An oscillating water column (OWC) wave energy converter (WEC) is a device designed to
extract energy from waves at sea by using the water to move trapped air and thus drive
an air turbine. Because the incident waves and the force caused by the power take-off
(PTO) interact, control of the power take off (PTO) system can increase the total energy
converted.
A numerical model was developed to study the interaction of an OWC with the water
and other structures around it. ANSYS AQWA is used here to find the effects on
the water surface in and around the central column of a five-column, breakwater-mounted
OWC. For open OWC structures, coupled modes were seen which lead to sensitivity to
incident wave period and direction.
The frequency-domain displacements of the internal water surface of the central column
were turned into a force-displacement, time-domain model in MATLAB Simulink using
a state space approximation. The model of the hydrodynamics was then combined with
the thermodynamic and turbine equations for a Wells turbine. A baseline situation was
tested for fixed turbine speed operation using a wave climate for a region off the north
coast of Devon.
A linear feedforward controller and a controller based on maximising turbine efficiency
were tested for the system. The linear controller was optimised to find the combination
of turbine speed offset and proportional constant that gave maximum energy in the most
energy abundant sea state. This increased the converted energy by 31% in comparison to
the fixed speed case. For the turbine efficiency control method, the increase was 36%.
Energy conversion increases are therefore clearly possible using simple controllers. If
increased converted energy is the only criterion for controller choice, then the turbine
efficiency control is the best method, however the control action involves using very slow
turbine speeds which may not be physically desirable
