Influence of Speed Regulation on the Power Performance of Small Wind Turbines Kyle K. Wetzel Wetzel Engineering, Inc. Lawrence, Kansas USA

Influence of Speed Regulation on the Power Performance of Small Wind Turbines

Kyle K. WetzelWetzel Engineering, Inc.Lawrence, Kansas USA

Variable –vs– Fixed Shaft Speed

WHICH IS BETTER?

WindPact Conclusion

From Poore & Lettenmaier, NREL/SR-500-33196, 2003

Commercial Small Turbine Configurations

Turbine Rated Power[kW]

Rotor Diameter

[m]

Speed Regulation/Generator Type

Rated Shaft Speed[rpm]

Rated Tip Speed Ratio

Skystream 3.7 1.9 3.7 Variable PM Alternator 325 7.0Proven 2.5 2.5 3.5 Variable PM Alternator 300 5.0ARE 110 2.5 3.6 Variable PM Alternator n/a n/aKestrel E400 3 4.0 Variable PM Alternator n/a n/aWhisper 500 3 4.5 Variable PM Alternator n/a n/aEndurance 5 5.5 Fixed Asynchronous 206 5.1Enertech E6 5 6.7 Fixed Asynchronous n/a n/aProven 6 6 5.5 Variable PM Alternator 200 4.8Bergey Excel 10 7.0 Variable PM Alternator 310 8.2ARE 442 10 7.2 Variable PM Alternator n/a n/aProven 15 15 9.0 Variable PM Alternator 150 5.9Enertech E10 25 10.4 Fixed Asynchronous 76 4.0Entegrity EW50 50 15.0 Fixed Asynchronous 65 6.1Northwind 100 100 21.0 Variable PM Alternator 59 4.6

Full Power ConversionUtility-Scale –vs– Small Turbines

• 2MW Turbine– $40-$60/kW – 3-4% of installed cost

• 10kW Turbine– $700-$1000/kW– 10-20% of the installed cost

Legacy Issues in Small Turbines

• Early small turbines were originally designed for off-grid operation– PM Synchronous Generators were ideal– High-pole-count PM generators also eliminated

the need for gearboxes (high-pole-count induction generators are not efficient)

• Current market for small wind is mostly grid-connected

Objectiveof the Current Study

• Determine whether variable speed operation of a small wind turbine delivers energy to the grid at a cost that is superior to that provided by fixed-speed operation.

• NOT to answer the question of whether variable-speed or fixed-speed operation is generally superior.

Common Platformfor Fixed and Variable Speed?

Rotor Speed Regulation

Generator Configuration Gearbox Overspeed Protection

Power Conversion for

Grid Connection

Variable

Low-speed synchronous (PM or wound rotor) NoneFurling at

V>Vrated

orPitch Control

Full AC-DC-AC

High-Speed synchronous 1- or 2-Stage Full AC-DC-AC

High-Speed wound rotor induction 1- or 2-Stage Partial AC-DC-AC

High-speed squirrel-cage induction 2- or 3-stage Variable Ratio

None

High-Speed Synchronous (wound rotor) 2- or 3-stage Variable Ratio Pitch Control None

Fixed

High-speed squirrel-cage Induction 1- or 2-Stage Stall + Furling at V>Vout

or Pitch Control

None

Low-speed squirrel-cage induction None None

Low-Speed synchronous (PM or wound rotor) NonePitch Control

None

High-Speed synchronous (PM or wound rotor) 1- or 2-Stage None

Not Typical for Turbines <20kW

Proprietary

Accept an Apples-to-Oranges Platform Comparison

1. Maintains high-efficiency of PM synch gen for variable speed

2. Eliminates gearbox losses for variable speed

3. Maintains low-cost & simplicity of induction machine for fixed speed

Turbine Configurations

Feature Variable-Speed Turbine

Fixed Speed Turbines

Configuration 3-bladed, Upwind, Grid-ConnectedRotor Diameter 7.0mRated Power 6000WLSS Speed 100-180 rpm 114 rpm 134 rpm 171 rpm

Generator 48-pole 3-phase PM Synchronous

4-pole 3-phase Squirrel-Cage Induction (Marathon 213TTFS6526FW)

Peak Gen Efficiency 96.0% 91.7%Generator Rating 6500W 6000WGearbox None 1:16.1 Planetary 1:13.7 Planetary 1:10.7 PlanetaryPeak GB Efficiency n/a 95.5% 95.7% 95.8%Peak Drivetrain Eff. 96.0% 87.5% 87.8% 87.8%Rectifier Polyphase n/aRectifier Peak Eff. 95.0% n/a

Power Converter Windyboy 7000US n/a

Peak Inverter Eff. 97.0% n/aPeak Electromech. Efficiency 88.5% 87.5% 87.8% 87.8%

6kW Wind Turbine

• Pitch Regulation used in Region 3 for power regulation

• Pitch fixed in Region 2• Difference in Region 3

power regulation is not unique to fixed or variable speed, so not part of the study

Component Efficiencies

Optimized Blade Planforms

0

100

200

300

400

500

600

0 500 1000 1500 2000 2500 3000 3500

Chor

d Le

ngth

--c

[mm

]

Radial Station -- r [mm]

