Note: Descriptions are shown in the official language in which they were submitted.
1082544
BACK~ROUMD
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(1) Field of the invention.
The present invention relates to the field of
solar collectors and more particularly to solar air heaters
having improved thermal efficiencie~.
(2) Description of the prior art.
A great deal of ef~ort has been devoted by
researchers in recent years to develop solar collectors
for the conversion of solar energy into heat energy.
These devices are potentially useful in many applications
where fossil fuels are now employed as the principal
source of energy. Such applications include for example
the heating of residential and commercial buildings, and
the generation of electric power. Solar collectors may
be widely used some day in the not too distant future
on roof tops of residential homes for supply~ng heat
turing periods of cold weather. It is of course of the
utmost importance in coming years to be able to manu-
facture solar collectors which are relatively inexpensive
and which have a high degree oftherm~l efficiency.
Solar collectors heretofore developed employ
a collector plate for converting ~olar energy into heat.
Typicallly, the collector plate is disposed inside a
housing having a transparent wall for passing incident
- solar radiation. The solar radiation passing through
the transparent wall is absorbed by the collector plate
and converted into heat. The converted heat energy
is then transferred to a fluid, i,e. a liquid or gas, by
conduction, convection and/or radiation, and heats the
fluid. The heated fluid is then conveyed away for
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1082S44
storage and subsequent utilization.
In one type of solar collector the fluid to be
heated is circulated through tubes or ducts for example,
positioned inside or ad~acent the collector plate. The
collector plate in these solar collectors is usually a
solid flat radiation absorbent plate, e.g. a darkened or
black metal plate which absorbs the incident solar energy
and transfers it as heat by conduction to the tubes or
ducts where heat exchange with the fluid occurs. Solar
collector devices of this type are, therefore, commonly
referred to as "flat plate collectors" and they may be
used to heat a liquid or gaseous medium.
When the converted heat is to be transferred
~niv to a gAseou~ medium such as a~r, other col~ector-de-
signs may be uæed. An excellent background study of
prior art solar air heaters is given in an article by
A. Whillier entitled "Black-Painted Solar Air Heaters of
j Conventional Designll, appearing in Solar Ener~Y Vol. 8,
No. 1, pages 27-31, Pergamon Press (19643.
In one type of solar air heater the gas is
passet throught the housing through the collector plate,
I where the collector plate is a porous, gas-permeable
-'~ plate, e.g. a porous black fiber mat, and the gas to be
heated passes directly through ~he solar energy absorb-
ing surface. Also, in this instance, the housing has
an inlet and outlet for esta~lishing a flow path for
, .
the ga~ to be heatet. In porous plate designs the
entire collector plate acts as a heat exchange medium
for transferring the absorbed or converted heat to the
gas or air flowing through the device. Thus, gas or
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air is drawn through the inlet and flows or transpires
through the collector plate and is heated. The heated
gas or air exits through the outlet and is conveyed to a
storage device for subsequent use. Solar collectors of
this type are referred to as so-called "transpiration
solar air heaters''.
A major problem with solar collectors is the loss
of absorbed heat by natural convection and re-radiation,
i.e. long-wave or infrared radiation, from the collector
plate towards the transparent wall.
It has been proposed in the literature to employ
certain types of cellular structures such as honeycombs
as a heat trap to reduce the loss of absorbed heat by
natural convection in flat plate solar heaters. The heat
trap is placed over the solid collector plate to guard
against the development of convective heat flow away from
~I the collector plate and toward the transparent wall. Any
heat that is conveyed by this convective flow to the trans-
parent wall can be readily lost through conduction or radi-
ation to the outside atmosphere. Thus Hollands in an
article entitled "Honeycomb Devices in Flat Plate Solar
Collectors", Solar Ener~y Vol. 9, No. 3 pps. 159-169,
Pergamon Press (1965) discloses the use of various types
of honeycomb structures, e.g. rectangular, square, tri-
angular, etc. as a heat trap to prevent convective losses
in a flat plate solar heater. The honeycomb trap can be ;
made o glass or plastics which transmit solar rays but
n 4
" 1082S44
are opaque to long-wave radiation. In transpiration solar
air heaters, of course, the loss of heat by natural convec-
tion does not occur if the gas or air to be heated con-
tinuously flows through the porous absorber in a direction
away from the transparent wall.
Various att~mpts have also been made in the prior
art to overcome the problem of re-radiation losses. In
flat plate collectors spectrally selective coating may be
applied to the absorber surface to reduce re-radiation
losses. A review of spectrally selective coating technol-
ogy is given by J. Jurisson, R.E. Peterson, and H.Y.B. Mar
in an article entitled "Principles and Applications of
Selective Solar Coatings" appearing in the Journal of
~Tacuum Science Technology Vol. 12, No. 5, pages 1010-1015,
1975. The coatings described, however, are not effective
in reducing re-radiation losses from transpiration air
heaters because the pores at the surface of a porous plate
act as black body cavities and limit the effectiveness of
any coating applied to the surface.
Various attempts have been made in the prior art
to overcome the problem of re-radiation losses from trans-
piration solar air heaters. Thus U.S. Pat. No. 3,102,532,
to Shoemaker dis41Oses a solar heat collector wherein air
to be heated is passed through a gas-permeable collector
composed of multilayers of slit and expanded aluminum foil.
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The expanded foil is coated with 8 black vinyl enamel on
the top surface facing the transparent wall. The bright
underside of the foil is highly reflective and acts æ a
trap to prevent loss of absorbed heat by re-radiation.
However, some radiation losses can occur through the open-
ing or slits in the foil and besides this foil collector
is difficult and expensive to manufacture.
