Tip Speed Ratio Research Articles

Design and analysis of a solar-wind hybrid renewable energy tree

A hybrid tree is an artificial structure resembling a natural tree with branches on top of which are mounted solar modules or wind turbines. It can help supply power to mobile phones, laptops, electric vehicles, home appliances and lighting loads covering small or large areas, which can be the best energy source for sustainable cities and modern societies. This paper presents a 3 kW hybrid tree design consisting of 2 kW solar and 1 kW wind to be installed at Vaddeswaram, Andhra Pradesh (16.26°N and 80.36°E) which can generate maximum energy using a two-axis tracking system. Different designs and applications of energy trees available worldwide are also presented. P–V and I–V characteristics of solar panels were obtained at different irradiance and temperature values. For wind turbine, power characteristics at different wind speed and co-efficient of power at different tip speed ratios were studied. Power generation study for the hybrid tree was carried out at different tilt angles from 10° to 20° for solar panels. Structural optimisations are performed to validate whether the structure can withstand the loads applied. The proposed solar-wind hybrid tree can generate 4709 kWh/year with the two-axis tracking system instead of generating 3763 kWh/year when solar panels are fixed at an 18.25° tilt angle.

Using vortex generators for flow separation control on tidal turbine profiles and blades

Tidal energy can play an important role in the Net Zero transition. Increasing tidal turbine performance through innovation is crucial if the cost of tidal energy is to become competitive compared to other sources of energy. The present investigation is a proof-of-concept study for the application of Vortex Generators (VGs) on tidal turbines in view of increasing their performance. The more mature wind energy industry uses passive VGs either as a retrofit or in the blade design process to reduce separation at the inboard part of wind turbine blades. Tidal turbine blades also experience flow separation and here we examine whether passive vane VGs can be used to reduce or suppress that separated flow. First, a wind tunnel investigation is performed to assess the performance of VGs on a 20% thick profile from the blade. Then, the VG effect on the 2D-profile is modelled in a Reynolds Averaged Navier-Stokes in-house solver. Results show that low profile VGs, i.e. VGs shorter than the local boundary layer, can increase the performance of the blade profile and successfully reduce flow separation. The VG effect on blade performance is examined in model scale and in full-size. VGs successfully suppress separation in both cases and it is shown that full-size information should be used for the placement of VGs. A maximum power coefficient increase of 1.05% is observed at a tip speed ratio of λ=3. The present proof-of-concept study demonstrates for the first time the potential of passive VGs to be included either in the design process of a tidal turbine blade or as a retrofit solution.

Hydrodynamic independence and passive control application of twist and flapwise deformations of tidal turbine blades

The load-induced deformations experienced by axial-flow rotor blades can result in significant hydrodynamic impacts on rotor operation. These changes in hydrodynamics are dominated by the flapwise and twist deformation components, affecting blade loading and performance. This work uses blade-resolved computational fluid dynamics simulations to explore the hydrodynamic interactions of coupled flapwise and twist deformations, and their potential for use in passive control strategies. The rotor blades were simulated under parametrically prescribed flapwise-only, twist-only and coupled flapwise–twist deformations. The results show that the hydrodynamic effects are adequately described by blade-element theory for twist deformations regardless of the presence of flapwise deformations, whereas flapwise deformations induce changes in the local lift and drag coefficients that are independent of twist. For moderate blade deflections, the hydrodynamic changes generated by the two deformation components can be approximated to be independent from each other. The observed hydrodynamic independence between the two deformation components is used to explore passive deformation strategies for a tidal rotor. By extrapolating an existing dataset containing CFD simulations of twist-only and flapwise-only deformation cases at different tip-speed ratios, control paths are designed within a tip-speed ratio, flapwise and twist deformation parameter space. These control paths demonstrate passive control strategies as a potential alternative to active pitch control on tidal turbines, showing similar performance and maximum loading, compared with an active pitch strategy, over a full tidal cycle. In particular, it is shown that flapwise deformations have an important role in power capping above rated flow speed.

