Note: Descriptions are shown in the official language in which they were submitted.
CONCENTRATED PHOTOVOLTAIC AND THERMAL SOLAR ENERGY COLLECTOR
[0001] CROSS-REFERENCE TO RELATED APPLICATIONS
[0002] This application claims priority per 35 U.S.C. 119(e)(1) to the
Provisional
Application No. 61/283,588, filed December 7, 2009, which is incorporated by
reference in its
entirety.
[0003] BACKGROUND OF TIIE INVENTION
[0004] Conventional solar collectors have a low energy conversion
efficiency. A typical
flat panel photovoltaic (PV) panel converts only 15-20% of the incident
radiant energy into
electricity, while a typical flat panel thermal energy collector converts
approximately 50% of the
incident radiant energy into heat. Because they do not concentrate the solar
energy, flat panel
thermal collectors are typically incapable of being used in applications where
it is desired to heat
a fluid to temperatures above 150 F. This results in a "low quality heat" as
it is referred to in the
industry. A representative flat panel device is disclosed in U.S. Patent No.
4,392,008
(hereinafter "Cullis").
[0005] Conventional flat panel solar collectors are also expensive,
primarily because they
contain a large number of silicon solar cells. A typical PV panel producing
approximately 250W
of electrical power contains approximately 20 square feet of silicon solar
cells, which require
solar grade silicon (e.g., 6N purity). Those cells are the most expensive
component of the typical
solar panel, even if the most inexpensive form of silicon suitable for solar
panel use.
[0006] Because of their low efficiency and corresponding need for
increased size,
conventional solar collectors are typically large and heavy. This reduces
their mounting options,
or increases the expense and flexibility of mounting. This leaves the user
limited in ability to use
an optimum number of solar cells and limited in the ability to optimally
locate the solar collector.
[0007] These disadvantages have led to a variety of attempted solutions
involving
concentrating the radiant solar energy. For example, Hines, et al. disclose
concentrating
"modules having a convenient size and market acceptance of traditional flat
photovoltaic solar
panels." Pub. No. U.S. 2007/0193620 Al. A lightweight, low-cost concentrating
solar energy
collector is disclosed by Hochberg and Costen (Fig. 1) that employs a
parabolic reflector in a
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cylindrical housing. U.S. Pat. No. 6,994,082. Gilbert discloses a low
concentrating photovoltaic
thermal solar collector employing "at least one elongated cross-sectionally V-
shape beam, a first
and second sunray light reflecting surfaces integral to the respective
interior faces of the V-shape
beam side legs." Pub. No. U.S. 2010/0282315. But a disadvantage of such
concentrating
systems is that concentrated photovoltaic "operates most effectively in sunny
weather since
clouds and overcast conditions create diffuse light, which essentially cannot
be concentrated."
U.S. Pub. No. U.S. 2010/0282315.
[0008] By way of background, United States Patent No. 6,111,190 discloses
a Fresnel
lens solar concentrator made of light weight materials that can be used in
space. United States
Patent No. 6,075,200 discloses a stretched Fresnel lens solar concentrator for
use in space.
United States Patent No. 6,031,179 discloses a color-mixing lens for solar
concentrator systems
that increases power output by chromatically dispersing light. United States
Patent No.
5,505,789 discloses a photovoltaic module using low-cost materials for high
performance using
an array of arched Fresnel lenses. United States Patent No. 5,498,297
discloses a photovoltaic
receiver with a PV cell coupled to a heat sink using a Tefzel film. United
States Patent No.
4,719,904 discloses a solar thermal receiver designed to minimize heat loss.
United States Patent
No. 4,711,972 discloses a PV cell for use with an optical concentrator. United
States Patent No.
4,672,949 discloses another solar energy collector designed to minimize heat
loss. United States
Patent No. 4,545,366 discloses a bi-focused solar energy concentrator. United
States Patent No.
6,990,830 discloses a system and method for supplying consumers with heat
energy or cooling
energy. United States Published Application No. 20010013207A1 discloses a
passive
collimating tubular skylight for collecting radiant energy. W02007109901A1
discloses a
support structure for a solar collector system. W02007103300A1 discloses a
solar collector with
a trough-like reflector and an absorber for receiving solar radiation.
W02007109899A1
discloses an energy supply system using a thermal storage container and one or
more solar
collectors for use therewith. W005090873A1 discloses a solar collector with a
linear reflector
and an absorber spaced from the reflector for receiving solar radiation and
conveying heat
therefrom to a fluid. United States Patent No. 4,224,082 discloses a multi-
functional solar
collector pole. United States Patent No. 4,323,052 discloses a solar energy
system. United
States Patent No. 4,392,008 discloses a combined electrical and thermal solar
collector. United
States Patent No. 4,491,681 discloses a liquid cooled, linear focus solar cell
for use with
parabolic or Fresnel optical concentrators. United States Patent No. 4,700,013
discloses a hybrid
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PV and solar heat energy system and concentrator. United States Patent No.
4,892,593 discloses
a solar energy collector using light funneling, a Fresnel lens, and a PV
panel. European
Published Application No. EP0384056A1 discloses solar collectors that generate
both thermal
and electric energy.
[0009] Thus, there presently exists the need for a solar energy collector
with increased
efficiency, a reduced need for silicon cells, a construction that increases
mounting options by
reducing weight and bulk, and improved performance in overcast conditions.
Furthermore, solar
panels are subject to failure, most commonly due to water damage to the panel.
