Two special coatings are available for application to an absorber plate installed below the cover glass described in Example 12.9. Each coating is diffuseand is characterized by the spectral distributions shown. Which coating would you select for the absorber plate? Explain briefly. For the selected coating, what is the rate at which radiation is absorbed per unit area of the absorber plate if the total solar irradiation at the cover glass is G s = 1000 W/m 2 ?
Two special coatings are available for application to an absorber plate installed below the cover glass described in Example 12.9. Each coating is diffuseand is characterized by the spectral distributions shown. Which coating would you select for the absorber plate? Explain briefly. For the selected coating, what is the rate at which radiation is absorbed per unit area of the absorber plate if the total solar irradiation at the cover glass is G s = 1000 W/m 2 ?
Solution Summary: The author explains that the coating A is selected for the absorber plate with explanation and also find the rate of radiation absorbed per unit area.
Two special coatings are available for application to an absorber plate installed below the cover glass described in Example 12.9. Each coating is diffuseand is characterized by the spectral distributions shown.
Which coating would you select for the absorber plate? Explain briefly. For the selected coating, what is the rate at which radiation is absorbed per unit area of the absorber plate if the total solar irradiation at the cover glass is
G
s
=
1000 W/m
2
?
A typical car's exterior consists of a thin layer of silica (SiO2) over an opaque painted metal panel.
Silica is transparent in the visible wavelengths but offers high reflectance in the near- to mid-
infrared wavelengths. The plot on the next page depicts the diffuse spectral reflectivity (pa) of the
car's surface:
Spectral reflectivity, P₂
0.8
0.6
0.4
ལ
0.2
0
0.1
1
1
10
Wavelength, λ(μm)
100
If the car's exterior temperature is T₁ = 77°C, determine both the total absorptivity (a) and the total
emissivity (a) of the silica-covered panel. Assume that the Sun's temperature is Tsun = 5800 K.
A proposed method for generating electricity from solar irradiation is to concentrate the irradiation into a cavity that is placed within a
large container of a salt with a high melting temperature. If all heat losses are neglected, part of the solar irradiation entering the cavity
is used to melt the salt while the remainder is used to power a Rankine cycle. (The salt is melted during the day and is resolidified at
night in order to generate electricity around the clock.)
9R =
Est-3.45 MW
i
Salt
Tsalt = 1000°C
Mirror
MW
qR
Consider conditions for which the solar power entering the cavity is asol = 7.10 MW and the time rate of change of energy stored in
the salt is Est = 3.45 MW. For a cavity opening of diameter D, = 1 m, determine the rate of heat transfer to the Rankine cycle, qr, in
MW. The temperature of the salt is maintained at its melting point, Tsalt = Tm= 1000°C. Neglect heat loss by convection and
irradiation from the surroundings.
Sun
Heliostats
6.4
Two large diffuse parallel plates are maintained at temperatures T₁ = 1400 K and T₂ = 700 K.
The plates are made from the same metal, and their spectral emissivities as a function of
wavelength, &, are approximated as shown by two constant values joined by a linear decrease
with wavelength. Compute the net radiant energy flux being transferred from plate 1 to plate 2.
What is the energy flux if both plates are assumed gray with an approximate average emissiv-
ity of 0.5 applied over the entire spectral range?
91
T₁ = 1400 K, Ex
T₂ = 700 K, EX
92-91
0.85
Ex
0.15
0
Answer: q₁106,850 W/m²; 91.gray = 68,070 W/m².
2
λ (μm)
7
Applied Statics and Strength of Materials (6th Edition)
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