Page 73 - Design of Solar Thermal Power Plants

64 2. THE SOLAR RESOURCE AND METEOROLOGICAL PARAMETERS

Spring has the longest sunshine duration of the year, lasting for
230e290 h; in summer, which is the rainy season, sunshine duration is
shortened to around 230 h; in autumn, monthly sunshine duration lasts
for 190e245 h; winter has the year’s shortest sunshine duration at
190e200 h (refer to Table 2.4).

2.5.1.4 Sunshine Percentage of Beijing
Sunshine percentage refers to the ratio of actual sunshine duration to
astronomical value at the same location and within the same period; the
larger the percentage, the more days with clear sky. Beijing has sufficient
sunshine. During a regular month, the respective sunshine percentage is
over 60%; only during July and August is the percentage lower, within a
range of 50%e60%. The sunshine percentages of Gubeikou, Tanghekou,
and Badaling Basin are the highest in the city, whereas the western area’s
is the lowest. Among the four seasons, winter enjoys the highest sunshine
percentage, whereas summer has the lowest value, and the spring and
autumn values fall between those of the other two. Monthly sunshine
durations and percentages for Beijing are shown in Table 2.4.
Daily utilization hours of solar direct radiation on a vertical surface in
Beijing are greatest during spring and autumn, averaging 6 h per day;
summer takes second place with 2e3 h available for daily utilization on
average during July and August, the decrease largely a result of the rainy
season. Daily utilization hours of global solar radiation on a horizontal
surface are greatest during spring; summer takes second place, and
winter has the least utilization hours.
At any time, the longer the continuous sunshine duration, the more
effective the solar energy received by the solar receiver. When sunshine is
interrupted constantly, the solar energy corresponding to this time may be
ineffective energy. For example, under the condition of 6 h of continuous
sunshine, all types of solar receivers can function effectively. The 6-h
continuous sunshine duration in Beijing is as much as 2287 h for the
year; the number in spring reaches as much as 661 h, which is 7.2 h per day
on average, whereas those of other seasons are all below 550 h, which is
below 6 h daily on average. From the perspective of the ratio of continuous
to actual sunshine duration during winter, the sunshine duration in Beijing
during spring and winter that can be effectively utilized by solar receivers
is quite long. If still referring to the standard of a 6-h duration of continuous
sunshine, the sunshine duration in these seasons that can be effectively
utilized by solar receivers accounts for about 85% of the actual sunshine
duration over the same period, whereas in summer it is only 70%.

2.5.1.5 Measured Value of Daily Mean Solar Direct Normal
Irradiance of Badaling
Fig. 2.6 shows measured data for Badaling during 347 days in 2009.
Based on the statistics, daily mean direct radiation for the year was
2
324 W/m .