Cross-flow Hydrokinetic Turbine Research Articles

Hybrid cross-flow hydrokinetic turbine: Computational analysis for performance characteristics with helical Savonius blade angle of 135°

Hybrid hydrokinetic turbine (HKT) performance can be maximized with the right design and flow characteristics, but they can only be determined through a methodical process. Specifically, a computational study of modified hybrid HKT has been conducted, considering the design factors (attachment angle; radius ratio) as well as the water flow speed. The computational analysis is performed on the different configurations of hybrid HKT under different flow parameters. The computational findings showed that these factors have an effect on the overall performance characteristics of hybrid cross-flow HKT. The hybrid HKT model performs optimally at a combination of parameters with attachment angle = 60°, v = 0 .5 m/s, radius ratio = 0.4, and TSR value = 0.9. It was also determined that compared to the models with a radius ratio of 0.2, 0.6, and 0.8, this hybrid HKT model is 40 %, 1.46 %, and 20.49 % more effective. The chaotic behaviour of flow across the rotor has also been visualized.

Design modification and performance prediction of ellipsoid cross-flow hydrokinetic turbine

In this study, an ellipsoid-shaped cross-flow hydrokinetic (ECFHKT) has been selected for the study. The performance of the ECFHKT was investigated for different design parameters, namely the number of blades, blade angle and blade length. The performance of ECFHKT was evaluated in terms of coefficient of torque (Ct) and coefficient of power (Cp) with respect to different tip speed ratios (TSR) by considering different flow velocities in the range of 0.5 m/s to 3.0 m/s, experimentally and numerically. Further, the Flow behavior across the ECFHKT was analyzed and discussed through pressure contour, velocity contour and velocity vector. It was found that design parameters play a vital role in enhancing the performance of the rotor and self-starting capabilities. For this analysis, the maximum Ct and Cp were obtained as 1.08 and 0.267, respectively. Further, It was observed that the ECFHKT performed well both in low and high flow conditions within a narrow range of tip speed ratio.

A Brief Study on the Implementation of Helical Cross-Flow Hydrokinetic Turbines for Small Scale Power Generation in the Indian SHP Sector

This article addresses the simulation and experiments performed on a Gorlov Helical Turbine (GHT) by altering the index of revolution of its helical blades. Gorlov Helical Turbine is a hydrokinetic turbine that generates energy from the perennial/tidal source. The paper serves a two-fold purpose: parametric optimisation of Gorlov Helical Turbine with respect to the index of revolution and viability of installing the turbines in river creeks. Nine models of turbines with a diameter of 0.600 m and a height of 0.600 m were generated with different indices of revolution and then subjected to simulation studies. A significant rise in the output torque of the turbine was not observed with the various indices of revolution, even as the probability of finding a section at every azimuthal position is likely to rise. Gavasheli's solidity ratio formula was used to formulate an expression for the output power. The output power as per analytical formulation is 1.11 W, which is of the order of output power obtained through simulation (0.951 W). The studies suggest that 0.25 remains the optimum value for the index of revolution of the helical blades. A model with 0.25 as the index of revolution was fabricated and tested at a river creek. The results were found to agree with the simulations accounting for the losses. The study results could encourage setting up hydrokinetic turbines in river creeks, thereby increasing the grid capacity of SHPs in India.

A DMST-based tool to establish the best aspect ratio, solidity and rotational speed for tidal turbines in real sea conditions

A Double Multiple Stream Tube (DMST) routine to predict the performance of cross-flow hydrokinetic turbines in real environments is presented, along with a site assessment application to identify the most efficient turbine aspect ratio, solidity and configuration (single, or paired) for a selected area of the Northern Adriatic Sea. The peculiarity of this DMST tool is its 3D character, since it allows to reproduce the vertical distribution of the torque generated by the turbine. To this end, correlations for fluid dynamic phenomena, based on high-fidelity fully CFD simulations, were implemented. The marine circulation code SHYFEM is adopted to obtain velocity profiles for a half lunar cycle period. The sites with the highest mean kinetic power were identified. The DMST routine is equipped with an iterative process able to establish which rotational speed maximizes the power output. Indeed, a spatially non-uniform velocity profile requires to determine the flow velocity more suitable to obtain the rotational speed via Tip Speed Ratio (TSR) definition. To this end, the section of the blades working at optimal TSR varies from top to bottom, until the maximum power is reached. It works as a virtual Maximum Power Point Tracking system able to adapt the turbine operating conditions for the different turbine geometries, and for changes in flow conditions. The results show that for the case study, the performance curve shape influences the optimal TSR blade section: the latter is often located in the upper part of the turbine for the low solidity, whereas, for high solidity turbines, in the bottom half part.

A numerical investigation on the influence of savonius blade helicity on the performance characteristics of hybrid cross-flow hydrokinetic turbine

One of the major drawbacks of having a hybrid cross-flow hydrokinetic turbine (HKT) is the alternative nature (positive and negative) of instantaneous torque generation by blades over one cycle of revolution, which will reduce the power coefficient and raise the pulsation level of instantaneous torque. Under the present study, the physical configuration of the proposed hybrid cross-flow HKT is altered by replacing the conventional Savonius HKT blade with a helical Savonius HKT blade. The numerical simulations have been carried out for five different hybrid HKT configurations with different Savonius blade helicity values as 0°, 45°, 90°, 135°, and 180° for the considered range of in-stream velocity from 0.5 m/s to 2.0 m/s. Based on the numerical study, the maximum power coefficient of hybrid cross-flow HKT having Savonius blade helicity of 45° was obtained as 0.208 corresponding to a TSR value of 0.9 and in-stream water velocity of 0.5 m/s. It is improved by 4.5%, in comparison to the maximum power coefficient obtained from the hybrid cross-flow HKT having conventional Savonius blade under the same flow conditions. The torque pulsation level for different configurations of hybrid HKT has also been analyzed in this study. The maximum reduction in TPF was observed as 31.54% for a hybrid cross-flow HKT model with a Savonius blade helicity of 180°. Further, it is observed that the negative magnitude of instantaneous torque generated by hybrid cross-flow HKT having Savonius blade helicity of 135° and 180°, completely nullified and positive instantaneous torque is achieved for all the values of rotor angle over one cycle of revolution. The transient nature of flow across turbine blades has also been visualized through velocity and pressure contours.

The effect of the blade number on a cross-flow hydrokinetic turbine

The effect of the blade number of a cross-flow hydrokinetic turbine is studied in this paper. To begin with, a blade cross-sectional shape NACA0025 is selected, as was used in a previous study. Secondly, after the turbine blade is modeled, the simulation cases are conducted numerically by use of a commercial package program, ANSYS Fluent. Before further analysis is conducted, validation is executed to make sure the calculation is reliable. The result shows that the blade number has effect on the turbine's efficiency or power coefficient (Cp). A 2-bladed, 3-bladed and 4-bladed turbine can obtain Cp values of 37.5%, 33.6% and 30.2%, respectively. Additionally, from the component torque yields gained by upper and lower surface of a blade respectively, it is revealed that the lower surface obtains much more torques than upper surface does during rotational range from 20° to 180°. Also, during this rotational range, the 2-bladed turbine proved to be more efficient than the other two cases. In contrast, the upper surface gained more torque yields than lower surface during rotational interval from 180° to 380°(=20°). And during this rotational interval, the torque yields obtained by the upper surface offset the lower surface torque yields, which made the total torque yields seem insignificant. The reduction of the blade number is thought to be the cause of efficiency enhancement due to less complex flow condition caused by the interaction of the blades upon rotation.

Numerical Study of the Dynamic Stall Effect on a Pair of Cross-Flow Hydrokinetic Turbines and Associated Torque Enhancement Due to Flow Blockage

An open-source 2D Reynolds-averaged Navier–Stokes (RANS) simulation model was presented and applied for a laboratory-scaled cross-flow hydrokinetic turbine and a twin turbine system in counter-rotating configurations. The computational fluid dynamics (CFD) model was compared with previously published experimental results and then used to study the turbine power output and relevant flow fields at four blockage ratios. The dynamic stall effect and related leading edge vortex (LEV) structures were observed, discussed, and correlated with the power output. The results provided insights into the blockage effect from a different perspective: The physics behind the production and maintenance of lift on the turbine blade at different blockage ratios. The model was then applied to counter-rotating configurations of the turbines and similar analyses of the torque production and maintenance were conducted. Depending on the direction of movement of the other turbine, the blade of interest could either produce higher torque or create more energy loss. For both of the scenarios where a blade interacted with the channel wall or another blade, the key behind torque enhancement was forcing the flow through its suction side and manipulating the LEV.

