1、INTERNATIONAL STANDARD IS0 9846 First edition 1993-l 2-01 Solar energy - Calibration of a pyranometer using a pyrheliometer hergie salaire - halonnage dun pyranom methods using pyrheliometers are the subject of this standard. The latter methods are more complicated than the former, because the pyran
2、ometers, which typically have a field-of-view angle of 2n, have to be compared with pyrheliometers, which are designed to measure direct so- lar radiation within a relatively small field of view. On the other hand, due to the relatively high accuracy of pyrheliometers, the latter methods are more ac
3、curate than the former ones. Since the WMO world radiometric reference (WRR), which represents the SI units of irradiance, is determined by a group of selected pyrheliometers, the transfer of the scale to pyranometers has to be accomplished by using standard pyrheliometers (see IS0 9060). Short desc
4、riptions of the cali- brations are given in I, 2 and S. It should be emphasized that “calibration of a pyranometer” essentially means the transfer of the WRR scale to the pyranometer under selected conditions. The determination of the dependence of the calibration factor (calibration function) on va
5、riable parameters is called “characterization”. The characterization of pyranometers is the subject of the appropriate International Standard for test methods for pyranometers. INTERNATIONAL STANDARD IS0 9846:1993(E) Solar energy - Calibration of a pyranometer using a pyrheliometer 1 Scope The objec
6、t of this International Standard is to promote the uniform application of reliable methods to calibrate pyranometers, since accurate calibration factors are the basis of accurate hemispherical solar radiation data which are needed for solar energy test appli- cations or simulations. This Internation
7、al Standard is applicable to all pyrano- meters in horizontal as well as in tilted positions. Its use is mandatory for the calibration of secondary standard pyranometers according to IS0 9060, and is recommended for the calibration of pyranometers which are used as reference instruments in compari-
8、sons. For other applications, the method using pyranometers as references may be used (see IS0 9847). This International Standard is intended for use by test institutions or test laboratories equipped with well- maintained pyrheliometers. 2 Normative references The following standards contain provis
9、ions which, through reference in this text, constitute provisions of this International Standard. At the time of publi- cation, the editions indicated were valid. All standards are subject to revision, and parties to agreements based on this International Standard are encouraged to investigate the p
10、ossibility of applying the most re- cent editions of the standards indicated below. Members of IEC and IS0 maintain registers of cur- rently valid International Standards. IS0 9060:1990, Solar energy - Specification and classification of instruments for measuring hemi- spherical solar and direct sol
11、ar radiation. IS0 9847:1992, Solar energy - Calibration of field pyranometers by comparison to a reference pyrano- meter. ISO/TR 9901: 1990, Solar energy - Field pyrano- meters - Recommended practice for use. 3 Definitions For the purposes of this International Standard, the definitions given in IS0
12、 9060 and the following defi- nitions apply. 3.1 calibration of a radiometer: Determination of the responsivity (or the calibration factor, as its re- ciprocal) of a radiometer under well-defined measure- ment conditions. 3.2 reference pyranometer: Pyranometer (see IS0 90601, used as a reference to
13、calibrate other pyranometers (see IS0 98471, which is a well- maintained and carefully selected instrument of rela- tively high stability and which has been calibrated using a pyrheliometer. 3.3 field-of-view angle of a pyrheliometer: Full angle of the cone which is defined by the centre of the rece
14、iver surface (see IS0 9060, 5.1) and the bor- der of the aperture, if the latter is circular and con- centric to the receiver surface; if not, effective angles may be calculated 4. 3.4 solar tracker; sun tracker: Power-driven or manually operated support which is employed to di- rect a pyrheliometer
15、 to the sun. “Equatorial trackers” are sun-following devices which have an axis of rotation pointing towards the elevated pole; the axes of motion are the hour angle and the declination of the sun. “Altazimuth trackers” are sun- following devices with the solar elevation angle and the azimuth angle
