Presentation on theme: "Influence of Speed Regulation on the Power Performance of Small Wind Turbines Kyle K. Wetzel Wetzel Engineering, Inc. Lawrence, Kansas USA."— Presentation transcript:
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 , 2003
Commercial Small Turbine Configurations TurbineRated Power [kW] Rotor Diameter [m] Speed Regulation/ Generator Type Rated Shaft Speed [rpm] Rated Tip Speed Ratio Skystream Variable PM Alternator Proven Variable PM Alternator ARE Variable PM Alternatorn/a Kestrel E Variable PM Alternatorn/a Whisper Variable PM Alternatorn/a Endurance55.5Fixed Asynchronous Enertech E656.7Fixed Asynchronousn/a Proven 665.5Variable PM Alternator Bergey Excel107.0Variable PM Alternator ARE Variable PM Alternatorn/a Proven Variable PM Alternator Enertech E Fixed Asynchronous764.0 Entegrity EW Fixed Asynchronous656.1 Northwind Variable PM Alternator594.6
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 ConfigurationGearboxOverspeed Protection Power Conversion for Grid Connection Variable Low-speed synchronous (PM or wound rotor)None Furling at V>V rated or Pitch Control Full AC-DC-AC High-Speed synchronous1- or 2-StageFull AC-DC-AC High-Speed wound rotor induction1- or 2-StagePartial AC-DC-AC High-speed squirrel-cage induction2- 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 Induction1- or 2-StageStall + Furling at V>V out or Pitch Control None Low-speed squirrel-cage inductionNone Low-Speed synchronous (PM or wound rotor)None Pitch Control None High-Speed synchronous (PM or wound rotor)1- or 2-StageNone 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
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
Optimized Blade Planforms NREL S822 Airfoil at All Stations
Optimized Blade Planforms
Cp –vs- TSR Re=5∙10 5
Aerodynamic Power 171rpm is Too High for Fixed Speed VS is only superior near cut-in
Generator Power This is the comparison that seems to vindicate variable speed
Power Delivered to the Grid
Relative Annual Energy Capture IEC Class 3 V avg =7.5m/s V cutout =25m/s Line losses = 5%Availability=95%
Cost of Energy
Relative Cost of Energy
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
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