Diffuser Augmented Wind Turbine Research Articles

CFD simulation of a multi-rotor system using diffuser augmented wind turbines by lattice Boltzmann method

A diffuser-augmented wind turbine (DAWT) achieves greater power generation efficiency by increasing wind speed through the diffuser. Nevertheless, scaling up this technology is difficult because of the considerable amount of wind drag on the diffusers. To overcome this difficulty, a multi-rotor system with two or more wind turbines on the same structure is one approach to increasing wind turbine power output. This research proposes a computational fluid dynamics (CFD) method to evaluate the hydrodynamic performance of a large-scale multi-rotor system of DAWTs. Compared to conventional wind turbines, CFD simulations of DAWTs necessitate higher computational costs because of the need of high-resolution meshes for the diffuser. Furthermore, the computational cost of a multi-rotor system increases with the number of rotors. To address the issue of high computational cost, we use the Lattice Boltzmann Method (LBM), which is well suited to large-scale CFD simulations. A wind turbine is modeled as an actuator line model and a diffuser as a wall boundary. An adaptive mesh refinement approach generates higher resolution meshes near the rotor and diffuser. LBM simulations were conducted for a single DAWT and a multi-rotor system with five DAWTs. The LBM results of the wake velocity and pressure distributions were in agreement with those obtained from wind tunnel experiments and general CFD methods in earlier studies. To investigate the diffuser gap effects, we simulated five DAWTs with diffuser gaps of 5%–25% of the diffuser diameter. The power gain of each DAWT was assessed. Great performance improvements were found with diffuser gaps of 20% and 25% of the diffuser diameter. On average, the five DAWTs achieved a power gain of more than 10%. These findings confirmed the accurate evaluation capability of the proposed CFD method for hydrodynamic characteristics of multi-rotor systems using DAWTs.

Optimization Based on Computational Fluid Dynamics and Machine Learning for the Performance of Diffuser-Augmented Wind Turbines with Inlet Shrouds

A methodology that could reduce computational cost and time, combining computational fluid dynamics (CFD) simulations, neural networks, and genetic algorithms to determine a diffuser-augmented wind turbine (DAWT) design is proposed. The specific approach used implements a CFD simulation validated with experimental data, and key parameters are analyzed to generate datasets for the relevant mathematical model established with the backpropagation neural network algorithm. Then, the mathematical model is used with the non-dominant sorting genetic algorithm II to optimize the design and improve the DAWT design to overcome negative constraints such as noise and low energy density. The key parameters adopted are the diffuser’s flange height/angle, the diffuser’s length, and the rotor’s axial position. It was found that the impact of the rotor’s axial position on the power output of the DAWT is the most significant parameter, and a well-designed diffuser requires accelerating the airflow while maintaining high-pressure recovery. Introducing a diffuser can suppress the wind turbine’s noise, but if the induced tip vortex is too strong, it will have the opposite effect on the noise reduction.

Optimization of Flanged Diffuser for Small-Scale Wind Power Applications

The development of renewable and clean energy has become more crucial to societies due to the increasing energy demand and fast depletion of fossil fuels. A state-of-the-art design for an augmented wind turbine has been introduced in the past years to increase the efficiency of compact horizontal axis wind turbines, exceeding the ideal Betz’s limit of the maximum energy captured from the wind. The optimization of the flanged diffuser - so-called diffuser augmented wind turbine DAWT - is investigated numerically using the multi-objective genetic algorithm “MOGA”. A 2D computational model is developed using ICEM CFD and solved by ANSYS Fluent. The Turbulence model selected is shear stress transport K-omega, with a pressure-based solver and a coupled algorithm scheme. The optimization objectives are to maximize the velocity ratio at the shroud throat and minimize shroud form dimensions. 517 design points were solved, and the design dimensions were categorized into four types: compact, small, medium, and large design. The results showed that the diffuser dimensions are the main parameters to increase velocity inside the shroud throat, where a long diffuser with a low converging angle drags more air inside the shroud, reaching in some cases more than double the upwind velocity. While the nozzle and flange are also effective in the different design types. It was found that a super long diffuser with a length ratio of 2.9 LD to throat diameter D is optimal with a diverging angle of 7.6˚, accompanied by a nozzle of ratio 1.2 LN/D and 12.6˚ converging angle and a flange length ratio of 0.6 LF/D. This optimal design increased the velocity ratio by almost 2.5 times.