Flow Field Measurement of Laboratory-Scaled Cross-Flow Hydrokinetic Turbines: Part II—The Near-Wake of Twin Turbines in Counter-Rotating Configurations

Cross-flow hydrokinetic turbines have sparked interest among fluid dynamicists for their potential for power enhancement in paired configuration. Following the first part of a laboratory-scaled turbine wake measurement project, this second part presents a monoscopic particle image velocimetry measurement of the near-wake of two cross-flow hydrokinetic turbines in six different counter-rotating configurations. The turbines operated in a small water flume at an average diameter-based Reynolds number of 2×104 with the incoming streamwise velocity of 0.316 m/s. The six configurations included two turbine separation distances, two turbine phase angles differences, and two different relative incoming flow angles. Similar to the observation of the single turbine configurations in part I, a correlation between flow field structures and the corresponding power output was observed. Effects of each parameter of the counter-rotating configurations are further discussed, which suggest guidelines for setting up multiple devices in a power farm. This article is accompanied by all full numeric data sets and videos of the results.

Case study of a cross-flow hydrokinetic turbine in a narrow prismatic canal

The performance of a high-solidity H-Darrieus cross-flow hydrokinetic turbine was tested in the subcritical flow of a narrow artificial canal. Turbine shape and swept area were selected to generate high blockage into the flow for mechanical power maximization, although a high axial thrust on the blades was attained. A detailed description of the support structure, probes, and the turbine is provided as a first step of further in-depth studies. After measurements of the undisturbed flow velocity with an acoustic doppler velocimeter, the turbine was submerged to operate at different rotor speeds, while measurements of torque, thrust, and rotor speed were acquired. The water depth at another crossbar was measured continuously to check the backwater profile spreading towards upstream, showing no flooding risk at each rotor speed. Experimental data were analyzed for the torque ripple investigation and high sensitivity to low tip speed ratios was demonstrated.

Flow Field Measurement of Laboratory-Scaled Cross-Flow Hydrokinetic Turbines: Part I—The Near-Wake of a Single Turbine

Recent developments in marine hydrokinetic (MHK) technology have put the cross-flow (often vertical-axis) turbines at the forefront. MHK devices offer alternative solutions for clean marine energy generation as a replacement for traditional hydraulic turbines such as the Francis, Kaplan, and Pelton. Following previous power measurements of laboratory-scaled cross-flow hydrokinetic turbines in different configurations, this article presents studies of the water flow field immediately behind the turbines. Two independent turbines, which operated at an average diameter-based Reynolds number of approximately 0.2×105, were driven by a stepper motor at various speeds in a closed circuit water tunnel with a constant freestream velocity of 0.316 m/s. The wakes produced by the three NACA0012 blades of each turbine were recorded with a monoscopic particle image velocimetry technique and analyzed. The flow structures with velocity, vorticity, and kinetic energy fields were correlated with the turbine power production and are discussed herein. Each flow field was decomposed into the time averaged, periodic, and random components for all the cases. The results indicate the key to refining the existed turbine design for enhancement of its power production and serve as a baseline for future comparison with twin turbines in counter-rotating configurations.

Passive flow control mechanisms with bioinspired flexible blades in cross-flow tidal turbines

State-of-the-art technologies for wind and tidal energy exploitation focus mostly on axial turbines. However, cross-flow hydrokinetic tidal turbines possess interesting features, such as higher area-based power density in array installations and shallow water, as well as a generally simpler design. Up to now, the highly unsteady flow conditions and cyclic blade stall have hindered deployment at large scales because of the resulting low single-turbine efficiency and fatigue failure challenges. Concepts exist which overcome these drawbacks by actively controlling the flow, at the cost of increased mechatronical complexity. Here, we propose a bioinspired approach with hyperflexible turbine blades. The rotor naturally adapts to the flow through deformation, reducing flow separation and stall in a passive manner. This results in higher efficiency and increased turbine lifetime through decreased structural loads, without compromising on the simplicity of the design.Graphic abstract

A Double Multiple Stream Tube (DMST) routine for site assessment to select efficient turbine aspect ratios and solidities in real marine environments

A MATLAB routine, based on a Double Multiple Stream Tube model, developed to quickly predict the performance of cross-flow hydrokinetic turbine, here is presented. The routine evaluate flow data obtained with the open-source marine circulation code SHYFEM. The tool can establish the best locations to place tidal devices taking into account bathymetric constraints and the hydrokinetic potential. Hence, it can be used to decide the best set of geometrical parameters. The geometrical variables of our analysis are turbine frontal area, aspect ratio and solidity. Several sub-models, validated with 3D and 2D CFD simulations, reproduce phenomena such as dynamic stall, fluid dynamic tips losses and the lateral deviation of streamlines approaching the turbine. As a case study, the tool is applied to an area of the northern Adriatic Sea. After having identified some suitable sites to exploit the energy resource, we have compared behaviours of different turbines. The set of geometrical parameters that gives the best performance in terms of power coefficient can vary considering several locations. Conversely, the power production is always greater for turbine with low aspect ratio (for a fixed solidity and area). Indeed, shorter devices benefit from higher hydrokinetic potentials at the top of the water column.

