Review of a Shrouded Wind Turbine for Low Wind Speeds

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Volume 1, Issue 5, May 2017 Available at: www.dbpublications.org
International e-Journal For Technology And Research-2017
IDL - International Digital Library 1 | P a g e Copyright@IDL-2017
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.
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.
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
type‟s ofrenewable energy systems [4].
2. CHALLENGES
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

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 are
explained through the figure below:
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
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

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]
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.
Figure2.2 Actuator disk model for a wind turbine
Conservation of mass inthe stream tube.
𝑚 = 𝜌𝐴1 𝑣1= 𝜌. 𝑆. 𝑣 = 𝜌. 𝐴2. 𝑣2
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 𝜌 and
the area of the turbine is given by S. The
forceexerted on the wind by the rotor:
F = m .a = 𝜌. 𝑆 . 𝑣. (𝑣1 − 𝑣2) (2.1)
Net work done,
dE =F.dx (2.2)
The power of the wind is
𝑃 =
𝑑𝑥
𝑑𝑡
= 𝐹.
𝑑𝑥
𝑑𝑡
= 𝐹. 𝑣 (2.3)
Substituting the force into the power equation will
yield the power extracted from the wind
𝑃 = 𝜌. 𝑆. 𝑣2
. (𝑣1 − 𝑣2) (2.4)
Power can also be computed by using the kinetic
energy
𝑃 =
∆𝐸
∆𝑡
=
1
2
. 𝑚. (𝑣1
2
− 𝑣2
2
)
(2.5)
Put the value of m from equation 2.1 then

𝑃 =
1
2
. 𝜌. 𝑆. 𝑣. (𝑣1
2
− 𝑣2
2
) (2.6)
Equating the two power equation 2.4 and eq.2.6
then
𝑃 =
1
2
. 𝜌. 𝑆. 𝑣. 𝑣1
2
− 𝑣2
2
= 𝜌 . 𝑆. 𝑣2
. (𝑣1 − 𝑣2)
(2.7)
𝑣 =
1
2
. (𝑣1 − 𝑣2) (2.8)
Put the value of v from eq. 8 in power based on
kinetic energy
𝐸 =
1
2
. 𝑚. (𝑣1
2
− 𝑣2
2
) =
1
4
. 𝜌. 𝑆. 𝑣1 + 𝑣2 . (𝑣1
2
−
𝑣2
2
) (2.9)
E=
1
4
. 𝜌. 𝑆. 𝑣1
3
. (1 −
𝑣2
𝑣1
2
+
𝑣2
𝑣1
− (
𝑣2
𝑣1
)3
(2.10)
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
𝑃𝑚𝑎𝑥 =
16
27
.
1
2
. 𝜌. 𝑆. 𝑣1
3
(2.11)
From a cylinder of fluid with cross sectional area S
and velocity v1Theobtainable power is
P =𝑐 𝑃.
1
2
. 𝜌. 𝑆. 𝑣1
3
(2.12)
The total power
𝑃𝑤 =
1
2
. 𝜌 . 𝑆. 𝑣1
3
(2.13)
Power coefficient
𝐶𝑝 =
𝑃
𝑃 𝑤
(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 𝑣1 in eq.13 to find out the total power
available.
5. Theoretical Analysis of Shrouded Rotor
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].
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].
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

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

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].
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.
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].
Major CFD models are based on the
incompressible Reynolds-Averaged Navier-
Stokes(RANS) equations derived from the main
principles of conservation of mass and momentum
Sumner et al. (2010)[20]:
𝜕𝑈 𝑖
𝜕𝑥 𝑖
= 0 2.15
𝑈𝑖
𝜕𝑈 𝑖
𝜕𝑥 𝑗
= −
1
𝜌
𝜕𝑃
𝜕𝑥 𝑖
+
𝜕
𝜕𝑥 𝑗
𝑣
𝜕𝑈 𝑖
𝜕𝑥 𝑗
+
𝜕𝑈 𝑗
𝜕𝑥 𝑗
− 𝑈𝑖 𝑈𝑗 + 𝐹𝑖
2.16
Where, 𝑈𝑖 𝑈𝑗 denote the mean velocity vector, p
represents modified mean pressure, 𝜌 is fluid
density, 𝐹𝑖is a body force.
8. CFD Code Structure
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].
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 user‟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].
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].
9. Finite-Volume Method
Most of the commercially viable CFD codes are
based on the method of a finite volume
discretization (Carcangiu 2008)[28]. The finite-

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].
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
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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