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BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to solar energy collecting systems, and particularly
to a new combination of an enclosure and an absorber or collector.
2. General Description of the Prior Art
In the past few years, and even before, many configurations of solar energy
collectors have been proposed and some of them marketed. The principal
problem today with solar energy collectors, either for heat or for
electricity conversion, is, as with most products, providing an acceptable
balance between cost, effectiveness, and durability. The fact that no
single configuration has really captured the market is an indication that
optimum designs are yet to appear. Considering the known types, perhaps
the most common one for heat collection is the flat plate collector
wherein a dark colored heat receiver is encased within an enclosure having
a transparent or translucent face through which solar radiation directly
impinges on the receiver and having a bottom side which is heavily
insulated. Typically, the receiver contains a passageway or passageways
through which a liquid, to be heated, is circulated. Depending upon the
material through which the receiver is constructed, and thereby often its
durability, a flat plate collector costs in the vicinity of $8.00 to
$14.00 per square foot of active surface, with typical installation costs
for a domestic hot water heater system running $800.00 to $2,000.00. This
high cost is in part because of a typical requirement that there be a
liquid-to-liquid heat exchanger to heat potable water and the use of a
special fluid which flows between the heat receiver and the heat exchanger
in order to avoid freezing or corrosion and deposits on the passageways of
the heat receiver, which would render the receiver inoperative or
ineffective after a relatively short period (in terms of the typical and
expected life of a heating system, or even a hot water system, of 5 to 15
years). For electricity conversion, perhaps the most common one is a flat
plate module arrangement of a number of photoelectric or photovoltaic
cells, or solar cells (terms used interchangeable), encased within an
enclosure having a transparent cover through which sunlight passes and
impinged directly onto the solar cells. Typically, each solar cell is
connected to electrical conductors which are brought to terminal
connectors from which the electrical power may be taken. These flat plate
solar cell modules, or photovoltaic arrays, are constructed such that the
heat from the photoelectric cells may be removed from the back side, which
are away from the sun, to keep the solar cells within the desired
operating temperature range. Typically, a number of these modules are
electrically connected together, as a photovoltaic system operational
arrangement, to get the desired power at a desired voltage level.
Currently, the cost of electricity using such module arrays is from $7.00
to 418.00 per watt. This high cost is due primarily to the expensive
manufacturing processes to produce the photosensitive semi-conductor
material for the solar cell. The delicate photoelectric cell
semi-conductor must be protected from the effects of the environment to
which it is exposed. A thin transparent covering is usually required to
protect the solar cell surface from handling during manufacturing and
assembly; beyond this, the amount of protective covering depends on the
planned application. For space applications, sufficient covering must be
used to protect the surface of the solar cells from micrometeorites;
generally glass is used as the protective covering to minimize degradation
from ultraviolet radiation. For terrestrial applications, the environment
is more harsh due to dust, rain, hail, and other projectiles; glass is
likewise preferred, but it is expensive and susceptible to breakage from
impacts and thermal stresses. Polymer coverings are less expensive than
glass and are more flexible but degrade in time due to ultraviolet
radiation effects. A technique to reduce the cost and provide some
protection to the solar cells is to utilize a photovoltaic system in
conjunction with an enclosed concentrator device. For terrestrial
applications, one such device is a linear trough-like arrangement in which
the solar cells are located at the bottom with the sun-sensitive surface
facing up toward the top of the trough, which is covered with a
transparent material, such as glass. The sides slope up and outward to the
top and are covered inside with a reflective material. In such an
arrangement, the solar cells can be covered with a thin layer of glass as
the trough top transparent covering protects them from the external
environment. A portion of the sunlight entering the trough would strike
the solar cells directly, and most of the remainder would strike the
reflective inner sides and, in turn, be reflected and concentrated down
onto the solar cells. Within limits, photoelectric cell power output is
proportional to the amount of light striking it. Consequently,
concentrators take advantage of this phenomenon getting more power out of
the solar cells than that obtainable if the solar cells were in the usual
flat plate arrangement. A significant problem with some types of known
photovoltaic concentrators is that they must be adjusted in tilt for the
sun's seasonal attitude and must track the sun throughout the day to be
effective.
