Vertical-axis Turbine Research Articles

Power Density Capacity of Tidal Stream Turbine Arrays with Horizontal and Vertical Axis Turbines

Power Density Capacity of Tidal Stream Turbine Arrays with Horizontal and Vertical Axis Turbines

Numerical Investigation of a Two-Bladed Vertical-Axis Turbine Operating in a Confined Channel

Numerical Investigation of a Two-Bladed Vertical-Axis Turbine Operating in a Confined Channel

다리우스와 사보니우스를 결합한 하이브리드 타입의 조류발전 터빈 설계

This study aimed to design a vertical axis turbine of tidal energy convertor by solving the unsteady Reynolds-averaged Navier-Stokes equation. Precise flow analysis was carried out to design a hybrid vertical dual turbine that combines the darrieus type and savonius type. The hexa- and tetra-hedral grids were used to construct the computational domain and the grid dependency test was performed to find the optimum grid system. Hybrid vertical turbine has been shown to further improve operational stability and power performance compared to the conventional darrieus turbine.

Investigation of the Configuration of Small Hydropower using a Novel Smoothed Particle Hydrodynamics Method

A novel Computational Fluid Dynamics (CFD) method utilizing Smoothed Particle Hydrodynamics (SPH) has been developed and applied to a simulation of flows in small hydropower systems. The simulation of the flow through a gravitational vortex turbine (GVT) small hydropower system where the flow is directed to a circular basin with a vertical-axis turbine, harnessing the rotational energy of the vortex formed to drive the turbine. Two modes of Fluid-Structure Interactions (FSI) were tested with identical flow conditions to evaluate the potential of this method to simulate complex FSI scenarios. It was found that simulation results for both one-way and two-way interactions produced reasonable results. The two-way interaction result proved to reflect more accurate FSI scenarios, but more studies are needed to provide validation.

The advantage of flow disturbance for vertical-axis turbine in low current velocity

Some findings start with inadvertent. One lack of a vertical-axis turbine is its low self-starting capability. This study aimed to improve the performance in low current velocities by adding a flow disturbance in front of the turbine. The idea has been inspired by the previous experimental study that was not specifically focused on this problem. The study was conducted using a numerical simulation method with Computational Fluid Dynamics (CFD). Firstly, the simulation results of the original turbine were validated with experimental data. Secondly, the turbine was modified by adding a flow disturbance in front of it. There were two foil types of flow disturbance modelled. The results showed that the flow disturbance with DU 06-W-200 had a slightly higher performance than NACA 63(4)021, with a coefficient of power (Cp) value of 0.52 at a current velocity of 0.54 m/s. At tip speed ratio (TSR) > 1, the turbine performance tended to decrease when using a higher angle of attack (AoA). Therefore, the recommended AoA for the flow disturbance is 0°. The flow disturbance has been proven to increase the performance of the vertical-axis turbine at a low current velocity.

Comparison of Sliding and Overset Mesh Techniques in the Simulation of a Vertical Axis Turbine for Hydrokinetic Applications

The application of Computational Fluid Dynamics (CFD) to energy-related problems has increased in the last decades in both renewable and conventional energy conversion processes. In recent years, the application of CFD in the study of hydraulic, marine, tidal, and hydrokinetic turbines has focused on the understanding of the details of the complex turbulent flow and also in improving the prediction of the performance of these devices. There are several complexities involved in the simulation of Vertical Axis Turbine (VAT) for hydrokinetic applications. One of them is the necessity of a dynamic mesh model. Typically, the model used in the simulation of these devices is the sliding mesh technique, but in recent years the fast development of the overset (also known as chimera) mesh technique has caught the attention of the academic community. In the present paper, a comparison between these two techniques is done in order to establish their advantages and disadvantages in the two-dimensional simulation of vertical axis turbines. The comparison was done not only for the prediction of performance parameters of the turbine but also for the capabilities of the models to capture complex flow phenomena in these devices and computational costs.

