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The present invention relates to solar energy collectors, and more
specifically to a solar energy collector including a target in the form of
a heat pipe and novel eflectors for collecting solar energy. Generally, a
heat pipe may be defined as a closed container housing a vaporizable
liquid as the heat transfer medium. The heat transfer medium is evaporated
at one location by absorption of heat and condensed at another location by
the extraction of heat. The condensed liquid is returned to the heat
absorption portion, e.g., via a wick. Heat pipes per se are well known in
the art, see for example, U.S. Pat. No. 3,229,759; 3,435,889; 3,602,429
and 3,746,081.
Various solar energy collector constructions are known in the art. U.S.
Pat. No. 1,802,635 discloses a solar water heater having a flat plate
collector. U.S. Pat. No. 3,125,091 discloses an inflatable pipe solar
energy collector. U.S. Pat. No. 3,203,167 discloses a system for
concentrating solar energy on a slit in an opaque cylinder. U.S. Pat. No.
3,262,493 discloses a system for collecting and storing solar heat within
the earth for future use. U.S. Pat. No. 3,321,012 discloses an array of
flattened tubes with semi-cylindrical reflectors.
A recognized problem in attaining widespread commercial use of solar energy
to provide heat and power has been the cost of collecting the energy. To
minimize such costs stationary approaches were considered using evacuated
tubes with solar energy absorbing means therein. Solar energy collectors
were then developed having a troughlike reflecting wall capable of
accepting solar radiation over a limited period of time and redirecting
all that was accepted to a target for complete utilization thereof. In
such improvements it was believed necessary to develop a cylindrical
reflector whose surfaces were parabolic in shape and spanned the target so
that the focal point of the parabolic surfaces was at the opposite edges
of the target.
Moreover, known solar energy collectors generally include tubes through
which a fluid flows as the intermediate heat transfer medium. When
installing an array of such collectors on a roof, a multiplicity of fluid
connections must be made at the site, causing an increase in installation
cost and risk of leakage.
It is an object of the present invention to expand the period of time that
a solar energy collector may be effectively used without increasing its
cost.
It is an object of the present invention to provide an improved solar
energy collector incorporating a low temperature heat pipe for heating
buildings and the like.
It is a further object of the present invention to provide an improved
solar energy collector having a dry thermal interface for transferring
heat to a manifold.
It is a still further object of the present invention to provide a highly
efficient solar energy collector.
It is a still further object of the present invention to provide a solar
energy collector which may be readily mounted on a manifold without the
need to make a large number of fluid connections.
It is a still further object of the present invention to provide solar
energy collectors which can be easily installed at the site, and removed
and replaced, as warranted.
It is a still further object of the present invention to provide improved
reflectors for collecting solar energy.
It is a still further object of the present invention to eliminate the need
for apparatus to mobilize a collector and/or reflector by use of a novel
reflector design for accommodating broader operating conditions.
It is a still further object of the present invention to provide a novel
fixed reflector construction having its surfaces shaped so as to capture
and reflect solar energy between low and high incidence and all
intermediate angles of incidence.
It is a still further object of the present invention combine with a
target, such as a heat pipe, a gull-wing shaped reflector to ensure
reflection of solar energy to the target.
Other objects, aspects, and advantages of the present invention will be
apparent when the detailed description is considered with the drawings.
Briefly, the solar energy collector of the present invention includes a
transparent envelope and a heat pipe containing a heat transfer fluid, the
heat pipe includes an evaporator portion disposed within the transparent
envelope and an emergent condenser portion with a flange forming a dry
thermal interface with a manifold for conducting heat energy from the heat
pipe to the manifold. Novel reflectors for collecting solar energy are
also described for use over a wider latitude of sun angles than heretofore
considered possible with fixed apparatus. Prior to the instant invention
those skilled in the art considered that a stationary solar energy
collector would only have limited effectiveness and that it was necessary
to provide means to track the sun with the collector or in some cases with
a collector-reflector combination in order to sufficiently increase the
amount of solar energy directed to the collector.