In an effort to solve some of the foregoing problems in collecting solar
energy to heat water and to directly convert to electricity, some design
improvements and research have been made. As an example, heretofore, it
has been proposed that where the object is to heat water, a potable hot
water tank itself be encased in a heat receiving enclosure, and that in
addition to utilizing direct radiation from the sun, some reflected
radiation be captured and furnished to the tank. One such system is
illustrated in the September 1976 issue of "Popular Science" magazine,
starting on page 101. This system employs an elongated tank in an
enclosure with an elongated front and with two of the sides forming a
light transmissive trapizoid. The back side, with a reflective inner
surface, is parallel to the front side, and the top and bottom sides are
perpendicular to the plane of the other sides and are heavily insulated. A
difficulty with this configuration is that for optimum performance, it
must be adjusted in attitude for the latitude of the location and as a
function of the altitude (varying with seasons) of the sun. Preferably,
some azimuth changes should be made through the day, i.e., tracking of the
sun, for best solar energy capture.
To achieve direct conversion of sunlight into electricity utilizing
photoelectric cells, solar cells, much research and development work has
been done and is still being sponsored by the U.S. Department of Energy
(DOE). The current mainstream effort by DOE is centered around their
"Low-Cost Silicon Solar Array" (LSSA) Project. The prime emphasis of the
LSSA Project is to develop low-cost silicon semi-conductor photoelectric
cells and to assemble the cells into low-cost modules, each having a power
output of approximately 10 to 15 watts. DOE is also doing some research
and development work on photovoltaic solar concentrators.
Considering the foregoing, it is an object of this invention to overcome
the stated problems, and particularly to provide an effective solar energy
collector which may be used to directly heat potable water and/or provide
an efficient photovoltaic system that is long-lasting, and is of a
configuration which provides a substantial measure of angular
compensation, enabling it to be constructed with a fixed orientation, and
yet be of improved effectiveness despite significant variations in both
azimuth and altitude (seasons and latitude) of the sun.
SUMMARY OF THE INVENTION
In accordance with this invention, an absorber is centrally positioned in a
pyramidal structure wherein the floor of the structure is made radiation
energy reflective, and the upper region of the structure is divided
between a side region, extending part way around, which has a radiation
reflective characteristic and a side region which is radiation energy
transmissive. Solar radiation would pass through the radiation
transmissive portion of the structure and a portion directly strike the
absorber and the balance significantly reflected onto the absorber. This
configuration does not require tilting. Its design is such that the base
is set in a horizontal plane, and the angle of the sloping sides may be
readily adjusted for latitude locations to improve solar collector
efficiency. However, without any adjustment for latitude, and with sides
permanently set at a fixed angle in the range of 40.degree. to 80.degree.,
the system is very efficient.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a plan view of an embodiment of this invention.
FIG. 2 is an elevational view with a portion of the side wall cut away to
illustrate the interior arrangement.
FIG. 3 is an elevational view of an alternate form of this invention
wherein the upper enclosure is formed of one piece and the absorber
protrudes through the base.
FIG. 4 is a plan view of an embodiment of this invention in which
photoelectric cells are mounted on a tank as an energy absorber.
FIG. 5 is an elevational view of an embodiment of the invention with a
portion of the side wall cut away to illustrate an interior arrangement of
photoelectric cells mounted on a tank.
FIG. 6 is a schematic illustration of a completed system in which
photoelectric cells are mounted on a tank.
FIG. 7 is an elevational view in which photoelectric cells are mounted on
an open grid structure, and a system is provided for automatic
introduction of cooling air into the enclosure to cool the photoelectric
cells.
FIG. 8 is an elevational view of an alternate arrangement in which a
transparent side of an enclosure is open to the atmosphere, but covered
with a protective wire mesh to allow free flow of air over the
photoelectric cells for cooling.
FIG. 9 is an oblique illustration of an embodiment of the invention when
used as a photovoltaic system for outer space application.
FIG. 10 is a cut-away elevational view of a structure on which the
photoelectric cells are mounted showing a truncated cone heat sink
structure employed to cool photoelectric cells in an outer space
application.
FIG. 11 is an underside view of an embodiment of the invention as would be
used in outer space and employing a truncated cone heat sink.
FIG. 12 is an elevational view of an embodiment of the invention used as a
photovoltaic system for outer space use and employing a cooling liquid
system to remove heat from photoelectric cells.
