U.S. patent application number 12/365549 was filed with the patent office on 2010-02-25 for solar trough and receiver.
Invention is credited to John Carroll Ingram.
Application Number | 20100043779 12/365549 |
Document ID | / |
Family ID | 41695162 |
Filed Date | 2010-02-25 |
United States Patent
Application |
20100043779 |
Kind Code |
A1 |
Ingram; John Carroll |
February 25, 2010 |
Solar Trough and Receiver
Abstract
A solar receiver unit comprises a generally tubular body having
an internal passage for conducting a heat-transfer fluid, and an
inlet chamber for receiving concentrated sunlight from a solar
reflector when in use. The inlet chamber is shaped to direct
reflection of concentrated sunlight at least one time in a
direction that provides the reflected sunlight with an additional
opportunity to heat the heat-transfer fluid.
Inventors: |
Ingram; John Carroll; (Simi
Valley, CA) |
Correspondence
Address: |
SELDON & SCILLIERI
12121 WILSHIRE BLVD., SUITE 1300
LOS ANGELES
CA
90025-1166
US
|
Family ID: |
41695162 |
Appl. No.: |
12/365549 |
Filed: |
February 4, 2009 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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61090505 |
Aug 20, 2008 |
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61090509 |
Aug 20, 2008 |
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61122304 |
Dec 12, 2008 |
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Current U.S.
Class: |
126/694 ;
126/678 |
Current CPC
Class: |
F24S 40/80 20180501;
F24S 40/20 20180501; F24S 23/74 20180501; F24S 20/20 20180501; Y02E
10/41 20130101; Y02E 10/45 20130101; Y02E 10/40 20130101 |
Class at
Publication: |
126/694 ;
126/678 |
International
Class: |
F24J 2/12 20060101
F24J002/12; F24J 2/04 20060101 F24J002/04 |
Claims
1. A solar trough and receiver unit comprising: a generally tubular
receiver having an internal passage for conducting a heat-transfer
fluid and an inlet chamber for receiving concentrated sunlight; a
generally parabolic trough for collecting and concentrating
sunlight into the inlet chamber of the receiver to cause the
heat-transfer fluid within the receiver to thereby be heated, the
inlet chamber being shaped to reflect said concentrated sunlight at
least one time in a direction that provides said reflected sunlight
with an additional opportunity to heat heat-transfer fluid within
the receiver.
2. The solar trough and receiver unit of claim 1 wherein the inlet
chamber is an integral part of the receiver.
3. The solar trough and receiver unit of claim 2 wherein the inlet
chamber has a sunlight-receiving surface that is shaped to direct
the majority of reflections back onto a the tube.
4. The solar trough and receiver unit of claim 1 wherein the inlet
chamber has a sunlight-receiving face that is shaped to direct the
majority of reflections back onto the receiving face.
5. The solar trough and receiver unit of claim 1 wherein the inlet
chamber has a solar ray-receiving surface that is shaped to direct
the majority of reflections back onto the tube.
6. The solar trough and receiver unit of claim 5 wherein the shape
of the receiving surface causes at least a substantial portion of
the concentrated solar rays from the trough to be incident on the
receiver at non-perpendicular angles so that reflections from the
receiving surface of concentrated sunlight from the trough are
incident on the receiver at least one more time.
7. The solar trough and receiver unit of claim 3 wherein the inlet
chamber includes at least at least one generally concave face
impinged by the concentrated sunlight.
8. The solar trough and receiver unit of claim 3 wherein the inlet
chamber includes at least a pair of generally concave faces
impinged by the concentrated sunlight.
9. The solar trough and receiver unit of claim 8 wherein at least
one longitudinally-extending channel is defined along the receiver
by the generally concave faces.
10. The solar trough and receiver unit of claim 9 including a
substantially transparent cover extending longitudinally along the
length of the receiver to generally enclose the inlet chamber.
11. The solar trough and receiver unit of claim 3 including a
substantially transparent cover extending longitudinally along the
length of the receiver to generally enclose the inlet chamber.
12. The solar trough and receiver unit of claim 11 including a
fluid within the covered inlet chamber space to substantially
insulate the receiver from heat loss through the cover.
13. The solar trough and receiver unit of claim 12 wherein the
fluid is a gas.
14. The solar trough and receiver unit of claim 13 wherein the gas
is air.
15. The solar trough and receiver unit of claim 12 wherein the
fluid is a liquid.
16. The solar trough and receiver unit of claim 11 wherein the
cover is slightly concave.
17. The solar trough and receiver unit of claim 1 wherein the
external receiver surfaces which are not exposed to the
concentrated sunlight are generally covered with a layer of thermal
insulation.
18. The solar trough and receiver unit of claim 17 wherein the
layer of insulation has an outer surface coated with a high solar
absorptivity coating so that the insulation absorbs sufficient
solar radiation to increase the insulation's surface temperature
and thereby decreases the temperature gradient across the
insulation in order to minimize heat loss through the insulation
layer.
