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Review of a Shrouded Wind Turbine for Low Wind Speeds Ajeet Kumar Yadav1, Devesh Kumar2 1, M. Tech Scholar, Mechanical Engineering Department, MMMUT, Gorakhpur, INDIA 2, Assistant Professor, Mechanical Engineering Department, MMMUT, Gorakhpur, INDIA ABSTRACT The use of renewable energy is promoted worldwide to be less dependent on fossil fuels andnuclear energy. Therefore research in the field is driven to increase efficiency of renewable energy systems. This study aimed to develop a wind turbine for low wind speeds. The extent of power increase, or augmentation, the factors influencing shrouded wind turbine performance, the optimal geometry and economical benefit remained unanswered.

The most important matter at hand when dealing with a shrouded wind turbine is to determine if the overall diameter or the blade diameter of the turbine should be the point of reference. As the wind turbine is situated in a shroud that has a larger diameter than the turbine blades, some researchers believe that the overall diameter should be used to calculate the efficiency Theory was revised to determine the available energy in the shroud after initial calculations showed that the power coefficients should have been higher than the open wind turbine with the same total diameter. A new equation was derived to predict the available energy in a shroud.

The worldwide increase in demand for energy and the obligation to protect the environment further rnecessitates the use of renewable energy. One such renewable energy resource that can be used iswind energy. The use of wind mills to produce energy from wind power dates back as far as 3000years.

From the late nineteenth century wind mills with generators (wind turbines) have been used to generate electricity. [1] As the demand for energy increased, it became clear that it will be necessary to locate windturbines at certain terrains and regions which previously have not been considered suitable. Theseterrains and regions may have gust, turbulence and low wind speeds or other physical constraints.Progressively more wind turbines tend to be installed at such complex terrains [2]. Also, recently more efficient designs have been introduced for low wind speeds as well asfor urban use where turbulence, noise levels and appearance needed to be considered and addressed [3]. Some new designs propose that the turbine forms part of a buildingand/or structures. Other designs apply turbines in conjunction with solar panels or other typeâ€&#x;s ofrenewable energy systems [4].

2. CHALLENGES

1. INTRODUCTION During the last years, significant progress has been made to understand the diffuser technology. Thus, new ideas have emerged on the origin of those technologies due to the potential increase in efficiency that diffuser devices produce in wind turbines, particularly for small wind turbines.Numerous investigations relative to shrouded Wind Turbine, or shrouded wind turbines concept over the last century were done.

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Most of the wind turbines that are on the market have been developed in countries that have higher mean wind speeds. The imported wind turbines are designed to have high Cp values at higher wind speeds. These wind turbines will not generate much energy except for the period of time that the wind velocity is high. Also, a wind turbine that is optimized for high wind speeds usually have reduced efficiency at low wind speeds. These wind turbines will fail to start rotating at low wind speeds [5]. Locally designed wind turbines also 1|P a g e

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International e-Journal For Technology And Research-2017 face a similar problem. The design for low wind speeds also react on the performance at the occasion the wind speed is high. Small wind turbines do not have pitch adjustment and the blade will have no optimum angles of attack at wind a speed that was not the design wind speed [5]. The available energy at low wind speed regions is a minimum; therefore the wind turbine should have high efficiencies at a wide range of wind speeds.

From this one can see the necessity for some new designs to enhance the Cp values of a wind turbines rotor for low wind speeds regions. One way to increase the Cp value of the wind turbine is to use structures like concentrators and diffusers. Both of these configurations are impractical to use in high wind speed regions because of structural

constraints [5]. In low wind speed regions it could be feasible to use them to increase the Cp values of a wind turbine. It should be noted that these shrouded wind turbines will probably be practical for micro and small wind turbines only. With a small, low wind speed wind turbine there is an even greater expectation to improve the Cp value, as the energy available is already minimal. To conclude it is evident that there is a definite need to improve the feasibility of small wind turbines in low wind speed conditions.

3. Aerodynamics Aerodynamic principles and condition explained through the figure below:

are

Figure 2.1: Two dimensional airfoil with labelled terminology Blade moving through the fluid develops different aerodynamic forces; the component of force which is acting perpendicular to the direction of moment is called lift force; and force acting in the direction of motion is known as drag force.The accurate models of aerodynamics aspects of wind turbines is one of the major key points to a successfully designing and analysing wind energy systems.

