aerodynamics and efficiency of wind turbines
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23/09/2014Aerodynamics / WEC
EFFICIENT WIND POWER GENERATION
Prepared for Dr. M. K. Deshmukh by Siddhant
Pardeshi, 2012A3PS200G
Siddhant Pardeshi2012A3PS200G
EEE F473: WIND ELECTRICAL SYSTEMS
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Contents
1.
Authors Note 3
2.
Key Concepts and Definitions 4
2.1 Aerodynamics
2.2 Efficiency
2.3 Aerodynamic Efficiency
2.4 Airfoil
2.5 Yaw
2.6 Pitch
2.7 Tip Speed Ratio
3. Research Questions 5
3.1 How does Aerodynamics influence Wind Turbine Blade Design?
3.2 What is the Effect of Number of Blades on Wind Turbine Efficiency?
3.3 How are Turbine Blades controlled for maximum Aerodynamic Efficiency?
3.4 How is Aerodynamic Power of Wind Turbines regulated?
3.5 How does the size of the Wind Turbine affect its Efficiency?
3.6 What is Wake Rotation? Describe its relationship with the Betz Limit.
3.7 How can we improve Aerodynamic Efficiency of Wind Turbines?
3.8 How does Aerodynamic Modelling Software work?
3.9 Is Wind Power alone sufficient to fulfil the demands of an industrial nation?
3.10 How can Emerging Technologies increase Efficiency of Wind Turbines?
4.
Conclusion 17
5.
References 18
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Authors Note
The primary application of a wind turbine is to extract energy from the wind. Therefore,
aerodynamics and the associated aerodynamic efficiency is a very important aspect of wind turbines.
My interest in Wind Energy is about the generation of Wind Power more efficiently. Hence, my
analysis of Aerodynamics and WECS in this report has been from the point of view of optimum
aerodynamic design with the objective of maximizing efficiency of the wind turbine subject to
practical and design constraints.
During the course of my analysis, Ive generated 10 sequential questions. Ive discussed 6 of
these in my presentation and have included them in this report after incorporating the changes
suggested by my guide and instructor, Dr. M.K. Deshmukh. I have tried to keep all questions basic in
order to promote clearer understanding of concepts and to make the material interesting for
readers of all interests.
I would like to thank Dr. M. K. Deshmukh for allotting me this interesting topic of
aerodynamics that is well suited to my interest, as I have discovered over the course of my research.
I would also like to thank my friends and colleagues for participating in the discussion questions of
my presentation and for their valuable feedback. I look forward to the readers suggestions for any
changes to be made in this report which will be duly incorporated if valid.
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Key Concepts Definitions
For the rest of this report, it has been assumed that the reader is familiar with the following
key concepts and definitions.
Aerodynamics
Aerodynamicsis the property of a solid object regarding the manner in which air flows around it.
Source: NASA
Efficiency
Efficiency is about achieving maximum productivity with minimum wasted effort or expense.
Source: Websters Dictionary
Aerodynamic Efficiency
Aerodynamic Efficiency is that property of a system by which efficiency is attributed to its
aerodynamic design. It is the interaction of aerodynamicsand efficiency.
Na = useful power extracted from the wind
Power supplied by the wind
Airfoil
An Airfoil or Aerofoil is a structure with curved surfaces designed to give the most favourable ratio
of lift to drag in flight, used as the basic form of the wings, fins, and tail-planes of most aircraft
Yaw
To yaw implies to twist or oscillate about a vertical axis. The yawsystem of windturbines is the
component responsible for the orientation of the windturbine rotor towards the wind.
Pitch
Blade pitchor simply pitchrefers to turning the angle of attack of the bladesof a propeller or
helicopter rotor into or out of the wind to control the production or absorption of power. Wind
turbines use this to adjust the rotation speed and the generated power.
Tip Speed Ratio
The tip-speed ratio, , or TSR forwindturbines is the ratiobetween the tangential speedof
the tipof a blade and the actual velocityof the wind. The tip-speed ratiois related to efficiency,
with the optimum varying with blade design.
