A review: diffuser augmented wind turbine technologies

ABSTRACT

Nowadays renewable energy sources play an important role in partially meeting the global energy demand and protecting the environment. Wind energy technology among the renewable sources is developing rapidly around the World as it becomes a challenging renewable energy technology. One of the latest developed technologies that enable small size wind turbines to operate in the most efficient way is the diffuser augmented wind turbine (DAWT) system utilization. In this study, concentrators involving nozzle-diffuser-flange systems were aimed to be reviewed thoroughly by studying the variety of geometries that are mostly available in the literature. Also, the diffuser augmented wind turbine technologies, from the viewpoint of designs, their aerodynamic characteristics, performances, and efficiencies were reviewed extensively. The benefits obtained from cased wind turbines in terms of wind speed and power coefficient enhancements were analyzed considering a variety of different studies found in the literature.

KEYWORDS:

• Power coefficient
• pressure drop
• renewable energy
• wind power
• wind speed
• wind turbine shrouding systems

NOMENCLATURE

Pav	=	:Maximum wind power that is available to be converted into electric power (W)
ρ	=	:The air density (kg/m3)
Ad	=	:Area of the rotor disc, i.e., the rotor swept area (Arotor) (m2)
Vi	=	:Wind speed having the potential of available wind power (m/s)
PTH	=	:Theoretical power delimitated by the Betz limit, maximum accessible turbine power (W)
P1	=	:Pressure at the immediate upstream of the rotor disc (kPa)
P2	=	:Pressure at the immediate downstream of the rotor disc (kPa)
U∞	=	:Free-stream wind speed (m/s)
Uw	=	:Wind speed at the far wake (m/s)
Ain	=	:Inlet area of the stream tube at the far upstream (m2)
Patm	=	:The atmospheric pressure (kPa)
Urotor	=	:Wind speed at the rotor disc (UD), i.e., the hub-height wind speed (m/s)
Frotor	=	:Thrust force of the rotor disc (N)
Aw	=	:Exit area of the stream tube at far wake (m2)
T	=	:Thrust force occurred by the pressure drop in the control volume (N)
a	=	:The axial flow induction factor
P	=	:Mechanical output power of the wind turbine, i.e.,the rotor shaft power (W)
Cp	=	:Power coefficient, i.e., the efficiency
Cpmax	=	:The Betz limit
Fdiffuser	=	:Thrust force of the diffuser excluding the rotor (N)
Uout	=	:Wind speed at the diffuser exit plane (Ub) (m/s)
Aexit	=	:Area of the diffuser exit plane without including the flange heights (m2)
Pexit	=	:Pressure at the diffuser exit plane (Pb) (kPa)
$\epsilon$	=	:Dimensionless area ratio to relate area of the diffuser exit plane (Aexit) and area of the rotor disc (Arotor)
$\mu$	=	:Dimensionless area ratio to relate inlet area of the stream tube (Ain) and area of the diffuser exit plane (Aexit)
$\mu \epsilon$	=	:Dimensionless area ratio to relate inlet area of the stream tube (Ain) and area of the rotor disc (Arotor)
Cp,rotor	=	:Power coefficient of the rotor with turbine in the case of casing utilization
$\dot{m}$	=	:Mass flow rate of the air passing through the rotor disc (kg/s)
L	=	:Casing full body length including the nozzle component, or full body length of a diffuser in the casing if there is not a nozzle component, or scoop full body length (m)
L1	=	:Length of the nozzle component in the casing, or entrance axial length of a scoop (m)
Ly	=	:Length of the diffuser component in the casing (m)
H	=	:The flange height (m)
D1	=	:Inlet diameter of the nozzle component or diffuser inlet diameter in the absence of the nozzle component in a casing (m)
D2	=	:The casing exit diameter (m)
D	=	:Diameter of the location where the wind turbine is installed in the casing (if the turbine is installed in the narrowest cross-sectional diameter in the casing, it is shown by the throat diameter - Dt) (m)
Drotor	=	:The rotor diameter (m)
Cpr	=	:Pressure drop that occurs along the inside of the casing, i.e., the dimensionless pressure coefficient
