Page 138 - Solar Power in Building Design The Engineer's Complete Design Resource

108 INTRODUCTION TO SOLAR POWER SYSTEM DESIGN

range from 88 to 96 percent. The mean value multiplier applied by the STC power
rating is 92 percent, which translates into a multiplier value of 0.92.
■ PV module array interconnection mismatch. As mentioned earlier, the dc output of
manufactured PV modules do vary, and when the PV modules are connected in tandem,
impedance mismatch results in power losses that may vary from 97 to 99 percent;
hence, a median degradation multiplier of 0.98 is applied during solar array power out-
put calculations.
■ Reverse diode losses. These losses are attributed to the voltage drop across diodes
that are used in each PV module to prevent reverse current flow into the unit.
Diodes are unidirectional electronic check valves that pass current only in one
direction and have intrinsic resistive characteristics; as a result they account for
energy loss due to heat dissipation.
■ DC wiring losses. A string of electrical wires that carry the dc output from PV mod-
ule to PV module and to the inverters are subject to ohmic resistive losses. Alhough
these losses could substantially be reduced by proper sizing of wires and conduits,
nevertheless, they account for 97 to 99 percent of performance efficiency and are
therefore 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 are also subject to voltage drop, conduit loss
derating, and conduit solar exposure. Theses losses likewise could be substantially
reduced by proper engineering design; however, a median loss multiplier value of
0.99 is generally applied to the calculations.
■ PV module dirt and soiling losses. When PV module surfaces are exposed to dirt,
dust, and snow, the efficiency of the performance can drop as much as 25 percent.
Solar power installations in windy, desert, and high vehicular traffic areas should
be cleaned periodically to maintain the optimum level of PV performance effi-
ciency. PV modules supported by tilted platforms or inclined terrain, in addition to
having a higher performance efficiency, are less susceptible to dirt collection and
are relatively easier to clean and maintain. Likewise in northern locations during
the winter, accumulated snow that blocks solar irradiance slides off the PV mod-
ules when PV arrays are tilted at an angle. Note that snow accumulation in north-
ern parts of the country can reduce solar power output performance by as much as
70 to 80 percent. Also for soiling a derating factor of 0.95 is recommended.
■ System availability and mean time between failures (MTBF). Solar power cogener-
ation configurations, whether grid connected or otherwise, are extremely reliable
systems since the most important active components, namely, PV modules, are man-
ufactured as hermetically sealed solid-state electronic devices with a life expectancy
of over 40 years. Inverters are likewise solid-state power conversion devices that are
guaranteed for at least 5 years by the 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 essentially
insignificant; however, a mean system availability multiplier of 0.98 is considered to