Page 137 - Solar Power in Building Design The Engineer's Complete Design Resource

SITE EVALUATION 107

Peak efficiency 97%

CEC efficiency 96%
Number of subarrays 6

Photovoltaic System Power Output Rating In general when designing a solar
power cogeneration system, the designer must have a thorough understanding of PV
system characteristics and associated losses when integrated in the array
configuration.
Essentially the power output rating of a PV module is the dc rating that appears on the
manufacturer’s nameplate. For example, SolarWorld SW175 mono is rated at 175 W dc.
The dc power output of the PV module is usually listed on the back of the units in watts
per square meter or kilowatts per square meter (watts divided by 1000). The rating of the
module is established according to international testing criteria referred to as the standard
2
test condition (STC) of 1000 W/m of solar irradiance at 25°C temperature discussed
previously.
Another testing standard that is used in the United States is based on the dc rating
2
of the nameplate which is defined as a 1000 W/m square plane of array irradiance at
20°C ambient temperature at a wind speed of 1 m/s, which is referred to as the
PVUSA test condition (PTC).
Note that the difference between the 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 the STC. As a result, under these PTC test conditions a
crystalline-based PV module will result in −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.
Note also that energy calculations of photovoltaic systems evaluated by the
California Energy Commission and State of Nevada for rebate consideration standards
take into account the PTC rating of PV modules and not the dc power output.
However, manufacturers always rate their photovoltaic product based on the dc output
power.

Photovoltaic System Losses When designing solar power cogeneration sys-
tems, the net energy output production must be calculated by taking into consideration
losses associated with the totally integrated system. In general, losses occur due to the
following design elements and environmental conditions:

■ PV dc nameplate derating. This is a loss resulting from dc power output from mod-
ules that vary from 80 to 105 percent of the manufacturer’s nameplate rating. Such
losses may result from solar cell physical dimensions, interconnecting cell solder
path bridge resistance, etc. The default value applied for such losses is 95 percent
of the dc nameplate value or a multiplier value of 0.95.
■ Inverter and matching transformer losses. These losses are a result of the conver-
sion of dc to ac power. The efficiency of inverters used in solar power cogeneration