Phenolic laminate solar absorber panel and method of making

A solar flat plate absorber panel is molded from a laminate consisting of a suitable web material impregnated with a thermosetting phenolic resin. The absorber plate is pressure formed from superimposed layers of material and mold-cured to produce a unitary structure having a first or solar radiation absorbing surface on one side and an integral closed hollow lattice work of fluid heat transfer passages on the other or reverse side of the panel. The laminate web may be made from any suitable material such as paper, cloth, canvas or wire mesh which is easily impregnated with a thermosetting phenolic resin. In the preferred embodiment a B-stage phenol-formaldehyde resin is used. The two superimposed sheets of impregnated material are molded between heated platens of an hydraulic press to form an integral structure and fluid pressure between the sheets is utilized to mold the desired passage shape 5.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates broadly to the field of solar energy 
utilization and, more particularly, to a mold-cured phenolic laminate 
solar absorber panel having an integral heat transfer passage system on 
the reverse side of the solar energy absorbing surface. 
2. Description of the Prior Art 
The rapid depletion of conventional sources of energy has resulted in an 
ever-widening search for alternatives to conventional sources such as 
petroleum and natural gas to meet the increasing demand for energy by our 
society today. One such viable source which is presently commanding a 
great deal of attention and research, development and in the deployment of 
experimental units is that of solar energy. The use of solar flat plate 
collectors to extract heat from solar energy is becoming increasingly 
important. Solar flat plate collectors may be employed, inter alia, as 
sources of heat for homes and buildings and for maintaining an adequate 
supply of hot water in such installations. 
In general, the prior art contains many examples of different ways to 
utilize solar energy absorbed by flat plate collectors of various types 
and configurations. Solar flat plate collectors normally consist of a 
solar absorber plate having a black body surface which absorbs heat from 
solar radiation combined with a heat transfer system which removes useful 
heat from the absorber plate and conducts it to a place where it is 
utilized or stored. Solar collector panels have been utilized to heat a 
variety of fluid media through heat transfer systems in conjunction solar 
absorber plates. The higher heat transfer coefficient of liquid media 
together with the higher heat capacity per unit volume exhibited by such 
materials as opposed to gaseous fluids results in the ability to obtain an 
efficient use of the solar energy absorbed. 
One of the greatest drawbacks to wide spread deployment of heating systems 
utilizing solar flat plate collectors has been the higher cost of such 
systems relative to the present cost of competing conventional sources of 
energy. Thus, an important goal of present solar energy research is to 
reduce the cost of solar systems to the point where they become 
economically competitive with other forms of energy. One of the most 
costly items in putting together a fluid-type solar energy heating system 
lies in the cost of materials and fabrication for a long-lived absorber 
plate and heat transfer system. Such problems as wide-swinging temperature 
variations, corrosion and other factors have been difficult to overcome 
short of utilizing expensive materials and fabricating techniques. 
SUMMARY OF THE INVENTION 
In accordance with the present invention solar absorber panels with 
integral fluid heat transfer systems may be molded from low-cost phenolic 
resin impregnated web materials to achieve a strong, corrosion-resistant 
absorber panel which is relatively insensitive to the degrading effects of 
solar radiation and temperature variations. The material constituting the 
reinforcing web is dictated by the particular application involved and may 
be paper, cloth, woven wire mesh, glass, asbestos fibers or other 
conventional material. The web material, before molding is impregnated by 
a thermosetting resin, normally, a B-stage phenolic resin to form the 
laminate which is utilized as the material of construction for the solar 
absorber panel of the present invention. In the preferred embodiment, a 
pair of thin sheets of the phenolic laminate material are pressed together 
between heated platens of an hydraulic press in a manner in which the 
lattice work of fluid heat transfer passages is formed and isolated on the 
reverse side of the solar absorbing surface. The phenolic resin is cured 
to a thermoset C-stage crosslinked polymer to produce a low-cost, 
corrosion-resistant relatively strong solar absorber panel. 
