IEEE Power & Energy Magazine - May/June 2018 - 79

To make better use of space, wind and solar plants can be
colocated, provided that conditions for both wind and solar power
generation are favorable at the same location.

in the case of an iwses, there were some additional considerations made regarding turbine placement to optimize the
placement of the solar array. this is discussed in the section
"design Considerations for an integrated wind-solar Plant."

Resource Assessment and Siting of Solar Arrays
good quality measurement of irradiance data is required
to perform a solar resource assessment. if usable measurements are not available, it is common industry practice to use
high-quality satellite-modeled data sets. For the two project
sites, satellite-modeled data were used to achieve the lowest
resource uncertainty. the long-term period of record from
the solargis database was used to estimate the long-term
resource and create a typical meteorological year (tMY). the
tMY represents long-term resource, temperature, and wind
speed at the site on an hourly basis during a typical year.
Based on available land area, setbacks, site conditions,
and industry-standard practices, optimal configurations were
developed for two PV solar technologies: crystalline and thin
film. a number of parameters were considered in the design,
such as tilt angle, azimuth, collector length, pitch, and dc-ac
ratio. the main objective of the optimization was to maximize the capacity factor and minimize shading losses, while
still achieving a cost-effective design that would meet the
capacity targets set for the sites.
the first step in the system design was to optimize the tilt
angle to the site latitude. the collector length was sized based
on module dimensions and industry-standard mounting approaches. the array azimuth was set to true south to optimize
energy production. a pitch/shading analysis was conducted to
determine the optimal design of the PV configurations prior
to energy simulation. the result of this analysis was used to
produce a plot of shading loss as a function of pitch (the space
between rows). the pitch/shading analysis resulted in groundcover ratios dependent on the latitude.
next, the solar arrays were laid out to optimize land use,
minimize intrusion into exclusion zones, and minimize the
potential shading loss caused by proximity to the turbines. the
solar arrays were set back from all exclusion zones and boundaries with a 10-m (33-ft) buffer zone. the buffer zones provide
setbacks from sensitive areas and a means of access to the solar
equipment. roads of 10-m width were outlined through and
around the solar plants to provide access.
a dc-ac ratio of 1.3 was used for the configurations. this
dc-ac ratio represents an industry standard that is value
optimized for current equipment costs to increase energy
may/june 2018

production and minimize energy cost. the oversized dc
array minimizes the losses upstream of the inverter, which
allows for greater energy production at low irradiance and
increases the ac capacity factor. this initial phase took the
design to the point where the mutual interaction of the wind
and solar equipment required further consideration.

Design Considerations for an Integrated
Wind-Solar Plant
when designing an integrated plant, it is important to ensure
that the interaction between the wind and solar plants is captured and taken into consideration. there are two principal
types of interactions that must be modeled: 1) the effect of
the turbines on the PV arrays and 2) the effect of the PV
arrays on the wind-flow field.
The Effect of PV Arrays on Wind Turbine Siting

at both project locations, the siting of the turbines was carried out first because the wind resource was more variable
across the site than the solar resource; therefore, the optimum
configuration of wind turbines is more sensitive to their siting
than that of the solar array. the presence of the PV panels can
impact the surface roughness and affect the wind flow through
the turbine array. an array of solar panels can be similar to
other topographic or locational features (trees, buildings, etc.),
in that it alters the surface roughness and affects wind flow
through the array. increased surface roughness changes the
profile of the atmospheric boundary layer as it flows across the
array, increasing the shear effect.
the approach for roughness modeling, turbine wakes, and
the impact on wind flow is based on a theory advanced by
sten Frandsen (see "For Further reading") that defines wind
farm equivalent roughness. Frandsen stipulates that an infinite
array of wind turbines is represented as a region of uniform
high-surface roughness. the roughness imposes drag on the
atmosphere, causing both a downstream change in the structure of the boundary layer and a reduction in the free-stream
wind speed at the turbine hub height. once the equivalent
roughness is defined, it is possible to calculate the hub-height
wind speed deep within the array, where the boundary layer
has reached equilibrium with the array roughness.
at the project sites, the effect was comparable to that of
the local vegetation. the wind-flow field model considered
the local roughness, assuming that the panels would closely
resemble the roughness of local vegetation to account for
this effect. the difference in the comparative hub height of
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Table of Contents for the Digital Edition of IEEE Power & Energy Magazine - May/June 2018