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Description  |
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BACKGROUND OF THE INVENTION
This invention relates to solar heating systems and more particularly to a
solar collector system which can be integrally incorporated in a building,
such as a single or multi-family residence or commercial building,
constructed in accordance with generally recognized building construction
practices.
In view of the increasing concern of diminishing supplies and increasing
costs of conventional energy forms, solar heating offers an attractive
alternate method for heating residences and other types of buildings
because of the general availability of solar energy and its non-polluting
characteristics. Basically, solar heating systems involve collecting
incident solar radiation, transferring this solar radiation into heat
energy, and then utilizing this heat energy for heating a building. Here,
we are particularly concerned with a non-concentrating immovable solar
collector which uses air as the heat transfer medium. Solar collectors,
such as above described, are usually installed on a stationary structure,
such as on the roof of a house or other building, and are aligned in an
optimum direction and at an optimum inclination for intercepting the
largest possible percentage of solar radiation available at all times of
the solar day and during all times of the heating season. These stationary
solar collectors are much less expensive than are movable collectors.
Of course, at any given latitude and at any given time of the day and the
year, only a given amount of solar radiation (or insolation as measured in
BTU/ft.sup.2 hr) strikes the earth's surface. Unfortunately, at higher
latitudes where the heating requirements are the highest, there is not
only less available insolation in the winter months but the time available
for each day for solar heating is descreased (i.e., there is less
daylight) thus requiring larger area collectors. Since in most
conventional solar heating systems the collectors are a major cost factor,
it is highly desirable that the collectors be as efficient as possible so
as to decrease their size and numbers required to heat the building.
The solar collector system of the present invention utilizes air as the
heat transfer medium. In most prior forced air solar collector systems,
solar radiation passes through a transparent panel and is absorbed within
the collector by various solar radiation absorption surfaces, such as the
walls of the collector or closely spaced cups or other means disposed
within the collector. Air is then forced over the absorption surfaces
within the collector and is heated by the absorption surfaces. The heated
air is then circulated to a large heat sink, such as a large volume of
water or crushed rock, where the heated air heats the sink. Air may then
be circulated through the sink for being heated thereby and this newly
heated air is then circulated through the building for heating purposes.
Oftentimes, the heat sink will store sufficient heat to adequately heat
the building for several days in the event cloudy weather blocks out solar
radiation. Auxiliary heating units in the heating system are usually
provided so that in the event solar radiation is not sufficient to heat
the building, the auxiliary heaters can be used to augment solar heating.
As previously mentioned, it is highly desirable that a forced air solar
collector be as efficient as possible in converting solar radiation into
heat energy and in heating the air circulated therethrough. The overall
efficiency of the solar collector may be expressed as the ratio of the
heat added to the air circulated through the collector in a given period
of time compared to the insolation available during that period in the
plane of the collector surface. In order to maximize the efficiency of any
solar collector, it is desirable that the temperature of the radiation
absorption surfaces be as low as possible so as to reduce reradiation and
conduction losses from the absorbing surface and thereby to insure that
the maximum available amount of heat possible is transferred to the air
circulated through the collector. It is also desirable that the ducting
losses of the air circulated through the collector be as low as possible
so as to reduce the energy required to circulate the air through the
collector. It is generally known that in order to increase the transfer of
heat to air that the air should be rapidly and turbulently circulated over
the heated surfaces thereby to increase the heat transfer coefficient
between the air and heated surfaces. This, however, requires a greater
expenditure of energy to circulate the air.
Reference may be made to such U.S. Pat. Nos. as 2,680,565 and 3,971,359
which describe solar heating collectors broadly similar to this invention.
SUMMARY OF THE INVENTION
Among the several objects and features of this invention may be noted the
provision of a forced air, non-concentrating solar collector system which
may be readily integrated into buildings constructed in accordance with
generally recognized building construction practices; the provision of
such a solar collector which has a high overall efficiency rating and yet
which has little restriction to the flow of air therethrough; the
proyision of such a solar collector system which serves as both a
weatherproof roof surface and as a structural sheathing for the roof for
the building on which it is installed; the provision of such a solar
collector system which minimizes thermal losses by reradiation and
conduction to the environment; the provision of such a solar collector
system which is light in weight; and a provision of such a solar collector
system which is of rugged and simple construction, which has no moving
parts, which can be fabricated with conventional tooling, which has a long
service life, and which is virtually maintenance-free.
