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BACKGROUND OF THE INVENTION
This invention relates in general to the collection of solar energy, and
more particularly to arrays of panels for heating air by trapping heat
from the sun under sheets of glass or other substantially transparent
sheet material. Heated air may then be circulated to warm occupied spaces
in buildings, for drying purposes, or stored for later use.
The growing awareness of fossil fuel shortages has sparked renewed interest
in the possibility of economically capturing solar energy for heating
purposes. Solar heating devices are not totally new, but previously have
found little use because of the abundance and low cost of high density
fossil fuels. While solar fuel is free, its low energy density requires
large collection areas for most applications. For example, 1500 square
feet of collection area facing the sun are typically required to fully
heat an average home. Consequently, collection equipment costs are high,
and most known solar heating systems cannot be justified economically.
Flat plate air heaters are known. The American Society of Heating,
Radiation, and Airconditioning Engineers (ASHRAE) shows several designs in
a recent chapter on Solar Heating and Cooling of Buildings. The simplest
design is a single pane of glass admitting solar energy into a cavity
where the incident radiation strikes a black absorbing surface which gets
warm. Air moving through the cavity either by natural convection or forced
air transfers the collected heat as desired. This inexpensive collector
has several shortcomings: first, with just a single pane of glass, the
collector loses considerable heat back to a cold outdoor environment, and
second, the flat back plate does not provide enough area for effective
heat transfer to the moving air stream. To overcome these problems, some
solar air heaters have at least two transparent surface sheets, with an
airspace therebetween, to minimize collector heat loss. Some solar air
heaters have multiple absorbing sheets to increase the area for heat
transfer to the moving airstream.
Solar air heaters are typically placed directly on a building surface,
either a wall or the roof, facing the sun, with thermal insulation placed
between the collectors and the space within the building. Since a large
surface area is usually needed for collection, arrays of identical panels
are usually used, the panels being fabricated in a size convenient for
handling and also for minimizing waste of materials. Currently fabricated
panels range in size from 2 feet by 4 feet up to 4 feet by 8 feet. The
glass surface of the collector may serve as the weather surface of the
building, but otherwise the collector panel is usually structurally
redundant. While a plurality of air heating collector arrays have been
installed between building framing members to reduce overall system costs,
such designs have not been very versatile in permitting variable ductwork
locations for air inlets and outlets to the panels.
An additional problem with flat plate heaters concerns their warm weather
performance. Economical equipment is not yet available for using the sun's
heat to drive absorption cycle airconditioning equipment, so it is
necessary to reject captured or incident solar energy during the summer.
Heat will build up rapidly in the air heating panel when air is not
flowing, and some of the heat will be transferred through the insulation
to indoor spaces, which is obviously an undesirable situation in the
summer. Overheating can also damage construction materials and possibly
even create a danger of a fire. Current techniques for preventing
overheating include shading the collectors and ventilating the air
cavities to the outdoors. Both of these approaches increase system costs.
Flat plate collectors (large glass-surfaced panels no more than 6-7 inches
thick) are generally known for use as solar energy collectors.
Concentrating collectors are capable of generating higher temperatures
than flat plate collectors, but the latter reach adequate temperatures for
space heating, are less expensive to build, and need not "track" the sun.
Either air or a liquid (usually water) may be used to transfer heat from a
flat plate collector to a storage medium or heated space. While water is
generally a more effective heat transfer medium than air, solar water
heaters are more expensive to build than solar air heaters since the
liquid must be contained in a flow network in close contact with an
absorbing surface. Also, the water heater faces maintenance problems with
respect to leakage and freeze damage. Costs and reliability currently
favor the air heater for space heating applications.
U.S. Pat. No. 3,863,621 discloses a solar wall system which includes two
transparent glass or plastic sheets behind which is mounted one or more
collector plates for absorption of the sun's rays, and the heat absorbed
by the collector plate is then transferred to air passing in the space
behind the collector plate. That patent discloses one embodiment wherein
the absorptive collector plate is a louvered plate which presents more of
its surface area to the sun's rays for absorption of solar energy.
U.S. Pat. No. 2,544,474 shows a solar water heater with water flow through
a parallel array of flattened tubes similar to louvers; the entire array
rotates to maintain the flattened surface perpendicular to the solar rays
and the tube surfaces are flat black to absorb solar energy. As absorbers,
the flattened tubes also radiate considerable energy back through the
glass, reducing collector efficiency.