COMPUTATIONAL AND EXPERIMENTAL ANALYSIS OF A DIFFUSER THAT CONSTITUTES A CONCEPTUAL AND UNPRECEDENTED DAWT FOR URBAN APPLICATIONS

This article presents numerical analyses of diffusers formed by different curved aerodynamic profiles and high lift-to-drag ratio airfoils, with and without the use of a ring flap (flanged diffuser) applied externally at the diffuser exit. The objective is to identify the optimal geometry for the fabrication of a full-scale prototype of a DAWT (Diffuser-Augmented Wind Turbine), which is a conceptual and innovative design developed for urban and rural use. The diffuser aims to increase the mass flow rate through the turbine, allowing for greater power extraction at low wind speeds caused by surrounding buildings. The mass flow increment factor is used as a performance parameter for the diffuser and it is found to have an approximately linear growth with the sum of radial forces, which is related to the lift coefficient of the employed aerodynamic profile. Both the application of the ring flap and the axial length of the diffuser generate higher radial forces and increase the mass flow rate through the diffuser. The diffuser with the NACA 2421 airfoil profile demonstrated a mass flow increment of 1.42 in a configuration with a flap and a configuration without a flap but with a longer axial length. The flap causes higher drag force and higher standard deviation. Therefore, for the fabrication of the turbine prototype, a flanged diffuser with the NACA2421 airfoil profile is chosen. By comparing the numerical and experimental results of diffusers with the NACA2421 airfoil profile, it is concluded that the experimentally obtained velocity increment is 7% lower than the increment indicated by numerical simulations.

Extensive design and aerodynamic performance investigation of diffuser augmented wind turbine (DAWT) guided by generalized actuator disc theory

Shrouding a turbine boosts power, lowers cut-in speed, but raises installation costs, and limits adaptability to wind shifts. A compact, wide-angle DAWT with competitive capacity is crucial for practical installations. Small turbines, often in fluctuating wind sites, necessitate thorough off-design performance analysis. A relatively short, wide-angle GOE431 diffuser is optimized by an efficient response surface method. Inverse blade element method, combined with actuator disc DAWT CFD considering wake swirl, shapes 90 cm-diameter rotor blade with minimal iterations. Implications of Generalized Actuator Disc Theory on DAWT design and performance are addressed with three-dimensional CFD for the first time. This approach unveiled substantial room for improving DAWT efficiency and uncovered key factors causing deviations from ideal performance. Tip leakage and diffuser losses constituted 9.5% of overall energy losses, with wake rotation, blade efficiency, and blade drag at 9.2%. Tip-hub losses, finite blade number, rotor-diffuser interaction, suboptimal rotor, and turbulence contributed to 12.1% in three-bladed DAWT, reaching CP,max = 0.746, with tip losses about one-third of bare turbine. Six-bladed DAWT raised CP by 93%, from 0.417 to 0.805, achieving 75% of ideal DAWT. Finite blade number led to reduced attack angles, higher tip losses, and limited flow expansion, contributing significantly to energy losses. As blade number and design tip-speed-ratio increased, blade Reynolds number decreased, suggesting an optimal combination to minimize energy losses. At off-design, a strong connection existed between thrust coefficient, diffuser efficiency, and Cp increase in wide-angle diffuser DAWTs. Maintaining CT near CT,opt (0.786) at high tip-speed-ratio led to significant Cp rise.