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Even though flow velocity is uniform along the length of the blade, blade velocity increases
linearly as we move to the tip. So angle and magnitude of relative velocity (apparent velocity) of
wind varies along the length of the blade. Apparent velocity becomes more aligned to chord
direction as we move to the tip.
Thus, there should be a continuous twist in the blade, so that at every airfoil cross section
angle of attack is optimum.
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Question 2: What is the Effect of Number of Blades on Wind Turbine
Efficiency?
As the number of blades in the wind turbine increases aerodynamic efficiency increases, but
in a diminishing manner. When we move from 2 blades to 3 blades design efficiency gain is about
3%. But as we move from 3 blades to 4 blades design, efficiency gain is marginal.
No. of blades From 1 to 2 From 2 to 3 From 3 to 4Many to
Infinite
% Efficiency
Increase6% 3% 0.5% ~0%
As we increase number of blades, cost of the system increases drastically. Along with that
mechanical design of blades also becomes a difficult affair. With more number of blades, blades
should be thinner to be aerodynamically efficient. But blades with thinner portion at the root may
not withstand bending stress induced due to axial wind load. So generally wind turbines with 3
blades which can accommodate a thicker root cross-section are used.
Wind turbine blades need a thicker root to withstand huge bending moment induced
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Question 3: How are Turbine Blades controlled for maximum
Aerodynamic Efficiency?
Listed below are some of the important factors that are used to design and control wind
turbine blades
1. Aerofoil Shape
The Aerofoil shape of wind turbine blades is usually similar to the Wings of an Airplane.
However the lift-to-drag ratio is 120, compared to 70 for sailplanes & 15 for airliners.
2. Yawing
Wind turbines are controlled to face the wind direction measured by a wind
vane situated on the back of the nacelle. By minimizing the yaw angle the power output is
maximized and non-symmetrical loads minimized.
Percent output vs Yaw Angle
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3. Pitching (Twist and Taper)
Wind condition can change at any time. It is possible to rotate a wind turbine blade on
its own axis, in order to achieve optimum angle of attack with varying wind conditions. This
is known as pitching of blades. A clever algorithm which uses wind condition and
characteristics of wind turbine as input, governs the pitch angle for the maximum powerproduction.
Pitching of Wind Turbine blades
4. Weight and Material
Material Wood Steel Aluminium Fibreglass
Strengths
Abundant
Relatively Cheap
Quite Strong
StrongLightweight
Strong
Lightweight
Inexpensive
Good Fatigue
Characteristics
WeaknessesNot Suitable for
Commercial use
Expensive
Heavy
Expensive
Subject to Metal
Fatigue
Relatively none
Fibreglass is the material used in most modern large wind turbines due to the above
advantages.
5. Tip Speed Ratio
High efficiency 3-blade-turbines generally have tip speed ratios of 6 to 7.
Variation of Cp with TSR
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Question 4: How is Aerodynamic Power of Wind Turbines regulated?
The power regulation of WPPs is done by aerodynamic, electric and electronic control. The
following aerodynamic control methods are popularly being adapted by turbine manufacturers to
regulate the power of large wind turbine rotors, although other types of aerodynamic control are
still under experimentation and research.
Sr.
No.Passive Stall Control Active Stall control Pitch control
1.
The stall profiled blades are
mounted at a fixed angle on the
hub
The stall profiled
blades are pivot able
for few angles in the
longitudinal axis
The rotor blades are
almost infinitely pivot
able in the opposite
direction to the active-
stall blades from 0 to 90
degrees in the
longitudinal axis
2.Cheapest due to simplicity in
constructionModerately Expensive
Expensive due to
sophisticated
construction
3. Less fluctuations in power outputConstant power output
at high wind speeds
Liable to fluctuations in
power output
4. Comparatively higher fatigue loads
Reduced aerodynamic
loads, peak torques and
lower fatigue loads
5.Responds most quickly to gusty
winds
Unstrained during
gusty winds and power
control more accurate
than passive stall
Mechanical pitch
adjustments require
finite response time to
effect power control
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Question 6: What is Wake Rotation? Describe its relationship with the
Betz Limit.
