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Description  |
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BACKGROUND OF THE INVENTION
The invention relates to a solar panel and more particularly to a high
efficiency solar panel for converting incident solar light into heat.
In some prior art types of solar panels, the incident light passes through
a window to a heat absorber. The window is intended to transmit solar
radiation while trapping the re-emitted infrared radiation from the
absorber. The window also reduces convection losses using "hot house" and
"window pane" techniques. The window sometimes consists of one or more
layers of transparent glass or plastic separated by air. The absorber is
designed to be heated by absorption of the energy incident from the sun
and to transmit this heat to a heat transfer fluid, which is typically
water or air. The absorber is insulated to minimize heat loss to the
surroundings. Each of the components and more particularly the window and
the absorber can be improved by the application of optical techniques.
The window portion should be higly transmitting to light having a
wavelength in the 0.3 to 2 micron range. Also the transmission of the
incident light should be independent of the angle of incidence or
polarization, thus making the solar panel useful even on a cloudy day
while avoiding the use of expensive mechanical tracking systems. Infrared
re-radiation from the absorbing portion should be trapped within the
panel. The window portion should be made of low cost materials without
sacrificing sturdiness.
Some of these requirements are contradictory. For example, in order to make
the window portion sturdy or in order to make it more effective in
trapping infrared re-radiation, some prior art panels are made with
relatively thick window portions which have lower light transmitting
qualities than relatively thinner sections. Thus, the light transmitting
capability is degraded to improve the capability to trap re-emitted
infrared radiation. Furthermore, when the window portion is made
relatively thick the acceptance angle properties of the window portin may
be degraded in some prior art arrangements.
The ideal absorbing portion must have a high absorptivity for light
wavelengths in the 0.3 to 2 micron range and a low emissivity for light
wavelengths in approximately the 10 micron range. For a black body, the
ratio of absorptivity to emissivity is approximately one whereas for a
polished metal surface this ratio is approximately three. Ratios on the
order of nine have been obtained by coating a thin absorption layer over a
reflecting surface. The layer is thick enough to absorb solar radiation
but thin compared to the wavelength of the infrared. In prior art devices
of this type, however, such coatings tend to be difficult to apply and
lose their high absorptivity to emissivity ratio with age. The
disadvantage of a simple polished metal surface is that it is highly
reflecting with a reflectivity constant of approximately 0.9. Further,
design requirements for such absorbing portions are that they should give
good contact with the fluid to be heated, be inexpensive to manufacture
and should reduce convection losses.
SUMMARY OF THE INVENTION
The above described disadvantages of the prior art are overcome and the
requirements for a high efficiency solar panel are met by the present
invention of a solar panel for converting incident light into heat
comprising a heat absorbing portion and a window portion interposed
between the incident light and the heat absorbing portion with at least
one of the heat absorbing and window portions having a plurality of
opposed reflecting surfaces which transmit the incident light by multiple
reflections to the heat absorbing portion. By "opposed reflecting
surfaces" is meant that that separate ones of the plurality of reflective
surfaces are spaced apart from and face each other, either directly or
obliquely, so that multiple reflections can occur.
In some preferred embodiments of the invention, the plurality of opposed
reflecting surfaces converge in the overall direction of the light travel
from the window portion to the heat absorbing portion. In one such
embodiment, for example, the opposed reflecting surfaces are arranged in
V-shaped, parallel grooves.
In some preferred embodiments means are also interposed between the
reflecting surfaces and the heat absorbing portion to reflect infrared
light emitted by the heat absorbing portion back to the heat absorbing
portion. For example, as part of the window portion of one such
embodiment, a plurality of parallel, triangularly shaped wedges are
provided for supporting the opposed reflecting surfaces. Each wedge has
one apex pointing away from the heat absorbing portion and supports a pair
of reflecting surfaces which diverge from the one apex in the direction
toward the heat absorbing portion. Thus, in effect, the reflecting
surfaces facing each other which are supported by each pair of adjacent
wedges converge in the direction of light travel to the absorbing portion.
