3 WIND Flashcards
*Advantages of wind energy generation
- Each wind turbine can produce up to 12MW
- Wind farms can be constructed quickly (once planning permission is obtained)
- Low cost in high-wind-speed onshore sites (4-5 p/kWh)
*Disadvantages of wind energy
- Noise and visual pollution
- Environmental impact
- Intermittent energy source
- Offshore- impacts fish/marine life (some research says they increase fish population as they act as artificial reefs)
- Onshore - impact on birds/bats
- Mining the copper ore emits CO2
*How to calculate power from wind turbine
Pwt = Cp.Pair = Cp(1/2)๐AU^3
*How to estimate energy output of wind turbine/farm
*What is the momentum theory of wind turbine rotors?
Uses Bernoulliโs principle to illustrate conservation of momentum.
Assumptions of momentum theory.
- rotor is a solid, infinitely thin disc
- disc is frictionless, without any rotational wake component (ie ideal)
- there is constant velocity induced along the axis of rotation on the disc
- the disk creates a flow around the rotor
Benefits of moving wind farms offshore
- minimal visual impact
- higher mean wind speeds (larger blades)
- lower turbulence (usually caused by obstructions on land)
Disadvantages of moving wind farms onshore
Higher capital and operating costs
Why is the power of wind turbine shut off at the Rated power?
As not economically desirable to have power higher than this.
Higher powers are generated by high wind speeds which only occur a few hours in the year.
Also need to ensure turbine can withstand all the forces.
How is wind turbine rotor shut off at rated power?
Power is cut off by rejecting the power in the wind before it gets into the turbineโs drivetrain for then weโd need to resist it with expensive equipment.
What is capacity factor?
The ratio of (how much energy is being made) / (how much energy would be made if wind farm was operating 24/7 at max output)
What is the potential difference between wind speeds in summer and winter?
Can be about double in winter as in summer
How to read hourly mean wind speeds chart using method of bins (/Weibull distribution)
y-axis: Hours per year
x-axixs: Wind speed
The most common wind speed will be the point on the x-axis with the highest corresponding y-value.
How to read binned power curve
y-aixs: power
x-axis: wind speed
Rated power occurs at the first x value (wind speed) for which the flat correlation occurs.
How to find useful energy out from binned charts
Useful Energy OUT = each bin from energy output chart (power for that wind speed) * number of hours
what is a streamtube?
A tube of constant mass flow rate
What is the general working of a wind turbine?
- the wind hits the blades
- the blades rotate with the rotor
- the rotor shaft rotates which is connected to the gearbox
- the gearbox increases the speed of the shaft
- the shaft goes into the generator, where the mech energy is converted into elec energy
Force-Momentum theory
the rate of change of momentum is a force caused by the pressure difference across the rotor disk
Limitation of momentum theory
When a > 0.5: momentum theory breaks down. As Wake velocity equation would become negative. An empirical modification is โด needed for high a values.
What is the cut off speed of wind turbines?
~25m/s wind speed
Which sides of the aerofoil have positive/negative pressure and why?
-ve pressure over upper surface as wind needs to accelerate to go a further way around than for the lower surface. (higher velocity -> higher Ek (Bernoullis))
+ve pressure over lower surface as wind is decelerating.
How to resolve resultant force from aerofoil section?
Resolve into lift (useful) and drag forces (need to resist)
Fl = perpendicular to direction of wind
Fd = parallel to direction of wind
How do drag and lift coefficients change with angle of attack?
Cl = Begins after 0 as the ๐ผ is whatโs causing the lift. Rises rapidly until stall point where acceleration is too much and flow detaches from upper surface.
Cd = initially very low then suddenly rises at stall point (where flow detaches). Then increases
How to test aerofoil for lift and drag coefs?
Test in wind tunnel by passing apparent wind over it. This gives the lift and drag forces which can be normalised to find Cl and Cd
At which angle of attack does stall occur?
~12-13ยบ
What is the angle of attack?
How does a blade stall?