This causes
added expense, loss of efficiency, and mounting limitations as traditionally
it is recommended to
not mount panels horizontally to avoid water collection. Water damage is also
caused by
commonly experienced environmental conditions, such as high humidity, rain,
and condensation,
making the use of solar cells in such environments challenging. There exists a
need to reduce
solar cell failure and provide an option that can minimize the challenge of
using solar energy
collectors in such environments.
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[0010] BRIEF SUMMARY OF PREFERRED EMBODIMENTS
[0011] In one preferred embodiment, the solar concentrator described herein
converts
incident solar energy into both heat and electricity. Summing both thermal and
electrical energy
outputs, the conversion efficiency of an embodiment of the present invention
is approximately
80%. Furthermore, a preferred embodiment of the present invention uses only
one-twentieth the
area of silicon cells to produce the same amount of electrical energy as a
conventional solar
panel, greatly reducing the material cost. This is accomplished by
concentrating approximately
twenty square feet of incident energy onto an approximately one square foot
photovoltaic cell
using a parabolic trough reflector and combining the reflector's photovoltaic
target and thermal
target into one device. In addition, according to a preferred embodiment, by
combining the
reflector's photovoltaic target and thermal target into one device and
integrating structural
elements of the PV and thermal solar energy collector, the user gains a
significant advantage in
flexibility of mounting location and positioning options, which leads to
increased efficiency in
use due to the ability to optimize location. The collectors herein are highly
modular in nature,
allowing for flexibility in design and utility in positioning, such as on a
rooftop where space is
available and obstructions are minimized. For example, a four unit
concentrator may be
designed to produce 1000 W output. If additional power output is preferred or
there is a desire to
supplement power output, additional units may be combined to meet the needs of
the application.
In a preferred embodiment, the geometry is aerodynamic in design to limit wind
resistance and
minimize the need to use high strength materials to compensate for
environmental stresses.
[0012] Furthermore, according to a preferred embodiment, by combining the
photovoltaic target and thermal target into one device, a receiver for
example, encasing that
device in a transparent tube, and evacuating the atmosphere from that tube,
the photovoltaic
target is isolated from moisture and other detrimental environmental elements,
and thermal
losses due to convection are reduced.
[0013] BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING
[0014] The foregoing and other aspects and advantages of the embodiments
described
herein will be better understood from the following detailed descriptions of
particular
embodiments with reference to the drawings.
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[0015] Fig. 1 is a block diagram of an embodiment of the solar collector
system
described herein;
[0016] Fig. 2 is a block diagram of another embodiment of the solar
collector system
described herein;
[0017] Fig. 3 is a drawing of the covered tube assembly of the solar
collector in Fig. 1;
[0018] Fig. 4a is a cross-section of the solar collector along line A¨A
of Fig. 1;
[0019] Fig. 4b is a cross-section of another embodiment of the solar
collector along line
A¨A of Fig. 1;
[0020] Fig. 5 depicts the solar collector in Fig. 1;
[0021] Fig. 6 depicts part of the solar collector in Fig. 5;
[0022] Fig. 7 is a drawing of a roof mounting bracket for the solar
collector embodiments
exemplified herein.
[0023] Figs. 8a ¨ 8i are cross-sectional views of stages of the
manufacture of solar
collector embodiments exemplified herein.
[0023A] Figs. 9a-9b are charts showing comparisons of collector's performance.
[0024] Like reference numerals refer to corresponding elements throughout
the several
drawings.
[0025] DETAILED DESCRIPTION
[0026] The embodiments herein preferably use a reflective or mirror
surface formed in a
parabolic trough, such that the reflective surface directs solar radiation
from the sun to a receiver
or set of receivers suspended above the reflective surface. The embodiments
herein are designed
to produce both electricity and thermal energy.
[0027] The receiver or receivers have solar cells, preferably on the
underside, such that
the solar cells or photovoltaics produce electricity. Solar cells operate more
efficiently when
cooled, In a preferred embodiment, a cooling fluid flows through the back of
the solar cells to
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extract heat from the solar cells and regulate their internal cell
temperature. The cooling fluid
removes heat in the form of heated fluid to facilitate a simultaneous dual
output: electricity and
hot fluid.
[0028] Due to the shape of the mirror or mirrored surface, the solar energy
is focused to a
point, a focal point where the solar energy is concentrated. One advantage is
that the silicon that
makes up the solar cells is relatively expensive while the mirrored surface
material is relatively
less expensive, such that if the mirrored surface is maximized and the silicon
is minimized, the
cost per unit of output power is minimized.
[0029] The solar cells used in the preferred embodiments operate at a
concentration of
from about 10 to about 100 times, more preferably from about 20 to about 50
times, more
preferably from about 25 to about 35 times.
[0030] In a preferred embodiment the system operates under substantially
direct sunlight.
Because the trough focuses the light to a point it is preferred to track the
sun as the sun moves
across the sky. It is also preferred to adjust the vertical orientation based
on the season to
maximize solar input. In a preferred embodiment, the tracking system uses a
microprocessor
which has an algorithm that knows exactly where the sun is at any time of any
day of the year
and determines the most effective positioning of the solar concentrator.
[0031] An example of one embodiment is a solar energy generating system
that has a
solar energy collector. The solar energy collector optionally has a cover in a
preferred
embodiment that provides protection from the elements and insulates from heat
loss. The solar
energy collector is constructed using one or more concentrating reflectors,
one or more
photovoltaic cells, one or more ribs to provide structural integrity, and a
photovoltaic cell
mounting structure, preferably in contact with a heat receiving and conveying
medium. The
conveying medium is a working fluid, such as a mixture of water and
antifreeze. The heat
receiving medium may preferably be a phase changing material at the preferred
temperature,
such as a wax or the like. An example of such a solid is wax with a melt
temperature from about
115 F to about 185 F, depending upon the preferred application. Where the
target use is
conventional heating, a lower temperature range of about 115 F to about 140 F
may be used.
