Solar collector having absorber plate formed by spraying molten metal

A solar energy collector whereby an absorber plate thereof is formed by providing a thin metallic foil for supporting sprayed molten metal, arranging fluid carrying tubes on the foil, and spraying molten metal using a source of pressurized inert gas onto a substrate and tubes to build up a coating of sufficient thickness to form a unified solar collector panel comprising the absorber plate, embedded tube and foil which have good thermal contact with each other. The method further includes forming a selective surface on the sprayed metal absorber plate.

BACKGROUND OF THE INVENTION 
The present invention relates to solar collectors and methods of producing 
the same. 
A typical solar collector absorbs radiant energy by way of an absorber 
plate to which tubes are mechanically attached or adhesively bonded. A 
working fluid in the tube transfers useful heat energy from the absorber 
plate. To attain high efficiency, one strives to achieve good thermal 
contact between the absorber plate and the tubes. As known in the art, the 
manner of connecting the tubes to the absorber plate affects heat 
collecting and transfer efficiency of the solar collector. U.S. patents to 
Boyd (U.S. Pat. No. 4,074,406), Andrassy (U.S. Pat. No. 4,089,326) Heinman 
(U.S. Pat. No. 4,245,620), Beckman (U.S. Pat. No. 4,369,836) and Grahman 
(U.S. Pat. No. 4,517,721) show typical absorber plates with tubes 
mechanically fastened or cemented into place. Use of dissimilar metals, 
such as copper tubes and aluminum plates, limits joining and connecting 
techniques and thereby presents some difficulties in the collector 
fabrication process. 
The present invention improves heat collecting and transfer efficiency 
regardless of tube configuration, and enables formation of an absorber 
plate and tube connection with greater thermal contact despite use of 
dissimilar metals or materials. The invention also obviates the need to 
use manifolds to connect respective tubes, unlike the welding and 
mechanical compression techniques used in prior art systems. In addition, 
an absorber plate having varying thicknesses throughout its surface may 
conveniently be fabricated. 
Solar collectors traditionally include a selective surface to increase 
absorption efficiency. A common high efficiency selective surface employs 
electro-plated black nickel or chrome. The process of forming a selective 
surface involves intricate surface preparation including cleaning, 
chemical treatment, etching, and electroplating. The present invention, if 
desired, enables application of a selective surface by conventional 
techniques, or alternatively, by using metallic foils having a surface 
which is pre-treated as a selective surface. 
SUMMARY OF THE INVENTION 
An absorber plate of a solar collector is formed by depositing sprayed 
molten metal on a substrate which includes heat transfer tubes until 
building up a sufficient thickness of solidified metal about the tubes 
whereby a unitary structure is formed including a collecting surface, the 
transfer tubes, and an absorber plate formed by the sprayed metal. The 
sprayed metal absorber plate may be separated from the substrate after 
solidification, or alternatively, may include a thin sheet of metallic 
foil placed over the substrate before spraying and solidification. The 
method preferably includes providing a selective surface on the absorber 
plate. Spraying preferably is achieved by depositing molten metallic 
particles sprayed in a vacuum chamber or by a pressurized inert gas in an 
inert atmosphere. 
These and other objects, features and advantages of the present invention 
will become more apparent from the following description when taken in 
connection with the accompanying drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring now to the drawings wherein like reference numerals are used to 
designate like parts and more particularly to FIG. 1, it is seen that a 
flat-plate collector comprises transparent glazing 3, collector frame 4, 
inlet port 5, and exit port 6. Solar radiation striking the collector 
passes through transparent glazing 3 and is absorbed by an absorber plate 
disposed beneath said glazing (FIG. 2). As the absorber plate heats up, a 
heat transfer medium, such as a gas or fluid, is introduced in the inlet 
port 5, and is brought into contact with said absorber plate 9, absorbing 
the available thermal energy and exiting through exit port 6 to be used or 
stored. In this manner, solar energy is transferred to the working heat 
transfer medium. Means are provided to insulate the absorber plate for 
maximum efficiency, by air or gas space 10 (FIG. 2) disposed above the 
absorber plate, and by an insulating material 11 (FIG. 2) on the edges and 
underneath said absorber plate. 
