Kippzonen BSRN Scientific Solar Monitoring System Manual de usuario descargar pdf (Pagina 48)

Major, G., 1992: Estimation of the error caused by the circumsolar radiation when measuring global

radiation as a sum of direct and diffuse radiation. Solar Energy, 48(4), 249-252.

designed system shading both a pyranometer and pyrgeometer along with measuring the normal 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 pyranometer and the pointing of a pyrheliometer, the elevation drive must be

mechanically translated so that it is horizontal and at the same height as the signal transducer of the

instrument to be shaded. In both cases, the shade disk(s) (or the shade sphere(s)) remain at a fixed

distance from the instrument sensor via the cantilevering-motion provided by the armature. Once installed,

care must be taken not to rotate the elevation drive of the tracker below the position where the cantilever

system binds on itself. This is accomplished by not allowing the elevation axis to go more than about

5 - 10° below the horizon before programming 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 number of steps

taken to move 360° must be checked. This maintenance procedure requires that the shade assembly

be disconnected from the tracker.

Whether using a single-axis or two-axes tracker, the instrument requiring shading must be placed

preciselyso that the shade of the diffusing disk completely covers the outer dome of the instrument.

The general rule presented by the WMO for pyrheliometry is that the ratio between the length and

the diameter of the opening angle of a pyrheliometer is 10:1. This rule can be used to approximate

the geometry of the disk or sphere used to block the irradiance from solar disk. Major (1992) discusses

the use of pyrheliometers and shaded pyranometers for calculating global radiation with respect to

the optimum 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 pyrheliometer in use. Major (Personal Communication)

has calculated that diffuse/direct irradiance measurements made at various BSRN stations using standard

systems may increase discrepancies between the global irradiance measured by a pyranometer and

the summation of the diffuse and direct irradiances by up to 5 W m . Optimized arm lengths and shade

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diameters improve this to better than 0.5 W m . Table 4.1 provides optimal geometry for several common

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combinations of pyranometers and pyrheliometer or cavity radiometer. Further work on this issue has

been done by the BSRN W orking Group on Diffuse Geom etry, culminating in a final report that is

reproduced as Annex C.While the advantages of using a two-axes tracking system for measuring both

diffuse and direct beam irradiance are numerous, there are several disadvantages that should be

considered:

(1) The use of a computer-controlled system increases the risk of failure over a synchronous

motor tracker because more complex equipment is involved that can fail (e.g. computer

components, tracker electronics).

(2) By using a single system to measure multiple components, a single failure can affect several

types of observations. This will remove the redundancy established in measuring 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 may be nearly offsetting so that normal quality assurance procedures may not

be adequate. The use of an active sensor (either directly connected to the tracking system

or simply monitoring the solar position) will provide the extra information necessary to determine

whether or not the tracker is following the sun. Errors in tracking are due to incorrect date

and time, or in the case of friction trackers slippage of the friction disk. This latter problem

is normally associated with hum an activity.

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