This document discusses the design and construction of wind turbine towers for maximum power generation. It begins by introducing the power output equation for wind turbines and the concept of optimum tower height, which aims to maximize average power output relative to total capital costs. Taller towers allow access to higher wind speeds but also increase construction costs. The document then examines various tower designs including steel tubular, concrete, hybrid, and lattice designs. It analyzes factors like costs, transportation, advantages and disadvantages of each design. The document also discusses loads on towers, stiffness requirements, and challenges in increasing tower heights for higher energy output.

1. Design and Construction of
Wind Turbine Towers for
Maximum Power Generation
Hardik Thakkar
Aakash Bagchi
Wind Engineering (Winter 2016)

2. Energy Output and
Construction

3. Introduction
Wind turbine Power output equation:
𝜌 = Density of air
𝐶𝑝 = Power coefficient (Bentz limit of 0.593)
A = Rotor swept area
U = Wind Speed
𝑃 = 𝐶𝑝 𝜌 A U3

4. Concept of Optimum Tower Height
• “Optimum tower height, defined as the height that maximises the ratio of
average power output to the total capital cost of the turbine and the
tower”.
• Increasing tower height is associated with increasing wind speed.
Higher
Energy
Output
High
wind
speed
High
altitude
TALLER
WIND
TOWERS

5. Continued …
• Two growth functions
1. Increase in height  Increase in cost
2. Increase in height  Increase in energy
• Theoretically, the optimum tower height lies at the point where the
two growth functions of construction cost and energy yield intersect.
• In larger turbines, construction costs rise more rapidly with
increasing tower height than in small turbines.
• In regions with a high degree of surface roughness, the wind speed
increases more slowly with height than at shore-based sites

6. Cost Break up
26%
74%
Tower
Other (Generator, Land,
Rotor hub etc.)

7. Comparison of Diameter of Rotor hub, Height of Tower and
Energy Production over years

8. Existing Scenario
Rotor Hub
Rotor
Blades
Tower >80m
Mostly used tower: Steel
Tubular Tower
Length: 80 m
Goal: Design 100m, 120m, 140m
towers
Material used: S355

9. Advantages and Disadvantages of Steel Tubular tower
• Advantages
Cost effective at 80m
Durable
Ductile – Can handle large deformation
High tensile strength
Smaller Foundation
• Disadvantages
More height More Cost
Significant Transportation Issue

10. New Technology in Wind Turbine Towers
• Concrete Turbine Tower
• Hybrid Turbine Tower
• UHCP
• Lattice Turbine Tower
http://www.windfarmbop.com/

11. Concrete Turbine Tower
• Advantages
• An ideal solution for high tower heights and large rotor
diameters
• Transport cost savings due to on-site manufacture
• Higher use of local content
• Readily available
• Success with 135m height
• Long life
• Disadvantages
• Heavier than steel
• Larger Foundation
• Weak in tension

12. Hybrid Turbine Tower
• The hybrid tower comprises a concrete tower with a
height of around 60 meters, which is mounted directly on
the base at the location and then prestressed. It bears the
three steel tower sections of the modular tower with a
total height of a further 60 meters.
• Advantages
• Easy to transport
• Lighter than concrete
• Smaller foundation
• 12% to 15% more energy
• Success with 140m height
• Disadvantages
• Heavier than steel
• Larger Foundation
http://www.evwind.es

13. UHCP-Ultra-High Performance Concrete
The concept of an UHCP is Lattice tower was first
developed by Lewin and Sritharan in 2010.
• Advantages
• Precast sections
• Minimal field labour and crane rental
• Easy to transport
• Stronger in tension
• Members are smaller and lighter
• Disadvantages
• Weaker in tension than steel

14. Lattice Turbine Tower
• Advantages
• Less steel than tubular design-Lighter design
• Easy to assemble
• Lattice structure allows for wide base
• Easy to transport
• PVC-polyester fabric coating encloses structure
• Disadvantages
• Bolted connections may cause issues
• Time consuming

15. Joints in Wind Turbine Towers
• Ring-flange connection
 Flange design
 Flange imperfection
 Ultimate Limit State (ULS)
 Fatigue Limit State (FLS)
• Alternative solution for bolted joints
Slip-on flange
Welding-on flange
Welding-neck flange
Welding-neck flange with defined contact area

