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Energies 2019, 12, 2791 2 of 22 The Wells and impulse turbines are the two extensively studied types of self-rectifying turbines, and the test results of their aerodynamic performance are most available compared to other OWC turbines. Extensive reviews and detailed analyses of Wells and impulse turbines can be found in [4–10]. Research on self-rectifying Wells and impulse turbines was performed considering their full-scale size and their performance under real operational conditions [3]. The analysis was based on a stochastic approach, and the pressure oscillations of the OWC were assumed to be random Gaussian. Falcao et al. conducted research on the optimization of air turbines for operation in a bottom-standing OWC [11], in which a Wells and a bi-radial impulse turbine were evaluated regarding their speed in energetic sea states. The bi-radial turbine is a highly efficient alternative to the self-rectifying axial flow turbines, which has a symmetrical geometry with respect to a plane perpendicular to its axis of rotation [5,12,13]. Different versions of unidirectional turbines have been studied in the twin-turbine concepts [12,14–17]. A twin-turbine topology employs two unidirectional turbines instead of a bi-directional turbine. Each turbine is active during a single direction of flow in the system (direct mode) and is considered idle during the reverse direction (reverse mode). The numerical and experimental studies of the twin-turbine configurations have mainly been performed on axial flow unidirectional turbines. Design modifications of this twin-turbine configuration have obtained a peak efficiency of around 70% in the direct mode [15,17–19]. However, the global efficiency of the twin-turbine also requires the efficient operation of the turbine in reverse mode [20]. Pereiras et al. [16] were the first to consider negative torque produced in the reverse turbine in the efficiency calculations of the twin-axial turbines. Research on the application of unidirectional radial impulse turbines in twin-turbine OWC configurations is still in progress. Rodriguez et al. [21] improved the geometry of a unidirectional radial turbine in twin-turbine topology for higher resistance to flow in the reverse mode, however, the turbine’s direct peak efficiency was significantly low (approximately 40%). They later focused on optimising this turbine’s design for efficiency maximization in the active mode, while keeping the strong flow blockage in the reverse mode [22]. A vented OWC is an alternative concept for rectifying airflow through the air turbine. The OWC column is fitted with an array of one-way air valves so that as the water column rises the air escapes from the chamber, bypassing the turbine with very little backpressure. Consequently, the water column rises further compared to that in a conventional OWC. The incoming wave energy is temporarily stored in water column heave which is added to outflow energy transferred to the air turbine when the water column falls, allowing capture of energy over the full-wave energy cycle with a unidirectional air turbine [23]. The power output of different air turbine and OWC configurations under irregular sea waves was evaluated by Ansarifard et al. [24]. Two unidirectional radial turbines, intended to be used in a vented OWC, were compared by a conventional turbine-OWC system regarding their full-scale size, rotational speed, and power conversion under real sea conditions using the experimental data of irregular wave in vented and bidirectional OWCs. Employing optimisation tools in the numerical studies can provide a refined approach for identifying different aspects of the turbine design. Employing response prediction algorithms can aid investigation of a larger design space and allow a more reliable solution. Understanding the impact of design variables on the objective functions can ensure a fast and reliable exploration of the optimum turbine designs. Several studies have focused on optimising the turbine designs for OWC and identifying the most sensitive parameters affecting the turbine performance [3,22,25]. Impact of the rotor blade profile was studied on performance of the Wells turbines [26–28], and optimisation methods were used to explore an optimum blade design by varying the camber line and blade thickness. A 2D blade profile of an impulse turbine was optimised in [29], a 3D blade geometry was created by stacking the 2D profile spanwise and a 5% efficiency improvement was obtained. Mohamed et al. [30] employed systematic optimisation to investigate the blade shape of a reaction turbine using the same parameters as is used in an airfoil design. The genetic algorithm and computational fluid dynamics (CFD) techniques were used to optimise the rotor blade shape of an axial turbine for efficiency maximization and improvement of the power output over a wide range of flowrates. The optimised airfoil obtainedPDF Image | Unidirectional Radial-Air-Turbine OWC Wave Energy Converters
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