Wind turbines while operatesinduces phenomenon like cross-flow components (when a rotor is not aligned with wind), where direction and magnitude relative to the rotor changes continuously as the blades rotate.Moreover, in such cases, phenomena like flow separation and other three-dimensional effects become more complex. Those instabilities interacting with hub tip and blade affects the overall flow field.Clearly, wind turbine IDL - International Digital Library

aerodynamics becomes more complex with all instabilities and flow interactions (Jonkman 2003)[2].In order to understand the complexities in wind turbine aerodynamics, there is need toanalyse a simple one-dimensional model first. According to thepast literature the flow velocity is an important factor that determines whether the flow is compressible or incompressible. Usually, as the blade tip speed do not exceeded the value of 100 m/s which is equivalent to Mach number of 0.3, and the flow around the rotor is supposed to be incompressible (Schlichting, 1979).[3] Drag on a 2-d aerofoil or body exerts a force in the direction of flow which can be divided into two parts, namely pressure drag and skin friction drag.

The latter; drag caused byshear stress. For example 2|P a g e

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International e-Journal For Technology And Research-2017 an infinite thin at plate with the flow parallel over surface willexperience friction drag only. Pressure drag can be understood as a plate oriented normal to the flow, this drag is due to the normal stress on the body. Thus total drag can be calculated as the combination of these two with change of the angle of attack (Shames 2003, 667) [4].

A lift force on a turbine blade can be calculated by integration of the pressure force to the surface of the blade (Bertin& Cummings 2009, 215 and 216) [5].

From the figure 2.1,the chord length can be seen as the distance between the leading edge to the trailing edge. The angle of attack is the angle between the chord line and the relative airflow. The camber is known as the asymmetry between the upper surface and lower surface of an aerofoil.

Separation starts to occur when the fluid flow did not follow the boundary layer over an adverse pressure gradient (Shames 2003, 666) [4] In case of an aerofoilwith high angles of attack flow it is called to attain stall condition (Wood 2011, 60) [6] The wind turbine blade is an aerodynamic body, in which efficiency of the blade is excessively affected by the aerodynamic performance.

4. Power Available The maximum power that can be extracted from the wind is explained below. This law is originated from the principles of conservation of mass and momentum which is generally attributed to Betz (1926)[7]

Figure2.2 Actuator disk model for a wind turbine Conservation of mass inthe stream tube. đ?‘š = đ?œŒđ??´1 đ?‘Ł1 = đ?œŒ. đ?‘†. đ?‘Ł = đ?œŒ. đ??´2 . đ?‘Ł2 Here v1 is the speed in the front of the rotor, v2 is the speed downstream to the rotor, and thespeed at the disc is denoted as v. The fluid density is đ?œŒ and the area of the turbine is given by S. The forceexerted on the wind by the rotor: F = m .a = đ?œŒ. đ?‘† . đ?‘Ł. (đ?‘Ł1 − đ?‘Ł2 )

(2.1)

Net work done, dE =F.dx

(2.2)

The power of the wind is Incompressible, homogeneous, , steady state fluid flow, No frictional drag ,An infinite number of blades, Non rotating wake, Uniform thrust over the rotor area, equal static pressure far upstream and downstream are the assumptions which are considered in order to derive the maximum power.

đ?‘ƒ=

đ?‘‘đ?‘Ľ đ?‘‘đ?‘Ą

đ?‘‘đ?‘Ľ

= đ??š. đ?‘‘đ?‘Ą = đ??š. đ?‘Ł

(2.3)

Substituting the force into the power equation will yield the power extracted from the wind đ?‘ƒ = đ?œŒ. đ?‘†. đ?‘Ł 2 . (đ?‘Ł1 − đ?‘Ł2 )

(2.4)

Power can also be computed by using the kinetic energy đ?‘ƒ=

∆đ??¸ ∆đ?‘Ą

1

= . đ?‘š. (đ?‘Ł12 − đ?‘Ł22 ) 2

(2.5) Put the value of m from equation 2.1 then

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đ?‘ƒ=

2

. đ?œŒ. đ?‘†. đ?‘Ł. (đ?‘Ł12 − đ?‘Ł22 )

(2.6)

Equating the two power equation 2.4 and eq.2.6 then 1

đ?‘ƒ = 2 . đ?œŒ. đ?‘†. đ?‘Ł. đ?‘Ł12 − đ?‘Ł22 = đ?œŒ . đ?‘†. đ?‘Ł 2 . (đ?‘Ł1 − đ?‘Ł2 ) (2.7) 1

đ?‘Ł = . (đ?‘Ł1 − đ?‘Ł2 )

(2.8)