19. The solar trough and receiver unit of claim 1 including a
turbulator to create sufficient turbulence within the heat transfer
fluid to decrease the thermal gradient within the receiver.
20. The solar trough and receiver unit of claim 19 wherein the
turbulator is disposed continuously within the receiver
passage.
21. The solar trough and receiver unit of claim 19 wherein a
plurality of turbulator surfaces are positioned generally serially
within the receiver passage.
22. The solar trough and receiver unit of claim 1 wherein the
receiver lacks glass shielding around the receiver.
23. The solar trough and receiver unit of claim 1 wherein the
receiver is longitudinally longer than the trough.
24. The solar trough and receiver unit of claim 1 wherein the
longitudinal length of the receiver is up to approximately 50
feet.
25. The solar trough and receiver unit of claim 1 including at
least one telescoping expansion joint for coupling the receiver to
an adjacent receiver, the joint comprising a generally tubular
member having an internal fluid passage and further having end
regions sized to fit within each of the adjacent receivers to
permit telescoping movement therewith in response to thermally
induced expansions and contractions of the adjacent receivers while
retaining the fluid passages of the adjacent receivers in
communication, and a seal member within the region between the
unit's receiver and the generally tubular member for sealingly
engaging the generally tubular member and receiver while permitting
said telescoping movement.
26. The solar trough and receiver unit of claim 1 including at
least one telescoping expansion joint for coupling the receiver to
an adjacent receiver, the joint comprising a tubular member having
an internal fluid passage and terminating in a pair of generally
longitudinally-spaced end regions, the receiver having at least one
end region sized to fit within an end region of the tubular member,
the tubular member permitting telescoping movement therewith in
response to thermally induced expansions and contractions of the
adjacent receivers while retaining the fluid passages of the
adjacent receivers in communication through the generally tubular
member, and a seal member within the region between the receiver
and tubular member for sealingly engaging the tubular member and
receiver while permitting said telescoping movement.
27. The solar trough and receiver unit of claim 1 further including
a first length of flexible tubing for coupling the fluid passage of
the receiver to a source of heat-transfer fluid at an upstream end
region of the receiver, and a second length of flexible tubing at a
downstream end region of the receiver for conducting the
heat-transfer fluid from the fluid passage of the receiver.
28. The solar trough and receiver unit of claim 27 wherein the
source of heat-transfer fluid is an adjacent receiver.
29. The solar trough and receiver unit of claim 1 wherein the
receiver includes a plurality of internal heat-exchanging fins
extending into the fluid passageway and structured to conduct heat
from the inlet chamber into the heat-exchange fluid.
30. The solar trough and receiver unit of claim 28 wherein the fins
extend substantially the longitudinal length of the receiver.
31. The solar trough and receiver unit of claim 1 wherein the
trough includes a back shell, a front reflective layer, and an
interjacent foam core 18 binding the back shell and the reflector
together.
32. The solar trough and receiver unit of claim 31 wherein the foam
core is a closed cell foam having substantially the same thermal
coefficient of expansion as the back shell and sufficiently high
modulus of elasticity to accommodate the differential strain
between the foam and the reflector.
33. The solar trough and receiver unit of claim 32 wherein the foam
is a polyurethane foam.
34. The solar trough and receiver unit of claim 33 wherein the foam
has a density of approximately 1 to 5 lbs/ft.sup.3.
35. The solar trough and receiver unit of claim 1 including a
support structure positioned on the trough to support the receiver
near the region of maximally concentrated sunlight reflected from
the trough.
36. The solar trough and receiver unit of claim 35 wherein the
trough has a pair of longitudinally-extending edge regions
separated by the width of the trough, and the support structure is
a bridge spanning the width of the trough.
37. The solar trough and receiver unit of claim 36 wherein the
bridge is coupled to the trough at the trough's outer
longitudinally-extending edge regions.
38. The solar trough and receiver unit of claim 37 wherein the
trough has a pair of longitudinally-spaced end regions, and the
bridge is coupled to the receiver at one of the receiver's end
regions.
39. The trough and receiver unit of claim 36 wherein the trough has
a pair of longitudinally-spaced end regions, and the bridge is
coupled to the receiver at one of the receiver's end regions.
40. The solar trough and receiver unit of claim 39 wherein a second
trough is adjacent the trough at their respective end regions, and
the bridge is coupled to the adjacent end regions on at least one
side of the adjacent troughs to secure the adjacent troughs
together.
41. The solar trough and receiver unit of claim 40 wherein the
receiver extends longitudinally along the pair of troughs.
42. The solar trough and receiver unit of claim 41 wherein the
receiver is supported by the pair of troughs.
