Wells turbines are among the most interesting power take-off devices used in Oscillating Water Column (OWC) systems for the conversion of ocean wave energy into electrical energy. Several configurations have been studied during the last decades, both experimentally and numerically. Different methodologies have been proposed to estimate the efficiency of this turbine, as well as different approaches to evaluate the intermediate quantities required. Recent works have evaluated the so-called second-law efficiency of a Wells turbine, and compared it to the more often used first-law efficiency. In this study, theoretical analyses and numerical simulations have been used to demonstrate how these two efficiency measures should lead to equivalent values, given the low pressure ratio of the machine. In numerical simulations, small discrepancies can exist, but they are due to the difficulty of ensuring entropy conservation on complex 3D meshes. The efficiencies of different rotor geometries are analyzed based on the proposed measures, and the main sources of loss are identified.

The hysteretic behavior of oscillating water column (OWC)-installed Wells turbines has been known for decades. The common explanation invokes the presence of unsteady aerodynamics due to the continuously varying incidence of the flow on the turbine blades. This phenomenon is neither new nor unique to Wells turbines, as an aerodynamic hysteresis is present in rapidly oscillating airfoils and wings, as well as in different types of turbomachinery, such as wind turbines and helicopter rotors, which share significant similarities with a Wells turbine. An important difference is the non-dimensional frequency: the hysteresis appears in oscillating airfoils only at frequencies orders of magnitude larger than the ones Wells turbines operate at. This work contains a re-examination of the phenomenon, using both computational fluid dynamics (CFD) and a lumped parameter model, and shows how the aerodynamic hysteresis in Wells turbines is negligible and how the often measured differences in performance between acceleration and deceleration are caused by the capacitive behavior of the OWC system. Results have been verified with respect to both spatial and temporal discretization, for unstalled and stalled operating conditions.

Ocean wave energy represents one of the most attractive renewable sources due to its high availability and predictability. Solutions based on the Oscillating Water Column (OWC) principle are one of the most promising for sea-wave energy conversion. The system is composed of two main units, an open chamber that converts the sea wave motion into an alternating airflow, and a turbine driven by this flow. The typical alternating airflow inside the OWC chamber requires a turbine with self-rectifying behavior. The Wells turbine is the simplest and most reliable turbine for this purpose in virtue of its rotor with symmetric blades staggered at 90 degrees relative to the axis of rotation. The non-stationary operating conditions of the Wells turbine strongly affect its performance when working away from its optimal efficiency point. By controlling the turbine rotational speed, the operating conditions can be kept closer to the maximum efficiency point. Recent works, based on dynamic simulations, have proposed control strategies for the turbine rotational speed, to avoid stall occurring under variable wave conditions. The present work investigates a rotational speed control in order to keep the operating conditions closer to the turbine’s maximum efficiency point. The analyses have been conducted in an experimental facility capable to simulate an OWC system with regular (sinusoidal) wave motion. Wells turbine performance has been evaluated for different control laws and it is compared to not-controlled turbine performance in order to evaluate the effectiveness of the control action.

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