FIG. 13 is a diagrammatic cut-away view of a liquid cooling system for
space application of the invention.
FIG. 14 is an elevational view of an embodiment of this invention as would
be used in conjunction with a space station.
FIG. 15 is an oblique illustration of an embodiment of the invention as a
large space photovoltaic power generator being free flying with the
ability to transmit power by radio frequencies.
FIG. 16 is a diagrammatic illustration of an embodiment of the invention as
a cluster of photovoltaic generators connected together in space for
supplying large amounts of power for radio frequency transmission.
DETAILED DESCRIPTION OF THE DRAWINGS
Referring initially to FIGS. 1 and 2, a pyramidal enclosure 10 is
constructed wherein triangular panels 12 and 14 are light transmissive,
and triangular panels 16 and 18 and opaque and constructed with an
interior reflective surface 20. Additionally, the surface 21 of base 24 is
reflective. The angle "a" (measured vertically from the horizontal) for
the panels would be in the range of 40.degree. to 80.degree.. The panels
are supported on frame members 22 which are supported at their bottom by
base 24, typically of plywood, this base then being supported by pillars
or elongated planks 26. The tops 29 of frame members 22 are connected, by
means not shown, to a flat rain cap or plate 27 which additionally
functions to generally prevent leakage at the point of connection of the
panels at the top. As a typical illustration, the base of each wall panel
would be approximately 64 inches, and the height of the unit would be
approximately 50 inches with flat rain cap 27, and 56 inches without rain
cap 27. Alternately, the whole upper structure may be formed of one piece.
In the center of enclosure 10 is positioned an energy absorber, such as a
water tank 30, typically holding approximately 48 gallons, and having a
diameter of 20 inches and a height of 32 inches. With this configuration,
the tank extends upward a generally maximum amount within the enclosure,
that is, to a point where there is a small clearance between the top of
the tank and closest engagement to side wall panels. For purposes of
interpretation, the term tank implies a single vessel of any shape, e.g.,
a cylinder or sphere, or a cluster of vessels that are interconnected, or
a spiral of tubing; all configured to contain a fluid or allow passage
thereof.
Wall panels of the reflective portion of the enclosure, panels 16 and 18,
would typically be constructed of wood, metal, fiberglass, or a plastic
material, with reflective surfaces 20 and 21 being a reflective coated
plastic, such as aluminized mylar, or of reflective aluminum foil. Light
transmission panels 12 and 14 are typically formed of a transparent or
translucent material, such as plastic or glass. Top rain cap 27 may simply
be of wood, plastic, or metal construction and may alternately embody or
support a cupola which would have thermostatically controlled vents to
enable excess heat in the enclosure to be discharged, if such should
occur. Alternately, in order to effect safe operating conditions when the
absorber is a tank to heat water, a pressure relief valve may be connected
to tank 30 which would simply open and discharge any unsafe pressure
condition arising from too high a temperature in the enclosure and causing
steam to be formed.
FIG. 3 illustrates a modified form of conical or pyramidal enclosure 40
wherein the upper structure is formed of one piece of light transmissive
material. One-half of the side walls, the front half 42, as shown, would
be of light transmissive material 44, and the other half, the back half 46
being opaque as a result of having an inner reflective surface 48. The
inside of base 47 would have an inner reflective surface 49. Enclosure 40
is mounted on a roof 41, and tank 43 is formed of spiralled tubing 43a
which extends into building 41a where it is supported on a base 45.
Alternately, referring to FIGS. 4 and 5, the energy absorber within
enclosure 10 is a tank 30 having photoelectric cells, or solar cells, 120
mounted on outer surface 123. Typically, this arrangement would be
identical to that shown in FIGS. 1 and 2 with tank 30 set upon base 24
above ground level 124 on pillars 26. Alternately, top portion 125
enclosure 10 would be covered by a form fitting cap 122 having a vent 129
for venting air in and out through and over water absorbing desiccant bed
131, e.g., silica gel, to minimize water vapor within enclosure 10. Solar
cells 120 would be electrically connected to a pair of power output leads
126. It is necessary to cool the solar cells, and this is accomplished by
heat transferring from the back side of the solar cells through the wall
of tank 30 and hence into a cooling liquid, such as water, within the
tank. The heated water would leave the tank through exit line 130 and be
replaced by cooler water entering inlet line 128 from a heat exchanger,
as, for example, as shown in FIG. 6.