It has also been proposed in the literature to
utilize a specularly reflecting honeycomb heat trap in a
solar air heater employing a porous collector plate. Thus,
Buchberg et. al. in an article entitled "Performance Char-
acteristics of Rectangular Honeycomb Solar Thermal Con-
verters", Solar Ener~y, No. 13, pps. 193-221, Pergamon
Press (1971), discloses a solar air heater employing a
rectangular honeycomb heat trap which is made from a
specularly reflectivematerial~ i.e. paper coated with a
clear aluminized layer.
So far as is presently known, there has been no
disclosure in the prior art literature of a æolar air
heater employing a porous collector plate for converting
solar radiation to heat and a transparent cellular struc-
ture such as honeycomb which is utilized as a trap to
prohibit losses of heat by re-radiation. Hollands supra
discloses the use of glass or plastic honeycomb but
these transparent honeycomb heat traps are used primarily
to prevent convective losses of heat in flat plate solar
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1 0 8
collectors,
It is an object of the present invention to
provide a solar air heater employing a porous collector
plate which has improved thermal efficiencies.
Another object of the present invention is to
provide a solar air heater e~ploying a porous collector
plate and a transparent radiation trap.
A further object of the present invention is to
provide a soLar air heater of the type described which is
easy to assemble and economical to manufacture.
SUMMARY OF THE INVENTION
It has been discovered in accordance with the
present invention that a significant improvement in
thermal efficiencies can be obtained in a solar air
heater employing a porous collector plate for converting
incident solar radiation to heat and transferring the heat
to a cont~nuous flow of gas such as air to be heated if a
transparent radiation trap which is made of a transparent
material which is opaque or black to lnfrared radiation
is interposed between the collector plate and the trans-
parent wall. The significance of this improvement was
totally unexpected over the teachings of the prior art.
This improvement amounts to about 15 percent or more
over solar collectors without the transparent radiation
.
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trap. Solar air heaters for applications such as space
heating with thermal efficiencies ranging between about
60 and 70 percent can now be produced. These thermal
efficiencies include of course the normal heat loss due
to transmission of solar radiation through the transparent
wall.
: .
Briefly then, the present invention resides in
a solar air heater comprising a housing having a trans-
parent wall and an inlet and an outlet for establishing
a flow path for a gas such as air to be heated, a porous
or gas permeable collector plate is disposed across the
flow path in the housing and arranged to accept the inci-
dent solar radiation passing through the transparent wall
and to transfer the absorbed heat to the gas or air pass-
ing along the flow path and through the collector plate
and a transparent radiation trap which is made of a trans-
parent material opaque or black to infrared radiation inter-
posed between the collector plate and the transparent wall.
BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 is a cross sectional, elevational view
showing a solar air heater embodying the present invention;
F~ures 2 and 3 are similar to Figure 1 but show
different embodiments of the present invention.
Figure 4 is a perspective view showing part of
a solar air heater employing a transparent honeycomb
radiation trap between a transparent wall and a porous
plate.
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Figure 5 is the same as Figure 4 but shows the
solar air heater employing a transparent fin radiation
trap between a transparent wall and a porous plate.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Referring now specifically to Figure 1 of the
drawing there is shown a solar air heater embodying the
present invention. The solar heater comprises a housing
10 having a transparent front wall 12 passing incident
solar radiation and a back wall 14. The housing further
includes an inlet 16 in one side wall and an outlet 18 in
the opposite side wall. The inlet 16 and outlet 18 are
arranged to establish a flow path for a gas such as air
~ to be heated as generally indicated by the arrows in the
j drawing. A porous collector plate 20 is mounted inside
the housing 10 in spaced apart parallel relation to both
the front wall 12 and back wall 14 and across the flow
path established between the inlet 16 and outlet 18.
The porous collector plate 20 is composed for example of
a porous darkened or black fibrous mat, woven screens or
~l 20 reticulated foam. A transpare~t radiation trap 22 is
3~; placed over the top æurfaces of the porous collector plate
20 facing the front wall 12. The radiation trap 22 is
an open structure such as a honeycomb or an array of fins
which permits the flow of gas or air through both the
trap 22 and the porous collector 20. The housing
10 may suitably be made of metal such as steel for
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~08ZS44 ~
ruggedness or the housing can be also be made of an insula-
ting material such as polymeric foam or fiber-glass if
desired.
Figure 2 shows basically the same solar collec-
tor design as described above except that the transparent
radiation trap 22ais interposed in spaced relation between
the porous collector plate 20 and the transparent wall 12.
Suitably the transparent radiation trap 22a may be supported
on an open structure such as a metal screen 24. The gas
or air passes through the open structure of the radiation
trap such as honeycomb in the same manner as described
hereinabove.
Figure 3 basically the same solar collector de-
sign is shown except that the transparent radiation trap
22b is mounted adjacent to the underside of the trans-
parent wall 12. However, the gas or air to be heated in ~ --
this case does not pass through the radiation trap 22b
but rather flows directly through the porous collector
plate 20.
Figure 4 shows a perspective view of one embodi-
ment of the transparent radiation trap 22 which is placed
over the porous collector plate 20 facing the transparent
front wall 12 which is spaced from it. In this embodi-
ment the radiation trap 22 is cellular in construction and
is made of a material which is transparent to visible
light and absorptive of infrared radiation such as
.
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082S44
polycarbonate or other clear polymers or glass. The
ratio of cell length and diameter is in the range of
about two and ten.
Figure 5 shows a perspective view of another
embodiment of the transparent radiation trap 22 in a con-
figuration similar to that described above. In this
embodiment the radiation trap 22 has a finned construction
and is made of a material with the same properties as those
described above. The ratio of fin height and fin spacing
is within the range of about four and twenty.
