5Full Power Conversion Utility-Scale –vs– Small Turbines 2MW Turbine$40-$60/kW3-4% of installed cost10kW Turbine$700-$1000/kW10-20% of the installed cost
6Legacy Issues in Small Turbines Early small turbines were originally designed for off-grid operationPM Synchronous Generators were idealHigh-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
7Objective 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.
8Common Platform for Fixed and Variable Speed? Rotor Speed RegulationGenerator ConfigurationGearboxOverspeed ProtectionPower Conversion for Grid ConnectionVariableLow-speed synchronous (PM or wound rotor)NoneFurling at V>VratedorPitch ControlFull AC-DC-ACHigh-Speed synchronous1- or 2-StageHigh-Speed wound rotor inductionPartial AC-DC-ACHigh-speed squirrel-cage induction2- or 3-stage Variable RatioHigh-Speed Synchronous (wound rotor)FixedHigh-speed squirrel-cage InductionStall + Furling at V>Voutor Pitch ControlLow-speed squirrel-cage inductionLow-Speed synchronous (PM or wound rotor)High-Speed synchronous (PM or wound rotor)ProprietaryAccept an Apples-to-Oranges Platform ComparisonMaintains high-efficiency of PM synch gen for variable speedEliminates gearbox losses for variable speedMaintains low-cost & simplicity of induction machine for fixed speedNot Typical for Turbines <20kW
106kW Wind TurbinePitch Regulation used in Region 3 for power regulationPitch fixed in Region 2Difference in Region 3 power regulation is not unique to fixed or variable speed, so not part of the study
21Impact of Configuration Changes Drop VS range to rpmIncreases energy capture 1%Doubles the generator cost (+5% on turbine cost)Geared High-speed synch gen for VSShaves 3% off cost of turbineIncreases losses 4-5% in the gearboxHalve cost of converter and increase efficiency to 95%Fixed speed still has a 5% advantage in COE
22ConclusionsVariable 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.
23ConclusionsIt 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.
24Recommendations Additional Study of: Wind Class Turbine Scale Alternative Drivetrain configurations