Turbine For Wave Energy Conversion Research Articles

Impact of tapered leading-edge micro-cylinder on the performance of wells turbine for wave energy conversion: CFD-optimization algorithms coupling study

Oscillating water column-based Wells turbine is one of the potential possibilities for wave energy conversion. The early turbine stall caused an early performance loss, limiting the power produced and the operating range. The present work intends to boost the output power and operating range of the Wells turbine by using a tapered leading-edge micro-cylinder. This design is unique in that the micro-cylinder's diameter and its distance from the leading-edge change along the blade span. A response surface optimization technique based on computational fluid dynamics (CFD) is used to determine the optimal dimensions of the new design. Utilizing the optimal new design in the upgraded Wells turbine significantly increases power output and operating range, specifically by 78.13% and 33.33%, respectively. According to comprehensive flow analysis, the novel micro-cylinder design generates a pair of vortices that successfully reduce axial flow velocities close to the suction side across a sizable portion of the blade span. This, in turn, results in the postponement of flow separation towards the trailing edge.

Acoustic noise emission of air turbines for wave energy conversion: Assessment and analysis

The oscillating-water-column (OWC) system is one of the most developed wave energy conversion technologies, and several prototypes have been deployed and tested on the sea. Noise emission is essential to this technology’s long-term environmental impact assessment. The power take-off system of an OWC comprises a self-rectifying air turbine that directly drives an electrical generator. The paper compares the noise emission of a biradial turbine with that of a Wells turbine. The noise emission of the biradial turbine was measured at the Instituto Superior Técnico variable-flow test rig, while the Wells turbine data were collected in the literature. Acoustic measurement procedures followed the guidelines of the ISO 3746 Standard. A definition of specific noise level is used to compare the noise emission from geometrically similar air turbines or other air turbines performing the same duty. The results show that the noise levels of the biradial turbine are significantly lower than those of the Wells turbine at off-design conditions for the same pneumatic power.

Influence of vortex interactions on the exergy transfer and performance of OWC wells turbine

To provide deep dives about aerodynamic loss mechanisms in Wells turbines for wave energy conversion, a loss audit analysis was performed by numerical experiments in a monoplane Wells turbine with guide vanes. The interactions between the tip-leakage and leading-edge vortices during the stall process were captured by an improved vortex identification method, which revealed the relationship between vortex interactions and stall mechanisms by identifying coherent structures and tracking the vortex core trajectory. Finally, the influence of vortex interactions on exergy transfer was quantified. The results indicate that the lost kinetic energy and mixing losses dominate the loss generation in the Wells turbine stage under stall conditions. Under the beneficial effect of tip leakage flow, leading-edge separation first begins at the equilibrium region between the tip-leakage and leading-edge vortices. As the leading-edge vortices expand toward the blade tip, the intensified leading-edge vortex interacts with the casing suction-side corner vortex and accelerates the dissipation of the tip-leakage vortices. Consequently, the contributions of viscous irreversibilities outweigh those of shaft work, being the dominant factor in the decrease in flow exergy, leading to a decrease in exergy utilization by 38.46% from the pre-stall condition to the stall condition.

Effect of guide vane on the performance of a sail wing turbine for wave energy conversion

Abstract In an Oscillating Water Column (OWC) based wave energy plant, the sea wave motion is used to drive an air column in an air chamber and that results in a bi-directional airflow. An air turbine for bi-directional airflow is used to convert the pneumatic energy into mechanical energy. The sail wing turbine can be used as an air turbine for wave energy conversion because it has a special structure in which the shape of the sail changes in reciprocating airflow and always can rotate in the same direction. In addition, the sail wing turbine does not experience the stalling effect and can acquire energy at a wide range of rotational speed and flow rate. In our previous studies, the performance of the sail wing turbine for bi-directional airflow has been investigated by using a wind tunnel test. In the present study, in order to further improve the performance of the sail wing turbine, guide vanes were installed, and the effect of the guide vane on the performance was investigated by a wind tunnel test and a computational fluid dynamics (CFD) analysis under a steady flow condition.

Sail Wing Turbine for Wave Energy Conversion

The studies on the oscillating water column (OWC) type wave energy converter has to be special geometry so that it can always rotate in the same direction in a bi-directional airflow, which makes it difficult to increase the efficiency and select a turbine geometry compared to the turbines for uni-directional flows. In order to overcome these difficulties, the use of a new air turbine for wave energy conversion, a sail wing turbine for bi-directional flow has been proposed by the authors. This turbine is operated in a bi-directional airflow by using some rotating sails made of a thin, flexible material such as cloth that changes shape in response to the airflow direction. The advantages of this turbine are that the rotor is lightweight, the turbine structure is simple, and it does not suffer from the stalling as seen in Wells turbine. However, it has been pointed out that the efficiency of this type of turbine is lower than that of the Wells turbine. In this study, in order to improve the performance of the sail wing turbine, the effect of the guide vane solidity on the performance was conducted through a wind tunnel test under steady flow condition.

