Page 116 - Solar Power in Building Design The Engineer's Complete Design Resource

86 SOLAR POWER SYSTEM DESIGN CONSIDERATIONS

Single-axis trackers usually have a single-axis tilt movement, whereas dual-axis trackers
also move in regular intervals adjusting for an angular position as shown in Figure 3.23
through 3.27.
In general single-axis trackers compared to fixed stationary tilted PV support sys-
tems increase solar power capture by about 20 to 25 percent. Dual-axis trackers on the
other hand can increase solar power production from 30 to 40 percent. Solar power
concentrators which use Fresnel lenses to focus the sun’s energy on a solar cell,
require a high degree of tracking accuracy to ensure that the concentrated sunlight is
focused precisely on the PV cell.
Fixed-axis systems orient the PV modules to optimize power production for a lim-
ited time performance and generally have a relatively low annual power production.
On the other hand, single-axis trackers, although less accurate than dual-axis tracker
applications, produce strong power in the afternoon hours and are deployed in appli-
cations such as grid-connected solar power farms that enhance power production in
the morning and afternoon hours.
Compared to the overall cost of photovoltaic systems, trackers are relatively inexpen-
sive devices that significantly increase the power output performance efficiency of the
PV panels. Even though some tracker systems operate with some degree of reliability,
they usually require seasonal position adjustments, inspection, and periodic lubrication.

Basic physics of solar intensity The amount of solar intensity of light that
impinges upon the surface of solar photovoltaic panels is determined by an equation
referred to as Lambert’s cosine law, which states that the intensity of light (I) falling
on a plane is directly proportional to the cosine of the angle (A) made by the direction
of the light source to the normal of the plane:

I = k × cos A

where k is Lambert’s constant. This equation is depicted in Figure 3.22. In other words,
during the summer when the angle of the sun is directly overhead, the magnitude of
intensity is at its highest, since the cosine of the angle is zero; therefore, cos 0 = 1,
which implies I = k.
The main objective of all solar trackers is to minimize the value of the cosine 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 Earth, hence the name. Essentially polar trackers are in general aligned per-
pendicular to an imaginary ecliptic disc that represents the apparent mathematical path
of the sun. To maintain relative accuracy, these types of tracker are manually adjusted
to compensate for the seasonal ecliptic shifts that occur with the seasons. Polar track-
ers are usually used in astronomical telescope mounts where high-accuracy solar
tracking is an absolute requirement.

Horizontal—axle trackers Horizontal trackers are designed to orient a horizontal
axle by either passive or active mechanisms. Essentially a long tubular axle is supported