Page 66 - Design of Solar Thermal Power Plants

58 2. THE SOLAR RESOURCE AND METEOROLOGICAL PARAMETERS

the solar concentrator and analyzing its concentration performance, in-
fluences of the cone angle on the concentration performance of the
concentrator must be considered.
Methods for calculating solar position vary; with different algorithms,
solar position values of varying precision can be obtained. A highly
precise solar position algorithm may consider more factors and tends to
be more complex. Eqs. (2.1)e(2.9) are the most basic methods for solar
position calculation without considering the influences of multiple
practical factors on solar position calculation, such as planetary pertur-
bation on Earth, axial procession of equatorial mean pole surrounding the
ecliptic pole, nutation of periodic motion of the equatorial true pole sur-
rounding the mean pole, and atmospheric refraction. The solar position
equation applied in astronomy proposed by Meeus can have a precision
as high as 0.0003 degrees, yet it requires massive calculation. Some solar
utilization facilities having low requirements for solar position precision
use the solar position equation with a high calculation speed for precision
in the range of 0.008e0.01 degrees. The Beijing Badaling 1-MW solar
tower thermal power plant (Badaling), in its heliostat tracking control
programs, applied a solar position algorithm proposed by Roberto Grena
that considers both precision calculation and time consumption calcula-
tion, and solar position precision fell within a range of 0.0027 degrees;
solar altitude and azimuth angle at present hours can be obtained through
calculation. The input parameters of the algorithm mainly include local
longitude, latitude, atmospheric pressure, ambient air temperature, date,
and local time.

2.4.2 Calculation of Tracking Angle

1. Light-receiving surface slope b, is the included angle of the sloped
light-receiving surface against the horizontal surface, 0 b 180 ,

where b > 90 refers to the surface facing downward.

2. Incidence angle, q, is the included angle between the solar incident
beam and the normals of a specific surface, the calculation
equation of which is as follows:

cos q ¼ sin d sin f cos b sin d cos f sin b cos g þ cos d cos f cos b
cos u þ cos d sin f sin b cos g cos u þ cos d sin b sin g sin u
¼ cos q z cos b þ sin q z sin b cos ðg gÞ (2.10)
s
In Eq. (2.10), g refers to the azimuth angle of the light-receiving surface
normal against the horizontal surface, and south by west is deemed the
forward direction.
For a horizontal surface with b ¼ 0 , according to Eq. (2.10), q ¼ q z ;

namely, zenith angle is the incidence angle of the incident solar beam
against the horizontal surface.