Absorber plate 9 is formed by depositing sprayed molten metal on a 
substrate which includes heat transfer tubes until building up a 
sufficient thickness of solidified metal about the tubes whereby a unitary 
structure is formed including a collecting surface, heat transfer tubes, 
and an absorber plate formed by the sprayed metal. Adhesion of the molten 
metal during spraying improves upon preheating the substrate. Cooling is 
allowed between layers to build up successive layers for thick deposits. 
When unconcerned with substrate damage due to overheating, the sprayed 
metallic absorber may be heated during or after spraying to a point where 
the sprayed metal particles are molten and flow together creating a more 
homogeneous material and consequently increasing the thermal conductivity. 
Spraying is preferably performed in a vacuum or inert atmosphere to reduce 
oxide formation. Such oxides lower thermal conductivity and impede the 
flow of the molten material. 
Spraying may be accomplished by a pressure differential between the sprayed 
source of molten metal and a chamber encasing the absorber plate. In some 
cases, an evacuated chamber has a negative pressure relative to the source 
of molten metal which may be at atmospheric pressure. The net pressure 
differential causes molten metal to flow into the evacuated substrate 
chamber. In this case, molten metal is directed through an orifice or 
atomizing device to induce a spray. Sometimes, it becomes necessary to 
increase the pressure differential by creating a positive pressure in the 
molten metal reservoir in order to induce a more rapid flow or final 
spray. Additionally, metal may be sprayed or deposited as a powder, flake, 
chip or similar size and shaped particles, and fused together by melting. 
After the powder, flake or chip has been deposited, it may be mechanically 
compressed before subsequent heating and melting. A binder or carrier 
agent may be used to hold together the deposited material which is then 
burned off during the heating and melting phase. To improve the adhesion 
of the melted metal to the absorber tubes, it may be desirable to first 
coat the tubes with a layer of sprayed metal. This may be of the same 
metal as the metal used to form the absorber plate. An alloy may 
alternatively be employed. 
Alternatively, the powder, chip or flake may be sprayed onto a heated 
substrate to effect melting on contact and solidification thereafter. 
The absorber plate may be formed as separate element distinct from the 
remaining parts of the collector unit. When the choice of metal, metal 
thickness, geometric configuration or other parameter produces a sprayed 
absorber plate having insufficient structural integrity without a 
substrate, the substrate on which metal is sprayed remains part of the 
assembled structure. The preferred embodiment of the present invention 
uses, but is not limited to, foils or wire mesh as a substrate material. 
When the sprayed absorber plate does have sufficient structural integrity, 
it may separated form the substrate to be placed as a distinct component 
within the collector housing. Alternatively, a reinforcing structure may 
be attached to the absorber plate by conventional means after the spraying 
step. 
By way of example, an absorber plate according to a preferred process is 
manufactured as follows. A thin aluminum foil having a thickness of 
approximately one mil is placed over a substrate. Aluminum or copper 
tubing is laid in the required pattern on the foil to provide a heat 
exchanger for the absorber plate. The substrate may include grooves to 
which the foil conforms and for receiving the tubing. The substrate can be 
heated prior to the spraying operation, if desired. After the entire 
assembly is laid in position, molten metal, such as aluminum, is then 
sprayed onto the tubing and the foil in order to build up a desired 
thickness of sprayed solidified molten metal across the foil which 
preferably embeds the tubing within. The resulting component, e.g., 
sprayed metal, foil and tubing, is then mounted as a single unit within a 
collector housing. It is evident that other variations of this process can 
be practiced. 
