How to Design a Wind Turbine a Challenge for the Mechanical and Electrical Engineer

How to design a wind turbine A challenge for the mechanical and electrical engineer

Staffan Engström

Lecture at Wind Power Systems, KTH Electrical Engineering, 5 October, 2015 Staffan Engström, Ägir Konsult

• Swedish Governmental Wind programme 1975 - 91 • Ägir Konsult 1991 – • Nordic Windpower 1991 – 2004

- PreferredÄgir manufacturing Konsult AB partner - Designing a wind turbine

 Conceptual design 1.Specification of requirements - For what purpose?

2.Principal solutions - What does it look like?

 Detailed design Withstand 20 (30) years operation Fulfil IEC-standard_

- PreferredÄgir manufacturing Konsult AB partner - Conceptual design

Specification of requirements

 Electricity production? (yes)  How large? (multi-MW)  Application? (grid)  Environment? (forest)  Wind conditions? (IEC class III, turbulence++, wind shear++)  And much more!_

- PreferredÄgir manufacturing Konsult AB partner - Conceptual design

Principal solutions

 Many!_

- PreferredÄgir manufacturing Konsult AB partner - A large number of possible? concepts...

- PreferredÄgir manufacturing Konsult AB partner - …and even more

- PreferredÄgir manufacturing Konsult AB partner - Vertical axis: no “new” technology

 Large resources invested in vertical axis technology in the US, Canada and Great Britain 1975-90  700 turbines of maximum 300 kW produced in series  Prototype 4 MW  Development during recent years at Vertical Wind, Uppsala/Falkenberg_

Eole 4 MW, Canada, 1990

- PreferredÄgir manufacturing Konsult AB partner - Why not vertical axis?

+ Harvests winds from all direc- tions – no need for yaw drive + No fatigue due to the weight of the turbine blades (+) Possible to put the generator on the ground - Low RPM, expensive gear-box or direct-drive generator - Large blade area - Exposed to fatigue from wind - Lower efficiency than HA - Only stall control - Noise problems?_ VAWT 850, 500 kW, Great Britain 1990

- PreferredÄgir manufacturing Konsult AB partner - Theory of horizontal axis wind turbines

How the air pressure varies Increase in front of the wind turbine Sudden pressure dip in the rotor plane, recovery after Mean pressure dip 1/1000 of atmospheric pressure Local at blade tips 1/10 – enough to hurt a bat!_

- PreferredÄgir manufacturing Konsult AB partner - Concept design cont.

Decision: horizontal axis wind turbine

- PreferredÄgir manufacturing Konsult AB partner - Concept design cont.

Next question:

 2 or 3 blades?

- PreferredÄgir manufacturing Konsult AB partner - Two or three blades? 1

 Three blades dominate totally today  History one explanation  ”Blade element theory” tell that two-bladers produce only 2-3 % less than three- bladers of same diameter  True?_

- PreferredÄgir manufacturing Konsult AB partner - Two or three blades? 2

Production per sq. metre of swept area of five three- bladed wind turbines compared with ditto of five two-  Measured production of two- bladed wind turbines at a mean wind speed of 7 m/s. bladed wind turbines in this Source H. Petersen (1997). case 8 % less than three- bladers.  A third blade costs less than 8 %.  The ”blade element theory” not fully true then!  But experience from Nordic Windpower indicates just 2-3 % loss of production  As blade element theory!  We still learn!_

- PreferredÄgir manufacturing Konsult AB partner - Two or three blades 3

Assembly is easier with two blades!_

- PreferredÄgir manufacturing Konsult AB partner - Two or three blades 4

Decision: three blades

- PreferredÄgir manufacturing Konsult AB partner - Concept design cont. Up-wind or Down-wind?:

Down-wind:  Slightly less energy  More fatigue  Severe disturbance from thumping noise Up-wind:  Keep distance to tower!  Need of more rigid blades  Possibly self-aligning_

- PreferredÄgir manufacturing Konsult AB partner - Up-wind or down-wind? 2

Decision: Up-wind

- PreferredÄgir manufacturing Konsult AB partner - Soft or stiff tower? 1

 Soft First bending frequency of tower below blade passage frequency (3p) at rated RPM = Resonant frequency is passed during start-up OK if no coupling between frequencies_