Solar rays strike the earth at a lower angle in winter than in summer. U.S.
Pat. No. 2,625,930 shows a roof top collector system designed to admit low
angle radiation into a collection area, yet reflect high angle radiation
from the roof surface. U.S. Pat. No. 2,918,709, utilizes louvered slats in
a window which may be oriented to admit radiation or reversed to reflect
radiation. But there is no structure in the prior art which utilizes a
selectively reflective and admissive geometry within a solar energy
collector cavity.
BRIEF SUMMARY OF THE INVENTION
It is, therefore, an object of this invention to provide a solar air heater
which overcomes the above-noted disadvantages.
It is another object of this invention to provide a solar air heater which
may be installed between structural framing members yet permit variable
duct inlet and outlet configurations.
It is another object of this invention to provide a solar air heater which
may be quickly preassembled for rapid installation at the construction
site.
It is another object of this invention to provide a solar air heater which
minimizes heat gain during hot ambient weather conditions.
These objects and others are accomplished in accordance with this invention
by providing a flat plate solar collector for installation between light
frame structural members, which comprises a pan of sheet stock such as
sheet metal with one or more substantially transparent surface sheets to
admit solar radiation into a sealed chamber between said pan and the
transparent surface sheets. A louvered structure placed behind the
transparent surface sheets reflects high angle solar rays yet admits all
low angle rays into the radiation absorbing chamber. The louvered
structure also increases heat transfer to a moving airstream within the
collector. Inlet and outlet ends of the panel may be chamfered or
otherwise adapted to permit alternative ductwork connections.
It is another object of this invention to provide a solar air heater which
uses selectively reflective and admissive geometry and materials with a
solar collector cavity to differentially admit and absorb more solar
energy in the winter months than in the summer months.
BRIEF DESCRIPTION OF THE DRAWINGS
For a better understanding of the invention as well as other objects and
further features thereof, reference is made to the following detailed
disclosure of preferred embodiments of the invention taken in conjunction
with the accompanying drawings thereof, wherein:
FIG. 1 is a partially schematic, partially cut-away isometric view of a
preferred embodiment of the louvered collector designed for location
between vertical structural members in a frame wall construction.
FIG. 2 is an enlarged, partially schematic, vertical cross-sectional view
of the top of a louvered flat plate collector like that shown in FIG. 1,
showing the reflection geometry of the louvers.
FIG. 3 is a partially schematic, partially cut-away vertical
cross-sectional view of the louvered collector installed within a wood
frame building structure, showing air supply and return ductwork.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
The partially cutaway isometric view of FIG. 1 shows the basic elements of
a preferred embodiment of the louvered flat plate collector of the present
invention. The basic structure of the collector is pan 1, constructed from
a flat sheet material such as sheet metal with long sides 3 which are
typically bent at right angles to the essentially planar ban bottom 2.
Short sides 4 (normally oriented as top and bottom) are here illustrated
as bent at about 45.degree. from the plane of the pan bottom 2 for reasons
described in conjunction with FIGS. 2 and 3. The typical depth of the pan
1 is somewhat exaggerated in the drawing FIG. 1 in order to more clearly
illustrate the relationships of the various parts of the system.
Intersections of sides 3 and 4 may be overlapped, and spotwelded, screwed
or otherwise connected together, then taped with a fabric duct tape to
make the system airtight. Narrow sides 4 are penetrated with holes 5 for
inlet and outlet connections. All four sides may have outwardly bent
flanges 6 for connection to structural members of a building. The pans as
described may be constructed of sheets of steel, aluminum, or copper, but
steel is presently preferred because of its greater strength and lower
cost. 22 gauge steel is presently particularly appropriate for a preferred
embodiment. The inner pan surfaces have a black coating (usually flat
black paint) to absorb incident solar radiation. Dimensionally, the pans
are sized to fit between parallel structural members located in standard
modular spacing patterns. In a preferred embodiment, about 1 1/2 thick
wood studs placed about 24 inches apart on center leave about 22 1/2 inch
wide spaces therebetween. Pans about 22 1/4 inches wide are recommended
for this spacing to allow about 1/4 inch for tolerance errors. About 3/4
inch wide flanges 6 make overall collector width about 23 3/4 inches. For
wall applications, collector height typically corresponds to total wall
height. For about 8 foot ceilings, pan height including flanges becomes
about 94 3/4 inches.
