43
Types of Solar Pointing Devices Used in the BSRN
Tracker Type Advantages Disadvantages
Synchronous Motor
(Equatorial Mount)
Figure 4.8
- least expensive
- self-contained
- very portable (light weight)
- easiest maintenance
- single axis requires adjustment
for solar declination
- pyrheliometer wiring must be
untangled every few days
- accuracy dependent upon
quality of the power line
frequency
Two-axis Passive
(algorithm controlled)
Figure 4.6
- follows solar disk by
translating accurate solar
position algorithms into stepper
motor control functions (using
either an internal or external
CPU)
- usually has larger payload
than synchronous motor
trackers
- does not require untangling of
cables
- may be able to attach an
unshaded pyranometer on the
azimuth axis to reduce the
azimuth uncertainty
- requires accurate clock for
accurate solar tracking
- more expensive than equatorial
mount
- may require a separate
computer control system to
operate
- pointing accuracy and
smoothness of position
dependent upon stepper motor
functions
Two-axis Active
(quadrant sensor controlled)
Figures 4.5 and 4.9
- similar load capabilities to the
two-axis passive tracker
- active tracking device
overcomes problems with clock
accuracy during line-of-sight
tracking (added accuracy over
algorithm control)
- most reliable accurate tracking
- may be able to attach an
unshaded pyranometer on the
azimuth axis to reduce the
azimuth uncertainty
- expensive
- active tracking eye must be
calibrated to ensure proper
tracking in complex sky
conditions
- may require a separate
computer control system to
operate
- requires accurate clock during
cloudy conditions to m aintain
accurate solar tracking
- pointing accuracy and
smoothness of position
dependent upon stepper motor
functions
Table 4.2. Advantages and disadvantages of common solar tracking instruments.
on an unlevelled tracker.
(4) The tracker needs to be aligned in the north-south direction. Depending on the type of tracker
the accuracy of this alignment varies. Equatorial trackers need to be precisely aligned, while
most two-axes passive and active trackers have correction algorithms built into the software
to allow alignment to be less precise. However, the greater the accuracy in aligning the tracker,
the easier it will be to initiate accurate tracking. The easiest manner of obtaining a north-south
line is to trace the shadow of a perpendicular object at solar noon. Several internet sites are
now available that provide such times upon entering the date and the station latitude and
longitude (e.g., http://titan.srrb.noaa.gov/highlights/sunrise/gen.html).
Details on the actual programming and set-up of individual features of the various types of trackers
is beyond the scope of this manual and the reader is referred to the technical manuals provided by
the manufacturers (Annex E provides a list of manufacturers of solar trackers).
Routine maintenance of solar trackers is can be found in Chapter 6.
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