Briefly, a solar collector of this invention comprises an inclined trough,
a solar radiation transmitting cover overlying the trough and enclosing
the latter thereby to constitute air passageway through the trough with
the bottom end of the trough constituting an inlet end, with the upper end
of the trough constituting an outlet end, and with air flowing through the
trough along the longitudinal axis thereof from the inlet to the outlet
end thereof, and solar radiation absorption means disposed within the
trough for absorbing solar radiation transmitted through the cover and for
heating the air flow through the trough. The solar radiation absorption
means comprises a plurality of louver panels disposed within the trough,
each of these louver panels extending transversely across the trough and
being inclined relative to the longitudinal axis of the trough. Each of
the louver panels further has a plurality of louver fins extending from
the louver panel with openings therebetween. Each of the louver fins is
generally parallel to the direction of the flow of air through the trough
and is disposed to intercept and absorb solar radiation transmitted
through the cover whereby the air flows through the louver openings and
over the louver fins in a direction generally parallel to the longitudinal
axis of the trough for being heated by the louver panels and the fins with
only a small restriction of the flow of air through the trough. Other
objects and features of this invention will be in part apparent and in
part pointed out hereinafter.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a perspective view of a portion of a roof having a solar
collector system of this invention incorporated therein with portions of
the collector covers removed to show details of the collectors;
FIG. 2 is a vertical cross-section taken along line 2--2 of FIG. 1;
FIG. 3 is an enlarged perspective view of a solar absorption panel of the
solar collector of this invention;
FIG. 4 is an enlarged exploded perspective view of a portion of the solar
collector system of this invention illustrating how adjacent collectors
are installed on the roof structural members of a building; and
FIG. 5 is an enlarged longitudinal cross-section of a portion of a
collector of this invention.
Corresponding reference characters indicate corresponding parts throughout
the several views of the drawings.
DESCRIPTION OF A PREFERRED EMBODIMENT
Referring now to the drawings, a solar collector system of this invention
is shown to include a plurality of solar collectors 1 incorporated in the
roof 3 of a building, such as in the roof of a house, which is constructed
in accordance with generally recognized building construction practices.
Solar collectors 1 may generally be characterized as fixed position,
non-concentrating solar collectors which absorb incident solar radiation
and heat air therewithin.
Each collector 1 comprises a trough or tray 5 made of sheet metal or the
like. The trough is generally channel-shaped in cross-section and has a
flat bottom 7 and opposite side walls 9a, 9b (see FIG. 4). Each of the
side walls has an outwardly extending flange 11a, 11b, respectively, for
purposes as will appear. A solar radiation transmitting cover, generally
indicated at 13, overlies the open through and encloses the spaces
therewithin the defined an air passageway P (see FIG. 2) extending
lengthwise of the trough. As best shown in FIG. 2, collectors 1 are
normally inclined at the same angle as roof 3 of the building on which
they are installed. Preferably, the inclination and direction of the roof
of the building incorporating solar collectors 1 of this invention is such
that the solar collectors have an optimum compass orientation and
inclination angle so as to intercept an optimum amount of direct solar
radiation during the heating season. Generally, a solar collector in the
northern hemisphere should face in a generally southerly direction. The
angle of inclination of the collectors will vary, depending on the
latitude in which the collector is located.
As shown in FIGS. 1 and 2, the lower ends of the collector trays 5 are in
communication with an inlet air plenum 15 and constitutes an inlet end for
the collector and the upper ends of the collector trays constituting an
outlet end are in communication with an outlet air plenum 17. Thus, air
may be ducted into plenum 15 by a blower (not shown) to be distributed to
the lower or inlet ends of each of the collector trays of the collector
system of this invention to flow generally longitudinally through air
passageway P in each of the collector trays and to be collected in outlet
plenum 17. It will be appreciated that air flow between the inlet and
outlet plenums and through the side-by-side collector trays is generally
parallel air flow. As is shown in FIG. 2, inlet air plenum is on the
inside of the building roof and is mounted on the underside of the roof
members. A boot B interconnects plenum 15 and the inlet end of each of the
trays and a damper D is provided for regulating the flow of air to each
tray. As is conventional with forced air solar heating systems, the air
heated by the collector is directed from outlet plenum 17 to a heat sink
(not shown), such as a large mass of crushed rock enclosed in an insulated
chamber located, for example, in the basement of the building. This heat
sink typically has passages incorporated therethrough through which heated
air from the collectors may be circulated to heat the heat storage mass.