As mentioned previously, the sprayed molten metal particles adhere best to 
irregularities in the surface of the substrate. In order to improve 
adhesion, the substrate surface is roughened by grit or sand blasting, 
such as with aluminum oxide. Soft substrate surfaces may be coated and 
then roughened to improve adhesion of the sprayed molten metal. The 
substrate surface may also be coated with another metal, such as zinc, to 
improve adhesion of the sprayed metal. 
Some examples of said coating would be adhesives or coatings of just about 
any kind, including, but not limited to, epoxy and polyester resins. The 
preferred embodiment uses a sodium silicate base or derivative because of 
its superior sealing qualities, longevity, wide temperature range, and low 
cost. It may be necessary to add a powder or other grit-type material to 
the coating to give the surface a textured or rough finish enhancing the 
adhesion of the sprayed molten metal. For best results, the powder or grit 
should be applied to the said coating while it is still in an uncured, 
semi-cured, or semi-liquid state. The preferred embodiment uses, but is 
not limited to, aluminum oxide as the grit to improve the adhesion of the 
sprayed molten metal. Adhesion of the sprayed molten metal may be improved 
sometimes by spraying the molten metal while said underlying coating is 
still uncured or semi-viscous, i.e., not hard or dry. 
Because the sprayed molten metal is sometimes carried by a pressurized 
stream of air or gas, its deposition characteristics are subject to the 
nature in which the stream of air or gas strikes the surface of the 
substrate. Inert gas may also be used as a source of pressurized gas. It 
is sometimes advantageous to change the geometry of the surface to be 
coated so that the sprayed molten metal is deposited where and in a manner 
that is desirable. An example where this is often applicable is where two 
or more objects or surfaces are to be joined or embedded with molten 
metal, such as, for example, to attach a round tube to a flat surface. 
Because it is difficult to direct the molten metal spray to the underside 
of the tube, it is often desirable to make a groove in the substrate to 
receive all or part of the tube. In some instances there may be a gap 
created between the edge of the groove and the edge of the tube, where the 
molten metal spray may be reluctant to fill in or bridge the gap. It is 
therefore necessary to fill the gap with a material that will ensure a 
smooth transition from the substrate to the tube. The preferred embodiment 
uses, but is not limited to, a sodium silicate base or derivative with 
aluminum oxide on the exposed surface. 
FIG. 2 illustrates a cross-sectional view of a version of FIG. 1, where the 
absorber plate 9 is formed by spraying molten metal onto a substrate 
according to the process described previously. The absorber plate 9 may be 
formed by spraying substrate 11 with the desired molten metal and securing 
it within the collector body, or alternatively, spraying the desired 
molten metal directly onto the insulating substrate 11 forming the 
absorber plate 9 directly thereon. Absorber 9 can be formed from, but is 
not limited to, copper and/or aluminum. Absorber plate 9 is insulated 
above from air space 10, said air space may be filled with an insulating 
gas. Additionally. absorber plate 9 is insulated on the bottom and sides 
by insulating material 11. Insulating material 11 may be a cellular or 
foamed insulation material. Solar collector housing 4 is shown which 
supports glazing 3 and houses the components of the collector. This member 
may be fabricated by existing techniques. Seal 13 is illustrated which 
seals transparent glazing 3 to collector frame 4 and may be constructed 
according to conventional practice. Clamp 12 holds the glazing securely 
within the collector. 
