Energy Storage Requirements for Near-Optimal Reactive Control of a Wave Energy Device

Abstract

This paper investigates an approach to near-optimal real-time reactive control of wave energy devices based on time-domain up-wave surface elevation information. The paper first presents the overall approach required for such control, and recalls the need for future force and response information due to the causality of the radiation force and the non-causality of the exciting force (in relation to the wave profile at device centroid). The present approach is a realistic approximation based on linearized wave propagation and device dynamics models. For a predominantly heaving submerged point absorber in an approximately uni-directional incident wave field, the amount of instantaneous reactive power is compared with the instantaneous absorbed power. Although the net reactive energy is zero in the absence of actuator losses, the instantaneous reactive energy requirement for small devices in swell-dominated spectra may be significant. The time-domain calculations here confirm this and show that substantial amounts of energy may need to be exchanged with an external energy storage unit or the grid until sufficient energy has been absorbed.