16、of the sun as coordinates of movement. 3.5 sun-shading disc device; shade disc device: Device which allows movement of a disc in such a way that the receiver of the radiometer (for example, a pyranometer) is shaded from the sun. IS0 9846:1993(E) For calibration purposes, particularly those described
17、 in clause 5, quick removal of the disc is mandatory. Further details on shade disc devices used in cali- brating pyranometers are given in 5.2.4. 3.6 direct solar radiation: That part of the extraterrestrial solar radiation which as a collimated beam reaches the earths surface after selective at- t
18、enuation by the atmosphere. to direct solar irradiance are derived from the differ- ence between the measured values of hemispherical solar irradiance and the diffuse solar irradiance (see note 1, 3.8). These values are measured periodically by means of a movable sun shade disc. For the cal- culatio
19、n of the responsivity, the difference in ir- radiance components is divided by the measured direct solar irradiance normal to the receiver plane of the pyranometer. The quantity measured is the direct solar irradiance, expressed in watts per square metre (see also IS0 9060). In the following subclau
20、ses the basic method is de- scribed. Modifications of this method, which may im- prove the accuracy of the calibration factors but require more operational experience, are mentioned 3.7 hemispherical solar radiation; global radi- ation: Combined direct solar radiation and diffuse so- lar radiation.
21、in annexes C and D. 5.2 Apparatus The quantity measured is the hemispheric solar ir- radiance, expressed in watts per square metre (see also IS0 9060). 3.8 diffuse solar radiation: That part of solar radi- ation which reaches the earth as a result of being scattered by the air molecules, aerosol par
22、ticles, cloud and other particles. The quantity measured is the diffuse solar irradiance, expressed in watts per square metre (see also IS0 9060). NOTE 1 For meteorological purposes, the solid angle from which the scattered radiative fluxes are measured shall be the total sky hemisphere, excluding a
23、 small solid angle around the suns disc. 4 Selection of methods Two calibration methods have been selected for standardization, because they are widely used and are reliable. Both methods use shade disc devices for measuring diffuse solar radiation and are based on the hemispherical solar radiation
24、being equal to the sum of direct solar and diffuse solar radiations. The derived calibration factors are representative of cloudless or scattered cloud conditions (see clause 8 for uncertainties). A modification of the calibration method in clause 5 for application during less stable sky conditions
25、is briefly described in annex C. Annex D contains a short description of an extended version of the calibration method in clause 6 to de- termine the dependence of the calibration factors on incidence angles. 5 Alternating sun-and-shade method 5.2.1 Pyranometer. In principle, this method can be appl
26、ied to any type of pyranometer. 5.2.2 Pyrheliometer. The choice of pyrheliometer used as the reference should be made according to the required accuracy and the operational conditions. Generally, secondary standard or first class instruments (see classification in IS0 9060) which are regularly compa
27、red with pri- mary standards represent a satisfactory level of accu- racy (see also clause 8). The pyrheliometer should produce at least one reference value every 2 min. 5.2.3 Solar tracker, power driven or manually oper- ated, employed to direct the reference pyrheliometer to the sun for the entire
28、 test period. A solar tracker of the altazimuth type should be used for pyrhelio- meters whose responsivity over the receiver surface is not circular-symmetrical. The required tracking ac- curacy depends on the slope angle (see IS0 9060) of the pyrheliometer. In the usual case the slope angle is abo
29、ut 1”. 5.2.4 Shade disc device, meeting the following re- quirements: a) The shade disc shall be positioned perpendicular to the suns ray and at a fixed distance d from the centre of the receiver surface of the pyranometer. b) The radius r of the shade disc should be larger than the radius of the ou
30、ter glass dome of the pyranometer by a minimum of d tan(0,5”) to allow for the divergence of the sun beam and small tracking errors. 5.1 Principle The pyranometer under test is compared with a pyr- heliometer measuring direct solar irradiance. The voltage values from the pyranometer that correspond
31、c) The ratio r/d should define an angle at the centre of the receiver surface which corresponds to the field-of-view angle of the pyrheliometer. NOTE 2 A fixed “shade slope angle”, corresponding to the slope angle of the pyrheliometer, can only be 2 IS0 9848:1993(E) d) e) stated for pyranometers whi
