Note: Descriptions are shown in the official language in which they were submitted.
1~25~'~5
3ACKGROUND OF THE INVENTION
(1) Fielt of the Invention
The present invent$on relates broatly to solar
heaters which convert solar ratiation into heat energy
and transfer the abosrbed heat either to a gas such as
air or a liquid such as water, the former type being
commonly referred to as a solar air heater and the latter
being com~only referret to as a solar water heater.
More particularly, the present invention relates to solar
heaters of either type which employ a heat trap between
the absorber and the light-transmitting front wall.
(2) Description of the Prior Art
Various proposals have already been made in the
prior art for employing a heat trap between the absorber
and front wall of either a flat plate solar heater or 8
transpiration solar air heater. Thus, Hollands discloses
the use of a transparent honeycomb heat trap in a flat
plate solar heater in an article entitled, "Honeycomb
Devices in Flat Plate Solar Collectors", Solar Energy,
Vol. 9, pp. 159-169, Pergamon ~ress (1965). The trans-
parent honeycomb heat trap, in this instance, serves tO
suppress the onset of natural convection currents and
additionally reduces heat losses by radiation.
- It has been further discovered that a transparent
honeycomb heat trap significantly increases the overall
efficiency of a transpiration solar air heater when
interposed between the porous absorber and front wall as
discloset ant ciaimed in our copending application Canadian
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Serial No. 307,619 filet on even tate herewith and
assigned to ~he common asslgnee hereof. As further
disclosed ant claimet in our copendlng application, it
has been fount that the honeycomb heat trap, when placed
ln at least firm mechanical contact with the front wall,
serves the additional function of prov~ding an air buffer
layer without the interposition of additional surfaces
from which incident sunlight can be reflected away from
the absorber and be lost.
Transparent cellular structures such as
clear plastic or glass honeycombs, which are now contem-
plated for use as heat traps in the solar heaters
described above, have been made by methods well known
in the prior art. In one method that has been used
heretofore, clear plastic or glass tubes are stacked
and bonded together by a suitable adhesive or solvent.
In another method, a multiplicity of elongated narrow
strips of plastic film are first coatet with an adhesive
at spaced apart intervals ant then adhered together.
This is followed by expansion into a hexagonal honeycomb
structure. The use of such bonded expanded honeycomb in
a flat plate solar heater has been described in an
article entitled, "Effect of a Mylar Honeycomb Layer on
Solar Collector Performance",by Chun and Crandall presented
at the 1974 Winter Annual Meeting of the ASME tPaper No.
74-WA/HT-ll).
A tisadvantage common to all honeycomb structures
fabricatet by the above described and other similar
techniques is the presence of adhesive bonds between
ad3acent cells. These adhesive bonds give rise to certain
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problems when the honeycomb is used as a heat trap in a
solar heater. One such problem is that the adhesive bonds
act as scattering sites for incident light and thus
reduce the overall transmission of solar rays through
the honeycomb at all sun angles different from normal
incidence. Thus, at all times different from solar noon,
the fraction of incident sunlight which reaches the solar
absorbing surface in the solar heater is reduced , with a
consequent reduction in conversion efficiency. Another
problem which can be associated with the use of any
- adhesive is that the adhesi~e will age with attendant
cracking, embrittlement and discoloration. Furthermore,
fabrication of clear plastic or glass honeycombs using
an adhesive is complicated by the fact that adhesives
require special handling and thus make the fabrication
process time-consuming and expensive.
SUMM~.RY OF THE INVENTION
The present invention is directed to a novel
and improved solar heater which may be either a flat
plate solar heater or a transpiration solar air heater
and which overcomes the above enumerated problems of the
prior art. The solar heater of the present invention
includes a housing having a light-transmitting front
wall and a radiation absorbent collector element arranged
to accept incident solar radiation passing through the
front wall. The solar heater further includes a heat
trap which is constructed at least in part from an open
cellular structure made in one piece from a thermoformable
material. Specifically, the open cellular structure from
~ 5
which the heat trap is constructed is composed of a
multiplicity of adjacent cells each having walls
which are common to other cells in the structure and
which walls are integrally formed with the walls of
other cells in one continuous piece. The heat trap
thus formed has no adhesive bonds joining the walls of
adjacent cells and consequently has a high transmission
of solar rays compared to bonded honeycombs of the
prior art. Furthermore, the heat trap is not subject to
deterioration due to the aging of an adhesive. As
shall be described in greater detail hereinafter, the open
cellular structure can be readily made from the product
of an expanded core process or by injection molding.