~j
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~08Z544
SUPPLEMEN~ARY DISCLOSURE
~t has been found that the most significant im-
proYement and consequentl~ the ~ighest thermal efficiencies
can be attained if the radiation trap is placed adjacent to
and in contact with the front wall. The radiation trap
should be composed of a multiplicity of open cells in com-
munication with the flow of gas or air through the solar
heater, the cells having walls substantially perpendicular
to the front wall which act as baffles to prevent the flow
of air through the radiation trap in a direction parallel
to the plane of the front wall but which at the same time
do not cause reflections of incident sunlight in a direction
toward the front wall during periods of normal operation.
In particular, the radiation trap may be made from plastic
or glass honeycomb with cells of various geometries, e.g.,
rectangular or hexagonal, or other cellular structures
such as those provided by spaced apart parallel fins arr-
anged across the flow of air through the collector.
In the accompan~ing drawing:
Figures 6 and 7 are perspective views of part of
a solar air heater employing different forms of a radiation
trap in accordance with the present invention;
Figures 8-12 are cross-sectional, elevational
j views of solar heaters showing a number of modifications
that can be employed in the embodiment of the present in-
vention shown in Figure 3, Figure 8 being located after
Figure 5;
~igures 13-16 are perspective views of part of a
,
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108ZS44
solar air heater sho~ing several different modifications of
t~e porous collector plate-radiation trap arrangement that
~ay ~lso ~e employed in accordance with the present invention;
Figure 17 ~s a graph showing the normalized rad-
iati~e heat loss through honeycomb and fin radiation traps,
Figure 17 being located after Figure 19;
Figure 18 is a schematic view showing transmission
of solar radiation through a transparent honeycomb structure
Figure 19 is a similar schematic view showing
transmission of solar radiation through a specularly re-
flecting honeycomb;
Figure 20 is a graph showing a comparison of the
transmission properties of transmparent and specular honey-
combs, Figure 20 being located after Figure 7;
; Figure 21 is a schematic view showing the in-
frared radiation exchange between the porous collector
plate and the infrared absorbing cell walls of a glass or
plastic honeycomb;
Figure 22 is a similar schematic view showing the
infrared radiation exchange between the porous collector
plate and the infrared reflecting cell walls of a metallic
honeycomb;
~: Figure 23 is a graph showing the effect of L/D
on radiative heat loss for cell walls of high and low
thermal conductivity, Figure 23 being located after Figure
24;
Figure 24 is a graph showing the relationship
between the thermal efficiencies and operating conditions
for water and air heaters both with and without a honey-
comb radiation trap;
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~3 ''
1082544
Figure 25 ~,s a g~aph showing the reIationship .
between the nor~al~zed ~ncrease ~n efficienc~ due to addi- -,
tion of a, transparent honeycomb radiation trap and oper-
at~ng conditions in both water and air heaters;
'' ~igure 26 is a graph similar to Figure 24 showing
the relationship between the thermal efficiencies and oper- :-
ating conditions for air heaters with an additional light- .'
transmitting layer beneath the honeycomb and without the
additional layer but with the honeycomb either bonded to
the transparent wall or supported between the transparent
wall and porous ab,sorber; ,
Figures 27a-b, 28a-b and 29a-b are schematic
views of the patterns of reflection losses and air flows in
different experimental solar air heaters with radiation
traps;
, Figures 30a-d are schematic views of the patterns
of transmitted and reflected rays from radiation trap cell
walls which are at several different orientations with res-
pect to the front wall; and
Figure 31 is a schematic view of a typical solar
~ heating system employing a solar air heater embodying the
,`: present invention, Figure 31 being located after Figure 25. .-
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: 108ZS44
Referring now to the drawing, there is shawn in
Figure 6 another form of radiation trap which is made from
a rect~ngular honeycomb panel 26. This form of radiation
trap is basically the same as that shown in Figure 4
except for the specific configuration of the honeycomb.~
In Figure 7, there is shown another form of
radiation trap which is made from a multip~icity of trans-
parent plastic or glass tubular segments 28. The tubular
segments 28 are glued or otherwise bonded together ~uch as
by solvent to form an elongated panel which i9 ~lso placed
on top of the porous collector plate 20 in spaced ~part
relation to the transparent wall 12. Suitably, the tubular
segments 28 may be cut from conventional plastic or glass
tubes or straws, for example. The ratio of tube length
to diameter is substantially the same as that for the
hexagonal honeycomb trap shown in Figure 4.
It will be understood that the construction
of the radiation traps used in accordance with the present
- invention is not restricted to the specific geometries
described hereinabove but that the traps may be made ~rom
other types of cellular geometries or open structures such
as triangular honeycomb cells or cells constructed from
corrugated or pleated sheets. Although the radiation '!
traps are most preferably made from cellular honeycomb
structures, other open structures having a high aspect
ratio (equivalent L~D for non-circular geometr~es)
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- 108ZS44
may be used in the practice of the present invention.
The radiation traps used in solar air heaters
may be made from any light-transmitting material which
is at the same time opaque or black to infrared radiation.
Ihe radiation traps may of course be translucent if desired
but preferably the traps are made of a transparent material
such as a clear plastic or glass, for example. There are
a number of clear plastic compositions which are black
or opaque to infrared radiation and which, therefore, can
~s 10 be used in the practice of the invention. These plastic
compositions include, for example, polyvinyl fluoride,
polycarbonate, fluorinated ethylene propylene, polymethyl
methacrylate, aromatic polysulfones, polyeth~lene
terephthalate, aromatic polyesters, polyvinylidene
fluoride, hexafluoropropylene, chlorotrifluoroethylene and
tetrafluoroethylene copolymers.
