How to design a wind turbine A challenge for the mechanical and electrical engineer Staffan Engström Lecture at Wind Power Systems, KTH Electrical Engineering, 5 October, 2015 Staffan Engström, Ägir Konsult • Swedish Governmental Wind programme 1975 - 91 • Ägir Konsult 1991 – • Nordic Windpower 1991 – 2004 - PreferredÄgir manufacturing Konsult AB partner - Designing a wind turbine Conceptual design 1.Specification of requirements - For what purpose? 2.Principal solutions - What does it look like? Detailed design Withstand 20 (30) years operation Fulfil IEC-standard_ - PreferredÄgir manufacturing Konsult AB partner - Conceptual design Specification of requirements Electricity production? (yes) How large? (multi-MW) Application? (grid) Environment? (forest) Wind conditions? (IEC class III, turbulence++, wind shear++) And much more!_ - PreferredÄgir manufacturing Konsult AB partner - Conceptual design Principal solutions Many!_ - PreferredÄgir manufacturing Konsult AB partner - A large number of possible? concepts... - PreferredÄgir manufacturing Konsult AB partner - …and even more - PreferredÄgir manufacturing Konsult AB partner - Vertical axis: no “new” technology Large resources invested in vertical axis technology in the US, Canada and Great Britain 1975-90 700 turbines of maximum 300 kW produced in series Prototype 4 MW Development during recent years at Vertical Wind, Uppsala/Falkenberg_ Eole 4 MW, Canada, 1990 - PreferredÄgir manufacturing Konsult AB partner - Why not vertical axis? + Harvests winds from all direc- tions – no need for yaw drive + No fatigue due to the weight of the turbine blades (+) Possible to put the generator on the ground - Low RPM, expensive gear-box or direct-drive generator - Large blade area - Exposed to fatigue from wind - Lower efficiency than HA - Only stall control - Noise problems?_ VAWT 850, 500 kW, Great Britain 1990 - PreferredÄgir manufacturing Konsult AB partner - Theory of horizontal axis wind turbines How the air pressure varies Increase in front of the wind turbine Sudden pressure dip in the rotor plane, recovery after Mean pressure dip 1/1000 of atmospheric pressure Local at blade tips 1/10 – enough to hurt a bat!_ - PreferredÄgir manufacturing Konsult AB partner - Concept design cont. Decision: horizontal axis wind turbine - PreferredÄgir manufacturing Konsult AB partner - Concept design cont. Next question: 2 or 3 blades? - PreferredÄgir manufacturing Konsult AB partner - Two or three blades? 1 Three blades dominate totally today History one explanation ”Blade element theory” tell that two-bladers produce only 2-3 % less than three- bladers of same diameter True?_ - PreferredÄgir manufacturing Konsult AB partner - Two or three blades? 2 Production per sq. metre of swept area of five three- bladed wind turbines compared with ditto of five two- Measured production of two- bladed wind turbines at a mean wind speed of 7 m/s. bladed wind turbines in this Source H. Petersen (1997). case 8 % less than three- bladers. A third blade costs less than 8 %. The ”blade element theory” not fully true then! But experience from Nordic Windpower indicates just 2-3 % loss of production As blade element theory! We still learn!_ - PreferredÄgir manufacturing Konsult AB partner - Two or three blades 3 Assembly is easier with two blades!_ - PreferredÄgir manufacturing Konsult AB partner - Two or three blades 4 Decision: three blades - PreferredÄgir manufacturing Konsult AB partner - Concept design cont. Up-wind or Down-wind?: Down-wind: Slightly less energy More fatigue Severe disturbance from thumping noise Up-wind: Keep distance to tower! Need of more rigid blades Possibly self-aligning_ - PreferredÄgir manufacturing Konsult AB partner - Up-wind or down-wind? 2 Decision: Up-wind - PreferredÄgir manufacturing Konsult AB partner - Soft or stiff tower? 1 Soft First bending frequency of tower below blade passage frequency (3p) at rated RPM = Resonant frequency is passed during start-up OK if no coupling between frequencies_ - PreferredÄgir manufacturing Konsult AB partner - Soft or stiff tower? 2 Example Rated RPM =12 RPM = 0,2 Hz = 1p (per rev) 3p = 0,6 Hz First tower bending frequency = 0,5 Hz Overcritical operation Soft tower_ - Preferred Ägir manufacturing Konsult AB partner - Soft or stiff tower? 3 Soft as before Stiff Frequency of tower above blade passage… = Excessive material use if not very low tower Soft-soft Frequency of tower below rotational frequency_ - PreferredÄgir manufacturing Konsult AB partner - Soft or stiff tower? 