- PreferredÄgir manufacturing Konsult AB partner - Soft or stiff tower? 2

 Example Rated RPM =12 RPM = 0,2 Hz = 1p (per rev)  3p = 0,6 Hz

First tower bending frequency = 0,5 Hz  Overcritical operation  Soft tower_

- Preferred Ägir manufacturing Konsult AB partner -

Soft or stiff tower? 3

 Soft as before  Stiff Frequency of tower above blade passage… = Excessive material use if not very low tower  Soft-soft Frequency of tower below rotational frequency_

- PreferredÄgir manufacturing Konsult AB partner - Soft or stiff tower? 4

Decision: Soft tower

- PreferredÄgir manufacturing Konsult AB partner - Power control 1

Power control is necessary!

- PreferredÄgir manufacturing Konsult AB partner - Power control 2

Pitch control: Start and operation

- PreferredÄgir manufacturing Konsult AB partner - Power control 3 Pitch control for controlling the power

Normal operation Power control

Increase of wind speed

Change of pitch angle

- PreferredÄgir manufacturing Konsult AB partner - Power control 4 Stall control Normal operation Power control

Increase of wind speed

Turbulence

•Passive stall: emergency braking with rotatable blade tip •Active stall: slow control and emergency braking with pitch control

- PreferredÄgir manufacturing Konsult AB partner - Power control 5

 Pitch control in all new large wind turbines

Source DEWI - PreferredÄgir manufacturing Konsult AB partner - Power control 6

Why is pitch control dominating?

 Stall control is hard to realise in the largest turbines  Small technical step from (active) stall  In combination with variable RPM  For a large turbine the pitch mechanism is a small cost  Slightly larger production  Increased demand for controllability_

- PreferredÄgir manufacturing Konsult AB partner - Power control 6

Decision: pitch control

- PreferredÄgir manufacturing Konsult AB partner - Pitch control – further development 1

 Already today a dedicated servo mechanism for each turbine blade  Simple and redundant solution  Alternative a giant

Enercon mechanical brake_

- PreferredÄgir manufacturing Konsult AB partner - Pitch control – further development 2

 Collective pitch is state of the art – all blades get same pitch angle  Cyclic pitch means that the angle of each blade is determined by its rotational position  Potential 15% less fatigue close to the hub*)  Individual pitch means that each Näsudden 1 1982 blade gets its individual angle  Potential 28% less fatigue*)  ”Only” soft-ware modifications  Planned already for Näsudden 1 in 1982!_

*) T J Larsen et al (2004)

- PreferredÄgir manufacturing Konsult AB partner - Pitch control – further development 3

Controllable trailing edge  The trailing edge of the outer part of the blade is flexible and possible to control  Piezoelectric actuators  Potential 64% less (maximum) blade root moment if 1/3 of the blade is controlled*)_

*) Buhl et al (2007)

- PreferredÄgir manufacturing Konsult AB partner - Pitch control – further development 4

Decision: collective pitch control, future up-grading to individual pitch investigated

- PreferredÄgir manufacturing Konsult AB partner - Fixed or variable RPM 1 Options for the rotational speed:

 Fixed  Variable

In combination with:  Pitch control  Stall control (already decided Pitch)_

- PreferredÄgir manufacturing Konsult AB partner - Fixed or variable RPM 2 Fixed RPM and pitch control – doesn’t work!

 Large power fluctuations even in a rather constant wind  Faster control creates instability_

Näsudden 1 (2 MW), 1983-88

- PreferredÄgir manufacturing Konsult AB partner - Fixed or variable RPM 3 Fixed RPM and stall control - works

NEG-Micon 1500/60 (1,5 MW), 1995-97

 Stall control is fast  Moderate power quality_

- PreferredÄgir manufacturing Konsult AB partner - Fixed or variable RPM 4 Variable RPM and stall control – works better stallreglering - fungerar bättre

Nordic 1000 (1 MW) 1995-  Some variation of RPM create less loads and better power quality_

- PreferredÄgir manufacturing Konsult AB partner - Fixed or variable RPM 5 Variable RPM and pitch control – works fine

 Control of both RPM and pitch angle creates small loads and a good power quality_

Enercon E66 (1,5 MW), 1997

- PreferredÄgir manufacturing Konsult AB partner - Fixed or variable RPM 6

 Variable RPM in all large new wind turbines

- PreferredÄgir manufacturing Konsult AB partner - Fixed or variable RPM 7

Why is variable RPM dominating?