Generally speaking, the trap may be located
in any of several different positions between the porous
collector plate 20 and the front wall 12. In the
`~ 20 embodi~ent of the solar air heater shown in Figure 1, the
radiation trap is located directly on top and in contact
with the porous collector plate 20 and in spaced apart
j~ :
; relation to the front wall 12. This embodiment of the
so~ar heater offers an advantage in that the flow of gas
- or ~ir directly through the honeycomb radiation trap tends
to recover some of the heat which is lost from the porous
collector plate 20 to the trap 22 by conduction and radiation.
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~082S44
However, the gas or air to be heated at the same time
passes directly underneath the front wall 12 and this
can increase the heat losses through the wall to the
ambient atmosphere.
A more preferred embodiment of the solar air
heater is shown in Figure 3. The radiatioD trap 22b in
this instance is located just beneath the front wall 12
and in spaced apart relation to the porous collector plate
_ 20. The gas or air to be h~ated enters the inlet 16,
passes through the space between the radiation trap 22b
and the porous collector plate 20, and then passes through
the collector plate 20 where the gas or air is heated by
absorbed radiation. It should be noted that in this
embodiment the gas or air does not flow through the
radiation trap. The radiation trap 22b serves the additional
function of providing a nearly stagnant air buffer layer
between the air flow and front wall 12. This further
reduces the heat losses to the surrounding or ambient
atmosphere. In order to effectively function as an air
buffer, the radiation trap 22b should be maintained in at
~~ lesst firm mechanical contact with the underneath side of
the front wall 12 and preferably should be b~nded to the
wall in order to prevent the flow of gas or air through
the trap and into contact with the front wall 12. Further,
it will be noted in those instances where it might not be
practical or feasible to bond the radiation trap 22b to
the front wall 12, the trap might be readily held in
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1082544
firm mechanical contact with the wall by an open support
structure such as open mesh positioned below the trap.
The support structure must of course be open to ~in~mize
the introduction of additional losses by reflection of
solar radiation back toward the front wall 12 from the
~upport structure.
Figure 8 shows a modification of the solar air
heater which is basically the same construction as that
of Figure 3 except that the flow of gas or air in this
instance is in the reverse direction. lhe gas or air to
be heated enters the inlet 30 which is located below the
porous collector plate 20 and passes through the space
between the collector plate 20 and ~ack wall 14. The gas
or air then passes ~hrough the collector plate 20 and is
heated by absorbed radiation. The heated gas or air exits
, through the outlet 32 which in this instance is located; between the radiation trap 23b and the collector plate 20.
j~ There are a number of additional modifications of
the solar air heater which are made possible by relocating
the radiation trap to another position other than on top
of the porous collector plate 20 such as by placing the
,~ trap directly underneath the front wall 12. Thus, it ispossible for example to locate the collector plate or
absorber ~n several different positions independently of
I the radiation trap.
1 Figures 9 and 10 sh~w two ~ ch modifications
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1082S~4
wherein the porous collector plate is positioned in non-
parallel relation to both the radiation trap 22b and the
front wall 12. The gas or air to be heated enters the
inlet 34 in one side wall of housing 10 and fLows through
the non-parallel porous collector plate in a direction
either first through the upper-surfaces 36a of the
collector plate 36 as shown in Figure 9 or first through
the bottom surfaces 38a of the collector plate 38 as shown
in Figure 10 and is heated by the absorbed radiation. The
.~s
heated gas or air then exits through an outlet 40. In
both of these modifications, it will be noted that the gas
or air flows directly through the collector plates 36, 38
w~thout changing direction as denoted by the arrows in the
drawing, thus assuring a more uniform flow through the
solar heater.
Figure 11 shows still another modification which
combines the features of the solar heaters illustrated in
Figures 9 and 10. In this modification, the two non-parallel
;. collector plates are combined into one solar heater withthe porous collector plate 42 being arranged in a V-shaped
configuration. The gas or air to be heated enters the
inlet 44, passes first through the non-parallel segment
i:
42a of the V-shaped collector plate 42 and then through the
other non-parallel segment 42b again without changing
direction and exits through the oùtlet 46. It will be
noted however that in this instance a two-stage heating
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~o~S44
effect is achieved in a single solar heater unit. Ihe
collector plate segments 42a and 42b may of course be
constructed in one piece or they may be made from two
pieces suitably joined together in the solar air heater.
Generally speaking, any number of porous collector element6
may be combined in non-parallel relation to the front wall
to provide a multiple stage heating effect in a single ~olar
heater unit.
A similar two stage heating effect can be achieved
by a further modification of the solar air heater as shown
in Figure 12. This modification similarly combines the
features of the solar heaters illustrated in ~igures 3 and
8. Thus, as shown, a baffle plate 48 is disposed intermediate
the length of the housing 10 and between the radiation trap
22b and the flat porous collector plate 50. An inlet 52
and outlet 54 are located on the same side of the collector
plate 50. The gas or air enters the inlet 52 and passes
,
through the space between the radiation trap 22b and the
. collector plate 50. The gas or air is then made to pass
through the porous collector plate 50 by the baffle plate
, ~
f ~ 48 and is heated by the absorbed radiation. The heated
~ I gas or air enters the lower space between the collector
: ~:
plate 50 and the back wall 14 and is again made to pass '`
~ ~ through the collector plate 50 being heated by absorbed
i~ radiation. The heated gas or air then exits through the
outlet 54.
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108~544
It may be mentioned that an advantage of the
solar air heater shown in Figure 2 wherein the radiatlon
trap 22a is located intermediate and spaced from both the
collector plate 20 and the front wall 12 is that the
radiation trap is not maintained in contact with collector
plate 20 but rather is spaced therefrom and conse~uently
there are no heat losses due to conduction of heat through
the collector plate to the radiation trap.
