Linear generator systems for wave energy conversion

Proceedings of the 7th European Wave and Tidal Energy Conference, Porto 09/2007;
Source: OAI
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    ABSTRACT: Among the various types of wave energy converters currently being developed, heaving point absorbers are one of the simplest and most promising concepts, possibly due to their ease of deployment and integration in larger arrays structures. A typical efficient energy conversion system for point absorbers is based on hydraulic power take-off (PTO) systems, consisting in a double-acting cylinder and two or more accumulators, reserving fluid at different pressures and linked between them by a hydraulic motor connected to an electric generator. For the purpose of control and modulation of the power output the hydraulic circuit might include a certain number of valves that can set the pressure levels within the accumulators. This paper presents a simple model of a heaving oscillating buoy, represented by a surface-piercing cylinder, extracting power by means of a hydraulic system. The hydrodynamic behaviour of the absorber is modelled through application of the linear water wave theory. Apart from the basic elements listed above, the model of the hydraulic system includes leakages and pressure losses and takes into account the compressibility of the fluid. Also possible extra accumulators are considered in order to improve the performance of the hydraulic system by means of properly controlled valves. The function of these extra accumulators consists in storing and releasing energy to the system when this is desirable for the improvement of the power output. Simulations were carried out through a time-domain approach making use of the Cummins equation and considering regular monochromatic waves and irregular wave-trains.
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    ABSTRACT: In this paper a linear switched reluctance gener-ator used in a direct drive wave energy converter is considered for modeling and model-based opti-mal control. Linear switched reluctance generators (LSRGs) are highly nonlinear electromechanical converters which must be adequately controlled in order to achieve a satisfactory energy conversion efficiency. In order to pave the way for a systematic design of a control strategy that directly addresses the wave energy converter efficiency maximization and respects system constraints, the discrete-time piecewise affine (DTPWA) hybrid model form is proposed to model the considered LSRG in the configuration with a power converter. The proce-dure of obtaining a hybrid DTPWA model itself is also pursued in a systematic way, by employing clustering-based piecewise affine fitting of nonlinear characteristics of the considered LSRG, which are in turn computed from the generator geometry using finite element analysis methods. The hybrid DTPWA model is validated on the starting nonlinear LSRG model.

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