 Softer dynamics create smaller loads and a better power quality  ”Free of charge” when using synchronous generators (direct drive or geared) + electrical converters  Increased production – if any – less important_

- PreferredÄgir manufacturing Konsult AB partner - Fixed or variable RPM 8

Decision: variable RPM

- PreferredÄgir manufacturing Konsult AB partner - Electrical systems for variable RPM 1

 Double fed induction State of the art for a long time generator (DFIG) and Low in investment frequency converter Slip rings need maintenance (30% of power), and gear Harder to fulfil today’s grid requirements_

- PreferredÄgir manufacturing Konsult AB partner - Electrical systems for variable RPM 2 New requirement – behave like a “power plant”

 The power system can not accept that a large wind turbine installation drops out due to grid disturbance  Also wind turbines have to support voltage and frequency control  Governing for the technical solutions selected_ Älvkarleby 530 GWh/år

- PreferredÄgir manufacturing Konsult AB partner - Electrical systems for variable RPM 3

 Double fed induction State of the art for a long generator (DFIG) and time frequency converter Small investment (30% of power), and gear Slip rings need maintenance Hard to fulfil the requirements of the grid On its way out!?

 Synchronous generator and Compact with permanent converter (100%) magnets = low cost (with/without gear) Converters getting cheaper

Fulfils grid requirements_

- PreferredÄgir manufacturing Konsult AB partner -

Electrical systems for variable RPM 4 But!

General Electric has

Re-Introduced DFIG - generators due to:

 Advancements in Low Voltage Ride Through – behaviour  The smaller=cheaper converters needed Conclusion: The technichal development is not always straight-forward!

- PreferredÄgir manufacturing Konsult AB partner -

Electrical systems for variable RPM 4 Permanent magnets is the (one) new solution for generators (and motors) 96,7 % at half power  More compact  Better efficiency, 95 % at full power especially at part load  In 2011 prices increased for Neodym magnets, see further discussion _

NewGen 177 kW, measured

- PreferredÄgir manufacturing Konsult AB partner - Electrical systems for variable RPM 5

Decision: synchronous generator with converter

- PreferredÄgir manufacturing Konsult AB partner - With gear or direct drive? 1

Available options:

 Synchronous generator and converter (100%) and gear

 Direct drive synchronous generator and converter (100%) _

- PreferredÄgir manufacturing Konsult AB partner - With gear or direct drive? 2 Generator with gear  Conventional alternative with gear and high-speed generator_

NEG Micon 750/48

- PreferredÄgir manufacturing Konsult AB partner - With gear or direct drive? 3

What’s wrong with gears?

 Gearboxes give too much downtime  (May be better today)_

Ref. P. J. Tavner (2008)

- PreferredÄgir manufacturing Konsult AB partner - With gear or direct drive? 4

Direct drive generator

 The gear is “substituted” by a large generator diameter.  And lots of electrically active material.  And a high weight of the mechanical structure  See man!_

Enercon E126 6 MW, DC excitation, diode rectifier

- PreferredÄgir manufacturing Konsult AB partner - With gear or direct drive? 5

 Increasing market share of direct drive  Offshore wind turbines especially vulnerable for gear problems  27 % of new wind turbines direct drive worldwide (2014)_

Source World Wind Energy Market Update 2015 - PreferredÄgir manufacturing Konsult AB partner - With gear or direct drive? 6

High weight of direct drive generators – kg/kNm

Comparison gear + generator

- PreferredÄgir manufacturing Konsult AB partner - With gear or direct drive? 7

Solution: invent a new generator! 90

40 kg/kNm

Global 2 MW Vestas 3 MW Enercon 2 MW Enercon 6 MW Darwind 5 MW NewGen 4 MW Enercon 2,3 MW Siemens Unison3 MW 0,75Vensys MW 1,5Vensys MW 2,5 MW HarakosanLeitwind 2 MW Scanwind1,5 MW 3,5 MW