In this preferred embodiment the means for reflecting the infrared
re-radiation comprises infrared reflecting surfaces which are disposed
between the other two apexes of each wedge, that is, the infrared
reflecting surfaces are arranged so as to face the heat absorbing portion
and are supported adjacent to the flat surface of the bottom of each
wedge. In other embodiments the opposed reflecting surfaces are on
self-supporting substrates arranged in parallel V-groove fashion. Both the
V-groove and wedge configurations preferably have their longitudinal axes
aligned along the direction of solar travel over the panel, for reasons
which will be explained in greater detail hereinafter. The infrared
reflecting surfaces also may have a variety of shapes such as flat,
inverted V-shaped grooves, or even concave, hemicylindrical grooves.
The window portion in other embodiments of the invention comprises a pair
of thin transparent sheets which are separated by a plurality of ribs made
of light transmitting material arranged to transmit the incident light
striking the window within a predetermied angle of acceptance to the heat
absorbing portion by means of multiple total internal reflections. In one
embodiment, for example, the ribs take the form of a plurality of
transparent columns while in other embodiments the ribs are in the form of
transparent zig-zag walls or a honeycomb structure. In these embodiments
the window portion preferably further includes means for segmenting air
spaces between the sheets to reduce convection losses and to distribute
better the mechanical load applied to the window. This form of
construction has the advantages of light weight, sturdiness, high light
transmission and low cost.
In these and other embodiments, the heat absorbing portion may include a
top panel of reflective surfaces arranged in honeycomb fashion to receive
the incident light passing through the window portion and to transmit the
light to the heat absorbing portion. The honeycomb panel has the
reflective surfaces preferably arranged to taper downwardly into the heat
absorbing portion so as to absorb the energy of the light during multiple
bounces and to trap the air above the heat absorbing portion, thereby
reducing convective heat losses.
In still other embodiments the top panel of the absorbing portion is
provided with a plurality of parallel grooves which may, for example, have
a V-cross-sectional shape. The longitudinal axis of such grooves is
preferably oriented along the axis of solar travel over the panel for
maximum receptivity and absorption of the incident light.
Many of the above described features of the invention may be utilized
independently of some of the others, but they are preferably combined into
one embodiment which is highly efficient in converting incident solar
radiation into heat.
It is therefore an object of the present invention to provide a panel which
is highly absorbing for incident light radiation having a wavelength in
the range of 0.3 to 2 microns;
it is another objecct of the invention to provide a heat absorbing panel
which has a low emissivity at radiation wavelengths corresponding to the
temperature of the absorber when the emitted light wavelength equals 5 to
15 microns;
it is a further object of the invention to provide a high efficiency panel
for converting incident light into heat wherein the panel is lightweight
and sturdy; and
it is a still further object of the invention to provide a high efficiency
heat absorbing panel which has low heat losses due to radiation,
convection and conduction.
The foregoing and other objectives, features and advantages of the
invention will be more readily understood upon consideration of the
following detailed description of certain preferred embodiments of the
invention, taken in conjunction with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is an exploded, perspective view, partly in section and with
portions broken away of a solar panel according to a first embodiment of
the invention;
FIGS. 2a and 2b are enlarged perspective views, partly in section and with
portions broken away of alternative window panels for the embodiment
depicted in FIG. 1;
FIGS. 3a and 3b are enlarged perspective view, with portions broken away
and in section of the rib portions of the window panels depicted in FIGS.
2a and 2b;
FIG. 4 is an enlarged vertical view, in section, and with portions broken
away of a window sheet of the embodiment of FIG. 1;
FIG. 5a is a perspective view of a modified window panel for the embodiment
of FIG. 1 with portions broken away;
FIG. 5b is an enlarged vertical view, in section, of the structure depicted
in FIG. 5a;
FIG. 5c is an enlarged vertical view, in section, of a modification of the
structure depicted in FIG. 5b;
FIG. 5d is a perspective view of a second modified embodiment of the
invention, with portions broken away and in section;
FIGS. 6a and 6b are diagrammatic illustrations for use in explaining the
operation of the window panels depicted in FIGS. 5a-5d;
FIG. 7 is a perspective view of a third modified window panel for the
embodiment of FIG. 1, with portions broken away; and
FIGS. 8 and 9 are perspective views partly in section and with portions
broken away of alternative top panels for the absorbing portion.
DETAILED DESCRIPTION OF CERTAIN PREFERRED EMBODIMENTS
Referring now more particularly to FIG. 1, the combined solar panel of the
invention includes a window panel portion 10 and a heat absorbing portion
12. The heat absorbing portion 12 further includes a fluid heat exchanging
chamber 14 and an insulating back 16 for the heat exchanging chamber 14.
The heat exchanging fluid, such as water or air, is continuously admitted
to the chamber 14 through an inlet pipe 15 and is exhausted from the
chamber by an outlet pipe (not shown). In operation, incident solar
radiation 18 passes through the window portion 10 to strike the heat
absorbing portion 12. The heat generated in the heat absorbing portion 12
by the solar radiation 18 is transferred to fluid passing through the heat
exchanging portion 14. This transfer of solar generated heat into the
fluid raises its temperature.
As mentioned above, the window portion must be designed to accept solar
radiation 18 over as wide an angle of incidence as possible. It also must
be designed to prevent heat loss through reradiation from the heat
absorbing portion 12 and heat loss due to convection currents above the
heat absorbing portion 12.
The window panel 10 includes a pair of thin, transparent sheets 20 and 22
which are spaced apart by a plurality of upstanding columns or ribs 24
which are also made of lightweight, transparent material and preferably of
a material which has high optical transmission qualities. In practice, the
material chosen for the sheets 20 and 22 and the ribs 24 may be
lightweight plastic. In other embodiment, the top sheet 20 may be glass
for ruggedness.
Means are provided for segmenting the air space between the sheets 20 and
22 to reduce heat loss due to convection currents which would otherwise
develop between the two sheets. In the embodiment depicted in FIG. 1, the
means for segmenting the space comprises a plurality of transparent
bubbles 26 on the upper surface of the sheet 22. The bubbles 26 may be
made of a transparent material such as plastic. The bubbles 26 can be
pressed flat against the flat sheets 20 and 22 to reduce reflection losses
at the spherical surface.
Referring now more particularly to FIGS. 2a and 2b, other types of columns
and air space segmenting are depicted. In the embodiment of FIG. 2a the
window sheets 20 and 22 are separated by a plurality of thin sheets 28
turned on edge to form a plurality of parallel ribs which extend the
length of the sheets 20 and 22. In the modified embodiment depicted in
FIG. 2b, the ribs are a plurality of sheets 30 turned on edge and bent in
a zig-zag fashion. In the embodiments of FIGS. 1, 2a and 2b, the ribs 24,
28 and 30 are made of lightweight, optically transparent material. In
still other embodiments (FIG. 7), the light transmitting structural
support between the sheets may be a honeycomb or other shapes.
The purpose of having the ribs 24, 28 or 30, in addition to separating the
sheets 20 and 22 to form an insulating air space, is also to provide means
for transmitting incident light striking the top sheet 20 to and through
the bottom sheet 22 so that the transmitted light will eventually strike
the absorbing portion 12. By providing these ribs, the incident light is
accepted over a wider incident angle than if the ribs were opaque and the
only light reaching the absorbing portion was light which managed to
penetrate both the top and bottom sheets 20 and 22. From Snell's law that
n.sub.1 sin.theta..sub.1 = n.sub.2 sin.theta..sub.2 (where .theta..sub.1
and .theta..sub.2 are the incident angle and the refracted angle,
respectively) and the principle of total internal reflection that the
angle (.theta..sub.3) for light leaving the material having the higher
index (n.sub.2) of refraction into a material (such as a coating) of a
lower refractive index (n.sub.3) must be 90.degree. (See FIG. 3b):
##EQU1##
Note that .theta. is measured with respect to the normal of the top
surface of the rib, and therefore the actual maximum acceptance angle in
the above examples is 2.theta..sub.1. In the case of wall type ribs, such
as ribs 28 and 30, there is some advantage in aligning their longitudinal
axis with the direction of travel of the sun over the panel. When this is
done the bulk of the incident light will always be within the acceptance
angle since the acceptance angle for light contained in all planes which
contain the longitudinal axis is substantially 180.degree..
n.sub.1 = refractive index for air or n.sub.1 = 1
n.sub.2 = refractive index of material of rib 28, and
n.sub.3 = refractive index of a protective dielectric coating 29 on rib 28,
where rib 28 is uncoated, n.sub.3 = 1 and
##EQU2##
While the above described mathematical relationships between the angle of
acceptance and the indices of refraction are given with respect to the rib
28, it should be apparent that substantially similar relationships apply
to the ribs 24 and 30. Thus, light is transferred through the area of the
window portion which is covered by the ribs by means of multiple, total
internal reflections within the ribs 24, 28 or 30.
In some embodiments it is preferable to coat the sheets 20 and 22 with a
transparent hard coating 32 in order to prevent scratching and an
anti-reflection coating 34 to minimize reflection losses (see FIG. 4).
Further, in some embodiments it is preferable to stack a number of window
sections 10 to minimize heat losses due to convection and reradiation from
the absorbing portion 12.
Referring now more particularly to FIGS. 5a and 5b, still another
embodiment of the window section 10 is illustrated. In the embodiment of
FIGS. 5a and 5b a panel of optical valves 36 are located beneath the
sheets 20 and 22. The optical valves 36 have the effect of admitting
incident light to the structure but blocking the escape of a large
percentage of the reflected infrared radiation emitted by the absorbing
portion 12. The basic structure of each optical valve in the panel 36 is
to provide a plurality of opposed reflecting surfaces which converge in
the overall direction of light travel from the window portion 10 to the
heat absorbing portion 12. The reflecting surfaces of each optical valve
of the panel 36 may be in the form of a plurality of reflecting V-grooves
or in honeycomb cells but both arrangements have wider openings at the top
surface, that is, the surface first struck by the incident solar
radiation, than at the bottom surface, that is, the surface closest to the
heat absorbing portion 12. The optical valve panel 36 may be composed of
combinations of metal and/or dielectric materials.
In the embodiment depicted in FIGS. 5a and 5b, a plurality of downwardly
converging V-grooves are formed by constructing the panel 36 of upwardly
pointing, triangularly shaped wedges 38 of low refractive index material
interspersed between downwardly pointing, truncated triangularly shaped
wedges 40 of high refractive index material. The incident solar radiation
is transmitted through the high refractive index medium 40 and is
reflected at each interface formed between a wedge of high refractive
index material 40 and a wedge of low refractive index material 38. Thus,
these interfaces formed by the discontinuities in the refractive indices
are effectively reflective surfaces 42 for the incident solar radiation.
Such a reflecting surface 42 is formed on the opposite, downwardly
converging sides of each wedge 40 and thus light entering the top of the
wedge 40 is multiply reflected downwardly towards the heat absorbing
portion 12 (not shown in FIG. 5b) which is below the optical valve panel
36.
In order to prevent the escape of infrared radiation emitted by the heat
absorbing portion, a plurality of infrared reflecting surfaces 44 which
face the heat absorbing portion 12 are provided on the bottom of each
wedge 38. Therefore this optical valve panel 36 is highly transmitting for
incident solar light and highly reflecting for the far infrared which is
re-emitted by the heat absorbing portion 12.
Referring now more particularly to FIGS. 5c and 5d, metal or multiple
dielectric layers are used for reflection in place of the wedges 38 and 40
of transparent material. In particular, in the embodiment of FIG. 5c a
plurality of opposed reflecting surfaces 46 are embedded in a transparent
medium 48. The opposed reflecting surfaces 46 are inclined to converge
downwardly towards the heat absorbing portion 12. The infrared reflecting
surfaces 44 are placed on the bottom of the panel of material 48 between
pairs of opposed converging surfaces 46 in a manner similar to the
embodiment of FIGS. 5a and 5b.
In the embodiment depicted in FIG. 5d, thin flat laminates 50 are attached
together along one edge to form a downwardly opening V-shaped channel 52.