114rpm

134rpm

171rpm

Variable Speed

-2

0

2

4

6

8

10

12

14

16

18

0 500 1000 1500 2000 2500 3000 3500

Twis

t D

istr

ibuti

on -

-q[°

]

Radial Station -- r [mm]

114rpm

134rpm

171rpm

Variable Speed

NREL S822 Airfoilat All Stations

Optimized Blade Planforms

Cp –vs- TSR

0

0.1

0.2

0.3

0.4

0.5

0.6

0 2 4 6 8 10 12 14 16 18 20

Stea

dy A

erod

ynam

ic P

ower

Coe

ffici

ent -

-Cp

Tip Speed Ratio -- TSR

Fixed Speed 114rpm Rotor


Variable Speed Rotor


Re=5 10∙ 5

Aerodynamic Power

0

1000

2000

3000

4000

5000

6000

7000

0 1 2 3 4 5 6 7 8 9 10

Mea

n Ae

rody

nam

ic R

otor

Pow

er --

P aer

o[W

]

Turbulent Mean Wind Speed -- Vavg [m/s]





IEC 61400-2 Class 3 Turbulence Distribution (18% Average)

171rpm is Too High for Fixed Speed

VS is only superior near cut-in

Generator Power

0

1000

2000

3000

4000

5000

6000

7000

0 1 2 3 4 5 6 7 8 9 10

Mea

n G

ener

ator

Pow

er --

P gen

[W]







This is the comparison that seems to vindicate

variable speed

Power Delivered to the Grid

0

1000

2000

3000

4000

5000

6000

7000

0 1 2 3 4 5 6 7 8 9 10

Mea

n El

ectr

ical

Pow

er --

P ele

c[W

]







Relative Annual Energy Capture29.20

26.69

25.64

26.00

26.15

0 5 10 15 20 25 30

Variable Speed Generator Power

Variable Speed Electrical Power

Fixed Speed 171rpm Electrical Power



Net Annual Energy Production [MWhr/yr]

100.0%

96.1%

97.4%

98.0%

90% 92% 94% 96% 98% 100%

Variable Speed Electrical Power

Variable Speed Generator Power




IEC 61440-2 Class 3

Vavg=7.5m/s Vcutout=25m/s

Line losses = 5% Availability=95%

Cost of EnergyVariable-

Speed Grid Connected

Fixed Speed 171rpm Grid Connected



Rotor 2,167$ 2,167$ 2,417$ 2,667$ Main Frame Integrated 300$ 300$ 300$ Main Bearing & Shaft Integrated 225$ 225$ 225$ Gearbox n/a 525$ 650$ 775$ Generator 2,658$ 625$ 625$ 625$ Controller 450$ 250$ 250$ 250$ Power Converter 5,450$ n/a n/a n/aAll other turbine hardware + OH + markup + S&H 8,721$ 8,721$ 8,721$ 8,721$ Turbine Retail Cost, Delivered 19,446$ 12,813$ 13,188$ 13,563$ 60' Tower, Delivered 11,500$ 11,500$ 11,500$ 11,500$ Balance of Station 6,500$ 6,500$ 6,500$ 6,500$

Turbine Installed Cost 37,446$ 30,813$ 31,188$ 31,563$

Turbine Annualized Cost 3,790$ 3,118$ 3,156$ 3,194$ Annualized Recurring Costs 400$ 400$ 400$ 400$

Total Annual Cost of Ownership 4,190$ 3,518$ 3,556$ 3,594$

Relative Cost of Ownership 117.8% 98.9% 100.0% 101.1%

Net Annual Energy (kWh/yr) 26,693 25,640 26,003 26,150

Levelized Cost of Energy 0.157$ 0.137$ 0.137$ 0.137$ Relative COE 114.8% 100.3% 100.0% 100.5%

Relative Cost of Energy

102.7%

98.6%

100.0%

100.6%

117.8%

98.9%

100.0%

101.1%

90% 95% 100% 105% 110% 115% 120%

Variable-Speed Grid Connected




Relative Energy Production and Cost

Energy Production

Cost of Ownership

Impact of Configuration Changes

• Drop VS range to 60-130 rpm– Increases energy capture 1%– Doubles the generator cost (+5% on turbine cost)

• Geared High-speed synch gen for VS– Shaves 3% off cost of turbine– Increases losses 4-5% in the gearbox

• Halve cost of converter and increase efficiency to 95%– Fixed speed still has a 5% advantage in COE

Conclusions

• Variable speed operation of a grid-connected small wind turbine offers small advantage in terms of energy capture compared to fixed speed operation when the losses associated with the AC-DC-AC power converter are considered. The advantage identified here is on the order of 2%.

• Variable speed operation of a small wind turbine delivers energy to the grid at a cost that is 15% higher than that from the best fixed-speed turbine.

Conclusions

• It is unlikely that improvements in the performance of the power converter, combined with reductions in the cost of the PM synchronous generator or the power converter alone could produce a cost-of-energy advantage for the variable-speed turbine.

Recommendations

• Additional Study of:–Wind Class–Turbine Scale–Alternative Drivetrain configurations

Documents

Influence of Speed Regulation on the Power Performance of Small Wind Turbines Kyle K. Wetzel Wetzel Engineering, Inc. Lawrence, Kansas USA