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Description  |
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FIELD OF THE INVENTION
The field of art to which the invention pertains includes the field of
solar energy collection means adjustably supported for the purpose of
maintaining the solar radiation at a given relationship with the
collection means as the Sun makes it apparent daily track about the earth,
said collection means having surfaces being so shaped and spaced along an
optical axis that they are capable of reflecting solar rays onto a
radiation absorbing surface remote from the collection means.
References of interest include U.S. Pat. Nos. 3,058,394, 3,982,527,
3,977,773, 4,079,724, 4,011,857, as well as "Applied Solar Energy," by
Meinel and Meinel, Addison-Wesley Publishing Co., 1976, "Non-tracking
Concentrating Collectors Utilizing Grazing Incidence Mirrors," a paper
presented by J. E. O'Meara et al. during the 1957 Conference of the
International Solar Energy Society in Los Angeles, and "Reduced Drag,
Paraboloid Type, Solar Energy Collectors," a paper presented by P. J.
Vermeulen et al. during the 1976 Conference of the International Solar
Energy Society in Winipeg, Canada.
BACKGROUND AND SUMMARY OF THE INVENTION
The intensity of solar radiation which is fairly low at ground level can be
increased significantly by paraboloid collectors which concentrate the
Sun's rays upon a small "point focus" target area. If the collectors are
combined with Sun-tracking devices which follow the Sun across the sky
along its daily and seasonally changing path, the concentration of
insolation at the target can result in temperatures which exceed by a wide
margin the 300.degree. to 400.degree. F. minimum that is required for most
of the process industries in the United States. It has been calculated
that a carefully designed tracking and concentrating device can produce
temperatures up to 7000.degree. F. and above which can be used for a wide
range of applications, e.g., to drive turbine generators for producing
electricity, to drive photovoltaic or thermionic-thermo-electric
generators of electricity, for metallurgical melting, for alloying, or
high-temperature fuel-producing chemical processes such as the release of
hydrogen from water.
Stationary as well as tracking prior art devices for the conversion of
solar into thermal energy through paraboloid collectors generally consist
of one or more cusps with reflective surface areas which reflect incident
light upon a focal zone that may be located intermediate the collector and
the Sun (front focus), or behind the collector (linear rear focus). In a
rear focus device the collector includes an opening for each cusp for the
transmission of the reflected light upon the focal zone. The concentration
of energy, that is the "equilibrium temperature" at the focal zone depends
upon the size and geometry of the collector-concentrator components as
well as on heat loss due to re-radiation to the environment which is
engendered during the energy-conversion process. Technical data commonly
available enable a direct correlation of the designed concentration factor
and the resultant "equilibrium temperature." To reduce wind drag it has
been proposed to build a collector from conical frusta which are
juxtaposed but spaced apart a sufficient distance to permit air flow
through the structure.
Front focus collectors have numerous drawbacks, such as the relative
inaccessibility of the focal zone which hampers the utility of the
devices. The absorber, thus of necessity, must be mounted on the collector
structure, where it adds weight to the tracking articulation and requires
reinforcement and stiffening of the support structure. Considerations of
weight as well as of shading effects limit the amount of focal zone
insulation that can be used. As a result, as much as 45% of the collected
energy is reportedly lost to the environment. A further disadvantage is
the need for frequent alignments of the collector relative to the absorber
when both move through a semi-circular tracking arc.
Prior art single-reflection rear-focus devices concentrate the insolation
on a linear focus, which means a dilution of the energy yield. Such
devices deliver only a fraction of the harvest which can be obtained in
point focus devices. An increase in their efficiency involves an increase
in the size of the components which presents structural problems as well
as higher costs for material and manufacturing. The more massive the
expanse, the greater the wind drag and the problems associated with it.
Multiple-reflection rear-focus devices, though they can produce a "point"
focus, also make substantial demands on material and construction
capability; the concentration of the insolation on a "point" requires
large surface areas from which the rays are reflected several times before
they hit the target.
The problems are aggravated in devices which track the Sun, because the
movement of heavy and voluminous bodies is expensive in terms of energy
and precision engineering. The drawback of stationary, that is,
non-tracking devices is that only parts of the collector surface are
accessible to incident light for the greater part of the day. The
reception of sunlight is near zero at sunrise, increases to a full value
at high noon and tapers off again to near zero at sunset. Thus, a
non-tracking collector-concentrator absorbs only some 60% of the otherwise
available insolation. Further, such devices are commonly tilted toward the
equator, and during some five months of the year (36.degree. N. latitude)
are self-shading near sunrise and sunset, thus, losing some 23% of the
annular daylight hours.
