Agriculture Reference
In-Depth Information
A.1
Chapter 2
A.1.2
Calculation of global radiation as a
function of insolation
(Jones, 1983; Villalobos
et al
., 2002)
Global solar radiation can be estimated if
insolation data are available (Rosenberg
et al
.,
1983) and for Spain it is calculated using the
following equation (Villalobos
et al
., 2002):
cos q
=
sin
(
LAT
)
sin
(
DEC
) +
cos
(
LAT
)
cos
(
DEC
)
cos
(
h
)
(2.5)
n
where:
q
= Zenith angle (Fig. 2.8)
LAT
= Latitude
DEC
= Solar declination: angular position of
the Sun at noon with respect to the plane of
the Equator (ranges between -23.5° y + 23.5°
approximately)
h
= Hourly angle (ranges from 0 to 360°)
being 0° at noon
Referred to the northern hemisphere
R
=
0.25 0.50
+
R
(2.7)
s
a
N
where:
R
s
= Solar global radiation
R
a
= Extraterrestrial radiation = solar constant
n
= Real number of sunshine hours per day
N
= Maximum number of sunshine hours
(see next section)
Villalobos et al., 2002)
[
]
360
DJ
−
172
DEC
=
23.5·
cos
(2.6)
365
The half of a day length (
h
s
) is:
DJ
= Julian day
For instance, for a latitude of 37°N:
In the summer solstice:
DJ
= 172
DEC
= 23.5
q
= 13.49°
In the winter solstice:
DJ
= 355
DEC
= -23.5
q
= 60.5°
In the equinoxes:
q
= 37°
p
h
= ×
arc.cos
(2.8)
180
( ) ( )
−
tan LAT
×
tan DEC
where:
h
s
=
Half of a day length, in radians.
=×
h
N
24
p
(2.9)