43. The solar trough and receiver unit of claim 36 wherein one of
the bridge and the receiver includes mounting tabs protruding from
its outer surface and the other of the bridge and receiver has
tab-receiving slots for supporting the receiver, the tabs and slots
being sized and shaped to prevent rotation about the receiver's
longitudinal axis of the inlet chamber with respect to the
trough.
44. The solar trough and receiver unit of claim 43 wherein the tabs
and slots are sized and shaped to allow expansion of the receiver
along that axis.
45. The solar trough and receiver unit of claim 43 wherein the
inlet chamber of the receiver has an aperture through which
concentrated sunlight from the trough enters the inlet chamber, and
the tabs and slots are positioned to keep the receiver's inlet
chamber's aperture generally aligned with the area of maximum
concentration of the incoming sunlight.
46. The solar trough and receiver unit of claim 36 including means
supported by the bridge for delivering cleaning liquid against the
reflector for periodic washing of the reflector surface.
47. The solar trough and receiver unit of claim 46 including a
generally tubular cleaning fluid member extending adjacent to at
least one of the longitudinally-extending edge regions of the
trough for conducting cleaning fluid, and at least one outlet from
the cleaning fluid member for directing at least some of the
conducted cleaning fluid against the trough in a direction
generally transverse to the longitudinal.
48. A solar receiver unit comprising: a generally tubular body
having an internal passage for conducting a heat-transfer fluid,
and an inlet chamber for receiving concentrated sunlight from a
solar reflector when in use; the inlet chamber being shaped to
reflect said concentrated sunlight at least one time in a direction
that provides said reflected sunlight with an additional
opportunity to heat the heat-transfer fluid.
49. The solar receiver of claim 48 wherein the chamber is an
integral part of the tube.
Description
PRIORITY CLAIM
[0001] Priorities of co-pending U.S. Provisional Patent Application
No. 61/90,505 filed Aug. 20, 2008, co-pending U.S. Provisional
Patent Application No. 61/90,509 filed Aug. 20, 2008 and U.S.
Provisional Patent Application No. 61/122304 filed Dec. 12, 2008
are claimed, and the content of these applications are hereby
incorporated by reference.
FIELD OF THE INVENTION
[0002] This invention relates to improvements in the capture and
use of solar energy. This invention relates more particularly to
solar energy collectors and, more particularly, to solar energy
collectors comprising a generally parabolic trough for
concentrating impinging sunlight on a receiver and a receiver for
receiving the concentrated sunlight.
BACKGROUND
[0003] The use of solar energy has been limited, in part, by the
costs associated with capturing and using it in a sufficiently
economic manner. Solar panels for converting sunlight into
electricity, as well as solar collectors for converting sunlight
into heat, are still relatively expensive when the costs of both
manufacture and installation are considered.
[0004] A solar collector typically comprises a solar receiver, such
as one or more solar cells or a heat-transmitting medium, and a
solar concentrator for collecting and concentrating the sunlight
onto the receiver, where it is converted to heat and/or electrical
power at an efficiency that is better than using un-concentrated
sunlight.
[0005] Parabolic solar concentrators are typically formed as
generally parabolic troughs that consist of a rigid space frame
holding parabola-shaped glass mirrors. These are highly engineered
structures designed for maximum efficiency. They often have large
apertures of up to 5 meters and thus require significant structures
that require significant assembly in the field.
[0006] State-of-the-art solar receivers comprise a generally
circular tube of steel, copper or alloy covered by glass. The space
between the glass and the tube contains a vacuum that is used as
insulation to reduce heat loss to the environment. This can be very
effective but requires high maintenance to maintain the vacuum and
check for glass breakage. The surface of the receiver tube is
typically covered with a special coating, called a selective
coating, that has high absorptivity in the solar spectrum (i.e., it
absorbs sunlight well) but has low thermal emissivity.
BRIEF DESCRIPTION OF THE DRAWINGS
[0007] In the following Detailed Description Of The Embodiments,
reference is made to the accompanying drawings which form a part
thereof, and in which are shown by way of illustration specific
embodiments in which the invention may be practiced. It is to be
understood that other embodiments may be utilized and structural
changes may be made without departing from the scope of the present
invention. In the accompanying drawings:
[0008] FIG. 1 is a top right perspective view of a preferred solar
trough and receiver unit;
[0009] FIGS. 2A and 2B are, respectively, a right front perspective
view in explosion and a left rear perspective view in explosion of
a preferred solar trough;
[0010] FIG. 3 is a schematic illustration of the preferred method
for fabricating the preferred solar trough;
[0011] FIG. 4 is a schematic illustration of sunlight being
reflected by the preferred solar trough to impinge on the receiver
of the preferred trough and receiver unit illustrated in FIG.