2

Put the value of v from eq. 8 in power based on kinetic energy 1

đ??¸ = . đ?‘š. (đ?‘Ł12 − đ?‘Ł22 ) = 2 đ?‘Ł22 ) 1

E=4 . đ?œŒ. đ?‘†. đ?‘Ł13 . (1 −

đ?‘Ł2 2 đ?‘Ł1

1 4

. đ?œŒ. đ?‘†. đ?‘Ł1 + đ?‘Ł2 . (đ?‘Ł12 − (2.9)

+

đ?‘Ł2 đ?‘Ł1

đ?‘Ł

− (đ?‘Ł2 )3 1

(2.10)

In order to extract energy from an air flow,a wake has been produced behind the rotor. This wake has somevelocity and pressure deficit relative to free undisturbed stream flow. In accordance (Igra 1981, Van Bussel 2007)[12], augmentation of a DAWT hasdirect consequences of the sub-atmospheric pressure around the exit plane of the shroud and rotor.

Shrouded rotors can combine with different systems with objective to concentrate and accelerate the wind. Hollow structures can be placed for surrounding a wind turbine to boost the wind flow. As it is clear from Figure 2.3, nozzle model section decreases the inside cross-section, in cylindrical model section may possess constant cross-section, and in diffuser model section can have cross-section at downstream that expands gradually (Ohya et al. 2008)[13].

E differentiating with respect to v1/v2find maximum or minimum value of E.Value of E is maximum when v1/v2 is equal to 1/3 putting this value in eq. 10 then result get 16 1

đ?‘ƒđ?‘šđ?‘Žđ?‘Ľ = 27 . 2 . đ?œŒ. đ?‘†. đ?‘Ł13

(2.11)

From a cylinder of fluid with cross sectional area S and velocity v1Theobtainable power is 1

P =đ?‘?đ?‘ƒ . 2 . đ?œŒ. đ?‘†. đ?‘Ł13

(2.12)

The total power đ?‘ƒđ?‘¤ =

1

. đ?œŒ . đ?‘†. đ?‘Ł13 2

(2.13)

Figure 2.3: Schematic representation of systems that concentrate and accelerate the wind,adapted from Ohya et al. (2008).

The principle of increasing the mass flow in the wind turbine can be conjugated with the turbulent mixing of the wake behind the rotor resulting in a power augmentation (Ten Hoopen 2009)[14].

Power coefficient đ?‘ƒ

đ??śđ?‘? = đ?‘ƒ

�

(2.14)

Maximum value of: Cp = 16/27 = 0:593 Eq. 2.13helps to determine total power availability in a concentrator or diffuser.This is proposed by Bernard Frankovic&Vrsalovic(2001)[8], Wang et al. (2007) [9],Orosa et al. (2009)[10], ,and Ohya&Karasudani (2010)[11] where the wind turbine in the shroud there velocity is average velocity on their measured and substituted on the place of đ?‘Ł1 in eq.13 to find out the total power available.

5. Theoretical Analysis of Shrouded Rotor IDL - International Digital Library

A mechanism to enhance air flow can be achieved by placing an annular lifting device around the rotor. This particular device is called a shroud or a diffuser of annular wing. The increase in velocities at diffuser exit plane combined with a decrement of static exit pressure and enhanced mass flow consequently leading to a higher extraction of energy potential from the wind.The principle behind a DAWT supposed tobe the cause of the air flow inside the diffuser to accelerate. Moreover, the suction is related with the lift of the aerofoil and according to the KuttaJoukowski theorem, which is related to the bound vorticity. The annular aerofoil causes a radial lift force that creates a ring vortex, based on Bio-Savartlaw it will induce a higher velocity in the suction side. Moreover, this higher 4|P a g e

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International e-Journal For Technology And Research-2017 velocity increases the mass flow through the rotor plane (Ten Hoopen 2009). It is well proven that if a bare wind turbine is operates at the maximum Betz limit, the airflow is retarded to 2-3 of the free stream velocity.This flow retardationresults into pressure increase in front of the rotor that pushes a small portion of the mass flow sideways around the rotor (Ten Hoopen 2009)[14].

condition, which is typical used in urban scenario (Kosasih&Tondelli 2012)[16].

The configuration of DAWT allows tip vortices to create at the blade tips to be significantly less due to closer proximities of the diffuser wall.

Therefore, mixing potential behind the exit plane of a DAWT is assumed to be higher from the case of a simple wind turbine (Ten Hoopen 2009)[14].

From Figure 2.4, the flange creates a low-pressure region at near wake of the diffuser by vortex generation. Moreover, high mass flow is drawn towards the inlet of shroud (Ohya et al. 2008, Ohya&Karasudani 2010, Takahashi et al.

2012)[13][11][17]. The flange induces vortices formation, which enhances the pressure drop and, subsequently, increases the air speed at the outlet.