Enhancement of a cycloidal self-pitch vertical axis wind turbine performance through DBD plasma actuators at low tip speed ratio

Enhancement of a cycloidal self-pitch vertical axis wind turbine performance through DBD plasma actuators at low tip speed ratio

Flexible blades to improve Darrieus turbine performance and reduce cost

Low cost “sailwing” aerofoils of flexible nylon mounted between a rigid leading edge of circular cross section and a rigid flat trailing edge were investigated in the 1970s and 1980s. Wind tunnel measurements of lift and drag of these aerofoils over the full range of angles of attack were published, but no predictions were made for a complete wind turbine incorporating them. Another paper stated that a sailwing turbine with aerofoils incorporating pivoting aerodynamic profile leading edges was found to exert considerable starting torque. In the present paper, published lift and drag data have been used to predict the performance of a small Darrieus hydrokinetic or wind turbine with sailwing aerofoils, using a standard multiple streamtube model over the full range of tip-speed ratios (TSRs) from 0.5 to 3. To provide a fair comparison, this method is also used to predict the performance of the turbine with symmetrical NACA0018 and cambered Gö420 blade profiles. At all examined TSRs, average torque and power of the turbine with flexible blades are higher than the turbines with rigid blades. For instance, at TSR of 2.5, the flexible blade turbine achieved 55 % and 165 % higher average torque than the turbines with symmetrical and cambered blades, respectively. This finding is attributed to the fact that the flexible blade camber reverses with reversing incidence, producing high lift coefficients at low incidence. Drag is higher for the flexible aerofoil, probably due to separation at the circular cross section leading edge, and measures are suggested to improve this. The flexible aerofoils are very cheap and simple to fabricate, making them an attractive option for affordable hydrokinetic turbines.

Improving the self-starting and operating characteristics of vertical axis wind turbine by changing center distance in part of blades

Compared with the horizontal axis wind turbine (HAWT), the vertical axis wind turbine (VAWT) has the advantages of low noise operation and no wind yaw device. However, the poor self-starting performance of VAWT is one of key problems restricting its development. In order to improve the self-starting capability of VAWTs in urban wind areas, this paper proposed a VAWT model with multi-blade local variable radius. The main object of this study is to research the effect of reducing the partial blade radius on the self-starting performance and power coefficient of the VAWT. In this study, the partial blade radius of the VAWT models was reduced to 0.375, 0.5, and 0.625 times of the outer layer. The dynamic torques and velocity contours of the different models were researched by experiments and numerical simulations to analyze the reason why reducing the partial blade radius affects the power coefficient of the VAWT. The static torque and static pressure contours of different models were researched to compare the self-starting capability of the models. According to the results, the minimum static torque values of the three innovative VAWT models were increased by 0.1, 1.8, and 2.8 times, respectively, compared to the traditional VAWT with only the outer layer. Furthermore, the power coefficients of three models were increased for tip speed ratios (TSR) less than 2.2. Therefore, the new models can improve the self-starting capability of VAWTs, and the higher power coefficients at low TSRs make them more advantageous for applications in urban buildings and low wind speed areas.

Wind turbine concepts for domestic wind power generation at low wind quality sites

An overview of extant wind turbine designs is presented. Each concept is evaluated for potential for micro-wind production under poor wind quality situations, such as those seen in densely populated, suburban, or sparsely forested regions. It was concluded that the Savonius turbine and the Savonius-Darrieus Hybrid turbines were the most suited designs out of the seven that were investigated. In addition, several types of materials for micro-wind turbines were explored. Two-dimensional Computational Fluid Dynamic (CFD) simulations were run with the Savonius, and Savonius-Darrieus hybrid turbines with wind speeds 3 m/s, 5 m/s, 7 m/s and 10 m/s. Power generated by the turbines under each wind speed as well as the power coefficients and Tip Speed Ratios of the turbines were determined, and it was found that the Savonius turbine simulated had the maximum power coefficient of 0.31 at TSR = 0.97 and the Hybrid turbine had 0.32 at TSR = 2.24. It was also concluded that the Hybrid turbine generates more power than the Savonius turbine for all wind speeds except for 3 m/s. The maximum power output by any turbine simulated in this study was 157 W.