It is the principal ob;ect of the present
invention to provide a solar heater having a heat trap
characterized by a high transmission of incident sunlight
as compared to heat traps of the prior art.
Another ob;ect of the present invention is to
provide a solar heater having a heat trap which is formed
in whole or in part by a cellular structure composed of
a multiplicity of cells having common walls which are
integrally formed as one piece without adhesive bonds.
Still another object of the present invention is
to provide a heat trap for a solar heater which contains
no adhesives and which is consequently easy and economical
to manufacture.
DESCRIPTION OF THE DRAWING
The present invention will now be described
in greater detail hereinafter with particular reference to
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the accompanying drawing which shows the preferred
embodiments thereof and wherein:
Figure 1 is an elevational schematic, cross-
sectional view of a typical flat plate solar heater made
in accordance with the present invention;
Figure 2 is a similar view showing a typical
transpiration solar air heater also made in accordance
with the present invention;
Figure 3 is an enlarged perspective view of a
part of the transpiration solar air heater shown in
Figure 2;
Figures 4a - 4c are elevational, schematic,
cross-sectional views of the platens used in the expanded
core process for forming articles from which heat traps
of the present invention can be made, the platens being
shown in their relative positions during different stages
of the process;
Figure 5 is a top plan view showing individual
film strips before bonding into a honeycomb structure
according to methods of the prior art;
Figure 6 is a similar view of the honeycomb
structure which is made by bonding the film strips shown
in Figure 5;
Figure 7 is a similar view of another type of
honeycomb structure fabricated from tubes bonded by
techniques of the prior art;
Figure 8 is a schematic view showing a typical
solar ray incident on a bonded honeycomb cell wall and
the resulting pattern of transmitted, reflected and
scattered rays;
Figure 9 is a schematic view showing a typical
solar ray incident on a honeycomb cell wall of the present
invention and the resulting pattern of transmitted and
reflected rays; and
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Figure 10 is a graph showing the relationship
between the angle of incidence and the solar transmission
of several heat traps;
DESCRIPTION OF THE PREFERRED EMBODDMENTS
It will be understood that the principles of
the present invention are applicable to both a flat plate
solar heater and a transpiration solar air heater, although
the heat trap performs somewhat different functions in
each type of solar heater. For purposes of convenience,
the principles of the present invention will be disclosed
independently with respect to each type of solar heater in
the following description.
Referring now specifically to Figure 1 of the
drawing,there is shown a flat plate solar heater embodying
the present invention. The solar heater comprises a housing
10 having a light-transmitting front wall 12 which passes
incident solar radiation and a back wall 14. The front
wall 12 is preferably made from a clear or transparent
material having a relatively low reflectivity and which
is non-porous and gas impermeable, e.g., clear plastic or
glass. A flat radiation absorbent collector plate 16 is
mounted within the housing 10 in spaced apart relation to
the front wall 12 and back wall 14. The collector plate 16
is arranged in the housing 10 so as to intercept solar rays
transmitted through the front wall 12. A tubular coil 18
or other passage means for a fluid such as air or water
is provided in contact with the flat collector plate 16.
Preferably, the coil 18 is located in the space below the
collector plate 16 as shown. The remaining space between
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the flat collector plate 16 and the back wall 14 may be
fiYlet with a suitable insulation, e.g., glass wool, as
denoted by the reference nume~al 20. The housing 10 may
suit:ably be made of a rlgid metal such as alum~num or
steel or other rigid material ~uch as plastic or fiberglass.
The housing 10 further include~ a heat trap 22
which is positioned ~ust beneath the front wall and which
is made from a cellular structure 8uch as transparent
honeycomb constructed in one piece in accordance with the
present invention. More specifically, the heat trap
is composed of a multiplicity of cells 24 having cell walls
26 which are substantially perpendicular to the front wall
12.
During operation of the solar heater, incident
solar rays pass through the front wall 12 and the heat
trap 22 and are absorbed by the flat collector plate 16
where they are converted to heat. This heat , in turn, is
transferred by conduction and convection to a fluid such
as air or water which is circulated through the coil 18
in contact with the collector plate 16.