The present invention is illustrated in the accompanying drawings, in
which:
FIG. 1 is a cross-sectional view of one embodiment of the solar energy
collector;
FIG. 2 is an end view of a building showing various possible arrangements
for mounting solar energy collector arrays thereon;
FIG. 3 is a side view in partial section of another embodiment of the solar
energy collector;
FIG. 4 is a sectional view taken along lines 4--4 of FIG. 3;
FIG. 5 is a partial top plan view of the solar energy collector of FIG. 3;
FIG. 6 is a sectional view of a solar energy collector, including a novel
gull-wing shaped reflector positioned below the transparent envelope;
FIG. 7 is a side view in partial section of another embodiment of the solar
energy collector;
FIGS. 8-16 are cross-sectional views of other embodiments for the solar
energy collector; and
FIG. 17 is an end view in partial section of a housing for an array of
solar energy collectors.
Referring to FIG. 1, the solar energy collector of the present invention is
illustrated generally at 20. The collector 20 includes a transparent
envelope or container 22, e.g., of glass or plastic, and a metal heat pipe
24 disposed therein, containing a conventional heat transfer fluid, e.g.,
acetone.
A preferred heat pipe 24 is the type disclosed in application Ser. No.
519,788, filed Oct. 21, 1974, and assigned to the same assignee as the
instant application. The heat pipe 24 extends through a closure member or
glass-to-metal seal 26 positioned at the open end of the container 22. The
heat pipe 24 includes an evaporator portion 30 and an emergent condenser
portion 32. A flattened metal member or flange 34 is affixed to the
emergent condenser portion 32 of the heat pipe 24.
The flange 34 is tightly coupled to a mating metal member 36 on a fluid
conduit or manifold 38. A fluid 39, e.g., a mixture of water and ethylene
glycol or other antifreeze mixture is continuously circulated through the
manifold 38. The flattened member 34 and mating member 36 are tightly held
together by bolts 37, see FIG. 5. However, it should be understood that
other clamps or fastening means may also be employed. The tight
metal-to-metal contact between the flattened member 34 and mating member
36 provides heat transfer which may be enhanced, as desired, by grease or
a grease-like material of high thermal conductivity, e.g., a silicone
grease.
Advantageously, the heat pipe 24 includes a heat absorbing coating 40,
e.g., black paint or a selective coating of the type described in U.S.
Pat. No. 2,917,817 (Tabor). However, it should be understood that the heat
absorbent coating is not limited to this type. The transparent container
22 is evacuated to an internal pressure of 10.sup.-4 mm. of mercury, or
less, to provide thermal insulation for substantially eliminating
convection and conduction losses from the heat absorbent coating 40. The
heat pipe 24 is positioned in the path of the incident solar radiation
generally indicated at 42.
In operation, incident solar radiation 42 is transmitted through the
transparent container 22 to the radiation absorbent coating 40 where the
radiant energy is converted into heat and conducted directly to the
evaporator portion 30 of the heat pipe 24. This heat causes evaporation of
the heat transfer fluid 25. The vapor flows toward the condenser portion
32 of the heat pipe 24 where it condenses, giving up heat to the manifold
38 through a dry thermal interface 44 between the flange 34 and mating
member 36. The condensed heat transfer fluid 25 flows by wick or by
gravity back to the evaporator portion 30 where the cycle is continuously
repeated. The heat pipe 24 and dry thermal interface 44 provide a highly
efficient heat transfer path between the radiation absorbent coating 40
and the manifold 38.
FIG. 2 illustrates various configurations for mounting an array of solar
energy collectors 20 on a building 46. When mounted on the roof 48, the
longitudinal axis of the collectors 20 may be arranged transversely to or
in the same same direction as the logitudinal axis of the roof 48. If
mounted on the exterior wall 50 of the building 46, the collectors 20 may
be oriented vertically.
