PLANT DESIGN AND ENERGY PRODUCTION ESTIMATION - Wind Resource Assessment: A Practical Guide to Developing a Wind Project - page 241

The analyst usually starts by manually placing turbines along obvious topographic
features or areas of strong resource. If the site has a strongly predominant wind
direction, it is a common practice to space the turbines relatively close together,
perhaps 3-4 rotor diameters apart, in the prevailing crosswind direction and much
farther apart, perhaps 6-10 or more rotor diameters, in the prevailing downwind
direction. This practice generally results in close to the maximum number of turbines
with manageable wake losses and turbulence-induced loads.
Most software programs come with an optimizing feature that automatically adjusts
the turbine locations through hundreds or thousands of iterations to find one that max-
imizes net energy production (including wake losses) while respecting all setbacks,
exclusions, and other objective constraints. These routines are effective, although
sometimes very slow for large layouts consisting of hundreds of turbines or more
(overnight or weekend runs are common). One drawback is that they do not account
for construction costs. The result can be stranded turbines that would be costly to
build. It is often necessary to manually adjust the layout by removing or moving such
turbines. Some software, such as the openWind Enterprise program, includes a cost-
of-energy optimizing feature, which designs road and electrical cabling networks on
the fly and takes their cost into account. Using this feature usually results in a more
compact layout.
However, no matter how satisfying the optimized solution might be, the developer
should expect to have to make adjustments. The GIS data layers used to establish
setbacks and other exclusions may not be perfectly accurate. Unmapped features such
as buildings and rock outcroppings can make it impossible to install a turbine where
planned. Community concerns about noise and appearance may also drive siting deci-
sions. Photo simulations such as that shown in Figure 16-6, which can be produced
by most wind plant design software, are often a useful tool for soliciting community
feedback and building community support for a project.

16.6 GROSS AND NET ENERGY PRODUCTION

The gross energy production is the annual output of the wind plant without wake
or other losses. It is calculated, for each turbine in the layout, from the following
equation:

N d N v

E k = 8766 F ijk P ijk (16.3)

i = j = 1 1

The sum is over the number of direction steps N d and speed bins N v . F ijk is the
frequency of occurrence (expressed as a fraction) and P ijk is the power output for
direction sector i , speed bin j , and turbine k . The factor 8766 is the average number
of hours in a year (taking into account that one in every 4 years is a leap year,
which has 24 extra hours). The units of energy are either kilowatt-hours or megawatt-
hours, depending on whether the turbine's power output is expressed in kilowatts or
megawatts.