In the preferred method, a pair of layers of paper or other web material 
impregnated with a B-stage phenol-formaldehyde resin are placed between 
the platens of an hydraulic press. One of the platens is recessed in the 
shape of the desired hollow fluid flow heat transfer passage lattice 
network and the other is planer to produce the flat solar energy absorbing 
surface. The platens are heated to a temperature between about 280.degree. 
F. to about 380.degree. F. and the press pressure utilized is between 
approximately 500 psi and 3,000 psi following conventional phenolic 
laminate molding techniques. To assure that the configuration of the 
internal fluid heat transfer passage system follow the detail of the 
recessed pattern on one platen, high pressure air or other suitable fluid 
is injected into the mold between the laminate layers as they are pressed 
so that the corresponding adjacent plated form is followed by each layer. 
The curing, crosslinking reaction is completed and the molded phenolic 
laminate absorber plates are removed from the mold. 
The desired solar energy absorbing surface coating is then applied to the 
absorbing surface. It should be noted that temperatures, pressures and 
times of reaction will vary with the particular resin formula utilized, 
the web or filler material, and the thickness of the laminar sheets. 
The internal strength imparted by the web material allows the sheets to be 
made in very thin sections, however, and these normally are in the range 
of from 0.010 inches to about 0.05 inches thick. The final pressed sheets 
may be a single laminate layer or made up of a plurality of layers of 
impregnated web laminated together to achieve the desired absorber plate 
thickness.

DESCRIPTION OF A PREFERRED EMBODIMENT 
FIGS. 1, 1A and 1B depict a solar absorber plate produced in the manner of 
the present invention. As best seen in FIGS. 1A and 1B the panel shown 
generally at 10 consists of a pair of superimposed sheets of material 11 
and 12. A hollow fluid flow passage lattice network includes inlet and 
outlet collectors 13 and 14 having connectors 15 and 16 respectively, 
which communicate with the interior. They are joined by a plurality of 
juxtaposed connecting passages 17. The entire structure is formed into a 
unitary member. 
FIGS. 2 and 3 depict opposite platens of an hydraulic press including an 
upper or top platen 19 utilized to form the molded configuration of the 
solar absorber plate 10. Accesses as at 23 and 24 are provided to allow 
the introduction of a fluid pressurizing medium between sheets 11 and 12 
through connectors 15 and 16. 
In making the solar flat plate absorber panels of the present invention, 
thin sheets of a web material impregnated with partially cured or B-stage 
phenolic resin are utilized for the material of construction. A pair of 
thin sheets of such material cut to the desired geometrical configuration 
are utilized to make each panel in the preferred embodiment. The pair of 
sheets are placed between the platens 18 and 19 (FIGS. 2 and 3) of an 
hydraulic press. As can be seen in the drawings, plate 18 is recessed in 
the form of a lattice network which is identical to that desired for the 
heat transfer fluid flow passage configuration of the finished solar 
absorber panel. Thus, recesses 20 and 21 corresponding to the desired 
inlet and outlet headers 13 and 14 are provided together with a large 
number of recesses 22 FIG. 3 which are shown at approximately right angles 
to recesses 20 and 21 and, in turn, are used to provide the shape of the 
passages joining the two headers to form a total heat transfer passage 
network. 
The platens are heated to a temperature of from between 280 and 380 degrees 
F. and the partially cured sheets 11 and 12 are superimposed between the 
platens. Connectors 15 and 16 have been previously secured to the sheet 
12. After the superimposed sheets 11 and 12 are placed between the platens 
18 and 19 the mold is closed and the pressure slowly increased. As the 
mold pressure is increased high pressure (200 psi-1000 psi) air or other 
suitably compatible fluid is caused to flow through one or more of the 
openings 23 and 24 to pressurize the two superimposed sheets internally 
such that sheet 12 is forced against the recesses in the platen 18 such 
that it assumes the form of the configuration of the recesses in the 
platen 18. Of course, if both accesses 23 and 24 are used to pressurize 
the system they will be connected to the same pressure source and if only 
one is utilized, the other will be suitably closed. As the desired 
configuration is reached, the press pressure is brought up a sufficient 
pressure such that mold-curing produces a unitary resulting structure. 
