Page 60 - Design of Solar Thermal Power Plants

2.4 CALCULATING METHODS FOR SOLAR POSITION 53

radiation, namely solar radiation that has not been scattered in the at-
mosphere. Solar direct radiation is described by direct normal irradiance
2
(DNI, measured in W/m ), which can be measured with a pyrheliometer
that automatically tracks and aligns with the sun. To improve the effi-
ciency of solar radiation utilization, most solar concentrators adopt a
single-axis or double-axis revolving method of tracking the movement of
the sun. Such concentrators are referred to as tracking concentrators.

2.4.1 Solar Angle

1. Declination angle, d, is the included angle of the line connecting
Earth’s core to that of the sun and the equatorial plane of Earth.
Its value varies yearly and changes daily. The respective variation
0

range is 23 27 (refer to Figs. 2.1 and 2.2).
The approximate declination angle for a specific day can be
calculated using the following equation:
284 þ n

d ¼ 23:45 sin 360 (2.1)
365
in which n is the date serial number that refers to the nth day of the
year; for example, n ¼ 1 refers to January 1.
The date serial number “n” can be easily obtained through calculation
according to Table 2.1.
2. Solar time is based on the time of apparent movement of the sun in
the sky. Midday (12:00 noon) in solar time (AST) is when the sun
passes perfectly through the local meridian line. At that moment, the
sun is at its zenith for the day. Solar time can be converted from
commonly used local standard time (LST) using the following
equation:

AST ¼ LST þ ET 4ðSL LLÞ (2.2)
in which LST refers to local standard time (unit: min); ET is the cor-
rected value (unit: min); SL is the longitude of the spot where the

FIGURE 2.1 Variation of declination angle within the Sun’s annual operational cycle.