Vertical Axis Hydrokinetic Turbine Research Articles

Large-eddy simulation and Co-Design strategy for a drag-type vertical axis hydrokinetic turbine in open channel flows

A drag-type, vertical axis hydrokinetic turbine, partially embedded in a relatively shallow channel streambank, has been introduced to mitigate side wall erosion while producing energy [1,2]. The turbine is deployed at river mid-depth to minimize the interaction with erodible bed and biota, floating debris, ice and logs, and it operates at low tip speed ratio, which is relatively safe for fish. To quantify the turbine performance and wake characteristics and to improve its design, we conduct high fidelity large-eddy simulations (LES) in an open channel flow under different operating conditions and blade geometry. The complex turbine geometry, including the rotor and housing structure, are captured by the immersed boundary method, while the coupled level-set and volume-of-fluid method is used to compute the free surface. The resulting power coefficients at different tip speed ratios are compared against results obtained in a reduced scale prototype experiment, carried out at the St. Anthony Falls Laboratory, for validation. Quantifying the near wake flow structures generated by the turbine and their contribution to the side wall shear stresses, responsible for potential streambank erosion, has become a critical design component. Blade geometry is indeed improved through multiple iterations using the reduced shear stress at the stream bank and the increased power coefficient as a combined metric in our co-design strategy. We believe such a procedure is essential to pursue diffused renewable energy extraction with positive environmental impacts.

Sensitivity analysis of the performance of a Darrieus hydrokinetic turbine in uncertain operating conditions

Unlike the previous scientific works on the vertical axis turbines which were mostly limited to design improvements, performance metrics and flow dynamics but significantly ignored the effect of operational uncertainties on the power output, this paper presents how the performance of a low-speed vertical axis hydrokinetic turbine can be attributed to the inherent uncertainties in the operating conditions. While a complete set of uncertainty sources in the design, development and operation of a water turbine is pictured with real and hypothetical relationships, this study focuses mainly on two uncertainties. Starting from the methodological illustration of a towing tank experiment, the sources of uncertainty are defined as ±0.7% in free-stream velocity V0 and ±1.06% in the turbine’s rotational velocity Ω. The stochastic space is constructed on a two-dimensional Clenshaw-Curtis grid and finite-volume-based deterministic computations are performed at each grid node. The sensitivity of the turbine’s performance to these input uncertainties is quantified using probability methods, including response surface, PDF and CDF. Flow-related non-linearities such as vortex release and blade-vortex interaction are directly reported in stochastic space and corresponding variations in the force coefficients of the turbine’s blade are identified. It is observed that the uncertainty in the turbine’s rotation is more influential on the power output compared to free-stream velocity.

Design and characterization of a vertical-axis micro tidal turbine for low velocity scenarios

Small tidal turbines are considered economical and reliable for distributed electricity generation. Low initial cost and ease of installation make them cover all sides of the economic viability triangle in the energy market. However, operating these turbines under realistic tidal velocities remains a great challenge. The work presented herein involves the design and assessment of a micro vertical axis hydrokinetic turbine, operating at low water velocities. The blade profile, solidity and aspect ratio of the turbine model have been selected looking for a self-starting and efficient operation. Thanks to the continuous development in the additive manufacturing technology, the model can be precisely fabricated at a fraction of the cost offered by traditional machining technologies. Experiments have been performed at three flow rates with a range of inlet velocities from 0.3 to 0.7 m/s. Power curves have been obtained for each operating condition, from zero load up to the point of maximum power. Additionally, the non-dimensional tip speed ratio and power coefficient have been used to compare the performance of the different parameters. It has been found that the upstream velocity has the most obvious effect on the turbine performance, and that the peak power coefficient is linked to the intensification in the blockage ratio. Furthermore, the actuator disc theory adjusted for open channel flow has been compared and found in consonance with the experimental results. This theory has also been employed to define the turbine efficiency which, from 0.45 m/s upwards, is over 70%, and as high as 81%. Finally, the performance in an inclined channel has been analysed, finding the correlations of the maximum power points and their corresponding tip speed ratios as a function of the slope.