1;
[0012] FIG. 5 is a cross-sectional view of the preferred receiver
in FIG. 1, taken along line 5-5 therein;
[0013] FIG. 6 is a cross-sectional view of the preferred receiver
as illustrated in FIG. 5, showing the addition of a preferred
optional cover;
[0014] FIG. 7 is a cross-sectional view of the preferred receiver
as illustrated in FIG. 5, showing the addition of preferred
heat-dissipating fins within the receiver;
[0015] FIG. 8A-C are respective side elevation views in schematic
of possible expansion joint arrangements that can be used as a
component of preferred solar trough and receiver units; and
[0016] FIG. 9 is a cross-sectional view in schematic of a modified
receiver.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
[0017] FIG. 1 is a top right perspective view of a solar trough and
receiver unit 10 (hereinafter referred to as the "collector unit
10"). The collector unit 10 comprises a generally parabolic trough
12 for collecting and concentrating sunlight onto a generally
central receiver 14 that extends along a longitudinal axis 11. The
receiver 14 can contain circulating fluid which is heated for
thermal application or photovoltaic ("PV") cells mounted on the
receiver 14 for concentrated PV applications. In concentrating PV
applications, the fluid in receiver 14 serves as a cooling fluid
for the PV cells.
[0018] As described in more detail below, the receiver 14 is
supported at or near the focal point of the generally parabolic
trough 12 by a support structure, the preferred structure being
illustrated at 28 and described in detail below. As will become
clearer, the preferred embodiment described herein reduces the
specific cost for constructing and installing the receiver,
simplifies the production and installation of parabolic troughs,
and reduces the delivered cost of energy from the resulting solar
system.
[0019] The trough 12 is preferably formed as a unibody structure
rather than as a distinct space frame with mounted components. The
preferred troughs are accordingly easily assembled in the factory,
require minimal field assembly, are less material intensive than
conventional solar collectors, weigh less than conventional
concentrators, are less fragile because they do not require glass
mirrors as reflective surfaces, and can be stacked for easy storage
and transportation.
[0020] The preferred receiver support structure is a "bridge"
design 28 as opposed to the conventional method of supporting the
receiver by means that extend from the center of the trough (which
can be used if desired). The preferred "bridge" structure 28 is
illustrated in FIG. 1 and described later. The preferred
bridge-type receiver support significantly increases the strength
of the trough unit, and reduces the mass of the material used in
the trough.
[0021] The preferred trough's reflective surface may be any
polished metal, any highly reflective material and/or a reflective
coating. It is also possible for the surface to be flat segments or
specifically curved mirrors.
[0022] The preferred individual trough units are designed to be
close coupled to form a row of solar collectors where desirable.
This minimizes the gap between reflective surfaces, thus reducing
losses.
[0023] The preferred solar collector system is designed for
automated cleaning.
[0024] In practice, the preferred troughs are preferably
transported without the receiver support attached to the trough so
that the trough units can be easily stacked or nested to allow for
more efficient transportation.
[0025] The preferred trough units are sufficiently light such that
two men can easily lift them and place them on their mounts.
[0026] Turning to FIGS. 2A and 2B, a right front perspective view
in explosion and a left rear perspective view in explosion of the
preferred trough unit 12 are respectively shown. The basic trough
unit preferably consists of three major components: a back shell
16, an interjacent foam core 18 and a front reflective layer 20.
The back shell 16 can be formed from steel or other metal, plastic
or composite material. Currently, the shell is preferably stamped
steel that is painted or coated to inhibit corrosion. Channels 16a
and/or ribs 16b, preferably formed integrally by means of the
stamping operation, are of sufficient dimensions to give the shell
a required degree of stiffness. A generally centered
longitudinally-extending spar (not illustrated) may also be placed
between the back shell and the front reflective layer to increase
the stiffness in the longitudinal direction. In practice, the
preferred back shell 16 also has pivot points, mounting and other
connection points for actuation and hardware.
[0027] The foam core 18 binds the back shell 16 and the reflector
20 together, provides spacing between the back shell 16 and the
reflector surface 20, and adds stiffness to the unit. The preferred
foam is introduced as a liquid during the construction of the
trough unit and solidifies during its curing process, becoming
permanently bonded to the surrounding surfaces. Once the foam has
set, the parabolic shape is permanently established in the
reflector.
[0028] The foam core is preferably a closed cell foam such as a
polyurethane foam. Polyurethane foam is preferred because it has
high bond strength and is easy to apply as a liquid. Additionally,
some polyurethanes have nearly the same coefficient of expansion as
mild steel so that there is minimal thermally-induced strain
between the back shell and the foam core. Since polyurethane foam
also has a high modulus of elasticity, it can easily accommodate
the small differential strain between the foam and the
reflector.
[0029] The foam may be of various densities, but is preferably 1 to
5 lbs/ft.sup.3. The foam-contacting surfaces of the back shell 16
and the reflector 20 may be unprepared prior to binding, or may be
prepared by etching or otherwise making the surface suitable for
bonding to the foam.