Performance characteristics of a cross-flow hydrokinetic turbine in current only and current and wave conditions

Testing of a 1:6 scale model of an H-Darrieus cross-flow hydrokinetic turbine (U.S. Department of Energy’s Reference Model 2 River Current Turbine) was conducted in the large towing tank facility at the United States Naval Academy. Tests were conducted in current only conditions at a range of diameter-based Reynolds numbers and at several turbine depths. Reynolds number independence was observed at or above ReD≈1.5×106, with an associated maximum power coefficient, CP of 0.36 at a tip speed ratio, TSR of 3.2. Turbine performance was shown to be fairly insensitive to turbine tip depth. Additionally, tests were conducted with incident waves in addition to current, which were scaled to be large enough to create a shear in velocity across the entire span of the turbine blade. The presence of waves degraded power production slightly when compared to the current only case. When viewed as a function of turbine angular position, the waves were found to amplify or diminish the cyclic signatures in the power measurement depending on the phase difference between the blade angle and wave phase angle. As cross-flow turbines are proposed to be surface mounted, this result suggests that active control strategies may be beneficial in improving power quality.

Two-way interaction between river and deployed cross-flow hydrokinetic turbines

This study focuses on the interaction between the water free surface, the riverbed, and some Darrieus-type hydrokinetic turbines deployed in river flows. As turbines offer a resistance to the flow, they affect the upcoming velocity, which in turn affects their performance. The proximity of the neighboring deformable free surface or rigid bed may also influence their power extraction. In this context, 2D and 3D URANS simulations of a cross-flow (H-Darrieus type) turbine are conducted with free-surface modeling and adapted boundary conditions allowing the capture of the interactions between the turbine and the resource. Different water depth immersions are considered in order to study local proximity effects. It is found, neglecting riverbed friction, that shallow immersion is detrimental to power extraction whereas bed proximity associated with deep immersion is favorable. This observation does not hold when considering a more realistic river with a velocity profile throughout the depth. Direction of rotation in high proximity cases also plays a role. Although the literature suggests a slight increase in power extraction with the Froude number, we find that when interaction with the resource is taken into account, the power extraction is rather independent of the Froude number for deep immersion or slightly decreasing for shallow immersion. Nonetheless, all the variations in power extraction reported in this study remain small compared to the ones associated with blockage effects. Finally, the shallow immersion case simulated in 3D behaves similarly to that simulated in 2D. Switching the orientation of the rotation axis from horizontal to vertical, despite changing the local interaction with the free surface, does not affect significantly the performance of the turbine.

Small cross-flow turbine:Design and testing in high blockage conditions

Obtaining electrical energy from water streams with velocities below one meter per second using hydrokinetic turbines presents an important challenge as it is significantly lower than the range of operations (higher than two meters per second) offered by units currently commercialized. This research goes one step further than the state-of-the-art studies regarding the degree to which energy extraction is influenced by the water flow blockage caused by turbines in water flows; presenting that blockage as a key factor in the procurement of electrical energy using hydrokinetic micro turbines from low-velocity flowing water. Primarily, it describes the design and characterization of a small vertical-axis cross-flow hydrokinetic turbine whose dimensions maximize the blockage of the water circulating in a channel at very low velocity. For the turbine characterization, a model has been tested experimentally in a water tunnel with different flow rates and complemented by simulations under the same conditions using an adjusted computational fluid dynamics model. The experimental tests contemplate the measuring of the turbine’s electrical power, hydraulic coefficient and rotational speeds. The simulations of the numerical model facilitate the procurement of the corresponding water pressure and velocity fields. Further analysis has allowed for a closer examination of the different type of forces exerted on the faces of the blades as well as their position. Upon even closer inspection there appear to be a variety of possible scenarios contingent on the turbine load.

