Kipp&Zonen BSRN Scientific Solar Monitoring System - Manual (Page 48)

M ajor, G ., 1992: Estim ation of the error caused by the circ um solar radiation when m easuring global
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radiation as a sum of direct and diffuse radiation. Solar Energy, 48(4), 249-252.
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designed system shading both a pyranom eter and pyrgeom eter along with m easuring the norm al incident
direct beam (see Section 4.4 for details on two-axes trackers). To use the tracker as a platform for
both the shading of a pyranom eter and the pointing of a pyrheliom eter, the elevation drive m ust be
m echanically translated so that it is horizontal and at the sam e height as the signal transducer of the
instrum ent to be shaded. In both cases, the shade disk(s) (or the shade sphere(s)) rem ain at a fixed
distance from the instrum ent sensor via the cantilevering-m otion provided by the arm ature. Once installed,
care m ust be taken not to rotate the elevation drive of the tracker below the position where the cantilever
system binds on itself. This is accom plished by not allowing the elevation axis to go m ore than about
5 - 10° below the horizon before program m ing the tracker to “go to sleep” and/or return to a pre-sunrise
position. Depending upon the type of tracker (especially if it is a friction drive), the num ber of steps
taken to m ove 360° m ust be checked. This m aintenance procedure requires that the shade assem bly
be disconnected from the tracker.
W hether using a single-axis or two-axes tracker, the instrum ent requiring shading m ust be placed
preciselyso that the shade of the diffusing disk com pletely covers the outer dom e of the instrum ent.
The general rule presented by the W MO for pyrheliom etry is that the ratio between the length and
the diam eter of the opening angle of a pyrheliom eter is 10:1. This rule can be used to approxim ate
the geom etry of the disk or sphere used to block the irradiance from solar disk. Major (1992) discusses
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the use of pyrheliom eters and shaded pyranom eters for calculating global radiation with respect to
the optim um design of the shade disk. The results indicate that the best equivalence can be expected
if the distance between the receiver and the shading disk is chosen so that the slope angle is larger
and the opening angle is less than those of the pyrheliom eter in use. Major (Personal Communication)
has calculated that diffuse/direct irradiance m easurem ents m ade at various BSRN stations using standard
system s m ay increase discrepancies between the global irradiance m easured by a pyranom eter and
the sum m ation of the diffuse and direct irradiances by up to 5 W m . Optim ized arm lengths and shade
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diam eters im prove this to better than 0.5 W m . Table 4.1 provides optim al geom etry for several com m on -2
com binations of pyranom eters and pyrheliom eter or cavity radiom eter. Further work on this issue has
been done by the BSR N W orking Group on Diffuse Geom etry, culm inating in a final report that is
reproduced as Annex C.W hile the advantages of using a two-axes tracking system for m easuring both
diffuse and direct beam irradiance are num erous, there are several disadvantages that should be
considered:
(1) The use of a com puter-controlled system increases the risk of failure over a synchronous
m otor tracker because m ore com plex equipm ent is involved that can fail (e.g. com puter
com ponents, tracker electronics).
(2) By using a single system to m easure m ultiple com ponents, a single failure can affect several
types of observations. This will rem ove the redundancy established in m easuring global, diffuse
and direct beam irradiance.
(3) If the tracker is not properly tracking the solar disk, errors in the direct beam and diffuse
irradiances m ay be nearly offsetting so that norm al quality assurance procedures m ay not
be adequate. The use of an active sensor (either directly connected to the tracking system
or sim ply m onitoring the solar position) will provide the extra inform ation necessary to determ ine
whether or not the tracker is following the sun. Errors in tracking are due to incorrect date
and tim e, or in the case of friction trackers slippage of the friction disk. This latter problem
is norm ally associated with hum an activity.