The present invention intends to overcome the limitations of prior art
described heretofore by providing an efficient single-reflection
rear-focus tracking solar collector which produces a "point" focal image
on a concentrator-absorber which is fixed in relation to the collector.
The entire structure is compact and lightweight and occupies a minimum of
space.
Accordingly, a rear-focus parabolic collector of solar energy is disclosed
which is formed as an array of nested annular conic frusta that are in
stepped relation to one another, and symmetrically disposed about a focal
axis passing through their geometric centers. The array which is mounted
on a support frame, has the profile of an inverted, truncated annular
trough that is open at both ends, and which ascends from a base, defined
by the outermost frustum, to a vertex defined by the innermost frustum.
Each frustum has an outer surface, and an inner surface which is defined
by a singular and unique parabola. The surfaces are inclined in an
upwardly and outwardly slanting direction relative to the common focal
axis of the parabolas. Thus, the upper rims of the various frusta have a
larger perimeter than the lower rims.
The area circumscribed by the lower rim of the innermost frustum at the top
of the array is substantially larger than the area required to transmit
rays reflected from the innermost frustum to the focus at the rear of the
collector. The ratio of the largest diameter of the outermost frustum to
that of the smallest of the innermost frustum is about 5:2, so that the
open area represents a loss of some 16% in available collection surface.
Although this subtraction appears as a sacrifice of a portion of the
fill-factor, it is fully compensated for by the configuration of the
present invention as will be shown hereinafter. In fact, the dimensions of
the open area are an essential item in the functional efficiency of the
device. The collector which includes a frame for connecting the frusta to
each other, is interposed between the Sun and an absorber. The absorber is
mounted on the support frame distal from the collector. The distance
between the collector and the absorber can be adjusted by moving the
latter along the focal axis toward, or away from, the collector.
Sun-tracking means operatively connected with the collector frame for
continuous movement of the collector and the absorber which is stationary
relative to the collector, constantly align the focal axis with the Sun as
it moves across the sky. The Sun-tracking means which are part of the
combination comprising the present invention, may include mechanical,
hydraulic, electric and electronic components such as are well-known in
the art.
Although the collector-absorber is operable with two frusta, spaced apart
to provide an annular air gap which reduces the wind drag, a preferred
embodiment comprises a larger number of frusta with intermediate air gaps.
The frusta, made of lightweight material, have a reflective inner surface
which may be formed of deposits such as polished foil, or a glossy
dielectric coating such as white paint, or aluminized film attached to the
frusta structure; alternately the base of the frusta may be a sheet
material such as pure aluminium which can be buffed or otherwise treated
by chemical or electrical polishing to obtain the desired brightness.
The attitude of the reflective surfaces toward the Sun is such that the
incident angle for solar radiation at any point is greater than 45.degree.
and preferably 60.degree. or greater. The magnitude of the incidence angle
is related to the reflectivity of the dielectric surface option in that
there is a marked increase in reflection when the incident light comes in
at 60.degree. (Daniels, Farrington: "Direct Use of the Sun's Energy," Yale
University Press, 1964). Such high reflection justifies the use of the
term "gloss lens" for the device. "Gloss" of opaque materials, according
to a test method of the American Standard for Testing Materials, is
measured by a reflection angle--corresponding to an incidence
angle--greater than 45.degree., and "lens" includes any device for the
concentration (or dispersion) of radiation.
The collection efficiency of the Sun-tracking collector-absorber of the
present invention resides in the combination of several factors: the
favorable angles of incidence and reflection which permit the convergence
of the rays onto a very narrow focal zone, the overall geometry which
compensates for the loss of fill-factor collector surface areas; the
compactness of structure and design, and the accessibility of the absorber
which is rotatably synchronized along the focal axis by the Sun-tracking
mode of operation of the collector, and which is stationary relative to
the collector and remote therefrom at its rear.