Development of a desmodromic variable pitch system for hydrokinetic turbines

The use of vertical-axis turbines is raising interest in the field of hydropower production from rivers or water channels, where suitable mass flows are available, without the need of high water jumps or large construction sites. Like their wind-based counterparts, these machines have many advantages in terms of installation and maintenance, but suffer from an inherently complex rotor hydrodynamics, characterized by a continuous variation of the blade angle of attack beyond the static stall limit and the corresponding onset of dynamic instabilities. In virtue of their typically lower rotational speed nonetheless, hydrokinetic machines can benefit from the possibility of active blade pitch control, which is barely feasible in case of wind turbines due to their high characteristic operation frequencies. Moving from this background, a novel control strategy, based on a double-cam desmodromic pitching system, has been developed in the present study by means of a combination of medium- and high-fidelity numerical approaches. In order to tailor the corresponding pitching law, here implemented in a parametric form, to this class of machines, a dedicated automatic optimization framework has been developed in the ANSYS® WORKBENCH® 20.2 suite. The latter combines a Multi-Objective Genetic Algorithm (MOGA) optimizer with an advanced two-dimensional Actuator Line Model (ALM) model of the turbine, developed by the authors. This optimization approach has then been applied to a test rotor with the objective of maximising the turbine power coefficient while containing the rotor stream- and cross-wise force oscillations. The resulting optimal configurations have been eventually verified via high-fidelity, fully-resolved CFD simulations. Results show that variable pitch can represent a promising solution for increasing the performance of vertical-axis hydrokinetic turbines, leading to a +35% efficiency enhancement as well as to a reduction of the machine unsteady loads.

Power Performance Assessment of Vertical-Axis Tidal Turbines Using an Experimental Test Rig

This article presents the characteristic curves of a vertical-axis hydrokinetic tidal turbine of the Darrieus subtype aimed at meeting the electricity demand of port facilities located at harbors and estuaries with low water-speed conditions. The turbine was tested in the water-current flume of the University of Santiago de Compostela for several flow conditions with different water heights and water speeds. Blockage conditions were tested by examining the results from two groups of tests: with and without an accelerator device that restricts the flow around the rotor. The tip speed ratio and the power coefficient were used to characterize the performance of the turbine for each test. Finally, the results for open-field conditions were obtained by applying empirical expressions, which allowed us to assess the performance of the device in estuaries and harbors with known water-flow regimes.

The influence of tidal turbine arrangement on the wake interaction in shallow water

The arrangement of tidal turbines in the tidal farm is known to be complicated due to the resistance to the tidal flow which causes the flow to be channeled around the individual devices. To successfully implement the tidal turbine, the wake interaction between the device and its implication needs to be fully understood. Typically, the wake interaction in the array depends on the arrangement and spacing between the device in the array. In this study, a numerical analysis is conducted through the use of Computational Fluid Dynamic (CFD) approach to investigate the influence of the array setup in shallow water application and to propose a suitable array layout for possible application in Malaysia. The numerical analysis is carried out with 2 combination sets of lateral and longitudinal spacing covering 15 turbines in staggered and squared array layout. Hypothetical ‘actuator’ disk and ‘actuator’ cylinder model are used to represent the Horizontal axis turbine (HATT) and Vertical axis turbine (VATT) respectively. The results showed that the VATT model has faster wake recovery and obeys the definition of the far wake. Staggered arrays with bigger spacing are preferable for application in shallow water due to the low probability of wake merging between the rows.

