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The energy of sea waves can be absorbed by wave energy converters in a variety of manners, but in every case the transferred power is highly fluctuating in several time-scales, especially the wave-to-wave or the wave group time-scales.

Author: António F. de O. Falcão IDMEC, Instituto Superior Técnico, Technical University of Lisbon.

In most devices developed or considered so far, the final product is electrical energy to be supplied to a grid. So, unless some energy storage system is available, the fluctuations in absorbed wave power will appear unsmoothed in the supplied electrical power, which severely impairs the energy quality and value from the viewpoint of the grid. Besides, that would require the peak power capacity of the electric generator and power electronics to greatly exceed the time-averaged delivered power. In practice, three methods of energy storage have been adopted in wave energy conversion.

An effective way is storage as potential energy in a water reservoir, which is achieved in overtopping devices, equipped with more or less conventional low-head hydraulic turbines, capable of attaining a peak efficiency close to 90%.

In the oscillating water column type of device, the size and rotational speed of the air turbine rotor make it possible to store a substantial amount of energy as kinetic energy (flywheel effect); this is particularly true for the Wells turbine, whose rotor diameter and blade tip speed are both substantially larger compared with the self-rectifying impulse turbine (that has been proposed as an alternative to the Wells turbine). These self-rectifying air turbines are relatively robust and mechanically simple pieces of equipment. However, they are subject to much more demanding conditions than the turbines in any other application, including wind turbines.

Indeed the flow through the turbine is reciprocating and is random and highly variable over several time scales, ranging from a few seconds to seasonal variations. It is not surprising that the time-average efficiency of an air turbine in an OWC has been found to be relatively low, in general not exceeding about 50%. This is a technical area with substantial room for improvement.

In a large class of devices, the oscillating (rectilinear or angular) motion of a floating body (or the relative motion between two moving bodies) is converted into the flow of a liquid (water or oil) at high pressure by means of a system of hydraulic rams (or equivalent devices). At the other end of the hydraulic circuit there is a hydraulic motor (or a high-head Pelton water-turbine) that drives an electric generator. The highly fluctuating hydraulic power produced by the reciprocating piston (or pistons) may the smoothed by the use of a gas accumulator system, which allows a more regular production of electrical energy. Naturally the smoothing effect increases with the accumulator volume and working pressure. High-pressure oil is the working fluid in the Pelamis, Wavebob, Powerbuoy, Wave Star devices, whereas sea water is used in the PTO of Aquabuoy and the Brazilian multi-body hyperbaric device. This type of PTO may be regarded as unconventionally using conventional equipment. Hydraulic motors (including variable displacement versions, particularly suitable for oil flow control) are commercially available up to several hundred kW, while Pelton turbines exist that cover a very wide range of power levels. In both cases peak efficiencies can reach close to 90%, although the efficiency can drop significantly at partial loads. The gas accumulator system may represent a substantial part of PTO cost.

In most wave energy devices, a more or less conventional electrical generator is used to produce electricity. Variable rotational speed is frequently adopted, the technology (and the power range) being basically similar to wind energy applicactions. Some devices use direct electrical energy conversion by means of linear electrical generators (this was pioneered in Holland for the Archimedes Wave Swing device). These machines are still at the development level. Such PTO systems do not require an intermediate mechanical system and may attain a high efficiency. On the other hand, the energy storage capability is small (or very expensive) which may result in a high peak-to-average power ratio and in poor
quality of the electrical power supplied to the grid.

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