Handbook of air conditioning and refrigeration / Shan K

3.28 CHAPTER THREE
National Climatic Data Center (NCDC), can be calculated as
I G � I D � I d � I DN � sin � � C
C n 2�
(3.60)
The reflection of solar radiation from any surface Iref, Btu/h�ft2 (W/m2 ), is given as
(3.61)
where �s � reflectance of the surface and Fsr � shape factor between the receiving surface and the
reflecting surface. The ground-reflected diffuse radiation falling on any surface Isg, Btu/h�ft2 (W/m2 ), can be expressed as
Isg � �g Fsg IG (3.62)
where Fsg � shape factor between the surface and the ground and �g � reflectance of the ground.
For concrete, �g � 0.23, and for bitumen and gravel, �g � 0.14. A mean reflectance �g � 0.2 is
usually used for ground.
The total intensity of solar radiation It,Btu/h�ft2 (W/m2 ), falling on a surface at a direction
normal to the surface on clear days, is given by
(3.63)
where I � component of reflected solar intensity in direction normal to surface, Btu/h�ft2 (W/m2 )
Iref. DN � component of reflected solar intensity in direction of sun ray, Btu/h�ft2 (W/m2 Iref � �sFsr (ID � Id) It � ID � Id � Iref. S
� (IDN � Iref.DN) cos � � Id )
Solar Radiation for a Cloudy Sky
For cloudy skies, the global horizontal irradiation IG*, Btu/h�ft2 (W/m2 ), usually can be obtained
from the NCDC. If it is not available, then it can be predicted from the following relationship:
I* G �� (3.64a)
Here Ccc indicates the cloud cover, on a scale of 0 to 10, and can be calculated by
1 � Ccc Q
P � C cc
2 R
P � IG C cc � C T � 0.5 � 4
Ccir. j
j�1
(3.64b)
where C T � total cloud amount
C cir. j � clouds covered by cirriforms, including cirrostratus, cirrocumulus, and cirrus, in j � 1
to 4 layers
Both C T and C cir. j values can be obtained from the major weather stations. The values of coefficients
P, Q, and R, according to Kimura and Stephenson (1969), are listed below:
P Q R
Spring 1.06 0.012 � 0.0084
Summer 0.96 0.033 � 0.0106
Autumn 0.95 0.030 � 0.0108
Winter 1.14 0.003 � 0.0082