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Hydrodynamic Systems One of the simplest hydrodynamic system, in concept at least, is the overtopping device. This device, shown in diagram to the right, is simply a reservoir with the bottom of the reservoir floating above the mean wave level (MWL). Wave crests splash over a ramp into the reservoir. Sea water flows from the reservoir through a turbine (the PTO), which is connected to an electric generator. This overtopping principle forms the basis of the Wave Dragon. In the Wave Dragon a set of “wings”, which float on the water surface, is used to funnel waves into the overtopping device and thus increase the wave amplitude.  Overtopping Device Diagram: Voith Hydro WaveGen Limited Another hydrodynamic system utilizes the oscillating water column (OWC) as shown in the figure at the right. The OWC device consists of a sealed box with an opening running along its bottom. The OWC box may be either floating or fixed on shore. If on-shore the position of the opening must always lie below the low tide level. If floating the opening must lie below the trough of the largest expected wave. The opening runs the entire width of the OWC box, perpendicular to the usual direction of wave propagation. The air pressure in the box will rise and fall as waves strike the box and the height of water over the opening changes. The varying air pressure drives the PTO device.  Oscillating Water Column Diagram: Voith Hydro WaveGen Limited Both the overtopping and oscillating water column devices belong to a class of wave energy convertors generically called “terminators”. Terminators are always wider along the dimension that is perpendicular to the wave propagation direction. The designation terminator arises from the fact that the waves usually terminate on the device. Terminators are either on shore or if floating act as a breakwater. Members of another generic class of WECs are called “point absorbers”. The length (along the direction of wave propagation) and width of a point absorber are small compared to the usual wave length. The majority of WEC designs are point absorbers. They are either floating or submerged devices. One type of point absorber is the Archimedes Wave Swing, shown in the diagram. The base of the device consists of an air-filled cylinder (the silo) attached to the sea floor and open on the top. The silo is enclosed by another movable cylinder (the floater). An airlock exists between the two cylinders. The floater can move up or down with pressure decrease or increase as a wave trough or crest passes above the device. The floater is attached to a linear electric generator (PTO). A number of Archimedes Wave Swings are usually situated together in an array. The individual units are connected together and to shore by underwater cables.  Archimedes Wave Swings Diagram: AWS Ocean Technology Another member of the class of point absorbers is the AquaBuoy. A diagram of the hydrodynamic system of the AquaBuoy is given in the figure. The AquaBuOY consists of four elements: a buoy, which floats on the surface, an acceleration tube, a piston and a set of hose pumps. The acceleration tube is a vertical, hollow cylinder rigidly mounted under the body of the buoy. The tube is open in both ends so seawater can pass unimpeded back and forth. Positioned at the midpoint of the acceleration tube is the piston, a broad, neutrally buoyant disk. When the buoy is at rest, the piston is held at the midpoint by the balanced tension of two hose pumps that are attached to opposite sides of the piston and extend to the top and bottom of the acceleration tube, respectively. The hose-pump is a steel reinforced rubber hose whose internal volume is reduced when the hose is stretched, thereby acting as a pump. As the buoy moves up and down with the wave action the piston alternately stretches the lower and upper hose pump. The pressurized sea water (the PTO method) is subsequently expelled into a high-pressure accumulator, and in turn fed to a turbine which drives a generator.  The AquaBuoy Diagram: Finavera Renewables Inc. Members of the third and final generic class of WECs are called “attenuators”. Attenuators are aligned with the incident wave direction with their beam, or width, much smaller than their length. Attenuators can be compliant or articulated structures. On of the most advanced and typically seen attenuator WECs is the Pelamis. A top and side view of the Pelamis is given in the figure. The Pelamis WEC is a semi-submerged structure composed of cylindrical sections linked by hinged joints. The wave-induced motion drives hydraulic rams in the articulated joints which force hydraulic fluid via control manifolds into high pressure accumulator tanks for short term energy storage. Hydraulic rams couple wave energy from both horizontal and vertical motion, thus extracting maximum energy from the ocean waves.  Pelamis WEC. Diagram: Pelamis Wave Power Before leaving hydrodynamic systems, the important concept of system performance should be discussed. A fundamental quantity used to evaluate system performance is “Capture Width”. In practice, the Capture Width of a particular WEC may be greater than the width of the device. If, for example, a particular site had waves with a power of 40 kw/m and a WEC of one meter width is properly setup for the wave frequency, then the power coupled to the intermediate medium (compressed air, pressurized fluid, etc.) may, in practice, be greater than 40 kW. This does not mean we are getting something for nothing, or are violating the conservation of energy principle. This is simply a result of the fact that the WEC extends deep into the water and this effectively increases the Capture Width to beyond the device width. What this means practically is that WECs set in an array can be spaced apart and still achieve optimal wave energy recovery. The Capture Width is dependent on the particular device, how it is set up and controlled, and on the wave characteristics at the site. Clearly one would like to maximize Capture Width for the devices in an array, so fewer devices are required. This maximization is obtained by careful advanced system modeling and tank testing, and careful onsite testing. |
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