[0030] The reflector component 20 is a structure to which a front
reflective layer 20a is attached or onto which it is deposited. The
reflector component 20 is preferably a thin sheet of metal,
preferably aluminum that is highly polished on its reflective
surface and coated to inhibit corrosion. Other possible materials
are stainless steel and, particularly with more highly reflective
layers attached to the reflection side of the reflector, plastics
and composite materials and glass mirrors. One example of a
reflective layer that can be attached to a substrate to form the
reflector component 20 is a reflective film such as ReflecTec.RTM.
mirror film (a product of ReflectTec, Inc. of Arvada, Colo.).
Alternatively, a reflective aluminum sheet such as Mirosun.RTM.
reflective sheets (a product of ALANOD Aluminum Veredlung of
Ennepetal, Germany) can be utilized with or without a
substrate.
[0031] Fabrication of the trough is relatively simple. Referring to
FIG. 3, the prepared reflector component 20 is placed in a tooling
fixture or jig such as a vacuum chuck 24, which holds the reflector
material or sheet in the correct alignment, position and shape. The
reflector component 20 is now prepared to be mated with the back
shell 16 in its fixture.
[0032] The prepared back shell 16 is fitted in a different tooling
fixture 22 (e.g., by vacuum, magnetic or othermeans). With both
pieces in their respective fixtures, they are ready to be brought
together for final assembly. The mating of the two fixtures
provides for the correct spacing between the back shell and the
reflector. If a spar is required in the unit it is placed in the
assembly before the two tooling fixtures are mated. The space 26
between them is filled with a liquid expanding polyurethane foam
material that expands to form the foam core 18. Because low
temperatures may result in an incomplete reaction within the foam,
and in poor bond strength and structural properties, the fixtures
are preferably heated so that the back shell 16 and reflector 20
are at a temperature that facilitates the reaction, distribution of
the foam, faster cure time and bonding of the foam. In accordance
with the preferred embodiment, a 2-part urethane foam is premixed
and injected into the cavity between the shell 16 and reflector 20.
One or more holes in the back shell can be provided as a means by
which the liquid foam is injected into the cavity between the two
pieces. The volume of the foam is preferably metered to assure the
correct amount of foam is introduced into the cavity, although a
slight overfill may be required. The unit takes approximately 3 to
5 minutes to cure completely. The trough's formation is then
complete and the trough unit is removed from the tooling fixture.
Cleanup, post processing and addition of small parts can be
completed at this time.
[0033] Referring to FIGS. 2A and 2B, it can be noted that the
reflector component 20 of the completed trough unit is held in the
correct shape by the cured foam layer 18. Accordingly, it may be
noted that a substantial amount of variation can be tolerated in
the back shell 16, since the foam 18 will compensate for the
variations by filling in the gaps between the shell and reflector
component during the formation process. Accordingly, to a
substantial extent, cost becomes the driving factor with respect to
the shell rather than precision so that less precise processes such
as stamping are feasible in manufacturing the shell.
[0034] The trough unit 12 can support the receiver 14 in the
conventional manner, but the preferred supporting structure is a
novel bridge (or truss) 28 (FIG. 1) that spans the width of the
trough and attaches to the trough's outer longitudinal edges 10a,
10b. Each trough unit has at least one receiver-supporting bridge,
and a bridge can also be used to lock the adjoining longitudinal
ends of adjoining troughs together to form a row of troughs. Linked
in this manner, a row would have a minimum number of
receiver-support bridge units equal to or greater than the number
of troughs plus one.
[0035] The full stiffness and strength of the trough is achieved
only after the receiver support is installed. Without the installed
receiver-supports, the troughs are easily stackable for convenient
shipping and movement, so it is preferred that the receiver support
is mated with the trough in the field, both to avoid damage to the
support and to also permit easier stacking of the troughs.
[0036] The receiver support can also carry provisions for
installation of a water hose/line (not shown) for periodically
washing the reflector surface. This hose/line is preferably
provided with nozzles or perforations at longitudinally-space
intervals along the trough. The receiver-support bridge accordingly
also supports a water (or special washing fluid) line, which
delivers water that sprays down the reflective surface of the
reflector 20 and, if present, a glass cover shielding the
receiver.
[0037] Preferably, the hose/line extends along or slightly above
one longitudinal edge region of the trough unit. During washing,
the trough is pivoted so that the edge region bearing the hose is
higher than the other longitudinally extending edge; the trough
units are typically mounted on structures allowing them to pivot in
this manner to enable the trough(s) to follow the sun's movement
during the day. The water/cleaning solution is deployed generally
laterally across the trough (or glass cover if present) from the
hose, preferably through a plurality of laterally-facing,
longitudinally disposed nozzles that impart sufficient velocity to
the water/cleaner to enable the water/cleaner to run across the
reflective surface and over the lower opposite edge of the trough,
carrying the accumulated dust and dirt away.