It will be noted of course that any one of the
different forms of the radiation trap shown in Figures 4-7
may be employed in the further embodiments and modi~;cations
of the solar air heater described. Thus, it is possible,
for example, to use an hexagonal, rectangular or tubular
honeycomb radiation trap such as shown in Figures 4, 6
and 7, respectively, or the radiation trap may be composed
of parallel fins such as shown in Figure 5. It should be
noted, however, that in those instances where the trap is
made from parallel fins, the fins must be oriented such that
they are arranged in a direction substantially perpendicular
. ~ 20 to the direction of flow of the gas or air through the solar
heater. If, on the other had, the fins are arranged in
' the same dlrection as tbe flow of gas or air to be heated,
'j the radiation trap.cannot function as an air buffer and
heat losses through the front wall 12 are likely to occur. ~.
As already indicated, the radiation trap used in th~se
embodiments must of course be made of light-transmitting
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108ZS44
material which can be translucent, clear or ~ransparent
and which must of course be black or opa~ue to infrared
radiation. In addition to the several different forms
of radiation traps already described and illustrated
herein, there are of course other types of materials wh~ch
will function as radiation traps such as plastic or glass
fiber batts or fused plastic films containing entrapped
gas bubbles. In this connection, it should be noted that
an open fibrous structure made of plastic or glass would
. .. .
function as a radiat~on trap but would not function as both
a radiation trap and air buffer without the interposition
of a non-porous, gas-impermeable layer between the fibers
and the gas or air flow through the solar heater. Also,
it should be noted that in any one of the above described
embodiments the porous collector plate or absorber can be
made of the same porous heat absorbing materials as already
described such as a black fibrous mat, woven or stamped
, screen or reticulated foam.
~ . In all of the embodiments illustrated, the
i~ 20 housing 10 may suitably be made of metal such as aluminum
X~ ~ or steel for ruggedness or the housing can also be made of
an insulating material such as polymer foam or fiberglass
l~ if desired. Preferably, although not neCessay, a layer
of insulators is placed adjacent to the back wall 14 as
~, indicated at 14a in Figure 8, for example.
- It may be practical and economical in some cases
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to incorporate both the radiation trap and porous collector
or absorber together in one element during manufacture
of the solar air heater. Thus, as shown in Figure 13,
the space between the parallel fins 56, which constitute
the rsdiat~on trap, may be partially filled with porous
heat absorbing material 58. SLmilarly, the voids defined
by the hexagonal honeycomb 60 may be partially filled
with the same porous heat absorbing material 62 as depicted
in Figure 14. It is also possible to construct a radiation
trap-absorber element by coloring a lower portion of the
fins 64 with a black or darkened paint or other coloring
agent as indicated at 66 in Figure 15. Figure 16 shows
the same type of radiation trap-absorber element ~sing the
hexagonal honeycomb 68 wherein the lowermost portion of
the honeycomb is colored with a black or darkened paint
or coloring-agent as indicated at 70 in ~igure 16. It
may be noted that all the embodiments shown in Figures
13-16 correspond substantially to the embodiments of the
present invention shown in Figure 1 wherein the radiation
trap i8 disposed on top of the porous collector plate.
It may be ~urther noted that the clear upper portion of
the radiation trap-absorber element must have an aspect
ratio which is in the same range as that described for the
radiation traps shown in Tigures 4-7. -.
s As hereinabove mentioned, the radiation traps
made from cellular honeycomb are preferably used in solar
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' ~ . " ~ ~ .' - '
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1082S44
air heaters although other types of geometries can be
employed as the radiation trap in the practice of the
present invention. Generally speaking, the amount of
reduction of heat loss that is achieved with radiation
traps of various geometries will depend on the aspect
ratio and will be within a range encompassed by tubular
or hexagonal honeycomb and parallel fin ra~iation traps.
In order to quantify this relationship, a theoretical
analysis was conducted to determine the amount of radiation
trapping achieved by honeycombs and parallel fins of
different aspect ratios. The results of this analysi~
are shown in Figure 17 where values for Q/QO were plotted
a~ainst the aspect ratio L/D (or the equivalent H/S for
- parallel fins). In the gràph, Q is the rate of heat 1O8S
from one black surface at 100C to another black surface at
0C with the radiation trap in place, while Qo is the rate
of heat loss between the same two surfaces without the
radiation trap present. The ratio of Q/QO is a measure
of the effectiveness of the radiation trap, with low
1~, . .
values of Q/QO indicating more effective heat loss reduction.
As seen from Figure 17, the honeycomb radiation trap is
more effective than the parallel fins. It may be further
seen that in order to achieve at least a 50 percent -
. reduction in radiation heat loss the aspect ratio L/D for
honeycombs must be greater than 2 and that the aspect
ratio for pæallel fins must be greater than 4. As also
~een from Figure 17, there is only a marginal additional
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~082S44
reduction in heat loss achieved by using honeycombs with
aspect ratios larger than lO or parallel fins with aspect
ratios larger than 20.
Figure 17 further shows the effect of cell wall
or fin thickness on heat loss reduction. ~us, the solid
curves represent a cell wall or fin thickness of 0.005
centimeters while the dotted curves represent a cell wall
or fin thickness of 0,016 centimeters. As will be clearly
seen from the curves the thinner cell wall and fin
.~ ,
dimensions provide more effective heat loss reduction.
Preferably, in the practice of the present invention,
the thickness of the honeycomb cell walls and fins should
be maintained in the range from about 0.0002 to about
0.~5 centimeters. It should be further noted that
the thickness of the cell walls and fins as shown for
example in Figures 4-7 have been exaggerated for purposes
of illustration.
