Kippzonen BSRN Scientific Solar Monitoring System Manual do Utilizador descarregar pdf (Página 142)

130

Figure C 4.2. Relationship between the geometry of

pyrheliometers and radiance (The relationship

applies for a diffusometer when the sun is at zenith).

C 4.2 Irradiance in the instrument: Numerical

solutions

Since the relationship between the radiation

and the sensor continuously changes with

the solar zenith angle, exact com parability

happens only when the sun is at zenith. This

rare occasion nevertheless is useful to

develop the general case for which most

measurements are carried out.

C 4.3 Sun at zenith

The geometrical condition between radiance

and an instrument is illustrated in Figure C

4.2 for the sun at zenith. The same figure

represents both pyrheliometers and

diffusometers by swiching the frontal circle

between the opening and the shading disc

(or sphere). We consider a point on the

sensor which is located at a distance of r'

measured from the sensor centre. Radiance

at this point falls at a zenith angle of z'. We

consider total irradiance F at this point

coming into the instrument (or blocked by the

shading disc). This calculation involves a

double integration, one from the normal to

the sensor to the edge of the opening (or disc)

whose angle to the normal is z, the other from

0º to 360º in azimuth:

where

This irradiance F expected at r' on the sensor must be integrated for the entire sensor surface to

obtain the absolute amount of radiative flux (or the amount blocked by the shading disc):

where r = the radius of the sensor.

This radiative flux can be converted for a unit area to obtain the irradiance expressed in the

conventional unit.

1 2 ... 137 138 139 140 141 142 143 144 145 146 147 ... 187 188

Sem comentários

Kippzonen BSRN Scientific Solar Monitoring System Manual do Utilizador Página 142