Performance investigation of Wells turbine for wave energy conversion with stall cylinders

The wave energy conversion of oscillation water column (OWC) systems is limited by the narrow range of efficient operation of Wells turbines. Effective control methods are critical to improving performance. In this study, spanwise stall cylinders were mounted near the blade leading edge to delay the stall using dynamic mode decomposition (DMD) analysis. The performance of the Computational Fluid Dynamics simulation with steady inflow conditions is in good agreement with the experimental data. The results indicate that the aerodynamics and turbine performance is significantly influenced by cylinder wakes, which are associated with the location and radius of the stall cylinders. The relatively optimal location of the stall cylinders is determined at a 10% chord of the blade with a radius of 1% of the chord, producing 19.15% and 26.57% increases in the operation range and maximum torque coefficient, respectively. The cylinders act by intensifying the turbulence in the boundary layer and weakening the tip leakage vortex, causing the cylinder wake vortices to interact with vortices near the boundary layer to guide the flow near the suction side. A larger cylinder radius produces stronger wake vortices, but the efficiency decreases due to increased dissipation losses.

Comparative Study on Starting Characteristics of Turbines for Wave Energy Conversion

Wells and impulse turbines for bi-directional airflow have been proposed as typical turbines for wave energy conversion. Each of them has some advantages and disadvantages. In recent years, the authors have proposed Wells turbine with booster and a counter-rotating impulse turbine for bi-directional airflow in order to overcome their drawbacks. In this study, the starting characteristics of four types of turbines for bi-directional airflow were compared by conducting a CFD analysis under a steady flow condition.

A Straight-Bladed Vertical-Axis Turbine for Wave Energy Conversion: Effect of Guide-Vane Geometry on Performance

This study proposes a straight-bladed vertical-axis turbine for ocean-wave energy conversion. The aim is to develop a novel air turbine suitable for an oscillating water column in a wave energy plant. To this end, the performances of straight-bladed vertical-axis turbines were investigated for different guide-vane geometries in a wind tunnel experiment. The rotor has four straight blades with an NACA0018 profile, a chord length of 80.5 mm, a pitch diameter of 460 mm, and a blade width of 490 mm. In this study, the guide-vane geometry was varied as arc, semi-arc, and semi-arc with a straight part. Under steady flow conditions, the turbine performance was evaluated by determining the torque, input and flow coefficients, and the turbine efficiency. The semi-arc blade geometry decreased the input coefficient and increased the torque coefficient. Moreover, the semi-arc blade with a straight part improved the starting characteristics of the turbine and its efficiency over a wide range of flow coefficients.

CFD Analysis of the Performance of a Double Decker Turbine for Wave Energy Conversion

The Double Decker Turbine (DDT) is a recent design introduced for oscillating water column (OWC) devices. Its major contribution is the combination of two typical solutions in just one prototype: a self-rectifying performance, to deal with the bidirectional flow, and the twin-turbine concept, allowing the use of unidirectional turbines. This is achieved by a set of two concentric turbines, called external and internal turbines (ExT—InT). In this work, Computational Fluid Dynamics (CFD) numerical model is developed to study in detail the performance of a DDT, where geometrical components for both turbines have been taken from previous works of the authors. The ANSYS-Fluent code was first executed by means of a URANS simulation with a realizable k-ε turbulence model to obtain the performance curve of the turbine under steady conditions. Results obtained reveal its potential with respect to other solutions in the current state-of-the-art of OWC solutions for Wave Energy Conversion. Following a non-steady analysis, we assumed a sinusoidal input from the chamber which also resulted in promising results. Finally, the flow analysis inside the DDT allowed the authors to envisage geometric improvements that could enhance the DDT efficiency on future works.

