Abstract

This paper presents the initial simulations and model tests into the ‘Green Water Concept’ for wave energy conversion. Instead of reducing the motions and green water as is done in normal offshore hydrodynamics, the ‘Green Water Concept’ tries to maximise the motions and green water through ‘inverse engineering’ to generate electric energy from the waves. First, results are presented of frequency domain calculations for the motion (de-) optimisation. Second, pilot model tests on the ‘Green Water Concept’ are presented. These tests also included the initial modelling of an electric and hydraulic Power Take Off (PTO). The accurate modelling of a PTO is an important aspect in testing of wave energy conversion concepts numerically and in a wave tank: at the moment that energy is converted into electricity in the PTO, the hydrodynamic behaviour of the structure is changing. The tests confirmed the high motions and large amount of green water of the Green Water Concept as predicted in simulations. The application of a real PTO gave important insight in the possibilities and challenges of PTO modelling at model scale. For the present concept a mean Power (at full-scale) close to 1MW was generated in a regular wave of H=3.0m for the maximum possible setting in the chosen test set-up. This setting was limited by the chosen mechanical and electronic motor set-up in this pilot test series, not the actual maximum of the Green Water Concept itself. Considering the test results, it is clear that the potential of the system is significantly larger.

Figure 1

Background

Wave energy is a concentrated form of wind energy: the wind transfers energy into the waves over a long fetch. Cruz [1] has estimated that Europe has an average wave power of 50kW per metre width of wave front [1]. Others [2] indicate that worldwide the economically exploitable amount of wave energy is estimated at 2,000 TWh/year, an average power of 200GW over a year. This is equivalent of 200 large power stations. The challenge is to generate a predictable amount of energy, in a reliable way, at a reasonable cost.

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