An increase in U (free wind speed) causes ๐ผ and ๐ to change. If ๐ผ increases to speeds of ~12-13, the blade will stall, whereby the lift forces will suddenly drop causing Fr to also decrease.
How to control the lift force acting upon a blade?
Can either reduce ๐ผ to 0, or increase ๐ผ to the stall point; both of which would cause Cl to drop, hence Fl would reduce.
Rotate the blades about their axis to vary ๐ผ.
How to control thrust and rotational forces acting on a blade?
By either
- controlling Fl and Fd (with ๐ผ), or
- operating rotor at a constant ๐. If ๐R is constant, then ๐ will automatically increase as U increases. (โ๐ => โ๐ผ => stall at some point, causing Fr to drop)
What is ๐?
What does an actuator do?
Rotates the blades if it reads too much power in the Power transducer and controller compared to Psetpoint
Why does the pitch regulated rotor have a cleaner and crisper power curve than for stall regulated?
As less power is lost as blade approaches stall.
What is pitch regulation?
โฆ..
All modern turbines are variable speed and use pitch regulation.
What are the two types of wind turbines?
- Fixed Speed. Induction generator locked onto the 50Hz mains. Generates power from the torque, which is then injected into the mains.
- Variable Speed. Uses power electronics to apply a controlled/restraining torque to the rotor shaft which controls the rotational speed.
What does the drivetrain of a fixed speed turbine consist of?
Rotor with blades fixed rigidly to the hub, gearbox, simple induction generator
Why do modern turbines operated at varying rotational speed?
- to operate at max power
- to allow the speed to vary a little in response to gusts
How is energy generation affected if rotor operates too slowly?
It would allow some wind to pass through the rotor disk without interacting with the blades. So no power would be generated from that wind
How is energy generation affected if rotor operates too quickly?
The blades will chop up the wind causing turbulence which is wasted energy.
How is energy generation affected if turbine operates at an ideal speed?
The blades interact with all the wind that passes through the rotor and can extract all energy possible from the wind; without any unnecessary turbulence.
What is power coefficient in relation to turbines?
The fraction of wind able to extract from turbine rotor
What is Betz limit?
The max value of Cp possible = 0.59
Itโs impossible for a system to be more efficient than this. = the best wind turbine design that could ever be obtained.
Why would you want to control torque applied to turbine rotor?
Need to break the link between mainsโ 50Hz and speed of generator.
How to control torque applied to turbine rotor
Using power electronics to vary ๐. - by passing all power into DC then inverting back to AC, using two power converters.
(Convert 50Hx mainsโ AC to DC. Invert the DC back to AC where the generator is and hence the rotor.)
How is speed controlled in variable speed rotor?
Indirectly control speed to control torque.
How is pitch angle control used to shut down a variable speed rotor?
A Pitch Controller twists the blades to reject some of the power before it reaches the generator and needs to be resisted. Resisting the power would be costly and inefficient.
What does the generator side converter of a variable speed rotor do?
Controls the rotational speed of the (synchronous) generator
What does the Grid side converter of a variable speed rotor do?
Moves power from capacitor to mains at 50Hz.
What frequency is mains power fixed at by power stations?
50Hz
What is measured on the shaft of the generator in variable speed rotor?
Rotational speed on the shaft which provides speed of rotation of synchronous generator. This is used to control the amount of torque applied to the generator to ensure we stay on the locus; to maintain optimum operation.
Wind speed canโt be measured.
What does the locus of variable speed operation show?
What torque to provide the rotor with for the generator speed to remain on the locus of the Power/Generator rotational speed graph.
(it gives operation at peak Cp over range of wind speeds)
What is pitch regulation?
Adjusting the angle of the blade to adjust ๐ผ. To keep the power captured by the rotor approximately constant.
How to find apparent wind on aerofoil?
Found from free wind speed (U) and rotational speed (๐).
At what windspeed does the Turbine power curve start at?
~4m/s
Usually no electricity generated below this.
Why is a rotor shut down?
To avoid mechanical damage.
