Page 146 - Alternative Energy Systems in Building Design

122 SOLAR POWER SYSTEM PHYSICS AND TECHNOLOGIES

nevertheless they account for 97–99 percent performance efficiency and therefore
are assigned a multiplier value of 0.99.
AC wiring losses. Similar to dc wiring, ac wiring from inverters to the switchgear
or service power-distribution hardware is also subject to voltage drop, conduit loss
derating, and wire-way solar exposure. Theses losses likewise could be reduced
substantially by proper engineering design. However, a median multiplier value of
0.99 generally is applied to the calculations.

PV module dirt and soling losses. When PV module surfaces are exposed to dirt, dust,
and snow, the efficiency of their performance can drop by as much as 25 percent.
Solar power installations in windy, desert, and high-vehicular-traffic areas should be
cleaned periodically to maintain optimal levels of PV performance efficiency. PV
modules supported by tilted platforms or inclined terrain, in addition to having higher
performance efficiency, are less susceptible to dirt collection and are relatively easier
to clean and maintain. Likewise, in northern locations in winter, accumulated snow
blocks solar irradiance slides of the PV modules when PV arrays are tilted at an angle.
It should be noted that snow accumulation in northern parts of the country can reduce
solar power output performance by as much as 70–80 percent. Even though soiling is
an empirical event, a derating factor of 0.95 is recommended.

System availability and mean time between failures (MTBF). It should be noted that
solar power cogeneration configurations, whether grid-connected or otherwise, are
extremely reliable systems because the most important active components, namely,
PV modules manufactured as hermetically sealed solid-state electronic devices
(with a life expectancy of over 40 years) and inverters, as well as solid-state power-
conversion devices, are guaranteed to last at least 5 years by manufacturers.
Since solar power systems do not make use of any moving mechanical devices, they
are not subject to wear and tear, like most energy-generating plants and equipment. The
only downtime that may result from periodic equipment and module tests is essen-
tially insignificant. However, a mean system availability multiplier of 0.98 is consid-
ered to be a safe derating factor. Other losses, such as shading and PV degradation
owing to aging and sun tracking, are, in general, not taken into account.
With reference to the preceding, overall calculated dc-to-ac losses amount to
0.77, that is,

DC-AC loss = 0.95 × 0.92 × 0.98 × 0.996 × 0.98 × 0.99 × 0.95 × 0.98 = 0.77

Array tilt-angle losses. The optimal tilt angle for PV module performance is the
latitude angle of the particular terrain. As discussed earlier, irradiance at latitude is
perpendicular to the solar PV module. At a latitude angle, annual solar power energy
output from PV modules is at its optimum. An increased tilt angle above the latitude
will increase power output production in the winter; however, it will decrease in
summer. Likewise, decreasing the tilt angle from the latitude will increase power
production in summer. The following table relates tilt angle and roof pitch, which
is a measure of the ratio of the vertical rise of the roof to its horizontal run.