System identification and centralised causal impedance matching control of a row of two absorber wave converters

Similar to offshore wind turbines, multiple point absorber wave energy converters (WECs) will be installed in an array configuration, to increase the total capacity, and to benefit from the economies of scale. Whereas wind turbines always interact destructively due to wake effects, WECs can interact constructively, since hydrodynamic interactions between the WECs occur through radiation and diffraction of waves, changing the direction of the incoming wave energy. This paper presents the dataset and results of the experimental modelling of a row of two 'WECfarm' heaving point absorber WECs at the wave basin of Aalborg University (AAU). Impedance matching enables maximum power transfer between two oscillatory systems, from the waves to the Power Take -Off (PTO) of the WEC. While literature covers this impedance matching approach for single, isolated WECs, the research discussed in this paper is unique by applying the experimental modelling of the impedance matching approach on a row of two WECs. Radiation system identification tests are executed to determine the intrinsic impedance of the isolated WECs, and the 2 x 2 impedance matrix of the two-WEC array. A causal impedance matching resistive and reactive controller are designed, implemented and tested for a selection of operational sea states. For centralised control, hydrodynamic interactions are taken into account by considering the complete impedance matrix, whereas for decentralised control no hydrodynamic interactions are taken into account by considering only the diagonal of the impedance matrix. The power absorption performance of the isolated WECs, the two-WEC array with decentralised control, and the twoWEC array with centralised control, are compared. Constructive interaction is identified, yielding enhanced power absorption for the array compared to the WECs isolated. Therefore, the layout of the WEC array and the control of the PTO should be optimised simultaneously, to maximise the power absorption of the array as a whole.