An increment in the air velocity in the diffuser, is therefore, obtained (Mansour &Meskinkhoda 2014).[18]

The effect of mixing on diffuser leeward provides one wake flow with higher volume. Moreover, a larger wake volume will result into lower exit pressures behind the rotor and therefore inducing more suction effects (Ten Hoopen 2009)[14].

The flange is a ring-type plane structure with a variable height which may affect the shroud performance. It‟s kept attached vertically towards the outer periphery of exit shroud (Ohya et al.

2008, Kosasih&Tondelli 2012)[13][16].

In Figure 2.4, the “throat” plane denotes the diffuser cross section perpendicular to the axisymmetric axis where the area inside the diffuser is found to be minimum (Hjort& Larsen 2014)[19].

6. Numerical Simulation (CFD) Computational fluid dynamics comprises of solving the Navier-Stokes equations with governing fluid flow equations using approximation method with numerical means (Sumner et al. 2010)[27].

Figure 2.4: Representative illustration of the flow around the shroud, considering thepresence of brim, adapted from Ohya&Karasudani (2010)[11].

An important characteristic that can be described from the application is a brimmed diffuser shroud.

Brim application assists the shroud tostay aligned towards the approaching wind. Another characteristic verifies that at low-tip speed ratios, the vortex generated from blade tip becomes suppressed throughout the interference with the boundary layer in the diffuser shroud. Therefore, aerodynamic noise is significantly reduced (Abe et al. 2006, Ohya&Karasudani 2010)[22][11].

Application of nozzle, with converging geometry at inlet of shrouded wind turbine, will become advantageous in variable wind direction flow

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CFD solvers are based upon following three basic fundamental conservations principles expressed in terms of mathematical equations: Conservations mass; conservation of momentum and conservation of energy (Sargsyan 2010)[21].

Extensive implementation of simulations in aerodynamic features, applied on various manners, ranging from Blade Element Momentum methods integrated by CFD solver to full 3D Navier-Stokes models became an important factor to evaluate performance of wind turbine (Sargsyan 2010)[21].

In Versteeg&Malalasekera (2007)[22]explained that one of the basic task of the CFD user is to design a grid which present a suitability between required accuracy and solution cost. Another concern in the numerical simulations is moving and stationary components that exists, that must be resolved separately (Bazilevs et al. 2011)[23].

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International e-Journal For Technology And Research-2017 Designing of wind turbine and aerodynamic performance is an important scientific field. In this area, number of researchers has developed numerical codes to support aerodynamic optimization to perform an upgrade to energy generation of wind turbine (Lanzafame et al.

2013)[24].

Where, đ?‘ˆđ?‘– đ?‘ˆđ?‘— denote the mean velocity vector, p represents modified mean pressure, đ?œŒ is fluid density, đ??šđ?‘– is a body force.

Performing CFD calculations provides enormousdetails information of the fluid flow, such as pressure, velocities, temperature, turbulence, etc.

Further, several type of graphics are expected to obtain, performing results in flow lines, contour lines and iso-lines, etc. At this level, is considered

by Castelli et al. (2013)[25], shows that these results can be compared with that obtained in a wind-tunnel study or an full-scale measurement.

CFD codes are developed around numerical algorithms that are constructed for resolution of various fluid flow problems. Aiming at providing intuitive tools for users of complex CFD codes, normally these are categorised in three elements: (i) Pre-processor, (ii) Solver, (iii) Postprocessor (Versteeg&Malalasekera 2007)[21].

3D CFD numerical codes are realistic, due to solving throughNavier-Stokes equations.

Nevertheless, in order to achieve these solutions, more computational times are needed. Also an appropriate preparation of geometry is important.

CFD codes are necessary mean to achieve information which is impossible to reach through experimental measurements (Lanzafame et al.

2013)[24].

7. Governing Equations The fluid dynamics involves complex relationships between the viscosity and how theflow develops, translating into mathematical models induces a high level of complexity for some problems (Massey 1996)[26].

The true fluid flow passing through and around a wind turbine is governed by the mainprinciplesof Navier-Stokes equations. Unfortunately, these equations are so complex thatanalytical solutions only have been found for simple cases. Although numerical solutionspresents abilities to solve these equations (Jonkman 2003)[27].

8. CFD Code Structure

Generally the precision of solution are governed by the number of cells in the grid. So higher the number of cells contained in grid domain, higher accurate will be the solution (Versteeg&Malalasekera 2007)[22].

Solver is the principal element of CFD code. The core of CFD code works with discretization of governing equations fluid flows. In this phase, unknowns are solved with a resolution of algebraic system of equations (Versteeg&Malalasekera 2007, Sargsyan 2010)[22][21].

