Note: Descriptions are shown in the official language in which they were submitted.
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BACKGROUND OF THE INVENTION
I. Field of the Invention
The present invention relates broadly to
solar heaters which convert incident solar radiation
into heat energy and transfer the absorbed heat ei~her
to a gas such as air or a liquid such as water, the
former being commonly referred to as a solar air heater
and the latter being commonly referred to as a solar
~: water heater. More particularly, the present invention
relates to solar heaters o either type which employ
a heat trap between the absorber and the light-transmitting
front wall.
II. Description of the Prior Art
Various proposals have already been made in
the prior art for employing a heat trap between the
absorber and the front wall of a flat plate solar
heater in order to reduce heat losses by natural con-
vection and radiation. An example o~ the foregoing
wherain a transparent honeycomb heat trap is used may
be found in an article by Hollands entitled,
"Honeycomb Devices in Flat Plate Solar Coll~ctors'9,
Solar Energy, Vol. 9, pp. 159-169, Pergamon Press (1965).
One problem encountered with such proposals is that it
may as a practical matter be necessary to provide space
between the honeycomb heat trap and the front wall to
allow for diferential thermal expansion o~ the solar
heater elements. It has been found, however, that the
presence of such a space can increase the heat loss by
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natural convection to the front wall. This lncreased
heat loss results from the development of-natural
convectLon currents be~ween adjacent cells of the
honeycomb structure.
It has been observed in the results of
recent work by Edwards, e~ al., in "End-Clearance
Effects on Rectangular-Honeycomb Solar Collectors"
presented at the 1975 International Solar Energv Gongress
and ~xposition, July 28 - August 1, 1975, UCLA,
Los Angeles; also published in Solar Energy, Vol. 18,
i~ pp. 253-257, Pergamon Press (1976), that this increasein heat loss may be reduced by providing a cpac~
between the honeycomb heat trap ~nd the fla~ plate
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. - absorber rather than between the heat trap and the
- front wall. The difficulty with this proposal, however,
-~ is that the honeycomb must of necessity be made with
relatlvely thin walls in order to act as an efficient
heat erap and, consequently, the honeycomb is either
;~ too flexible to ~e self-~upporting if made of clear
plastic or extremely fragile if ~ade of glass and,
therefore, separate means for ~upporting ehe honeycomb
struc~ure over lts e~tire area must be-provlded.
It has al o ~een proposed in the prior art to
employ a honeycomb or other open cellular structure in
contact with the front wall ~o act es a radiation trap
and ~ir ~uffer in ~ transpiration solar air heater. Thus,
in our cope~d~ng ~pplic~tion Canadiarl Serial No. 307,618
filed on even date herew~th and a~signed ~o the con~non
~signee hereo~" there i8 disclosed and claimed a
3~ transplration ~olar ~Ir heater wherein the cellular
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7834
radiation trap is held in at least firm mechanical
contact with the front wall in order that it may
sdditionally act as an air buffer layer. It has been
demonstrated by experimental work, as reported in our co-
pending Canadian application, that a significant improve-
ment in thermal efficiency i~ attained when the radiatlon
trap is held finmly in contact with or is actually bonded
to the front wall. When the radiation trap, on the
o~her hand, ~ only loosely held in place adjacent to the
front wall, the problem arises that convection currents
may pass through the heat trap and contact the front
wall where heat losses may occur.
In the case of both the flat plate solar heater
and the transpiration solar air heater described above,
the heat trap may be supported in contact with the front
wall by one or more rigid bars below the cellular or
honeycomb heat trap or by the provision of adhesive
joints between the heat trap and the front wall.
Although this type of support means effectively solves
the problem of oonvective air flow throug~ the heat
trsp, the drawback of support means of this type is that
the bars or ~dhesive joints or other support elements
lntroduce additional surfaces from which incident solar
rays ~ay be re1ected or scattered in a direction away
from the solar absorber and be lost.