Experimental investigation of the wake characteristics behind twin vertical axis turbines

Vertical axis wind and tidal turbines are a promising technology, well suited to harness kinetic energy from highly turbulent environments such as urban areas or rivers. The power density per occupied land area of two or three vertical axis rotors deployed in close proximity can notably exceed that of their horizontal axis counterparts. Using acoustic Doppler velocimetry, the three-dimensional wake developed downstream of standalone and twin vertical axis turbines of various shaft-to-shaft distances and rotational direction combinations was characterised in terms of mean velocity and turbulence statistics, with their impact on momentum recovery quantified. Results show that the wake hydrodynamics were more impacted by turbine rotational direction than lateral distance between devices for the range of lateral spacing considered. In the cases with turbines operating in a counter-rotating forward configuration, the wake mostly expanded laterally and attained the largest velocities that exceeded those in the single turbine case, with full momentum recovery at 5 turbine diameters downstream. The wake developed by the counter-rotating backward setup notably extended over the vertical direction, whilst devices rotating in the same direction featured the greatest lateral wake expansion with reduced velocities. Linear wake superposition of the single turbine wake provided a good representation of the mean velocity field behind twin-turbine setups. The presented results indicate that, in the design of twin-turbine arrays moving in counter-rotating forward direction, a lateral spacing of, at least, two turbine diameters should be kept as this allows the kinetic energy in the wake to be fully recovered by five turbine diameters downstream.

Hydrokinetic turbine array analysis and optimization integrating blockage effects and turbine-wake interactions

In order to maximize the energy extraction of a hydrokinetic turbine array deployment, turbines must be positioned judiciously within the array. In fact, it is possible to optimize the array configuration in order to benefit from blockage effects and to avoid negative turbine-wake interactions. The Effective Performance Turbine Model (EPTM) that has recently been proposed is a suitable tool for this purpose, allowing to test and analyze a large number of different configurations at a practical computational cost. In this work, assuming a turbulent flow environment, many vertical-axis turbine array configurations (VAT arrays) are tested to study the effect of local blockage, lateral and longitudinal spacing, array staggering and direction of rotation on turbine performance. It is found that the wake deflection associated to the specific turbine tested and the lateral force it induces has a great impact on the array optimization. Following this, results show that there is no advantage to stagger VATs and that aligned configurations should be preferred. In this context, turbines should rotate inward (looking downstream) with respect to the central plane of the array. Some guidelines are also provided in terms of lateral and longitudinal spacings, with the support of a novel parameter indicating the marginal power provided by the addition of a row of turbines.

Impact of some design considerations on the wake recovery of vertical-axis turbines

Using delayed detached-eddy simulations, the objective of this paper is to show the effects of some design parameters on the wake recovery of vertical-axis turbines. To that end, the wake dynamics of a single-blade turbine is analyzed and compared for two different positions of blade attachment point and for two designs of blade support structures (struts). The results show that the chordwise position of the blade attachment point affects the temporal evolution of the blade circulation during the turbine revolution. More precisely, we demonstrate that an attachment point positioned near the trailing edge of the blade leads to a temporal vorticity distribution that is detrimental to the wake recovery. Moreover, we show that the use of tip struts, that are highly beneficial to the turbine efficiency, affects the spatial distribution of the vortex structures shed by the turbine as well as the wake recovery. The results presented in this paper suggest that the velocity recovery in the wake is highly dependent on the very near-wake vortex dynamics. Therefore, we can infer that any geometric parameter affecting the temporal and spatial distributions of the vorticity shed by the blades can significantly alter the wake recovery of vertical-axis turbines.

Study of a Bi-Vertical Axis Turbines Farm Using the Actuator Cylinder Method

Vertical axis turbines, which extract kinetic energy from currents, can produce electricity independently from a current’s direction. Hence, this type of turbines raises interest for harvesting energy from tidal currents, where flow changes direction during flood and ebb tides, and where currents present large variation of direction during tide. Methods for representing vertical axis turbines in tidal farms should be implemented in order to predict correctly power production with an acceptable time cost. The Actuator Cylinder (AC) is one of them. Numerical results in terms of wakes, with the study of velocity profiles, and efforts are compared to experiences, as well as showed that the method is sufficiently accurate and for a reasonable computing time, which is of prime importance for tidal turbines farms studies. The Actuator Cylinder method is implemented in ANSYS Fluent in a 2D stationary resolution. The method is then applied to a double levels of two counter-rotating rotors marine turbine designed by Hydroquest. Wake and power production of a single turbine and several farm configurations are studied under different current conditions (magnitude and direction).

