Environmental Engineering Reference
In-Depth Information
I
I
T
a
Collector plate
U
(
T
c
-
T
a
)
q
Cover plate
T
c
Figure 7.7
A flat plate collector absorbs incident sunlight on a thermally conducting collector plate that is
cooled by a flow of liquid delivering the absorbed heat to a storage tank. A glass cover plate reduces the heat
loss to the ambient environment.
7.5.1
The Flat Plate Collector
The most common and economical form of solar thermal energy collector is the flat plate collector,
illustrated in Figure 7.7. It consists of a thermally conducting collector plate equipped with passages
through which a heat transfer fluid passes, transferring heat from the collector plate to a fluid
storage tank. The collector plate surface facing the incoming sunlight is treated to absorb as much
of that sunlight as possible. A transparent cover (usually glass) is placed parallel to the collector
plate, forming an enclosed space that reduces the heat loss to the surrounding atmosphere, whose
temperature
T
a
is less than that of the collector plate,
T
c
.
Of the incoming solar irradiance
I
that falls on the cover plate, some is reflected and absorbed
by the cover plate, and only a portion of what is transmitted through the cover plate is absorbed by
the collector plate. Overall, if the fraction of the incoming solar irradiance
I
that is absorbed by the
collector plate is
I
.
18
The warm collector plate will lose
heat to the surrounding atmosphere by heat conduction, convection and radiation at a rate that is
proportional to the temperature difference
T
c
−
β
, then the solar heat flux to the plate is
β
T
a
between the plate and the ambient atmosphere.
The proportionality constant
, called the overall heat transfer coefficient, depends in part upon
the heat transfer properties of air and the radiant heat transfer properties of the collector and cover
plates. As a consequence, the net unit heat flux
q
that is collected in the storage system is the
difference between the absorbed irradiance and the loss to the atmosphere:
U
q
=
β
I
−
U(
T
c
−
T
a
)
(7.2)
Note that the higher the temperature of collection, the smaller the amount of the heat collected,
18
The value of
β
depends upon the angle of incidence of the incoming solar radiation, the refractive and
transmissive properties of the glass cover plate (see Figure 7.7), and the absorptive properties of the collector
surface for solar wavelengths. For the best designs, about 80 % of the incident solar radiation is absorbed by
the collector plate.