Seen from above, the collector represents a continuous, uninterrupted
annular surface area circumscribing the noncollecting central portion,
whereas an elevational view shows the assembly as a spaced-apart,
staggered configuration of parabolic frusta. In the plane normal to the
focal axis the focal zone consists of a multiplicity of very tight
overlapping ellipses--each derived from one reflection spot--which
approach a "point" and represent the maximum concentration of the
reflected radiation. The greater the concentration the higher the
temperature at the focus which can be utilized through conventional heat
transfer or conversion means for a multitude of purposes.
Another factor which contributes to the efficiency of the solar energy
conversion is the spacing of the frusta relative to one another as well as
to the focal axis. The slightly vertical clearance between proximate
frusta prevents areas of shading, so that light incident upon any surface
portion is transmitted by single reflection onto the absorber without the
energy loss inherent in multiple reflections.
A further advantage is that the center of focus is fixed at a point and
does not require gross arcuate large radius translations of the energy
absorber such as would be necessary in a Sun-tracking front focus device.
The Sun-tracking operation of the device is automatic and needs only a
one-time adjustment at the time of installation to align the
concentrator's diurnal axis with the true North (or South), and to
position the diurnal axis relative to the horizontal plane, so that it is
at an angle which corresponds to the local latitude of the site. This
one-time adjustment places the diurnal axis in parallel with the earth's
axis.
The compact structure and the light weight of the device constructed in
accordance with the present invention are further advantages which make it
possible to position it economically in a spatially limited area, either
on a pole in the ground or on top of existing buildings. It has been
calculated that a four-frusta collector with an overall diameter of some 9
feet, equivalent to the diameter of a backyard picnic table umbrella, can
deliver more than the annual heating/cooling requirements of the average
household, even if performance penalty and thermal losses are taken into
account. The yield can be improved by increasing the collection area of
the device as fabricated, and/or by using multiple units.
Still another advantage is that the highest delivered "equilibrium
temperatures" on the absorber can be scaled down to provide lower
temperatures appropriate for a particular use. This can be done by moving
the absorber away from or toward the collector along the focal axis, in
order to diffuse the sharp focal image and dilute the concentration on the
absorber.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a perspective view of the solar energy collector-concentrator in
accordance with the present invention.
FIG. 1A is a schematic isometric view of a detail shown in FIG. 1.
FIG. 2 is a cross sectional view, partly broken away, of a detail shown in
FIG. 1.
FIG. 3 is a schematic view of a detail shown in FIG. 1.
DETAILED DESCRIPTION
Referring to FIG. 1, there is shown a solar collector 10 consisting of a
concentrically nested array of four conical frusta 12a, 12b, 12c, and 12d
which are in stepped relation and separated by air gaps from one another.
The frusta are symmetrically disposed about a focal axis A passing through
their geometrical centers. The array has the profile of an inverted
truncated, annular trough (FIG. 2) which is open at both ends and which
ascends from a base, circumscribed by the outermost frustum 12a, to a top
circumscribed by the innermost frustum 12d. Each of the frusta has an
outer surface, and an inner, reflective, surface which is defined by a
singular and unique parabola, and which is inclined in an upwardly and
outwardly slanting direction toward the Sun. Thus, the upper rim of each
frustum has a larger perimeter than its lower rim. A pair of thin
cross-ribs 14 and 16 are attached to the upper rims of the four frusta to
connect the units to each other.
The spaced air gaps between the frusta 12a, 12b, 12c, and 12d (FIG. 2),
have the dual function of reducing wind drag against the structure, and of
transmitting solar rays, incident upon and reflected by, the inner
surfaces of the frusta, onto the focal zone located along the axis A. The
parabolic surface areas of the frusta, and the intermittent air gaps are
calculated to provide, in a plane view, the appearance of a virtually
unbroken annular surface, spanned by the ribs 14 and 16, and surrounding a
central aperture that is equivalent to the area circumscribed by the lower
view of the frustum 12d.
A strut 18, attached at one end of the rib 14 at the outer surface of the
frustum 12a is connected at its other end to an arm 20. The opposite end
of the arm 20 is affixed to a housing 22 of a declination tracking motor
whose rotor 26 is connected, via linkage means 28, to an elongated rod 24.
