Aerodynamics of Wind Turbine Control Systems By Chawin

Aerodynamics of Wind Turbine Control Systems By Chawin Chantharasenawong 21 August 2009

Chawin Chantharasenawong ชวน จนทรเสนาวงศ Academic staff at Department of Mechanical Engineering King Mongkut’s University of Technology Thonburi (KMUTT) มหาวทยาลยเทคโนโลยพระจอมเกลาธนบร

Our work with wind energy

Today’s topic � Large wind turbines � Types of wind turbines � Turbine blades � Types of power controls � How they work? � Pros and cons � Conclusions

Large wind turbines � Large wind turbines are wind turbines with rated power over 1 MW � Wind turbines are typically classified into � Horizontal axis wind turbines (HAWTs) � Vertical axis wind turbines (VAWTs)

Horizontal Axis Wind Turbines

Vertical Axis Wind Turbine

Advantages of large HAWTs and VAWTs HAWTs � Due to the atmospheric boundary layer, wind speeds are usually higher with altitude. The power available is proportional to the wind seed to the power of three. VAWTs � Easy maintenance as it is close to ground � Wind direction does not affect the power production � No high tower required, hence lower initial cost

Disadvantages of large HAWTs and VAWTs HAWTs � Large modern wind turbines consist of over 90 m high towers and 40 m long blades. Transportation is difficult and costly. � Dangers to birds in the vicinity VAWTs � Lower efficiency when compared to an equivalent HAWT because part of the VAWT is always turning against the wind without producing power � VAWTs cannot be installed on a high tower to take advantage of the high speed wind due to vibration problems � The turbine must be dismantled for maintenance

Optimum number of blades � For �B optimum number of blades we need to consider : number of blades � CL/CD : aerofoil characteristics � Lambda : tip speed ratio i c i f Ef y c en

Optimum number of blades More blades 1 blade * Calculation of Cp max is performed using information from ‘Wind Energy Explained’ by Manwell et al. and Wilson et al. (1976)

Optimum number of blades

Optimum number of blades Consider a wind turbine operating at tip speed ratio = 5. 1 2 3 4 5 6 7 Power coefficient 0. 467 0. 507 0. 525 0. 536 0. 543 0. 548 0. 552 Improvement 0, 56 0, 54 8. 57% 3. 55% 2. 10% 1. 31% 0. 92% 0. 73% 0, 52 Power coefficient Number of blades 0, 5 0, 48 0, 46 0, 44 0, 42 0 1 2 3 4 5 6 Number of blades 7 8

Efficiency of blade section � Blade section characteristics � Preference High lift to produce torque � Low drag to prevent flapwise damages and reduce losses � � Aerofoil shape is the most suitable, ie. high lift-to-drag ratio

Preliminary Conclusion � Modern megawatt class wind turbines are almost exclusively equipped with three wing-shaped rotor blades mounted on a horizontal axis.

Wind turbine operation Cut-out Cut-in Rated * Actual power curve of CPC NEWUNITE FD-77 -1500 -III 1. 5 MW wind turbine which has been installed at Huasai, Nakonsrithammarat

Excess energy 6000 kilowatts 5000 4000 Excess energy 3000 2000 1000 0 0 5 Wind turbine power curve 10 15 Wind speed (m/s) 20 Available wind power (Efficiency = 0. 587) Calculated with air density = 1. 225 kg/m 3 25

Power control � In case of stronger winds it is necessary to waste part of the excess energy of the wind in order to avoid damaging the wind turbine. All wind turbines are therefore designed with some sort of power control. � There are two different ways of doing this safely on modern wind turbines. � Stall control � Pitch control

Stall controlled wind turbine � Rotor blades are fixed to the hub on a stall control wind turbine � No mechanisms, no moving parts

How does stall control work? Local angle of attack is a function of wind speed, ie. High wind speed = high angle of attack

Lift coefficient How does stall control work? Stall controlled wind turbine operation conditions during high wind speeds Angle of attack

How does stall control work? � High wind speed � High angle of attack � Flow separation over upper surface � Stall � Lift force is broken � Rotor torque is reduced � Power is controlled

Pitch controlled wind turbine � Electronic controller monitors the wind turbine power output � When the power exceeds the limit, it sends a signal to the blade pitch mechanism to turn the blade. � Reduce local angle of attack

Lift coefficient How does pitch control work? Angle of attack Pitch controlled wind turbine operation conditions during high wind speeds

Advantages and disadvantages � Aerofoil is most efficient (maximum lift-to-drag ratio) when operating at its designed angle of attack, which is usually below the stall angle � Pitch controlled turbines operate below the stall angle during high wind speeds Drag Lift

Advantages and disadvantages � Aerofoils entering stall are associated with formation of leading edge vortices, which cause large changes in aerodynamic forces leading to strong structural vibration and ultimately reducing fatigue life � Pitch controlled turbines are designed to operate below stall angles at all times

Advantages and disadvantages Stall controlled turbines are safer during sudden changes in angle of attack � Gusts � Wind shear Lift coefficient • Angle of attack

Advantages and disadvantages � Almost all modern large wind turbines are equipped with pitch control systems, hence its maintenance services and spare parts are readily available. � Moving parts in the pitch mechanism require regular maintenance at a cost

Conclusions � Modern multi-megawatts wind turbines are designed with 3 -blade HAWT configurations � Pitch control system is favourable � Stall control system has several minor advantages

Thank you for your attention � Chawin Chantharasenawong � Mechanical Engineering Department � KMUTT � chawin. cha@kmutt. ac. th � http: //Dr. Chawin. com