Solar collector

A solar collector panel is made from glass fiber reinforced concrete using a dissolvable core of polymer foam to form the internal passageways. The core is dissolved in a solution of solvent and polymer which impregnates and coats the concrete surfaces of the passageways to seal the passageways and to isolate the concrete from the heat transfer fluid.

BACKGROUND 
The conversion of solar radiation into usable heat at adequate temperatures 
is the subject of much interest, but, heretofore, has been of relatively 
limited practical application. The basic technology is not new; however, 
realization of that technology in the form of commercial hardware has been 
retarded by the cost of the hardware, and by the cost of solar collector 
panels in particular. Most collector panels available today use expensive 
materials, require sophisticated manufacturing techniques available only 
in highly industrialized countries, and result in an apparatus which often 
is aesthetically unacceptable. An efficient, attractive, very inexpensive 
collector which is easily made near or at the sites of use, with low 
capital investment, and from cheap, common materials by labor of minimum 
skill is essential to the growth of practical solar heating. The present 
invention provides such a solar collector. 
SUMMARY 
Typical applications of the solar collector of the present invention are 
use as the heat source for heating room air or domestic hot water, as the 
energy source for room air cooling, for heating swimming pools, as a heat 
source for operating a vapor cycle engine to pump water or generate 
electricity, and many other applications in which heated water or heat 
generated vapor pressure are desirable. The collector panels of the 
present invention are useable as roofing by themselves, or can be placed 
on existing roofs. The panels can be formed in colors, patterns, or 
textures to closely resemble roof tiles or shingles. The panels can be 
used as paving for driveways, sidewalks, patios, swimming pool surrounds 
and the like. The panels can be used to form exterior walls. 
According to the present invention, a solar collector panel is made 
entirely from concrete reinforced with alkali resistant glass fiber to 
result in an extraordinarily thin walled, light weight panel structure of 
great strength. Since the collector panel can be made in inexpensive molds 
by hand or with fairly basic machinery, it can be made with labor of 
relatively low skill in non-industrialized areas. Little or no energy is 
required for the manufacture. The panels can be made at or near the site 
of use. 
A desire for relatively high collection efficiency has been a focus of much 
of the prior design effort for collector panels. This desire has been 
responsible in part for the complexity and cost of those panels. The 
desire for high efficiency is perhaps misguided, for the heat source is 
free and the only penalty of lower efficiency is the need for more 
collector area; a penalty easily accomodated if the collector cost is low. 
Comparative testing of collector panels of the present invention with 
several different more complex, allegedly highly efficient prior art 
collector panels revealed that the panels of the present invention were 
far more efficient than the compared panels, thereby reducing, rather than 
increasing, the collector area requirements. 
DETAILED DESCRIPTION 
The following description is of a preferred embodiment of a solar collector 
according to the present invention. The described embodiment is a flat 
collector suitable for use as a roofing panel. The materials required are 
cement, sand, alkali resistant glass fiber, water, foamed polymer sheet, 
and a solvent for the polymer. The equipment required is that required to 
mix concrete, a suitable open mold, and hoses and a storage tank for the 
polymer solvent. The invention is preferably carried out with apparatus 
for spray application of glass fiber reinforced concrete (GRC), which 
apparatus is well known and widely used. 
In the drawings; 
FIG. 1 is a perspective view of a collector panel according to the present 
invention, 
FIG. 2 is a cross-sectional view taken along line II--II of FIG. 1, 
FIG. 3 is a plan view of the styrene foam core used in making the panel of 
FIG. 1, 
FIG. 4 is a schematic view of the plumbing connections for an array of a 
plurality of the panels of FIG. 1, 
FIG. 5 is a cross-sectional view taken along line V--V of FIG. 4, 
FIG. 6 is an enlarged view of the circled zone of FIG. 5, 
FIG. 7 is a cross-sectional view taken along line VII--VII of FIG. 4 and 
showing a modification of the panel of FIG. 1, and 
FIG. 8 is a perspective view of a dwelling having a roof mounted array of 
collector panels in accordance with the present invention.