Performance testing of Darrieus turbine in hydrokinetic power plant model

Darrieus Turbine is a type of vertical axis hydrokinetic turbine that is often used in hydroelectric power plant. This research aims to performance test of Darrieus Water Turbine in hydro power plant experimentally. Darrieus Turbines uses blades with type NACA 0018 and Polylactic Acid material. In performance test of the Darrieus Turbine, 5 variations of fluid flow velocity are: 0.77 m/s, 0.89 m/s, 0.99 m/s, 1.17 m/s, and 1.25 m/s are used. Also the test uses variations in the number of turbine blades are: 2 pieces, 3 pieces and 4 pieces with 0º, 10º, and 15º angle of attack. The test results obtained the highest rotation at 195.17 rpm with a turbine mounted 2 blades and an angle of attack 0˚, the power generated at the rotation is 0.18 Watt. But the optimal power was obtained 0.1 Watt for 4 blades turbine, because the turbine could to do self-starting at fluids velocity 0.77 m/s for all angle of attack. The optimum result was produced by 4 blades turbine, because the turbine could do self-start at 0.77 m/s water velocity either at 0º, 10º, or 15º. The optimum power was produced by 4 blade turbine, which was 0.1 Watt.

Performances of vertical-axis hydrokinetic turbines with chordwise-flexible blades

This paper presents a parametric study of Vertical-Axis Turbines (VAT) equipped with chordwise-flexible blades. The objective of this study is to assess the possibility of increasing the efficiency of VATs by allowing passive foil deformations that take advantage of the periodically changing hydrodynamic conditions inherent to VATs. The study is carried out with numerical simulations based on a partitioned Fluid–Structure Interaction (FSI) code implemented within OpenFOAM. Two-dimensional URANS numerical simulations are carried out at a Reynolds number of 107 based on the turbine diameter, which is representative of hydrokinetic applications. A single-blade rotor equipped with a chordwise-flexible NACA 0015 profile is considered. The flexibility is located in the rear part of the foil, between the attach point and the trailing edge. The tip-speed ratio range extends from 2 to 8 and the solidity is set to 0.286. The characteristics of the blade are specified with a dimensionless flexibility parameter and a so-called pressure-to-inertia ratio. The pressure-to-inertia values investigated are larger than one and are representative of hydrokinetic applications in water. The flexibility is varied from zero (rigid blade) to large values involving large deformations. The FSI effects related to these two parameters are investigated and the mechanisms that allow efficiency improvements are presented. It is shown that flexibility introduces a stall mitigation mechanism that helps in improving the efficiency of low tip-speed ratio configurations. Flexibility also reduces blades’ drag in the downstream portion of the cycle when the turbine operates at high tip-speed ratios. More specifically, relative efficiency improvement of about 12% to 15% are observed with moderate flexibilities at tip-speed ratios of 2 (lower than optimal) and 6 (higher than optimal). However, at the optimal tip-speed ratio, flexibility strictly reduces the efficiency, although this reduction remains small for moderate flexibilities. Overall, results also show that blade flexibility tends to extend the tip-speed ratio range of high-efficiency configurations, which may provide interesting advantages when the turbine operates in changing flow conditions.

Performance evaluation of vertical axis hydrokinetic turbine (VAHT) helical blade using duct system for low current speed application

This study evaluate the performance of vertical axis hydrokinetic turbine (VAHT) - Helical Blade using duct system. Experimental study has been performed in the open channel with measured current speed of 0.7 m s−1, 0.9 m s−1, and 1.1 m s−1. Rotational speed and torque were measured to obtain the performance factor (C P ) and tip speed ratio (TSR). The gain of current speed has good response on rotational speed and torque. VAHT - Helical Blade using duct system has greater C P than non-duct system at three different current speed. The maximum C P is 0.44 reached on TSR of 2.92. The addition of duct system has good response on C P of VAHT - Helical Blade due to increasing current speed around the turbine.

