Control Strategies for Arrays of Wave Energy Devices

In this paper, we investigate the differences between two control strategies for a two-device linear array of wave energy converters (WEC) for device spacings of 4 to 80 times the device diameter. The WECs operate in heave only and are controlled in real time. The control strategies, called the independent device and global array control, estimate the excitation forces and calculate the optimum vertical velocity trajectory and reactive power take off force to achieve the velocity for a given sea state. The independent device controller assumes that each device is in isolation and only computes the excitation forces based on the incident wave, whereas the global array controller does take the radiation and diffraction effects from the other devices into account. Whatever controller is used in the simulations, all hydrodynamic coupling effects are accounted for. For a cylindrically shaped single-body device with diameter of 5 m and a draught of 25 m we simulate the device motions for 5 regular wave cases and one irregular sea state representing a Bretschneider spectrum. These first test sets illustrate the different control needs depending on the separating distance of the array members. A global control algorithm is advantageous for small separating distances, where radiation is larger