Wave energy:   Appendix 2 [ main text | hovedtekst ]

Wave energy converters Classification Wave Energy Converters (WECs) is often divided into three classes; terminators, attenuators and point absorbers, depending on their size and orientation with respect to the waves. A terminator has a horizontal extension substantially larger than the typical wavelengths, and is aligned parallel to the wave crests. Attenuators are of the same order of magnitude, but are oriented along the direction of the wave propagation. Both terminators and attenuators typically have a rather flat frequency response curve. This means that they can utilize energy from waves of significantly different wavelength with virtually the same order of efficiency. Point absorbers are converters with a horizontal extension much smaller than the typical wavelengths. They have a narrow frequency response curve, with a large peak. This means that point absorbers can utilize energy efficiently from waves within a small variation of wavelengts, but are rather unefficient when exposed to waves outside this narrow spectrum. The wavelength for which the energy absorption is maximum, is called the resonance wavelength. Point absorbers are of particular interest for several reasons. Firstly, a point absorber can absorb a considerable amount of energy per volume of the converter, provided that it oscillates in resonance. In fact, an axisymmetric wave converter of this type can theoretically absorb all the incoming wave energy in a wave front of width d, where: and lambda is the wavelength. In other words; it can absorb more energy than the energy transported within the width of the converter itself. This is a result of a diffraction phenomenon, and we say that the WECs absorption width is greater than its physical width. Another important point is that the small physical size of a point absorber makes it rather unexpensive compared to attenuators and terminators. Finally, the fact that the resonance wavelength of a point absorber is smaller than the wavelengths of the incoming waves, makes such devices suitable for phase control. The small size combined with the possibility of controlling the oscillations means that the power to volume ratio may become large, which is of crucial importance to cost efficiency. Note that the division into the classes terminators, attenuators and point absorbers is a rather crude one. Some WECs, e.g. with a horizontal extention comparable to one wavelength, or utilizing several mechanisms of conversion, may not obviously belong to one particular class. Small converters Some years ago the wave energy research focused mainly on large converters, emphasizing the importance of a large power output. During the later years focus has shifted towards smaller units, as one has realised that the main objective is to minimize the costs per power output, not to maximize the power output itself. One can always extract any wanted amount of energy simply by putting more units into the ocean. To sum up, smaller WECs are interesting for four reasons: They may function as point absorbers. That is, power is absorbed from a width of the incoming wave larger than the width of the absorber itself, increasing the power/cost-ratio substantially. Small units are the most suitable for phase control, due to their high resonance frequency. This feature may increase the power/cost-ratio even more. Small units respond to small waves, which are present most of the time. Finally, small units require less construction material. Hence, investment costs are lower. Consepts and devices The three classes of WECs can be further divided into groups according to the technical principles applied to convert the wave energy into mechanical and electrical energy. The converters may also be classified according to their modes of oscillation (rotational or translatory, and along/around the three coordinate axes). Below we only give a short description of some of the most important devices. One of the most common concepts is a floating or semi-submerged buoy, where the energy is extracted from the oscillatory motion of the buoy relative to some rigid or inert structure, e.g. a concentric axis moored to the seabed or a submerged reference plate. The energy extraction system may be mechanical, pneumatic or hydraulic. Buoys typically belong to the class of point absorbers. Another interesting device is the so-called Tapchan (Tapered channel) invented by the Norwegian company Norwave. It consists of a water basin a few meters above the ocean surface level and a tapered channel leading into the basin. Incoming waves are led through the channel. As the channel narrows the wave height increases, and eventually the waves spill over the sides of the channel and into the basin. From the basin the water runs back to the sea through a conventional water turbine. The so-called Salter Duck, invented by Stephen Salter in Edinburgh during the seventies, is an assymetrical device moving in pitch mode relative to a horizontal stationary spine as the waves passes. The power take-off mechanism is placed in the interior of the spine. This device may be considered as a point absorber or a terminator, depending on the number of units on each spine. Yet another popular device is the oscillating water column (OWC), which typically belongs to the class of point absorbers. Some OWC devices consist of several oscillating water columns placed side by side, hence constituting a terminator. An OWC consists of a chamber entrapping a volume of air above a water column. The chamber is open towards the sea below the surface. The incoming wave causes the water column to oscillate vertically, creating a dynamic air pressure in the air chamber. The resulting air flow is pumped through an air turbine, driving a generator. Typically, a so-called Wells turbine is used in this case, since it rotates in only one direction, regardlessly of the air flow direction. One particular variant of OWC is the Twin OWC. In this case the air chamber is divided by a vertical wall, resulting in two parallell air chambers, side by side. The Japanese inventor Yoshio Masuda originally designed the twin OWC in order to rectify the air flow through the turbine. He equipped each chamber with a check valve, one of them letting air only in from the outer atmosphere, and the other one only out to the atmosphere. The turbine was placed between the two chambers. Hence, when the water columns were rising, the valve in one of the chambers (let's call this chamber A) would close, and the air in this chamber would flow through the turbine into the adjacent chamber (chamber B) where it would be let out through the valve. When the water columns were dropping, the valve in chamber B would close, and air would would be sucked in through valve in chamber A and through the turbine to chamber B, in the same direction as in the first case. After the unidirectional Wells turbine came into use, it was no longer necessary to rectify the air flow. The twin OWC has however become subject to new interest during the last decade, for another reason. If one replaces the passive check valves with controlled valves, this device is very well suited for phase control. In this case the oscillatory motion is held back in the appropriate parts of the oscillation period by using signals from wave measurements to open and close the valves. It is of course possible to phase control a single OWC in a similar manner, using a single valve between the chamber and the atmosphere, but this has shown to give a rather uneven power output, being difficult to utilize. Employed on a twin OWC, phase control can increase the efficiency of the device considerably with almost no additional costs. Of course, the rectification of the air flow is an additional advantage, as this allows use of a conventional air turbine, which is more efficient than the Wells turbine. Other important concepts include: The Pelamis, which is a snake-like construction oriented along the direction of wave propagation. It consists of several cylindrical sections linked by hinged joints, where the wave energy is extracted by means hydraulic rams. These may be controlled to tune the resonance response of the device. The ConWec - a phase-controlled OWC where an internal piston drives a water pump, pumping sea water into a basin above sea level. The water runs back into the sea through a conventional water turbine. The Hose pump, a hydraulic system consisting of flexible hoses connected to buoys and submerged moorings. The hoses are designed to chance volume as they stretch due to the wave oscillations, hence pumping fluid through turbines. The BBDB (Backward Bent Ducted Buoy), a buoy with an internal water column where the inlet is pointing away from the incident wave. The Rotating sylinder. While oscillating in the waves, the cylinder is being rotated as well, due to a motor. The combined motion produces an assymmetric flow around the cylinder. This yields the so-called «Magnus-Budal effect», providing an additional force, enhancing the power extraction. The Pendulor, consisting of a flap placed in the inlet of a chamber, hinged in the upper end, and connected to a hydraulic pump. Although pointing out some common principles, the above selections do not necessarily give a representive picture of the various ideas world wide. Arne Brendmo Latest rev. 2007-03-01