The wave climate is location dependent in the sense that it is affected by the wind action, the water depth, the topography, the wave-current interaction, the tidal action, the angle wave of incidence and other factors. Here the various locations for the deploy- ment of WECs are defined, and some of their advantages and disadvantages are explained.
2.2.1 Onshore zone
WEC structures placed on the shoreline, rarely experience the action of extreme events to the extent offshore structures might experience during the project’s lifecycle. These locations are advantageous in the sense that they are close to the utility network, therefore their Operation and Maintenance can be undertaken within reasonable costs. However, the energy levels at these locations appear to be relatively low compared to the offshore
environment. In addition, recent concerns with regards to climate change flooding led to the development of other MRE technologies for coastal protection (Wadey et al., 2013). This zone is very well known for its contribution to the stability of the coast and ecology (i.e. saltmarsh vegetation) (Pringle, 1995). The deployment of WECs is not very often undertaken in this zone, as it is a zone of stake holder interest, and other human activities.
2.2.2 Nearshore zone
In oceanographic terms, the location close to the coast where wave breaking occurs is known as “the surf zone” (Svedsen et al., 1978). As a wave enters shallow water, it begins to feel the bottom and slows down. Consequently, the wavelength and phase velocity decrease whilst the wave period remains constant, causing the wave height and steepness to increase. When a wave’s steepness (a ratio of its height to its length) exceeds 1:7, wave breaking and energy dissipation takes place (Thurman and Trujillo, 2001). The reduction in wave energy in shallow water areas has encouraged WEC developers to pursue offshore technologies. However, the zone just before the occurrence of wave breaking appears to be quite beneficial for wave energy conversion: during storms, the reduction in deep water wave energy due to shoaling effects reduces engineering requirements on surviv- ability design. Several WECs concepts have been designed to operate in the nearshore environment, and a promising approach in recent years has been the integration of WECs into breakwaters (e.g. the Mutriku Breakwater Wave Plant (Torre-Enciso et al., 2009)). Unfortunately, devices deployed in the nearshore in some cases may negatively affect sediment transport, by interfering with circulation patterns (e.g. locations with rip currents, sandbars, etc. (Davis and Fitzgerald, 2003)). Existing morphodynamic patterns are important for the stability of the coast (Masselink et al., 2011). Previous studies have looked at the impact of nearshore WEC farms on the dynamics of the coastal hydrodynamics (Rusu and Guedes Soares (2013), Porter et al. (2014)), making use of well-established numerical models.
2.2.3 Offshore zone
High energy levels available in the offshore environment, along with a limitation in marine usable space in the nearshore environment, has led to an interest in the deployment of WECs at greater depths. Large arrays of WECs could be deployed further offshore minimising visual impact. The deployment of WECs in an offshore environment could also act as protection for the nearshore, by reducing the wave energy incident on the coast. It is likely that the presence of WECs offshore will contribute to a reduction in the nearshore wave height. A disadvantage of offshore WEC deployments is the increased cost and complexity of operation and maintenance (R´emouit et al., 2018); similar procedures from offshore oil and gas and offshore wind need to be thoroughly studied, since no op- erational experience has been recorded for a WEC array deployment (Rinaldi et al., 2018).
WECs can be free to move in three translational modes (1 - surge, 2 - sway and 3 - heave) and in three rotational modes (4 - roll, 5 - pitch and 6 - yaw) (Ibrahim and Grace, 2010). The six Degrees of Freedom (DoF) are depicted in Fig. 2.1 for an axisymmetric WEC. In most cases, energy is extracted from the heaving motion of the WEC. According to the European Marine Energy Centre (EMEC)1, WEC technologies can be classified into the following main categories:
1. Attenuators 2. Point absorbers
3. Oscillating wave surge converters 4. Submerged pressure differential devices 5. Bulge wave devices
6. Rotating mass devices
7. Oscillating Water Columns (OWCs) 8. Overtopping/ terminators
The following sections briefly describe these WEC technologies.
2.3.1 Attenuators
These are floating structures, placed perpendicular to the incoming wavefront. They consist of several joints and hinges, whose relative motion due to wave action captures the incident energy. A representative example is the well-known Pelamis. This device was deployed in the form of an array in Agu¸cadoura, Portugal in 2008, but only lasted for two months before needing to be decommissioned (Dalton et al., 2010).
2.3.2 Point absorbers
These are buoy-shaped designs. During initial numerical research on WEC hydrodynamics using linear wave theory, it was discovered that bodies with characteristic dimensions that are small compared to typical incident wavelengths could capture energy contained in a wave crest length, greater than their own diameter (Falnes, 1980). The majority of point absorbers are axisymmetric, and operate in heave. However, some point absorbers have also been designed to operate in pitch (Folley, 1993), surge (Bhinder et al., 2009), yaw, roll or a combination of these. Point absorbers are small and modular, allowing for gradual expansion in array capacity rather than risking significant capital investment with a single large device. One of their main advantages is that they are not affected by the wave direction, due to their small size and axial symmetry. However the directionality of the incident waves will affect the hydrodynamics when studying WEC arrays (Wolgamot
1
Figure 2.1: The axisymmetric WEC, denoted as spar-buoy OWC, oscillating in six DoF, source: Malvar Ferreira (2016).
et al. (2013), Oikonomou et al. (2017)). This is possibly the most popular device type in terms of the number of concepts under development, with substantial commercial and academic research devoted to devices of this type.
2.3.3 Oscillating wave surge converters
These devices consist of a hinged arm close to the seabed, connected to a near-surface collector. The arm oscillates due to the surging action of waves. Characteristic examples are the Waveroller and Oyster.
2.3.4 Submerged Pressure Differential devices
This is a sea bottom mounted submerged point attenuator, usually located in the nearshore environment. The sea level rises and falls due to the waves’ motion, and a pressure differential with respect to hydrostatic equilibrium is formed above the device. An example of this technology is the mWave2.
2.3.5 Bulge wave devices
This is a rubber tube filled with water, moored to the seabed. As the water approaches the stern, the passing wave causes pressure variations along the tube (e.g. the Anaconda WEC3). Whilst the bulge travels across the tube, it grows, gathering energy. This energy
2http://www.bomborawave.com/mwave (Accessed in June 2018) 3
the sea.
2.3.6 Rotating mass devices
Heaving and swaying motions of a floating device internally drive either an eccentric weight or a gyroscope. This movement is attached to an electric generator inside the device. Not many of the existing devices fall into this category, with the exception of the Penguin, developed by the Finnish company Wello Oy4.
2.3.7 OWCs
This is a partially submerged structure with a hollow spar tube. It is open to the sea below the water line, enclosing a column of air on top of a column of water. The waves’ action causes the rise and fall of the device relative to the water column, leading to compressions and expansions of the air chamber. The upper part of the structure is in contact with the atmosphere. The enclosed air flows to and from the atmosphere via an air turbine, which is often designed to allow bi-directional air flow by means of self-rectification. This is the most investigated WEC technology to date. The floating version of the OWC is presented in Falc˜ao et al. (2012).
2.3.8 Overtopping/ terminator devices
These structures are placed parallel to the incoming wavefront (perpendicular to the prevailing wave direction), and physically intercept waves. An example of a termina- tor device is Salter’s Duck, developed at the University of Edinburgh (Salter, 2016). Overtopping devices are highly non-linear, and hence difficult to model numerically.