Page 187 - Fluid Power Engineering

160 Chapter Eight

The choice of turbine classes can get complex. In cases that involve
parameters that straddle two classes or cases that involve insufficient
data to compute the parameters, turbine manufacturers should be
consulted to determine the WTG Class.

Estimation of Losses
There are two key concepts that have not been discussed up to this
point: Losses and uncertainty. These two mutually exclusive concepts,
however, are often mentioned together, which causes confusion. A
wind resource assessment is not bankable without a rigorous analysis
of losses and uncertainty.
Losses are estimates of decrease in energy output that is known.
As an example, consider energy loss because of transmission of elec-
trical energy from generator to grid. Suppose an electrical engineer
estimates the electrical losses to be 2.2%. This is the expected loss in
energy. Uncertainty, on the other hand, is a statistical concept that
describes the unknowns associated with estimates. In the same ex-
ample, several unknowns may cause the losses to be 2.1 or 2.3%. The
uncertainty in the loss estimate is ±0.1%. In addition to uncertainty in
loss estimates, there are uncertainties associated with estimates of the
annual average energy production. This section will focus on losses
and the next section will focus on uncertainty.
Following are categories of losses:

Wake losses. This is loss in energy production because of re-

duced energy and increased turbulence in the wake of tur-
bines. Wake was covered in the second section of this chap-
ter. Wake losses may be due to internal turbines within the
wind farm, or because of external turbines in adjacent wind
farms.Windresourceassessmentsoftware,likeWindPROand
WindFarmer, contain a variety of wake models to compute
wake losses for individual turbine and for the entire wind
farm.
Plant availability. This is loss in energy production because of

unavailability of the plant even though the wind resource is
available. Reasons for unavailability of plant are:
1. Scheduled and unscheduled maintenance of turbine
2. Scheduled and unscheduled maintenance of balance of
plant
3. Grid unavailability, which may be caused by fault on the
grid because of external reasons
Electrical losses. The difference in energy produced at the gen-

erator and the energy delivered to the grid is the electrical
loss. Subcategories of electrical losses are: