Page 135 - Solar Power in Building Design The Engineer's Complete Design Resource

SITE EVALUATION 105

the electric energy use. Even though optimum tilting of PV arrays results in superior
average yearly energy production for the same number of PV modules, lower effi-
ciency resulting from flat array installation may constitute a reasonable alternative.

DC-to-AC power conversion calculations After completing the preceding
steps, the designer must evaluate the PV module’s electrical performance parameter
and configure PV strings in a manner most appropriate for use with a dc-to-ac inverter
system. Note that upon preliminary configuration of the PV arrays and subarrays, the
design engineer must coordinate solar power dc and ac wiring details with the inverter
manufacturer. In view of specific electrical design performance characteristics of PV
modules, inverter manufacturers provide dc input boundary limitations for various
types of array configurations.
In general, the maximum allowable dc power voltages produced by a string of PV
modules for a specific type of inverter may be limited within a 300- to 600-V dc band-
width at which the inverter may perform power conversion within safe margins and
yield the highest conversion efficiency. The excursion of output voltage produced by
PV strings beyond the safe boundaries is determined by Vmp, or the combined series
PV string maximum peak voltage when measured in an open-circuit condition. For
instance, 11 SolarWorld PV modules (AG SW 175 mono) when connected in series at
an average ambient temperature of 90°F (Vmp = 35.7 V) produce a swing voltage of
387 V, which will be within the input voltage boundaries of the inverter. Upon deter-
mining the allowed number of series PV string, the designer will be in a position to con-
figure the topology of the PV array and subarrays that would conform to inverter power
input requirements.
In most instances inverter manufacturers provide a Web-based solar array power
calculator where the designer is allowed to choose the type of inverter power rating,
PV module manufacturer, and model number. The preceding data along with the
ambient operational temperature, tilt angle, and array derating coefficient figures (as
outlined earlier in this chapter) can be inserted into the calculator. The calculations
provide accurate inverter string connectivity and ac power output performance
results.
The following is an example of a SatCon PV calculator used to determine the allow-
able string connectivity and power output performance for a 75-kW dc solar power
system.

PV Module Specification

PV module SolarWorld AG SW 175 mono
STC W (standard test conditions) 175 W

CEC W (California Energy Commission test) 162.7 W
Voc 44.4 V
Vmp 35.7 V