| 1. How Can I Calculate the Amount of Power Available at a Given Wind Speed? |
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Because air has mass and it moves to form wind, it has kinetic energy. You may remember from science class that: kinetic energy (joules) = 0.5 x m x V2 where:
m = mass (kg) (1 kg = 2.2 pounds)
V = velocity (meters/second) (meter = 3.281 feet = 39.37 inches)
Usually, we‘re more interested in power (which changes moment to moment) than energy. Since energy = power x time and density is a more convenient way to express the mass of flowing air, the kinetic energy equation can be converted into a flow equation:
Power in the area swept by the wind turbine rotor:
P = 0.5 x rho x A x V3
where:
P = power in watts (746 watts = 1 hp) (1,000 watts = 1 kilowatt)
rho = air density (about 1.225 kg/m3 at sea level, less higher up)
A = rotor swept area, exposed to the wind (m2)
V = wind speed in meters/sec (20 mph = 9 m/s) (mph/2.24 = m/s)
This yields the power in a free flowing stream of wind. Of course, it is impossible to extract all the power from the wind because some flow must be maintained through the rotor (otherwise a brick wall would be a 100% efficient wind power extractor). So, we need to include some additional terms to get a practical equation for a wind turbine.
Wind Turbine Power:
P = 0.5 x rho x A x Cp x V3 x Ng x Nb
where:
P = power in watts (746 watts = 1 hp) (1,000 watts = 1 kilowatt)
rho = air density (about 1.225 kg/m3 at sea level, less higher up)
A = rotor swept area, exposed to the wind (m2)
Cp = Coefficient of performance (.59 {Betz limit} is the maximum theoretically possible, .35 for a good design)
V = wind speed in meters/sec (20 mph = 9 m/s)
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 (depends, could be as high as 95% if good)
If there is any single equation that the beginning wind enthusiast should memorize, this is it. Contributed By Eric Eggleston, 5 February 1998 |
| 2. Is there a difference between intermittency and variability? |
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Variability implies a fluctuation around a certain baseline, such as the variation of electricity demand through the day. Intermittency implies something that frequently starts and stops.
Wind power is both intermittent and variable. |
| 3. Are bigger turbines more efficient? |
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No, they are just bigger. Output depends on wind speed and the combination of blade diameter and generator size. Bigger blades on a taller tower can capture more wind to run a bigger generator, but they don't do so more efficiently than smaller models. The grid has to adjust supply in response to the fluctuations of wind power as well as those of demand. |
| 4. Can wind turbines help avoid blackouts? |
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No. Wind turbines themselves need power from the grid to work. A blackout knocks them out, too. If they were providing power at the time, that loss aggravates the effect of the blackout. |
| 5. Doesn't a unit of electricity produced by wind turbines reduce a unit from another source? |
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Because the grid must continuously balance supply and demand, yes, it must reduce the supply from somewhere else when the wind raises enough to start generating power. If there is hydropower on the system, that is the most likely source to be reduced, because it can be switched on and off the most readily. Otherwise, the output from fuel-burning plant is ramped down or it is switched from generation to standby. In either case, it still burns fuel. |
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