This paper evaluates the technical feasibility and performance characteristics of an ocean-wave energy to electrical energy conversion device that is based on a moving linear generator. The UC-Berkeley design consists of a cylindrical floater, acting as a rotor, which drives a stator consisting of two banks of wound coils. The performance of such a device in waves depends on the hydrodynamics of the floater, the motion of which is strongly coupled to the electromagnetic properties of the generator. Mathematical models are developed to reveal the critical hurdles that can affect the efficiency of the design. A working physical unit is also constructed. The linear generator is first tested in a dry environment to quantify its performance. The complete physical floater and generator system is then tested in a wave tank with a computer-controlled wavemaker. Measurements are compared with theoretical predictions to allow an assessment of the viability of the design and the future directions for improvements.

This paper evaluates the theoretical application of nonlinear model predictive control (NMPC) to a model-scale point absorber for wave energy conversion. The NMPC strategy will be evaluated against a passive system, which utilizes no controller, using a performance metric based on the absorbed energy. The NMPC strategy was set up as a non-linear optimization problem utilizing IPOPT package to obtain a time varying optimal generator damping from the power-take-off (PTO) unit. This formulation is different from previous investigations in model predictive control, as the current methodology only allows the power-take-off unit to behave as a generator, thereby unable to return energy to the waves. Each strategy was simulated in the time domain for regular and irregular waves, the latter taken from a modified Pierson-Moskowitz spectrum. In regular waves, the performance advantages over a passive system appear at frequencies near resonance while at the lower and higher frequencies they become nearly equivalent. For irregular waves, the NMPC strategy lead to greater quantities of energy absorption than the passive system, though strongly dependent on the prediction horizon. It was found that the ideal NMPC strategy required a generator that could be turned on and off instantaneously, leading to sequences where the generator can be inactive for up to 50% of the wave period.

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One of the primary challenges for wave energy converter (WEC) systems is the fluctuating nature of wave resources, which require the WEC components to be designed to handle loads (i.e., torques, forces, and powers) that are many times greater than the average load. This approach requires a much greater power take-off (PTO) capacity than the average power output and indicates a higher cost for the PTO. Moreover, additional design requirements, such as battery storage, are needed, particularly for practical electrical grid connection, and can be a problem for sensitive equipment (e.g., radar, computing devices, and sensors). Therefore, it is essential to investigate potential methodologies to reduce the overall power fluctuation while trying to optimize the power output from WECs. In this study, a detailed hydraulic PTO model was developed and coupled with a time-domain hydrodynamics model (WEC-Sim) to evaluate the PTO efficiency for WECs and the trade-off between power output and fluctuation using different power smoothing methods, including energy storage, pressure relief mechanism, and a power-based setpoint control method. The study also revealed that the maximum power fluctuation for WECs can be significantly reduced by one order of magnitude when these power smoothing methods are applied.

The aim of this research is to use spectral techniques in evaluating the irregular wave performance of a novel wave energy converter concept that combines an oscillating surge wave energy converter with active control surfaces. The control surfaces allow the wave energy converter to have a time-varying geometry that enables the hydrodynamic exciting and radiation coefficients to be altered. In the current state of development the device geometry is controlled on a sea-to-sea time scale and combined with control of the power take-off (PTO) on a wave-to-wave time scale to maximize power capture, increase capacity factor, and reduce design loads. Analysis begins with the application of linear hydrodynamic theory to evaluate the device performance in terms of absorbed power, foundation loads, and accumulation of fatigue damage on the PTO. To determine the linear PTO damping coefficient that maximizes the time-averaged absorbed power for a given sea state, an optimization problem was constructed while incorporating a motion constraint on the maximum pitch amplitude of motion. The inclusion of the motion constraint prevents linear scaling of the performance results with the significant wave height. Previous studies on the modeling of oscillating surge wave energy converter designs have included the consideration of nonlinear hydrodynamics. Therefore, a quadratic viscous drag moment was added to the system dynamics through use of a quasi-linear viscous damping coefficient. The same performance quantities were calculated for both the linear and nonlinear models while assuming an irregular wave surface elevation described by a Bretscheider spectrum. One major effect of including the viscous drag moment was flattening of the capture width and structural load curves with respect to the wave spectrum peak frequency while reducing the sensitivity with respect to the significant wave height compared to the linear analysis.