Page 61 - Design of Solar Thermal Power Plants

54 2. THE SOLAR RESOURCE AND METEOROLOGICAL PARAMETERS

25
20
15
Solar declination angle/(°) -5 5 0
10

-10
-15
-20
-25
0 30 60 90 120 150 180 210 240 270 300 330 360
Date serial number
FIGURE 2.2 Variation of solar declination angle during the year.

measuring point has been located in mean time; and LL refers to local
longitude, which is positive for east and negative for west.
Two corrections have been made in Eq. (2.2); the first corrects the
procession and revolving speed variation during Earth’s revolution
around the sun, and the second corrects the difference between local
longitude and the longitude of the measuring point at mean time:
ET ¼ 9.87 sin ð2BÞ 7.53 cos ðBÞ 1.5 sin ðBÞ
in which B ¼ 360 (n 81)/364 and n is the date serial number (refer to

Table 2.1).
Fig. 2.3 indicates the variation curve of ET over time within a year.
3. Duration of sunshine, H dl , represents the difference between the daily
sunrise and sunset times and is determined by local latitude and
daily solar declination angle. Its calculation is:
H dl ¼ 2 cos 1 ð tan f tan dÞ=15 (2.3)
in which f refers to local latitude.
4. Solar hour angle, w, is the angular deviation of the sun corresponding
to the local meridian line caused by rotation of Earth. At midday
solar time, w ¼ 0 ; w has a negative value in the morning and a

positive value in the afternoon. The variation speed of the solar hour
angle is 15 degrees per hour. The solar hour angle w can be
calculated with the following equation:

w ¼ 0:25 ðAST 720Þ (2.4)