For air conditioning applications, a higher temperature range of about 170 F
to about 185 F may
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be preferred. Where both such uses are the goal, an intermediate temperature
of from about
140 F to about 170 F. Where a phase change material is used, the temperature
is more easily
regulated and a greater amount of heat may be absorbed, making such a material
the preferred
heat receiving medium.
[0032] Where the embodiment has a cover, the cover is translucent or
transparent to solar
radiation.
[0033] In operation, the concentrating reflector directs concentrated solar
radiation onto
the one or more photovoltaic cells, which convert part of said concentrated
solar radiation into
electrical energy and, in a preferred embodiment, a larger part of said
concentrated solar
radiation into thermal energy. The one or more photovoltaic cells conduct the
thermal energy to
the photovoltaic cell mounting structure and the cell mounting structure
conducts said thermal
energy into the heat absorbing material, such as a working fluid which can
transfer the absorbed
heat to a number of mediums, such as a phase changing solid or other heat
absorbing means.
[0034] Structurally, in a preferred embodiment, the cover, reflector, and
structural ribs
are integrally connected together such that they support said solar energy
collector. The reflector
surface can be made of a number of materials that act as mirrored surfaces.
The structural ribs
may be translucent, transparent or reflective in some embodiments. In other
embodiments the
structural ribs are formed in a perimeter with no material in the middle
portion to allow sunlight
to directly contact a maximum surface area of the reflector and reflect a
maximum amount of the
sunlight to the receiver.
[0035] In a preferred embodiment, the solar energy system is mounted with a
tilt
mechanism that provides the ability to tilt said solar energy collector on a
vertical plane. This
compensates for the incident angle of sunlight during different times of year
based on the
trajectory of the sun. In addition, a preferred embodiment has a rotating
mechanism to provide
the ability to rotate the solar energy collector to increase, optimize or
maximize the incident light
throughout the course of the day.
[0036] In a preferred embodiment, the solar energy system uses a control
module in
communication with the tilt mechanism wherein the control module directs the
tilt mechanism to
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tilt said solar energy collector to a specified tilt angle. The control module
preferably determines
the specified tilt angle using a latitude and longitude associated with the
solar energy collector,
and a date and time said solar energy collector is to be tilted. In a
preferred embodiment, the
control module of the solar energy system receives environmental data and
determines when to
place said solar energy collector in a protected orientation based on the
environmental data. The
environmental data may be vibrational data. Excessive vibration may be caused
by buffeting of
the collector in high winds of a storm. In such a case, vibration data could
cause the control
module to turn the collector face down to protect it from wind born debris,
for example. The
environmental data may also be a measure of solar radiation incident on the
solar collector.
[0037] In an embodiment, the control module receives operating data from the
solar
collector and adjusts said specified direction based on that operating data.
The operating data
may be voltage data from the photovoltaic cell and said specified tilt angle
and rotation may be
chosen to maximize the voltage data. The operating data may also be
temperature data from the
solar collector and the specified tilt angle may be selected to optimize said
temperature or reduce
the temperature data if the system is overheating. The operating data may also
be a fluid flow
rate and the specified tilt angle is chosen to position the solar collector in
a protected orientation.
One embodiment of the solar energy systems herein uses a switch that causes
the control module
to position the solar collector in a protected orientation.
[0038] In a preferred embodiment the solar energy system uses a transparent or
translucent covered tube and two end caps positioned around the cell mounting
structure and the
one or more photovoltaic cells. The transparent covered tube, end caps, and
cell mounting
structure create an airtight volume about said photovoltaic cell.
[0039] In one embodiment, the solar energy system uses a cooling system that
contains a
heat absorbing media, such as wax, and a working fluid, such as a fluid
mixture of water and
glycol to move the heat which can be stored in said heat absorbing media,
which is preferentially
a phase changing media to regulate temperature and maximize heat storage. In a
preferred
embodiment, the solar energy system the covered tube is airtight and is
evacuated of air to
decrease the convection of thermal energy away from the cell mounting
structure and the one or
more photovoltaic cells. The solar energy system may also use a reflective
coating applied to an
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inner portion of the transparent covered tube, such that the reflective
coating reflects the
concentrated solar radiation toward the cell mounting structure.
[0040] In another embodiment, the solar energy system also may contain a
plurality of
cell mounting structures and a plurality of photovoltaic cells, wherein the
plurality of cell
mounting structures are positioned linearly and the structures may also
contain bores axially
aligned with one or more fluid tubes running through the bore for the length
of the aligned
plurality of cell mounting structures. In an alternative structure, a phase
changing medium is
used in place of the fluid. In another alternative, the fluid tubes are in
contact with the mounting
structures to cool the photovoltaic cells or to absorb and collect heat from
the collector.
[0041] The solar energy system embodiments preferably uses a reflector that
concentrates solar radiation onto an area of the plurality of photovoltaic
cells that is from about
one tenth to one hundredth of the surface area of the reflector thereby
creating about a 10X to
about a 100X concentration of solar radiation, preferably from about one
twentieth to about one
fortieth, more preferably from about one twenty-fifth to about one thirty-
fifth, most preferably
about one thirtieth or about one twentieth.
[0042] In one embodiment, the plurality of photovoltaic cells comprise single
junction
silicon solar cells, with spacings of less than 100 microns between the P+ and
N+ regions in said
single junction silicon solar cells, to allow linear operation of the single
junction solar cells at
about a 20X to 30X concentration of solar radiation. Operation at
concentration ranges above
about 10X requires such small spacing. The solar energy system may have a
plurality of
photovoltaic cells that comprise single junction silicon solar cells with nano-
structures, such as
nano-tube structures, between the P+ and N+ regions to allow linear operation
of said single
junction solar cells at the particular concentrations of solar radiation.
[0043] In another embodiment, the plurality of photovoltaic cells comprises
multi-
junction gallium arsenide (GaAs) photovoltaic cells, such as those available
from Spectrolab, Inc.
Though they are more expensive, GaAs cells have efficiencies that can exceed
40%, significantly
higher than the single junction silicon solar cell. A still further embodiment
employs photocells
with gallium indium phosphide (GaInP). GaInP photovoltaic cells have
efficiencies higher than
silicon cells, such as boron-doped Czochralski (CZ) silicon wafers or floating
zone (FZ) doped
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wafers. Other alternatives include gallium-doped CZ (Ga:CZ), magnetically
grown CZ (MCZ),
and boron-doped FZ silicon (B:FZ). All three of these materials have been
shown to exhibit no
measurable degradation after light exposure (LID) and result in a longer
efficient life cell.
[0044] Alternative efficiency techniques may be employed with the embodiments
herein,
such as the use of preferred textures such as random pyramid textured surface
to reduce
reflection, the use of an antireflective coating such as a silicon nitride
antiretlection coating,
narrow laser-machined line width may be used to minimize shading, high
conductivity copper
metallization may be used as well.
[0045] A preferred embodiment herein includes a method of manufacturing a
solar
energy collector for later assembly. The manufacturing includes creating a
cover, a reflector, a
plurality of ribs, and a covered tube assembly. The parts may be assembled
before or after
shipping. If, for example, the parts are assembled after shipping, the
reflector may be positioned
about said ribs to form a concentrating reflector. The ribs may be positioned
about the covered
tube assembly to place the covered tube assembly at a focal point of the
concentrating reflector,
and said cover may be positioned about the covered tube assembly at a position
determined by
the concentrating reflector.
[0046] Referring now to Figs. 1-6, which show preferred embodiments of a
concentrating photovoltaic and thermal solar energy collector, the following
describes such
figures in further detail. In Fig. 1, a block diagram shows solar collector 1
held by frame 33 and
oriented to receive solar radiation. Cool fluid supply 2 is connected to solar
collector 1 to direct
fluid through covered tube assembly 3. Reflector 4 directs solar radiation
onto covered tube
assembly 3. Reflector 4 particularly directs solar radiation onto photovoltaic
cell 5 (see Fig. 3),
an element of covered tube fluid assembly 3. Covered tube assembly 3 absorbs
and transfers
part of the energy from the solar radiation into the fluid supplied by cool
fluid supply 2.
Warmed fluid exits covered tube assembly 3 into warm fluid return 6. Warm
fluid return 6
directs warmed fluid to devices (not shown) utilizing warm fluid, such as
radiators, storage tanks,
or other devices known to those of skill in the art. Covered tube assembly 3,
by way of
photovoltaic cell 5, also converts part of the solar radiation into electrical
energy. Covered tube
assembly 3 outputs DC electrical power via power leads 7.
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[0047] Referring to Fig. 2, in an embodiment power leads 7 are connected to DC
¨ AC
inverter 8, which outputs AC electrical power. Rotation actuator and sensor 9
connects solar
collector 1 to base 12 in a manner allowing rotation actuator and sensor 9 to
rotate solar collector
1 about an axis parallel to that of covered tube assembly 3. This rotating
allows rotation actuator
and sensor 9 to position solar collector 1 to track at least a portion of the
daily movement of the
sun across the sky. Tilt actuator and sensor 11 connect solar collector 1 to
base 12 in a manner
allowing tilt actuator and sensor 11 to pivot solar collector 1 about an axis
perpendicular to that
of covered tube assembly 3. This pivoting allows tilt actuator and sensor 11
to position solar
collector 1 for seasonal tracking of the sun.
[0048] Cool fluid supply 2 is equipped with input temperature sensor 14 and
fluid flow
sensor 15. Warm fluid return 6 is equipped with output temperature sensor 16.
Power leads 7
are equipped with DC voltage sensor 17 and DC current sensor 18. Tilt actuator
and sensor 11,
rotation actuator and sensor 9, input temperature sensor 14, fluid flow sensor
15, output
temperature sensor 16, DC voltage sensor 17, and DC current sensor 18 are
placed in
communication with control and interface module 13. Control and interface
module 13 regulates
fluid flow to maintain fluid temperature in a design range of 150 F ¨ 175 F.
In an embodiment,
control and interface module 13 is in communciation with personal computer 19
via USB cable
20. One of skill in the art would understand that the communications between
the sensors and
control and interface module 13, and between interface module 13 and computer
19, could be
performed wirelessly.
[0049] Fig. 3 depicts one end of covered tube assembly 3 and the side that
receives solar
radiation from reflector 4. An electrically insulating heat conducting
elastomeric material may
be used to mount the PV to the heat sink. Photovoltaic cell 5 is mounted onto
cell mounting
structure 21. Core fluid tube 22 passes through end cap 23 via bore 28 (see
Fig. 4a). A thermal
connection between core fluid tube 22 and cell mounting structure 21 is made
by minimizing the
space between the two, and filling what space remains with thermally
conductive grease (not
shown) or an electrically insulating heat conducting elastomer material (not
shown). Cover tube
24 is transparent to solar radiation and surrounds photovoltaic cell 5, cell
mounting structure 21,
and core fluid tube 22. Cover tube 24 slides into and seals against end cap
23. Power leads 7
pass through end cap 23 via lead spout 25.
