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SOLAR TRACKING MECHANISMS AND PLATFORMS
platform mainly performs rotational movement around this one Polar axis to track the sun
on the solar disc circumference (date dependant Diurnal Circle solar tracking).
2.6
Azimuth and Elevation Drive Mechanisms
In this section, aspects of solar tracking functionality and the transmission solution required
for a solar tracking platform is discussed. The transmission or actuator solution is detailed
in terms of the motion platform concept and the transmission drive components. The picking of this equipment is required as it centres around the integration of the transmission
system onto a mechanical platform suitable for accommodating solar tracking.
We therefore describe various lines of modular gearbox systems available for solar
tracking design controls. While learning from past solar tracking system experiences, this
study also highlights the features, benefits and disadvantages of the various transmission
and gearbox systems in solar tracking applications.
Figure 2.13 presents the reader with a gallery of random images of various linear and
rotational gear and slew drives. This may help in serving the purpose of assisting readers
new to the field of solar tracking to grasp certain of the design concepts described thus far.
Figure 2.13 Image gallery of linear and rotational gear drives, transmission systems, actuators
and slewdrives typically used in solar tracking applications (Prinsloo, 2014b)(Siemens, 2013a)(SKF,
2013).
Available literature discuss various linear and rotational transmission and gear drive
mechanisms used and tested in solar tracking applications. This includes practical experiences with both linear and rotational type actuators, such as for example screw drives,
worms drives, slew drives, spur gear drives, hypoid drives, helical gear drives, bevel gear
drives and cycloidal drives.
2.6.1 Sun Tracking: Drive Speed and Gear Ratios
The sun angle plots for the azimuth angle (and elevation) angle can now be used to determine the solar tracking speed and gear ratio requirements. It was noted before that the
partial differential of the solar path movement angle curves (slope at each point) equates to
the solar tracking speed (degrees per minute), as illustrated in Figure 2.14. The sun path on
the azimuth axis typically moves faster, and the point of maximum sun movement speed
can be identified on the graph.
AZIMUTH AND ELEVATION DRIVE MECHANISMS
47
Figure 2.14 Solar azimuth and elevation angles of the daytime sun path for a certain geographical
location (Ray, 2012), with the slope of the azimuth curve representing of the maximum sun movement
speed superimposed.
With reference to Figure 2.14, one can determine the speed of the sun in degrees per
minute by using the parameters obtained from the figure (at the point of maximum slope)
in the formula given in Equation 2.1 below.
SunSpeed(degree/min) =
∆SunAngle (degrees)
δtime (minutes)
(2.1)
Equation 2.1 computes the speed of the sun in degrees per minute. However, to relate
the speed of the sun to motor speed, we need to convert the sun speed to revolutions per
minuted (rpm or RPM). Still referring to Figure 2.14, one can therefore determine the speed
of the sun in rpm by dividing by 360◦ as in Equation 2.2 below.
SunSpeed(degree/min)
(2.2)
360◦
Depending on the location of the observer, Equation 2.2 will show that the sun is moving on average at an angular speed of around 0.25◦ per minute (Stine and Geyer, 2001).
Thus, on the fastest moving solar tracking axis, namely the azimuth axis (see Figure 2.14),
er axis should achieve an angular rate of movement of at least 0.25◦ /mi
/min
the solar tracker
to keep up with the relative sun movement. To achieve an angular movement rate of
0.25◦ /min,, Equation 2.2 shows that a minimum rotational motion speed of 0.000694 rpm
(0.25
25◦ /360
360◦ = 0.000694 rpm) is required to accomplish successful solar tracking.
Electrical motors typically move at a rate of around 1750-2000 rpm. This means that
an electrical motor on its own would thus not achieve such slow rate of movement with
adequate torque to drive solar tracking. Therefore a gear drive or transmission system
is required to gear-down motor speed while providing sufficient torque at slower solar
tracking speeds.
With a gearbox on the fastest moving axis, namely the azimuth axis on Figure 2.14, the
motor shaft still needs to turn at a certain minimum required speed in order for the tracker
to keep up with the movement of the sun. To determine this minimum required rotational
SunSpeed(rpm) =
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SOLAR TRACKING MECHANISMS AND PLATFORMS
speed for a tracking motor, one can use Equation 2.3 with the sun speed (rpm, determined
in Equation 2.2) as follows:
M otorShaf tM in (rpm) = SunSpeed(rpm) × Gearratio
(2.3)
where the Gearratio is determined as follows:
Gearratio =
(M otorinput speed )
(Gearboxoutput speed )
(2.4)
If the motor and gearbox combination cannot reach the minimum required speed calculated in Equation 2.3, then a different gear ratio (gearbox or transmission system) or higher
speed motor needs to be selected.