[0038] Alternatively, an automated cleaning machine may be used to
clean each multi-unit row of collector units. Due to the bridge
design and the close coupling of the collector units, the path is
clear down the center of the collector units for an automated
cleaner to traverse the length of a row of collectors. A cleaning
machine may be placed at one end of a row and travel the length of
a row, typically 100 to 200 meters. The automated cleaner can be
used to clean and/or polish the reflective surface of the reflector
20 using air, water, brushes or any combination of those, and
preferably cleans the reflector and the receiver glass, if any,
simultaneously.
[0039] The receiver 14 (FIG. 1) may be of the type utilizing a
heat-transfer liquid or may instead be of the photovoltaic type.
The receiver 14 comprises a tube situated at or near the focal
point of the parabolic concentrator 12 so that concentrated
sunlight is focused on the receiver's face, or just slightly inside
or outside as needed to disburse the concentrated sunlight over the
correct area.
[0040] FIG. 4 is a schematic illustration of sunlight being
reflected by the preferred solar trough to impinge on the receiver
14 of the preferred trough and receiver unit illustrated in FIG. 1.
FIG. 5 is a cross-sectional view of a preferred receiver of the
heat-transfer type taken along line 5-5 in FIG. 1. While the
reflector 12 is described and illustrated as having a generally
parabolic shape, the shape may not be a true parabola owing to a
correction factor that changes the shape slightly from a true
parabola to compensate for the fact that the sun's rays are not
exactly parallel and thereby attain a better focus of the
concentrated rays.
[0041] Turning to FIG. 5, the receiver 14 comprises a generally
longitudinally-extending tube 30, the interior of which functions
as a fluid passageway 31 that accommodates the generally
longitudinal flow of heat-transfer fluid 34 that is pumped through
the tube to absorb the solar energy as heat. The receiver may be
formed from steel, aluminum, stainless steel, copper or any other
metal or metal compound having the desired heat transfer properties
and material properties that can withstand the temperatures and
environmental conditions to which the receiver is subjected. The
heat-transfer fluid is typically a liquid but may be a gas.
[0042] The trough-facing surface of the tube 30 is shaped to form a
receiver chamber 42 that receives the concentrated solar rays from
the trough so that the sunlight can impinge on the wall of the
receiver chamber portion of the tube and heat the heat-transfer
fluid within the fluid passageway A portion of the impinging
sunlight from the trough is not absorbed, but is instead reflected
from the receiver chamber face and would represent inefficiency in
energy transfer; however, the shape of the receiver chamber is such
that partially redirected (reflected) concentrated rays are mostly
redirected (i.e, reflected) within the receiving chamber to allow
for another opportunity for solar energy absorption. In the
preferred receiver, the receiver chamber is an integral part of the
tube 30.
[0043] As illustrated in FIG. 5, the preferred receiver chamber 42
is integral with the tube, and is formed by a recess having a
generally longitudinally-extending concave face or, as illustrated
here, faces 36, 38. The receiver chamber 42 extends longitudinally
along the trough-facing side of the receiver to receive the
concentrated solar rays from the trough. The tube may have one such
face or more than 2, but two is preferred. The particular shape of
the tube serves to approximate a black body, which is designed to
limit the emission of thermal radiation. Those skilled in the art
will recognize that one could utilize two or more contiguous tubes
instead of one; e.g., a tube with face 36, and a second tube with
face 38. Alternatively, several relatively smaller tubes could be
arranged to achieve the same purpose.
[0044] As illustrated in FIG. 6, the faces of the receiver are
aligned so that the concentrated solar rays received from the
reflector strike the tube surface at angles that cause a
substantial number of reflected rays to remain internally within
the receiver chamber 42 for at least one reflection, resulting in
initially non-absorbed energy being reflected back onto the
receiver chamber's absorbing face, and thereby giving the solar
energy of the rays an additional opportunity to be absorbed to
thereby increase efficiency.
[0045] The tube's surface is preferably coated with a selective
coating, which is typically designed to selectively absorb
wavelengths within the solar spectrum with the greatest energy
content and emit as little heat as possible. Such coatings are well
known in the art. A typical selective coating may have an
absorption rate of 93%, the remaining 7% being reflected back.
Accordingly, the rays initially reflected from the receiver
chamber's absorption face after entering the receiver chamber,
undergo at least one internal reflection so they are incident again
on the tube's surface and can be further absorbed as heat. Those of
ordinary skill in the art will recognize that the faces themselves
need not be concave but only need be configured to retain a
substantial proportion of the reflected rays within the solar
chamber.
[0046] It is desirable to minimize heat loss from the receiver so
that the heat-transfer fluid retains a maximal amount of solar
energy. Heat loss from the receiver chamber happens primarily by
two mechanisms: radiation and convection. Radiation losses are
roughly constant for a constant receiver temperature. Convective
losses to the air surrounding the receiver are variable depending
primarily on ambient environmental conditions such as temperature
and wind speed, while second order conduction losses occur through
mounts and insulation.