The movement of the rotor 26 about the axis C in the direction of the
arrow C.sub.1 moves the arm 20 relative to the arm 24. The arm 24
continues into an arm 44 which is received in sleeve bearing 30 that
include a perforated bracket 32. A tubular stanchion 34, mounted on a base
36, rotatably houses a shaft 38 which includes at its upper end an opening
(not shown). A pin 40, passing through the opening of the shaft 38 and the
adjacent perforation of the bracket 32, pivotally connects the sleeve
bearing 30 to the stanchion 34. The strut 18, the arm 20, the rods 24 and
44, the sleeve bearings 30, the stanchion 34 and the shaft 38 as well as
their linkage means comprise the support structure for the collector 10 as
well as the tracking articulation system for continuously following the
Sun's virtual motion along two axes as will be described hereinafter.
A solar energy concentrator in the form of an absorber 42 is disposed at
the common focal zone of the frusta 12a, 12b, 12c, and 12d. The area of
acceptance of the absorber for the reflected solar rays may be a plane or
an arcuate surface, neither of which needs to be larger than approximately
one inch in diameter for high temperature collection. The absorber is
placed a sufficient distance away from the collector structure and the
tracking system to avoid deterioration of the latter by the high
temperatures developed on the absorber and interference with the mechanism
of the system. At the same time, the distance, both from the collector and
the tracking system, increases the accessibility of the absorber and
facilitates the transfer of energy from it.
The absorber 42, shown as a "bead," is mounted at one end of a shaft 46
which is centered on the axis A and which is threadedly movable, along the
axis through a perforation 29 that is provided in the arm 44. The absorber
may be positioned relative to the center of focus by advancing or
retracting the shaft 46 along the axis A. The location of the center of
focus so obtained relative to the absorber 42 remains fixed during daily
and seasonal tracking.
The transfer or conversion of the energy concentrated on the absorber is
effected by methods well-known in the art. In one embodiment (not shown)
the absorber 42 includes a cavity into which a heat transfer fluid is
admitted. In another embodiment the absorber is immersed in an absorber
tube which is filled with a heat-transfer medium. In yet another
embodiment, the thermal energy is converted into electrical energy by
photovoltaic cells.
As shown in FIG. 1, the base 36 is disposed on the ground which is taken to
be parallel to the horizon at the site. In an alternate embodiment which
dispenses with a base, the stanchion 34 may be mounted on flat or sloping
roofs or on any variety of existing structures.
The frusta 12a, 12b, 12c, and 12d are made of lightweight material. They
may be manufactured separately or as one unit, by casting, molding, vacuum
forming, or other conventional manufacturing methods. The assembly is
compact and stable and presents no structural load problems. For example,
if the frusta are made of 0.016" aluminum, the entire collector stack
would weigh only approximately 50 pounds. The upwardly and outwardly
facing surfaces of the frusta are made reflective either by integral
formation with a reflective metallic dielectric material, or by vacuum or
electrostatic deposition of such material, by a dip, a chemical polish or
a coating of metallized film, of aluminized paint or similar means.
Preferably, the material used in the construction of the collector should
be corrosion resistant and impervious to breakage due to wind-driven
objects.
The absorber 42, comprising material which has a high melting point such
as, for instance, tungsten, may be provided with a dielectric surface
coating which allows the reflected solar energy to penetrate. Underneath
this may be a layer made of energy absorbent material. It is to be
understood, however, that the material components of the absorber are not
to be considered part of the present invention but are recited by way of
example only.
The dimensions of the collector 10, its elevation and its distance from the
focal zone are shown in FIG. 1 by way of comparison next to the drawing of
a human figure taken to be 6 foot 2 inches tall. The diameter of the
outermost frustum 12a, respectively of the collector 10, is 9.7 feet which
is approximately the diameter of a larger-size backyard picnic table
umbrella. It is to be understood that this, and other, dimensions of the
drawing according to FIG. 1 do not present limitations in size or scale
but are shown merely to demonstrate the feasibility of placing the
collector-concentrator unobtrusively in a location where it satisfies
architectural and aesthetic standards. If the site permits, much larger
units may be installed, although additional structural support means would
be required.
