Page 136 - Alternative Energy Systems in Building Design

112 SOLAR POWER SYSTEM PHYSICS AND TECHNOLOGIES

EARTH’S AXIS OF ROTATION EARTH’S AXIS OF ROTATION

SOLAR DECLINATION ANGLE –23.5° SOLAR DECLINATION ANGLE +23.5°

SUN RAYS SUN RAYS
i SUN i

SUMMER SOLSTICE
WINTER SOLSTICE
i = −23.5° i = +23.5°
DECEMBER 21/22 JUNE 21/22
Figure 3.56 Solar declination angle in the northern hemisphere.

As our planet rotates around the sun on an axis tilted at approximately 23.5 degrees,
the solar declination angle I (shown in Fig. 3.56), constantly varies throughout its rev-
olution in oval-shaped orbit and changes from +23.5 degrees on June 21–22, when the
earth’s axis is tilted toward the sun, to −23.5 degrees by December 21–22, when the
earth’s axis is tilted away from the sun. The tilt of the earth’s axis at these two sea-
sonal changes, referred to as the summer and winter equinoxes, is 0 degrees.
The solar declinations (shown in Figs. 3.56 and 3.57) described below result in sea-
sonal cyclic variations in solar insolation. For the sake of discussion, if we consider
earth to be a sphere of 360 degrees, within a 24-hour period, it will have rotated 15
degrees around its axis each hour (commonly referred to as the hour angle). It is the
daily rotation of the earth around its axis that gives the notions of sunrise and sunset.
The hour angle H (shown in Fig. 3.58) is the angle that the earth has rotated since
midday or solar noon. At noon, when the sun is exactly above our heads and does not
cast any shadow on vertical objects, the hour angle equals 0 degrees. By knowing the
solar declination angle and the hour angle, we can apply geometry and find the angle
of the observer’s zenith point looking at the sun, which is referred to as the zenith
angle Z (shown in Fig. 3.59).

EARTH’S AXIS OF ROTATION

SOLAR DECLINATION ANGLE 23.5°

SUN RAYS
i SUN

Figure 3.57 Solar declination angle.