- PreferredÄgir manufacturing Konsult AB partner - With gear or direct drive? 8

Conventional direct drive generator

 Air gap ca 5 mm, tolerance ca 0,5 mm.  Long load path means a heavy design

- PreferredÄgir manufacturing Konsult AB partner - With gear or direct drive? 9

New generator, NewGen

 Generator bearings adjacent to air-gap  Minimised load-path and stiffness need  With reduced need for construction material  Enables further increase of diameter  With further reduced need for electrical material _

- PreferredÄgir manufacturing Konsult AB partner - With gear or direct drive? 10

Cross section NewGen  Outside rotor of solid steel with Neodym magnets Pairs of steel wheels support rotor Stator with conventional windings

- PreferredÄgir manufacturing Konsult AB partner - With gear or… 11 Small NewGen ready for testing

- PreferredÄgir manufacturing Konsult AB partner - With gear or direct drive? 12

Chinese monopoly on rare-earth metals problem for direct drives

Neodymium used in advanced magnets

Wind turbine direct drive generators and electric car motors mean increasing need Chinese interests control 97 % of rare earth metal mines, restricts export Rare-earth minerals not ”rare” (more abundant than copper) It takes time to open a new mine, e.g. Stora Kärr, Sweden Prices now stabilising at a higher level (NewGen needs ¼ of magnets of conventional designs)_

- PreferredÄgir manufacturing Konsult AB partner -

With gear or direct drive? 13

 Prices had recovered in 2014

- PreferredÄgir manufacturing Konsult AB partner - With gear or direct drive? 14

Decision: NewGen direct drive

- PreferredÄgir manufacturing Konsult AB partner - How large? 1

D 100 m D/H 125 m H 95 m

 High wind shear due to forest  Increased hub height from 95 to 125 m means +33% kWh/m2  Large turbine cheapest way to increase hub height_

- PreferredÄgir manufacturing Konsult AB partner - How large? 2

 Largest turbine 164 m diameter, offshore (Vestas)  Highest power 8 MW, offshore (Vestas, same turbine)  Danish SSP Technology builds blades for Samsung 171 m diameter turbine (offshore)  LM Windpower foresees 225 m diameter - 20 MW !  Fatigue due to weight will set a limit  Onshore transportation create difficulties over 4? MW_ Repower/Senvion 6 MW

- PreferredÄgir manufacturing Konsult AB partner - How large? 3

 Length of 5 MW blade is 61,5 m (truck additional).  Maximum allowable length of transport 50 – 55 m (on ordinary Swedish roads, with special permit).  Joint adds 10 % to composite blade cost.  Spanish Gamesa builds blades for 4.5 MW turbine in two parts_

- PreferredÄgir manufacturing Konsult AB partner - How large? 4

Possible solution for 5 MW, 125 m

 Partial blade pitch control  Inner steel blade, 20 m  Outer composite blade, 40 m  Cheap steel replaces expensive composite  Blade bearing, pitch mechanism at 20 m radius  Smaller mechanisms cost less_

- PreferredÄgir manufacturing Konsult AB partner - How large? 5

Decision: Ca 5 MW, with partial pitch control

- PreferredÄgir manufacturing Konsult AB partner - Summary of concept design

 Horizontal axis wind turbine

 Three blades  Upwind  Soft tower  Pitch control  Collective pitch  Variable RPM  Synchronous generator  Direct drive, NewGen  5 MW  Partial pitch control _

- PreferredÄgir manufacturing Konsult AB partner - Detailed design 1

 Conceptual design 1.Specification of requirements

2.Principal solutions

 Detailed design

- PreferredÄgir manufacturing Konsult AB partner - Detailed design 2

Purpose of detailed design:

 Meet required specification during 20 (30) years of operation  = Fulfil IEC-standard  With a Lifecycle Cost that is as low as possible  Investment and Maintenance! _

- PreferredÄgir manufacturing Konsult AB partner - Detailed design 3  The design work is governed by the IEC*) standard IEC 61400-1 Wind Turbines. Part 1: Design requirements  Calculations etc. checked by a certification body (e.g. DNV GL, merger of Germanische Lloyd and Det Norske Veritas).  Certification mostly takes years  Mostly design changes are needed to fulfil the standard  Also other standards for components etc _ *) International Electrotechnical Commission

