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Experimental development of offshore wave energy converters

More:  Institutes 
Portuguese title: Desenvolvimento experimental de sistemas offshore de energia das ondas
Reference no: PTDC/EME-MFE/66999/2006
Period: June 2007 till May 2010
Status: Completed

Institutes (4)  Top 

Abstract:
The ocean waves may be regarded as a major energy resource worldwide, and especially in Portugal, where natural conditions (high level of wave energy, narrow continental shelf i.e. deep water near the coast) and others (electrical grid close to the coast, government-imposed high purchase price of wave-generated electrical energy) are particularly favourable. Offshore devices are basically oscillating bodies, either floating or (more rarely) fully submerged. If large-scale exploitation of wave energy is to be performed, large arrays of such devices (like wind farms) are to be deployed offshore. Compared with shoreline and near-shore devices, offshore devices exploit the more powerful wave regimes available in deep water (typically more than 40m water depth). However they are in general more complex, which, together with additional problems associated with mooring, access for maintenance and the need of long underwater electrical cables, has hindered their development, and only recently some systems have reached, or come close to, the full-scale demonstration stage. The theoretical and numerical modelling is the first stage in the development process of wave energy converters (WECs). The hydrodynamic modelling of the wave energy absorption is usually based on linear water-wave theory. This theory ignores non-linear effects that are known to be important in the more energetic sea states, and also in small WECs (“point absorbers”), whose motion amplitude is relatively large. The standard way to account for non-linear wave effects is by model testing in wave basin, usually at scales between about 1:80 to 1:10. The main function of the mooring system of a floating WEC is to keep it in place. This results in an interactive process, since the mooring forces will depend on the motions of the WEC, and such motions in turn depend on mooring forces in addition to hydrodynamic forces and power take-off forces. The mooring configuration should be adapted to the type of WEC. The capability to suitably model the energy conversion chain (from waves to electrical energy) and mooring system, and its validation by physical model testing are essential in the conception, basic studies, and design of wave energy converters, and in the optimal design/specification of their structural, mechanical and electrical components. This is addressed in five tasks. The class of devices to be addressed in the project is that of a two-body point absorber oscillating in heave: body 1 is a floater oscillating with respect to a submerged mass that consists of the water (body 2, that acts as a reference inertia system) contained in a vertical tube open at both ends, located under the floater (possibly piercing it to the free-surface). The energy is extracted through the relative motion between the floater and a piston moving inside the tube. There are several devices based on this or on a similar concept that are being or have been investigated in several countries, and constitute a promising class of devices. The object of Task 1 is to define the configuration or configurations to be studied, including new ones, taking into account their technical interest and/or because they are regarded as strategic from the national point of view (e.g. Portuguese institutions involved in their development or fabrication or deployment). This will include the definition of the power take-off system as far as this is relevant to the theoretical and the experimental modelling. The theoretical/numerical modelling of the device performance including the energy conversion chain from the wave to useful energy is to be done in Task 2 where extensive use will be made of the results from the ongoing FCT project “Modelling, optimization and control of offshore wave energy systems”, 2004-2007, with some of the same partners. This task will also provide numerical results from a fully non-linear hydrodynamic Volume-of-Fluid (VOF) numerical code, including non-small amplitude waves and real-fluid effects, to assess the non-linear hydrodynamic effects that cannot be modelled by the theoretical numerical modelling referred to above, based on linear water-wave theory. Slack mooring is the most suitable type of mooring in the case under consideration. A suitable configuration and design for the mooring line system will be the object of Task 3, where use will be made of existing design rules and numerical codes with account being taken of specific conditions and constraints. Model testing is to be performed in Task 4 in a wave flume (scale about 1:80) and in Task 5 in a wave tank (scale about 1:40). Detailed velocity field measurements concerning the diffracted wave field (fixed floater) will be performed in Task 4 with PIV technique. These tests will provide an experimental validation of theoretical/numerical results from Tasks 2 and 3, and will allow an assessment of important non-linear hydrodynamic effects that cannot be accounted for by the linear water wave theory that underlies the available device performance modelling.

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