Site Test Performance and Numerical Study of Vertical Axis Hydrokinetic Turbine Straight Blade Cascaded (VAHT–SBC)

The Vertical Axis Hydrokinetic Turbine – Straight Blade Cascaded (VAHT-SBC) is a type of energy generation technology developed to meet the increasing demand for renewable energy. Previous studies have been carried out to enhance the efficiency of the turbine through several aspects. To deploy a turbine on site, a study on its power generation and conversion is needed. In this research, the VAHT–SBC was integrated with a permanent magnet synchronous generator (PMSG) by a pulley and belt transmission system. This study was conducted by experimental and numerical analysis. The CFD simulation result showed that the highest torque of the turbine was found at 0° and 360°, with an average value of 23.923 Nm and with current velocity at 0.92 m/s. The experimental data showed that the voltage and frequency were proportional to the generator rotational speed and inversely correlated with the load given to the integration system. In the inverter, the voltage and frequency values were stable at 230 V and 56 Hz respectively. The VAHT-SBC prototype was able to produce a maximum power of 50 W on site, with a current velocity of 0.82 and 0.92 m/s.

CFD-based improvement of Savonius type hydrokinetic turbine using optimized barrier at the low-speed flows

A Savonius turbine is a vertical axis hydrokinetic turbine (VAHT) utilizes in low-speed channels and rivers. In comparison with the other hydrokinetic turbines, the Savonius turbine is simple in construction and installation, and involves less installation cost; however, these turbines have low torque and power coefficients in comparison with other hydrokinetic turbines. The idea of this paper is utilizing a simple barrier to deviate the fluid flow from the reversing bucket of the Savonius turbine to enhance its generated power. In order to investigate the most appropriate length of the barrier, numerical modeling has been performed by applying computational fluid dynamics (CFD). The continuity, Reynolds Averaged Navier-Stokes – RANS equations and the SST transition turbulence model are numerically solved. The validation of the numerical simulation is assessed based on the experimental data of Sandia laboratories, and the results indicated good agreement with experimental data. Thereupon, the model is utilized for optimizing the length of the barrier in various cases. The power coefficient of different cases is compared with the obstacle-less conventional Savonius turbine. The results of this analysis reveal that utilizing a barrier in its optimum length increases the maximum generated power by about 18 percent.

Experimental and numerical study of a flapping-blade vertical-axis hydrokinetic turbine under free surface deformation and blockage effects

Hydrokinetic turbine proximity to deformable free surface in shallow open surface flume has significant impacts on flow hydrodynamics around the turbine, affecting the hydrodynamic performance of the hydrokinetic turbine. Also, the free surface deformation extent depends on the flow restriction created by the turbine, which works as a barrier to the fluid flow. This phenomenon is called solid blockage or formally the blockage. The present investigation aims to assess the effects of blockage and free surface deformation on flow hydrodynamics around a flapping-blade vertical-axis hydrokinetic turbine called “Hunter turbine.” A 1:20 Hunter turbine model was fabricated and experimentally investigated in a laboratory altogether with transient CFD simulations. The simulation was carried out for both rigid lid surfaces and free surface assumptions, while k–ω SST turbulence model was used for both cases, and the volume-of-fluid method was employed for the free surface model. Simulations results were verified by empirical data, showing a good agreement. The power coefficient reached 0.23 in the best-case scenario, and the maximum power coefficient occurs at a flow coefficient between 0.39 and 0.43 for all investigated flows. CFD results demonstrated that the flow deformation blockage ratio increment leads to greater exerted torque on the turbine blades. While the mean torque coefficient for the rigid lid is bounded up to 0.18, the mean torque coefficient in the presence of the free surface reaches 0.4 at the same blockage. Also, the power coefficient is increased by 10%. Furthermore, rigid lid surface simulation shows that increasing the blockage ratio also increases power coefficient such that the power coefficient at blockage ratio 0.32 is approximately three times larger than the power coefficient at blockage ratio 0.2.

Performance analysis of two vertical-axis hydrokinetic turbines using variational multiscale method