Figures 18 and 19 schematically show the
~- different mechanisms that are involved in the transmission
~. .
of incident sunlight through transparent honeycombs and
' specularly reflecting honeycombs such as already employed
,~ in the prior art by Buchberg et al supra. The solar rays
are transmitted through the honeycombs in either of two
ways, namely by reflection or direct transmission of the
solar rays. In the case of the specularly reflecting
honeycomb 72, the solar rays are transmitted solely by
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1082544
reflection as clearly depicted by the ar~ows in the
schematic view of Figure 19. Conversely, the solar rays
are transmitted by both reflection and direct transmission
in the case of the transparent honeycomb 74 as shown
by the arrows in the schematic view of Figure 18. .,
The dual mecbanisms of combined reflection and
transmission results in a higher overall transmission
efficiency for transparent honeycomb compared to specularly
reflecting honeycomb. Thus, when the transparent honey-
comb is used as a radiation trap versus a reflective
honeycomb, a higher fraction of the incident sunlight will
be transmitted to the collector plate or absorber where
it is converted into heat. To quantify this difference,
a theoretical analysis of the transmission efficiency of a
clear plastic honeycomb with an aspect ratio of 10 and
a highly reflecting metallized honeycomb of the same
aspect ratio was performed. The results of the analysis
are shown in Figure 20 where the overall transmission
efficiency of the two honeycombs is shown as a function
of incident sunlight angle. It can be clearly seen that
the clear or transparent honeycomb has a higher transmission
~ than the reflecting honeycomb at all incident angles
'~ above zero. Although the dual mechanism of combined
reflection and transmission has been hereinabove described ~-
in connection with transparent or clear honeycomb, it will
of course be understood that the concept is valid for
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honeycombs made of any light-transmitting material.
While it is advantageous to make the radiation
trap of a material which is transparent to solar rays, it
is also necessary as indicated that the radiation trap
must be absorptive of infrared or thermal radiation.
Tbe ~echanism by which infrared absorption in a honeycomb,
parallel fin or similar structure produces a radiation
trap effect is shown schematically in Figure 21. ~hermal
or infrared radiation is emitted in a diffuse manner
from a given point on the collector plate or absorber 76
such as the single point depicted at 78 in the view of
Figure 21. In a radiation trap of sufficiently high
aspect ratio (e.g., greater than 2 in the case of
honeycomb), the greatest fraction of the emitted radiat~on
will strike the walls of thè trap 80 as shown for instance
at the point 82 and will be absorbed. If any reradiation
occurs from the point 82, the greatest fraction will again
strike the walls of the trap 80 at another point such as
at the point 84 and will also be absorbed. C~nversely, as
shown in Figure 22, the emitted infrared radiation from a
given point 86 on the collector plate or absorber 88
striking the walls of an infrared reflecting honeycomb,
parallel fin or similar structure 90 will continue in a
direction away from the collector plate 88 by means of
multiple reflections as generally depicted by the arrows
and there will be substantially little or no trapping of t~e
~nfrared radiation
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108ZS~4
In addition to the optical property requir ements
of the radiation trap described above, it is also necessary
that the trap be made from a material that possesses a
low thermal conductivity, e.g., most plastics and glass.
To demonstrate the importance of using a low conductivity
material for the radiation trap, a theoretical analysis of
the radiation trapping properties of blac~ened aluminum
honeycomb (i.e. high conductivity material) and plastic
honeycomb (i.e. low conductivity material) was performed.
._s
In this analysis, the relationship between the Q/QO ratio
as previously defined and the aspect ratio LID was studied
and the results are shown in the graph of Figure 23.
It will be seen from the graph that the honeycomb which
is made from a high conductivity material does not function
as an effective radiation trap. This is due to the fact
~ that large amounts of heat are conducted through the
¦ walls and offset the reduction in radiation heat transfer
due to radiation trapping. The honeycomb which is made
from the low conductivity material on the other hand does
not suffer from this limitation and is therefore a
~j :
3~ superior radiation trap.
j~ A series of experiments were conducted to show
the unexpected results that are obtained by the use of ;
transparent radiation traps in transpiration air heaters
compared to their use in flat plate water heaters ~uch
as disclosed by Hollands supra. In the experiments, two
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.
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108~S44
solar heaters were constructed, one being a flat plate
water heater and the other being a transpiration air heat~r.
Both solar heaters were constructed with a single glazing
(light-transmitting front wall) and an equivalent amount
of thermal insulation. The two solar heaters were first.
tested acc~rding to procedures developed by the National
Bureau of Standards described in NBSIR 74-635 to determine
their thermal performance without any honeycomb between
the glazing and the collector plate or absorber. The
- 10 test data was recorded pursuant to procedures set up by
the National Bureau of Standards wherein the thermal
conversion efficiency ~ is plotted against a collector
performance parameter defined as
P* - ~avg amb~
Io
ln this definition TaVg is the average of the inlet and
outlet temperature of the fluîd (e.g. air) flowing through
the solar heater and Tamb is the temperature of the
~surroundings. Also in the definition, Io is the magnitude
of the flux of incident solar radiation. Thus, it will be
seen that the performance parameter P* is defined as the
~ difference between average overall temperature in the
;'; ; collector and the ambient temperature divided by the
magnitude of incident solar radiation. For space heating
i applications using solar air heaters, this parameter
; typically lies between about 0.04 and 0.08 square meters -
!1 29
. ,
.
. . ~
~` ~08~S44
degree Celsius per Watt. ~e results of the test for ~the
flat plate water heater and the transpiration air heater
with~ut the honeyc~b trap ~re shown by the curves labelled
A and B, respectively, in the graph of Figure 24.
The experiments were continued by modifying
each of thertwo solar heaters to include a tubular honeycomb
radiation trap between the glazing and the collector plate
or abs~rber. The tubular honeycomb had an L/~ ratio of
10 and was made of clear polycarbonate. The wall
thickness of the tubular honeyc~mb was 0.009 centimeters.