Air can be circulated through the heat sink storage mass to be heated at a
later time so that this heated air can then be circulated into the
building to heat the latter. It is, of course, well known to incorporate
an auxiliary heater, such as a gas or oil fired unit and with appropriate
thermostatic controls, in a solar heating system to augment solar heating
and to automatically control the temperature within the building.
In accordance with this invention, solar radiation absorption panels 19 are
incorporated in the passageway P of each trough 5 for absorbing solar
radiation transmitted through cover 13, to transform into heat energy, and
to efficiently heat the air flowing through the passageway. The solar
radiation absorption panels are shown to be a series of sheet metal
louvered panels, each of which extends transversely across trough 5
between side walls 9a, 9b and which extends up from the bottom 7 of the
trough to the upper edges of the side walls in close proximity to the
bottom face of cover 13. The panels are each inclined relative to the
longitudinal axis LA of the trough with the upper ends of the end panels
inclined toward the outlet or upper end of the trough. Each louver panel
19 has a plurality of louver fins 21 extending therefrom with openings 23
therebetween. As previously mentioned, louver panels 19 are preferably
made of sheet metal or the like and louver fins 21 are punched therefrom
to extend transversely across the panel and to be generally parallel to
one another. The fins are generally flat, and, as shown in FIG. 5, are
generally parallel to the longitudinal axis LA of collector trough 5 and
extend out from the front face of the panel. In other words, these fins
are generally parallel to the inner face of the cover parallel to the
direction of the flow of air through the passageways. Openings 23 are
formed by the vacant spaces left by the fins struck from the sheet metal
panels. With collectors 1 being inclined at the above discussed optimum
inclination angle, the louver fins are disposed generally at right angles
to the solar radiation striking the collector under optimum conditions.
In accordance with this invention, louver fins 21 are so sized and
positioned that nearly all of the solar radiation transmitted through
cover 13 is absorbed by the fins and the openings 23 are so sized as to
insure that the flow of air through the collector trough is in good heat
transfer relation with the louver panels and with the louver fins for
readily transferring heat from the louver panels and fins to the air and
yet so that the flow of air through the trough is not substantially
restricted. As heretofore pointed out, the object of insuring good heat
transfer between the absorption surfaces of the collector and the air
heated thereby and the object of minimizing the flow resistance to the air
through the panel often run counter to one another. In order to provide
both a good solar radiation absorption surface and to permit the air to
flow freely through the collector with little restriction, openings 23 in
the louver panels preferably constitute a relatively large percentage of
the cross-sectional area of trough 5. The length L (see FIG. 5) of the
louver fins, as measured in the direction of the longitudinal direction of
the flow of air through the trough, is preferably maintained within a
desired range so that the fins have adequate surface as to intercept most
of the radiation striking the collector. It will be understood that the
fins of the collector of this invention are so sized that they do not
shade one another. The fins also must provide adequate surface area to
transfer heat to the air moving through the collector and the openings
between the fins must be small enough so as to insure that the flow of air
through the collector is sufficiently turbulent (even at relatively low
flow velocities, for example, less than 100 feet per minute) so as to
insure good heat transfer from the heated louver panels to the air but
without imposing high pressure drops within the collector. The width of
louver fins 21 in the direction between the trough side walls 9a, 9b is as
long as possible. The length L of fins 21 in the direction of the air flow
preferably ranges between about 1/8 and 5/8 inches, and even more
preferably ranges between about 1/4 and 3/8 inches. As shown in the
drawings, the louver openings are arranged in three sets extending
transversely across the louvered panels with a bridging section 25 between
the sets. These bridging sections are desirably sized so as to give
sufficient structural support to fins 21, but maximize the amount of the
panel having fins 21 and openings 23 therein. It will be understood that
any other desired arrangement or grouping of the louvered slots in the
panel may be used. With louver panels 19 constructed as above-described, a
majority of the solar radiation passing through cover 13 is absorbed by
fins 21 and the air flowing through passageway P passes freely through
openings 23.
