Wave Periods and the Calculation of Wave Power
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An informed, and accurate, characterization of the wave energy resource is an essential aspect selection of suitable sites for the first commercial installations of wave energy converters. This paper describes a detailed study on the variability of wave climate and measured spectral shapes and how they differ from the standard formulations prescribed by theory. In particular dissonance between the ratio of the energy period (TE) to the average zero-crossing period (T₀₂) is investigated at a range of open ocean locations. This relationship is important in the context of resource assessment as many previous works, lacking in detailed spectral data, have often assumed an incorrect ratio. This in turn has influenced the accuracy of the resulting estimates of wave energy. It is demonstrated that the earlier use of the frequently-employed wave period ratios is erroneous and more suitable relationships are presented for the Bretschneider and JONSWAP theoretical spectra. Furthermore, analysis of measured buoy data from real sea-states is used to illustrate that this relationship can in fact vary significantly in practice, depending on geographical location and the prevalent wave conditions. The variability that exists in spectral shape and bandwidth, and the effect this has on the relationship between TE and T₀₂, is illustrated through the comparison of recorded spectra with the Bretschneider spectrum. Analysis of a fifteen year dataset measured by a buoy off the coast of Southern California is presented to illustrate how the wave period ratio fluctuates on a seasonal and interannual basis, while it is also shown that previous studies of the Irish wave energy resource may have underestimated the theoretical power available by as much as 18%.