16. Comparative cost analysis
• Design cost and scaling model: Projecting the cost
of wind turbine components and subsystems for different
sizes and configurations of components
Factors for comparison
• Height, Power capacity and radius of rotor
• Cost and weight for each component
• Cost per kW-hr for cost efficiency of design
Assumption and limitation
• Operation and Maintenance, Salvage, and other
capital costs were not considered
• Capacity factor may increase as tower height increases

17. Combination cost analysis
• Cost comparison of steel tubular, precast concrete,
hybrid, and UHCP designs
• Cost and scaling model for turbine blades and rotors
• Material, Transportation,
• Cost per kW-hr to compare cost efficiency of designs

18. Cost of Energy
• 𝐶𝑂𝐸 =
𝐹𝐶𝑅×𝐼𝐶𝐶 +(𝐿𝐿𝐶+𝑂&𝑀+LRC+Fees)
𝐴𝐸 𝑛𝑒𝑡
FCR = Fixed Charge Rate
ICC = Initial Capital Cost (includes installed system cost)
LLC = Land Lease Cost
O&M = Operations and Maintenance cost
LRC = Levelized Replacement and overhaul Cost
Fees = Annual insurance warrantees
AEPnet = net Annual Energy Production.

19. • Fixed charge rate : FCR includes construction financing, financing
fees, return on debt and equity, depreciation, income tax, property
tax and insurance.
• Initial capital cost : The initial capital cost is the total cost and the
balance of station cost. Primary Cost elements also include in Initial
cost.
• Annual Operating expenses :
It includes;
• Land lease cost
• O&M cost
• Replacement cost

20. Component of cost
• Control , safety system, condition monitor
• Tower and foundation
• Transportation, Roads and civil works
• Assembly installation
• Electric interface
• Levelized replacement cost

21. Operation and Maintenance
• Day-to-day scheduled and unscheduled maintenance and operations
cost of running a wind farm.
• Periodic monitoring: Periodic monitoring is a specified technical
inspection of the whole structure, tower and foundation.
• Corrosion: tower-shell, flanges and bolts
• Cracks: concrete and welds
• Retrofitting of flange imperfections: Imperfections in form of a
flange-sided taper or parallel gap

22. Design of Turbine Towers

23. Historical Development of the Tower
• First Windmills appeared in China
around 200BC, pumping water
• Other types: Guyed Steel, Mixed
concrete/Steel, FRP etc.
Millhouses
•Low Height
•Voluminous
construction
Lattice (3-D
Truss)
•Taller, Stiffer
•Coming back into
practice
Concrete
•Experimental,
used in Denmark
•Finding favour for
>80m height
towers now
Steel Tubular
Most commonly
used
Knowledge of the
tech results in cost-
effectiveness

24. Codes and Specifications for Wind Tower design
• IEC (International Electrotechnical Commission) 61400-2
• Groups Wind Turbines into classes – I, II, III – based on typical wind
speeds
• Further, each class is classified into – A, B, C – based on wind turbulence
character
• Special class – S – for areas with tropical storms/hurricanes
• CSA-C61400-2 (CSA, 2008)
• (Derived from IEC 61400-2)
• ASCE7 – Wind Loads on structures
• European codes
• GL Rules (Germanischer Lloyd - Guidelines for the Certification of Wind
Turbines) [Germany]
• Guidelines for Design of Wind Turbines – DNV/Riso [Denmark]

25. Goal of the Design
• Increase ‘hub height’
• Correspondingly, increase
tower stiffness
• Minimum cost
• Aesthetics!

26. Loads
Static
• Tower-head weight
• Tower own weight
• Aerodynamic rotor thrust
• From an uniform, steady wind
speed
• Generally, a 50-year return period,
3-second gust taken as extreme
load
Breaking Strength
Analysed by studying the Max
BM distribution
In standard case, the BM acting
at the Tower Base
Dynamic
• Tower Coupling: Rotor
resonating with the tower’s
natural bending frequency
• Vortex-induced vibrations: Of
concern only during
construction, when nacelle and
rotor not yet mounted
Fatigue
• Directly proportional to wind
speed and turbulence
Stiffness (contd..)

