aerodynamics and efficiency of wind turbines

8/11/2019 Aerodynamics and Efficiency of Wind Turbines

1/18

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


2/18

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

2


3/18

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.

3


4/18

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.

4


5/18


6/18

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.

6


7/18

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

7


8/18

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

8


9/18

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

9


10/18

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

10


11/18


12/18

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.

12


13/18

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.

13


14/18

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.

14


15/18

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.

15


16/18

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.

16


17/18

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.

17


18/18

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/

aerodynamics and efficiency of wind turbines

Documents