CA1082544A

CA1082544A - Solar air heater

Info

Publication number: CA1082544A
Application number: CA283,355A
Authority: CA
Inventors: Robert W. Mccullough; Thomas A. Hewett
Original assignee: Union Carbide Corp
Current assignee: Union Carbide Corp
Priority date: 1976-08-06
Filing date: 1977-07-22
Publication date: 1980-07-29

Abstract

SOLAR AIR HEATER

Abstract of the Disclosure A solar air heater is constructed with a housing having a transparent wall and an inlet and an outlet for establishing a flow path for a gas such as air to be heated and a porous collector plate positioned across the flow path and arranged to accept incident solar radiation pass-ing through the transparent wall wherein a transparent radiation trap is interposed between the collector plate and transparent wall.
S P E C I F I C A T I O N

Description

BACK~ROUMD
-(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 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 i -3-., '. ,:

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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.

, i 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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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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:, -` ~082S44 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.

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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.

-" ~08ZS44 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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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 .

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.

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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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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;

~3 '' 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) .. . .

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 -17_ ,; ~j.,~;, .. .. --. --~ .. ,. .. ..... ..... ., . . .. ... ~ .. ... ... . . . .. .

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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 l -18-.
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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 . . . .

~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 ` -21 - .

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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 ; --23-.

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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 ~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 .

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 .

~' , `` ~08;~S44 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 .A~, , .. , ~, . :

.
,; ~ .

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 . ~
.

~ ::

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 .
, ' ~ , ~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 .i~

: ' . '.' ". ' `' ,. ', ~'. ' ,. ~ " " ':
: . . .

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 '~ `'`'` '` .

` 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 .

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 `~ 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 ~,; "
, 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 . .
;

: .

~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 ~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 , .
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.

t~
~ .

_ _ _ __ , .. . . ' ' . . - ~ . . . . : .

Claims

WHAT IS CLAIMED IS:

1. A solar heater comprising, in combination:
a housing having a transparent wall for passing incident solar radiation and including an inlet and an outlet for establishing a flow path for a gaseous medium to be heated;
a gas permeable radiation absorbent collect-or element positioned across the flow path in said housing and arranged to accept incident solar radiation passing through said transparent wall and to transfer the absorbed heat to said gas-eous medium passing along said flow path and through said element; and a transparent radiation trap interposed be-tween said element and said transparent wall, said radiation trap comprising a cellular structure opaque to infrared radiation emitted from said element in a direction toward said transparent wall.

2. A solar heater according to claim 1 wherein said collector element comprises a porous opaque mat made from a material selected from the group consisting of re-ticulated foam, woven screen, and pressed fibers.

3. A solar heater according to claim 1 wherein said transparent radiation trap is juxtaposed with the surface of said collector element facing said transparent wall.

4. A solar heater according to claim 1 wherein said transparent radiation trap is interposed between and spaced from said collector element and said transparent wall.

5. A solar heater according to claim 1 wherein said transparent radiation trap is juxtaposed with the surface of said transparent wall facing said collector element.

6. A solar heater according to claim 1 wherein said transparent radiation trap comprises a transparent honeycomb structure.

7. A solar heater according to claim 6 wherein said transparent honeycomb structure is made of glass or a clear polymeric material.

8. A solar heater according to claim 7 wherein the length and diameter of each opening in said honeycomb structure is maintained at a predetermined ratio within the range of between two and ten.

9. A solar heater according to claim 1 wherein said transparent radiation trap comprises an array of closely spaced, transparent, parallel fins.

10. A solar heater according to claim 9 wherein said transparent fins are made of glass or a clear poly-meric material.

11. A solar heater according to claim 9 wherein the width and spacing between said transparent fins is maintained at a predetermined ratio within the range of about four and twenty.