Numerical analysis of damping induced by impulse turbines for wave energy conversion

The oscillating water column (OWC) based wave energy devices require a turbine for the conversion of wave energy to electrical energy. Customarily, Wells and impulse turbines have been widely accepted for this energy conversion. However, the damping (i.e. the pressure drop developed based on volume flow rate) induced by the turbine significantly influences the performance of the OWC. The present study involves two different impulse turbines that is, reference and optimized. Initially, the reference turbine was numerically validated with two different turbulence models ( k- ε and SST). It was found that the SST predicted better than k- ε in terms of flow separation and efficiency. This numerical methodology was extended to the optimized turbine and detailed flow physics has been presented in this article. Furthermore, both the turbines were compared in terms of pressure drop, turbine pressure coefficient, and turbine damping coefficient. It was observed that the induced damping by the optimized turbine is less than the reference turbine. Finally, different diameters were arrived from the optimized turbine by the principle of dynamic similarity to match with an available optimum range of damping.

Aerodynamic analysis of a biradial turbine with movable guide-vanes: Incidence and slip effects on efficiency

The paper presents a numerical study on the aerodynamics of a biradial turbine for wave energy conversion. The turbine is symmetrical with respect to a plane perpendicular to its axis of rotation. The two inlet/outlet rotor openings are axially offset from each other facing the radial direction and two rows of radial-flow movable guide-vanes surround the rotor. The existence of a counter-rotating eddy in the relative flow between rotor blades affects both the flow incidence angle at the rotor inlet and the outlet flow angle. A model was used to predict the incidence at the rotor inlet and the slip at the outlet, and the resulting data was employed to redesign the rotor inlet/outlet blade metal angle for the original guide-vane set. Numerical results for the new geometry were compared with data for the original turbine. The rotor and diffuser losses were reduced especially at operating conditions away from the peak efficiency point, resulting in higher average turbine efficiency.

Experimental Investigation on a Fixed Oscillating Water Column with an Impulse Turbine for Wave Energy Conversion

Experimental Investigation on a Fixed Oscillating Water Column with an Impulse Turbine for Wave Energy Conversion

Study on Performance of a Straight-Bladed Vertical-Axis Turbine for Wave Energy Conversion by CFD

Study on Performance of a Straight-Bladed Vertical-Axis Turbine for Wave Energy Conversion by CFD

FSI analysis and simulation of flexible blades in a Wells turbine for wave energy conversion

In this paper a preliminary design and a 2D computational fluidstructure interaction (FSI) simulation of a flexible blade for a Wells turbine is presented, by means of stabilized finite elements and a strongly coupled approaches for the multi-physics analysis. The main objective is to observe the behaviour of the flexible blades, and to evaluate the eventual occurrence of aeroelastic effects and unstable feedbacks in the coupled dynamics. A series of configurations for the same blade geometry, each one characterized by a different material and mechanical properties distribution will be compared. Results will be given in terms of total pressure difference, supported by a flow survey. The analysis is performed using an in-house build software, featured of parallel scalability and structured to easy implement coupled multiphysical systems. The adopted models for the FSI simulation are the Residual Based Variational MultiScale method for the Navier-Stokes equations, the Total Lagrangian formulation for the nonlinear elasticity problem, and the Solid Extension Mesh Moving technique for the moving mesh algorithm.

Wave Pump System by a Combination of Centrifugal Pump with Impulse Turbine for Wave Energy Conversion

Wave Pump System by a Combination of Centrifugal Pump with Impulse Turbine for Wave Energy Conversion

Efficiency improvement of a self-rectifying radial impulse turbine for wave energy conversion

This paper aims to improve the performances of a self-rectifying radial impulse turbine used in wave energy conversion. The turbine design involves blades of circular profiles. The Design of Experiments method has been implemented for the efficiency optimization. The objective function aims to maximize the average turbine efficiency between the inhalation and the exhalation modes. A 3D viscous flow modelling has been performed with the ANSYS Fluent code. Simulations allow the turbine efficiency computation accordingly to the changes in the turbine geometry. Influence of ten geometrical parameters has been considered in the optimization process. According to a confidence degree of 95%, it has been found that only four of them have significant influence on the turbine efficiency. The rotor geometry admits the highest influence, particularly the blade angle. Additional analysis has been conducted by considering the rotor blade stagger angle. It has been found that the efficiency can be kept high in the both modes with a stagger angle of 4°. Efficiencies more than 50% have been reached. The present turbine efficiency has been compared to that of a turbine of elliptical profiles. An increase of about 19% in the efficiency has been reached with the present turbine.