In this embodiment of the present invention, the
heat trap 22 serves the dual function of reducing the
radiative heat loss from the solar heater and of suppressing
the onset of natural convection in the air space between
the flat collector plate 16 and the front wall 12. In
order for the heat trap to effectively reduce heat loss
by radiation the cells 24 must be of sufficlently high
aspect ratio as described i~ detail in our copending appli-
cation Canadian.Serial No. 307,619 supra, i.e., in the range
of 2 to 10 for honeycomb cellular structures. As shown
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in Figure 1, the cell walls 26 divide up the air space
between the flat collector plate 16 and the front wall 12
and inhibit the development of natural convection currents.
To allow for differential thermal expansion of the solar
heater elements, a gap may be provided either above or
below the heat trap.
Figure 2 shows a transpiration solar air heater
embodying the present invention. As shown, the transpira-
tion solar air heater comprises a housing 28 having a
transparent front wall 30, e.g., clear plastic or glass,
and a back wall 32. The housing 28 further includes an
inlet 34 in one side wall and an outlet 36 in the opposite
side wall. The inlet 34 and outlet 36 establish a flow
path through the housing 28 for a gas such as air to be
heated as generally indicated by the arrows in the drawing.
A porous, radiation absorbent collector plate 38 is mounted
inside the housing 28 in spaced apart parallel relation
to the front wall 30 and the back wall 32 and across the
flow path established between the inlet 34 and the outlet
36. The porous collector plate 38 may be composed, for
example, of a porous darkened or black fibrous mat, woven
or stamped screens, or reticulated foam. Although the
porous collector plate 28 is shown in spaced parallel
relation to the back wall, it will be understood that
the collector plate may be positioned in non-parallel
relation to the back wall as disclosed and claimed in our
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copenting application Canadian-Serial No.307,619 ~upra. If
desired, a layer of insulation 40 may be placed at~acent
to the back wall 32 and in spaced apart relatlon to the
porous collector plate 38. Again, the housing 28 may
be made of rigid metal such as aluminum or steel or other
rigid materials such as plastic or fiberglass.
In accordance with the present invention, a
heat trap 42 is positioned ~ust beneath the front wall 30.
The heat trap is made from a cellular structure such as
honeycomb which is also constructured in one piece as
shall be described in greater detail hereinafter. The
heat trap in this embodimentis basically the same con-
struction as that shown in Figure l having cells 44
defined by cell walls 46 which are positioned substantially
perpendicular to the front wall 30. In this instance,
however, the assembly of the heat trap 42 in the housing
28 is such as to provide an englarged space 48. This
space 48 provides a passage for the gas such as air to
be heated between the lower surface of the heat trap and
the porous collector plate 38.
The operation of the transpiration solar air
heater is similar in that incident solar rays pass through
the transparent iront wall 30 and the heat trap 42 and
are absorbed by the porous collector plate 38 and converted
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to heat. However, in this lnstance, the g88 or air to
be heated enters the inlet 34 and follows the flow path
indicated by the arrows in the drawing. The gas or air
then passes or transpires through the entire porous
collector plate 38 and iB heated. The heated gas or air
then exits via the space 50 below the porous collector
plate 38 and through the outlet 36. It should be noted
that `in this embodiment, the heat trap 42 serves the dual
function of reducing the radiative heat loss from the
solar heater and of proviting an air buffe~ layer composed
of baffles which prevent the forced convective flow of
the gas or air to be heatet ad~acent to the front wall
where heat losses may occur. As discloset and claimed in
our copending application Canadian Serial No. 307,619 supra,
the heat trap ls preferably maintained in at least firm
mechanical contact with and may actually be bonded to
the front wall 30 in order to improve its effect as an
air buffer. With this specific structure it is not
necessary to employ an additional gas impermeable layer
to prevent the flow of gas through the honeycomb and into
contact with the front wall. The aspect ratio of the
heat trap in this instance is basically the same as that
descrlbed above, that is, in a range of between about 2
and 10 for honeycomb cellular structures.