A computational investigation to analyze the effects of different rotor parameters on hybrid hydrokinetic turbine performance

Cross flow hydrokinetic turbines have been considered a decent choice for riverine applications. Under the present work, performance of hybrid rotor cross flow hydrokinetic has been investigated for different rotor parameters. The proposed hybrid rotor is the combination of two conventional turbine rotors (Savonius and Darrieus). Investigations have been performed by considering flow velocity varying in the range from 0.5 m/s to 2.0 m/s. The performance of hybrid rotor was assessed in term of power coefficient (CP) and torque coefficient (CT). Further, comparison of hybrid rotor and single Darrieus rotor has been carried out on the basis of their performance. Flow behaviour across the turbine vicinity is analyzed and discussed with the pressure, velocity contours and velocity streamlines. It has been found that radius ratio and attachment angle play a crucial role in performance enhancement with reduction in torque fluctuations. For the considered range of rotor parameters, the maximum power coefficient (CPmax) and average torque coefficient (CTavg) were obtained as 0.109 and 0.1276 corresponding to radius ratio of 0.6 at an attachment angle of 90°. Further, power coefficient and torque coefficient of hybrid rotor are found to be improved by 37.97% and 35.88% respectively as compared to single Darrieus rotor.

CFD analysis of performance improvement of the Savonius water turbine by using an impinging jet duct design

Abstract The majority of research on water turbines focuses on design improvement of large-scale hydrokinetic turbines for power generation, which may have delayed the utilization of kinetic energy contained in rivers and canals. The aim of this paper is to improve the efficiency of a two bladed Savonius type cross-flow hydrokinetic turbine, which can be used as an energy converter to harness free-stream kinetic energy of water. An impinging jet duct design is presented for improving performance of the Savonius turbine in wind application as seen from literature. The performance of the modified turbine is evaluated using CFD software Fluent, and is compared with that of a simple two bladed Savonius water turbine and some of the prominent literature designs of the Savonius turbine. It is shown that the present design exhibits improved performance compared to the selected designs of the Savonius turbine. Further an insight of the improved performance of the modified turbine is also obtained from flow physics study.

Numerical evaluation of helical hydrokinetic turbines with different solidities under different flow conditions

In the present study, the effect of the turbine solidity on the performance of helical cross-flow hydrokinetic turbines is numerically investigated under different flow conditions. Four turbines, with different solidities of 0.15, 0.2, 0.25 and 0.3, were investigated in five dissimilar flow conditions with Froude numbers of 0.0714, 0.143, 0.214, 0.286 and 0.357. The devices employed in the analyses were three-bladed with the diameter of 1 m and length of 1.5 m. They were positioned horizontally in a rectangular-shaped channel with a perpendicular alignment to the flow direction. Symmetrical NACA0018 hydrofoils, which are twisted with an angle of 120°, were used in the blade design. The investigation results have shown that the turbines with the solidity values 0.15 and 0.2 are more efficient than the other two cases. Considering all five flow conditions, the models have displayed the worst performance at a tip speed ratio of 4.0.

Simulation and Comparison of Helical and Straight-bladed Hydrokinetic Turbines

In this article simulation analyses have been performed using computational fluid dynamics techniques to investigate the performance of helical and straight-bladed cross flow hydrokinetic turbines with a horizontal layout and similar dimensions. The length and diameter and the type of the hydrofoil of both turbines are selected to be the same. A symmetrical NACA hydrofoil has been used in blade design for both turbines. The simulation analyses were performed for a 2-D NACA0018 hydrofoil and 3-D four bladed turbines. The skewness coefficient and the orthogonal quality tests have been dependent as statistical tools to examine the quality of the mesh for all models. The obtained results have shown that the helical turbine reaches the stall condition at the tip-speed ratio (TSR) of 3.75, while the straight-bladed turbine stops converting energy at 3.1. The highest power coefficient for the helical turbine was about 0.37, while it was found to be around 0.29 for the straight-bladed turbine. These results demonstrate that a helical turbine of 1.5 m in length and 1 m in diameter is more efficient than the same-sized straight-bladed turbine under the same flow conditions.

Energy extraction from ocean currents using straight bladed cross-flow hydrokinetic turbine

Harvesting marine renewable energy remains to be a prime focus of researchers across the globe both in environmental and in commercial perspectives. India is blessed with a long coastline, and the seas around Indian peninsula offer ample potential to tap various ocean energy forms. National Institute of Ocean Technology carries out research and various ocean energy technologies, out of which harnessing kinetic energy in seawater currents is one. This article presents the open sea trials recently carried out on National Institute of Ocean Technology’s cross-flow hydrokinetic ocean current turbine in South Andaman. The turbine was designed to generate 100 W electricity at 1.2 m/s current speed and was built in-house. The turbine was initially tested in a seawater channel and then was deployed in Macpherson Strait in Andaman. It was fitted below a floating platform designed especially for this purpose, and the performance of the turbine was continuously logged inside an on-board data acquisition system. The trials were successful and in line with computations.