A plurality of the V-shaped channels 52 are aligned parallel to each other
and extend along the length of the solar panel. The opposed reflecting
surfaces of each adjacent pair of reflecting channels 52 form a pair of
downwardly converging reflecting surfaces as in the other optical valve
embodiments described above with reference to FIGS. 5b and 5c. The bottom
of each V-shaped channel 52 is open so that infrared radiation emitted
from the heat absorbing portion 12 strikes the interior surfaces of the
V-shaped channel 52 which face the heat absorbing portion 12 and is
re-reflected back to the heat absorbing portion 12 by multiple reflections
within the V-shaped channels 52 as indicated by the dashed line path in
FIG. 5d. In other embodiments, the infrared reflecting surfaces may be
flat, curved or even corner cubed and are not necessarily merely the
backside of the laminates 50.
The reflective surface 46 and 50 may be polished metal or multiple layers
of dielectric coatings over the channel surfaces to form a reflective
laminate. One advantage of dielectric coatings over metal surfaces is that
dielectric coatings can be made with a higher reflection coefficient than
the metal surfaces alone but generally for smaller angles and wavelength
ranges.
One important consideration in the design of these optical valves 36 is the
acceptance angle for solar radiation. If the light striking the top of the
panel 36 is at too large an angle with respect to the normal to the top of
the panel 36, then the incident light will be returned out the top or
refracted out of the panel rather than be passed through it by multiple
reflections. In all planes containing the apex line of the V-groove, all
incident angles of solar radiation are accepted. Thus, with reference to
FIG. 5a, incident light striking the panel 36 in a plane which is normal
to the panel 36 and which passes through the lower apex of any given wedge
40 will be accepted.
Referring now more particularly to FIG. 6a, which is an enlarged and
perspective view of the embodiment depicted in FIG. 5d, the apex line is
defined as a line 54 which is centered between the opposed reflecting
surfaces 50 at their closest points and parallel to the top surface of the
panel 36. For light striking the top of the panel 36 in a plane which is
normal to the apex line 54, it can be shown that the acceptance angle of
the incident light is:
##EQU3##
and the full acceptance angle, 2.theta..sub.max (i.e. the light striking
from either side of a line normal to the top of the panel 36) is:
##EQU4##
where d.sub.2 is the width of the smaller end, and d.sub.1 is the width of
the larger end of the V-shaped groove formed by the opposed reflecting
surfaces 50. For any larger angle the incident light will not be multiply
reflected toward the apex but instead will be reflected upwardly of the
V-groove.
When the groove consists of two different dielectrics, as depicted in FIGS.
6b and 5b, for example, the full acceptance angle can be shown to be:
##EQU5##
where n = refractive index of wedge 40, and n' = refractive index of wedge
38. See Fiber Optics, pp. 18-21, by N. S. Kapany, the present applicant,
(Academic Press, New York 1967).
While in the above described embodiments, the optical valve panel 36 is
illustrated in the figures as comprising a plurality of V-grooves, in
other embodiments (FIG. 7), as mentioned above, it has a honeycomb cell
structure 60 wherein the opening closest to the incident light is wider
than the opening which is closest to the absorbing portion 12. The opposed
plane surfaces inside each honeycomb cell of the structure 60 provide a
plurality of opposed reflecting surfaces which operate in substantially
the same manner as the V-groove arrangement shown above so that a detailed
description of the honeycomb arrangement will be omitted.
The honeycomb structure 60 may be situated between the sheets 20 and 22 (as
shown in FIG. 7) or it may be a separate panel below the sheet 22. The
plane surface area 62 of the structure 60 below and between the honeycome
cells is preferably a reflecting surface for the infrared radiation
emitted by the absorbing portion 12.
One advantage of this structure over the V-groove or wedge configurations
is that it is non-axially directional. The V-groove or wedge structures of
FIGS. 5a-5d preferably are utilized with their longitudinal axes aligned
with the sun's direction of travel over the panel so that the bulk of the
incident light will always strike within the incident angle of acceptance.