- Preferred Ägir manufacturing Konsult AB partner -

Detailed design 4

 The IEC standard defines three wind turbine classes with a Mean wind speed of I. 10 m/s II. 8,5 m/s III.7,5 m/s  And Turbulence levels of A. 0,16 B. 0,14 C. 0,12  Other values: Class S (Special) – specified by designer  Forest – possibly high turbulence and wind shear! _

- PreferredÄgir manufacturing Konsult AB partner - Detailed design 5

 The IEC standard defines 22 Load cases, covering normal operation, fault conditions and transportation, assembly etc.

- PreferredÄgir manufacturing Konsult AB partner - Detailed design 6

 Calculations for detailed design in time-domain computer models of the entire wind turbine.  Typical models:

Vidyn (Teknikgruppen, Sweden)

x Bladed (GarradHassan, UK) y Flex (Risö, Denmark) Duwecs (Delft University, the Netherlands) _

- PreferredÄgir manufacturing Konsult AB partner - Detailed design 7

 Also the wind field has to be modelled in a proper way.

 Note that the turbine rotation frequency modulates the response to turbulence.

As seen at a stationary point

As seen by a rotating blade_

- PreferredÄgir manufacturing Konsult AB partner - Detailed design 8

 Result of simulation a time series of wind, power, flap moment, yaw moment etc.

 Evaluate for extreme load, fatigue and stiffness/buckling_ _

- PreferredÄgir manufacturing Konsult AB partner -

Detailed design 9

Design drivers:

Component Fatigue Extreme load Stiffness Blades x Machinery bed x Hub x Tower x (x) (x)

 Understand the design by identifying the design drivers  Depends on concept, size etc _

- PreferredÄgir manufacturing Konsult AB partner - Detailed design 10 Bending modes of a three-bladed wind turbine: 2d

es-drive train ec1-pitch/tilt ec2-yaw

 Bending modes of wind turbine couples to rest of installation  Fundamental to understand that a wind turbine is a very slender and vibration prone construction _

- Preferred Ägir manufacturing Konsult AB partner -

Detailed design 11 You should know the following:

You do not know anything until you know everything!

Everything influences everything!

A wind turbine is the most elaborated fatigue machine that has ever been invented!

If anything can go wrong, it will! Murphy

- PreferredÄgir manufacturing Konsult AB partner -

Detailed design 12

How is it then at all possible to do something called a “conceptual design”?

Understandable if engineers tend to be conservative.

Design has to be iterative_

- PreferredÄgir manufacturing Konsult AB partner - Detailed design – materials 1

Composite materals

 Necessary for blade design due to fatigue resistance and weight  At least in the outer parts of the blade  Based on glass fibre (GRP), carbon fibre (CRP) and (today rarely) wood  Use of CRP increasing due to stiffness and weight  Turbine blades largest GRP items produced_

- PreferredÄgir manufacturing Konsult AB partner -

Detailed design – materials 2

Welded steel

 Welded steel shell towers are state of the art  Welds determine fatigue resistance  No use for advanced steels  Transportation problem above ~100 m height  Alternatives bolted designs, precast concrete elements, lattice towers, wood_

- PreferredÄgir manufacturing Konsult AB partner - Detailed design – materials 3

Nodular iron

 Low cost material with a high fatigue resistance  Large use for turbine hubs and machinery beds_

WinWind

- PreferredÄgir manufacturing Konsult AB partner - Detailed design – final! Today it is easy to be a wind turbine manufacturer! (when you know the recipe)

Small amount of work in workshop assembly (~ manmonth/turbine). Most work in component manufacture. Many competent component suppliers today. A vast difference from 1980! The large turbine manufacturers tend to increase vertical integration (but also the opposite tendency).

Workshop manufacture in large series basis for cost reduction.

A large industry that is expanding at a rapid rate!_end

- PreferredÄgir manufacturing Konsult AB partner - The End

(History of Swedish Wind Power. In Swedish.)

- PreferredÄgir manufacturing Konsult AB partner -