Investigation to Air Compressibility of Oscillating Water Column Wave Energy Converters

Abstract

It has been suggested that for full scale oscillating water column (OWC) devices, the pressure in and the volume of the air chambers can be large to create air compressibility in the air chamber. Due to compressibility, its density and temperature are different from those in atmosphere. When in exhalation, the pressurized air is driven out of the air chamber and mixes into the atmosphere outside the air chamber; whilst in inhalation, the atmosphere is sucked through the power take-off (PTO) system into the air chamber, and mixes with the de-pressurized air in the chamber. This paper presents a study on air compressibility in OWC air chambers by theoretical analyses and the relevant experimental studies. The theoretical analysis is based on the first-order differential equation for the flowrate and the chamber pressure, which has been derived for the air flow under the assumptions of the isentropic process and the known power take-off characteristics. In the study, an orifice type of PTO and a porous membrane type PTO, which are supposed to represent a typical nonlinear and linear PTO for small models, respectively, are both investigated. The investigation has shown the feasibility of the theoretical method on the air compressibility and the possible power loss due to the air compressibility.