The wind turbine described by Betz does not actually exist.It is merely an idealized wind
turbine described as an actuator disk. It's a disk in space where fluid energy is simply extracted from
the air. In the Betz turbine the energy extraction manifests itself through thrust. The equivalentturbine described by Betz would be a horizontal propeller type operating with infinite blades at
infinite tip speed ratios and no losses. The tip speed ratio is ratio of the speed of the tip relative to
the free stream flow. This turbine is not too far from actual wind turbines. Actual turbines are
rotating blades.They typically operate at high tip speed ratios. At high tip speed ratios three blades
are sufficient to interact with all the air passing through the rotor plane. Actual turbines still produce
considerable thrust forces.
One key difference between actual turbines and the actuator disk, is that the energy is
extracted through torque. The wind imparts a torque on the wind turbine, thrust is a necessary by-
product of torque. Newtonian physics dictates that for every action there is an equal and opposite
reaction. If the wind imparts a torque on the blades then the blades must be imparting a torque on
the wind. This torque would then cause the flow to rotate. Thus the flow in the wake has two
components, axial and tangential. This tangential flow is referred to as wake rotation.
Torque is necessary for energy extraction. However wake rotation is considered a loss.
Accelerating the flow in the tangential direction increases the absolute velocity. This in turn
increases the amount of kinetic energy in the near wake. This rotational energy is not dissipated in
any form that would allow for a greater pressure drop (Energy extraction). Thus any rotational
energy in the wake is energy that is lost and unavailable. This loss is minimized by allowing the rotor
to rotate very quickly. To the observer it may seem like the rotor is not moving fast; however, it is
common for the tips to be moving through the air at 6 times the speed of the free stream.Newtonian mechanics defines power as torque multiplied by the rotational speed. The same amount
of power can be extracted by allowing the rotor to rotate faster and produce less torque. Less
torque means that there is less wake rotation. Less wake rotation means there is more energy
available to extract.
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Question 7: How can we improve Aerodynamic Efficiency of Wind
Turbines?
3D Software Modelling is a highly useful tool that can be used for blade design and siting of
wind power projects without having to actually implement them. The following different types of
software are available depending on the needs of the WPP designer.
Turbine Design
FOCUS6:For design of Wind turbine components like rotor blades. Developed in
Netherlands.
FAST:An aero-elastic simulator developed by NWTC (USA) that uses Blade Element
Momentum (BEM) theoryto model turbine performance in turbulent flow of air.
QBlade:An open source software developed by Hermann Fttinger Institute of TU Berlin. It
is a BEM code coupled with the airfoil simulation code XFOIL.
Air Flow Modelling
WAsP: Uses apotential flow model to predict how wind flows over terrain at a site.
WindSim: Uses CFD. More accurate, but computationally expensive
Farm Modelling
OpenWind, WindPRO, WindSim: Useful for simulation and prediction of Site Behaviour.
Using these software suites, designers can optimizeand observe blade geometry, twist, yawangle, angles of wind attack and wind velocity for the required efficiency. Thus, Aerodynamic
Efficiency of Wind Turbines can be improved specific to the site.
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Question 8: How does Aerodynamic Modelling Software work?
3D Modelling software generally applies BEM (Blade Element Monitoring) theory due to its
simplicity and overall accuracy, but its originating assumptions limit its use when the rotor disk is
yawed, or when other non-axisymmetric effects (like the rotor wake) influence the flow.Limited
success at improving predictive accuracy has been made using computational fluid dynamics(CFD) solvers based on Reynolds-averaged NavierStokes (RANS) and other similar three-
dimensional models such as free vortex methods. These are very computationally intensive
simulations to perform for several reasons. First, the solver must accurately model the far-field flow
conditions, which can extend several rotor diameters up- and down-stream and include atmospheric
boundary layer turbulence, while at the same time resolving the small-scale boundary-layer flow
conditions at the blades' surface (necessary to capture blade stall).
