Performance - Wind Energy: Renewable Energy and the Environment - page 149

8 Performance

It is important to remember that the load is part of the wind energy conversion system ( Figure 8.1 ) .
The most common application is the generation of electricity, which is a good match between the
characteristics of the rotor and the load. The other major application for wind power is pumping
water, which is a poor load match when the rotor is connected to a positive displacement pump
(constant torque device). However, the farm windmill is well designed to pump low volumes of
water with a positive displacement pump, even though it is inefficient.
Overall, performance of a system is measured by annual energy production and annual average
power for that wind regime. Compromises on efficiencies for each component of the system should
be combined to maximize annual energy production within the initial costs and the life cycle costs.
The last two factors may be opposed, as reducing the initial costs could increase life cycle costs. The
comparison will be for wind turbines that generate electricity.
Power curves and power coefficients have been measured experimentally, and peak efficiencies for
the system are around 0.40 for vertical-axis wind turbines to 0.50 for horizontal-axis wind turbines
(see Figure 6.14 ). For constant rpm operation, such as an induction generator, the rotor will operate
at peak efficiency at only one wind speed (see Figure 6.13 ). Also, for a variable speed rotor, the effi-
ciency will decrease above rated wind speed as power output is limited to the rated value. To increase
generator efficiency, some units have two generators, with one operating at low wind speeds and the
other operating at high wind speeds. The Vestas V27 had a 50/225 kW asynchronous generator with
synchronous speeds of 750/1,000 rpm. Another possibility to increase generator efficiency is to change
the number of poles of the generator between low and high wind speeds. The Mitsubishi, 1 MW rated
power, has an induction generator rated at 250/1,000 kW, with wind rotor speeds of 21/14 rpm.

8.1 MEASURES OF PERFORMANCE
Capacity factor: Capacity factor is the average power, which is equivalent to an average efficiency
factor.

CF average power/rated power (8.1)

In general, capacity factors are calculated from the kilowatt-hour produced during a time period,
since power energy/time. The time periods vary; however, the most representative time period
would be 1 year, although capacity factors for a month and a quarter have been reported. Capacity
factors of 0.3 would be good, 0.4 would be excellent, while those of 0.10 would be too low.
For wind sites and wind farms with class 4 and above winds, annual capacity factors should be
0.35 or greater, and during windy months, the capacity factors can exceed 0.50. Capacity factors are
somewhat arbitrary because of the different sized generators for the same rotor diameter. For the
month of February 2002, Lake Benton I, Minnesota, reported a capacity factor of 0.49, and Lake
Benton II, Minnesota, reported a capacity factor of 0.60. The difference was the wind turbines at
Lake Benton II had a larger diameter, more swept area, for the same size generator.
Availability: The availability is the percentage of time the unit is available to operate and is
a measure of reliability. For prototypes and early production models, the availabilities were low,
0.50 or even lower. Third-generation models have availabilities of 0.95-0.98. Manufacturers may
define availability differently, so be careful in comparing availability of different wind turbines.
Reliability and operation and maintenance affect the system performance.

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