From Equation 2.3, the maximum allowable gear ratio or reduction gearing allowed to
convert
vert a typical rotational speed of a motor of 1750 rpm to the minimum required tracking
speed of 0.000694 rpm (solar tracking speed), then the gear ratio of the transmission or
gear drive system is computed to be around 2,000,000:1 (1750/0.000694). With such an
abnormally high gear ratio, a solar tracker gear drive system and a 1750 rpm motor will
just be able to keep up with the sun movement during maximum sun movement.
Typically, a more practical and realistic gear drive system for solar tracking uses a
transmission system with a gear ratio between 10,000:1 and 30,000:1. With such gear
ratios, solar tracker rotational movement is normally faster than the rotational movement
speed of the sun (0.000694 rpm). This is the reason why on/off type solar tracking control
systems are used, to synchronise the solar tracker angular rotational movement on the
ground with the sun’ss movement in the sky (solar tracking control described later in Section
8.3.2)
Knowing that one can determine the maximum angular speed of the sun in rpm (Equation 2.2), one can alternatively determine the minimum required rotational motor speed for
a given gear ratio that is more realistic or practical. This makes it possible to select a typical solar tracking gearbox or transmission system and then select a motor with sufficient
speed to meet the requirement in Equation 2.3.
In this regard, Equation 2.5 can be used to relate the speed of the motor and gear drive
axles to the eventual rotational speed of the solar tracking system axis. This formula is
valuable to determine the rotational speed of the tracker on either axis from the motor shaft
rpm and the gear ratio of the gearbox or transmission system on that axis, and is very handy
when the motor speed is fixed or if the motor gear drive can only operate within a certain
rpm range.
SunT rackerSpeed(rpm) =
M otorShaf t(rpm)
Gearratio
(2.5)
Using Equation 2.5 in a typical practical example, we will show how to compute the
rotational speed of the solar tracking axis shaft (rpm) from the motor shaft speed (rpm) and
the gear ratio. Assume we have a transmission system with a gear ratio of 15,000:1 and
a motor speed around 1750 rpm, then the rotational solar tracking movement calculated
from Equation 2.5 will be around 0.175 rpm. This means the rotational solar tracking
speed would be roughly 250 times faster than the point of fastest movement of the sun on
the azimuth axis (Equation 2.2).
Continuing with this example, we can compute the solar tracking speed by selecting a
slower speed motor or by slowing down the speed of the motor with PWM or VFD drives
(as discussed later in Sections 9.2.1 and 9.2.2) to operate at a different efficiency point
AZIMUTH AND ELEVATION DRIVE MECHANISMS
49
(see Figure 2.15). Say we reduce the motor speed down to around 20 rpm and still use
the same gearbox with gear ratio of 15,000:1, then the rotational solar tracking movement
speed calculated from Equation 2.5 will be around 0.00133 rpm. This means a rotational
solar tracking speed roughly double the maximum speed of the movement of the sun on
the azimuth axis (Equation 2.2). This motor gearbox/transmission system combination
will therefore be able to keep up with the maximum solar movement as the motor shaft
rotational speed will me above the minimum of Equation 2.3.
Figure 2.15 Example of motor performance curves and optimal operation points for a typical DC
motor (left) and a typical DC motor running at a reduced voltage (right) (Johnson Electric, 2014).
In order to reach an optimum solar tracking motor/gearbox solution, the designer should
strive to select a motor/gearbox combination that is able to deliver an acceptable solar
tracking and motor speed (Motor Shaft rpm in Equation 2.5), such that the electrical motor
operates as close as possible to its point of maximum torque or maximum efficiency as per
the motor performance curve (see Figure 2.15). The motor performance curve or test graph
is thus a crucial resource during this part of the design phase. The designer should further
ensure that the tracking speed or rotational solar tracking angular movement of the solar
tracker is at least within the same order or a higher speed than the rate of movement of the
sun on the azimuth axis at the point of maximum solar movement (as per Equation 2.1 and
Figure 2.14), otherwise the tracker may lag the sun at certain stages.
The remaining discussion will now focus features of gear drives and transmission systems typically used in solar tracking applications.
2.6.2
Sun Tracking: Linear Drives
Linear drives can provide the necessary mechanical movement and torque to enable realtime solar tracking and for the controller to accurately follow the sun as it moves in it’s
trajectory across the sky.
Some linear actuators integrate a motor drive with a screw, gearbox, control board,
position sensor, limit switches, in a lubricant dust sealed housing. This makes these drives
a popular choice of drive in photovoltaic solar tracking systems. Linear drives are often of
the ball screw jack type. These drives inherently offer large transfer rations with limited
ge transfer ratios in turn ensure movement control at lower levels of current
backlash. Large
consumption.
An important practical consideration in using linear drives in solar automation or tracking applications is the relevant industry specification (Bisenius, 2012). The Since the linear
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