The Sun-tracking system for the collector-absorber in accordance with the
present invention is based on methods and operational components
well-known in the art. It includes means for a one-time adjustment of the
collector axis A by rotating the shaft 38 in the stanchion 34 to point it
to the true North (in the Northern hemisphere) or to the true South (in
the Southern hemisphere), as well as means for continuous adjustments to
follow the seasonal and diurnal excursions of the Sun. The one-time
adjustment task also defines the angle of inclination of the diurnal axis
D at the site to become parallel with the earth's rotational axis. It is
effected at the time of installation by rotating the bracket 32 about the
equatorial-adjusting axis B in the direction of the arrow B.sub.1,
bringing the sleeve bearing 30, and hence the axis A, to rest at an angle
with the horizontal which corresponds numerically to the latitude of the
site. Seasonal adjustments are made by changing the attitude of the arm 20
relative to the rod 24. This is accomplished by moving the housing 22
about the axis C in the direction of the arrow C.sub.1 to follow the Sun
from North to South. The total excursion in six months amounts to
47.degree. which corresponds to a daily change of 0.2574.degree.. Diurnal
adjustments are made by moving the rod 24 at constant speed, relative to
the sleeve bearing 30, about the axis D in the direction of the arrow
D.sub.1 to follow the Sun from East to West at the rate of 15.degree. per
hour. The total excursion amounts to approximately 180.degree.. The axes
A, C, and D which intersect at right angles (FIG. 1A), at the focus on the
absorber 42, are offset from one another to permit adequate clearance.
Because absorber 42 is supported on an arm 44 which is centered on the axis
D and affixed to the arm 24 at the sleeve bearing 30 the center of the
absorber acceptance area is always fixed as adjusted relative to the
center of the focal zone.
Drive means for the non-linear motions of the Sun-tracking system may be
controlled by a microcircuit computer which instructs an appropriate motor
in accordance with solar ephemeris data stored in its clock memory;
alternately they may be coupled to a closed-loop system which, upon
detection of an error between the position of the Sun and the focal zone
initiates corrective movements to compensate for deviations due, for
example, to wind conditions or mechanical irregularities (or a combination
of the two). The hydraulic, mechanical or electronic components of the
tracking system may include Sun sensors, a timing mechanism as well as
appropriate circuits for the control and coordination of the various
movements as are well-known to one skilled in the art.
The operation of the collector-concentrator in accordance with the present
invention is based upon the single reflection of solar rays from the inner
parabolic surfaces of the frusta 12a, 12b, 12c, and 12d onto a focal zone
which is normal to the focal axis and common to all four frusta. The focal
image is formed of a multiplicity of overlapping tight ellipses which is
the closest approach to a point that is attainable. Tightness of the
ellipse grouping is affected by the fabricated accuracy of the location of
each reflective x-y point on the frusta. Even with a precise x-y
fabrication accuracy, the location of the center of focus and the size of
the focal zone may be affected by a misalignment of the focal axis with
the center of the Sun. In this, the location of the center of focus is
disturbed more than the focal size because an unwanted decrease in
incidence angles on one side of the collector tends to be neutralized by a
desired increase in incidence angles on the opposite side. Tracking
misalignments of this nature which are caused by manufacturing errors can
be overcome by calibrating the collector through adjustment means mounted
on the collector itself. Also, the tracking means can be calibrated to
deliver perfection in operation. Assuming a reasonably perfect tracking
mechanism, the concentration of solar energy is thus a function of the
collection area and of the resulting focal zone. The higher the
concentration, the greater the amount of useful energy that is harvested
from a predetermined collection area exposed to insolation.
In order to enhance the usefulness of the collector-concentrator for the
operation of thermionic, magnetohydrodynamic and other high-temperature
devices, it is desirable to produce the highest possible temperature at
the absorber, especially since this maximizes the usefulness of the
collected energy and because heat lost to the environment in the form of
convection or re-radiation must be subtracted from a potentially
achievable maximum gain. It has been reported that such heat loss is
substantially inversely proportional to the concentration and hence also
to the temperature. Lower temperatures such as may be required for
domestic purposes and other applications can be produced by diffusing the
sharp focal image through a movement of the absorber 42 away from or
towards the collector 10 along the axis A. In accordance with the
illustration of FIG. 1, this can be done by manipulating the threaded
shaft 46 relative to the perforation 29.
The larger focal zone provided for lower temperatures reduces the demands
on fabrication accuracy and calibration of the tracking mechanism which
are more stringent for the "point" focus required for higher temperatures.