Diffuser augmented wind turbines: A critical analysis of the design practice based on the ducting of an existing open rotor

The study investigates the soundness of a popular uncoupled design strategy for diffuser-augmented wind turbines (DAWTs), namely the use of an annular wing to enclose an existing open-rotor. To this aim, the paper presents a numerical analysis of the NREL-Phase-VI rotor enclosed into a shroud whose cross-section consists of the Selig-S1223 airfoil. Particular attention is devoted to the analysis of the blade pressure fields, velocity triangles, blade forces, tip-vortex and wake development. The data show that the duct induces a gain in the rotor inlet axial velocity and, therefore, in the local flow-angle. Consequently, the blade forces, the extracted work, and the risk of flow separation considerably rise. Thanks to the simultaneous increase in the ingested mass flow rate and extracted work, the DAWT experiences a higher power coefficient (CP,exit) which, however, would be further improved if a coupled design-procedure was used. Indeed, in the present case, the maximum CP,exit is obtained for the wind-speed value corresponding to the duct optimal flow behaviour. However, in this condition, the rotor operates at off-design with an extensive flow-separation on the blade suction-side. Finally, while the inefficiencies magnitude is specific of the analysed case, the conceptual relevance of the achievements remains valid in general.

Investigations on Integrated Funnel, Fan and Diffuser Augmented Unique Wind Turbine to Enhance the Wind Speed

Wind energy is an alternative future energy to fossil fuels since it is abundant and green energy. As a result, high performance unique design is proposed that has a diffuser augmented wind turbine including intake funnel with guide vanes, natural fan, straight flow section, exit splitter with air openings and end flange. This proposed design is called as Integrated omni-directional Intake funnel, Natural fan, Straight diffuser, Splitter and Flange (I2NS2F) Design. To construct this I2NS2F configuration, four distinct wind turbines were developed: a) bare wind turbine, b) wind turbine diffuser design with single rotor turbine, c) bend diffuser, intake funnel, natural fan, splitter and flange and d) I2NS2F design. The proposed designs are numerically studied using MATLAB Simulink and Ansys Fluent. These designs are optimized by Random Search Optimization with Supervisory Control and Data Acquisition technique to evaluate the wind velocity and the performance is comparatively estimated. From this analysis, I2NS2F design achieves 53m/s of wind velocity at turbine region for 5.5m/s inlet wind and it could be considered as highest wind velocity than other three designs. It is evidently proved and concluded that proposed I2NS2F design augments natural wind, resulting in greater green power generation.

EFFECT OF FLANGE VIBRATION ON AIRFLOW PASSING THROUGH A DIFFUSER WITH KNOWN GEOMETRY

A diffuser-augmented wind turbine is used to enhance the overall performance characteristic of the wind turbine. Flanges near the trailing end of the diffuser create a pressure difference that accelerates the free-stream velocity into the diffuser, increasing the performance of the wind turbine. In the present analysis, a numerical method is used to analyze the effect of flange vibration at different amplitude and operating frequencies. The diffuser is operated at no-load (<i>C</i><sub>t</sub> = 0) condition for all cases. An optimum opening angle is determined in the absence of flange vibration at a constant airflow rate and is used for further analysis. Performance characteristics such as diffuser speed-up ratio and coefficient of power are estimated for all cases. It is found that the velocity at the centerline oscillates with the same frequency as that of the flange.

Diffuser augmented wind turbines – a case study for hybrid energy system applications and comparison with horizontal axis wind turbines

ABSTRACT Hybrid energy system that combines diffuser-augmented wind turbines with photovoltaic panels and batteries are proposed in this research. ANSYS fluent® is used to simulate diffuser performance and ratified experimentally. HOMER® is used to perform a techno-economic analysis of the proposed hybrid energy system in terms of the levelized cost of electricity and the net present cost. Units of 1 kW wind turbines, 1 kW solar PV units, and 1 kWh Li-ion battery units are being used in the simulation. Input parameters like solar radiation, wind speed, and electricity tariff are considered for a typical location in Palakkad, India (10.7976° N, 76.7430° E). The proposed hybrid system with off-grid and on-grid mode are analyzed, and the results are presented. For 20 years project life, the proposed optimal on-grid hybrid energy system based on diffuser-augmented wind turbines has a levelized cost of electricity of $0.0044/kWh and a net present cost of $22,857. The corresponding values for the optimal on-grid hybrid energy system based on wind turbines are $0.0096/kWh and $41,143, respectively. Sensitivity analysis is performed on wind speed, solar radiation, and export tariff. According to this study, the usage of diffuser augmented wind turbine benefits low-speed wind regimes.