The performance prediction of two counter-rotating vertical axis hydrokinetic turbines is presented in this paper. The flow field is governed by the 3D time-dependent incompressible Navier–Stokes equations. The system of equations is discretized using the Arbitrary Lagrangian-Eulerian Variational Multi-scale formulation for turbulence modeling on moving domains. Sliding interfaces are used to handle the rotor-stator interactions. Weak enforcement of essential boundary conditions is used to relax the requirement of boundary layers resolution. A grid convergence study based on the evaluation of the grid convergence index for the computed torque and the time-averaged axial wake velocity is performed. The grid convergence study shows good convergence with grid refinement in our formulation. Experimental validation of the averaged torque is performed for two different flow conditions and different turbine rotational velocities with good agreement. The finest mesh resolution from the grid convergence study is used for the multiple turbines simulation. The simulation shows almost 25% deficiency in the averaged torque of the downstream turbine. A multi-domain method is introduced to predict the performance of the turbine array at a lower computational cost than the full array simulation. The results from the multi-domain method show good matching on the averaged torque with the full array simulation. The initial study on the effect of struts on the torque generation is presented. The results assure the robustness of the ALE-VMS formulation and how it can be used to simulate multiple turbines at full-scale and full geometric complexity.

CFD based analysis of flow-driven vertical axis hydrokinetic turbines: blade hydrofoil and interactions effects

In last years, interest in hydrokinetic energy conversion (HEC) technology has grown. However, HEC technology requires advancements to become successful for practical in-situ conditions. In this study we investigated the Darrieus type vertical axis hydrokinetic turbines (HKT). Simulations are carried out using commercial CFD package to study the hydrodynamic behavior of a three bladed turbine. Two HKT with same solidity equal to 0.17 were considered, a 1st with symmetric-NACA0018 blade hydrofoils and a 2nd with cambered-NACA4415 blades. Our simulations show a shift in instantaneous torque coefficients for two different tip-speed ratio (TSR) values. For TSR = 1, the 2nd HKT has a delay compared to the 1st HKT, while for TSR = 2 it has a lead. NACA0018 provides highest troque coefficients. For a hydrofarm use, it is important to consider the interactions between hydrokinetic turbines. We investigated the effect of streamwise distance on performance of a Darrieus turbine. A distance of about 11 times the diameter of HKT along the streamwise direction seems essential.

Experimental Study of Hydrokinetic Power Generation System Integration – Straight Blade Cascaded (SBC) Vertical Axis Darrieus Turbine

The utilization of hydrokinetic energy as renewable energy is an effort in meeting the increasing demand for energy. The development of vertical axis hydrokinetic turbine straight blade cascaded (VAHT-SBC) at previous research has successfully extract energy with high-performance coefficients. However, the developed VAHT-SBC has not yet been integrated with generators as a power system generator. This research was conducted experimentally on an open channel, including an integrated system designed using VAHT-SBC and Permanent Magnet Synchronous Generator (PMSG) with a belt-pulley mechanism. The result shows the proportional relation of generator rotational speed to voltage and frequency. On the contrary, the generated voltage experience drop due to the loading in the integration system. The value of the voltage and frequency of inverter is not affected by rotational speed and load. The resulting inverter voltage has a value of 230 V and a frequency of 56 Hz. The installation of the integration system on hydrokinetic turbines prototype with the current velocity of 0.82 m s–1 and 0.92 m s–1 could produce 50 W of power.

Design and numerical analysis of an efficient H-Darrieus vertical-axis hydrokinetic turbine

Hydrokinetic turbines are one of the technological alternatives to generate and supply electricity for rural communities isolated from the national electrical grid with almost zero emission. The Darrieus turbine is one of the options that can be used as a hydrokinetic turbine due to its high power coefficient (Cp) and easy manufacture. In the present work, the design and hydrodynamic analysis of a Darrieus vertical-axis hydrokinetic turbine of 500 W was carried out. A free stream velocity of 1.5 m/s was used for the design of the blades. The diameter (D) and blade length (H) of the turbine were 1.5 m and 1.13 m, respectively. The blade profile used was NACA0025 with a chord length of 0.33 m and solidity () of 0.66. Two (2D) and three dimensional (3D) numerical analyses of the unsteady flow through the blades of the turbine were performed using ANSYS Fluent version 18.0, which is based on a Reynolds-Averaged Navier-Stokes (RANS) model. A transient 2D simulation was conducted for several tip speed ratios (TSR) using a k-ω Shear Stress Transport turbulence (SST) scheme. The optimal TSR was found to be around 1.75. Main hydrodynamic parameters, such as torque (T) and CP, were investigated. Additionally, 3 geometrical configurations of the turbine rotor were studied using a 3D numerical model in order to identify the best configuration with less Cp and T fluctuation. The maximum Cp average was 0.24 and the amplitude of Cp variation, near 0.24 for the turbine model with 3 blades of H equal to 1.13 m. On the other hand, for the turbine models with 6 and 9 blades of H equal to 0.565 m and 0.377 m, respectively, the maximum Cp averages were 0.51 and 0.55, respectively, and the amplitude of Cp variation, near 0.07 for the model with 6 blades and 0.17 for the model with 9 blades. This revealed that the hydrokinetic turbine with a geometrical configuration of 6 blades greatly improves the performance of the turbine due to this model has advantages compared to models with 3 and 9 blades, in terms of the reduction of their T curve fluctuation.

