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www.sname.org/sname/mt October 2012 pressure within the chamber as each wave passes. e turbine would be mounted at the small hole in the top of the can. Some devices use turbines that continually spin in one direction, others use reversing turbines. Oscillating water columns can be oating or shore mounted. A unique advantage to OWC technology is that most, if not all, of the prime mover equipment is above the water line, making equipment access for service signicantly easier. Overtopping device. Overtopping devices use wave energy that has crested and fallen over a structure. ese devices also can be either oating or shore based. Perhaps a crude analogy of an overtopping device is to think of a parking lot near a seawall on a stormy day. As large waves crash over the sea- wall, the parking lot, which likely has some sort of drainage system, will ll with water. A turbine placed in the drainage system captures the energy of the waves that have crashed over the seawall and onto the park- ing lot and are now returning to sea. Attenuator. An attenuator is a wave energy device that typically oats on the surface and is oriented perpendicular to the incident waves. A commercial device called Pelamis is a good example of an attenuator. Its akin to a large, mechanical snake that exes with each passing wave. PTO units at each joint convert this motion to electric energy via mechanical or hydraulic systems. Oscillating wave surge converter. is is a hinged ap device that converts the hori- zontal component of wave energy (surge) to electric power. Designs are varied within this category and generally involve devices that are fully submerged or that break the surface. Additionally, there is variation on where the electricity is generated?either on the device and exported to shore via cable, or the devices motion is used to pressurize a working uid that is sent ashore and spins a generator via a traditional hydropower turbine. Axial ow turbine. An axial ow turbine typ- ically involves some sort of bladed device, resembling a propeller design. Axial ow tur- bines can be shrouded or unshrouded and have various combinations of blade designs. While the land-based wind turbine industry has generally settled on an upwind design (the turbine nacelle is always downstream of the blades), the MHK industry has several upstream and downstream nacelle designs that are being pursued. In addition, some devices have no nacelle at all and may use a rim-driven generator. Within the rim-driven generator designs, some have an open cen- ter while others have a fairing in the center. Cross-ow turbine. is describes the set of turbines whose axis of rotation is perpendicular to the current flow. These turbines can be vertically oriented or hori- zontally oriented. Reciprocating device. Another class of current devices are suspended in the ow and designed to capture vortex-induced vibrations. Readers may recall the collapse of the Tacoma Narrows Bridge in 1940, in which the wind owing over the bridges roadway excited an oscillating motion (both vertical and torsional) in the roadway. The oscillations increased in magnitude until eventually the bridge span failed. Reciprocating devices seek to harness that instability and convert the motion to elec- tricity. Devices vary in design and shape; some devices have circular foils while oth- ers resemble a hydrofoil type design. Current Category The Ocean Power Technologies PB150 PowerBuoy was deployed o the coast of Scotland in 2011.