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

Variable –vs– Fixed Shaft Speed WHICH IS BETTER?

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

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

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

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

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.

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

Turbine Configurations Feature Variable-Speed Turbine Fixed Speed Turbines Configuration 3-bladed, Upwind, Grid-Connected Rotor Diameter 7.0m Rated Power 6000W LSS 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 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%

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 NREL S822 Airfoil at All Stations

Optimized Blade Planforms

Cp –vs- TSR Re=5∙105

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

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

Power Delivered to the Grid

Relative Annual Energy Capture IEC 61440-2 Class 3 Vavg=7.5m/s Vcutout=25m/s Line losses = 5% Availability=95%

Cost of Energy

Relative Cost of Energy

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