CLAIMS SUPPORTED BY SUPPLEMENTARY DISCLOSURE

12. A solar air heater comprising, in combination:
a housing having a light-transmitting front wall for passing incident solar radiation and including an inlet and an outler for establishing a flow path for a gaseous medium to be heated;
a gas-permeable radiation absorbent collector element positioned across the flow path in said housing and arranged to accept incident solar radiation passing through said front wall and to transfer the absorbed heat to said gaseous medium passing along said flow path and through said collector element; and a radiation trap disposed in said housing adjacent to the surface of said front wall facing said collector element, said radiation trap comprising a cellular structure containing a multiplicity of open cells in communication with said flow path and having cell walls which are substantially perpendicular to said front wall and which serve as baffle elements to inhibit the flow of said gaseous medium through said radiation trap in a direction substantially parallel to the plane of said front wall, said cellular structure being composed of a light-trans-mitting material which is opaque to infrared radiation emitted from said collector element in a direction toward said front wall.

13. A solar air heater according to claim 12 wherein said cellular structure is made from a glass or clear plastic composition selected from the group consisting of polyvinyl fluoride, polycarbonate, fluorinated ethylene propylene, polymethyl methacrylate, aromatic polysulfones, polyethylene terephthalate, aromatic polyesters, polyvinylidene fluoride, hexafluoropropylene, chlorotrifluoroethylene and tetrafluoroethylene copolymers.

14. A solar air heater according to claim 12 wherein said cellular structure comprises a transparent honeycomb.

15. A solar air heater according to claim 14 wherein said transparent honeycomb is composed of a multiplicity of cells having a hexagonal cross-section.

16. A solar air heater according to claim 14 wherein said transparent honeycomb is composed of a multiplicity of cells having a rectangular cross-section.

17. A solar air heater according to claim 14 wherein said transparent honeycomb is composed of a multiplicity of cells formed by tubes stacked side-by-side and bonded to adjacent tubes by an adhesive or solvent.

18. A solar air heater according to claim 14 wherein said transparent honeycomb is composed of a multiplicity of cells having a length to diameter ratio of between about 2 and 10.

19. A solar air heater according to claim 14 wherein said transparent honeycomb is composed of multiplicity of cells having walls of a thickness within the range of from about 0.0002 to about 0.05 centimeters.

20. A solar air heater according to claim 12 wherein said cellular structure is formed by an array of transparent spaced apart, parallel fins disposed in a direction substantially perpendicular to said flow path.

21. A solar air heater according to claim 20 wherein said transparent spaced apart, parallel fins have a height to spacing ratio of between about 4 and 20.

22. A solar air heater according to claim 20 wherein said transparent spaced apart, parallel fins have a wall thickness within the range of from about 0.0002 to about 0.05 centimeters.

23. A solar air heater according to claim 12 wherein said cellular structure is maintained in at least firm mechanical contact with said front wall.

24. A solar air heater according to claim 23 , wherein said cellular structure is held in firm contact against said front wall by open support means.

25. A solar air heater according to claim 24 wherein said support means comprises an open mesh placed against the side of said cellular structure opposite to said front wall.

26. A solar air heater according to claim 24 wherein said support means comprises a plurality of spaced apart, parallel bars placed against the side of said cellular structure opposite to said front wall.

27. A solar air heater according to claim 23 wherein said cellular structure is bonded to said front wall.

28. A solar air heater according to claim 12 wherein said radiation absorbent collector element comprises a porous opaque mat made from a material selected from the group consisting of pressed fibers, woven screen, stamped screen and reticulated foam.

29. A solar air heater according to claim 12 wherein said radiation absorbent collector element is positioned in substantially parallel, spaced apart relation to said front wall.

30. A solar air heater according to claim 29 wherein said inlet and outlet are arranged in said housing such that said gaseous medium to be heated passes through said radiation absorbent collector element in a direction away from said front wall.

31. A solar air heater according to claim 29 wherein said inlet and outlet are arranged in said housing such that said gaseous medium to be heated passes through said radiation absorbent collector element in a direction toward said front wall.