Peak performance in terms of maximum concentration at lowest cost requires
the careful coordination of all parameters of the device, such as the x-y
dimensions, the height, shape and number of the frusta, their spacing
relative to each other and to the absorber, the focal length, and the size
of the open area circumscribed by the lower rim of the innermost frustum
12d. For a collector with a diameter of 9.725 ft. a central circular area
with a diameter of 3.947 ft. has been determined experimentally and
theoretically to be the necessary and best fit. This central circular
"hole" with a diameter which is some 40% of the outside diameter of the
device, is a requisite for the feasibility of the collector-absorber of
the present invention. Without it, it would not be possible to construct
an operational device of the necessary size and characteristics.
The deficiency in inner collection surface is overcome by a slight increase
in the outside diameter of the collector. The resulting enlargement of the
collector restores equality, in terms of collection surface, with prior
art type parabolic rear-focus devices, although the latter approaches
cannot be compared with the present invention in terms of versatility and
efficiency.
FIG. 2 illustrates the arrival of the rays R.sub.1, R.sub.2, R.sub.3, and
R.sub.4 on the inner surfaces of the frusta 12a, 12b, 12c, and 12d, and
their reflection, through the air gaps and the inner empty hole onto the
absorber 42 at an incidence--and reflection-angle which lies between
54.degree. and 72.degree.. Consistent with an earlier standard set by the
American Standard for Testing Materials for measuring the reflectivity of
opaque surfaces, incidence angles in this range produce what is called a
"gloss." This definition of high reflective power, especially for
dielectric, that is, non-conducting surfaces, justifies the description of
the present device as a Gloss Lens, if lens is taken as the definition for
any device which concentrates (or disperses) radiation.
The drawing of FIG. 2 is a simplified view in that it shows a parallel
arrangement of incoming solar rays. In reality, rays coming from the Sun's
edge subtend a half-angle which can be taken to be 0.27.degree., so that
the focal image from each point of the reflected insolation is not a sharp
point but a spread-out elliptical area. The radius of deviation .DELTA.x
from a point focus assumes a different value for the reflections from each
frustum, with an average of a 0.425 inch radius for a four-frusta
collector. The calculation of the deviation .DELTA.x is based on the fact
that the major axis of each elliptical image from each reflection point
lies on the common center of focus but is of different length on opposite
sides thereof. A conservative estimate of the size of the focal zone may
be obtained by averaging the larger segments of all the ellipses' major
axes and by calculating the focal area as if it were a circle with a
radius represented by the average length of the segments.
In an approximation which represents the surface S which receives the
insolation as a plane, and the acceptance area A.sub.f of the reflected
rays as a circle, the concentration factor K for the device is given by
the equation
##EQU1##
For a 9.7 ft. collector with a radius of 58.348 inches and an empty inner
area of 23.682 inch radius
##EQU2##
which corresponds to a temperature of 7052.degree. F. that is well in the
solar furnace category. (See V. B. Veinberg, "Optics for the Utilization
of Solar Energy," or "Standard Handbook for Mechanical Engineers," 7th
ed., McGraw-Hill.)
The parabolas which define the shape of the frusta 12a, 12b, 12c, and 12d
(FIG. 3) according to the equation x.sup.2 =2py, where p is the semi-latus
rectum, differ from each other by the value of p, so that each point on
their surface areas lies on a unique intersection of the x and y axes.
This enables the full utilization of the surfaces for the collection of
solar energy. As shown in FIG. 3, rays, such as R.sub.4 which grazingly
pass beyond the acceptance surface of the frustum 12b are still caught by
the next lower frustum 12a and reflected therefrom onto the focal zone.
The data which specify the parabolic surfaces and other variables, such as
the number of frusta or the distance between the collector and the
absorber, are obtained by mathematical calculations and may be fed into a
computer program for the numerically controlled manufacture of the device.
Although an operable collector-absorber can be constructed of two frusta,
a four-frusta unit offers the most favorable energy yield in terms of
minimal stack height. A decrease in height of individual frustra which is
obtainable by units with a greater number of frusta, is offset by an
increase in wind drag and the disadvantage of stronger and heavier support
structures.
Similarly there is an optimal distance between the collector and the focal
zone, respectively the center of the empty "hole" in the plane of the
lower rim of the frustum 12a. This may vary depending on the size of the
collector for a 9.7 ft. collector, for instance, the predetermined optimal
distance is 16 inches.
A 9.7 ft. diameter collector-absorber, as described heretofore, at a
location of 36.degree. N and assuming 100% efficiency of the device as
well as 100% sunshine can produc | | |
