Wave energy conversion in a random sea

The response of a three-dimensional oscillating water column (OWC) wave energy converter in monochromatic and random seas was investigated to determine the potential power available. Probabilistic models of available average wave power in a random sea state were formulated through the development of probability density functions transformed from theoretical distributions of wave heights and periods in a random wave field and a functional relationship describing the wave power. The computed available average wave power in a random sea state was in good agreement with measured wave spectra data. A hybrid-finite element method was used to numerically model the fluid-device interaction and to establish the radiation diffraction hydrodynamic force coefficients for the device. Six OWC devices with varying wall configurations, modelled in finite depth and monochromatic seas, showed considerable variation in peak performance and frequency response bandwidth with a consistent variation in performance between devices for obliquely incident wave directions. The numerical model for monochromatic seas was verified using the results of wave tank experiments. Based on a transformation of random variables using a functional relationship developed for wave power extraction in monochromatic seas, a probabilistic density function of wave power extraction was derived. Based on a linear superposition principle, overall device performance in random seas with narrow and broad banded wave spectra characteristics was determined. In random seas, the performance of the device is influenced by the frequency bandwidth of the wave spectra such that an increase in the spectral bandwidth parameter causes a reduction in peak performance, but effectively broadens the frequency response of the device. Predicted overall device performance in random seas were in agreement with experimental tests.