This is true because for light contained in a plane passing through their
longitudinal axes the V-groove or wedge structures have an acceptance
angle of nearly 180.degree.. With the honeycomb structure 60, however, the
sun's direction of travel is not material since light is accepted in the
same manner for any given incident angle within the acceptance angle with
respect to the normal to the panel for all radial directions about the
panel.
Another advantage of the honeycomb cell structure 60 over some other
designs is that it is very effective in segmenting the air space above the
absorbing portion to reduce convection heat losses.
Referring again more particularly to FIG. 1, as mentioned above, the ideal
absorber has a high absorptivity in the wavelength range of 0.3 to 2
microns and a low emissivity in the wavelength range of approximately 10
microns (the far infrared). In the embodiment depicted in FIG. 1, the
absorbing portion 12 has a tapered honeycomb structure 56 embossed
directly on the upper face of the top panel 58 of the heat exchanger 14.
The embossed honeycomb structure 56 provides multiple bounce absorption of
the incident light ray transmitted through the window portion 10. The
multiple bounce absorption is substantially similar in operation to that
described above with reference to the reflecting surfaces for the optical
valve structure 36, that is, the incident light passing through the window
10 is reflected a multiple number of times within the hollows of the
honeycomb structure 56 until it strikes the upper surface of the heat
exchanger 14 where the incident light is converted into thermal energy.
The honeycomb structure 56 gives good contact between the heat exchanger 14
and the fluid to be heated and it is inexpensive to manufacture in large
quantities. In other less advantageous embodiments, the honeycomb
structure 56 is not tapered downwardly towards the heat exchanging portion
14 and has a substantially uniform cross section. The advantage of having
a multiple bounce absorption structure is that it provides high absorption
with low emissivity of infrared. This is because although a polished
reflector has a low absorptivity, .rho., for a single bounce of the
incident radiation, the fractional absorption for n bounces is (1 -
.rho.).sup.n.
For example, if .rho. equals 0.8 and n = 3, approximately 99% of the
incident energy is absorbed by the absorbing portion 12. In this manner a
high ratio of absorptivity to emissivity can be obtained while still
maintaining appreciable absorption. In addition, the honeycomb structure
illustrated in FIG. 1 reduces convection losses because the air above the
heat exchanging portion 14 is compartmentalized. Furthermore, the top of
the honeycomb structure 56 can be covered with a light transmitting panel
which may, for example, be the bottom sheet 22 of the window portion to
enhance the compartmentalization of the air above the heat exchanging
portion 14.
In other embodiments, the top panel 58 of the absorber may have V-grooves
similar to those shown for the optical valves 36, for example. In still
another embodiment, referring more particularly to FIG. 8, the top panel
58 has a plurality of upstanding metallic fibers 64. These fibers 64
absorb the heat energy from the incident light and conduct it to the panel
58. They also reduce heat loss due to convection by trapping the air
immediately above the panel 58. The fibers 64 are preferably coated to
more effectively absorb the heat energy of the light while reducing the
infrared emission.
The panel embodiments have been described above as generally flat but in
other embodiments they may be curved to give a concentrated effect. In
such embodiments, either or both the window and heat absorbing portions 10
and 12, respectively, may be curved. Thus, for example, referring more
particularly to FIG. 9, the honeycomb dimpled top panel 66 of the heat
absorber 14 may have a curved cross section.
Although the insulation backing for the heat exchanger 14 is shown as being
fiberglass in FIG. 1, in other embodiments the insulation may be composed
of a multilayer structure of alternating sheets of reflective and
dielectric material. In still other embodiments the insulating portions
may be segmented plastic sheets of the type described above in reference
to the window portion 10 with a reflecting surface below.
While the panel of the invention has been described above for use in
converting solar radiation into heat, it should be apparent that it may
also be used for converting incident radiation from other sources, both
natural and man made, into heat.
The terms and expressions which have been employed here are used as terms
of description and not of limitation, and there is no intention, in the
use of such terms and expressions, of excluding equivalents of the
features shown and described, or portions thereof, it being recognized
that various modifications are possible within the scope of the invention
claimed.
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