Wind Turbines - Wind Energy: Renewable Energy and the Environment - page 84

that power until the unit shuts down at the cut-out wind speed. Some wind turbines with fixed-pitch
blades and induction generators continue to operate at any wind speed. Above the rated wind speed
the power output is constant or even decreases somewhat because of the decreasing aerodynamic
efficiency with increasing wind speed.
The most important parameter in determining energy production is the rotor area, as energy
production will increase as the square of the radius. A larger generator does not necessarily mean
more energy production because the efficiency at low wind speeds will change with generator
size. Some large wind turbines have two generators, one a smaller generator for lower wind speeds
to increase overall efficiency. Although a larger generator is probably desirable in the best wind
regimes, the optimum size for a given rotor radius for a given wind regime is still undetermined.
Manufacturers are now offering different size generators (rated power) for the same rotor diameter,
or the same size generator for different rotor diameters. Jay Carter, Sr. designed and built a wind
turbine for both medium and good wind regimes, which is done by only changing the size of the
induction generator (30 kW, six poles; 50 kW, four poles).

5.6.2 F AULTS
Wind turbines are shut down for faults such as loss of load, vibration, loss of phase, current or volt-
age anomalies, etc. Each of these safety features could save the unit, but the most important feature
is a method of controlling the rotor when there is a loss of load (fault on the utility grid) during high
winds (overspeed control). If the unit is not shut down within a few seconds, it will reach such high
power levels that it cannot be shut down and will self-destruct. The large torque excursions and also
the emergency application of mechanical brakes may damage the gearbox. Faults result in power
spikes, large current, and voltage drops.

5.7 ENERGY PRODUCTION
Annual energy production is the most important factor for wind turbines. Of course, that is com-
bined with economics to determine feasibility for installation of wind turbines and wind farms.
Approximate annual energy can be estimated by the following methods:

1. Generator size (rated power)
2. Rotor area and wind map
3. Manufacturer's curve of energy versus annual wind speed

5.7.1 G ENERATOR S IZE
This method gives a rough approximation because wind turbines with the same size rotors can have
different size generators:

AKWH CF*GS*8,760 (5.2)

where AKWH annual energy production, kWh/year; CF capacity factor; and 8,760 number
of hours in a year.
The effect of the wind regime and the rated power for the rated wind speed can be estimated
by changing the capacity factor. The capacity factor is the average power divided by the rated
power (generator size). The capacity factor is estimated from energy production over a selected time
period, and in general, capacity factors are quoted on an annual basis, although some are calculated
for a quarter of a year. Capacity factors can also be calculated for wind farms, and they should
be close to the same values as capacity factors calculated for individual wind turbines. However,