Small Scale Wind Technologies Part 2 Centre for Renewable Energy at Dundalk IT CREDIT 1
Part 2 Small and large scale wind turbine technologies 2
Overview of small scale grid connected system Wind Turbine with PMG Rectifier/Controller Grid Tie Inverter Fuse Panel Grid Variable (wild) AC Dump Load Grid quality AC Meter DC Note: Isolator/protection switches not shown
Airfoil Shape Just like the wings of an airplane, wind turbine blades use the airfoil shape to create lift and maximize efficiency. The Bernoulli Effect
Basic Aerodynamics The Lift Force is perpendicular to the direction of motion. We want to make this force BIG. α = low α = medium <10 degrees The Drag Force is parallel to the direction of motion. We want to make this force small. α = High Stall!!
Lift/Drag Forces on rotating blade section Head Wind V ΩR L W Direction of rotation D V = Wind speed W = Apparent wind speed Ω= rotational speed ΩR = speed of blade section at a distance R from hub L = Lift D =Drag
Tip-Speed Ratio Tip-speed ratio is the ratio of the speed of the rotating blade tip to the speed of the free stream wind There is an optimum angle of attack which creates the highest lift to drag ratio Because angle of attack is dependant on wind speed, there is an optimum tip-speed ratio TSR = ΩR U Where, Ω = rotational speed in radians/sec R = Rotor Radius U = Wind Free Stream Velocity R ΩR
Coefficient of Performance Coefficient of performance varies with Tip Speed Ratio Characterized by Cp vs Tip Speed Ratio curve
Betz Limit All wind power cannot be captured by rotor or air would be completely still behind rotor and not allow more wind to pass through Theoretical limit of rotor efficiency is 59% (impossible to achieve) Most modern wind turbines are in the 35 45% range
Rotor Solidity Solidity is the ratio of total rotor planform area to total swept area Low solidity (0.10) = high speed, low torque a R High solidity (>0.80) = low speed, high torque A Solidity = 3a/A
Power from the rotor Wind Turbine Mechanical Output Power at rotor shaft: P = 0.5 x p x A x C p x U 3 C p = Coefficient of performance (varies with wind speed!!) = Ratio of wind turbine mechanical output power of the wind turbine to power in the wind that intercepts the rotor 11
Generator Overview Synchronous Generators Asynchronous Generators Synchronous means the rotor magnetic field rotates exactly the same rate as the grid frequency The rotor magnetic field in an asynchronous generator does not rotate exactly at the same rate as the grid frequency
Generator Overview- Permanent Magnet The permanent magnet generator (PMG) -Most popular in small scale wind systems at present -There is no control over the magnetic field strength of the magnets -Voltage amplitude and frequency varies with RPM (i.e. wild AC) => cannot be connected directly to grid
Generator Overview- Permanent Magnet
Inverters Is a device that converts DC power to AC power Two Types Non grid tie: modified sine wave, square wave and pure sine wave Grid tie Only grid tie inverters should be used to connect to grid
Non Grid Tie Inverters Modified Sine Wave Inverters They only suit certain types of load where power quality is not a major issue (e.g. light bulbs) Pure Sine Wave Inverters Note: These type must never be connected to grid
Micro Generation Interface Protection Requirements (ESB Networks -EN50438)
Grid Tie Inverters
Grid Tie Inverters
Eff (%) Grid Tie Inverters Efficiency 100 90 80 70 60 50 40 30 20 10 0 0 10 20 30 40 50 60 70 80 90 100 P/Pnom (%) SB50 SB25
Grid Tie Inverters Inverter Power vs Input Voltage Curve e.g. Windy Boy 1100LV The input voltage vs Power can be programmed Some inverters have maximum power point tracking MPP capability
ESB Grid connection of micro-generation systems (www.esb.ie) Single phase 230V connection 25 A => 6kW (rated power) Three Phase (230/400V) 16A => 11kW (rated power) Applications are made on NC6 notification form Micro-generation Small wind, Solar PV, Micro CHP, Microhydro http://www.esb.ie/esbnetworks/en/generatorconnections/micro_gen_connections.jsp No grid connection application fee for NC6 Need to install and interval meter to measure imports ands exports
Eff (%) Overview of small scale grid connected system 0.4 Cp 0.3 100 0.2 90 80 70 0.1 60 50 0.0 40 0 2 4 6 8 Tip Speed Ratio 10 12 30 20 10 Wind Turbine with PMG 0 0 10 20 30 40 50 60 70 80 90 100 P/Pnom (%) SB50 SB25 Grid Rectifier/Controller Fuse Panel Grid Tie Inverter Variable (wild) AC Dump Load Grid quality AC Meter DC Note: Isolation/protection not shown
Power from the Wind Turbine Wind Turbine Electrical Output Power: P = 0.5 x p x A x C p x U 3 x N b x N g Ng = generator efficiency (50% for car alternator, 80% or possibly more for a permanent magnet generator or grid-connected induction generator) Nb = gearbox/bearings efficiency (a good quality gearbox could be as high as 95% ) 24
Overview of small scale standalone system Basic idea- other variants exist Rectifier/Controller Inverter AC loads Master Control (System Manager) Battery Bank Generator (optional) Note: Isolator/protection not shown
Health and Safety Aspects Rotor Overspeed! Vibrations (e.g. tower) Electrical Safety- Protection and Signage
Health and Safety Aspects Rotor over speed is a very significant issue If a turbine rotor is allowed to run out of control into over speed it can self destruct and have costly (even fatal!) consequences Commons methods of over speed prevention Furling tail Electric braking dump loads Bade pitching mechanisms Each of these methods has shortcomings
Furling tail Furling tail is hinged off centre to rotor shaft. In high winds above rated power the tail folds and yaws turbine out of wind It is designed so that that a spring or gravity returns the turbine back to the normal upwind position at normal operating wind speeds
Electrical braking dump loads As the rotor RPM increases beyond the normal operating range the voltage increases to a level where the controller switches on dump resistor loads rated above rated power. These loads slow down the rotor preventing overspeed. Shorting the output of a permanent magnet generator will stop the rotor altogether (can be done when parking/ servicing the turbine)
Blade pitching mechanisms Changing pitch of blade to reduce the lift and preventing rotor over speed Controlled active blade pitching mechanism commonly used on large scale wind turbines Hinged spring loaded mechanisms used on some scale wind systems
Blade pitching mechanisms
Blade pitching mechanisms