32. A solar air heater according to claim 29 wherein baffle means are provided for directing said gaseous medium to be heated first through one segment of said radiation absorbent collector element and then through another segment thereof.

33. A solar air heater according to claim 12 wherein said radiation absorbent collector element is positioned in non-parallel relation to said front wall.

34. A solar air heater according to claim 33 wherein said inlet and outlet are arranged in said housing such that said gaseous medium to be heated passes through at least one radiation absorbent collector element without changing direction.

35. A solar air heater according to claim 34 wherein said inlet and outlet are arranged in said housing such that said gaseous medium to be heated passes through said radiation absorbent collector element in a direction toward the surface facing said front wall.

36. A solar air heater according to claim 34 wherein said inlet and outlet are arranged in said housing such that said gaseous medium to be heated passes through said radiation absorbent collector element in a direction away from the surface facing said front wall.

37. A solar air heater according to claim 34 wherein said radiation absorbent collector element comprises two non-parallel segments arranged in a V-shaped configuration.

38. A solar air heater according to claim 37 wherein said inlet and outlet are arranged in said housing such that said gaseous medium to be heated passes first through one of said non-parallel segments and then through the other of said non-parallel segments forming said V-shaped collector element.

39. A solar air heater according to claim 12 wherein said housing includes a bottom wall and side walls and wherein said inlet and outlet are located in opposite side walls of said housing.

40. A solar air heater according to claim 39 wherein a layer of insulation is provided adjacent to said bottom wall.

41. A solar air heater according to claim 39 wherein said housing is made of metal.

42. A solar air heater according to claim 39 wherein said housing is made of a rigid insulating material.

43. A solar air heater comprising, in combination:
a housing having a light-transmitting front wall for passing incident solar radiation and including an inlet and an outlet for establishing a flow path for a gaseous medium to be heated;
a gas-permeable radiation absorbent collector element positioned across the flow path in said housing and arranged to accept incident solar radiation passing through said front wall and to transfer the absorbed heat to said gaseous medium passing along said flow path and through said collector element; and a radiation trap disposed in said housing adjacent to the surface of said collector element facing said front wall, said radiation trap comprising a cellular structure made of a light-transmitting material which is opaque to infrared radiation and containing a multiplicity of open cells arranged to pass said gaseous medium to be heated therethrough and along said flow path.

44. A solar air heater according to claim 43 wherein said cellular structure is made from glass or clear plastic composition selected from the group consisting of polyvinyl fluoride, polycarbonate, fluorinated ethylene propylene, polymethyl methacrylate, aromatic polysulfones, polyethylene terephthalate, aromatic polyesters, polyvinylidene fluoride, hexafluoropropylene, chlorotri-fluoroethylene and tetrafluoroethylene copolymers.

45. A solar air heater according to claim 43 wherein said cellular structure comprises a transparent honeycomb.

46. A solar air heater according to claim 4 wherein said transparent honeycomb is composed of a multiplicity of cells having a hexagonal cross-section.

47. A solar air heater according to claim 45 wherein said transparent honeycomb is composed of a multiplicity of cells having a rectangular cross-section.

48. A solar air heater according to claim 45, wherein said transparent honeycomb is composed of a multiplicity of cells formed by tubes stacked side-by-side and bonded to adjacent tubes by an adhesive or solvent.

49. A solar air heater according to claim 45 wherein said transparent honeycomb is composed of a multiplicity of cells having a length to diameter ratio of between about 2 and 10.

50. A solar air heater according to claim 45 wherein said transparent honeycomb is composed of a multiplicity of cells having walls of a thickness within the range of from about 0.0002 to about 0.05 centimeters.

51. A solar air heater according to claim 43 wherein said cellular structure is formed by an array of transparent spaced apart, parallel fins.

52. A solar air heater according to claim 51 wherein said transparent spaced apart, parallel fins have a height to spacing ratio of between about 4 and 20.