Power augmentation of ducted wind turbines for urban structures: Experimental, numerical, and economic approaches

AbstractRecent development in using wind turbines for urban areas results in inserting turbines inside buildings. As buildings' walls may act as a duct for the turbine, this study focuses on a ducted wind turbine with a fixed duct geometry. A method is organized for achieving the improved generated power and the wind speed augmentation with fixed geometry of duct regardless of the type of the turbine, which is the aim of building designers. Using a porous disc (PD) instead of a wind turbine rotor makes the study cost and time effective. PDs within a duct help estimate any given duct's maximum available power extraction capability. In addition, experimental and numerical tests examine the effect of PDs solidity on the performance of diffuser augmented wind turbines and the corresponding economic analysis. Both experimental and numerical results agree that the power coefficient highly depends on the solidities of the PD. The power coefficient of a ducted PD with a solidity of 0.3 is augmented by up to 30%. Nevertheless, in some cases, employing a duct can contribute to the power reduction if the solidity exceeds a critical value. A smoke visualization technique helps vortex study. Economic assessment of a ducted turbine for three scenarios belonging to Germany and Italy shows a 15.3% decline in cost per electricity production. The payback period decreases by 3.42 years, 7.68 months, and 6.36 months for Scenarios 1, 2, and 3.

Aerodynamic comparison of slotted and non-slotted diffuser casings for Diffuser Augmented Wind Turbines (DAWT)

The ability of Horizontal & Vertical Axis Wind Turbine (HAWT & VAWT) to harness the incoming wind is limited due to several factors such as structural constrains, fatigue problems, geographical and ecological issues, noise pollution, vibrational & acoustic losses. Diffuser Augmented Wind Turbine (DAWT) exploits the concepts of velocity augmentation to maximize power production efficiently when compared to conventional approach. The optimization of the geometric parameters can help the augmentation capability of the diffuser. This study investigates the aerodynamic effect of slits on diffuser casings and the performance of slotted and non-slotted diffuser casings is compared using 3D CFD analysis and validated experimentally. The performance of the studied diffusers is analysed in terms of velocity at the rotor plane, augmentation ratio, and swallowed mass flow rate. At tip speed corresponding to 62 m/s, the performance parameters of slotted DAWTs showed an increase by 27.4% and 46.8% compared to non-slotted DAWTs and open turbines, respectively. It is also observed that the inclusion of slotted and non-slotted diffusers enclosing the rotor resulted in an average augmentation of 2.01 and 1.36 in both power and torque produced, respectively.

The Joukowsky rotor for diffuser augmented wind turbines: design and analysis

Diffuser-augmented wind turbines are known for the potential improvement in power extraction in comparison with open wind turbines. Despite the large number of research works dealing with this subject and unlike the open rotor case, an optimum ducted rotor model is still missing. Since the Joukowsky (free-vortex) optimum rotor exhibits the best power coefficient in the open configuration, this paper presents a newly developed ring-vortex free-wake approach for the performance evaluation of an optimum Joukowsky rotor enclosed in a duct of general shape. The method, which is extensively verified, relies on the exact solution of the steady, incompressible, inviscid and axisymmetric flow, and it naturally takes into account the wake divergence and rotation. The procedure is used to obtain, for the first time, the maximum-power-coefficient/tip-speed-ratio characteristic curve for a diffuser augmented wind turbine. The proposed ducted rotor beats the Betz limit by 14.5% when the power coefficient is referred to the device frontal (exit) area. Additionally, the device experiences a slower decrease in performance with the reduction of the tip-speed ratio, thus extending the design range of ducted rotors in comparison with the open ones. Finally, taking into account the mutual influence of the disk and duct, a new rotor design strategy, capable to evaluate the optimum distribution of the chord and pitch-angle along the blade span, is also proposed. A complete design exercise is carried out and the rotor geometry is obtained for three different values of the nominal tip speed ratio. The paper also proves that a two-dimensional design procedure, which strongly couples the duct and the rotor induced flow, is mandatory to properly evaluate the optimum rotor geometry.