Multi-fidelity vortex simulations of rotor flows: Validation against detailed wake measurements

Two flow models with different fidelity of the DTU vortex solver MIRAS have been used to simulate the wake generated by a model wind turbine with various levels of asymmetry. Predictions are validated against experimental Particle Image Velocimetry measurements and dye visualizations. The experiments were conducted in a recirculating free-surface water channel with an immersed two-bladed rotor mounted on a shaft. The blades were designed to approximate a Joukowsky rotor. A detailed comparison between the measurements and the simulations is first performed for an unperturbed baseline case at different tip speed ratios. The analysis consists of a tip vortex characterization, including the vortex core profile, and a comparison of the instantaneous and mean velocity and vorticity fields, along with the mean wake profile at different downstream locations. Overall, good agreement is obtained between measurements and simulations, especially with the higher-fidelity particle-mesh model, which is capable of very closely predicting many of the flow features observed in the experiments. Rotor asymmetry triggers a vortex instability, commonly known as leapfrogging, which accelerates the breakdown of tip vortices, enhancing the mixing of wake structures and promoting a faster wake recovery. The prediction accuracy of this instability by the solvers is analyzed for different tip speed ratios and perturbation amplitudes. This work aims at setting the groundwork for future flow instability studies with the MIRAS solver.

3D CFD Modelling of Performance of a Vertical Axis Turbine

Recently, wind turbine research has switched focus to vertical axis wind turbines due to the extensive research that has been performed on horizontal axis wind turbines and the potential of vertical axis wind turbines in built-up areas. This study aims to analyse the performance of a small-scale hybrid vertical axis wind turbine that can switch from functioning as a Darrieus (lift) turbine to a Savonius (drag) turbine by rotating the blades. The turbine was analysed using 3D computational fluid dynamics (CFD) simulations in ANSYS Fluent as the primary method, and the findings were verified using wind tunnel experiments. During the analysis, design parameters such as the blade length, diameter, and number of blades were varied to determine if the design had room for improvement. It was found that the current design of the turbine has an optimal efficiency of 12.5% in the Darrieus configuration, which was found to increase when the diameter or blade length was increased. The Savonius configuration was found to be more efficient at low tip-speed ratios (<0.14), and its efficiency could be increased by adding more blades. The experiments found similar trends to the simulations; however, the efficiencies obtained were on average a tenfold increase from the simulation. Implementing the changes that increased efficiency leads to an increased wake recovery distance, making it less suitable for use in a wind farm.

Model-scale experiments of passive pitch control for tidal turbines

Tidal currents are renewable and predictable energy sources that could prove fundamental to decrease dependency from fossil fuels. Tidal currents, however, are highly unsteady and non uniform, resulting in undesirable load fluctuations on the blades and the drive train of turbines. A passive morphing blade concept capable to reduce the load fluctuations without affecting the mean loads has recently been formulated and demonstrated with numerical simulations (Pisetta et al., 2022). In this paper, we present the first demonstration of this morphing blade concept, through experimental tests on a 1.2-m diameter turbine. We show that fluctuations in the root-bending moment, thrust and torque are consistently reduced over a broad range of tip-speed ratios. This work also highlights some critical design aspects of morphing blades. For instance, it is showed that the friction resistance can substantially decrease the effectiveness of the system and thus must be minimised by design. Overall this paper demonstrates for the first time the effectiveness of morphing blades for tidal turbines, paving the way to the future development of this technology.