In the transpiration air heater, the position of the
honeycomb radiation trap was similar to that shown in
Figure 3. ~gain, the construction of the two dified
solar heaters was basically the same using a single
glazing and the same insulating material. The two heaters
', were then again tested using the same procedures outlined
above. The results of the test for the flat plate water
heater and the transpiration air heater using the honeycomb
radiation trap are shown by the curves labelled C and D,
respectively, in the graph of Figure 24. By reference to
the two sets of curves A, B and C,D, it will be readily
seen that the increase in performance efficiency is
significantly larger in the case of the transpiration
air heater as compared to the flat plate water heater.
In fact, it will be further seen from the curves that
without a honeyc~mb radiation trap the water heater has a
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, '
~ ,
~08ZS44
higher efficiency than the transpiration heater over the
entire range of operating conditions, whereas the converse
is true in the case where the honeycomb radiation traps
are incorporated in the two solar heaters.
In order to better show the magnitude of ~the .
difference in efficiency improvement resulting from the
inclusion of the honeycomb radiation trap in the two
solar heaters, a graph showing the fractional efficiency
increase over the efficiency of the solar heaters without
_ .
the honeycomb is presented in Figure 25. It will be
noted that over the entire range of operating conditions
the thermal efficiency increase for the transpiration air
heater is significantly greater than that for the flat
plate water heater.
Another series of experiments was conducted to
demonstrate the importance of maintaining the transparent
radiation trap in at least firm mechanical contact with
the front wall 12 in those embodiments where the radiation
trap is positioned adiacent to the front wall and the
trap serves the additional function of providing an air
buffer layer. As mentioned above, the transparent radia-
tlon trap is preferably bonded directly to the underneath
side of the front wall 12 o~, alternatively, may be held
in firm mechanical contact by an open support structure
wh~ch minimizes reflection losses of solar radiation
back toward the i~ront wall. The experiments were conducted
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: ' . '.' ". ' `' ,. ', ~'. ' ,. ~ " " ':
: . . .
1082S44
with a single collector of a construction similar to that
shown in Figure 11 wherein the porous collector plate or
absorber had a V-shaped configuration and was tested
using the same tubular honeycomb.radiation trap positioned
adjacent to the underneath side of the front wall. The
honeycomb radiation ~rap was made from polycarbonate tubes
with an aspect ratio of 7 and a wall thickness of 0.009
centimeters. The radiation trap was held in place by different
means in each test. In the first test the radiation trap
was held loosely against the front wall by an open support
structure consisting of thin spaced apart parallel bars.
In the second test the radiation trap was held against
the front wall by a continuous sheet of light-transmitting
air impermeable material, i.e. a fiber glass reinforced
polyester sheet with a high solar transmittance of between
0.85 and 0.90. In the third test, the radiation trap was
bonded to the front wall with a silicone rubber adhesive
sealant. The bond was such that air could not pas6
through the honeycomb into contact with the front wall.
` 20 In all other respects, the solar heaters remainet the
same throughout the experiments. The performance tests
were conducted in accordance with the ~ational Bureau of
Standards procedure outlined hereinabove. The results
of these tests are shown in the graph of Figure 26. -
Curve A represents the results of the test wherein the
honeycomb radiation trap was held loosely against the
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'~ `'`'` '` .
` 108Z544
front wall by an open support structure while curve B
represents the results wherein the radiation trap was held
in place by the continuous sheet of air impermeable light-
transmitting material. Curve C represents the results of
t~e tests wherein the honeycomb trap was actually bonde~
to the front wall with the adhesive sealant in accordance
with the present invention. It will be observed from the
curves that at a low value of P* corresponding to low
temperature operation of the solar heaters, the solar
- 10 heater using the additional air impermeable layer (curve
8) exhibits a lower efficiency than either of the others
due to additional reflection losses of some of the incident
solar rays. It will be further observed that the solar
heater in which the honeycomb is only loosely held
against the front wall (curve A) exhibits a faster
degradation of performance with increasing temperature
(corresponding to high values of P*) than the other
~ heaters. This effect is due to increased heat losses
resulting from the passage of some air through the honey-
comb and in contact with the front wall. However, neither
of these effects are observed in the case where the
honeycomb trap is actually bonded to the front wall
(curve C) using an adhesive sealant. As a result, high .
efficiencies are attained over the entire range of
operating conditions.
The differences in performance noted above can
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1()82544
be better understood by reference to the schematic views
"a" and '~" in Figures 27, 28 and 29. In particular,
Figure 27 shows the pattern of reflection losses in
view "a" and the pattern of air flow in view "b" for the
~olar heater wherein the honeycomb trap 22b is loosely ''
held against the front wall 12 by an ~pen support structure.
As denoted by the arrows in view "a", reflection losses
in a direction away from the collector occur only at the
front wall. As further depicted by the arrows in view
'b", a portion of the air flow through the solar heater
passes through the honeycomb trap 22b and comes into contact
with the front wall 12 where heat losses may occur. The
performance of this solar heater is represented by curve A
in Figure 26.
Figure 28 shows the pattern of re~lection
losses in view "a" and the pattern of air flow in view
"bi' for the solar heater wherein the honeycomb trap 22b
is supported by an air impermeable light-transmittinæ
layer 92. Again as depicted by the arrows in view "a"
reflection losses occur at the front wall 12 and in
addition they also occur at the air impermeable layer 92.
Also, as depicted by the arrows in view '~", all of the
~ir flow is prevent'ed from passing fnto the honeycomb
trap by the presence of the air impermeable layer 9~ and
tbus the honeycomb acts in addition as an air buffer.