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[0050] Covered tube assembly 3 has an end cap 23 at each end.
End caps 23 are made of
aluminum and join cover tube 24 and core fluid tube 22 to create an air tight
seal. Though other
materials, including molded plastic, can be used for end cap 3 care must be
taken to match the
chosen material's thermal expansion coefficients with those of cover tube 24
and core fluid tube
22. In the preferred embodiment, matching and sealing is done using "0" rings
(not shown)
made of ethylene propylene diene rubber (epdm) or ethylene-propylene rubbers
(EPR).
Alternatively, the sealing may be accomplished by metalizing a portion of
cover tube 24 and
soldering end cap 23 to it, or by using a melted glass frit for the bond. In
one embodiment one
end cap 23 can be removable while the other is permanently bonded. This allows
disassembly of
covered tube assembly 3 for maintenance or upgrade. One end cap 23 provides
lead spout 25,
for power leads 7 (see Fig. 1). Preferably, lead spout 25 is sealed after
power leads 7 are routed
through it. One end cap 23 provides a tube and valve (not shown) for
evacuation. Alternatively
the tube and valve could be replaced by a copper tube that can be cold welded
post-evacuation,
as is commonly used in refrigeration systems.
[0051] Fig. 4a is a cross-section of solar collector 1 along
lines A ¨ A of Fig. 1. Covered
tube assembly 3 is positioned with photovoltaic cell 5 near the focal point of
reflector 4 and held
in place by reflector ribs 31 (see Fig. 5). Front cover 27 is transparent to
solar radiation and is
connected to reflector 4, with covered tube assembly 3 contained in the
created space. Incident
solar radiation passes through front cover 27. Part of that incident radiation
also passes through
=
cover tube 24 to strike cell mounting structure 21. Cell mounting structure 21
absorbs and
transforms much of this radiation into thermal energy. The thermal energy is
conducted
throughout cell mounting structure 21 to core fluid tube 22, which is in
thermal contact with cell
mounting structure 21, and contributes to warming fluid from cool fluid supply
2 (Fig. 1).
Radiation not immediately passing through cover tube 24 continues and reflects
off reflector 4, is
concentrated by the focusing shape of reflector 4, passes through cover tube
24, and strikes
photovoltaic cell 5. Photovoltaic cell 5 converts the solar radiation into
electrical energy
according to its efficiency, absorbs much of the remaining solar radiation as
thermal energy, and
conducts it to cell mounting structure 21, core fluid tube 22, and the fluid
inside.
[0052] In a preferred embodiment, core fluid tube 22 is a
single copper tube,
approximately 8' long and 1/2" nominal ID. External pumps (not shown) pump
fluid to be heated
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through core fluid tube 22. By confining core fluid tube 22 to a single copper
tube for the length
of solar collector 1, fluid leaks inside the collector are minimized.
Fabricating core fluid tube 22
from copper provides a long life, though other materials may be suitable
including stainless steel.
Materials prone to corrosion should be avoided. Also, in this embodiment cell
mounting
structure 21 is an aluminum extrusion with bore 28 through it. Core fluid tube
22 is slid inside
bore 28 to provide a thermal path between cell mounting structure 21 and the
working fluid from
cool fluid supply 2, which is water and anti-freeze fluid in a preferred
embodiment. Cell
mounting structure 21 provides a flat mounting surface for photovoltaic cell
5.
[0053] In this embodiment, some solar radiation is converted into electricity.
Much more
is captured as heat and transferred to the working fluid. Transferring heat to
the fluid generates
heat output from collector assembly 1. Removing heat from cell mounting
structure 21 lowers
the temperature experienced by photovoltaic cell 5, which makes it more
efficient. In a preferred
embodiment, the thermal connection between cell mounting structure 21 and core
fluid tube 22 is
augmented by thermal conductive grease or paste (not shown). Alternatively
cell mounting
structure 21 could be press fit or crimped onto the core fluid tube 22. In
addition to extruding
cell mounting structure 21 from aluminum, other metals such as copper or any
good heat
conductor could be used. Alternately, structure 21 could be molded using a
good thermal
conductivity plastic.
[0054] In one embodiment, covered tube assembly 3 eliminates core fluid tube
22. Fluid
from cool fluid supply 2 flows to warm fluid return 6 through bore 28 in cell
mounting structure
21.
[0055] One feature of the present invention is that the working fluid is
heated by
concentrated solar radiation from reflector 4. Some radiation is converted
into electricity while
much more is captured as heat and transferred to the working fluid. This
allows the working
fluid to reach and be maintained at 150-175 F. This high temperature is
referred to in the
industry as a "high quality" heat. Such temperatures are not achievable in
thermal collectors that
do not concentrate the solar rays, such as that disclosed by Cullis. Solar
concentration at or
above 20X is required to achieve fluid temperatures considered to have "high
quality".
Concentration above 10X requires modifications to the PV solar cells such as
smaller junction
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spacing as discussed above. Concentration above about 50X requires even
further and more
costly modification of the PV solar cells. Thus, concentration of optimally
20X provides
significantly high enough fluid temperature for the collected thermal energy
to be considered
"high quality" and can be accommodated by only minor modifications to the PV
solar cell,
keeping costs of generating electrical energy low.