Wake Characteristics and Power Performance of a Drag-Driven in-Bank Vertical Axis Hydrokinetic Turbine

Preliminary design of a new installation concept of a drag-driven vertical axis hydrokinetic turbine is presented. The device consists of a three-bladed, wheel-shaped, turbine partially embedded in relatively shallow channel streambanks. It is envisioned to be installed along the outer banks of meandering rivers, where the flow velocity is increased, to maximize energy extraction. To test its applicability in natural streams, flume experiments were conducted to measure velocity around the turbine and power performance using Acoustic Doppler Velocimetry and a controlled motor drive coupled with a torque transducer. The experiment results comprise the power coefficient, the spatial evolution of the mean velocity deficit, and a description of the flow structures generated by the turbine and responsible for the unsteadiness of the wake flow. Applying a triple decomposition on the Reynolds stresses, we identify the dominant contribution to such unsteadiness to be strongly associated with the blade passing frequency.

Large Eddy Simulations on Vertical Axis Hydrokinetic Turbines - Power coefficient analysis for various solidities

Large Eddy Simulations (LES) are performed on a Vertical Axis Hydrokinetic Turbine (VAHT) at two different solidities, so as to enable a more complete physical description of the flow than for classic statistical calculations. To analyze the turbine performances, local quantities are defined to evaluate the contribution of the different turbine elements to the global VAHT power coefficient. For a deeper analysis of the major losses, the real turbine is also compared with an ideal turbine composed of only three infinite blades. It is observed that the ideal turbine with the lower solidity provides the best performance, but the losses due to the blade tips and the arms strongly increase for the real turbine at the same solidity. Consequently, for the considered real turbine, there is no clear gain to decrease the solidity. Simulations of the ideal turbine are performed for various solidities at their optimal Tip Speed Ratio (TSR) to study the evolution of optimal power coefficient as a function of solidity. A maximum power coefficient is obtained for a small value of the solidity. This is explained because the optimal TSR of this optimal solidity leads to angles of incidences on the blade which avoid a penalizing dynamic stall phenomenon but are high enough to produce an important positive torque The design of an efficient turbine has then to limit losses, to be able to use small solidity and then to avoid dynamic stall phenomenon by having a high optimal TSR.

Improvement of sinusoidal pitch for vertical-axis hydrokinetic turbines and influence of rotational inertia

In previous study (Chen et al., 2018), performances of a fixed pitch and a sinusoidal pitch H-Darrieus vertical-axis turbine (VAT) are compared by numerical simulations. With an appropriate amplitude of sinusoidal pitching, the power efficiency could be improved by 33%, and the fluctuation in power output, rotation speed and shaft torque are suppressed meanwhile. In this study, to reduce the hydrodynamic resistant torque, the original sinusoidal pitching is modified to keep the pitch angle at zero or at a small initial angle in the downstream half of rotation cycle. Numerical simulation results show this modification increases power efficiency by 20% over that of an original sinusoidal pitch turbine. Furthermore, systemically study confirms that as rotational inertia of rotor increases, the power coefficient and tip-speed ratio do not change much, and the fluctuation in output torque can be dramatically reduced. However, a very large rotational inertia turbine would perform very poor unless the rotor be accelerated over a threshold speed with exterior assistance.