Cprmax	=	:Maximum pressure drop ratio determined in the pressure drop distribution
ɣ	=	:Nozzle taper angle of the casing (o)
Ѳ	=	:Half cone angle of the diffuser component of the casing (o)
H/D	=	:The flange-height ratio, i.e., the dimensionless flange height
x/D	=	:Dimensionless axial coordinate system defined with respect to the cross sectional diameter of the wind turbine location
u/U∞	=	:Wind speed ratio determined along the inside of the casing
$C_{pmax}^{\ast }$	=	:The theoretical estimation of the maximum power coefficient
${\lambda }_R$	=	:Blade tip speed ratio
n	=	:Number of blades in a wind turbine
Cd	=	:Drag coefficient
Cl	=	:Lift coefficient
G.M.T.	=	:General momentum theory
T1	=	:Turbine 1
T2	=	:Turbine 2
T3	=	:Turbine 3
T4	=	:Turbine 4
T5	=	:Turbine 5
u(x)	=	:Axial velocity distribution inside the casing (m/s)
2Ѳ	=	:Full cone angle of the diffuser component of the casing (o)
x/L	=	:Dimensionless axial coordinate system inside the casing
Umax/U∞	=	:Maximum wind speed ratio (K)
L/D1	=	:Dimensionless axial full body length of the casing, or diffuser length ratio in the casing in the absence of the nozzle component
µ*	=	:The diffuser exit to inlet area ratio, i.e., D22/ D12 (For this expression to be valid for this definition of µ*; the diffuser casing should not involve an inlet shrouding and the condition D1=D=Dt should be satisfied)
x	=	:Axial coordinates inside the casing (m)
rdir	=	:Radial coordinates perpendicular to the axial coordinates (m)
u	=	:The axial wind speed (m/s)
v	=	:The radial wind speed (m/s)
Umax	=	:Maximum axial wind speed obtained inside the casing (m/s)
Vmax	=	:Maximum radial wind speed obtained inside the casing (m/s)
Vx	=	:Magnitude of the axial wind velocity (u) (m/s)
Vy	=	:Magnitude of the radial wind velocity (v) (m/s)
Uuv/U∞	=	:The y-axis demonstrating u and v wind speeds by the ratio of U∞
AFSCS	=	:Airfoil structured casing system
CT	=	:Thrust coefficient
xt	=	:The axial location of the wind turbine inside the casing shroud (m)
W.T.	=	:The abbreviation for “Wind Turbine”
Dh	=	:Turbine hub diameter (m)
Ls	=	:Turbine axial length including whole mechanical mechanism (m)
α	=	:The angle formed in clockwise direction between the diameter line of a single Savonius blade with the axial axis (o)
d	=	:Diameter of a single Savonius blade (m)
s	=	:Piece of diameter length in a Savonius rotor, which both blades intersect (so, s=2.d-Drotor) (m)
Фp	=	:The diameter of the pivot axle around which both Savonius blades rotate (m)
L/D	=	:Dimensionless full body length of the casing
Ct	=	:The loading coefficient
Lst	=	:Axial length to measure the distance of the point where the cross section of the truncated triangle shroud is cut, measured with respect to the flange cross-sectional plane (m)
Ф1	=	:Entrance diameter of a scoop (m)
Ф2	=	:Exit diameter of a scoop (m)
L2	=	:Axial length given for the exit section of the scoop (m)
Фc	=	:Diameter of the cylindrical section of the scoop (m)
Rc	=	:Radius of the cylindrical section of the scoop without the thickness (m)
Rct	=	:Radius of the cylindrical section of the scoop including the thickness (m)
Lc	=	:Axial length of the cylindrical section of the scoop (m)
D1/D	=	:Dimensionless inlet diameter of the casing
D2/D	=	:Dimensionless exit diameter of the casing
D0	=	:Exit diameter of the casing, including the flange heights (m)
rdir/D	=	:Dimensionless radial coordinate system inside the casing
U1	=	:Fluid velocity at the immediate upstream of the rotor disc (m/s)
U2	=	:Fluid velocity at the immediate exit after the rotor contact (i.e., it is existed in the plane where P2 is existed) (m/s)
Uout	=	:Wind speed determined at the diffuser exit plane (Ub) (m/s)
P0	=	:Pressure at far upstream, i.e., before the casing interaction (kPa)
Ltr	=	:Length of the perpendicular edge of the triangular section of the diamond cross sectional shroud (parallel to the axial axis) (m)
L/D2	=	:Dimensionless full body length of the casing according to the exit diameter
p	=	:Local pressure value (kPa)