Page 128 - Alternative Energy Systems in Building Design

104 SOLAR POWER SYSTEM PHYSICS AND TECHNOLOGIES

directly overhead, intensity is at its highest because the cosine of the angle is zero.
Therefore, cos 0 = 1, which implies that I = k or equals Lambert’s constant.
The main objective of all solar trackers is to minimize the value of the cosine of the
angle and maximize the solar intensity on the PV planes.

POLAR TRACKERS

Polar trackers are designed to have one axis rotate in the same pattern as the earth. As
such, they are referred to as polar trackers. In essence, polar trackers in general are
aligned perpendicular to an imaginary ecliptic disk that represents the apparent math-
ematical path of the sun. To maintain relative accuracy, these types of trackers are
adjusted manually to compensate for the seasonal ecliptic shifts that occur in autumn,
winter, spring, and summer. Polar trackers frequently are used in astronomical tele-
scope mounts, where high-accuracy solar tracking is an absolute requirement.

HORIZONTAL-AXLE TRACKERS
Horizontal trackers are designed to orient a horizontal axel by either passive or active
mechanisms. A long tubular axle is supported by several bearings that are secured to some
type of wooden, metallic, or concrete pylon structure. The tubular axes are installed in
north-south orientation, whereas the PV panels are mounted on the tubular axel that
rotates on an east-west axis and tracks the apparent motion of the sun throughout
daylight hours. It should be noted that single-axis trackers do not tilt toward the equator,
and as a result, their power-tracking efficiency is significantly reduced in midwinter.
However, their productivity increases substantially during spring and summer seasons
when the sun’s path is directly above. Horizontal-axel single-axis trackers, because of the
simplicity of their mechanism, are considered to be very reliable and easy to clean and
maintain and are not subject to self-shading.

PASSIVE TRACKERS

The rotational mechanism of a passive tracker is based on use of low-boiling com-
pressed gases that are moved or displaced from east to west side by solar heat, which
converts liquid to gas, causing the tracker to tilt from one side to another. The imbal-
ance created by movement of the liquid-gas material creates the fundamental principle
of bidirectional movement. It should be noted that various climatic conditions such as
temperature fluctuations, wind gusts, and clouding adversely affect performance of
passive solar trackers. As a result, they are considered to have unreliable tracking effi-
ciency. However, they do provide better solar output performance than fixed-angle solar
support platforms. Figure 3.53 shows various tilt angles for a passive solar tracker.
One of the major passive solar tracker manufacturers is Zomeworks, which manu-
factures a series of tracking devices known as Track Rack. Tracking devices begin
tracking the sun by facing the racks westward. As the sun rises in the east, it heats an
unshielded west-side liquid-gas-filled canister, forcing the liquid into the shaded east-
side canister. As the liquid moves through a copper tube to the east-side canister, the
tracker rotates so that if faces east.