Modulated circulation control around the blades of a vertical axis hydrokinetic turbine for flow control and improved performance

The circulation control around the Darrieus turbine blades is considered as a significant practical solution so as to improve the machine's performance. This paper describes a kinematics-based method, through which the pitching motion of the blades varies in such a way that the flow is controlled around them as well as across the rotor. For this purpose, the potential flow approach is used to derive the blade pitching laws being compatible with the unsteady Kutta-Joukowski condition at the trailing edge of the blade. By means of this, a constant circulation around the blade is ensured. Preliminary CFD investigations have evidenced the operating conditions at which the constant-circulation framework may be applied to turbulent flows in order to control the vortex shedding and avoid blade-vortex interactions. Nonetheless, a strict application of this concept prevents the turbine from harvesting the energy. As an alternative strategy, the application of a locally variable-circulation is proposed in order to enhance the power output without compromising on flow control. Three major accomplishments of the variable-circulation-based blade pitching proposition are the following: a. flow control around the turbine blades with definite patterns of vortical structures, b. consistent increase in the turbine's coefficient of power, and c. widened operating range of the turbine.

A CFD investigation of a variable-pitch vertical axis hydrokinetic turbine with incorporated flow acceleration

This paper presents the numerical modelling of a novel vertical axis tidal turbine that incorporates localised flow acceleration and variable-pitch blades. The focus is to develop a computational fluid dynamics model of a 1:20 scale model of the device using ANSYS® Fluent®. A nested sliding mesh technique is presented, using an outer sliding mesh to model the turbine and additional inner sliding meshes used for each of the six blades. The turbine sliding mesh is embedded in an outer static domain which includes the flow accelerating bluff body. Modelled power performance and velocity data are compared with experimental results obtained from scale model tests in a recirculating flume. The predicted power curves show general agreement with the measured data; the relative difference in maximum performance coefficient for example, is just 5.7%. The model also accurately reproduces measured flows downstream of the turbine. The verified and experimentally validated model is subsequently used to investigate the effects of the variable-pitching and number of blades on device performance.

Potential Evaluation of Vertical Axis Hydrokinetic Turbine Implementation in Equatorial River

In rural areas, small villages are often disconnected from the electrical grid because of lack of economic resources. Long extensions of the grid, only contributing small rural areas with electricity, are not economically beneficial. The temporary solution for the disconnection is to contribute with diesel-generators. These have become common for remote electricity distribution because of their quick installation and mobility. However, the high expenses of the fuel for the diesel generators, along with the challenging fuel transportation because of the scarce to non-existing infrastructure, are not a sustainable solution for electricity distribution in the rural areas. Hydrokinetic turbine is an attractive option for harnessing energy from natural flow of rivers where no static head exists. However, equatorial river system is subjected to high amount of sediment and flowing objects due to the large quantity of vegetation in the area. This leaves the turbine exposed to a higher degree of erosion on the blades and a higher risk of getting clogged. A field study was conducted to attain the velocity and characteristics of sediment in the Niah River located in Miri of Sarawak state, Malaysia. The collected data was analyzed and used as input values during simulations of a turbine model by the Computational Fluid Dynamics. Areas on the turbine exposed to the water with a high velocity and containing a high concentration of sediment were examined. The investigation states that areas on the blade’s outer sides are most exposed to erosion and will be in need of protection in form of a harder surface. Adaptation of hydrokinetic turbine for the conditions prevailing in the equatorial river is essential to improve the efficiency and lifetime of the turbine.

Performance investigation of an innovative Vertical Axis Hydrokinetic Turbine – Straight Blade Cascaded (VAHT-SBC) for low current speed

Research on the development and innovation of Vertical Axis Hydrokinetic Turbine (VAHT) to improve performance has been done. One of the important indicator that affects VAHT’s performance is Coefficient of Performance (Cp). Theoretical Cp value for the VAT (Darrieus) turbine is 0.45. This paper presents the results of a performance investigation for an innovative Vertical Axis Hydrokinetic Turbine – Straight Blade Cascaded (VAHT-SBC) by modifying the number and the arrangement of blades using CFD simulation. Symmetrical NACA 0018 is used for this study, each model is simulated with current speed variation (U - m/s) of 0.5, 1 and 1.5. An increase in Cp value is shown in variation of 9 blades (3 blades cascaded in each arm) with Cp value of 0.396 at TSR of 2.27 which is reach 88% of the theoretical value. Furthermore, the streamline velocity of the pressure contour, velocity streamline and torque fluctuations are also presented in this paper to gain in deep information.