Solar Radiation - Understanding Renewable Energy Systems - page 56

Table 2.8 Different Definitions of Solar Azimuth Angle

Reference Symbol N NE E SE S SW W NW

ISES, 2001;
NREL, 2000 γ 0° 45° 90° 135° 180° 225° 270° 315°

ϕ γ

CEN, 1999 for >0 a 180° 225° 270° 315° 0° 45° 90° 135°

ϕ γ CEN, 1999 for <0 b 0° 270° 270° 225° 180° 135° 90° 45°

α S DIN, 1985, this topic 0° 45° 90° 135° 180° 225° 270° 315°

Note: ϕ , latitude; a north of the equator; b south of the equator

in this topic defines the sun height as the angle between the centre of the sun
and the horizon seen by an observer. The azimuth angle of the sun describes
the angle between geographical north and the vertical circle through the centre
of the sun. EN ISO 9488 defines the solar azimuth as the angle between the
apparent position of the sun and south measured clockwise at the northern
hemisphere and between the apparent position of the sun and north measured
anticlockwise in the southern hemisphere (CEN, 1999). This can cause
confusion when comparing calculations with different angle definitions. Table
2.8 shows the different solar azimuth angle definitions of various sources.
The sun height and solar azimuth depend on the geographical location of
the observer, the date, time and time zone. The position of the sun is strongly
influenced by the angle between the equatorial plane of the Earth and the
rotational plane of the Earth around the sun, called the solar declination. The
solar declination

varies between +23°26.5´ and -23°26.5´ over a year. Since
the orbit of the earth around the sun is not circular, the length of a solar day
also changes throughout the year. Usually, the so-called equation of time eqt
takes this into consideration. Many algorithms have been developed to
calculate the position of the sun. A relatively simple algorithm is described
overleaf (DIN, 1985).

δ

Zenith

E

N

S

W

Sun meridian

Figure 2.10 Definitions of the Angles Describing the Position of the
Sun Used in this Topic