~ 5
Although the heat trap has been shown in
Figure 2 as being just beneath the front wall 30 of the
transpiration solar air heater where it functions as
an air buffer and radiation trap, it will be understood
that the present invention is not restricted to this
location of the heat trap. Thus, as disclosed and
claimed in our copending application Serial No.
supra, which is incorporated herein by reference,
it may also be advantageous to position the heat trap
on top of the porous collector plate or to locate the
heat trap in a position intermediate the collector plate
and front wall.
Figure 3 shows in greater detail the heat trap
42 used in the transpiration solar air heater of Figure 2.
Although the heat trap may be made from a variety of
cellular structures, it is preferred to employ a
hexagonal honeycomb configuration as shown in the
drawing. As shown, the honeycomb structure is composed
of a multiplicity of hexagonal cells 44 which are arranged
adjacent to one another and are defined by the walls 46
which are common to other cells in the structure and
which walls are integrally formed with walls of other cells
in one continuous piece. It may be noted that the cell
walls 46 are oriented substantially perpendicular to the
plane of the front wall. As already indicated, the heat
trap used in the flat plate solar heater of Figure i may
be of basically the same hexagonal construction.
The heat trap may be made in one piece from
the products of conventional forming or molding techniques
as shall be described hereinafter in greater detail.
Suitably, the heat trap may be made from glass or clear
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plastic compositions such as polyvinyl fluoride, poly-
carbonate, fluorinated ethylene propylene, polymethyl
methacrylate, aromatic polysu:Lfones, polyethylene
terephthalate, aromatic polyesters, polyvinylidene
fluoride, hexafluoropropylene, chlorotrifluoroethylene
and tetrafluoroethylene copolymers.
In the embodiments of the present invention
shown schematically in Figures 1-3, the heat traps are
shown with cells having relatively thick walls for purposes
of illustration. It will, however, be understood that
in order to effectively function as heat traps the cell
walls must be made relatively thin, i.e., in the range
of 0.0002 and 0.05 centimeters.
The heat trap can be made by conventional
forming or molding techniques well known in the art.
For instance, heat traps can most advantageously be made
from articles made by the expanded core process disclosed
and claimed in U.S. Patent No. 3,919,446 issued to W.H.
Smarook on November 11, 1975, and assigned to the common
assignee hereof. Variations and improvements of this
process and apparatus for carrying out the process are
disclosed in the following patents: U.S. Pat. Nos.
3,765,810, 3,919,379, 3,919,380 and 3,919,445.
Figures 4a - 4c show different stages of the
expanded core process. In the first stage of the process,
a blan~ 52 of thermoformable material is placed between
two heated platens 54, 56 provided with a pattern of vent
holes 58 as shown in Figure 4a. In the next stage the
heated platens 54, 56 are brought into contact with the
blank 52 as depicted in Figure 4b. The platens are then
allowed to separate under the force of the compressed
bias springs 60, 62 expanding the cross-section of the
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of the blank 52 in a manner whereby voids such as
cells 64 of hexagonal shape, for example, are formed
from the surfaces of the blank. As shown in Figure 4c
the cellular structure thus formed is made in one
continuous piece with cell walls 66 common to adjacent
cells 64. These walls are integral with other cell walls in
the cellular structure as more particularly shown in the
perspective view of Figure 3. Ordinarily, articles
formed by this process include a perforated skin on one
or both sides such as shown at 68 in Figure 4c. The skin
or skins must be removed before the article is suitable for
use as a heat trap. The perforated skins, if not removed,
would be oriented such that incident solar rays reflected
from them would be directed away from the solar absorber
and be lost. The perforated skins may be removed by
passing an electrically heated wire through the cell walls
immediately ~djacent to the skinsor by use of a reverse-
cutting,knife-edge band saw blade such as that used for
the cutting of metallic honeycomb~. For a better under-
standing of the process, reference should be made to the
specifications of the aforementioned U.S. patentC to
W.H. Smarook. It will be further understood, of course,
that the heat trap can be formed by other processes such
as injection molding wherein thermoformable material is
injected under pressure into a mold having the desired
cellular configuration.
