of the air turbine (the Wells type) is a simple way to rectify the bidirectional flow and thereby convert the oscillating power from waves, due to the fact that the need for check-valves can be omitted and the structure thus constructed with less moving parts. Voith Hydro WaveGen Limited has been developing this type of turbines.
The LeanCon WEC is floating and also based on the concept of oscillating water columns (Fig. 2.8). It is a large structure covering more than one wave length and it consists of a large number of OWC chambers. This entails that the resulting vertical force on the WEC is limited. The downward forces from the negative pressure on parts of the WEC prevent it from floating up on the top of the waves and due to this the device can have a low weight (constructed from high strength fiber reinforced material). Before the air flow reaches the power take off system (PTO) the air flow is rectified by the non-return valves. Thus, LeanCon uses uni-directional air turbine, while most other OWC use Wells turbines.
Wave Activated Bodies
The category of wave activated bodies (WABs) encompasses a very large field of WEC concepts. In this section a number of examples are given to give an impression of the plurality, but it cannot be considered complete as the number of concepts in this category can be counted in hundreds.
The Pelamis WEC is a floating device, made up of five tube sections linked by universal joints which allow flexing in two directions (Fig. 2.9). The WEC floats semi-submerged on the surface of the water and inherently faces into the direction of the waves, kept in place by a mooring system. As waves pass down the length of the machine and the sections bend in the water, the movement is converted into electricity via hydraulic power take-off systems housed inside each joint of the machine tubes, and power is transmitted to shore using standard subsea cables and equipment .
Like Pelamis, the Crestwing is a moored device utilizing the relative motion between wave activated bodies (Fig. 2.10). While Pelamis is harvesting the energy from 2 degrees of freedom (DOF) in a total of 4 joints, Crestwing is just using a single DOF for power production. The hinged rafts of the Crestwing are closed box structures. And the PTO of the Crestwing is a mechanical system using a ratchet mechanism and a fly wheel for converting the oscillatory motion between the rafts into a rotating motion on an axle, which can be fed into a gear and generator system. Other concepts have been tested using relative rotation between floating bodies includes Dexa (Fig. 2.11), Martifer, MacCabe Wave Pump and Cockerell’s Raft.
Another group of floating WABs includes translating (often heaving) bodies. This includes devices Ocean Power Technologies (OPT) (Fig. 2.12), which is one among a number of technologies utilizing a point absorber. The OPT PowerBuoy is using a reference plate as point of reference for the PTO. OPT has used different solutions for PTO, including oil hydraulics. OPT is working on a range of deployment projects, and have conducted sea trials using both a 40 kW and 150 kW version of their technology. Other devices using similar approaches include Wavebob (using a submerged volume rather than a damping plate for reference) (Fig. 2.13) and SeaBased (using a fixed reference point at the seabed, where also the PTO, a linear generator, is placed (Fig. 2.14).
Other types of point absorbers also exists, such as Fred Olsens Lifesaver, which not only utilizes the heave (translation) but also the pitch and roll (rotation), as it consists of a torus connect to the seabed through winches with integrated PTOs (Fig. 2.15).
As an example of a submerged WAB Carnegies CETO buoy can be mentioned. In the CETO device the buoy itself is completely submerged and kept in place by a tether fixed at the seabed and with a hydraulic pump based PTO in line (Fig. 2.16).
Besides the above, another group of fixed WABs, specifically submerged flaps hinged at the seabed, can be mentioned. This type includes Oyster, developed by Aquamarine, which was announced in 2001 by Professor Trevor Whittaker’s team at Queens University in Belfast (Fig. 2.17). The flap is moved back and forth by the waves, and power is taken out through hydraulic pumps mounted between the flap and the structure pinned to the seabed. The latest generation Oyster 800 has an installed capacity of 800 kW. It has a width 26 m and height of 12 m was installed in a water depth of 13 m approx. 500 m from the coast of Orkney at EMEC.