Referring to FIGS. 3, 4, and 5, the collector 20A includes a heat pipe 24A
having four absorbing fins 52, 54, 56 and 58 extending outwardly
therefrom. The absorbing fins 52, 54, 56 and 58 are of substantially equal
size and extended to a position adjacent the inner wall of the transparent
container 22A. Advantageously, the absorber fins 52, 54, 56 and 58 are
coated with a heat absorbent coating like that shown in FIG. 1. A
metal-to-glass seal 26A is provided at the open end of the glass container
22A to maintain vacuum integrity and the flattened portion 34A of the
emergent end of the heat pipe 24A is mounted on the mating portion 36A of
a manifold 38A with suitable fastening means such as bolts 37.
Referring to FIG. 6, the solar energy collector 20A advantageously includes
an novel inverted elongate gull-wing shaped reflector 60 positioned
externally of the transparent container 22A and extending along its length
for reflecting incident solar energy indicated by the dash-lines 62 and
focusing it on the finned heat pipe 24A. Reflector 60 may be made of metal
or of a transparent material having reflective material embedded therein.
An advantage of the latter construction is that the reflector 60 permits
partial reflection and partial transmission. As shown in FIG. 6, the
incident solar energy 62 does not pass around the reflector 60, but due to
the novel cross-sectional shape, is redirected to the target, or heat pipe
24A. The gull-wing shape of the reflector 60 can be provided by involute
curves. The gull-wing shaped reflector 60 is generally positioned at the
center line of the angle of maximum incident energy on the heat pipe 24A,
e.g., the vertical line through fins 54 and 58. It should be understood
that the reflector 60 could also be formed of planar surfaces, hyperboloid
surfaces, paraboloid surfaces, and surfaces of revolution which intersect
at a central point or apex of the reflector 60. The reflector 60 increases
the amount of solar energy focused on the finned heat pipe 24A by
concentrating the solar energy from an area wider than the heat pipe 24A.
Specifically, the reflector 60 is shaped to direct solar energy to the
entire surface of the finned heat pipe 24A, even the surface area below
lateral fins 52 and 56 which is not in the direct path of the solar
energy. Other reflectors and their location will be described with
reference to FIGS. 9, 10, 11, 13, 14, 15 and 16. Specifically, the
reflectors may be positioned externally of the transparent container as
shown in FIGS. 6, 11 and 15, within the transparent container as shown in
FIGS. 9, 10 and 16, and between the walls of a double-wall transparent
container 22B as shown in FIGS. 13 and 14.
With specific reference to the embodiment shown in FIG. 6, the reflector 60
is bilaterally symmetrical and shaped so that nearly all of the incident
solar energy is reflected to impinge on the heat pipe 24A. Specifically,
considering the distance from the center 55 of the heat pipe 24A to the
extremity of one of the fins 52, 54, 56, or 58 as unity, the central
portion 57 of the reflector 60 is located at a distance of 1.36 units from
the center 55 of the heat pipe 24A. The outer portions 59 and 61 of the
reflector 60 are in the form of curved surfaces having a radius of 3.06
units from a common center located on the axis of symmetry a distance of
2.86 units above the central portion 57 of the reflector 60. The curved
surfaces 59 and 61 extend from the central portion to imaginary lines 2.75
units from the axis of symmetry. The center of the central portion 57 of
the reflector 60 is curved upwardly to form a cusp 63. The cusp 63 is
formed by intersecting arcs having radii of 0.6 units and centers located
0.43 units on opposite sides of the axis of symmetry 0.6 units above the
central portion 57.
Referring to FIG. 7, a double-wall transparent container 22B is shown,
including an inner wall 64 and a spaced outer wall 66 forming an annular
space 68 therebetween. The annular space 68 is evacuated and the interior
space 70 within the inner wall 64 of the transparent container 22B is not
evacuated. A closure member 26B, e.g., of cork, plastic, and the like, is
positioned at the open end of the transparent container 22B to provide
support and insulation for the heat pipe 24B.
