Analysis of the Ocean Falls Wave Energy Converter in Regular Waves

Abstract

The world’s oceans contain a vast wave energy potential. Many promising technologies of wave energy conversion have been developed, such as the Oscillating Water Column. BAM Infraconsult B.V. has developed the Ocean Falls concept of an oscillating water column that has the addition of a tube and a back wall which is able to move in order to change resonant characteristics of the structure to match incoming wave frequencies.

A numerical model has been made to capture the dynamic behaviour of the oscillating water column represented as a "rigid piston", where the system is modelled as a linear mass-spring-damper. The internal water surface is assumed to be horizontal and flat, both valid approximations for small chamber widths with respect to the wavelength of the incoming wave. The hydrodynamic coefficients, as well as the excitation force on the piston have been obtained using the ANSYS AQWA software. Thermodynamics in the chamber have been included by assuming a linear, adiabatic and reversible air flow. Furthermore, pressure oscillations in the chamber due to the oscillation of water are assumed small compared to the atmospheric pressure. The equation of motion of the piston is coupled to the pressure oscillations inside the air chamber, which includes linear turbine damping and air compressibility. The response of the system has been studied in the frequency domain for regular and irregular waves.

Model experiments have been performed to compare the linear model to experiment results for regular waves. The turbine damping has been modelled by adding a lid on the air chamber containing an opening where air can flow through. The area of the opening is related to the level of damping induced on the water surface inside the chamber. Two back wall positions and three different damping cases have been studied.

Air compression has been negated from the linear model while comparing to experiments due to scaling complications and the relatively small effect of air compression. For the case with no turbine damping the model experiments indicate a longer resonant period compared to the linear model. This difference, to a smaller extent, is also visible in the damping cases. The measured flow in the damping cases is slightly larger than the linear model. Pressure measurements compare well with the linear model. Efficiency is defined to be the power in the waves divided by the power available to a power take-off system, such as a turbine. For the low damping case the efficiency measured and efficiency computed in the linear model compare well and both give a peak efficiency of around 60\%, contrary to the high damping cases where the model under-predicts the measured efficiency, indicating a higher damping input into the model possibly due to the linearisation of the damping. For high damping the experiments also give an efficiency of around 60\% while the numerical model predicts this to be circa 50\%.