This is normally between 500 and 3,000 psi depending upon the precise 
nature of the sheets being molded. 
The pressure is held until the thermosetting phenolic resin is fully 
crosslinked or cured to form a C-stage resin. In this state the resin is 
fully crosslinked and a rigid, strong, heat-resistant material formed. The 
press pressure is then reduced, the platens opened and the fully cured, 
molded phenolic resin solar absorber panel removed and cooled. The in situ 
curing of the phenolic laminate produces a unitary structure wherein the 
areas 25 between the passages 17 along with the periphery of the unit are 
actually crosslinked into a single unitary structure. The internal 
pressure preserves the hollowness of the entire fluid passage system and 
assures its continuity. 
While many different formulas of phenolic resins are possible, the 
preferred embodiment utilizes a canvas web impregnated with a B-stage 
phenol-formaldehyde resin such as that obtainable from Spaulding Fiber Co. 
of Wheeling, Illinois. The resin in the partially cured or B-stage state 
is solid and insoluble but swelled by solvents. It is infusible but 
softened by heat such that at the 280.degree. F. to 380.degree. F. 
temperature utilized the resin is sufficiently softened such that it may 
be shaped and reformed by the applied internal pressure. When heat is 
applied, the crosslinking of the B-stage resin continues until the fully 
cured infusible, insoluble crosslinked state is reached. While the speed 
with which the phenolic resin reaches the required molecular weight and 
degree of crosslinking varies with temperature, pH, resin formulation, or 
structure, resins of the type preferred in the present invention will cure 
at the termperature involved in a few minutes. 
The cured unitary body of the solar absorber plate of the present invention 
is then ready for placement as the absorbing surface and heat transfer 
mechanism of a solar flat plate collector after the solar absorbing 
surface of the original sheet 11 is coated, if necessary, with the desired 
solar absorber coating medium. 
While the mold-cured solar absorber plate of the invention is normally 
coated with a selective solar radiation absorbing material after 
formation, the techniques may be employed. For example, a material such as 
a pre-coated or treated metallic member may be inserted between the lower 
laminated sheet 11 and the platen 19 to produce an integral solar 
absorbing surface to the solar absorber. 
While the preferred web material is suitable paper or canvas as other 
cloth, depending upon the particular structural and environmental 
requirements of the solar absorber panel being fabricated, other webbing 
materials such as woven wire mesh, glass or asbestos fibers or other 
suitable material may be used. Also other formulations of phenolic resins 
may be utilized. The sheets may be in the form of a single thickness of 
impregnated web material as a plurality of such thickness pressed together 
in superimposed relationship. 
One of the distinct advantages of the solar absorber panel of the present 
invention lies in the generally low cost of mass produced, commonly 
available phenolic resin laminates together with the relatively little 
amount of labor required to mold the absorber panel. The panels appear to 
be a great deal cheaper than panels fabricated of welded or brazed metals 
and are highly resistant to corrosion by commonly used liquid head 
transfer media than conventional materials such as steel. The panel should 
have an extremely long life in a normal solar absorbing environment as the 
phenol-formaldehyde resin is not only corrosion-resistant, but also is not 
affected by ultra-violet light as is the case with many other polymeric 
molecular structures. 
The solar absorber panel of the present invention also overcomes concerns 
about the heat conducting of phenol-formaldehyde resin materials. For many 
years phenolic resins have, of course, been used to insulate materials 
both from heat and electrical conductivity. However, inasmuch as phenolic 
laminate sheets as thin as from about 0.02 inches to about 0.05 inches can 
be utilized to produce solar absorber panels in accordance with the 
present invention, and because a very large number of small passages 17 
are normally molded into the structure, the conductivity of heat through 
the phenol-formaldehyde resin does not present a serious problem to the 
efficient collection of solar energy by the absorber plate of the 
invention.