As indicated above, heat traps for use in solar
heaters of the prior art have been made using a bonded
expanded honeycomb. Such honeycomb structures can be
made by the bonding technique illustrated in Figures
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3L~Z5~25
5 and 6. In this technique a multiplicity of strips
of plastic film, as shown at 70a, 70b and 70c, are
crimped or otherwise shaped into corrugations and placed
adjacent to one another with the flat surfaces 72a,
72b and 72c aligned. A suitable adhesive is then
applied to at least one of the flat surfaces and the
surfaces are adhered to one another to form an hexagonal
honeycomb as shown in Figure 6. The aligned surfaces
72a, 72b and 72c when adhered together form a multiplicity
of cemented joints as at 74 throughout the honeycomb
structure.
A similar honeycomb structure for use in solar
heaters can be made from a multiplicity of transparent
tubes 76 as shown in Figure 7. The tubes 76 are placed
in like manner adjacent to one another and then coated
with an adhesive along their longitudinal surfaces. The
tubes 76 are then adhered together through a multiplicity
of bonded j oints 78.
Any type of bonded honeycomb such as those
described above suffers from the disadvantage that the
bonded joints act as scattering sites for incident light.
Figure 8 shows a typical solar ray incident on a bonded
honeycomb cell wall and the resulting pattern of trans-
mitted, reflected and scattered rays. The incident ray
80 is partly transmitted as at 80a and partly reflected
as at 80b. Part of the incident ray 80 is also scattered
into a multiplicity of diffuse rays as indicated at 80c.
The diffuse rays 80c result from the scattering of light
at the adhesive bonds 74 of the bonded expanded honeycomb
shown for example in Figure 6. Similar scattering of
5~ ~ 5
incident ~olar rays occurs at the bonted ~oints 78 of
the tubular honeycomb 6hown in Figure 7. lt will thus
be seen thst the presence of adhesive bonds in honeycomb
structures csn significantly reduce the overall
transmiss$on of solsr rays through the honeycomb.
This retuction in light transmission will occur at all
sun angles different from normal incidence, resulting in
reduced conversion efficiencies at all times different
from solar noon.
Figure 9 shows a typical solar ray incitent
on a cell wall of a honeycomb structure used as a heat
trap in accrodance with the present invention. It will
be noted that the honeycomb in this case is constructed
in one continuous piece with integrally for~ed cell walls
as depicted at 82 and that therefore there are no bonded
~oints to act as scattering sites for incident light.
The incident ray 84 is partly transmitted as at 84a and
partly reflected as at 84b. Since there are no scattering
sites in the honeycomb cell walls, there are no scattered
diffuse rays as depicted in the view of Figure 8. It
will be further noted that regardless of the type of
honeycomb used, the transmitted and reflected rays continue
in a direction through the heat trap while,on the other
hant, at least part of the scattered rays are in directions
out of the heat trap 8nd away from the absorber where they
are lost.
- A series of experiments were conducted to compare
the overall solar transmission of honeycomb heat traps
of the present invention with that exhibited by bonded
honeycomb traps of the prior art. Two types of bonded
honeycomb structures were compared in the experiments.
One type conslsted of bonded expanded strip honeycomb sub-
stantially as shown in Figure 6. This honeycomb was made
of clear Mylar (TM) from Dupont and had hexagonal cells
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1 ~ ~ 5~ ~ 5
with a length of 2.0 inches and an effective diameter
of 0.375 inches. The other type of bonded honeycomb used
in the experiment was made from tubes bonded together
in the manner shown in Figure 7. This bonded honeycomb
was made of~-learpolycarbonate and had circular cells
with a length of 2.0 inches and a diameter of 0.1875
inches. The heat traps constructed in accordance with
the present invention were made from honeycombs produced
by the expanded core process described hereinabove. The
honeycombs were made in one continuous piece of poly-
carbonate and had hexagonal cells with a length of 1.0
inch and an effective diameter of 0.250 inches. The
exact composition of the adhesive used in fabricating
the bonded expanded strip or bonded tube honeycombs was
not known.
The experiments were conducted in the following
manner: Two calibrated pyranometers were mounted on the
same flat surface which could be moved to change its
orientation with respect to the sun. The honeycomb to be
tested was then mounted in a plane parallel to the flat
surface and over one of the pyranometers. The radiation
flux incident on each pyranometer was then measured
for solar incidence angles of between about 0 and 50
degrees, and the ratio of the flux under the honeycomb o
that measured with no honeycomb was determined. This
ratio represents the overall solar transmittance of the
honeycomb.
Figure 10 shows the results of these experiments.