[0047] The shape of receiver chamber 42 described herein aids in
the retention of heat, which can be radiated and convected from the
receiver surface. First, the aperture 44 of the preferred chamber
42, through which the concentrated solar rays from the trough
enter, is preferably narrower than the receiver's diameter, leaving
only a narrow window for radiation to escape and thereby
approximating a blackbody. Second, and for the same reason, the
convective heat window is smaller. Additionally, the shape of the
preferred chamber 42 helps to maintain a pocket of heated air close
to the receiver face within the chamber, aiding significantly in
the reduction of convective heat transfer.
[0048] The receiver chamber 42 may be protected by a transparent
cover 40 that extends along the length of the tube to generally
close the receiver chamber 42. The cover isolates a pocket of air
next to the tube's faces in the receiver chamber 42, and acts as
insulation to minimize heat losses to the environment by
essentially minimizing free and forced convection with the
environment. Those of ordinary skill in the art will appreciate
that gasses other than air can be utilized within the receiver
chamber for the same purposes.
[0049] The cover 40 is preferably slightly concave, with respect to
the outside, as an additional heat-transfer limiting mechanism.
Specifically a small boundary layer of hot air will stay in the
enclosed region thus defined and act as additional insulation.
Additionally, the concave face protects the faces 36, 38 from
direct exposure to wind-forced convention. The cover 40 is
preferably a narrow piece of borosilicate glass, and is preferably
coated to decrease reflectance within the solar spectrum; such
coatings are commercially available and well known to those of
ordinary skill in the art.
[0050] It may be noted that the inclusion of the cover is optional,
and may or may not be used depending on the application. There is a
trade-off between decreasing the heat loss through the use of the
cover while causing some loss owing to sunlight reflection from the
cover.
[0051] The external receiver surfaces which are not exposed to the
concentrated sunlight are preferably covered with a standard
industrial pipe insulation 32. The exposed outer surface of the
insulation is preferably coated with a black paint or a high solar
absorptivity coating so that the insulation absorbs as much direct
and diffuse solar radiation as possible. This increases the
insulation's surface temperature, and thereby decreases the
temperature gradient across the insulation and, consequentially,
the heat losses from the fluid chamber that is conducted through
the insulation, thereby making the receiver more efficient.
[0052] In summary, the preferred receiver's fluid-conducting tube
is thereby shaped and fabricated to maximize the absorption of
solar energy by the heat-transfer fluid flowing within the fluid
passageway therein, while associated heat losses to the environment
are minimized. Moreover, the configuration of the receiver is such
that thermally induced stresses are reduced.
[0053] Referring to FIG. 5, thermally induced stresses are induced
by temperature differences between the outside or top of the
receiver and the trough-facing side of the receiver. The
temperature within the receiver chamber 42 can reach approximately
400.degree. to 550.degree. C. or more, causing expansion of the
concave faces that tries to bend the receiver along its
longitudinal axis, while the lower temperature of the top surface
of the receiver causes an expansion of the top side that is
different than that of the receiving chamber. Thus, there is an
imbalance of exerted forces due to thermal expansion. The receiver
may accordingly be configured with a moment of area of the receiver
chamber, which is at roughly one uniform temperature, and a moment
of area of the top section which is at a second lower but roughly
uniform temperature, that are balanced such that thermal stress
created in each of these chambers are substantially balanced to
minimize induced bending. In other words, the stress created from
differential thermal expansion resulting from the solar chamber
faces being hotter than the adjoining top section is substantially
minimized by this configuration, permitting the receiver to grow
substantially evenly about its longitudinal axis to yield minimum
thermally-induced bending.
[0054] The preferred receiver may also be provided with internal
"turbulators"; i.e., surfaces, partial flow obstructions or other
devices designed to cause a mixing action within the heat-transfer
fluid that causes the relatively hotter fluid near the receiver
chamber wall to be mixed and/or entrained into the relatively
greater volume of flowing fluid. In laminar flow, the fluid along
the walls of the fluid chamber tends to flow more slowly then the
remaining fluid owing to its contact with the walls, and creating a
"boundary layer" of fluid that is hotter than the main fluid body.
Turbulators are intended to disrupt the boundary layer effect,
causing a mixing of the heat-transfer fluid as it flows through the
receiver, and thereby increasing the heat-transfer efficiency
because heat is carried away from the receiver chamber walls more
efficiently since the temperature gradient from the wall to the
fluid is greater.