53. A solar air heater according to claim 51 wherein said transparent spaced apart, parallel fins have a wall thickness within the range of from about 0.0002 to about 0.05 centimeters.

54. A solar air heater according to claim 43 wherein said radiation absorbent collector element comprises a porous opaque mat made from a material selected from the group consisting of pressed fibers, woven screen, stamped screen and reticulated foam.

55. A solar air heater according to claim 43 wherein said radiation absorbent collector element and said radiation trap are combined together in a single unit.

56. A solar air heater according to claim 55 wherein said combined radiation absorbent collector element and radiation trap comprises a cellular structure containing a multiplicity of open cells arranged to pass said gaseous medium to be heated therethrough and along said flow path, said cells being partially filled with a porous opaque, heat absorbing material.

57. A solar air heater according to claim 56 wherein said cellular structure is made from glass or clear plastic composition selected from the group consisting of polyvinyl fluoride, polycarbonate, fluorinated ethylene propylene, polymethyl methacrylate, aromatic polysulfones, polyethylene terephthalate, aromatic polyesters, polyvinylidene fluoride, hexafluoropropylene, chlorotrifluoroethylene and tetrafluoroethylene copolymers.

58. A solar air heater according to claim 56 wherein said cellular structure comprises a transparent honeycomb.

59. A solar air heater according to claim 58 wherein said transparent honeycomb is composed of a multiplicity of cells having a hexagonal cross-section.

60. A solar air heater according to claim 58 wherein said transparent honeycomb is composed of a multiplicity of cells having a rectangular cross-section.

61. A solar air heater according to claim 58 wherein said transparent honeycomb is composed of a multiplicity of cells formed by tubes stacked side-by-side and bonded to adjacent tubes by an adhesive or solvent.

62. A solar air heater according to claim 58 wherein the portion of the honeycomb cells which is left open has a length to diameter ratio of between about 2 and 10.

63. A solar air heater according to claim 58 wherein said transparent honeycomb is composed of a multiplicity of cells having walls of a thickness within the range of from about 0.0002 to about 0.05 centimeters.

64. A solar air heater according to claim 56 wherein said cellular structure is formed by an array of transparent spaced apart, parallel fins having the spaces between said fins partially filled with the porous opaque, heat absorbing material.

65. A solar air heater according to claim 64 wherein the portion of the spaces between said transparent spaced apart, parallel fins which is left open has a fin height to spacing ratio of between about 4 and 20.

66. A solar air heater according to claim 64 wherein said transparent spaced apart, parallel fins have a wall thickness within the range of from about 0.0002 to about 0.05 centimeters.

67. A solar air heater according to claim 55 wherein said combined radiation absorbent collector element and radiation trap comprises a cellular structure containing a multiplicity of open cells arranged to pass said gaseous medium to be heated therethrough and along said flow path, said cells having walls part of which are made opaque and heat absorbent by coloring with a dark paint or coloring agent.

68. A solar air heater according to claim 67 wherein said cellular structure is made from glass or clear plastic composition selected from the group consisting of polyvinyl fluoride, polycarbonate, fluorinated ethylene propylene, polymethyl methacrylate, aromatic polysulfones, polyethylene terephthalate, aromatic polyesters, polyvinylidene fluoride, hexafluoro-propylene, chlorotrifluoroethylene and tetrafluoroethylene copolymers.

69. A solar air heater according to claim 67 wherein said cellular structure comprises a transparent honeycomb.

70. A solar air heater according to claim 69 wherein said transparent honeycomb is composed of a multiplicity of cells having a hexagonal cross-section.

71. A solar air heater according to claim 69 wherein said transparent honeycomb is composed of a multiplicity of cells having a rectangular cross-section.

72. A solar air heater according to claim 69 wherein said transparent honeycomb is composed of a multiplicity of cells formed by tubes stacked side-by-side and bonded to adjacent tubes ty an adhesive or solvent.

73. A solar air heater according to claim 69 wherein the surfaces of the honeycomb cell which are left transparent have a length to diameter ratio of between about 2 and 10.

