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.

The influence of scale on the air flow and pressure in the modelling of Oscillating Water Column Wave Energy Converters

“…The single frequency assumption is reasonable because of the analysis is based upon regular waves, and therefore only one excitation frequency transmitted through the system. This is confirmed from the experimental data shown in Section 5 and similar rationale is also used by [14,16] in their analysis. We can therefore write any such flowvariable f as:…”

“…Compressible air flow in OWC chambers is studied further by Sheng et al [14], assuming isentropic air flow. Their study does not consider the wave interaction problem, as the free-surface motion is represented as a rigid, flat boundary with a given first-order oscillatory motion.…”

“…A benefit is better designs of prototype OWC using small scale models, and recommendations about including air scaling in the design of small scale experiments. To achieve this, initially, the gas (air) flow problem will be investigated by applying a linear analysis to the 1D equations presented in [14]. The linear analysis is a well-know tool used to study OWC interaction with waves and air pressures [19], and despite the simplifying assump-tions excluding nonlinear and viscous effects, it has been proven a satisfactory tool to investigate these processes [20].…”

Assessment of primary energy conversions of oscillating water columns. II. Power take-off and validations

“…And again, the numerical method predicts the air compressibility well. In scale models, the porous membranes (Figure 4) have been used to approximate the linear air turbine PTO (see details in Sheng et al 11 ). Practically, a weak nonlinear relation between the chamber pressure and the air volume changing rate can be seen (Figure 7).…”

“…The test rig could provide a much powerful drive to the piston than that from the normal tank test. Hence, it is possible to obtain air compressibility in the well-controlled laboratory conditions (Sheng et al 5,11 ). The validation has shown the developed numerical method is capable of predicting the compressibility.…”