[0056] The preferred embodiment uses eight cell mounting structures 21, each 1
foot
long, in solar collector 1. This facilitates mounting photovoltaic cells 5.
One would understand
that longer cell mounting structure 21 sections could be used up to the length
of solar collector 1.
This would facilitate eliminating core fluid tube 22. Cell mounting structure
21 needs to have
good heat transfer properties. It should also allow for the differing thermal
expansion properties
of photovoltaic cell 5 and the material comprising cell mounting structure 21.
In the preferred
embodiment, this is achieved using a flexible high temperature conductive
adhesive. Alternate
bonding process could involve press fitting or bendable tabs to secure the
photovoltaic cells.
Matching the thermal expansion of the photovoltaic cell 5 and the material
comprising cell
mounting structure 21 could also be done ¨ allowing the use of a rigid bond
between the two.
[0057] Reflector 4 is a thin piece of polished stainless steel sheet metal
bent and held in a
parabolic shape focusing the incident radiation onto covered tube assembly 3
and photovoltaic
cells 5 and cell mounting structure 21 housed within. Reflector 4 can also be
made of other
materials such as aluminum or plastics. The preferred embodiment incorporates
reflector film 30
bonded to the reflector using pressure sensitive adhesive to produce a highly
reflective surface at
low cost. This is preferable to polishing the surface of reflector 4 itself.
The particular film used
in the preferred embodiment is: ReflecTech Mirror FilmTM.
[0058] In the preferred embodiment cover tube 24 is clear all the way around.
Concentrated radiant energy from reflector 4 enters from approximately the
half of cover tube 24
nearest reflector 4. In one embodiment, depicted in Fig. 4b a reflective
coating 29 or physical
reflector is added to the half of cover tube 24 that is opposite from
reflector 4. Reflective coating
29 reflects concentrated solar radiation back onto cell mounting structure 21.
[0059] Referring to Fig. 5, assembly of the preferred embodiment involves
sliding cell
mounting structures 21, eight of them with photovoltaic cells 5 attached, over
core fluid tube 22.
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Cell mounting structures 21 arc secured in place with set screws and good
thermal contact is
insured with a conductive paste or grease (not shown). Once all cell mounting
structures are in
place, electrical connections are made that place photovoltaic cells 5 in
electrical series (not
shown). Core fluid tube 22 is used as the conductor returning the connection
from one end
photovoltaic cell 5 to the opposite end's end cap 23, so that both electrical
contacts can be made
from the same side of covered tube assembly 3. Multiple solar collectors 1 can
be connected in a
system, preferably with each supplied with a DC-AC inverter 8 (see Fig. 2,
alternatively this
could be a DC-DC converter). This eliminates the risk of shaded collectors 1
shunting current
from fully illuminated units. In an embodiment, such inverters are added to
each individual cell
mounting structure 21 to improve performance.
[0060] Photovoltaic cell 5 is the element that converts incident radiation
into electricity.
In the preferred embodiment, photovoltaic cell 5 is operating with a 20x or
higher concentration
of the incident radiation. In the industry this is referred to as a medium
concentration. High
concentrations are on the order of 100 ¨ 1000X. In the preferred embodiment,
photovoltaic cell
is a single junction silicon solar cells because they are the most cost
effective. Other
technology cells can be used, such as GaAs, Ga-doped silicon or other
materials discussed herein
or multi junction technologies, each having a particular cost ¨ performance
trade off.
[0061] Typical single junction silicon solar cells made of medium resistivity
material do
not operate well at a 20X concentration ¨ they work better up to approximately
5X. Above 5X
they are said to become non-linear. Their output current drops as the incident
energy
concentration rises in a phenomenon that resembles increasing internal shunt
resistance. The
physical limitations are traceable in part to the rise of the minority carrier
recombination in the
PN junction and the physical resistance of the electrical contacts.
[0062] Still regarding Fig. 5, in the preferred embodiment photovoltaic cells
5, cell
mounting structures 21 (see Fig. 4a), and core fluid tube 22 all contained
within cover tube 24.
Cover tube 24 is made of BoroSilicate Glass, which is transmissive of the
terrestrial solar spectra,
sustains high temperatures, and is strong. Aluminum adapter plates (not shown)
are placed
between the cell mounting structures 21 (see Fig. 4a) and around core fluid
tube 22 to maintain
the position of core fluid tube 22 within the cover tube 24. The aluminum
adapter plates could
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be of another material, but care must be taken in selecting the material for
the elements that are
within cover tube 24 because they are also in the 20X intensified beam of
incident energy and
will get hot. Alternately the function of the adapter plate could be
incorporated into cell
mounting structure 21, eliminating the need for these adapter plates.
[0063] To improve the overall thermal efficiency of solar collector 1 the air
space inside
covered tube assembly 3 is evacuated. This minimizes convective losses and
maximizes the heat
flowing into the fluid in core fluid tube 22. In the preferred embodiment, the
evacuation is
carried out to "roughing pump" levels, typically 10-3 mmHg absolute pressure.
Alternately
higher vacuum levels could be achieved using high vacuum pumps or getters or a
combination of
both. Higher vacuum will lead to even less convective heat loss. In one
embodiment covered
tube assembly 3 is filled with a gas having a lower thermal conductivity than
air, which increases
the thermal efficiency without the need for creating and maintaining a high
vacuum.