Performance enhancement of an H‐Darrieus vertical axis wind turbine via moving surface boundary‐layer control

AbstractThe moving surface boundary‐layer control method (MSBC) is a kind of active flow control methods that overcomes the adverse pressure gradient and suppresses the flow separation by directly energizing the boundary layer near the moving surface. In this study, computational fluid dynamics simulations were performed to investigate the feasibility of using MSBC to improve the aerodynamic performance of a straight‐bladed H‐Darrieus vertical axis wind turbine (VAWT). In doing so, a segment of the original surface of the VAWT blade was then replaced with a moving surface. Influences of different geometric and motion parameters of moving surface, including location, length, velocity ratio, and groove depth on the net power coefficient of VAWT, were systematically studied. Moreover, the sensitivity of different parameters to the net power coefficient of VAWT at the optimal tip speed ratio was analyzed via the orthogonal design method. As a result, the optimal combination of moving surface parameters was able to be determined. The present results show that the maximum net power coefficient of VAWT having blades with partially moving surface can be increased by 22.8% compared with that of the baseline VAWT. In addition, the effectiveness of two active flow control methods, co‐flow jet and moving surface, in improving aerodynamic performance of a VAWT was also compared. It is found that MSBC method has the potential to achieve superior control performance and promote the energy utilization coefficient of the VAWT more effectively by using comparatively less external energy input.

Performance improvement of the Savonius turbine using a novel augmentation device with the Taguchi optimization method

This paper proposes a new augmentation device to improve the coefficient of power (Cp) of a Savonius turbine using a computational fluid dynamics method. The device is a rotating cylinder synchronized with a rotating turbine using a timing belt mechanism without needing any external power source. Unlike conventional flow deflectors, the device is not placed in front of the turbine. Instead, it is placed parallel to the turbine position. The optimum device diameter and position are determined using the Taguchi optimization method. The device diameter and its position (lateral distance, longitudinal distance, and location; left and right) are selected as controllable process parameters and the Cp as the performance output. The Taguchi L16 orthogonal array is selected as the design of experiment. From the Taguchi analysis, the optimum device parameter improves the Cp by up to 0.3877 at a tip speed ratio (TSR) of 0.9, a 55.64% improvement relative to the case without the device. The highest Cp achieved is 0.4450 at TSR 1.1 with an 83.73% improvement relative to the case without the device. Analysis of variance results suggest that the Cp is greatly influenced by both the device diameter and lateral distance. The proposed device improves the overall performance of the Savonius turbine over a wide range of TSR.

Sensor fault-tolerant control for wind turbines: an iterative learning method

The combined wind speed estimator and tip speed ratio (WSE-TSR) tracking control scheme is widely used to regulate power production for large-scale modern wind turbines. Although very effective, such an advanced control scheme, based on the prior model information, is highly dependent on external measurements. For partial-load region control, the only external information involved is commonly the measured rotor or generator speed. Inaccuracy in such sole measurement results in an unintended turbine operation and might lead to sub-optimal power production and instability. This paper presents a fault-tolerant control (FTC) method, which aims to eliminate the sensor fault effects for modern wind turbine systems. To fulfil this goal, an iterative learning scheme is proposed to detect and estimate the multiplicative sensor fault, on which an adaptive FTC law is formulated such that the effects of the sensor fault are eliminated. Case studies show that the proposed iterative learning FTC method performs well in detecting, estimating, and accommodating the sensor fault under realistic turbulent wind conditions. The advanced wind turbine controller can maintain its control performance even under faulty conditions, preventing further damage to other turbine components and allowing for continuous power production.