In addition, many CFD solvers have difficulty meshing parts that move and deform, such as
the rotor blades. Finally, there are many dynamic flow phenomena that are not easily modelled by
RANS, such as dynamic stall and tower shadow. Due to the computational complexity, it is not
currently practical to use these advanced methods for wind turbine design, though researchcontinues in these and other areas related to helicopter and wind turbine aerodynamics.
Free vortex models (FVM) and Lagrangian particle vortex methods (LPVM) are both active
areas of research that seek to increase modelling accuracy by accounting for more of the three-
dimensional and unsteady flow effects than either BEM or RANS. FVM is similar to lifting line theory
in that it assumes that the wind turbine rotor is shedding either a continuous vortex filament from
the blade tips (and often the root), or a continuous vortex sheet from the blades' trailing
edges. LPVM can use a variety of methods to introduce vorticity into the wake.
BiotSavart summation is used to determine the induced flow field of these wake vorticies'
circulations, allowing for better approximations of the local flow over the rotor blades. Thesemethods have largely confirmed much of the applicability of BEM and shed insight into the structure
of wind turbine wakes. FVM has limitations due to its origin in potential flow theory, such as not
explicitly modelling model viscous behaviour (without semi-empirical core models), though LPVM is
a fully viscous method. LPVM is more computationally intensive than either FVM or RANS, and FVM
still relies on blade element theory for the blade forces.
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Question 9: Is Wind Power alone sufficient to fulfil the demands of an
industrial nation? What are the limitations to Wind Turbine Efficiency?
The following illustration describes the situation.
According to Betz Limit, Maximum Efficiency of Wind Turbine < 59 %.
Currently, Maximum Efficiency obtainable with a Propeller-Type windmill is roughly 47 %.
This occurs Propeller Tip Speed = 5 times or 6 times wind velocity.
For a given rotor speed, efficiency drops rapidly as the wind velocity decreases.
Power obtainable (rotor diameter)^2 and Power obtainable (wind velocity)^3
Based on the above data, consider the following scenario:
For rotor diameter = 30m & wind speed = 14 m/s, Max energy obtainable at 59%
efficiency = 690 kW
Due to variations, if Wind Speed reduces to 7 m/s, the same Max energy obtainable
drops to = 86 kW
At this lower wind speed, we would require 17000 turbinesof 30m diameter
operating at 40% efficiency to match the output of a single 1000 MW central power
station.
With current technology, if the site is unsuitable, wind turbines alone can never meet the
power demands of an industrialized nation.
The above calculations are only subjective. Wind power alone is anyway not intended to
serve as the sole power source of an industrial nation. Our intention is to use it optimallyalong with other sources of renewable energy. However, the above question exposes
where we stand at current levels of technological developments.
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Question 10: What are the Emerging Technologies to increase
Aerodynamic Efficiency of Wind Turbines?
A radical development has been expressed in terms of The Saphonian A Wind Turbine
with ZeroBlades.
Non-Rotational. Not restricted by Betz Limit.
2.3 times more efficient than an average wind turbine.
Costs 45% less than conventional wind turbine
No blades, no hub and no gearbox. No noise.
Working: Kinetic Energy of wind is stored via a hydraulic accumulator or is converted to
electricity via a hydraulic motor or generator.
The above development is an initiative in an entirely new direction to implement a bladeless
design of wind turbines. The validity of the Companys claims is currently subjected to testing and
results are expected within the next 24 months.
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Conclusion
The Key Observations of my Report are as follows
1. The shape and orientation of blade cross sectionis an important factor that affects wind
turbine performance. Aerofoil cross sections of blades are designed for optimum values of
angle of attack for maximum performance.
2. The optimum number of blades for a wind turbine that generates electricity is 3. Increasing
the blade count beyond this yields negligible improvements in aerodynamic efficiency,
unfavorable increase in blade stiffnessand undesirable increase in overall cost.