Where a high electrical power output capability is desired, a plurality of
solar energy absorbers of the type shown in FIGS. 4 and 5 would be
connected together. In such an arrangement, power output leads 126 of each
would be electrically connected in a desired series and/or parallel
arrangement to obtain a power output at a desired voltage level. In such
use, cooling inlet lines 128 and exit lines 130 of each unit may be
connected in a parallel arrangement to obtain adequate flow through each
tank for proper cooling.
A typical arrangement for removing heat from within solar cells 120 on tank
30 is illustrated in FIG. 6. In this case, pump 140 would pump liquid
through normally open cut-off valve 142 through inlet line 128 into solar
cell covered tank 30 within enclosure 10. In inlet line 128 is located a
drain valve 144 to effect draining of the liquid cooling circuit through
line 131 into sump 162. The heated liquid would leave solar cell covered
tank 30 through exit line 130 and pass through normally open cut-off valve
156 and into heat exchanger 146 located in large tank 147. The liquid
flows through heat exchanger 146 and is cooled by transferring heat to
liquid 145, such as water, within large tank 147 and then out of heat
exchanger 146 into line 148, through normally open cut-off valve 150 and
into pump 140. The heated liquid 145 in large tank 147 is removed through
exit line 152 and replaced with cooler liquid through inlet line 154.
Connected in line 130 is a reservoir/surge tank 158 which upon the top is
located a fill/vent valve system 160 to allow filling of the cooling
circuit or to admit air to effect draining of the cooling circuit, when
drain valve 144 is opened, into sump 162. The liquid in the cooling
circuit may be pure water, an anti-freeze solution, or an oil. Electric
power from the solar cells is collected by power output leads 126 which
are connected, typically, to a conventional power output control unit 164;
from there the power would be sent to a power storage or distribution
system through power lines 166. The cooling circuit heat exchanger 146 may
be immersed in a liquid, as indicated in FIG. 6, for liquid-to-liquid heat
exchange or, alternately, may be in the air and connected to a blower
system for liquid-to-air heat exchange. Normally, enclosure 10 would be
external to a building, represented by wall 168, which houses the valves,
pump controls, and other related equipment.
The systems thus far described are for the purposes of heating liquids
using solar energy, and for the combination of efficient means of
converting sunlight directly into electricity using photoelectric cells,
and heating water in the process of keeping the photoelectric cells
cooled. However, arrangements are contemplated for direct electrical
conversion within a pyramidal enclosure with solar cell cooling
accomplished with air. A typical such arrangement is shown in FIG. 7 in
which photoelectric cell arrays 120 are mounted on the periphery of an
open grid structure or shell 180, being hollow in the center, and
constructed with air gaps 181 between the arrays, located in central
region of pyramidal enclosure 10. Installed on top of enclosure 10 in a
cupola 182 that provides a means to allow air, by convection, to flow
through vent doors 185 as controlled by linkages 186 and bi-metallic
temperature sensor 187. At a predetermined minimum temperature, the cupola
vent doors 185 open, thereby allowing warm air to flow out by convection
forces and be replaced by cooler air entering through filler 184, flowing
through duct 186, through vent holes 188 in enclosure base 24 and through
the inside of enclosure 10. Vent holes 188 are positioned underneath shell
180, whereby cooling air would flow up inside the hollow portion of shell
180 and out between the arrays through air gaps 181 and effectively
cooling the solar cells of arrays 120. Cupola vent doors 185 automatically
close at night by return spring 183 in cold climates to reduce the
temperature cycling of the solar cells. Power would be extracted from
photoelectric cell arrays 120 through power output leads 126. Although the
temperature controlling action described herein is an automatic mechanical
system, it is appreciated that this could be done by electrical sensors
and electrical activation of vent doors 185, and, in addition, an inducted
draft or forced draft fan could be employed.