[0064] Ribs 31 and end ribs 32 form a mounting structure for reflector 4 that
holds it in
the proper parabolic shape. Each rib 31 and end rib 32 is fabricated from
aluminum sheet,
approximately 1/8" thick and incorporates features for securely attaching to
reflector 4 and to
front cover 27. Slot 35 is created in ribs 31 to receive cover tube 34 (see
Fig. 5). At each end of
solar collector 1, end rib 32 includes features for securing to each end cap
23 and this holds
covered tube assembly 3. End ribs 32 provide features for mounting the entire
collector in a
frame 33. End ribs 32 also provide mounting for the rotation bearings (not
shown) of the system.
Alternate manufacturing techniques could be employed such as a molded or
extruded metal or
plastic assembly incorporating the reflector and rib structures.
[0065] Together ribs 31, end ribs 32, reflector 4, and front cover 27
integrate to form the
supporting structure of solar collector 1. Now regarding Fig. 6, strategically
placed fastening
points 33 between reflector 4 and ribs 31 and or end ribs 32, and fastening
points (not shown)
between front cover 27 and ribs 31 and or end ribs 32, provide structural
integrity. The
aluminum used for ribs 31 and end ribs 32 is formed to be a 95% reflection.
The preferred
embodiment uses poprivets at points 33 to fasten reflector 4 to ribs 31 or end
ribs 32, and
grommet screws to fasten front cover 27 to ribs 31 or end ribs 32. Alternate
fastening means
include welding, crimping, or adhesives. The structural integrity achieved by
integrating
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together ribs 31 and end ribs 32, reflector 4, and front cover 27 allows solar
collector 1 to be
fabricated in a manner that minimizes its weight and bulk. This greatly
expands its mounting
options, particularly making it available for non-industrial installations.
[0066] Assembled, the preferred embodiment of solar collector 1 is 30 inches
wide and
94 inches long, with a collection area of 2,820 square inches and a focal
distance of one foot. In
the orientation shown in Fig. 5, the combined height of frame 33 and solar
collector 1 is 24".
And solar collector 1 employs 32 photovoltaic cells 5, each 1.3" wide, on the
eight cell mounting
structures 21.
[0067] In the preferred embodiment front cover 27 is clear and without lens
features.
Front cover 27 protects covered tube assembly 3 and reflector 4 from dust,
rain, and damage.
Front cover 27 is structurally attached to both ribs 31 and end ribs 32, and
reflector 4, and is
made of clear polycarbonate approximately 1/8" thick. Polycarbonate is a ultra-
violet ("UV")
stabilized material. Care must be taken in material selection because of the
long term UV
exposure and structural aspects. Typically UV stabilized polycarbonate has an
"in sun" lifetime
of 10-15 years. Photovoltaic cells 5 and other system components may last 15-
25 years. Front
cover 27 may be replaceable to allow solar collector 1 a longer service life.
And to maintain the
efficient transmission of solar radiation, front cover 27 may be regularly
cleaned or equipped
with a disposable transparent sheet (not shown).
[0068] Returning to Fig. 2, concentrating collection systems perform optimally
when
equipped with rotational tracking that keeps the system directed toward the
sun's rays. In a
preferred embodiment tilt actuator and sensor 11 may use a worm gear drive
mechanism (not
shown) with large driven gear (not shown) attached to end rib 32 (see Fig. 5).
The small worm
gear (not shown) is mounted tangent to the large driven gear. A large
reduction ratio is used
allowing a small 12v DC electric motor (not shown) to effect the motion. The
large reduction
ratio also provides resistance to wind pressures and prevents the panel form
moving
inadvertently. Alternate embodiments use a chain or belt drive mechanism, or
linear drive
mechanisms acting on a tangent to the arc of motion. In the preferred
embodiment the rotation
range is much wider than that required for simply tracking the sun. The
rotation range is great
enough to allow solar collector 1 to be positioned with front cover 27 facing
"down," and
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protected by reflector 4 from the elements. This might allow the system to
survive storms, or
prevent over-heating that could damage front cover 27 or covered tube assembly
3, or simply
extend system life by protecting front cover 27 at night. In one embodiment,
such a "down" or
safe position is made to further protect solar collector 1 by adding a parking
structure (not shown)
to frame 33 and "nesting" solar collector 1 into the parking structure.
[0069] Tilt tracking provides optimal solar collector 1 alignment throughout
the year.
The range of tilt is less than the range of rotation. Tilt tracking is
sometimes referred to as
second axis tracking. In the preferred embodiment the tilt actuator and sensor
11 (Fig. 2) uses a
rack and pinion drive (not shown). Alternatively, a linear drive mechanism
could be used.
[0070] One function of control and interface module 13 is to maintain a
desired solar
collector 1 alignment using the rotation actuator and sensor 9 and tilt
actuator and sensor 11.
Once solar collector 1 has been installed, the proper tilt and rotation for a
given time and place
can be computed. No feedback is required. Based on the date and time of day
control and
interface module 13 adjusts the tilt and rotation for optimal alignment of
solar collector 1 with
the sun's rays. Local features may shade solar collector 1 and in an
embodiment a feedback loop
based on "peaking" the power output is employed to position solar collector 1
at the tilt and
rotation that provides the peak power output.
[0071] Another function of control and interface module 13 is to protect solar
collector 1.
