Several types of solar tracking devices exist, from the single-axis synchronous m otor tracker to the com puter-controlled dual-axis active-sensor tracker. Each type of tracker has advantages and disadvantages which m ust be balanced by the individual researcher before installing the device of choice. Table 4.2 indicates the advantages and disadvantages of som e of the m ore com m on types of trackers. It goes well beyond the scope of the m anual to provide the installation and m aintenance
instructions for each of these devices. A broad overview, however, is im portant because of the significance solar tracking plays in the m easurem ent of direct and diffuse radiation.
The following general characteristics are com m on to all solar pointing devices.
(1) The tracker location m ust be known. The m ore precisely the location can be determ ined, the easier the setup of the tracker. W ith m odern GPS receiver system s the position of the tracker can be determ ined (latitude and longitude) to within ±3 m . From a 1:50,000 topographic m ap the location can be determ ined to within better than 50 m . Depending upon the size of the installation care should be taken so that the actual location of the tracker is determ ined and not sim ply the central location of the BSRN observatory.
(2) In all cases a reliable power supply is necessary. Synchronous trackers not only require a constant power supply, as do other trackers, but also a constant and accurate electrical frequency if the tracker is to m aintain accurate alignm ent on the solar disk at all tim es. Changes in power line frequency will alter the speed at which the solar disk is tracked. Most utility com panies are required by law to m aintain the power line frequency to som e stated accuracy within a 24-hour period with a m axim um excursion from that stated frequency at any given m om ent.
Stepper-m otor-controlled trackers are less susceptible to such changes because of their internal conversion from AC to DC power. The use of UPS system s on synchronous m otor trackers is also of lim ited utility because m any inverter system s output frequency as square waves rather than the sinusoidal wave required by the tracker.
Figure 4.7. The contribution of the solar disk to the irradiance of pyrheliom etric sensors depending on the pointing error. (A) Case of m ountain aerosol and 60°
solar elevation. (B) Case of continental aerosol and a 20°
solar elevation. (Calculations and graph courtesy of G.
Major)
Figure 4.9. Brusag two-axis active tracker. Active tracking is accom plished by balancing the signals from the quadrant sensor that is found on the flat of the elevation disk. The pyrheliom eter is a Kipp & Zonen CH1.
(3) The base on which the tracker is to be placed m ust be stable. W hile active trackers and som e passive trackers are able to correct for a non-level surface, all trackers perform better if they are m ounted such that the instrum ent base is level. Trackers m ounted on pedestals should be levelled such that the vertical axis of the tracker, and not just the pedestal post, is perpendicular to the horizon. A three-point base that allows easy adjustm ent is recom m ended.
The use of spring tensioners, lock washers, or bolts that are tightened using double nuts, will reduce the problem of the connection between the m ounting post and the tracker base loosening and causing the tracker to tilt from level. The tracker should be rotated about the vertical axis during the levelling process to ensure that the axis is vertical. A num ber of active-tracking and com puter-controlled passive active-tracking system s are capable of m athem atically correcting for out-of-level conditions so that solar tracking is m aintained. These corrections, however, do not correct for the tilt error of any pyranom eter or pyrgeom eter that is m ounted
Figure 4.8. A single-axis synchronous m otor tracker.
This m odel is an Eppley Model ST-1 Equatorial Mount.
Types of Solar Pointing Devices Used in the BSRN
Table 4.2. Advantages and disadvantages of com m on solar tracking instrum ents.
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 alignm ent varies. Equatorial trackers need to be precisely aligned, while m ost two-axes passive and active trackers have correction algorithm s built into the software to allow alignm ent to be less precise. However, the greater the accuracy in aligning the tracker, the easier it will be to initiate accurate tracking. The easiest m anner 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 tim es upon entering the date and the station latitude and longitude (e.g., http://titan.srrb.noaa.gov/highlights/sunrise/gen.htm l).
Details on the actual program m ing and set-up of individual features of the various types of trackers is beyond the scope of this m anual and the reader is referred to the technical m anuals provided by the m anufacturers (Annex E provides a list of m anufacturers of solar trackers).
Routine m aintenance of solar trackers is can be found in Chapter 6.
5.0 Data Acquisition