Another air cooling arrangement allowing free flow of air could be employed
in embodiments of this invention as shown in FIG. 8. As envisaged,
pyramidal enclosure 10 would not have a covering on the light transmissive
side 42, that is, it would be open to the atmosphere. For mechanical
protection of insides of the enclosure, a wire mesh cover 192 is placed
over light transmissive open side 42. In this arrangement, photoelectric
cell arrays 120, mounted on shell 180, are cooled by the free flow of air
over the solar cells and through air gaps 181. A connection cap 190,
positioned at the top apex of enclosure 10, connects adjoining ends of
adjoining sides and the wire mesh cover 192.
Thus far, emphasis on uses of the invention have been terrestrial
applications. However, this invention is equally applicable for solar
energy collection in space, particularly for generating electrical power.
Of major concern for space operations is the total amount of mass that
must be sent up from earth to perform this or any other function. A
feature of this invention is that it provides means for higher solar
energy collection for a given mass in space than systems currently in use
or as far as is known, now contemplated. An example of the employment of
the present invention in space is illustrated in FIG. 9. As shown,
pyramidal enclosure 200 is constructed of framework members 210 about the
periphery of base 214, the two triangular-shaped sides 216 and 218, and
diagonally across the base, for structural integrity. Stretched between
the framework members 210, forming base 214 and the two sides 216 and 218,
is a thin plastic covering 200 having an inward facing reflective surface.
Sides 216 and 218 extend approximately one-half way around the enclosure;
the balance, side 223, is open. A stabilizing strut 224 extends from the
top apex 226 of the enclosure to opposite corner 228 of base 214,
bisecting the open side 223. In the central region of enclosure 200 is a
cylindrical support structure 222 upon which is mounted photoelectric cell
arrays 225. With the open side 223 of enclosure 200 oriented toward the
sun, sunlight would directly strike the solar cells of arrays 225 facing
the sun and also reflective surfaces of covering 220 on base 214 and sides
216 and 218. The resulting reflected radiation would be concentrated and
directed onto all of the solar cells of arrays 225 covering structure 222.
In such an arrangement, the typical dimensions of each the diameter and
height of structure 222 would be in the range of 1/4 to 1/2 the length of
one side of base 214. By proportionally selecting the height of the
enclosure, and hence the resulting dimensions of sides 216 and 218,
concentration ratios (the ratio of the projected reflective inner surfaces
to the total area of solar cells) much greater than one can be obtained,
allowing high power outputs from the solar cells and resulting in fewer
solar cells being required for a particular power output level over that
required from a flat plate arrangement. Since the mass of enclosure 200 is
about 10% to 15% of the mass of the photoelectric cell arrays 225 and
structure 222, this is a mass efficient combination particularly adapted
for space applications.
FIGS. 10 and 11 illustrate means for removing heat from photoelectric cell
arrays 225 in a space application. Sides 216 and 218 are omitted in FIG.
10 for purposes of simplification of illustration. As shown in this
cut-away view, cylinder support structure 222 includes an internal heat
sink 230 constructed as a truncated cone of solid thin material having
good thermal conductive properties, such as aluminum. The inner and outer
surfaces 231 and 232 are colored black. The small end 233 of heat sink 230
is connected to the underside of the top of structure 222 which supports
the photoelectric cells arrays 225. The larger conical end 235 of heat
sink 230 extends through an opening 234 in base 214. In this arrangement,
heat from the photoelectric cells would radiate to a cooler outer surface
232 of heat sink structure 230, be conducted through its thin wall, and
then be re-radiated from the inside surface 231 out to black space. The
electric power from solar cell arrays 225 would be collected and brought
out through output power leads 237 to power output control unit 236 which,
in a conventional manner, would convert and/or distribute the power for
use.
Another arrangement for removing heat from solar cells is to utilize a
liquid coolant system, as illustrated in FIGS. 12 and 13. In this
arrangement, photovoltaic arrays 225 on a structure 222 are cooled by
coolant coils 244 attached to the inside wall of structure 222. In
general, a liquid coolant is circulated by means of a pump 240 of coolant
control system housing 238 through distributor line 242 to coolant coils
244 and then through exit line 246. As a further feature of this
invention, and as illustrated in FIG. 13, the coolant fluid as heated by
the solar cells is fed through power generating system 247 including means
(not shown) to convert the heat energy in the coolant to electrical power.
