Page 145 - Alternative Energy Systems in Building Design

SOLAR POWER SYSTEM DESIGN CONSIDERATIONS 121

Another testing standard used in the United States is based on the dc rating of the
2
nameplate, defined as 1000 W/m of solar irradiance at 20°C ambient temperature and
wind speeds of 1 m/s. This is referred to as PVUSA test conditions, or simply, PTC.
It should be noted that the difference between PTC and STC is that in the former,
the ambient temperature and wind speed can result in PV module temperatures of
about 50°C as opposed to 25°C for STC. As a result, under PTC test conditions, crys-
talline-based PV modules will show −0.5 percent degradation per each degree Celsius.
Hence the power rating of silicon-type PV modules is reduced to 88 percent of the
nameplate rating.
It also should be noted that energy calculations for PV systems evaluated by the
CEC and the state of Nevada for rebate consideration take into consideration the PTC
rating and not the dc power output. However, manufacturers always rate their PV
products based on the dc output power.

PV SYSTEM LOSSES
When designing solar power cogeneration systems, the net energy output production
must be calculated by taking into consideration losses associated with the totally inte-
grated system. In general, losses occur owing to the following design elements and
environmental conditions:

PV dc nameplate derating. Losses results from dc power output from modules that
vary from 80 to 105 percent of the manufacturers nameplate rating. Such losses may
result from solar cell physical dimensions, interconnecting cell solder path bridge
resistance, and the likes. The default value applied for such losses is 95 percent of
the dc nameplate value, which translates to a multiplier value of 0.95.
Inverter and matching transformer losses. This loss is a result of the conversion of
dc to ac power. The efficiencies of inverters used in solar power cogeneration range
from 88 to 96 percent. The mean value applied by 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 does vary, and when such modules are connected
in tandem, impedance mismatch results in power losses that may vary from
97 to 99 percent. Thus a median degradation multiplier of 0.98 is applied during
solar array power output calculations.

Reverse diode losses. These losses are attributed to voltage drops across diodes,
which 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 direc-
tion and have intrinsic resistive characteristics. As a result, they account for energy
loss owing to heat dissipation.
DC wiring losses. Strings of electrical wires, which carry the dc output from PV to
PV modules and to the inverters, are subject to ohmic resistive losses. Even though
these losses could be reduced substantially by proper sizing of wires and conduits,