SUMMARY_OF~ INVENTION
The present invention is directed to a novel
~nd improved solar heater which overcomes the above
enumerated problems in either a flat plate solar hester
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or a transpiration solar air he-ater. The solar heater
of the present invention includes a housing having a
light-transmitting or transparent 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
integrally formed in one piece with the front wall in
accordance with the principles of the present invention.
The heat trap thus fo~med is significantly stronger
than the unsupported cellular heat trap by itself and
requires no further support to maintain it in position
against the ront wall. The problems described above
~`
for both flat plate solar heaters and transpiration
solar air heaters are thus solved without the intro-
duction of support elements or adhesive bondq or joi~ts
from which incident solar rays may be reflected away
from the radiation absorbent element and be lost. In
addition~ sînce there are no adhesive~bonds in the heat
trap of the present invention, there are no problems with
aging of the adhesive, i.e., dis oloratîon9 embrîttlement,
cracking, etc.
For purposes of simplicîty, the combîned
integrally formed heat trap and front wall shall be
hereinafter reerred to as an "integral heat trap glazing"
throughout the following description.
The prîncipal object of the present invention
is therefore to provide a solar heater having an integral
heat trap glazing whîch îs self-supporting and which has
a hîgher overall transmission of incîdent solar rays
than heat traps supported adjacent to the front wall
~L7834
of prior art solar heaters.
Another object of the present invention is
to provide a novel and improved integral heat trap
glazing for use in solar heaters which is strong, durable,
easy to handle and economical to manufacture.
A
DESCRIPTION OF THE DRAWING
The present invention will now be described
; in greater detail hereinafter with particular reerence
to the accompanying drawing which shows the preferred
` 10 embodiments hereof and wherein:
Figure 1 is an elevational, schematic, cross-
sectional view of a typical flat plate solar hea~er
made in accordanc~ 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; and
Figure 3 is a partially cut away perspective
view of a preferred embodiment of the integral heat
trap glazing of the present invention.
D2SCRIPTION OF THE PREFERRED EMBODIMENT5
~` 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 integral heat trap glazing performs some-
what d~fferent 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.
83~
Referring now specifically to Figure 1 of
the drawing, there is shown a flat plate sol~r heater
embodying the present invention. The solar heater
comprises a housing 10 including a flat radiation
absorbent collector plate 12 spaced from a back wall 14.
A tubular coil 16 or other passage means for a fluid
such as air or water is provided in the space below and
in contact with the 1at plate absorber 12. Preferably,
although not necessarily, the remaining space between
the flat plate absorber 12 and the back wall 14 is
filled with a suitable insulation as denoted by the
reference numeral 18. The housing 10 may suitably be
made of a rigid metal such as aluminum or steel or
oth~r rigid material such as plastic or fiberglass.
The housing 10 further includes an integral
heat trap glazing 20 which serves as the light transmit-
ting front wall of the solar heater in accordance with
the present invention. As shown, the integral heat
trap glazing 20 is made in a single piece but has a
flat upper surface 20a which faces outwardly from the
solar heater and a lower surface 20b which faces
inwardly and which i9 spaced a short distance above
the flat absorber plate 12 to provide a small gap 22
of, for example, about S mm. or less. The lower surface
20b is formed by a multiplicity of cellular openings with
walls which are substantially perpendicular to the upper
surface 20a. The cellular openings which constitute the
lower surface 20b may be in the form of a honeycomb or
other cellular structure such as that formed by parallel
fins. The integral heat trap glazing 20 may be made in
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7 ~ ~ ~
one piece by conventional forming ~r molding technlqués
as shall be described hereinafter in greatPr detail.
Suitably, the glazing 20 may ~e made from glass or
clear plast~c c~mp~sitions such ~s polyvinyl fluoride,
polycarbonate, fluor~nated ethylene propylene, polymethyl
methacrylate, aromat~c p~lysulfones~ polyethylene
terephthalate, aromatic polyesters, polyvinylidene
1uorite, hexafluoropropylene, chlor~trifluoroethylene
and tetraflu~rethyl~ne copolymers.