Effect of pitch angle on power and hydrodynamics of a vertical axis turbine

The use of vertical-axis turbines in marine-current applications for electric energy generation is still in early developments, and one of the key factors for assessing the applicability of such technology is the power coefficient. To contribute towards the highly competitive market of renewable energy conversions, the turbine system requires good outcomes in terms of energy yield. In this scenario, one of the main challenges regarding the design process to improve the blade performance is to find the best trade-off between the maximization of the power output and the minimization of the structural loadings.In the current work, the influence of blade pitch angles on the hydrodynamics of a vertical-axis five-blade water turbine has been studied. The pitch angles from −5° to +5° were investigated using Computational Fluid Dynamics (CFD). The simulations were validated against experimental data for the power coefficient collected in a river. Overall, a good agreement was found in terms of computed power between simulations and experiments for a wide range of tip speed ratios. The CFD model was proven to be suitable for exploratory analyses and an optimized design was found, providing a 2.3% higher power coefficient by adopting a pitch angle of +2° compared to the zero-referenced pitch angle. Besides validating with the experiment, the CFD simulations were compared with the results of a vortex model. The effect of different pitch angles on the performance prediction and on the blade and turbine loadings was also discussed. It is becoming vital to develop an understanding of the complex interaction of vertical-axis turbines, especially in tidal-current areas where there is a lack of detailed experimental data.

Recent Development in the Design of Wind Deflectors for Vertical Axis Wind Turbine: A Review

Developments in the design of wind turbines with augmentation are advancing around the globe with the goal of generating electricity close to the user in built-up areas. This is certain to help lessen the power generation load as well as distribution and transmission network costs by reducing the distance between the user and the power source. The main objectives driving the development and advancement of vertical-axis wind turbines are increasing the power coefficient and the torque coefficient by optimizing the upstream wind striking on the rotor blades. Unlike horizontal-axis wind turbines, vertical axis turbines generate not only positive torque but also negative torque during operation. The negative torque generated by the returning blade is a key issue for vertical-axis wind turbines (VAWTs) that is counterproductive. Installation of wind deflectors for flow augmentation helps to reduce the negative torque generated by the returning blades as well as enhance the positive torque by creating a diversion in the upstream wind towards the forwarding blade during operation. This paper reviews various designs, experiments, and CFD simulations of wind deflectors reported to date. Optimization techniques for VAWTs incorporating wind deflectors are discussed in detail. The main focus of the review was on the installation position and orientation of the deflectors and their potential contribution to increasing the power coefficient. Topics for future study are suggested in the conclusion section of the paper.

Vertical-Axis Wind Turbine Steady and Unsteady Aerodynamics for Curved Deforming Blades

Vertical-axis wind turbines’ simpler design and low center of gravity make them ideal for floating wind applications. However, efficient design optimization of floating systems requires fast and accurate models. Low-fidelity vertical-axis turbine aerodynamic models, including double multiple streamtube and actuator cylinder theory, were created during the 1980s. Commercial development of vertical-axis turbines all but ceased in the 1990s until around 2010 when interest resurged for floating applications. Despite the age of these models, the original assumptions (2-D, rigid, steady, straight bladed) have not been revisited in full. When the current low-fidelity formulations are applied to modern turbines in the unsteady domain, aerodynamic load errors nearing 50% are found, consistent with prior literature. However, a set of steady and unsteady modifications that remove the majority of error is identified, limiting it near 5%. This paper shows how to reformulate the steady models to allow for unsteady inputs including turbulence, deforming blades, and variable rotational speed. A new unsteady approximation that increases numerical speed by is also presented. Combined, these modifications enable full-turbine unsteady simulations with accuracy comparable to higher-fidelity vortex methods, but over faster.

