Page 161 - Alternative Energy Systems in Building Design

SOLAR POWER SYSTEM DEPLOYMENT 137

150 mi northeast of Los Angeles. The 550- × 380-ft land provided by the SWD is rela-
tively flat, with no trees, which required minimal grading and brush clearing. As shown
in the figure, PV arrays are mounted in a north-facing-south orientation.

Mechanical description PV array design for this single-axis tracking system was
based on the use of 7200 Shell Solar Industries Model SQ-160 modules that were
assembled into 1800 panels. The proposed configuration of the array provides 60 rows
approximately 170 ft long and spaced at 21 ft on center. The proposed 21-ft row-to-row
spacing extends the array’s operational day and maximizes energy output by minimizing
shadowing effects.
The north-south axis trackers use a 20-ton screw drive jack to provide 45 degree
east to 45 degree west single-axis tracking to maximize the daily energy output from
the array. The screw jacks are controlled by a square-D (or Allen Bradley) program-
mable logic controller (PLC). A clock-based controller provides ±2 percent tracking
accuracy for the flat-plate PV arrays and allows backtracking to eliminate row-to-row
shadowing.
The system was installed on top of 720 wooden utility-grade ground-embedded
poles as foundation for the array structure. Each pole is 15 ft in length and is buried
in the ground at a depth of approximately 7–8 ft. The panel support structure for the
array uses square galvanized steel torque tubes that are free to rotate at ±45 degrees,
which in turn are supported by galvanized steel bearing plates. The precise motion of
this torque tube is provided by screw jacks that are regulated by the controller system.
Prewired solar panels are clamped directly to the steel structure with two panel clamps
per panel.
The steel subassemblies form 60 rows consisting of 30 rows of 2-pair matrices.
These rows then are divided electrically to form five equal-sized subsystems consisting
of 360 prewired panels. Each panel is factory prewired with four Shell Solar SQ160
modules and delivered to the site in reusable shipping racks. The panels are also equipped
with factory quick disconnects to ease field wiring.
DC to ac power conversion is accomplished with five Xantrex Model PV-225208
inverters that are centrally located on a 12- × 74-ft concrete pad placed adjacent to five
step-up transformers, collected output of which is connected to a low-voltage metering
system. Centrally located power accumulation allows for shorter conductor runs to all
five of the inverters.

Electrical description As described earlier, the array is electrically divided into
five equal subsections consisting of 1440 SQ160 modules, dc circuit combiners, one
225-kW inverter, and a 225-kVA, 208 to 12.47-kV step-up transformer. Thirty panels
each containing four modules are used on each of the 60 single-axis tracking rows.
The dc collectors feed underground current to each inverter’s dc interface that incor-
porates prefabricated fusing and a manually operated disconnect switch. The ac out-
put of the inverters also includes manually operated disconnect switches that feed the
low-voltage section of the step-up transformer. The low-voltage winding of the trans-
former includes a metering section that is fitted with an energy-production meter. The