FIG. 3 is a cross-sectional view of yet another version of FIG. 1, similar 
to that in FIG. 2, except that absorber tubes 16 have been added to the 
absorber plate 17. The absorber plate 17 is formed in the same way as 
absorber 9 in FIG. 2, except that molten metal is sprayed over absorber 
tubes 16 and its substrate embedding them within absorber plate 17. The 
preferred embodiment uses copper tubes sprayed with aluminum. As discussed 
in the background of the invention, copper tubing is extremely difficult 
to attach to an aluminum absorber, soldering, brazing or welding according 
to conventional practice being just about impossible. However, this 
combination of metals is extremely desirable because of the corrosion 
resistance of the copper tubes to the heat transfer fluids, which usually 
have water as the major component, and because of the low cost of the 
aluminum for the absorber plate. The process described herein, of spraying 
molten metal to form the absorber plate works extremely well by embedding 
the copper tubing within the aluminum or attaching it thereto, with bond 
strengths of 8000 psi or higher easily attained. Because the copper 
absorber tube is wrapped in the aluminum absorber plate, the copper tubing 
is sealed off from environmental factors such as moisture and salt ions 
that might promote galvanic corrosion. In addition, no fluxes as with the 
conventional, soldering, brazing and welding processes, are needed, fluxes 
usually containing high concentrations of metal salts which promotes 
corrosion. 
FIG. 4 is a cross-sectional view of a variation of FIG. 1 and is very 
similar to FIG. 3, where the absorber tubes 16 are embedded in the 
absorber plate 17 by spraying molten metal 15 directly on substrate 11 and 
absorber tubes 16, resulting in a continuous absorber plate 17, where the 
absorber tubes 16 are an integral part of the absorber plate. 
Additionally, grooves 18 are formed in the substrate 11 to receive part of 
the absorber tubes 16. 
FIG. 5 is an expanded cross-sectional view of a variation of an absorber 
plate such as that in FIG. 4, where the absorber plate 20 is formed 
directly on the insulation material 11 by spraying molten metal as 
described herein. Groove 25 is formed in insulating material 11 to receive 
all or part of absorber tubes 16. In order to improve adhesion of the 
sprayed metal absorber plate 20 to insulating material 11, an initial base 
coating 21 has been added to the surface of 11. To further improve 
adhesion of the sprayed metal absorber plate to insulating member 11, an 
additional layer or coating 23 of powder or grit has been added to the 
surface of base coat 21, as described previously. Base coatings 21 and 22 
should bridge or close the gap at 24 created between the edge of groove 25 
and absorber tube 16. This will assure that the sprayed metal absorber 20 
will make a smooth transition to the absorber tube 16. This greatly 
enhances ease of manufacturing as described previously. The preferred 
embodiment uses a sodium silicate base or derivative for 21 with aluminum 
oxide added to the surface to form 22. 
FIG. 6 is an expanded cross-sectional view of an absorber plate formed 
according to the process of spraying molten metal described herein, 
utilizing a foil or metal sheet 27 as the substrate for sprayed metal 
coating 26. Foil or sheet 27 has grooves 35 formed therein to receive 
absorber tube 16. Sheet or foil 27 also is prepared, as previously 
mentioned, to improve adhesion of the sprayed molten metal layer to it. 
The preferred embodiment uses, but is not limited to, grit blasting as the 
said preparation method. Sprayed molten metal is applied preferably in 
thin multiple layers, building up coating 26 onto foil 27 and absorber 
tube 16 to the desired thickness forming a continuous uninterrupted 
absorber plate. Sprayed metal coating 26 may be concentrated on the joint 
29 where the substrate sheet 25 or foil 27 meets absorber tube 16, coating 
29 and the area to either side of 29 on 16 and 27 with sprayed molten 
metal, hence attaching said absorber tube 16 to substrate sheet 27. 
Some methods may damage the substrate by excess heat developed during 
spraying molten metal. The present invention overcomes this problem by 
applying sprayed molten metal layer 26 in multiple thin layers and 
building up to the appropriate thickness, allowing any excess heat to 
dissipate after each layer. In this manner, the absorber tubes 16 can be 
efficiently attached to the substrate 27 without damaging the pre-coated 
surface 28. 