Experimental tests and CFD simulations of a horizontal wave flow turbine under the joint waves and currents

This paper focus on the comparison of traditional vertical-axis turbines and a newly proposed horizontal wave flow turbine. Firstly, the major design parameters of this new concept are presented. Then a series of experimental tests of a prototype in the towing tank have been performed to investigate its hydrodynamic performance. It is found that the highest energy conversion rate is about 24.83% and the average impeller rotation power also reaches its maximum value of 352 W among all the six design cases. In addition, the Computational Fluid Dynamics (CFD) simulations have been performed to investigate its hydrodynamic characteristics and energy harvesting efficiency under the combined action of waves and currents. The results also show that the amplitude of the turbine's thrust and torque moment fluctuations increase with increasing motion amplitude. Furthermore, the wave dynamics cause instantaneous fluctuations in the flow field distribution for both types and the horizontal wave flow turbine has a more stable flow field. The comparison of both types of turbine based on the CFD simulations show that the torque moment, thrust force, and lateral force coefficients of the horizontal wave flow turbine are approximately 1.2–1.5 greater than those of the vertical axis turbine. Therefore, the horizontal wave flow turbine shows better energy harvesting efficiency if compared with the traditional vertical one.

Lagrangian coherent structures and material transport in unsteady flow of vertical-axis turbine wakes

In this study, the unsteady phenomena of vertical-axis turbine wake flow were analyzed in two rotational speed conditions from a Lagrangian perspective. The wake flow field was obtained using particle image velocimetry. The spectral proper orthogonal decomposition method was used to decompose the wake modes with different frequencies. The Lagrangian coherent structure theory based on the finite-time Lyapunov exponent was used to study the material transport and mixing in the wake evolution process. The results show that the Lagrangian coherent structures can capture the boundary of the wake vortex. At a low rotation speed, a vorticity–flux window is found, through which the fluid is transported to the rotation-induced structures. At a high rotation speed, the increasing material transport between the separation bubbles and the incoming flow eventually leads to shedding of the large-scale vortex structures, accompanied by a decrease in the length of the separation zone. The material transport and mixing revealed in the unsteady flow of turbine wakes have significance in guiding the study of the flow control mechanism.

Experimental investigation into the improvement of self-starting capability of vertical-axis tidal current turbine

Indonesia has a lot of tidal current energy resources that can be harnessed using a vertical-axis turbine. However, one of the disadvantages of the vertical-axis turbine is its low self-starting capability. Under the actual condition, tidal current velocity is fluctuated and sometimes is very low, making the turbine unable to rotate. The purpose of this study is to investigate and improve the self-starting capability of a vertical-axis turbine. The study was conducted experimentally using a towing tank in Institut Teknologi Sepuluh Nopember (ITS) at Surabaya, Indonesia. The speed of towing carriage was set with the starting condition of 0 up to 0.4 m/s with an increment of 0.1 m/s. The stages of the test are as follows: first, the original vertical-axis turbine, a turbine with three straight-blades, was investigated; second, the modified vertical-axis turbine (inclined-blade) was proposed to increase the self-starting capability. The inclined-blade turbine has smooth self-start characteristics compared with the straight-blade turbine. It can start rotating at 0.2 m/s with a turbine rotation speed of 2.25 rpm. The original vertical-axis turbine with straight blades can start rotating at 0.3 m/s with a turbine rotation speed of 1.93 rpm. Therefore, a vertical-axis turbine modified with inclined-blades has better self-start capability that is suitable to implement in an area with low tidal current velocity.

Adaptation of a container tank to a savonius vertical axis turbine analyzed by reverse engineering

Due to the alarming climatic situation experienced by all nations, we are in the need to urgently develop and improve sustainable technologies for power generation in order to supply the growing demand for power generation in power systems worldwide. This project aims to design and build a prototype vertical axis wind turbine Savonius analyzed by reverse engineering process by adapting the structure of a tank and taking advantage of its shape and dimensions, with the intention of building a wind turbine with the lowest possible budget, In addition, data from the psychrometric chart of the state of Puebla (winter-spring) will be used so that by means of numerical simulations of fluid flow using CFD software we can obtain the parameters of linear velocity, angular velocity, drag coefficient and projected power that the wind rotor will be able to develop.