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.