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

2. Variable –vs– Fixed Shaft Speed WHICH IS BETTER?

3. WindPact Conclusion
From Poore & Lettenmaier, NREL/SR , 2003

4. Commercial Small Turbine Configurations
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
7.0
Proven 2.5
2.5
3.5
300
5.0
ARE 110
3.6
n/a
Kestrel E400 3
4.0
Whisper 500
4.5
Endurance 5
5.5
Fixed Asynchronous
206
5.1
Enertech E6
6.7
Proven 6 6
200
4.8
Bergey Excel 10
310
8.2
ARE 442
7.2
Variable PM Alternator
Proven 15 15
9.0
150
5.9
Enertech E10 25
10.4 76
Entegrity EW50 50
15.0 65
6.1
Northwind 100
100
21.0 59
4.6

5. Full Power Conversion Utility-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

6. 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

7. Objective of 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.

8. Common Platform for 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)
None
Furling at V>Vrated or
Pitch Control
Full AC-DC-AC
High-Speed synchronous
1- or 2-Stage
High-Speed wound rotor induction
Partial AC-DC-AC
High-speed squirrel-cage induction
2- or 3-stage Variable Ratio
High-Speed Synchronous (wound rotor)
Fixed
High-speed squirrel-cage Induction
Stall + Furling at V>Vout
or Pitch Control
Low-speed squirrel-cage induction
Low-Speed synchronous (PM or wound rotor)
High-Speed synchronous (PM or wound rotor)
Proprietary
Accept an Apples-to-Oranges Platform Comparison
Maintains high-efficiency of PM synch gen for variable speed
Eliminates gearbox losses for variable speed
Maintains low-cost & simplicity of induction machine for fixed speed
Not Typical for Turbines <20kW

9. Turbine Configurations
Feature
Variable-Speed Turbine
Fixed Speed Turbines
Configuration
3-bladed, Upwind, Grid-Connected
Rotor Diameter
7.0m
Rated Power
6000W
LSS Speed
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
Gearbox
None
1:16.1 Planetary
1:13.7 Planetary
1:10.7 Planetary
Peak GB Efficiency
n/a
95.5%
95.7%
95.8%
Peak Drivetrain Eff.
87.5%
87.8%
Rectifier
Polyphase
Rectifier Peak Eff.
95.0%
Power Converter
Windyboy 7000US
Peak Inverter Eff.
97.0%
Peak Electromech. Efficiency
88.5%

10. 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

11. Component Efficiencies

12. Optimized Blade Planforms
NREL S822 Airfoil
at All Stations

13. Optimized Blade Planforms

14. Cp –vs- TSR
Re=5∙105

15. 171rpm is Too High for Fixed Speed VS is only superior near cut-in
Aerodynamic Power
171rpm is Too High for Fixed Speed
VS is only superior near cut-in

16. This is the comparison that seems to vindicate variable speed
Generator Power
This is the comparison that seems to vindicate variable speed

17. Power Delivered to the Grid

18. Relative Annual Energy Capture
IEC Class 3
Vavg=7.5m/s Vcutout=25m/s
Line losses = 5% Availability=95%

19. Cost of Energy

20. Relative Cost of Energy

21. Impact of Configuration Changes
Drop VS range to 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

22. 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.

23. 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.

24. Recommendations Additional Study of: Wind Class Turbine Scale
Alternative Drivetrain configurations