~The performance of this solar heater is represented by
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`~ 108XS44
curve B in Figure 26. As shown by curve B, at low
temperature the increased reflection l~sses in the solar
heater result in a lower efficiency than that of the
solar heater represented by curve A while at higher
temperatures the presence of an air buffer layer results
in a higher efficiency than that of the solar heater
represented by curve A.
Figure 29 shows the pattern of reflection losses -
in view "a" and the pattern of air flow in view "b" ~or
~.
the solar heater wherein the honeycomb trab 22b is bonded
to the front wall 12 by an adhesive sealant. The pattern
of reflection losses is basically the same as that shown
in view "a" of Figure 27 but differs from the pattern of
reflection losses shown in view "a" of Figure 28 in that
no additional reflection losses occur below the front wall.
Conversely, as shown in view "b", the pattern of air flow
is basically the same as that for the solar heater shown
in view "b" of Figure 28 in that there is no air flow
through the honeycomb to the front wall. Thus the bonded
honeycomb trap acts as an air buffer by providing a
stagnant layer of air in all but the lowermost portion of
~` the honeycomb as shown by the arrows in the drawing.
Although the radiation trap has been depicted
in the accompanying drawing as having walls which are
disposed perpendicular to the front wall, it will be under-
stood of course that the present invention is not 80
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108Z544
restricted and that the radiation trap may in fact be
made with walls that are disposed at other angles with
respect to the front wall so long as any solar rays
reflected from the cell walls are n~t directed ~ack toward
the front wall during normal periods of operations. For
most practical purpoæes, the normal period of operation
- m~y be considered to include a period of ab~ut three
hours before and after solar noon. Uithin this period,
solar rays will be incident on the solar heater at an
angle within about 45 degrees of the perpendicular to
the front wall. For any given range of angles of incidence,
the cell walls must be disposed at an angle less than
some critical angle ~easured with respect to the perpendicular
to the front wall in order to insure that any reflected
solar rays are not directed back toward the front wall.
The development of the cr~tical angle for the
range of incidence angles ~p to 45 degrees from the
~ perpendicular to the front wall is schematically depictedin the views of Figures 30a to 30d. As shown in all the
- 20 ~iews "a" to "d" of Figure 30, the solar rays incident
within an angle of 45 degrees from the perpendicular to
the front wall are partly reflected from the front wall
il 12 as depicted by the arrow 94 and partly transmitted ..
directly through the front wall where the solar rays strike ~
the cell wall 94 and are again partly transmitted and
partly reflected as denoted by the arrows 98 and 100
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. .
;
: .
~082S44
respectively. As shown schematically in Figure 30a when the
cell wall 96 is aligned wi~h the perpendicular to the
front wall~ the reflected ray 100 will be directed away
from the front wall 12 and toward the absorber. Figure
30b shows the pattern of transmitted and reflected rays
in the instance where the cell wall 96 is disposed at an
angle el which is less than the critical a~gle ec. It
will be noted that the reflected ray 100 is still in ~
direction away from the front wall and toward the absorber.
Figure 30c shows the pattern of transmitted and reflected
rays in the instance where the cell wall 96 is disposed at
the critical angle ec and the reflected ray 100 is directed
parallel to the front wall 12. For the incident angle of
45 degrees shown in the drawing, the critical angle ec is
22 5 degrees with respect to the perpendicular to the front
wall. When the cell walls are disposed at angles of e2
greater than ~c~ the reflected rays 100 as shown in Figure
~ 30d will be directed back toward the front wall and away
from the absorber. Thus the phrase "substantially
`~ 20 perpendicular to the front wall" as used herein and in theappended claims to define the orientation of the cell walls
is intended to mean that the cell walls may be disposed at
any angle less than the critical angle measured with respect
to the perpendicular to the front wall, e.g. at angles
of less than about 22.~ degrees when the normal period of
operation is taken from about three hours before and
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~082544
after solar noon.
A typical solar space heating syste~ incorporating
a transpiration air heater in accordance with the present
invention is shown schematically in Figure 31. As shown,
air is drawn first through the solar air heater 102 via
duct 104 by means of a mechanical blower 106 in duct 108
and is heated by absorbed radiation when sufficient sunlight
is available. With the dampers 110, 112 and 114 in the
positions as shown in the drawing, the heated air i6
forced through the furnace 116 and then passes into the
space to be heated indicated at 118. ~he furnace 116 may
by any conventional gas, oil or electric ~urnace ~r
other heating source. When the temperature of the air
leaving the solar heater in the duct 104 is below the
temperature required for heating the space 118, additional
heat may be added by the furnace 116. During periods
when no heating of the space 118 is re~uired, the positions
of dampers 110 and 112 may be changed to those shown in
the dotted lines in order to allow the solar heated ~ir to
~' .
. 20 pass through a rock bed thermal storage bin 120 via duct
122. ~eat stored in the storage bin 120 may be utilized to
heat the space 118 during periods when sufficient sunlight
is not available to provide ade~uately heated air directly
. from the solar air heater 102. To utilize the stored heat,
the dampers 110, 112 are moved to the position indicated
by the solid l~nes and damper 114 is moved to the position
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, .
S;~
. .. . . . . . . . .. . . . .
..
,
~08Z544
indicated by the dotted lines such that air to be heated
is drawn from the space 118 via duct 124, then through the
thermal storage bin 120 wherein the air is heated. The
heated air then passes through the blower 106 via the
ducts 122, 126. Heated air passes through the furnace al6
where additional heat may be added to the air if the
temperature of the heated air is not sufficient to maintain
the space 118 at the desired temperature.
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t~
~ .
_ _ _ __
,
.. . . ' ' . . - ~ . . . . : .