FIG. 7 shows an expanded cross-sectional view of an absorber plate similar 
to that described in FIG. 6. If it is not desirable to form grooves in the 
substrate to receive the absorber tubes, alternatively one can modify the 
shape of the tube 30 so that, when attached to a flat substrate plate 32 
and sprayed with molten metal 34 according to the process described 
herein, the sprayed metal layer 34 makes a smooth transition from 
substrate 32 to absorber tube 30 in a triangular shape, however, the 
present invention is not limited to any particular size or shape. Also 
shown is surface 33 which may be a selective or non-selective surface 
coating, said coating may be on either side of the absorber plate which is 
comprised of elements 30,32, and 34 restricted only in that it be on that 
side of the absorber plate facing the sun. 
FIG. 8 is an expanded cross-sectional view of one variation of an absorber 
plate formed according to the process of spraying molten metal disclosed 
herein. As in FIG. 7, FIG. 8 shows a flat substrate 32 with no grooves to 
receive absorber tubes 40, hence tubes 40 have had their shape modified to 
facilitate the spraying of molten metal coating 34 to ensure that joints 
37 are properly filled, securing said tubes 40 to said substrate 32. 
Coating 33 shows a selective or non-selective surface coating applied to 
the absorber plate, comprised of elements 32,34 and 40 restricted only in 
that it be on the side of the absorber facing the sun. FIG. 8 shows 
absorber tubes 40 in a half-round shape, said tubes not limited to any 
shape or size. 
FIG. 9 is an expanded cross-sectional view of an absorber plate 20 
manufactured according to the process described herein of spraying molten 
metal. Molten metal is sprayed onto insulating substrate 11 with grooves 
25 receiving tubes 16 to form absorber plate 42. Underlying base coats 21 
and 22 may be used, as described in FIG. 5, to improve the adhesion of 42 
to 11. 
FIG. 9 is a variation of FIG. 5, in that the absorber plate 42 thickness is 
varied in order to increase the rate of heat flow from the absorber plate 
to the absorber tube. The molten metal spray can be controlled as it is 
deposited so as to precisely control the thickness of the absorber plate 
in predetermined areas for more efficient use of the metal and better 
thermal efficiency. The preferred embodiment shows absorber 42 thicker in 
the vicinity of the absorber tubes 16 and 5 tapering off to the midpoint 
between the tubes 44, where the thickness increases until it meets the 
adjacent absorber tube. 
FIG. 10 shows an absorber plate 50 with the heat transfer tubing 47 
embedded in or attached thereto, in a sinusoidal pattern. Inlet port 48 
and exit port 49 are located next to each other with remaining, following 
tube sections run parallel or near parallel so that the hot and cold tubes 
are in a heat exchange relationship with each other, via the absorber 
plate 50. This has the effect of moderating hot sports on the absorber by 
keeping any one particular area from getting considerably hotter than the 
rest of the absorber, which would decrease efficiency because of increased 
conduction, convection and radiation from the hot spot. Additionally, the 
tubes may be arranged with a gentle slope so that the entire tubing 
configuration may drain by gravity. A vacuum breaker may be installed, if 
necessary for breaking a vacuum in the tube 47. Because the absorber plate 
50 is formed from sprayed liquid metal, it can easily conform to almost 
any absorber tube arrangement. 
FIG. 11 shows absorbing tubing 57 in a more conventional arrangement, said 
tubes 57 running parallel to each other, connected at the ends by a pair 
of manifolds 56. This absorber tubing arrangement may be embedded, coated 
or attached to absorber plate 50 by spraying molten metal as previously 
described. Inlet port 54 and exit port 55 are provided to introduce and 
remove the heat transfer fluid. The tubing may be arranged so as to drain 
by gravity, if desired. 
Accordingly, while I have shown and described plural embodiments in 
accordance with the present invention, it is understood that the same is 
not limited thereto, but is susceptible to numerous changes and 
modifications as known to one having ordinary skill in the art, and I, 
therefore, do not wish to be limited to the details shown and described 
herein, but intend to cover all such modifications as are encompassed by 
the scope of the appended claims.