FIGS. 1 and 2 show a collector panel 10 according to the present invention. 
The illustrated panel is of a flat surface configuration suitable for use 
as a roofing panel or for application to an existing roof. The panel is on 
the order of one or two square meters in area and has a thickness of one 
or two centimeters. Within the interior of the panel is a pattern of 
conduits or passages comprising a plurality of parallel ducts 12 which 
terminate at either end in header or manifold chambers 14, 16. The 
concrete on the interior is impregnated and coated with a polymer to 
render the concrete impervious to water, gas, or other fluid heat transfer 
media. 
FIG. 4 shows the interconnection of a plurality of the panels of FIG. 1 to 
form a collector array for a dwelling roof. The dotted arrow on the right 
side indicates the down slope direction. A heat transfer medium such as 
water is circulated through inlet 18 of panel 41 to the lower manifold 16, 
upwardly through the parallel ducts 12 to the upper manifold 14 and thence 
through outlet 19. The heat transfer medium then circulates through a "U" 
pipe 20 to the next panel 42 in similar fashion. The interconnections 
illustrated in FIG. 4 are a series-parallel arrangement such that pairs of 
adjacent panels 41, 42 are connected in series. Other pairs of panels 43 
and 44, 45 and 46, etc. are connected in series and each such pair of 
panels is connected in parallel with the other pairs of the array. The 
overall path of circulation is up slope to take advantage of natural 
connection. 
FIGS. 5-7 show in greater detail the application of the collector panels to 
a roof. The embodiment illustrated in FIGS. 5-7 is a modification of that 
of FIG. 1. As can be seen in FIGS. 5 and 6, the panels are adapted to lap 
one another in shingle or clapboard fashion. As can be seen in FIG. 7, the 
flat central portion of the panel 10 is bordered by an integral rim 70 
which extends below the flat panel 10 to provide an insulating air space 
below the panel and extends above the panel to provide a rabett 72 to 
receive a sheet of glass 75 or transparent plastic. Although glass or 
plastic significantly impedes solar insolation by reflection and by 
blocking radiation, particularly that outside the visible spectrum, glass 
reduces conduction and convection losses to the air. Where the ambient air 
temperature is low or where winds are strong, glass may be desirable to 
achieve adequate temperatures. FIGS. 6 and 7 also show a lid 76 which 
covers the piping in the space between panel arrays. A completed 
installation of collector panels on a roof is shown in FIG. 8. 
The following description of the method of making solar collector panels in 
accordance with the present invention is directed to the flat panel of 
FIG. 1, although the method is applicable to more complex configurations 
such as panels simulating roof shingles or terracotta roof tiles. The 
dimensions and materials are appropriate for a preferred embodiment. 
Although the method is described in the context of spray application of 
the concrete, the method can be practiced by hand using hand tools. 
The panels are cast in an open mold which imparts the upper face 
configuration of the panel. The mold can provide a textured surface or can 
provide a simulation of shingles or tiles. The mold can be of any durable 
material suitable for concrete casting. Preferably, the mold is made of 
glass reinforced plastic or glass reinforced concrete (GRC). To aid in 
release of the cast panel, the mold can be coated with a conventional 
concrete mold release agent by brush or spray. 
When prepared, the mold is laid out on a horizontal surface and a layer of 
glass reinforced concrete (GRC) 3 or 4 mm. thick is sprayed into the mold. 
Preferably, the concrete mix is about 3/4 cement by weight and 1/4 sand by 
weight before the addition of water. At least 3% and preferably more than 
5% by weight of alkali resistant glass fiber, known as AR fiber, is added 
to the concrete. Such concrete reinforcement fiber is available from 
Pilkington Bros., Ltd. of England or Owens Corning of the United States. 
The spray apparatus simultaneously sprays the concrete slurry and projects 
the glass fibers chopped to the appropriate length of 2 to 10 cm. The 
first layer of GRC is vibrated or troweled with a float to release air 
bubbles. 