Design and construction of an offshore diffuser augmented wind turbine with a high efficiency alternator

This paper describes the design and the assembly of a new type of a ducted wind turbine with an electric power generator adopting permanent high coercive magnets embedded in the peripheral ring of the rotor. The nominal power is 20 kW, and the maximum diameter of the external duct is 9 meters. The project has been carried out within the Marine Energy Laboratory (MEL) funded by the Italian Ministry for Education, University and Research (MIUR) and collects the most advanced technologies of naval maritime engineering and combines them with energy and turbomachinery technologies. The presence of a divergent duct enables the interception of a greater air mass flow rate, allowing the reduction of the rotor diameter and the deflections of the blades; hence, at constant tip speed ratio, a higher rotational speed compared to conventional turbines and a better efficiency of the permanent magnet power generator.

Effect of Rotor Blades Number and Rotor Position on the Performance of a Diffuser Augmented Wind Turbine

Effect of Rotor Blades Number and Rotor Position on the Performance of a Diffuser Augmented Wind Turbine

Comparative Analysis of Diffusers for Micro Wind Turbine

With the increase in demand for clean energy, a micro wind turbine would be the best option for remote and urban residential areas. Installing wind turbines is not feasible in most land areas due to low or inadequate wind speed. The energy generated by the wind turbine is directly proportional to the cube of wind velocity. So, if we manage to increase wind speed slightly, it would increase energy significantly. One approach to solving problems in areas with low wind speed is using Diffuser Augmented Wind Turbine (DAWT). In DAWT, the turbine blades are typically surrounded by a duct which increases the cross-sectional area in the stream-wise direction. Since a diffuser encloses the wind turbine, the pressure behind the turbine will drop, which results in an increasing wind velocity. Different types of diffusers have been introduced to increase wind velocity. The main aim of the research is to perform a comparative analysis of four different types of diffusers to increase the power output of wind turbines. The CFD simulation of Plain Diffuser, Plain Diffuser with Inlet Shroud, Flanged Diffuser, and Flanged Diffuser with Inlet Shroud is performed to determine the maximum velocity each diffuser produced. With the solution from the simulation, a comparative analysis of each diffuser is conducted, and the results are further verified with the previous studies on DWAT. And Flanged Diffuser is found to be optimum with an increase in power generation up to 3.6 times compared to a bare wind turbine.

SENSITIVITY ANALYSIS OF ANGLE, LENGTH AND BRIM HEIGHT OF THE DIFFUSER FOR THE SMALL DIFFUSER AUGMENTED WIND TURBIN

This paper presents the performance of the diffuser augmented wind turbine (DAWT) with the various diffuser shapes using the numerical investigations. DAWT is also a type of wind turbine and the diffuser shapes, the nozzle shapes and the cylindrical shapes are commonly inserted around the horizontal axis wind turbine (HAWT) to become the more efficient wind turbine. The aim of this study is to find the more efficient design of the diffuser for the horizontal axis wind turbine using the numerical investigations. In this research, the converging and diverging diffuser shape is inserted and the airfoil design is calculated by using the Blade Elementary Momentum Theory. The airfoil type NACA 4412 is chosen because it is suitable for the low wind speed area and easy to produce. The turbulent model k-ω is combined with the Navier Stoke equation to solve the 3-dimensional steady flow simulation of the diffuser augmented wind turbine using the Computational Fluid Dynamics (CFD) simulations. The numerical investigation is used to compare and predict the power coefficient of the DAWT with various shapes. The baseline design of the diffuser (L = 170 mm, H = 57 mm and α = 11̊) is firstly investigated. To predict the power coefficient of the various diffuser shapes, the range of the length of the diffuser is (L/D = 0.5 to 1.5), the range of the brim height of the diffuser (H/D = 0.1 to 0.35) and the range of the angle of the diffuser (α = 5̊ to 15̊ ) are also investigated. The parameters of the diffuser shapes are assigned by using the Central Composite Design Face Centered Method. The response surface method is also used to predict the most efficient diffuser design. The performance of the horizontal axis wind turbine, that of the diffuser augmented wind turbine and that of the diffuser augmented wind turbine with various shapes of diffuser are compared. The performance of new diffuser augmented wind turbine (IND_009) is 50% and 55% higher than the baseline diffuser augmented wind turbine and the horizontal axis wind turbine at rated velocity. The flow visualization of the HAWT, DAWTs are also discussed.