As heretofore mentioned, louver fins 21 are generally parallel to the
longitudinal axis LA of trough 5. These fins may, however, be so arranged
to at least partially deflect the flow of air downwardly away from the
inner face of cover 13 while still permitting the air to flow through the
collector in a direction generally parallel to the longitudinal axis of
the collector trough. By slightly deflecting the air downwardly away from
the inner face of cover 13, the thermal losses to the cover may be
minimized.
Louver panels 19 are preferably made in modules or units incorporating
several louver panels arranged one after the other. The modules are
designed to fit within trough 5 and to be secured (e.g., riveted) to the
trough as a unit. The tops of the louvered panels are connected to the
next adjacent upper louver panel in the module by legs 27, the latter
being spaced relatively far apart from one another with large openings
therebetween through which the air may flow substantially without
restriction by the legs. In this manner, the panels may be installed in
their respective troughs as a unit with the arrangement and spacing of the
individual louver panels of the unit being preestablished. This greatly
reduces the assembly time of the collector of this invention. Bridging
portions 25 and legs 27 may be crimped along their length to resist
longitudinal bending.
It, of course, will be understood that all surfaces of louver panels 19 and
the inner surfaces of trough 5 are preferably highly absorptive to solar
radiation. For example, these surfaces may be painted with a conventional
high absorptivity paint, such as is commercially available from the
Minnesota Mining and Manufacturing Company of Minneapolis, Minnesota under
their trade designation 3M BLACK NEXTAL. Thus, these surfaces have a flat
black surface which is highly absorptive to solar radiation.
Cover 13 preferably includes a pair of spaced panes 13a, 13b substantially
transparent to solar radiation. These panes are held together by a
structural peripheral frame 28 (see FIG. 4). Panes 13a, 13b are separated
by one another to provide a dead air space therebetween for thermally
insulating the panes from one another. As shown in FIG. 4, panes 13a, 13b
are joined by frame 28 in a unitary panel which has a width somewhat
greater than trough 5 so that the side margins of the cover overlie side
flanges 11a, 11b of the trough for purposes as will appear. While any
suitable material may be used for panes 13a, 13b the material should have
a relatively high transmissivity factor so that the maximum amount of
insulation striking the panel is transmitted into the trough 5 for heating
the air flowing therethrough. The cover should also have adequate
structural strength to withstand snow and wind loading on the roof, and
have adequate weather resistance to serve a double function as the weather
surface for the roof. Preferably, cover 13 is a commercially available
unit sold by Kalwal Corporation of Manchester, New Hampshire. It will be
understood that in applications where snow loading and thermal insulation
of the covers is not a concern, a single pane cover may be used.
Further in accordance with this invention, solar collectors 1 form an
integral part of the roof structure of a building built in accordance with
generally recognized building construction practices. As shown in FIG. 1,
roof 3 includes inclined roof members 29, such as the upper chords of a
wood roof truss or the like. As is typical, these roof members are equally
spaced from one another. For example, the roof members may be spaced on 24
inch centers and may be of 2 .times. 8 lumber. Inlet and outlet plenums 15
and 17 are incorporated in the roof. Collector troughs 5 are so sized that
they fit closely between adjacent roof members 29 with their flanges 11a,
11b fitting on the upper edges of the roof members. As is shown in FIG. 4,
the flanges of adjacent collector trays may be overlapped on the upper
surface of the roof members. Covers 13 are then installed over their
respective trough so that the edges of cover frame 28 are so supported on
flanges 11a, 11b with a gap G between the side edges of adjacent covers.
Covers 13 for each trough may be as long as the trough, or, as shown in
FIG. 1, may be shorter than the trough and a plurality of these shorter
covers may be sealingly secured together in end-to-end abutting relation
to extend the length of the trough. While the collectors have been
above-disclosed incorporated in a conventional frame building, it will be
understood that the collectors may be readily incorporated in virtually
any conventional building in a manner that would be apparent to a skilled
artisan.