This generating system may, for example, be a Brayton Cycle power
generator system. Discharge coolant line 249 from the power generating
system feeds the coolant through radiant heat exchanger 248 located
beneath base 214, which thus performs the function of a heat sink for
generating system 247 as well as the solar cell cooling circuit. Line 250
connects the outlet of radiant heat exchanger 248 to the inlet of pump 240
which recirculates the cooled fluid back to coolant coils 244. A surge
tank 252 is connected to line 250 to permit any necessary fluid expansion
in the system without damage. The cooling circuit systems functions would
be electrically controlled by control unit 254, which along with pump 240,
reservoir/surge tank 252, and other elements of the system would be housed
in housing element 238 located beneath base 214. Power from photovoltaic
arrays 225 would be carried through output power leads 237 to power output
control unit 236, in this arrangement located beneath housing 238, which
in a conventional manner would convert and/or distribute the power for
use. In a space operation, the open side of enclosure 200, bisected by
stabilizing strut 224, would be oriented toward the sun, whereby radiator
248 would always be facing toward black space.
Applications of the invention for space power generation are many and
varied. Typically, a thus powered generator could be connected to a
satellite or to a space station to provide necessary power. As an example,
shown in FIG. 14 is enclosure 200 with liquid cooled photoelectric cell
arrays 225 mounted on structure assembly 222, and with cooling control
system housing 238 beneath. A power output control unit 236 is positioned
beneath housing element 238, and it is connected to a space station 300 by
adapter 302. With the open side 223 of enclosure 200 oriented toward the
sun, radiator 248 would face toward black space. From power control unit
236, the available power would be routed through adapter 302 to space
station 300 for use.
This invention is also applicable for large space power generating
concepts, such as a configuration for a Space Power Satellite (SPS) as
being studied by NASA. Typically, such an arrangement would be a free
flying configuration as illustrated in FIG. 15. In this configuration, the
dimensions of the base and height of enclosure 200 would be measured in
hundreds or thousands of feet, with photovoltaic arrays 225 and structure
222, upon which they are mounted, proportionally sized. The construction
would be similar to that described in FIG. 9, except additional support
members 401 would be required over base 214 and opaque sides 216 and 218
to properly support the thin plastic covering 220, having an inward facing
reflective surface, to the large spans. To maintain proper orientation and
position, station keeping modules 400 would be located at three of the
base corners. These station keeping modules would contain control moment
gyroscope systems (not shown) and liquid rocket reaction control motor
systems (not shown) which would respond to signals received from
orientation control unit 406, located under corner 228 of base 214, and by
their appropriate actions maintain proper orientation. Under control unit
406 would be located a power station 402. Electric power generated in the
solar cells would be brought to power station 402, from where a portion
would be used to operate orientation control unit 406 and modules 400 and
other systems of the SPS, and the remainder would be converted to radio
frequency energy to be beamed to a desired location, such as a receiving
station on earth, through antenna 404 which can swivel to maintain desired
orientation. Cooling of the solar cells may be effected by either the heat
sink concept shown in FIGS. 10 and 11 or the liquid cooling system shown
in FIGS. 12 and 13. A number of the configurations shown in FIG. 15 may be
connected together, as illustrated in FIG. 16, for a very large power
generating system. In this arrangement, each of the enclosure units 200
would be delivered to the desired earth orbit, such as geosynchrous orbit,
there the units would be connected to one another at corners 504 of bases
214 where opaque sides 216 and 218 intersect with open sides 223. A
station keeping module 400 would be located at this juncture with
additional modules located at the other base 214 corners. A power station
402 would be under the open side base corner 228 of each enclosure unit
200 to collect the power from the photovoltaic arrays of each unit and to
provide power for that unit's operation. A central orientation control
unit 420, located under power station 402 of a centrally located enclosure
unit 200, would provide the necessary orientation control signals to all
of station keeping modules 400. By such an arrangement, each unit can be
maintained in proper position and orientation relative to each other. The
remaining power from each unit would be sent by wires to a central power
station 500 located beneath central orientation control unit 420, from
where the power would be converted to radio frequency energy for
transmission to earth by antenna 502.
While the structures thus far described have utilized photovoltaic cells
for, for example, photocells 120 shown in FIG. 8, or photocells 225 shown
in FIG. 14, which function to generate electricity, it is to be
appreciated that the structures may be employed to focus energy onto a
photocell or photocells of the photoresistive type as where employed in a
signalling system.
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