Environmental Engineering Reference
In-Depth Information
Table 16-2.
Loss categories and typical values
Loss category
Low
Typical
High
Wake effects, %
3
6.7
15
Availability, %
2
6.0
10
Electrical, %
2
2.1
3
Turbine performance, %
0
2.5
5
Environmental, %
1
2.6
6
Curtailments, %
0
0
5
Total losses, %
7.8
18.5
37.0
The ranges apply to plants in mature operation; losses, especially availability losses, may be greater in the
first 6-12 months after construction. Some plants may have smaller or larger losses than indicated here.
The total loss is the product of the efficiencies; see Equation 16.4.
Source:
AWS Truepower.
to be greater in the first 6-12 months of operation as problems are identified and
resolved. Turbine performance, too, can change over time as turbine components such
as blades become worn and pitted.
16.6.1 Wake Effects
This is the reduction in wind speed and increase in turbulence that occurs downstream
of a wind turbine. In projects involving more than a handful of turbines, wake effects
typically reduce power production by anywhere from 3% to 15%. Keeping this loss
manageable is the main reason turbines are rarely spaced closer than 6 rotor diameters
in the prevailing wind direction. Furthermore, wake-induced turbulence can cause
wear on the components of turbines, and for this reason, turbines are usually spaced
no closer than 3 rotor diameters in crosswind directions, and they may have to be shut
down under certain conditions to satisfy the manufacturer's warranty.
Since wake effects change with the layout, all wind plant design software must
contain a wake model of some kind, and most have more than one. Three main
types in regular use are described later in this chapter: the Park and modified Park
model, the eddy viscosity (EV) model, and the deep-array (or large-array) wake model.
Other models based on CFD and LES (large-eddy simulation) codes, which are under
development, are outside the scope of this topic.
16.6.2 Downtime
A wind plant or turbine is said to be available when it is capable of generating its
full rated output, given sufficient wind. Availability losses occur when some turbines
in a project, or the entire project, are inoperative for some reason. They can also
occur because of a failure or shutdown of the power grid or substation. An overall
plant availability of 97-98% (2-3% loss) is frequently assumed in energy production
studies, but is likely to be optimistic unless there is good evidence that the plant
operator has regularly achieved such high performance with the turbine model in
question. Plant start-up problems, repair delays, fleet-wide turbine issues requiring
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