It will be noted from curve A of the graph that the 2.0
inch bonded expanded strip honeycomb exhibits a rapid
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decrease in overall solar transmittance with increasing
angle of incidence, beginning at a value above 0.90 for
normal incidence and dropping to a value below 0.70
for an incident angle of 50 degrees. This range of
incident sun angles corresponds to periods of solar
heater operation approximately three hours before and
after solar noon forsolar heaters oriented to face the
sun at solar noon. The transmission of the 2.0 inch
bonded tube honeycomb, shown as curve B in the graph,
exhibits a similar dependence on incident angle,having a
value greater than 0.85 at normal incidence and dropping
to a value below 0.70 for an incident angle of about
50 degrees. In comparison thereto, the 1.0 inch
integrally formed honeycomb exhibits a high transmission
over a wide range of incident angles, as indicated by
curve C in the graph. Since 2.0 inch expanded core
honeycomb was not available at the time of the experi-
ments for comparison with the 2.0 inch bonded honeycombs,
a hybrid 2.0 inch expanded core honeycomb was constructed.
This hybrid honeycomb was made by stacking two 1.0 inch
honeycombs over the test pyranometer. It was determined
beforehand, however, that this hybrid honeycomb would
not exhibit the same performance as a 2.0 inch expanded
core honeycomb due to the presence of additional light
scattering sites at the upper end of the lower honeycomb.
Test data was taken on this hybrid honeycomb and is shown
as curve D in the graph of Figure 10. The anticipated
results for a single piece 2.0 inch expanded core honeycomb
may be extrapolated from the above results by dividing out
the end losses for the second honeycomb as shown by
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cllrve E. It may be noted that the transmission of light
in the case of the extrapolat:ed results for the 2.0 inch
expanded core honeycomb begins at the same high value as
the 1.0 inch expanded core honeycomb at normal incidence.
The transmission of light for the extrapolated results
falls somewhat more rapidly with increasing incident
angle, but remains above 0.80for incident angles up to
about 50 degrees. This more rapid decrease in transmission
is due to the additional thickness of the honeycomb
which is desirable for its effective use as a heat trap.
It will be further noted that even in the case of the
2.0 inch stacked hybrid honeycomb with additional end
losses, the overall transmission of light is still far
superior to either of the bonded honeycomb structures of
the prior art.
It should be noted that the heat trap has been
shown in the drawing with walls perpendicular to the
plane of the front wall. The present invention, however,
is not so restricted, and the cell walls may be disposed
at angles other than perpendicular so long as any solar
rays reflected from the cell walls are not directed back
toward the front wall during normal periods of operation.
Thus, the term "substantially perpendiuclar to the front
wall," as used herein and in the appended claims is
intended to include such other angles with reference to
the orientation of cell walls. It has been determined
that the cell walls may be disposed at angles with respect
to the perpendicular of up to about 22.5 degrees without
incident solar rays being reflected away from the solar
absorber when the normal period of operation is taken
to be about three hours before and after solar noon.
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For a more detailed explanation of the cell wall angle
and how it ~s derived, reference is made to our copending
application Canadian Serial No. 307,619
The present invention is likewise not restricted
to the construction of heat traps which are made entirely
in one piece. In fact, the heat traps may be constructed
from two or more of the integral cellular structures,
suitably 30ined together,for example,by an adhesive. This
adhesive would be the only adhesive used throughout the
entire heat trap assembly.
The heat trap of the present invention may also
be made with a transparent front wall or glazing formed
integrally therewith as described and claimed in our copend-
ing application Canadian Serial No. 307,619 filed on even
date herowith and assigned to the common assignee hereof.
From the foregoing it will be readily seen that
the present invention provides a solar heater having a
heat trap characterized by a high transmission of incident
sunlight as compared to heat traps of the prior art.
Specifically, the present invention provides a heat trap
for a solar heater which is made from a cellular structure
composed of a multiplicity of cells having co~mon walls
which are integrally formed in one piece from the same
clear or transparent thermoformable material. The
heat trap made in accordance with the present invention
does not contain any adhesive bonds or ~oints, except
those necessary to join large sections of the integral
cellular structure, which ~oints can act as sites for
scattering of incident sunlight and which can discolor
or otherw~se det~riorate with age.
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