Real-time control of wave energy converters requires knowledge of future incident wave elevation in order to approach optimal efficiency of wave energy extraction. We present an approach where the wave elevation is treated as a time series and it is predicted only from its past history. A comparison of a range of forecasting methodologies on real wave observations from two different locations shows how the relatively simple linear autoregressive model, which implicitly models the cyclical

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... or of waves, can offer very accurate predictions of swell waves for up to two wave periods into the future. Index Terms-Time series, wave energy, wave forecasting. I. INTRODUCTION T HE energy conversion in most wave energy converters (WECs) is based either on relative oscillation between bodies or on oscillating pressure distributions within fixed or moving chambers. Oscillators generally have pronounced resonances, which enable efficient power absorption only over a restricted range of frequencies. In order, however, to cope with the variations of wave spectra, a control system can be designed to alter the oscillator dynamics such that the efficient energy conversion occurs over a wide range of wave conditions [1]. The control approach, in the early stages of wave energy conversion, consisted of frequency domain relationships regulating the dynamics of the system to be tuned for maximum energy absorption at different peak frequencies corresponding to different incoming wave spectra [1], [2] . Although being an advantageous approach for real sea spectra, frequency domain techniques do not generally allow real-time control on a wave-bywave basis, which can significantly raise the device productivity and, therefore, its economical viability. Real-time optimal control can be directly derived from the aforementioned optimal frequency relationships [1], [2] . The main difficulties arise from the fact that the transformation into the time domain results in noncausal transfer functions, so that the conditions for optimal power absorption can be realized only if future motion of the device, or of the future incident wave profile, are known [1]- [3] . The problem of short-term wave prediction, for some seconds into the future, of the actual wave elevation profile at a specific point of the sea surface, is central, therefore, to the more general Manuscript