Page 95 - Design of Solar Thermal Power Plants

2.6 SOLAR IRRADIANCE PREDICTION METHODS 83

in monthly atmospheric circulation and the nature of the underlying
surface, the constitution of the two components shows significant
monthly variations that are mainly determined by differences in cloud
cover and atmospheric transparency for various months.

2.5.8.3 Sunshine Duration in Harbin
The mean sunshine duration of Harbin is 2641 h, which is longer in
summer and shorter in winter, with values in spring and autumn falling
in between the two; however, the amplitude of variation of sunshine
duration for the year is less than that of the possible sunshine duration. As
the temperature is low, humidity is low and the cloud cover is low in
winter, while the sunshine percentage (percentage of actual sunshine
duration to possible sunshine duration) is high; in February, it can be as
high as 67%. Although daylight time is short, total sunshine duration
during NovembereMarch still accounts for about 34% of the yearly total.
With plenty of rainy days in summer, the sunshine percentage decreases,
to just 53% in July. The actual sunshine duration for July is lower than that
of May and June.
The maximum value of annual sunshine duration in Harbin over the
years is 2878 h (1978), and the minimum value is 2315 h (1968). The
maximum value of monthly sunshine duration is 327 h (August, 1975),
and the minimum value is 87 h (January, 1985). The maximum value of
monthly sunshine duration is normally not in June when the summer
solstice occurs. This mainly results from the arrival of summer winds in
June, increased cloud cover, and a reduction in sunshine percentage; the
summer half-year’s monthly sunshine duration is significantly influenced
by cloud cover.

2.6 SOLAR IRRADIANCE PREDICTION METHODS

2.6.1 Estimation Method for Solar Direct Normal Irradiance

As required by optical principles, site selection for concentrating solar
power (CSP) plants shall rely on solar DNI resources. Meteorological
stations in China don’t possess such test data. Nevertheless, we can acquire
global irradiation data and sunshine duration on the horizontal surface
corresponding to the period of 1994e2003 from domestic meteorological
stations for DNI estimation, a method based on global radiation and
sunshine duration on the horizontal surface that is described as follows.
When astronomic radiation coming from outside Earth’s atmosphere
passes through to Earth, due to the influences of scattered radiation, ab-
sorption, and reflection by Earth’s atmosphere, radiation finally reaching