Materials and environmental compatibility
Aesthetics and architectural integration
Economic factors and costs
Consider all of the factors above when designing a PV system. Numerous
publications make it easier to understand mechanical design issues and apply
solutions. In this section we focus on standardization, safety, and accessibility to
the roof system in relation to mechanical design.
The installation of PV arrays on standard types of roofing does not require
excessive site-specific information. The contractor coordinates with the subcon-
tractors to facilitate easy installation. Mounting systems must be flexible enough
to accommodate various conditions and applications. Installation is made simple
by adhering to structural and safety standards. As with any mechanical design,
steps taken to ensure the associability and safety of the system make a system that
has longevity and stability.
Mechanical Design and Electrical
and Thermal Performance
The effects of temperature on electrical components, on all PV technologies, and
on the performance of crystalline silicon PV modules and arrays are fairly well
known. But the concerns tend to be dismissed in general by most of the industry.
This creates numerous problems for performance and system life. Less is under-
stood about the impacts on newer thin-film products. It is assumed that due to
the increased potential for degradation of materials, the life of a PV array is
reduced at higher operating temperatures.
The electrical performance of all PV arrays is affected by
temperature in different ways. There are also issues associated
with temperature rating for electrical components of the
system. Temperature coefficients typically reduce the output
of a crystalline silicon array by about 5 percent for each 10
degree Celsius increase in cell operating temperature. The
temperature coefficients for thin-film materials may be less;
however, they have their own challenges with durability and
lifetime service in hot environments. Standard design prac-
tices suggest minimizing array temperatures whenever
possible.
Standoff and rack-mounted designs usually operate at cooler temperatures
with better airflow. The height, array pitch, and airflow capability underneath the
array all affect operating temperature. Standoff mounted arrays have an increase
in temperature rise by as much as 50 percent when the standoff height is reduced
NOTE
Standard derating numbers are
based on properly operating technology
that is meeting at least the NEC
requirements. Equipment designs that
do not meet the minimum requirements
tend to fall dramatically in output.
CHAPTER 9
PV System Mechanical Design 187

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