The pre-processor phase contains the introduction of physical flow model with the aim of converting it into a mathematicalmodel (Sargsyan 2010)[21].

The principle activities of userâ€&#x;s are: to define of computational domain; grid generation; physical/chemical modelling of phenomena (e.g. turbulence models, relative heat transfer, combustion models); defining and specifying fluid properties and boundary conditions of cells relative to another boundary (Versteeg&Malalasekera 2007)[22].

Major CFD models are based on the incompressible Reynolds-Averaged NavierStokes(RANS) equations derived from the main principles of conservation of mass and momentum Sumner et al. (2010)[20]:

At last, post-processor phase analyses the solution results. With the development of CFD packages results in a number of ways of conceptualization of solver outputs. So it is possible to set contours and graphs, perform domain and grid visualizations, visualise vector plots and path-lines, and to perform also dynamic representations using different animations (Sargsyan 2010)[21].

đ?œ•đ?‘ˆ đ?‘–

9. Finite-Volume Method

đ?œ•đ?‘Ľ đ?‘–

đ?‘ˆđ?‘–

=0

đ?œ•đ?‘ˆ đ?‘– đ?œ•đ?‘Ľ đ?‘—

2.15 =−

1 đ?œ•đ?‘ƒ đ?œŒ đ?œ•đ?‘Ľ đ?‘–

+

đ?œ• đ?œ•đ?‘Ľ đ?‘—

đ?‘Ł

đ?œ•đ?‘ˆ đ?‘– đ?œ•đ?‘Ľ đ?‘—

+

đ?œ•đ?‘ˆ đ?‘— đ?œ•đ?‘Ľ đ?‘—

− đ?‘ˆđ?‘– đ?‘ˆđ?‘— + đ??šđ?‘– 2.16

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Most of the commercially viable CFD codes are based on the method of a finite volume discretization (Carcangiu 2008)[28]. The finite6|P a g e

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International e-Journal For Technology And Research-2017 volume method is responsible for sub division of the domain into a different finite number of continuous control volumes, and thus the conservation equations are imposed to those control volumes (Ferziger&Peric 2002).These methods canhandleany type of grid, so it is justified for complex geometries (Ferziger&Peric 2002). A detailed explanation of the finite-volume method is presented in Ferziger&Peric (2002)[29] To summarise, the control-volume technique applied by FLUENT consists in: Dividing the domain into different discrete control volumes using computational meshing; integrating the basic governing equation over the control volumes in order to produce algebraic equations for the discrete variables and use of linearization of the discrete equations and solving them for the resultant equation system (Carcangiu 2008, Versteeg&Malalasekera 2007, Fluent 2011a)[28][21][30].

In ANSYS FLUENT core are available with two numerical methods, which are applied as per several conditions.

Pressure-based solvers wereintroduced for low-speed incompressible flows.Although, the second solver designed as density-based solver, was introduced for application in high-speed compressible flows.The pressure-based solver applies an algorithm for group of methods which were designed to be projection method. In this method, the restriction of mass conservation for velocity field is attained by solving a pressure equation. The pressure equation is originated from the continuity and momentum equation in such a way that velocity field, improved by the pressure fulfils the continuity equation. The overall solution process requires iterations wherein the entire groups of governing equations are solved continuously until the solution converges (Fluent 2011b)[31].

expresses conservation laws based upon the logic of a closed control volume (Fleck 2012)[32].

10. Turbulence Modelling The flow field were defined with the Reynolds averaged navies-stokes equation. the equation were completed with the use of additional turbulent models. This additional transport equation that was solved along with RANS flow equation was the k-ɛ turbulence or k-ω turbulence model.

The flow layer k-ɛ model with standard wall function was used to obtain cell independence but near wall performance is unsatisfactory. Thus for increase accuracy a k-ω model with a Gamma RE theta transition model was introduced after cell independence was reached. The model was implemental with afield function that defines the free stream edge. The k-ω model required more computing resources therefore cell independence was initially reached with the two layer k-ɛ model.

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FLUENT uses a cell-cantered finite-volume technique based on multi-dimensional linear reconstruction scheme. Allowing the application of computational elements with arbitrary polyhedral topology (triangular, quadrilateral, tetrahedral, hexahedral, pyramidal, prismatic) (Mo et al. 2013).

FLUENT also applies a control-volume-based technique to remodel the governing flow equations into algebraic equations that can be numerically solved (Makridis& Chick 2013). This technique consists of conjugating transport equation in each volume, resulting in a discrete equation that

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