Numerical Analysis of the Influence of Design Parameters on the Efficiency of an OWC Axial Impulse Turbine for Wave Energy Conversion

Oscillating water column (OWC) axial impulse turbines permit the conversion of wave energy into electrical power. Unlike other hydropower units with a mature and well established technology, such turbines have been recently developed, there are still few prototypes operating and therefore there is a large space for optimizing its design. Many recent studies focus on the improvement of the efficiency and transient characteristics by means of experimentation and also simulation techniques. In the present paper we use a 3D numerical simulation model (computational fluid dynamics model with ANSYS-Fluent 18) to analyze the influence of different geometrical parameters on the efficiency of the turbine, which have been less discussed yet. A reference configuration case has been used to validate our simulation model by comparing it with previous experimental results. Then, parametric variations in the guide vane number and type, gaps between the rotating and stationary part and hub to tip ratio have been introduced in the model to discuss the influence of these effects. It is found that some of these parameters have an important influence on the efficiency of the turbine and therefore, the results presented in this paper can help to optimize future designs of OWC impulse turbines.

A study of counter-rotating impulse turbine for wave energy conversion - Effect of middle vane on the performance -

In an oscillating water column (OWC) wave energy converter, the oscillating water column caused by the sea wave motion is used to drive an air column in the air chamber. An air turbine for this bi-directional airflow is used to convert the pneumatic energy into the mechanical energy. A counter-rotating impulse turbine for wave energy conversion has been proposed by M. E. McCormick in 1978. In a previous study, authors investigated the effect of turbine geometry on the performance of the counter-rotating impulse turbine and clarified that the efficiency of the turbine is higher than a single rotor impulse turbine in a range of high flow coefficients. However, the counter-rotating impulse turbine has a disadvantage. Its efficiency in a range of low flow coefficients is remarkably lower due to the deterioration of flows between two rotors. In this study, middle vanes were installed between two rotors in order to achieve a further improvement in the performance, and its effect was investigated by using the computation fluid dynamic (CFD) analysis. As a result, it was found that the efficiency of the turbine was greatly improved by installing the middle vanes, and a favourable turbine geometry was clarified.

A straight-bladed vertical axis turbine for wave energy conversion: Effect of guide vane position on the performance

A straight-bladed vertical axis turbine for wave energy conversion has been proposed in order to develop a novel air turbine suitable for an oscillating water column based on a wave energy plant. This study aims at investigating the effect of guide vane position on the performance of a straight-bladed vertical axis turbine. The experimental study was performed using a wind tunnel. The rotor comprises four straight blades with a profile of NACA0018, chord length of 80.5 mm, pitch radius of R = 230 mm, and a blade width of 490 mm. The guide vane has a profile of a circular arc and a chord length of l = 87 mm. The distance between these two guide vanes w and the gap between the rotor blade and guide vane G were altered to investigate the effects of the guide vane solidity l/w and position G/R. In addition, we performed computational fluid dynamics (CFD) on exactly the same configuration as the experimental system to study the effect of the guide vane position. The experimental results indicated that the guide vane solidity l/w and position G/R suitable for the proposed turbine are obtained in the case of l/w = 1.09 and G/R = 0.11. The flow visualization was determined using CFD. Moreover, we confirmed the control of the flow separation by the setting of the guide vane.

A Counter-Rotating Impulse Turbine for Wave Energy Conversion

Wave energy can be converted to the electrical energy by using a wave energy converter. The wave energy converter with oscillating water column (OWC) is one of the most promising devices because of its simple structure and easy maintenance. In this device, an oscillating water column due to the wave motion is used to drive an air column. An air turbine is used to convert the pneumatic energy of this bi-directional airflow into the mechanical energy. The counter-rotating impulse turbine for wave energy conversion has been proposed and tested so far, and the average efficiency has been shown to about 0.3. On the contrary, in another offshore experiment, it has been reported that the power generation efficiency of this turbine is larger than Wells turbine in case of small waves. However, there is a scarcity of the detailed characteristics data of counter-rotating impulse turbine. In a previous study, the authors investigated the effect of rotor blade solidity and setting angle of guide vane on the performance of this turbine, and they clarified that the efficiency of this turbine is higher than impulse turbine with single rotor in the range of high flow coefficients. The present study aimed to investigate the effect of rotor blade profile on the turbine performance by using the computation fluid dynamic (CFD) analysis. The inner and outer angles of turbine rotor blade are changed in the range of 50° to 70°. The commercial CFD software of SCRYU/Tetra of Cradle Co. Ltd. was used in the present work. The Reynolds averaged Navier-Stokes (RANS) equations were used as the governing equations and the low Reynold’s number SST k-ω model was used to predict the turbulent stresses. As a result, it was found that the inner angle of γ = 70° and the outer angle of γ = 60° of the turbine rotor blades can give the best turbine efficiency and it shows the efficiency close to the impulse turbine with single rotor, even in the range of low flow coefficients.