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Fundamental Radiation
an average solar absorptivity of 0.8 and an average emissivity
of 0.8. Assume that the collector plate is so large that you may
treat the problems as 1-D heat flow with heat sources and/or
sinks.
a. Identify and label all significant heat transfer resistances and
flows and draw the steady-state thermal network diagram for
the collector.
""
b. Write the heat balance equations needed to solve for q . Do not
c
solve.
c. If the absorber plate is replaced with a black chrome surface
with an average solar absorptivity of 0.95 and an average emissivity of 0.1, what values will change in the thermal network
""
diagram? How will q change and why?
c
11.9. A solar collector consists of an insulating back layer, a fluid conduit through which a water–glycol solution flows to remove heat,
an absorber plate and a glass cover plate. The external temperatures T air , T sky , and T ground are also shown in the diagram. Solar
""
radiation of intensity q (W/m 2 ) is incident on the collector and
s
""
collected heat q (W/m 2 ) is removed by the fluid. The absorber
c
plate is painted black with an average solar absorptivity of 0.9
and an average emissivity of 0.9. Assume that the collector is so
large that you may treat the problems as 1-D heat flow with heat
sources and/or sinks.
a. Identify and label all significant heat transfer resistances and
flows and draw the steady-state thermal network diagram for
this collector.
""
b. Write the heat balance equations needed to solve for q . Do not
c
solve.
c. If the absorber plate is replaced with a black chrome surface
with an average solar absorptivity of 0.9 and an average emissivity of 0.1, what values will change in the thermal network
""
diagram? How will q change and why?
c
11.10. An opaque, gray surface at 27 ◦ C is exposed to an irradiation of 1000 W/m 2 , and 800 W/m 2 is reflected. Air at 17 ◦ C
flows over the surface and the heat transfer convection coefficient is 15W/m 2 K. Determine the net heat flux from the
surface.
11.11. Consider an opaque, horizontal plate with an electrical heater
on its backside. The front side is exposed to ambient air that is
at 20 ◦ C and provides a convection heat transfer coefficient of
10 W/m 2 K, a solar irradiation (at 5800 ◦ K) of 600 W/m 2 , and an
effective sky temperature of −40 ◦ C. What is the electrical power
(W/m 2 ) required to maintain the plate surface temperature at
T s = 60 ◦ C (steady state) if the plate is diffuse and has designated
spectral, hemispherical reflectivity (reflectivity = 0.2 for wavelength less than 2 μm, reflectivity = 0.7 for wavelength greater
than 2 μm)?
Fundamental Radiation
an average solar absorptivity of 0.8 and an average emissivity
of 0.8. Assume that the collector plate is so large that you may
treat the problems as 1-D heat flow with heat sources and/or
sinks.
a. Identify and label all significant heat transfer resistances and
flows and draw the steady-state thermal network diagram for
the collector.
""
b. Write the heat balance equations needed to solve for q . Do not
c
solve.
c. If the absorber plate is replaced with a black chrome surface
with an average solar absorptivity of 0.95 and an average emissivity of 0.1, what values will change in the thermal network
""
diagram? How will q change and why?
c
11.9. A solar collector consists of an insulating back layer, a fluid conduit through which a water–glycol solution flows to remove heat,
an absorber plate and a glass cover plate. The external temperatures T air , T sky , and T ground are also shown in the diagram. Solar
""
radiation of intensity q (W/m 2 ) is incident on the collector and
s
""
collected heat q (W/m 2 ) is removed by the fluid. The absorber
c
plate is painted black with an average solar absorptivity of 0.9
and an average emissivity of 0.9. Assume that the collector is so
large that you may treat the problems as 1-D heat flow with heat
sources and/or sinks.
a. Identify and label all significant heat transfer resistances and
flows and draw the steady-state thermal network diagram for
this collector.
""
b. Write the heat balance equations needed to solve for q . Do not
c
solve.
c. If the absorber plate is replaced with a black chrome surface
with an average solar absorptivity of 0.9 and an average emissivity of 0.1, what values will change in the thermal network
""
diagram? How will q change and why?
c
11.10. An opaque, gray surface at 27 ◦ C is exposed to an irradiation of 1000 W/m 2 , and 800 W/m 2 is reflected. Air at 17 ◦ C
flows over the surface and the heat transfer convection coefficient is 15W/m 2 K. Determine the net heat flux from the
surface.
11.11. Consider an opaque, horizontal plate with an electrical heater
on its backside. The front side is exposed to ambient air that is
at 20 ◦ C and provides a convection heat transfer coefficient of
10 W/m 2 K, a solar irradiation (at 5800 ◦ K) of 600 W/m 2 , and an
effective sky temperature of −40 ◦ C. What is the electrical power
(W/m 2 ) required to maintain the plate surface temperature at
T s = 60 ◦ C (steady state) if the plate is diffuse and has designated
spectral, hemispherical reflectivity (reflectivity = 0.2 for wavelength less than 2 μm, reflectivity = 0.7 for wavelength greater
than 2 μm)?