Handbook of air conditioning and refrigeration / Shan K

sin � and the horizontal component OQ � I DN cos�. In Fig. 3.7c, for the right triangle OPQ, angle
POQ � �, and hence OP � I DN cos� cos�. In Fig. 3.7d, OPL is a right triangle. From point P, a
line PT can be drawn perpendicular to the line normal OU, and hence, two right triangles are
formed: OTP and LTP. In OTP, because angle OPT ��, the horizontal component of I DN along the
line normal of the tilting surface OU is OT � I DN cos� cos� sin �. In Fig. 3.7e, draw line O�U�
parallel to line OU. Again, line QM can be drawn from point Q perpendicular to O�U�. Then the
right triangles PTL and QMR are similar. Angles PLT and QRM are both equal to �. In the right
triangle QMR, the component of RQ that is parallel to the line normal to the tilting surface is MR �
I DN sin� cos�.
The intensity of solar rays normal to a tilted surface I �,Btu/h�ft 2 (W/m 2 ) is the vector sum of
the components of the line normal to the tilted surface, or
3.8 SOLAR RADIATION
HEAT AND MOISTURE TRANSFER THROUGH BUILDING ENVELOPE 3.25
I � � IDN cos � �
� I DN (cos � cos � sin � � sin � cos �
(3.53)
Solar radiation provides most of the energy required for the earth’s occupants, either directly or
indirectly. It is the source of indoor daylight and helps to maintain a suitable indoor temperature
during the cold seasons. At the same time, its influence on the indoor environment must be reduced
and controlled during hot weather. The sun is located at a mean distance of 92,900,000 mi,
(149,500,000 km) from the earth, and it has a surface temperature of about 10,800°F (6000°C). It
emits electromagnetic waves at wavelengths of 0.29 to 3.5 �m (micrometers). Visible light has
wavelengths of 0.4 to 0.7 �m and is responsible for 38 percent of the total energy striking the earth.
The infrared region contains 53 percent. At the outer edge of the atmosphere at a mean earth-sun
distance, the solar intensity, called the solar constant I sc, is 434.6 Btu/h�ft 2 (1371 W/m 2 ). The
extraterrestrial intensity I o,Btu/h�ft 2 (W/m 2 ) varies as the earth-sun distance changes during the
earth’s orbit.
Based on the data from Miller et al. (1983), the breakdown of solar radiation reaching the
earth’s surface and absorbed by the earth is listed in Table 3.5. As listed in Table 3.5, only 50 percent
of the solar radiation that reaches the outer edge of the earth’s atmosphere is absorbed by the
clouds and the earth’s surface. At any specific location, the absorption, reflection, and scattering of
solar radiation depend on the composition of the atmosphere and the path length of the sun’s rays
through the atmosphere, expressed in terms of the air mass m. When the sun is directly overhead,
m � 1.
TABLE 3.5 Components of Solar Radiation That Traverse the Earth’s
Atmosphere
Components Breakdowns
Scattered by air 11% Reflected to space 6%
Absorbed by water vapor,
Scattered to earth 5
dust, etc. 16
Intercepted by clouds 45 Reflected to space 20
Absorbed by clouds
Diffused through clouds
4
and absorbed by earth 21
Traversed through air 28 Absorbed by earth 24
Reflected by earth 4