Page 162 - Alternative Energy Systems in Building Design

138 SOLAR POWER SYSTEM PHYSICS AND TECHNOLOGIES

meter includes a cellular modem so that it can be read remotely. The electronic meters
are designed to store daily, weekly, and monthly energy-production parameters.
The transformers step up the 208 V ac to 12.47 kV ac. The high-voltage output of
each transformer includes fusing and a “hot stick” disconnect. All five transformers
are loop fed, and the final underground feed from the transformer pad extends 200 ft
to the north section of the array, where it is terminated at a riser pole.
The Xantrex inverters used in this installation meet IEEE 929 and UL1741 standards,
and as such, they do not require any anti-islanding hardware.

Energy performance Generally speaking, it is estimated that the annual energy
production from a single-axis tracking system can be as much as 20 percent than that
from a comparable fixed-tilt system. In general, single-axis tracking modeling soft-
ware used in this project calculates energy production of a single north-south axis
row of PV modules from sunrise to sunset (90 degrees east to 90 degrees west). The
most popular software currently used for calculating solar array output performance
such as PV Design Pro or PV Watts use a 90-degree east to 90-degree west algorithm
to calculate the maximum available annual energy. As discussed earlier, when calcu-
lating energy output performance, shadowing effects must be accounted for in the
annual energy production model.
When tracking multiple rows of solar panels, it should be noted that the higher the
tracking limit angle (in this case 90 degrees), the larger will be the shadow cast in the
morning and afternoon hours. This shadowing effectively will shut down energy pro-
duction from all the rows located behind the eastern-most row in the morning and the
western-most row in the evening. This effect can be reduced by limiting the tracking
limit angle to 45 degrees. From a practical standpoint, the linear actuators used in the
most popular systems easily accommodate a 45-degree limit angle and is the hardware
used in the proposed system. To further improve the energy performance of the system,
a backtracking scheme is used in the morning and evening hours of each day to
eliminate the row-to-row shadowing.
Backtracking begins by adjusting the tilt angle of each row to 10 degrees east just
before the sunrise in the morning. As the sun rises, each row begins tracking east just
enough so that no row-to-row shading occurs. This backtracking continues until the
tracker limit angle of 45 degrees is reached, at which time the tracker controller waits
until the sun catches up with the 45-degree tilt angle and then begins to follow the sun
throughout the day. In the afternoon, the controller will repeat the backtracking scheme
until the sun sets.
Shell Solar Industries is including typical energy profiles for tracking arrays in
December and June (the winter and summer solstices). These profiles illustrate the
effects just described and the impact they have on the annual energy production of a
multirow single-axis tracking system.
This project was constructed by Shell Solar Industries for customer Semitropic Water
District and placed in service in April 2005. Owing to market conditions at the time, Shell
SQ-85 modules were used in place of the SQ-160 modules. Owing to other project con-
straints (related to the California incentive funding program at the time), the project size
ultimately was reduced to 979.2 kW (from 11,520 kW for the SQ-85 modules).