In early stages of the power plant development, several tests with different prototype variants were performed. These tests proved the basic concept of the device and supplied the team with the necessary experience for the further research.
Small scaled model tests (scale 1:30) were performed in MARINTEK in Trondheim, Norway in May 2010. The model was tested in regular and irregular waves of the JONSWAP spectrum and the buoy’s motion (heave, roll, pitch and yaw) was measured. Electrical output of the model was also monitored.
The tests proved favourable effects of the innovative elements of the device: the water captured in the buoy chambers and the transformation of the buoy’s motion into a unidirectional and a relatively smooth rotation by means of inertial clutches. Several particularities of the devie were change since these experiments in accordance with the later numerical analyses, therefore these tests could be accepted today only as concept-proving.
The model tested in Denmark was roughly 1:35 in scale. It was incorporated with the patented PTO system and a simplified electrical systems. Sensors for movements and forces were installed to measure the most vital parameters of the model.
The model, tested in Denmark (1:35), and some of the measured outputs of Aalborg tank tests: float heave ζb , float pitch ψb and dynamic forces in tendons FT (Aalborg, Denmark, September 2013).
A typical transfer function of the buoy heave, obtained by analysis of recorded data left), and the power, obtained from the recorded data as function of the buoy vertical speed.
The latest model (scaled at 1:18), was tested at Ifremer deep water basin in Brest, France, under the MaRINET EU incentive for renewable technology trials. This model was already fully functional with a built-in original PTO system and a full electrical system with various control options and full monitoring (electrical parameters, including power output, forces in tendons and the PTO system, movement of the device). During the trials, the model was already returning some of the generated power back to the public electric grid. This device transformed 45 % of available wave energy into electrical energy.
The model, tested in France(1:18), and some of the recorded outputs: wave ζw , float heave ζb , spar surge ξs, generator rating nG, generator torque MG, tension forces in tendons Q1,2 and force in the toothed rack Ft (Brest, France, February 2014).
A typical transfer function of the float heave (top left), a typical transfer function of the spar surge (bottom left) and typical captured power captured as curves as functions of the generator loading.
In addition to prototype tests and model tests in wave tanks, Sigma Energy constructed a dry rig for testing of the mechanical PTO system and the generator regulations. These tests are continuously performed and, in spite of the principle shortcoming of such tests that they cannot properly cover complex interaction between waves and structure, several very useful information have been obtained. These outcomes cover friction and other loses in the system, generator efficiency in changeable regimes, performance of unidirectional clutches in low and high multiplier rotation ratio, etc.
The dry rig for the PTO system testing in Sigma workshops (top left and bottom left) and some results of the dry tests and the numerical comparison (right). The force in the toothed rack, torque on the generator shafting and the generator rating are presented. The input motion simulated regular waves.
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