As heretofore mentioned, the solar collectors of this invention also
constitute the weather surface for roof 3 in the area of the roof that the
collectors cover. It is therefore necessary to seal the collectors
relative to the building roof so as to keep weather and water out of the
building and to seal the covers relative to collector troughs 5 to prevent
the escape of heated air flowing through the troughs and to prevent water
or other contaminants from entering the trough. A seal 31, such as a
continuous bead of a commercially available silicone caulking compound, is
provided between flanges 11a, 11b and the side margins of cover 13 to both
seal the cover relative to the trough and to be least partially bond the
cover in place overlying the trough. As is shown in FIG. 4, batt strip 33
of extruded aluminum or the like overlies the adjacent sides of adjacent
covers 13 and spans gap G therebetween. Batt strip 33 is sealed to the
covers by beads of sealant 35 applied to the outer faces of the cover
frame. The batt strips are then positively secured to roof members 29 by
screws 37 inserted through the batt strips and the overlapped flanges 11a,
11b of the collector troughs and into the roof members. The screws firmly
clamp the covers 13 to the roof members and secure the troughs to the roof
members. Thus, the covers constitute a structural sheathing for the roof
of the building. The upper and lower ends of covers 13 are sealed relative
to the roof structure by any suitable conventional flashing technique. A
center weather-proofing or sealant strip 39 is held in position in a
groove 41 on the under face of the batt strip. The center insulation strip
seals screws 37 as the latter are inserted therethrough.
EXAMPLE
Efficiency measurements have been conducted on the solar collector of this
invention wherein collector trough 5 was about 24 inches wide and 2 3/8
inches deep. The collector was tested in Southern Illinois at a latitude
of about 38.degree. N. The collector was oriented to face south and was
inclined to the horizontal at an inclination angle of about 55.degree..
The air flow through the conductor was regulated to be about 4.4 cubic
feet per minute per square foot of collector area. The transmissivity
factor of cover 13 was determined to be about 0.76. The heat output of the
collector for unit time was determined by measuring the average flow rate
of the air circulated through the collector trough and by measuring the
difference in temperature of the air entering and exiting the collector
trough. The instantaneous efficiency e of the collector may be calculated
from the following equation:
e = Cp m .DELTA. T/A.sub.c S
where Cp is the specific heat of air, m is the mass flow rate of the air,
and .DELTA. T is the difference between the inlet and outlet air
temperatures, A.sub.c is the collector area, and S is the isolation in the
collector plane at the time in question. It was found that the peak
efficiency of the collector was about 69% during normal incidence of solar
radiation and its average efficiency was about 66% for an entire sunny
day. It should be noted, however, that the transmissivity fact of the
cover was about 0.76 and thus the indicated maximum possible efficiency of
the collector was 76%. Thus, the measured maximum efficiency of the
collector was about 92% of the maximum possible efficiency when the
transmissivity of the cover was taken into account. The maximum outlet air
temperature was measured to be about 110.degree. with an inlet air
temperature of about 60.degree. and thus the outer surfaces of the
collector trough exposed to atmospheric air were relatively cool and thus
decreased conduction and reradiation heat losses to the atmosphere.
The same tests were conducted with the same collector trough 5 but the
louvered panels 19 were removed. The inside of the trough was, of course,
a highly absorptive surface. Here the measured maximum efficiences of the
collector was found to decrease to about 40-50%.
The pressure loss or drop in the flow of air through a 12 feet long
collector of the invention at an average velocity of about 100 feet per
minute was measured to be about 0.1 inches of water. This pressure drop is
low compared to other flow restrictions in the air circulation system and
thus the solar absorption panels 19 within the trough impose little
restriction of the flow of air through the collector of this invention.
In view of the above it will be seen that the several objects of the
invention are achieved and other advantageous results attained.
As various changes could be made in the above methods without departing
from the scope of the invention, it is intended that all matter contained
in the above description or shown in the accompanying drawing be
interpreted as illustrative and not in a limiting sense.
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