Louvered structure or sheet 8 included between the pan bottom 2 and
substantially transparent surface sheets 9 and 10 provides the seasonally
selective performance of the collector. Its characteristics are more fully
described in conjunction with FIG. 2.
Transparent sheets 9 and 10 may be either glass or a transparent plastic
material such as Rohm and Haas "plexiglas". Glass is somewhat lower in
cost, but is heavier and more subject to breakage. Also, glass cannot
easily be drilled for penetration by fasteners, making it more difficult
to hold in place. A preferred embodiment of the louvered collector uses
damage resistant clear acrylic sheet as the weather surface 10, and less
expensive glass for the rear sheet 9. Aligned holes 11 in the pan flanges
6 and 12 in the acrylic sheet 10 are regularly spaced around the perimeter
for screws or other fasteners to be driven into the structural members.
Recommended fastener spacing is about 12 inches on center on the
structural members.
The enlarged cross-sectional view of FIG. 2 shows details of assembly at
the top portion of the collector as well as the reflection geometry of the
louver system. The louver sheet 8 is held in place with angle clips 19
spotwelded or otherwise attached to pan sides 3. Rubber tabs 18 made for
example of dense neoprene, are adhered to pan sides 3 to space glass sheet
9 between louver sheet 8 and acrylic sheet 10, giving soft support to
prevent glass breakage. Latex or butyl caulk 17 may be applied
continuously around the glass edge to create a tight seal between air flow
cavity 21 and insulating chamber 20 which is between the two transparent
sheets. For maximum insulating value, chamber 20 should be about 3/8 to
about 1/2 inch thick. Overall pan depth or thickness is typically about 3
1/2 inches, leaving an air flow cavity 21 slightly less than about 3
inches thick. Within the airflow cavity, the center plane of the louver
sheet 8 is displaced toward the outside of the panel, and is preferably
located about 3/4 to about 1 inch behind glass sheet 9. In this location
most of the air flow occurs in the absorbing portion of the chamber,
between pan surface 2 and the louver sheet 8.
The louver surfaces are designed to reflect high angle rays 16 occurring in
the summer or hot months, yet permit low angle rays 14 and 15 either to
pass directly through the louver sheet as illustrated by ray 14 or be
reflected from the upper surface of louver strip 13 onto the rear
absorbing surface as illustrated by ray 15. The reflective upper surfaces
of the louver strips 13 may comprise any highly light and heat reflective
material such as polished bare metal such as steel or aluminum, an
aluminum coating, a silvered mirror coating or any other suitable
reflective surface. The under sides of louver strips 13 may be either
reflective or absorbing (i.e. either shiny or black). Reflective
undersides cause more solar energy to be reflected onto the absorbing rear
pan surface, and result in lower temperatures on the louvers. Absorbing
undersides will result in higher louver temperatures, hence better heat
transfer to the air flow through the chamber. For this reason, absorbing
louver undersides are preferred.
From the above discussion it will be appreciated that the performance of
the louvers depends more on the relationship between louver angle and
incident radiation angle than on louver depth and spacing. While the
louvers may be individual strips as wide as about two inches individually
fastened to the pan edges, it is usually more economical to produce the
louvers from a single sheet as typified by heat register manufacturing.
Long parallel slits extend nearly across, but not to the edges of, a metal
sheet, with louvers then bent to the desired angle while remaining
interconnected at the edges of the sheet. With this production approach,
depth of the louver strip must be equal to the spacing between strips. For
this geometry, reflective characteristics are virtually independent of
louver strip spacing. For an incoming ray at angle .beta. from the
horizontal, striking a louver pattern at an opposed angle .alpha. from the
horizontal, all incident radiation is reflected when angle .beta. is
greater than (90.degree..alpha.), and all radiation is admitted to the
absorbing chamber when .beta. is less than (90-2.alpha.).degree.. In
between, the percentage of incident energy reflected may be computed from
the formula:
##EQU1##
Since the angle of incoming solar rays varies with latitude, time of day,
and time of year, a mathematical integration process may be used to
determine total solar energy capture performance of the inventive system.
A digital computer has been used for this purpose, and results indicate
that at 40.degree. north latitude, preferred angle .alpha. of 25.degree.
will admit substantially all radiation from Oct. 19 to Feb. 23, and will
reflect a maximum of 47% of incident radiation on the longest day of the
year, June 21.