Referring to FIG. 8, a heat pipe 24C is shown centrally positioned within a
transparent container 22C and having a pair of horizontally absorbing fins
72 and 74 of substantially equal size.
Referring to FIG. 9, a metal reflector 76 is positioned within the
transparent container 22 of the collector 20 shown in FIG. 1.
Referring to FIG. 10, another solar energy collector 20D is illustrated in
which the heat pipe 24D includes a single vertical fin 78 extending
downwardly to a position adjacent the interior surface of the transparent
container 22D. The lower half of the interior surface of the transparent
container 22D includes a reflective coating or silvering 80 to focus solar
energy incident thereon onto the heat pipe 24D or its fin 78.
Referring to FIG. 11, another solar energy collector 20E is illustrated in
which the transparent container 22E includes a reflective coating or
silvering 80E on its exterior surface. The heat pipe 24E is eccentrically
mounted within the transparent container 22E and includes a pair of
horizontal fins 82 and 84 of substantially equal size extending from a
position adjacent to the interior surface of the transparent container 22E
to the center thereof.
Referring to FIG. 12, a solar energy collector 20B having a double-wall
transparent container 22B is illustrated in which the heat pipe 24F
includes horizontal fins 86 and 88 which are off center. The fins 86 and
88 extend outwardly from the exterior surface of the heat pipe 24F to a
position adjacent the interior of the inner wall 64.
Referring to FIG. 13, a solar energy collector 20D having an eccentric
double-wall transparent container 90 is illustrated. The inner wall 92 is
eccentric relative to the outer wall 94 and the evacuated space
therebetween is sufficient to accommodate a metal reflector 96. The heat
pipe 98 includes a pair of horizontal fins 100 and 102 which are of
substantially equal size and extend to a position adjacent the interior
surface of the inside wall 92.
Referring to FIG. 14, a solar energy collector 20B having a double-walled
container 22B is illustrated with silvering 104 on the lower half of the
exterior surface of the inside wall 64. The heat pipe 106 includes three
fins 108, 110 and 112 of substantially equal size which extend outwardly
from the exterior surface of the heat pipe 106 to a position adjacent the
interior surface of the inside wall 64.
Referring to FIG. 15, a solar energy collector 20B having a double-walled
container 22B is illustrated with a gull-wing shaped reflector 60
positioned below the container 22B and the cusp 63 of the reflector 60
positioned on the vertical axis of the heat pipe 24A which corresponds to
the sun angle of maximum incident energy on the heat pipe 24A. The heat
pipe 24A is of the same configuration as that shown in FIG. 6.
Referring to FIG. 16, another double-wall transparent container 22B is
illustrated. The heat pipe 114 includes four equally spaced fins 116, 118,
120 and 122. Fins 116, 118 and 120 are of substantially equal size.
Downwardly extending fin 122 is of reduced size to accommodate a gull-wing
shaped metal reflector 60A positioned within the interior of the container
22B so that its ends contact the interior surface of the inside wall 64.
Referring to FIG. 17, an array of collectors 20A are shown housed within a
protective housing 124 having a transparent protective cover 126 which may
be made of glass, plastic, or the like. The cover 126 protects the array
from damage from the elements, e.g., hail, prevents accumulation of dirt
and debris on or around the collectors 20A, and aids in reducing heat
losses. The collectors 20A include gull-wing shaped reflectors 60;
however, it should be understood that the reflectors 60 may be eliminated,
if desired.
The aforedescribed solar energy collectors should be considered as
illustrative. Representative embodiments have been illustrated in the
drawings; however, it should be apparent that reflective coatings may be
used on the interior and exterior surfaces of the collectors, the heat
pipes may include various fin configurations, and the metal reflectors may
assume various configurations, symmetrical as well as asymmetrical, and be
positioned within or without the transparent container, as desired.
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