ON THE SELF-STARTING COMPARATIVE PERFORMANCE EVALUATION OF DARRIEUS AND HYBRID HYDROKINETIC ROTOR

Darrieus rotor is a promising technology for hydrokinetic and wind energy harvesting applications. However, the Darrieus rotor suffers from the problem of poor starting performance. The present research highlights solutions to improve the poor starting performance of the Darrieus rotor by introducing the hybrid rotor. Further, a comparative performance evaluation of conventional vertical axis Darrieus and hybrid rotors has been investigated numerically. The most widely used S-series S-1046 hydrofoil has been utilized by hybrid and Darrieus rotors. Further, two semicircular blades are used for the Savonius part of the hybrid rotor. The size of the Savonius part is optimized to obtain maximum performance from the hybrid rotor. Analyzing the flow field distributions across the turbine vicinity has highlighted various possible reasons. The study results have demonstrated that the hybrid rotor yields an exceptional increment of about 159.41% in the torque coefficient under low tip speed ratio (TSR) regimes (during initial starting) compared to the Darrieus rotor. However, due to the Savonius rotor's presence, the hybrid rotor's maximum power coefficient is reduced slightly compared to the maximum operating point of the Darrieus rotor. Further, the hybrid rotor yields a wider operating range than the single maximum operating point by the Darrieus rotor. The present investigations will assist the designers in selecting the site-specific hydrokinetic technology suitable for efficient and optimum use of hydrokinetic potential.

The effect of pitch angle variation on performance of hybrid Darrieus-Savonius dual shaft vertical wind turbine

All Vertical turbines use a fixed pitch angle; the determination of the pitch angle is very influential on the characteristics of a vertical wind turbine. In the initial stages of planning a Savonius Darrieus vertical turbine, an experiment was carried out to determine the best pitch angle, and the best pitch angle was 7 degrees. In its development, many designs have changed from the initial design, so it is necessary to carry out field experiments to get the best pitch angle. The purpose of this study is to recommend the best pitch angle for hybrid Savonius Darrieus dual shaft vertical wind turbines. That suit each wind characteristic with the variable performance tip speed ratio (TSR) also (l), coefficient of performance (Cp), mechanical power and mechanical energy. Data is collected for 8 hours in real-time, using proximity sensors, anemometers, and an Arduino Mega to record and store data on a computer. The data collection interval at every 10 seconds. The data obtained is the RPM of the rotor and the RPM of the anemometer. The results of this study are the recommended pitch angle according to the wind speed, pitch angle of 5 degrees for 1 m/s wind speed, pitch angle of 8 degrees for 2 m/s a, and pitch angle of 8 degrees for 3 m/s wind speed.

Numerical investigation and improvement of the aerodynamic performance of a modified elliptical-bladed Savonius-style wind turbine

<abstract> <p>The Savonius turbine has an advantage over other types of vertical axis wind turbines (VAWT), which have speeds ranging from the lowest wind speed to the highest. However, the main problem is the negative torque on the rotary blades. This paper used computational fluid dynamics to numerically investigate the two-dimensional flow analysis of a modified elliptical Savonius wind turbine. This study investigated and compared five rotor blades: Classic, elliptical, and their three modifications. The behavior of wind energy was studied explicitly by changing the angle of the axis of the elliptical blade from the concave side, which leads to a convex shape to increase the area affected by the thrust force and increase the positive torque. The ANSYS (previously known as STASYS Structural Analysis System) Fluent version 15 software solves the unstable Reynolds-Naiver-Stokes (URAN) equation. The coupling algorithm solves the pressure-based coupling pressure velocity using the ANSYS Fluent. In the simulation, the drag, lift, and moment coefficients on the Savonius turbine were calculated directly at each change in the axis angle. The test results at wind speeds of up to nine m/s showed that the modified elliptical turbine with an axis angle of 50° had the highest coefficient power (Cp) among other elliptical blade modifications. In comparison, the test results with variations in wind speeds of 4–12 m/s showed that turbines with an axis angle of 55° performed better with a higher tip speed ratio (TSR) than other models.</p> </abstract>

The Behavior of Vertical Axis Water Turbine With Flexible Blades: Self-Start, Ventilation, and Cavitation