Other relevant WECs utilizing same operating principles includes Waveroller (Fig. 2.18), Resolute Marine Energy (Fig. 2.19) and Langlee (Fig. 2.20). However, the latter is not fixed to the seabed, but a structure with two flaps attached to a floating reference frame.
In addition to the above mentioned WECs in the WAB category, also a number of devices exist where multiple bodies are combined into one larger structure. An example here is the Wavestar device, which consists of two rows of round floats—point absorbers—attached to a bridge structure, fixed to the sea bed by the use of steel piles, which are cast into concrete foundations (Fig. 2.21). All moving parts are therefore above normal seawater level. The device is installed with the structural bridge supporting the floats directed towards the dominant wave direction. When the wave passes, the floats move up and down driven by the passing waves, thereby pumping hydraulic fluid into a common hydraulic manifold system which produces a flow of high pressure oil into a hydraulic motor that directly drives an electric generator. A prototype with a total of two floaters (diameter of 5 m) has been undergoing sea trials at DanWEC, Hanstholm, Denmark.
Another multi-body device is the Floating Power Plant (Fig. 2.22). This device is a moored structure utilizing multiple WABs aligned parallel to the wave crests. Thus, the operating principle resembles to some extent the Wavestar, except the reference structure here is floating and not bottom mounted. Furthermore, the floating structure is used as a floating foundation for wind turbines. Floating Power Plant has carried out sea trials at a benign site with a reduced scaled prototype, and is currently preparing its first full scale prototype deployment.
The Weptos WEC is another floating and slack-moored structure, composed of two symmetrical frames (“legs”) that support a multitude (20) of identical rotors (Fig. 2.23). The shape of these rotors is based on the shape of Salter’s duck WEC (invented and intensively developed since 1974 ). All rotors on one leg are connected to the same frame are driving a common axle. Each axle is connected to an independent PTO. The torque, resulting from the pivoting motion of the rotors around the axle, is transmitted through one-way bearings on the up- and down-stroke motion of the rotor. The angle between the two main legs is adaptable. This allows the device to adapt its configuration relative to the wave conditions, increasing its width relative to the incoming wave front in operating wave conditions and reducing its interaction with excessive wave power in extreme wave conditions.
The Wave Dragon is a slack moored WEC utilizing the overtopping principle (Fig. 2.24). The structure consists of a floating platform with an integrated reservoir and a ramp. The waves overtops the ramp and enters the reservoir, were the water is temporarily stored before it is led back to the sea via hydro turbines generating power to the grid, and thereby utilizing the obtained head in the reservoir. Furthermore, the platform is equipped with two reflectors focusing the incoming waves towards the ramp, which thereby enhance the power production capability.
Other overtopping based approaches do also exist, including the SSG, which is a fixed structure acting as a combination of a WEC and a breakwater (Fig. 2.25). In order to still being able to harvest the wave power with good efficiency, while not having the option of adjusting the ramp height through the floating level, SSG consists of multiple reservoirs with different heights. However, simpler approaches with just a single reservoir integrated into (existing) breakwaters are also being explored.
The Development of WECs
As seen above a large variety of WECs exists, and more are still appearing. EquiMar (an EU FP7 funded research project ), along with others, has promoted the use of a staged development approach to the development of WEC’s, and thus, the stage of development can also be used for characterization of the WEC’s. EquiMar uses 5 stages to describe the development of a WEC from idea to commercial product. These 5 stages are illustrated in Fig. 2.26.
Each stage should provide specific valuable information to inventor and investors, before going to the next step, and hereby avoid spending too many resources before having a reliable estimate on the concepts potential.
This topic will extensively be addressed in the corresponding Chap. 4 entitled: Techno-economic development of WECs.
As seen from the above examples of WECs and the staged development approach, an important element of the development is the initial real sea testing of the WEC prototype, which paramount prior to commercial introduction to the market. This has called establishment of test sites in real sea, which is the topic for the next section.