Abstract
Oscillating water column (OWC) wave energy converters (WECs) are a popular type of wave energy devices, due to their advantages over many other WECs. For example, OWC WECs normally have no moving components in sea water, and have a small torque and a high rotational speed for a certain power take-off. Practically, some foundation-type pioneer plants of OWC WECs have been very successful in generating electricity to grids continuously. In order to obtain higher yields of wave energy production, it is proposed to move the OWC WECs to open and deep water regions, and for the purposes of economics and reliability, the OWC WECs are designed to be floating devices, with a potential of utilizing the device motions to improve wave energy conversion capacity. To further understand the OWC WEC performances in waves, a floating cylindrical OWC has been designed and tested in an ocean wave tank. In the model test, five different size orifices are designed to represent different damping levels of the air flow. In the experimental study, a systematic series of tests in both regular and irregular waves has been conducted to help understand the hydrodynamics and aerodynamics of the generic OWC device.
In the model test, the interior water surface motion and the pressure in the air chamber are measured and based on them the primary power take-off by the device can be calculated. Alternatively, the power take-off can be calculated by the pressure measurement only or by the interior water surface measurement only due to the unique relation of the pressure drop and the airflow passing through the orifices. In addition, in the experiment, the motions of the floating structure have also been measured, from which it is possible to correlate the motions and the wave energy extraction.
As expected, the orifices exhibit a quadratic non-linear relation between pressure and the flowrate. Though simple, the orifice power take-off system may exhibit a similar flow feature to that of an impulse turbine, thus an appropriate model to the impulse turbine.