During operation of thç ~olar heater,incident
solar rays pass l~hrough the integral heat trap glaz~ng
20 and are absorbed b~ the flat collector plate 12 and
are co~verted to heat. This heat ~n turn ~s transferred
by conduction and convection t~ a fluid ~uch as air or
water which ls circulated through the co~l 16 in contact
wi~h the collector plate 12.
In this ~mbodiment of the present ~nvention,
the integral heat trap glazing 20 6erves the dual
function of reducing the radiative heat loss from ~he
601ar heater and of ~uppressi~g the onset of ~atural
c~nvection ~n ~he alr ~pace ~tween the,flat co~lector
plate and the ~ront ~all. In order for ~be cellular
lower ~urface 20b of the integral heat trap glazing 20
to effectively reduce heat loss by ra~iation, the
c~11ular ~penlngs ~ust be of a sufficiently high
a~pect ratio as described and cl~imed ~n detail in
our Canadian Pstent 1,082,544, ~n ~he range of 2 to
10 for hone~comb cellular structures and 4 to 20 for
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1~7834
finned cellular structures. hs shown ~n Figure 1, the
walls of ehe cellular openings divide up the air space
between the flat collector plate and the front wall and
inhibit the development of natural convection currents.
At the s&me time, a small gap 22 is maintained between
the lower surface 20b of the glazing 20 and the flat
absorber plate 12 to allow for differential thermal
expansion of the solar heater elements.
Figure 2 shows a transpiration solar air heater
embodying the present invention. As shown, the transpira-
tlon solar air heater comprises a housing 24 having a
back wall 26, and an lnlet 28 in one ~ide wall and an
outlet 30 in the opposite s~te wall which establish a
flow path for a gas such as air to be heated as generally
indicated by the arrows in the drawing. A porous,
radiation absorbent collector plate 32 is mounted inside
the housing 24 in spaced apart parallel relation to the
back wall 26 and across the flow path established between
the inlet 28 and the outlet 30. The porous collector
plate 32 may be composed for example of a porous darkened
or black fibrous mat, woven or stamped screens, or
reticulated ~oæm. Although the porous collector plate 32
~s shown ln spaced parallel relation to the back wall 26,
it will be understood that ehe collector plate may be
positioned in non-parallel relation to the back wall as
disclosed and claimed in our copending application Serial NQ-
307,618. If desired, a layer of insulation 34 may be
placed adjacent to the back wall 26 and in spaced apart
relation to the porous collec~or plate 32. Again, the
housing 24 may be made o, arigid metal ~uch as aluminum
~; or steel or other rigid material such as plastic or
fibergl~s~.
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The housing 24 ~urther includes an integral
- heat trap glazing 36 which also serves as the light-
transmitting front wall of the transpiration solar air
heater. The integral heat trap glazing 36 is of
basically the same construction as that shown in Figure
1, including a flat upper surface 36a and a lower
surface 36b which is formed by a multiplicity of
cellular openings. In this instance, however, the
assembly of the integral heat trap glazing 36 in the
housing is such as to provide an enlarged space 38~
This space 38 provides a passage for the gas such as
air to be heated betweeen the lower surface 36b of
the heat trap glazing and porous collector plate 32.
The integral heat trap glazing may be abricated using
the same conventional molding or forming techniques
and the same glass or plastic compositions mentioned
above.
The operation of the transpiration solar
air heater is similar in that incident solar rays pass
through the integral heat trap glazing 36 and are
: absorbed by the porous collector plate 32 and converted
to heat. However, in this instance, the gas or air
to be heated enters the inlet 28 and follows the flow
path between the inlet 28 and outlet 30. The gas or
air passes or transpires through the entire porous
collector plate 32 and is heated. The heated gas or air
then exits via the space 40 below the collector plate 32
and through the outlet 30. It should be noted that
in this embodiment, the integral heat trap glazing 36
serves the dual function of reducing the radiative heat
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:
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-~ loss from the solar heater and o providing an air
buffer layer composed of baffles which prevent the
forced convective flow of the gas or air to be heated
adjacPnt to the front wall where heat losses may occur.