Solar collector 1 may be damaged if photovoltaic cells 5 are overheated. This
could occur if the
fluid flow within core fluid tube 22 were interrupted. With input from fluid
flow sensor 15
indicating reduced flow, control and interface module 13 can adjust the tilt
or rotation to move
solar collector 1 away from optimal alignment with the sun's rays, thus
protecting photovoltaic
cells 5 from damage. In an embodiment, temperature sensors (not shown) are
incorporated in or
on cell mounting structure 21 that input to control and interface module 13,
which determines the
operating temperature of photovoltaic cells 5 and adjusts tilt and rotation of
solar collector 1 as
needed. Control and interface module 13 is also programmed with sunrise and
sunset
calculations that cause it to rotate the solar collector "down" at night ¨
with front cover 27
beneath reflector 4. And, in case of a potentially damaging storm, a switch
(not shown) allows
an operator to cause control and interface module 13 to tilt the solar
collector 1 "down."
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Alternatively a motion sensor such as a multi-axis accelerometer can be fixed
to solar collector 1
to indicate vibrations, such as those caused by high winds. In such a
situation, should the
vibrations exceed a set threshold, control and interface module 13 could
automatically park the
Solar Collector in the "down" position.
[0072] Fig. 6 shows part of the solar collector of Fig. 5, featuring fastening
points 34 and
frame 33.
[0073] Fig. 7 shows a roof mounting bracket useful with the solar collector
embodiments
described herein and particularly useful for mounting efficiently on a surface
to permit full range
of motion both in the rotational axis and the tilt axis. As shown on Fig. 7,
750 is the mount pipe,
751 is the bracket plate, 752 holes in pipe and plate, 753 holes in plate, and
754 is the seal boot.
The collector frame can be mounted by using a pipe of standard diameter size
flattened on one
end and drilled with several holes so it can be fastened to the roof joist
under the roof. The round
end of the pipe will protrude thru the roof and act as a mount point for a
tilt pivot elbow for the
collector frame assembly. A standard plumbing vent pipe roof seal boot will be
used to at all four
protruding pipes to seal the pipe to the roof. These vent seal boots are the
standard in the
plumbing industry for pipe roof protrusions. The resulting mount pipes will
also allow the
electrical and plumbing for the collector to be passed thru under the roof
where they will be
protected from weathering and heat loss.
[0074] Figures 8a ¨ 8i show cross-sectional layered structures of a solar
receiver of an
embodiment described herein. Figure 8a depicts a wafer 899 with a{111} crystal
orientation,
about 100 to about 150 mm in diameter, about 2 to about 5 mm thick, boron
doped about 0.2 to
about 0.5 ohm-cm. Alternatively, the silicon can be crystalline,
polycrystalline, black
amorphous, gallium doped, or other silicon known to one of ordinary skill.
Figure 8a shows an
oxide layer 898 on the top and bottom that is 5000 Angstroms thick with a
silicon layer 897
between. Figure 8b shows the top oxide layer removed. In this step, a pyramid
shape may be
etched in the top 5 microns deep. Other orientations may be used as known to
one of ordinary
skill in the art. Figure 8e shows a doped silicon layer 896, which may be
doped with phosphorus
for example and may be 0.1 micron deep to a resistance of 0.01 ohm-cm2 for
example. Figure
8d shows a 600 Angstom thick nitride top textured surface 895. Figure 8e shows
a top coating of
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microns thick photoresist 894 over the top of the nitride surface 895. Figure
8f shows a wet
etch of the top layer through the nitride surface. Figure 8g shows a vapor
deposition of titanium,
palladium on the top and bottom with layers of about 200 to about 250
Angstroms. Figure 8h
shows the photoresist removed with an acetone ultrasonic clean. Figure 8i
shows an
electroplating step where a 35 micron thick layer of silver is deposited
thereon.
[0075] An advantage of the embodiments herein is shown in the charts of Figs.
9a-9b.
The Fig. 9a is a comparison of the same collector's performance on two
separate days. The first
day is sunny and at full solar incidence on a collector trough. The second day
is hazy with
high and low clouds in the sky. As can been seen from the two lines, the
electrical performance
is almost the same. A drop of about 7% is determined by considering the area
under the two
curves. Collectors consistent with the embodiments herein hold nearly the same
electrical output
due, at least in part, to the wide angle of solar radiation acceptance.
[0076] In contrast, a high concentration collector system demonstrates a
dramatic power
fall-off on hazy or cloudy days. Fig. 9b shows two lines that represent the
heat collection of a
high concentration solar collector. As can be seen, the amount of power
collected is lower on the
hazy day by 25%. This difference in power output between sunny and hazy days
is mostly
attributed to reduced portions of the spectrum reflected and absorbed by the
high clouds.
[0077] While the foregoing description and drawings represent the preferred
embodiments of the present invention, it will be understood that various
additions, modifications
and substitutions may be made therein without departing form the spirit and
scope of the present
invention as defined in the accompanying claims. In particular, it will be
clear to those skilled in
the art that the present invention may be embodied in other specific forms,
structures,
arrangements, proportions, and with other elements, materials, and components,
without
departing from the spirit or essential characteristics thereof The presently
disclosed
embodiments are therefore to be considered in all respects as illustrative and
not restrictive, the
scope of the invention being indicated by the appended claims, and not limited
to the foregoing
description, port structure.
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[0077] While the foregoing description and drawings represent the preferred
embodiments of the present invention, it will be understood that various
additions, modifications
and substitutions may be made therein without departing form the spirit and
scope of the present
invention as defined in the accompanying claims. In particular, it will be
clear to those skilled in
the art that the present invention may be embodied in other specific forms,
structures,
arrangements, proportions, and with other elements, materials, and components,
without
departing from the spirit or essential characteristics thereof. The presently
disclosed
embodiments are therefore to be considered in all respects as illustrative and
not restrictive, the
scope of the invention being indicated by the appended claims, and not limited
to the foregoing
description.
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