Nearshore oscillating wave surge converters and the development of Oyster

Abstract

Oscillating wave surge converters (OWSCs) are a class of wave power technology that exploits the enhanced horizontal fluid particle movement of waves in the nearshore coastal zone with water depths of 10–20 m. OWSCs predominantly oscillate horizontally in surge as opposed to the majority of wave devices, which oscillate vertically in heave and usually are deployed in deeper water. The characteristics of the nearshore wave resource are described along with the hydrodynamics of OWSCs. The variables in the OWSC design space are discussed together with a presentation of some of their effects on capture width, frequency bandwidth response and power take-off characteristics. There are notable differences between the different OWSCs under development worldwide, and these are highlighted. The final section of the paper describes Aquamarine Power's 315 kW Oyster 1 prototype, which was deployed at the European Marine Energy Centre in August 2009. Its place in the OWSC design space is described along with the practical experience gained. This has led to the design of Oyster 2, which was deployed in August 2011. It is concluded that nearshore OWSCs are serious contenders in the mix of wave power technologies. The nearshore wave climate has a narrower directional spread than the offshore, the largest waves are filtered out and the exploitable resource is typically only 10–20% less in 10 m depth compared with 50 m depth. Regarding the devices, a key conclusion is that OWSCs such as Oyster primarily respond in the working frequency range to the horizontal fluid acceleration; Oyster is not a drag device responding to horizontal fluid velocity. The hydrodynamics of Oyster is dominated by inertia with added inertia being a very significant contributor. It is unlikely that individual flap modules will exceed 1 MW in installed capacity owing to wave resource, hydrodynamic and economic constraints. Generating stations will be made up of line arrays of flaps with communal secondary power conversion every 5–10 units.

Constrained control of arrays of wave energy devices

“…Examples of larger individual devices include Pelamis [1], Oyster [2] and Powerbuoy [3], while closely-spaced arrays include those by Wavestar [4], Fred Olsen Lifesaver [5] , Manchester bobber [6] and Trident Technologies [7]. In particular, compact circular arrays have been shown to be capable of extracting more energy than a single device of the same volume, and presenting a larger spectral bandwidth [8].…”

The Wave Energy Resource

“…Thus, wave power associated with particular wave frequencies may be more significant than wave power at other frequencies. For example, the power capture per unit wave height of many surging/pitching devices tends to increase with wave frequency because the incident wave force/torque also tends to increase with wave frequency [8,9], whilst the incident wave power decreases with wave frequency (see Eq. 3.18).…”

Technological Cost-Reduction Pathways for Oscillating Water Column Wave Energy Converters in the Marine Hydrokinetic Environment.

“…17 DTI evaluated OWC devices in a specified deployment climate using experimental model testing and frequency domain numerical modeling. The economics of an offshore OWC in a point absorber configuration with an 11.0 m diameter as shown in Figure 4 was evaluated.…”

“…At this time the authors know of no work resulting in a similar formulation to Yemm et al, 16 however there is work that clearly shows the dependence between actual capture width and width of device. 17,18 These theoretical capture widths were derived using linear analyses (linear potential flow theory) which cannot be achieved in reality. Thus, there are many factors that reduce the actual output of a device from the theoretical limit including: viscous and friction losses, motion limitations, and nonlinearities that move beyond the applicability of linear potential flow theory.…”