Power Enhancement of a Vertical Axis Wind Turbine Equipped with an Improved Duct

Efforts to increase the power output of wind turbines include Diffuser Augmented Wind Turbines (DAWT) or a shroud for the rotor of a wind turbine. The selected duct has three main components: a nozzle, a diffuser, and a flange. The combined effect of these components results in enriched upstream velocity for the rotor installed in the throat of the duct. To obtain the maximum velocity in the throat of the duct, the optimum angles of the three parts have been analyzed. A code was developed to allow all the numerical steps including changing the geometries, generating the meshes, and setting up the numerical solver simultaneously. Finally, the optimum geometry of the duct has been established that allows a doubling of the flow velocity. The flow characteristics inside the duct have also been analyzed in detail. An H-Darrieus Vertical Axis Wind Turbine (VAWT) has been simulated inside the optimized duct. The results show that the power coefficient of the DAWT can be enhanced up to 2.9 times. Deep dynamic stall phenomena are captured perfectly. The duct advances the leading-edge vortex generation and delays the vortex separation.

A design of experiment approach as applied to the analysis of diffuser-augmented wind turbines

Since the 1930s ducted-wind-turbines have received considerable attention due to the possibility of obtaining a significant increase in the extracted power compared to an open turbine with the same rotor swept area. The incomplete knowledge of the flow phenomena occurring in these devices and the large design-space to be explored generally hampers the development of an effective and robust design-procedure. This work investigates the performance of ducted wind turbines through a fully automated analysis procedure based on a Computational-Fluid-Dynamics-Actuator-Disk approach. The method duly takes into account the effects of the design parameters, the rotor-duct coupling, the wake rotation and expansion, and the spanwise variability of the rotor load. A Design-of-Experiment analysis is carried out to quantify the impact of the change of all geometric parameters and their mutual interactions. The latter explicitly account for the simultaneous variation of all design parameters. As such, it is a powerful pre-design tool that allows to reduce and confine the design space. It is found that the chord and stagger angle of the duct contribute more than 85% to the improvements of the turbine performance, while the effects of the thickness are negligible. Finally, the sensitiveness of the performance to the variation of the operating conditions is also analysed revealing that the optimal configuration is also the most robust.

Diffuser Total Efficiency Using Generalized Actuator Disc Model and Its Maximization Method

The diffuser total efficiency was formulated and defined based on the generalized actuator disc model for the index of the efficiency of the diffuser-alone of the diffuser-augmented wind turbines. An optimization method to maximize the diffuser total efficiency was developed using a genetic algorithm and axisymmetric computational fluid dynamics. A case study was conducted for a 10% chord-to-diameter ratio, 2% thickness-to-chord plate, and the crest position at 50% chord of the diffuser. The optimal result showed a diffuser total efficiency of 1.087. Furthermore, 1392 (=48 population × 29 generations) simulation cases of the optimization process showed that high diffuser total efficiency appears at a low-drag coefficient, high-lift coefficient, and 15–25% low diffuser height-to-chord ratio.

Computational analysis of high-lift-generating airfoils for diffuser-augmented wind turbines

Abstract. The impetus towards sustainable energy production and energy access has led to considerable research and development on decentralized generators, in particular diffuser-augmented wind turbines. This paper aims to characterize the performance of diffuser-augmented wind turbines (DAWTs) using high-lift airfoils employing a three-step computational analysis. The study is based on computational fluid dynamics, and the analysis is carried out by solving the unsteady Reynolds-averaged Navier–Stokes (URANS) equations in two dimensions. The rotor blades are modeled as an actuator disk, across which a pressure drop is imposed analogous to a three-dimensional rotor. We study the change in performance of the enclosed turbine with varying diffuser cross-sectional geometry. In particular, this paper characterizes the effect of a flange on the flow augmentation provided by the diffuser. We conclude that at the end of the three-step analysis, Eppler 423 showed the maximum velocity augmentation.