The geometry of the integral heat trap glazing 36 in
this instance is basically the same as that described
above ;that is, the aspect ratio may be in the same
range of between about 2 and 10 for honeycomb cellular
structures and 4 and 20 for finned cellular structures.
In the two embodiments of the present invention
~- illustrated schematically in Figures 1 and 2, the
integral heat trap glazings 20 and 36 are shown with
a lower surface composed of cells having relatively
thick walls for purposes of illustration only. It will
however be understood that in order to effectively
function as a heat trap the walls must be made relatively
thin, i.eO, in the range of between about 0.0002 and 0.0~
centimeters. The thickness of the remainin~ upper planar
portion of the glazing may vary depending on the overall
size of the glazing and also on the cell wall thickness
- ~ selected. Obviously in those instances where the cell
wall is very thin the upper planar portion of the glazing
must carry a higher fraction of the total load te.g., its
own weight, wind and snow loads, etc.) and therefore
must be thicker. On the other hand~ where the cell walls
are relatively thick,the upper planar portion can be
made thinner than if the glazing were constructed without
the lower cellular surface which constitutes the heat
trap. Generally speaking, the thickness of the upper
planar surface will vary between about 0.075 and 0.60
centimeters.
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Figure 3 shows the preferred form of the integral
heat trap glazing of the present invention. As shown,
the integral heat trap glazing 42 is formed with an
upper planar portion 42a having a substantially flat
surface and a lower cellular portion 42b comprising
cells which have a hexagonal cross section. The walls
of the individual cells are integrally formed with the
upper planar portion as at 44, providing a continuous
distribution of the glazing and heat ~rap material with
no internal surfaces, joints, or adhesive bonds from
which incident solar rays might be scattered or reflected.
The integral heat trap gl~zing can be made by
co~ventional forming or molding techniques well known
in the art. For instance, the integral heat ~rap glazing
can most advantageously be made by an expanded core
process as 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 and claimed in
the following patents: U.S. Pat. Nos. 3,765,810,
3,~19,379, 3,919,380 and 3,919,445. In the basic process,
articles such as the integral heat trap glazing can be
made by expanding the cross-section of a blank of
thermoformable material in a manner whereby voids such
as hexagonal cells are formed from one sur~ace of the
~ blank. The remaining unexpanded portion of the blank
; forms the upper planar portion of the integral heat
- trap. Ordinarily, articles formed by this proc~ss include
a perforated skin on the side opposite to the unexpanded
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planar por~ion which must be removed before the
article is suitable for use as an integral heat trap
glazing. The perforated skin, if not removed, would be
oriented such that incident solar rays re1ected from
it would be directed away from the solar absorber and be
lost. The perforated skin may be removed by passing
an electrically heated wire through the cell walls
immediately adjacent to the skin or by use of a reverse-
cutting,knife-edge band saw blade such as that used for
the cutting of metallic honeycombs. For a better
understanding of the process, reference should be made
to the specifications of the aforementioned patents.
It will be further understood, ofcourse,that the integral
heat trap glazing can be formed by other processes such
as injection molding wherein thermoformable material is
injected under pressure into a mold having the desired
cellular coniguration.
It may be noted that the cellular portion of the
integral heat trap glazing 42 has been shown in the
drawing with cell walls perpendicular to the upper planar
portion 42a. 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 ~ell walls are not directed back toward
- the upper planar portion during normal periods of opera-
tion. Thus the term "substantially perpendicular to
the upper planar portion", 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 dispos~d
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,
at ~ngles 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 operat~on is taken to be about three hours
before and after solar noon. For a re detailed
explanation of the c~ll wall angle and how it is derived,
reference is made to our Canadian Patent No. 1,082,544.
From the foregoing, it will be readily seen that
the present invention provides a solar heater in the
lC fonm of either a flat plate heater or a transpiration
. . air heater having an integral heat trap glazing which
is self-supporting and which has a high ove~all trans-
mission of incident solar rays. In addition, the
present invention provites an integral heat trap glazing
for use in solar heaters which has a high strength to
weight ratio and which is durable, easy to handle, and
economical to manufacture.
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