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12-61
12-105E A surface is exposed to solar and sky radiation. The equilibrium temperature of the surface is to be determined.
Properties The solar absorptivity and emissivity of the
surface are given to s = 0.10 and = 0.6.
Analysis The equilibrium temperature of the surface
in this case is
R 444=
−=
−=
=−−=
−
s
s
skyssolars
skyssolarsradnet
T
T
TTG
TTGq
4
4
4282
44
44
,
R) 0()RBtu/h.ft 101714.0(6.0)Btu/h.ft 400(10.0
)(
0)(
12-106 Water is observed to have frozen one night while the air temperature is above freezing temperature. The effective sky
temperature is to be determined.
Properties The emissivity of water is = 0.95 (Table A-18).
Analysis Assuming the water temperature to be 0C, the value of
T = 4C
Tsky = ?
Water
Ts = 0C
Tsky = 0 R
Ts = ?
s = 0.1
= 0.6
Gsolar = 400 Btu/h.ft2
Insulation
12-62
12-107 A surface is exposed to solar and sky radiation. The net rate of radiation heat transfer is to be determined.
Properties The solar absorptivity and emissivity of the surface are given to s = 0.72 and = 0.6.
Ts = 350 K
s = 0.72
= 0.6
Gd = 400 W/m2
12-63
12-64
12-110 Prob. 12-108 is reconsidered. The the net rate of solar energy transferred to water as a function of the
absorptivity of the absorber plate is to be plotted.
Analysis The problem is solved using EES, and the solution is given below.
"GIVEN"
12-65
Special Topic: Solar Heat Gain through Windows
12-111C (a) The spectral distribution of solar radiation beyond the earth’s atmosphere resembles the energy emitted by a
12-112C A window that transmits visible part of the spectrum while absorbing the infrared portion is ideally suited for
12-113C A low-e coating on the inner surface of a window glass reduces both the (a) heat loss in winter and (b) heat gain in
12-114C A device that blocks solar radiation and thus reduces the solar heat gain is called a shading device. External shading
devices are more effective in reducing the solar heat gain since they intercept sun’s rays before they reach the glazing. The
12-115C The SC (shading coefficient) of a device represents the solar heat gain relative to the solar heat gain of a reference
12-116C The solar heat gain coefficient (SHGC) is defined as the fraction of incident solar radiation that enters through the
glazing. The solar heat gain of a glazing relative to the solar heat gain of a reference glazing, typically that of a standard 3
12-66
12-67
12-68
12-69
12-122 The net annual cost savings due to installing reflective coating on the West windows of a building and the simple
payback period are to be determined.
Assumptions 1 The calculations given below are for an average year. 2 The unit costs of electricity and natural gas remain
constant.
= 615 kWh/year
Then the decrease in the annual cooling load and the increase in the annual heating load due to reflective film become
Cooling load decrease = Qsolar, summer Aglazing (SHGCwithout film - SHGCwith film)
= (482 kWh/year)(60 m2)(0.766-0.35)
= 12,031 kWh/year
The implementation cost of installing films is
Implementation Cost = ($15/m2)(60 m2) = $900
This gives a simple payback period of
years22.5=== $40/year
900$
savingscost Annual
costtion Implementa
periodpayback Simple
Air
space
Glass
Sun
12-70
12-71
12-124 A house located at 40º N latitude has gray-tinted double pane windows. The total solar heat gain of the house at 9:00,
12:00, and 15:00 solar time in July and the total amount of solar heat gain per day for an average day in January are to be
determined.
Assumptions The calculations are performed for an average day in a given month.
Properties The shading coefficient of a gray-tinted double pane window with 6-mm thick glasses is SC = 0.58 (Table 12-5).
incident solar,glazingincident solar,glazinggainsolar 5046.0 qAqASHGCQ
==
Then the rates of heat gain at the 4 walls at 3 different times in July become
North wall:
W236
) W/m(117)m 4(5046.0
22
22
00:9 gain,solar
==
Q
Double-pane
12-72
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