Page 168 - Solar Power in Building Design The Engineer's Complete Design Resource

138 SOLAR POWER GENERATION PROJECT IMPLEMENTATION

Figure 5.18 Large-scale single-axis solar farming system,
Semitropic Water District, Wasco, California. Photo courtesy of SolarWorld.

The proposed configuration of the array provides 60 rows approximately 170 ft long
and spaced at 21 ft on center as shown in Figure 5.18. The proposed 21-ft row-to-row
spacing extends the array’s operational day and maximizes energy output by minimiz-
ing shadowing effects.
The north-south axis trackers utilize 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) programmable 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 is installed on top of 720 wooden utility-grade ground-embedded poles as
foundations for the array structure. Each pole is 15 ft in length and is buried in the ground
at a depth of approximately 7 to 8 ft. The panel support structure for the array utilizes
square galvanized steel “torque” tubes that are free to rotate at ±45 degrees, which are in
turn supported by galvanized steel bearing plates. The precise motion of these torque
tubes 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 two-pair matrix.
These rows are then divided electrically to form five equal-size 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 by use of five Xantrex model PV-225208
inverters that are centrally located on a 12 ft × 74 ft concrete pad placed adjacent to five
step-up transformers, the 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.