Kippzonen BSRN Scientific Solar Monitoring System Bedienungsanleitung PDF herunterladen (Seite 23)

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. (See Table 4.1 for common

combinations of pyrheliometers and pyranometers.)

Bush, B.C., F.P.J. Valero and A.S. Simposn, 2000: Characterization of thermal effects in pyranometers: A

data correction algorithm for improved measurement of surface insolation. Jour. Atmos. Ocean. Tech., 17,

165 - 175.

A solar tracker with an accuracy of ±0.10° or better, is needed to accommodate the

pyrheliometer, the ACR and, during calibrations, a second ACR. It is recommended that the

tracker pointing be monitored using a four-quadrant sensor because pointing accuracy is

important in determining the quality of the direct beam measurement. The sampling rate should

be the same as that of the instruments attached to the tracker. A variety of high-quality trackers

are now available that use four-quadrant sensors for actively positioning the tracker on the solar

beam during periods of high irradiance and a solar position algorithm during low intensity

conditions.

The parameters to be monitored are: output of the pyrheliometer thermopile; outputs of the ACR

(U, I or thermopile signals for a passive instrument); body temperatures of the pyrheliometer and

ACR; output of the four-quadrant sensor. Only the values associated with the calculated

irradiance (mean, maximum, minimum and standard deviation) are required by the BSRN archive.

All other raw data should be archived at the centre responsible for the measurements.

2.2.1.2 Diffuse Radiation

The original instrument configuration associated with the BSRN target uncertainty of 4% (5 W m )

-2

was a ventilated pyranometer with a sensor to monitor the instrument thermopile temperature (to

be used to correct for temperature-related changes in thermopile responsivity and thermal

offset). Shading of the instrument from the direct sun was to be by a shading disk. The shade

geometry of the combination of the sensor and the disk was to replicate the geometry of the direct

beam sensor when pointing toward the zenith (5° full-angle from the centre of the detector) . The

instrument’s sensor and dome must be completely shaded. Incorrect geometry alone can lead to

errors of up to 5 W m depending on instrumentation and atmospheric conditions.

-2

A relationship has been shown between pyranometer thermal offsets and diffuse irradiance that

can significantly affect the quality of the measurement . A possible solution to overcome the offset

problem is to use a ‘black and white’ type pyranometer (B&W) for the measurement of diffuse

radiation. This type of instrument does not exhibit the thermal offset of ‘black’ thermopile

instruments because both hot and cold junctions are exposed to the same thermal regime. B&W

instruments, currently, do not have the same optical qualities (spectral and directional) as the

black thermopile instruments recommended for use at BSRN stations and cannot be used for the

measurement of global radiation. Using a B&W instrument for the measurement of diffuse

irradiance would therefore mean a second type of instrument would have to be used for the global

irradiance measurement, which may increase the overall uncertainty of the three-component

measurements.

Research is presently ongoing to determine whether a correction factor can be applied to the

‘black’ pyranometer thermal offset. In Section 9.2.2 experimental methods of correcting this offset

are presented. Further research continues into the design of a ‘black’ pyranometer that does not

exhibit thermal offset. Several instruments that use other technologies (e.g., PRT) are available

that do not exhibit offset problems associated with radiative cooling.

The BSRN has yet to recommend a standard method of correcting for thermal offset or selecting

a particular type of instrument for measuring diffuse irradiance. A careful uncertainty analysis of

any diffuse system will determine the quality of the measurement if each component is properly

addressed.

The parameters to be acquired are: output of pyranometer thermopile; pyranometer body

temperature. Only the values associated with the calculated irradiance (mean, maximum,

minimum and standard deviation) are required by BSRN archive. All other raw data should be

archived at the centre responsible for the measurements.

1 2 ... 18 19 20 21 22 23 24 25 26 27 28 ... 187 188

Kommentare zu diesen Handbüchern

Keine Kommentare

Kippzonen BSRN Scientific Solar Monitoring System Bedienungsanleitung Seite 23

Kommentare zu diesen Handbüchern