ABSTRACT: Optimal control procedures for maximum energy absorption by OWC devices require the incident wave to be known in the future, mathematically up to infinity. This is a major limitation for practical on-line control procedures, since the incident wave cannot be predicted, except for very short time intervals. The present paper presents a sub-optimal control procedure which requires only the time-history of the diffraction flow. Together with the theory, the paper presents results in the frequency domain. Linear hydrodynamic theory is assumed throughout the paper.
INTRODUCTION Activities on wave energy R, D&D have been recently reviewed in a paper by Falnes and L0vseth (1991). In January 1992 an European Community project in Wave Energy started, under JOULE programme. Among the aims of this project is the construction of an European Pilot Plant of the Oscillating Water Column (OWC) type. This kind of device for extracting energy from ocean waves is based on the exchange of energy from the water into the air that is trapped above the water free-surface by an air chamber open at the bottom. The air chamber, which we assume to be formed by a fixed, partly submerged structure, is connected with the atmosphere by a duct containing an air turbine. As a result of an incident, in general irregular wave field, the internal free surface as well as the air pressure are made to oscillate, and a flux of air is driven back and forth through the turbine. This allows to introduce a new degree of freedom in the control of the device, which may lead to a considerably increase in energy absorption. The control problem in irregular seas is to find out the instantaneous flow across the turbine and air pressure in the pneumatic chamber that maximize energy absorption.