Abstract Three-bladed Darrieus-type vertical axis water turbine is a promising solution for producing electricity with minimal impact on the environment. Although considered a viable option, straight-bladed Darriues-type turbines have not been used commonly due to various operational issues; self-start and stall at low water speeds while ventilation and cavitation are limiting at high water speeds. In this study, the use of flexible blades with an aspect ratio of 2.21 is investigated at water velocities of 0.34, 0.51, 0.68, and 0.85 m/s experimentally. A stiffer turbine that has an 85–95 Shore A hardness blade starts to rotate at 0.51 m/s flow velocity. The more flexible turbine that has a 75–85 Shore A hardness blade starts to rotate at lower water velocities and experiences low rotational speeds resulting in an improved self-start. However, low rotation speed will cause a reduction in the coefficient of performance (Cp). High-speed imaging of the flow field also shows that a low tip speed ratio (TSR) helps to prevent the occurrence of ventilation and cavitation for the turbine with 75–85 Shore A hardness blades.

Three-phase AC-DC Converter for Direct-drive PMSG-based Wind Energy Conversion System

In this paper, a wind energy conversion system (WECS) is presented for the electrification of rural areas with wind energy availability. A three-phase AC-DC converter based on a bridgeless Cuk converter is used for power extraction from the permanent magnet synchronous generator (PMSG). The bridgeless topology enables the elimination of the front-end diode bridge rectifier (DBR). Moreover, the converter has fewer components, simple control, and high efficiency, making it suitable for a small-scale WECS. A squirrel cage induction motor (SCIM) is used to emulate a MOD-2 wind turbine to implement the PMSG-based WECS. A direct-drive eight-pole PMSG is used in this study; thus, a low-input-voltage system is designed. The converter is designed to operate in the discontinuous inductor current mode (DICM) for inherent power factor correction (PFC) and the maximum power point tracking (MPPT) is achieved through the tip-speed ratio (TSR) following. The performance of the developed system is analyzed through simulation, and a 500 W hardware prototype is developed and tested in different wind speed conditions.

Wind Turbine Fatigue Load Analysis for Derating Control Strategies Aimed at Grid Balancing

With the exponential growth of wind energy worldwide, wind power plant owners are eager to participate in grid balancing, in order to compete with traditional power plant owners. In this way, they take part in frequency control which notably amounts to reacting to grid faults by an adequate injection of power. The latter calls for power reserve. For wind farms, this is achieved by derating methods which consist in extracting from the wind less power than the available power, in order to be able to increase the produced power when needed. The objective of this paper is to compare two wind turbine (WT) derating methods in terms of lifetime damage equivalent load (DEL). The first strategy is based on a modification of the generator torque set point to achieve derating, while keeping the standard pitch angle control. The second strategy achieves derating by pitch control while keeping the standard tip speed ratio set point. Both strategies were simulated in OpenFAST under various wind conditions covering all the WT operating regions. Focusing on having a constant power reserve, the simulation results on a single WT show that the pitch-based strategy is the best, among the two considered ones, when it comes to derating while reducing lifetime DEL.

Performance Enhancement of VAWT using Diffuser for Energy Extraction from Cooling Tower Exhaust Air

Renewable energy generation need to be accelerated to battle climate change and depletion of fossil fuel resources. Innovation to design wind recovery system which are efficient is vital to contribute green energy production. Many advancements in vertical axis wind turbines (VAWT) were made over the years however, it is still not as efficient as conventional turbines, and some countries does not have the luxury of strong consistent wind throughout the year. Therefore, this study focuses on extracting wind energy from unnatural sources, specifically for cooling tower exhaust air energy recovery. In this study, cycloidal diffuser with different shroud lengths was used to study the performance of a 3-bladed H-Darrieus VAWT (HDWT) with S-1046 airfoils under accelerated wind conditions in a 3-dimensional numerical study using shear stress transport k-ω turbulence model. The cycloidal diffuser with shroud length of 0.48D increased the HDWT power coefficient by 26.66% compared to the bare HDWT at tip speed ratio of 2.0. Aerodynamics around the energy extractor system was also discussed and this investigation has provided good understanding of the flow behaviour of the wind augmented HDWT under cooling tower exhaust air.