32、ch are operated in a position normal to the suns ray. For other pyranometers, the shade slope angle varies according to the angle of inci- dence of the ray on the receiver plane. Those parts of the disc holder which obscure the field-of-view angle of the pyranometer should be as small as possible in
33、 order to restrict the dis- turbance of the signal to less than 0,5 %. Similar regard to interference with other neighbouring in- struments should be considered. The shade disc must be easy to remove and re- place, so that the change from the shade phase to the hemispheric solar irradiance phase, or
34、 vice versa, takes less than 5 % of the phase duration. The five types of shade disc devices briefly described in annex A are the designs of different institutions; only one of them is commercially available at present. 5.2.5 Data acquisition system. To acquire the values, in millivolts, of the radi
35、ometer readings, the system should be equipped with a pre- cise digital voltmeter with a resolution of 1 .LV and an uncertainty of 0.1 % of the pyranometers calculated output at 1 100 W m-*. High temperature stability is required for outdoor operation. The data sampled from all radiometers should be
36、 recorded within about 1 s. A time resolution for calculating the correspond- ing solar elevation angle with an uncertainty of less than 0.1” is required. For documenting the variation of the measured values during the calibration period, the data should be appropriately recorded. 5.3 Measurement co
37、nditions Clear sky conditions are essential for reduced variance in the results. However, clouds are tolerable if they are at a large angular distance from the sun ( 45”) and have a low angular velocity, to guarantee stable values of diffuse solar radiation within the cycle time of the measurement p
38、rocedure (see 5.7.1); i.e. the change in the diffuse solar irradiance must be negli- gible. In the case of tilted pyranometers, clouds which are outside the field of view have minimal influence on the measurement procedure. In principle, the other environmental conditions during calibration should b
39、e similar to the typical conditions during normal use of the pyranometer. The most im- portant parameter is the range of solar elevation, fol- lowed by the ambient air temperature, level of hemispherical solar irradiance and tilt angle. During calibration, wind conditions are also important, since p
40、yrheliometers operating with open tubes are disturbed by strong wind speeds, especially gusts coming from the suns azimuthal direction. It is rec- ommended that pyrheliometers are operated with wind screens if wind-induced instability of the measurements is intolerable. 5.4 Measurement site The meas
41、urement site shall offer rigid supports to in- stall the instruments and be of convenient access. In the case of horizontal pyranometers, obstructions on the horizon are tolerable provided they do not ob scure the sun during the calibration period and their effect on the measurements varies monotoni
42、cally at a small rate (see 5.7.1). Specular reflection by ob- structions should be avoided. In the case of inclined pyranometers, signal contributions from the radiation reflected by the foreground should vary in the sense mentioned above. Space must be provided around the pyranometers for the movem
43、ent of the shade disc. The distance to other instruments should be large enough so that possible interference can be neglected. The distance between the pyrheliometer and the pyranometer should be less than 30 m, otherwise both radiometers may not be similarly affected by the same atmospheric events
44、 (for example, structured turbidity elements). 5.5 Installation The installation of the pyrheliometer and the solar tracker as well as the pyranometer with the shade disc device shall be carried out as described in the appropriate operation and manufacturers manuals, and considering 5.4. Pyranometer
45、s which are used in combination with a ventilation device should also be ventilated during the calibration procedure. In the case of inclined pyranometers, the cable outlets should point downwards to avoid interference from rain or direct solar radiation (see lSO/TR 9901). 5.6 Calibration procedure
46、5.6.1 Preparatory phase Start the preparatory phase about 30 min before the measurement phase to allow for: - acclimatization of the radiometers, electronics and data acquisition system; - adjustment of radiometers, solar tracker and shade disc device; - checking of the electrical connections, test volt- ages and zeroing tests; - final cleaning of the optical windows. 3
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