3. Turbine blades are designed for maximum aerodynamic efficiency while considering design
constraints. Optimum values of design parameters like TSR, blade dimensions, turbine size,
etc. are chosen based on site analysis.
4. Passive Stall Control, Active Stall Control and Pitch Stall Control are 3 methods by which
Aerodynamic Power of Wind Power Plants can be regulated. Each methods has its
characteristic advantages and drawbacks
5. Bigger turbines are more efficient and allow for more energy conversion, however they do
have certain practical limitations. Turbine size has quadrupledover the past 30 years.
6. The wind turbine described by Betz does not actually exist.It is merely an idealized wind
turbine described as an actuator disk. The same amount of power can be extracted by
allowing the rotor to rotate faster and produce less torque. Less torque means that there is
less wake rotation. Less wake rotation means there is more energy available to extract.
7. 3D Software Modelling can help improve Aerodynamic Efficiency by allowing designers to
model wind turbine parts and tweak its parameters for optimal performance.
8. Aerodynamic Modelling Software uses computational theories like BEM, CFD, RANS, FVM
and LPVM to simulate Wind Turbines and their operational conditions.
9. Although we have achieved 47 % efficiency in wind turbines, increased reliability on wind
energy will require further advancements in technology.
10.Future Wind Turbine designs can be bladelessand will have twice the efficiency of todays
turbines, while being cost-effectiveand noise-free.
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References
Eric Hau (ed.), Wind Turbines Fundamentals, Technologies, Applications, Economics 2nd
Edition, Springer 2006. Page 121
Jamieson, Peter. Innovation in Wind Turbine Design sec11-1,John Wiley & Sons, 5 July 2011.
Earnest, Joshua: Wind Power Technology, First Edition, PHI 2014.
Bhadra, S.N., Kastha D. and Banerjee, S.: Wind Electrical Systems, OUP 2005.
www.wmc.edu
www.ecn.nl
www.learnengineering.org/2013/08/Wind-Turbine-Design.html
www.nasa.gov/audience/forstudents/k-4/stories/what-is-aerodynamics-k4.html
http://www.britannica.com/EBchecked/topic/609552/turbine/45698/Wind-farms
en.wikipedia.org/wiki/Wind_turbine_design
www.saphonenergy.com/site/en/the-saphonian.2.html
http://www.wmc.edu/http://www.wmc.edu/http://www.ecn.nl/http://www.ecn.nl/http://www.learnengineering.org/2013/08/Wind-Turbine-Design.htmlhttp://www.learnengineering.org/2013/08/Wind-Turbine-Design.htmlhttp://www.nasa.gov/audience/forstudents/k-4/stories/what-is-aerodynamics-k4.htmlhttp://www.nasa.gov/audience/forstudents/k-4/stories/what-is-aerodynamics-k4.htmlhttp://www.britannica.com/EBchecked/topic/609552/turbine/45698/Wind-farmshttp://www.britannica.com/EBchecked/topic/609552/turbine/45698/Wind-farmshttp://en.wikipedia.org/wiki/Wind_turbine_designhttp://en.wikipedia.org/wiki/Wind_turbine_designhttp://www.saphonenergy.com/site/en/the-saphonian.2.htmlhttp://www.saphonenergy.com/site/en/the-saphonian.2.htmlhttp://www.saphonenergy.com/site/en/the-saphonian.2.htmlhttp://en.wikipedia.org/wiki/Wind_turbine_designhttp://en.wikipedia.org/wiki/Wind_turbine_designhttp://www.britannica.com/EBchecked/topic/609552/turbine/45698/Wind-farmshttp://www.britannica.com/EBchecked/topic/609552/turbine/45698/Wind-farmshttp://www.nasa.gov/audience/forstudents/k-4/stories/what-is-aerodynamics-k4.htmlhttp://www.learnengineering.org/2013/08/Wind-Turbine-Design.htmlhttp://www.ecn.nl/http://www.wmc.edu/
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