74. A solar air heater according to claim 67 wherein said cellular structure is formed by an array of transparent spaced apart, parallel fins part of which are colored with the dark paint or coloring agent.

75. A solar air heater according to claim 74 wherein the portion of said transparent spaced apart, parallel fins which is left transparent has a fin height to spacing ratio of between about 4 and 20.

76. A solar air heater according to claim 74 wherein said transparent spaced apart, parallel fins have a wall thickness within the range of from about 0.0002 to about 0.05 centimeters.

77. A solar air heater according to claim 43 wherein said housing includes a bottom wall and side walls and wherein said inlet and outlet are located in opposite side walls of said housing.

78. A solar air heater according to claim 77 wherein a layer of insulation is provided adjacent to said bottom wall.

79. A solar air heater according to claim 77 wherein said housing is made of metal.

80. A solar air heater according to claim 77 wherein said housing is made of a rigid insulating material.

81. A solar air heater comprising, in combination:

a housing having a light-transmitting front wall for passing incident solar radiation and including an inlet and an outlet for establishing a flow path for a gaseous medium to be heated;
a gas-permeable radiation absorbent collector element positioned across the flow path in said housing and arranged to accept incident solar radiation passing through said front wall and to transfer the absorbed heat to said gaseous medium passing along said flow path and through said collector element; and a radiation trap disposed in said housing between and spaced from said collector element and said front wall, said radiation trap comprising a cellular structure made of a light-transmitting material which is opaque to infrared radiation and containing a multiplicity of open cells arranged to pass said gaseous medium to be heated therethrough and along said flow path.

82. A solar air heater according to claim 81 wherein said cellular structure is made from a glass or clear plastic composition selected from the group consisting of polyvinyl fluoride, polycarbonate, fluorinated ethylene propylene, polymethyl methacrylate, aromatic polysulfones, polyethylene terephthalate, aromatic polyesters, polyvinylidene fluoride, hexafluoro-propylene, chlorotrifluoroethylene and tetrafluoroethylene copolymers.

83. A solar air heater according to claim 81 wherein said cellular structure comprises a transparent honeycomb.

84. A solar air heater according to claim 83 wherein said transparent honeycomb is composed of a multiplicity of cells having a hexagonal cross-section.

85. A solar air heater according to claim 83 wherein said transparent honeycomb is composed of a multiplicity of cells having a rectangular cross-section.

86. A solar air heater according to claim 83 wherein said transparent honeycomb is composed of a multiplicity of cells formed by tubes stacked side-by-side and bonded to adjacent tubes by an adhesive or solvent.

87. A solar air heater according to claim 83 wherein said transparent honeycomb is composed of a multiplicity of cells having a length to diameter ratio of between about 2 and 10.

88. A solar air heater according to claim 83 wherein said transparent honeycomb is composed of a multiplicity of cells having walls of a thickness within the range of from about 0.0002 to about 0.05 centimeters.

89. A solar air heater according to claim 81 wherein said cellular structure is formed by an array of transparent spaced apart, parallel fins.

90. A solar air heater according to claim 89 wherein said transparent spaced apart, parallel fins have a height to spacing ratio of between about 4 and 20.

91. A solar air heater according to claim 89 wherein said transparent spaced apart, parallel fins have a wall thickness within the range of from about 0.0002 to about 0.05 centimeters.

92. A solar air heater according to claim 81 wherein said radiation absorbent collector element comprises a porous opaque mat made from a material selected from the group consisting of pressed fibers, woven screen, stamped screen and reticulated foam.

93. A solar air heater according to claim 81 wherein said housing includes a bottom wall and side walls and wherein said inlet and outlet are located in opposite side walls of said housing.