4 Decision: Soft tower - PreferredÄgir manufacturing Konsult AB partner - Power control 1 Power control is necessary! - PreferredÄgir manufacturing Konsult AB partner - Power control 2 Pitch control: Start and operation - PreferredÄgir manufacturing Konsult AB partner - Power control 3 Pitch control for controlling the power Normal operation Power control Increase of wind speed Change of pitch angle - PreferredÄgir manufacturing Konsult AB partner - Power control 4 Stall control Normal operation Power control Increase of wind speed Turbulence •Passive stall: emergency braking with rotatable blade tip •Active stall: slow control and emergency braking with pitch control - PreferredÄgir manufacturing Konsult AB partner - Power control 5 Pitch control in all new large wind turbines Source DEWI - PreferredÄgir manufacturing Konsult AB partner - Power control 6 Why is pitch control dominating? Stall control is hard to realise in the largest turbines Small technical step from (active) stall In combination with variable RPM For a large turbine the pitch mechanism is a small cost Slightly larger production Increased demand for controllability_ - PreferredÄgir manufacturing Konsult AB partner - Power control 6 Decision: pitch control - PreferredÄgir manufacturing Konsult AB partner - Pitch control – further development 1 Already today a dedicated servo mechanism for each turbine blade Simple and redundant solution Alternative a giant Enercon mechanical brake_ - PreferredÄgir manufacturing Konsult AB partner - Pitch control – further development 2 Collective pitch is state of the art – all blades get same pitch angle Cyclic pitch means that the angle of each blade is determined by its rotational position Potential 15% less fatigue close to the hub*) Individual pitch means that each Näsudden 1 1982 blade gets its individual angle Potential 28% less fatigue*) ”Only” soft-ware modifications Planned already for Näsudden 1 in 1982!_ *) T J Larsen et al (2004) - PreferredÄgir manufacturing Konsult AB partner - Pitch control – further development 3 Controllable trailing edge The trailing edge of the outer part of the blade is flexible and possible to control Piezoelectric actuators Potential 64% less (maximum) blade root moment if 1/3 of the blade is controlled*)_ *) Buhl et al (2007) - PreferredÄgir manufacturing Konsult AB partner - Pitch control – further development 4 Decision: collective pitch control, future up-grading to individual pitch investigated - PreferredÄgir manufacturing Konsult AB partner - Fixed or variable RPM 1 Options for the rotational speed: Fixed Variable In combination with: Pitch control Stall control (already decided Pitch)_ - PreferredÄgir manufacturing Konsult AB partner - Fixed or variable RPM 2 Fixed RPM and pitch control – doesn’t work! Large power fluctuations even in a rather constant wind Faster control creates instability_ Näsudden 1 (2 MW), 1983-88 - PreferredÄgir manufacturing Konsult AB partner - Fixed or variable RPM 3 Fixed RPM and stall control - works NEG-Micon 1500/60 (1,5 MW), 1995-97 Stall control is fast Moderate power quality_ - PreferredÄgir manufacturing Konsult AB partner - Fixed or variable RPM 4 Variable RPM and stall control – works better stallreglering - fungerar bättre Nordic 1000 (1 MW) 1995- Some variation of RPM create less loads and better power quality_ - PreferredÄgir manufacturing Konsult AB partner - Fixed or variable RPM 5 Variable RPM and pitch control – works fine Control of both RPM and pitch angle creates small loads and a good power quality_ Enercon E66 (1,5 MW), 1997 - PreferredÄgir manufacturing Konsult AB partner - Fixed or variable RPM 6 Variable RPM in all large new wind turbines - PreferredÄgir manufacturing Konsult AB partner - Fixed or variable RPM 7 Why is variable RPM dominating? Softer dynamics create smaller loads and a better power quality ”Free of charge” when using synchronous generators (direct drive or geared) + electrical converters Increased production – if any – less important_ - PreferredÄgir manufacturing Konsult AB partner - Fixed or variable RPM 8 Decision: variable RPM - PreferredÄgir manufacturing Konsult AB partner - Electrical systems for variable RPM 1 Double fed induction State of the art for a long time generator (DFIG) and Low in investment frequency converter Slip rings need maintenance (30% of power), and gear Harder to fulfil today’s grid requirements_ - PreferredÄgir manufacturing Konsult AB partner - Electrical systems for variable RPM 2 New requirement – behave like a “power plant” The power system can not accept that a large wind turbine installation drops out due to grid disturbance Also wind turbines have to support voltage and frequency control Governing for the technical solutions selected_ Älvkarleby 530 GWh/år - PreferredÄgir manufacturing Konsult AB partner - Electrical systems for variable RPM 3 Double fed induction State of the art for a long generator (DFIG) and time frequency converter Small