27. Buckling
• Most common mode of failure of turbine towers is buckling
• Caused due to extreme loading or fatigue loads
• Trend is to reduce the thickness of tubular steel towers to make
them economical
• In turn, making them susceptible to buckling failure

28. Tower Stiffness
• Rotor exciting forces of two types:
• Occurring with Rotor’s rotational frequency. Arising from mass
imbalances.
• Rotor’s rotational frequency multiplied by the no. of rotor blades.
Aerodynamic imbalances (due to Tower Shadow Effect or Vertical Wind
Shear).
• In a turbine with a three-bladed rotor, the aerodynamic
frequency of excitation occurs at thrice the rotational frequency
of the rotor (3 P).
• Stiffness is kept as low as allowed by the design, for practical
reasons. (economics!)
• Also, as heights rise, it becomes more and more difficult to make stiff
towers

29. Stiff Tower: Natural Frequency greater
than blade-passing frequency(3P)
Soft Tower: Natural Frequency between
blade-passing frequency(3P) and
rotational frequency(P) – [The Working
Range]
Soft-soft Tower: Natural Frequency less
than rotational frequency(P)

30. First Natural Frequency
𝑓 𝑛 =
1
2𝜋
3𝐸𝐼
0.23 𝑚𝑡𝑜𝑤𝑒𝑟 + 𝑚𝑡 𝑢𝑟𝑏𝑖𝑛𝑒 𝐿3
fn = fundamental natural frequency (hz)
E = modulus of elasticity
I = moment of inertia of tower cross-section
mtower = mass of tower
mturbine = mass of turbine
L = height of tower
• For a simple case, the tower is an uniform cantilever with point
mass on top

31. First Natural Frequency
• The first natural bending frequency of the tower should not
coincide with the critical exciting forces
• Also depends on stiffness of foundation and elasticity of soil
• Should be 10% higher (3.1P) than the rotor excitation frequency
• As height of tower increases, natural frequency of the tower
decreases
• Challenge is to keep it higher than the rotor excitation
frequency even for higher (>100m) towers
• But, increasing stiffness would mean very costly towers
• So, instead of stiffer, designers have made towers softer
• Soft-soft towers
• But, what about when starting or stopping the turbine?

32. Tower Shadow effect
• When the turbine blade passes by the tower it creates a shadow
effect
• Each time a blade passes the tower, the air flow experienced by
the blade is disrupted
• Upwind turbines have a reduced a tower shadow effect for the
same blade-tower spacing
• Risk of blade-tower strike
• Requires accurate prediction of blade deflection
• Methods to calculate Tower Shadow effect
• Physical – Using hot-wire anemometers
• Empirical – Steady Wake model, Powles’ model etc.
• CFD – Computational Fluid Dynamics models
Blade-Tower
spacing

33. Other Tower Vibrations
• Torsion frequency
• Coupled with Yaw system of turbine
• Significantly higher than first bending
frequency
• Reduced when using down-wind
turbines with free yaw compared to
rigid hubs

34. Newer Design Methods
• Computation Fluid Dynamics to model Tower Shadow effect
• VMS (Variable Multi-Scale) method to model the complex
aerodynamics of the rotor-tower system
• LIDAR (Light Detection and Ranging) – To provide feed-forward
and feed-back controls for reduced fatigue in towers and energy
maximisation

35. Conclusion
• More power output per turbine will mean building higher
towers to access higher wind speeds
• Higher towers will require new materials to be explored
• It means coming out of the comfort zone wrt steel tubular
towers
• Materials like Prestressed Concrete, UHCP or Lattice structures
• Higher towers will require “stiff” design instead of “soft”
towers designed at present

36. Recommendation
• Considering the various factors having an impact on the
selection of material and design concept:
• Construction cost
• Construction time
• Transportation cost and feasibility
• Stiffness of towers to withstand tower-rotor coupling
• Lattice structures are stiff and economical in terms of material,
but not easy to construct
• Hybrid structures (Base and foundation of concrete and rest of
the tower of steel) or UHCP seem to be the future of wind
turbine towers

37. Thank You for your
attention