A core to form the passages or waterways is then placed on the first layer 
of GRC. The core 30 is illustrated in FIG. 3. The core is formed from a 
sheet of polymer foam such as polystyrene foam. A sheet of polystyrene 
foam approximately 3 or 4 mm. thick is cut in the pattern illustrated by 
any appropriate technique such as a steel rule die, a hot wire cutter, or 
by hand using a template and knife. The pattern comprises bars 13 integral 
with end pieces 15, 17. The bars 13 correspond to the ducts 12 of the 
finished panel and the end pieces 15, 17 correspond to the manifold 
chambers 14, 16 of the panel. The end piece 17 at the bottom of the panel 
is cut at an angle a to provide the lower manifold chamber with a slope 
for more complete drainage. The inlet 18 and outlet 19 pipes are affixed 
to the foam and are thereby properly positioned in the completed panel in 
communication with the manifold chambers 14, 16. 
After the foam core 30 with the inlet and outlet pipes 18, 19 has been 
positioned in the mold over the first layer of GRC, concrete slurry 
without glass fiber is sprayed into the mold to fill all the spaces in and 
around the core 30 with concrete. The mold is again vibrated or troweled 
with a float to release any air bubbles. 
The casting is completed by the spray application of a final layer of GRC 3 
or 4 mm. thick and the mold vibrated or troweled to release entrapped air 
and to provide a smooth surface for the bottom face of the panel. 
The cast panel can be released from the mold after the concrete has set, 
usually the next day. The cast panel is then cured under very high 
humidity conditions to develop maximum strength. The cure requires about a 
week of a constant wet environment. The time for cure can be shortened at 
elevated temperatures. For example, curing at 50.degree. C. requires about 
two days. When fully cured, the panels are dried to remove all excess 
water. A week in the sun is sufficient. 
When the panels are fully cured and thoroughly dried, the polymer core can 
be removed. The core is dissolved in an industrial solvent for the 
polymer. For polystyrene foam suitable solvents include perchloroethylene, 
trichloroethylene, methyl ethyl ketone, methyl isobutyl ketone, toluene, 
carbon tetrachloride, benzine, carbon disulphide, ethylene dichloride, 
methylene chloride, ethyl acetate, and others. Preferably, the solvent is 
chosen on the basis of availability, cost, toxicity, flamability and low 
boiling point. Methylene chloride, Methyl ethyl ketone, or 
trichloroethylene are preferred for polystyrene. Because the density of 
polymer foams such as polystyrene foam is low, only a relatively small 
volume of polymer is present in a relatively large volume foam core. 
Consequently, dissolution of the foam is prompt. 
The polymer core is dissolved by circulating the solvent through the panel 
from a container and back to the container. Preferably a pressure head of 
a meter or more is employed. The percentage of polymer in the solvent 
solution increases as cores are dissolved. Preferably, the solution 
contains a high percentage of dissolved polymer. The polymer-solvent 
solution wets and penetrates the dry concrete to deposit the polymer 
solution in the air bubbles and intersticies of the concrete thereby 
impregnating and sealing the panel passageways against leakage or seepage 
of water. Further, some of the polymer solution remains as a coating on 
all of the interior waterway passages of the panel after the solution has 
been drained from the panel. When so much polymer is dissolved that the 
viscosity of the solution becomes too great for adequate permeation, more 
solvent is added. The polymer coating hardens as the solvent evaporates 
and forms a barrier between the concrete and the water or other heat 
transfer medium. Thus, the panel is protected against leakage or seepage, 
and against leaching of the concrete by circulating water. The water, in 
turn, does not contact the concrete and so does not become alkaline. The 
completed panel is impervious to water, antifreeze solutions, gases such 
as propane, butane, flurocarbons, and other fluid heat transfer media. 
The solvent from the solution adhering to the passageways can be evaporated 
into the atmosphere or can be recovered in a closed air circulation system 
by use of a simple condenser. When solvent is recovered, the solution 
approaches a steady state ratio of solvent to polymer wherein the volume 
of polymer left in each panel approximates that brought to the solution by 
each core. 
The completed panel can be painted or stained for appearance and to improve 
heat absorbtion. Excess polymer solution may be used as a vehicle for the 
paint, thereby utilizing all byproducts of the process.