The above formula is correct for any parallel slat geometry in which:
s.gtoreq.h sin.alpha.
wherein s is the slant length of the slat and h is the spacing between
slats. For the preferred angle .alpha. of 25.degree. and for the s/h ratio
of 1 resulting from the convenient production method described, more than
twice the slant length required for reflection geometry is provided. Other
manufacturing techniques may be used to reduce this excess material, or it
may be left to increase heat transfer surface area.
It will also be appreciated that the presence of the louvered sheet tends
to reduce the loss of energy radiated from the absorbing surface. The
greater the louver angle .alpha., the less reradiated energy is lost from
the chamber, according to the relationship (for slant length equal to
spacing):
% reradiated energy lost = 1-sin .alpha..
For the 25.degree.louver angle, about 42% of the reradiated energy is
captured by louvers.
From the above discussion it is apparent that the performance of the
louvered collector could be further improved if louver angle were
variable. However, production costs for an adjustable louver arrangement
would be high, since individual slats with a connecting linkage would be
required and a manual or automatic control mechanism would be needed as
well. The cost of such additional features does not presently appear to be
justified by the potential return of being able to completely block
sunlight in the summer.
FIG. 3 shows a partially cutaway, sectional view of the louvered collector
installed in a wall of wood 2 .times. 6 construction. The structure
consists of floor framing surfaced with plywood subfloor 22. The vertical
2 .times. 6 studs are spaced about 24 inches on the center and rest on 2
inch .times. 2 inch sill plates 23 and 24 spaced about 2 1/2 inches apart.
A 2 inch .times. 6 inch top plate 25 is attached atop the studs and
supports the floor or roof structure above. The fully assembled collector
pan 1 is fastened through flanges 6 to the wood structural members. About
1 1/2 inch thick rigid insulation 26 may be preadhered to the back of pan
1 to generate additional insulating airspace 28 between the rigid
insulation 26 and interior surface panel 27 (usually gypsum wallboard)
fastened to the inside of the wood framing members. A layer of reflective
aluminum foil 29 may be used to further improve the insulating value of
the wall.
FIG. 3 demonstrates the advantages of the sloping pan ends 4. Ductwork
serving the collector may be channeled either vertically, as shown by
flexible duct 31 inserted through duct opening 5, or horizontally, as
shown by sheet metal boot 34 inserted through top duct opening 5, or at an
intermediate angle if desired. Because it is desirable to locate duct work
within heated spaces, the configuration shown in FIG. 3 could be used in
many applications of the present invention. Cooler air flowing into the
collectors enters from the bottom to take advantage of the upward gravity
flow as the air is warmed by the sun. In many cases the air inlet ductwork
is too large to be placed at the wall-floor joint within the room, as can
be done at the ceiling where it does not interfere with furniture
placement and occupant activities in the room. Thus, the supply duct must
be placed below the floor. Round duct 30 supplies an array of panels each
with its own flexible inlet duct 31. To keep the air return duct 33 below
insulated ceiling 36, boot 34 is run substantially horizontally into the
return duct which is surfaced with gypsum board 27. Boot 34 may be
equipped with flanges 35 and 36 for connecting with sheet metal screws to
the return duct and the collector panel, respectively.
It will be appreciated that for other constructions, different air inlet
and outlet systems may be used. For example, a typical two-story collector
wall would have a vertical boot from the top of the first floor collector
into the bottom of the second floor collector, so that the two could be
connected in series. It will also be appreciated that the invention
disclosed here may easily be adapted for panel placement atop sloped
roofs, with louver angles varied to provide the proper ratio or reflected
energy as the seasons change.
The louvered flat plate collector as disclosed clearly satisfies the stated
objectives of providing an economical solar heating panel which reduces
costs by its ease of installation within a light frame wall, by using a
maintenance free fixed louver geometry to capture solar energy on a
seasonally selective basis, and having inlet-outlet ports located to
maximize duct location alternatives.
Although specific components, proportions and arrangements of elements have
been stated in the above description of preferred embodiments of this
invention, other equivalent components and arrangements of elements may be
used with satisfactory results and various degrees of quality, or other
modifications may be made herein to synergize or enhance the construction
of the invention to thereby increase its utility. It will be understood
that such changes of details, materials, arrangements of parts, and uses
of the invention described and illustrated herein, are intended to be
included within the principles and scope of the claimed invention.
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