94. A solar air heater according to claim 93 wherein a layer of insulation is provided adjacent to said bottom wall.

95. A solar air heater according to claim 93 wherein said housing is made of metal.

96. A solar air heater according to claim 93 wherein said housing is made of a rigid insulating material.

97. In a solar heating system including a solar air heater, means for passing a gas to be heated through said solar air heater and for directing the heated gas to a space to be heated, a storage device for storing heat over a prolonged period of time, means for periodically diverting said heated gas away from said space and into said storage device when said space has reached a pre-determined temperature and means for periodically directing the gas to be heated through said storage device for heating said gas and then directing said heated gas into said space; the improvement wherein said solar air heater comprises, in combination:
a housing having a light-transmitting front wall for passing incident solar radiation and including an inlet and an outlet for establishing a flow path for a gaseous medium to be heated;

a gas-permeable radiation absorbent collector element positioned across the flow path in said housing and arranged to accept incident solar radiation passing through said front wall and to transfer the absorbed heat to said gaseous medium passing along said flow path and through said collector element; and a radiation trap disposed in said housing adjacent to the surface of said front wall facing said collector element, said radiation trap comprising a cellular structure containing a multiplicity of open cells in communication with said flow path and having cell walls which are substantially perpendicular to said front wall and which serve as baffle elements to inhibit the flow of said gaseous medium through said radiation trap in a direction substantially parallel to the plane of said front wall, said cellular structure being composed of a light-transmitting material which is opaque to infrared radiation emitted from said collector element in a direction toward said front wall.

98. A solar heating system according to claim 97 wherein said means for directing said heated gas to the space to be heated includes an additional heating source.

99. A solar heating system according to claim 97 wherein said storage device comprises a rock bin.

100. In a solar heating system including a solar air heater, means for passing a gas to be heated through said solar air heater and for directing the heated gas to a space to be heated, a storage device for storing heat over a prolonged period of time, means for periodically diverting said heated gas away from said space and into said storage device when said space has reached a pre-determined temperature and means for periodically directing the gas to be heated through said storage device for heating said gas and then directing said heated gas into said space; the improvement wherein said solar air heater comprises, in combination:
a housing having a light-transmitting front wall for passing incident solar radiation and including an inlet and an outlet for establishing a flow path for a gaseous medium to be heated;
a gas-permeable radiation absorbent collector element positioned across the flow path in said housing and arranged to accept incident solar radiation passing through said front wall and to transfer the absorbed heat to said gaseous medium passing along said flow path and through said collector element; and a radiation trap disposed in said housing adjacent to the surface of said collect-or element facing said front wall, said radiation trap comprising a cellular structure being composed of a light-transmitting material which is opaque to infrared radiation emitted from said collector element in a direction toward said front wall.

100 wherein said means for directing said heated gas to the space to be heated includes an additional heating source.

102. A solar heating system according to claim 100 wherein said storage device comprises a rock bin.

103. In a solar heating system including a solar air heater, means for passing a gas to be heated through said solar air heater and for directing the heated gas to a space to be heated, a storage device for storing heat over a prolonged period of time, means for periodi-cally diverting said heated gas away from said space and into said storage device when said space has reached a pre-determined temperature and means for periodically directing the gas to be heated through said storage device for heating said gas and then directing said heated gas into said space;
the improvement wherein said solar air heater comprises, in combination:
a housing having a light-transmitting front wall for passing incident solar radiation and including an inlet and an outlet for establishing a flow path for a gaseous medium to be heated;
a gas-permeable radiation absorbent collector element positioned across the flow path in said housing and arranged to accept incident solar radiation passing through said front wall and to transfer the absorbed heat to said gaseous medium passing along said flow path and through said collector element; and a radiation trap disposed in said housing between and spaced from said collector element and said front wall, said radiation trap com-prising a cellular structure being composed of a light-transmitting material which is opaque to infrared radiation emitted from said collector element in a direction toward said front wall.

104. A solar heating system according to claim 103 wherein said means for directing said heated gas to the space to be heated includes an additional heating source.

105. A solar heating system according to claim 102 wherein said storage device comprises a rock bin.