[0055] The turbulators have multiple functions. First, they stir
the fluid, thereby decreasing its thermal gradient and removing the
hottest fluid from the relatively hotter absorption surfaces of the
receiver. Lower fluid temperature near the absorption surface
results in an increase in heat transfer effectiveness. Second, some
turbulators can effectively act as heat transfer fins, increasing
receiver efficiency. Third, turbulators can allow for lower flow
rates of the heat transfer fluid, thus reducing pumping power
requirements, because a higher flow rate is not needed to create
the turbulence necessary to mix the relatively hotter heat-transfer
fluid along the wall into the relatively cooler fluid. Turbulators
may be installed continuously or periodically in the receiver tube
fluid section.
[0056] A turbulator may be one or more a suitably bent wires, one
or more bodies of porous or mesh material, or other flow-disrupting
surface member(s) disposed within the fluid path of the fluid
chamber to disrupt laminar flow and create sufficient turbulence to
mix the relatively hotter fluid with the rest of the flowing fluid.
A turbulator may be disposed continuously throughout the fluid path
or a plurality of turbulators may be serially disposed along the
fluid path.
[0057] The configuration of another heat transfer enhancement
mechanism is illustrated in FIG. 7 wherein a plurality of fins 52
extend inwardly from the inner, ray-receiving wall of the fluid
passageway to increase the heat transfer area along the fluid path.
These will also cause a mixing action and increased turbulence, but
not to the extent of the turbulators. Moreover, the preferred fins
are in thermal contact with the fluid tube wall and thus are also
thermally coupled to the receiver's optical chamber's absorption
face to aid in the conduction of heat from the solar chamber faces
into the heat transfer fluid. In the preferred embodiment, the fins
are integral to the receiver chamber wall. The fins not only
function as heat exchanger fins that increase the effectiveness of
heat transfer to the portions of the receiver body from which they
extend, but can also increase the stiffness of the receiver and may
thereby allow thinner walls to be provided around the receiver
chamber and fluid chamber for at least some applications.
[0058] Since the receiver is not normally glass shielded or vacuum
insulated, as are conventional receivers, the solar collector units
and rows of solar collectors can be very long. The preferred
receivers described herein may be as long as practically
transportable to the installation site; e.g., lengths of up to 50
feet or more. This has several advantages: first it reduces the
number of connections between receiver sections by a factor of 3 to
4 when constructing rows of solar collectors. Because connections
are dead spaces which cannot be used to absorb sunlight, losses are
accordingly decreased. Second, the time taken to install the
collector units is reduced, thereby reducing installation costs,
because the long receiver requires fewer connections.
[0059] Third, fewer expansion joints are required by the longer
receivers. The change in temperature from cold non-operating
conditions to hot peak power conditions may be 350.degree. C. or
more on average, and the receiver length will accordingly grow
significantly, requiring the use of expansion joints to accommodate
the growth. Since the receiver joint connections are coincident
with expansion joints, the number of expansion joints is greatly
reduced by using long receivers. Moreover, expansion joints may be
installed less frequently in connection with the receiver disclosed
herein by placing hard connectors at some coupling joints.
Moreover, since expansion joints are associated with losses in the
system, the use of longer receivers is more efficient because they
provide greater area available to receive concentrated light. Very
long receivers would not be possible in the more standard glass
insulated receiver. The preferred absence of glass shielding and
vacuum seals herein, however, removes limitations on how much any
section of receiver may grow.
[0060] Because the preferred receiver does not use a vacuum
insulated shield and, as such, has options available for expansion
joints other than the standard bellows joint used with state of the
art receivers, the preferred receiver can conveniently use a
simpler expansion joint. FIG. 8C is a schematic illustration of the
preferred configuration for an expansion joint wherein neighboring
and regions of adjacent receivers 14', 14'' are coupled together at
opposite ends of a flexible tube or hose 56 formed from a material
that can withstand in the temperature of the heat-transfer
fluid.
[0061] Another configuration is a telescoping configuration, which
is sealed with o-rings 58 or other shaft seal mechanism. FIGS. 8A
and 8B schematically illustrate one such expansion joint wherein
two receivers 14', 14'' are coupled together via a tube 54 having
an internal fluid passageway through which the fluid chambers of
both receivers communicate. The tube 54 is inserted into the
adjacent end regions of the two receivers through a seal or O-ring
58 that permits longitudinal movement of each receiver 14', 14''
with respect to the tube 54 without blocking the flow of
heat-transfer fluid from one fluid chamber to the other. As the
receivers expand (FIG. 8B) or contract (FIG. 8A), the change in
receiver length is accommodated by the seals 58, which permit the
consequential relative movement between the receivers.
[0062] The expansion joints are designed as to be fully collapsed
(maximum expansion) when the receiver is at its maximum operating
temperature. This has the effect of further minimizing the "dead
area", which cannot be used as absorber surface.
[0063] The preferred receiver also preferably includes mounting
guides that prevent rotation of the receiver about its longitudinal
axis when mounted on the trough. For example, tabs 62, 64, 66 (FIG.
7) protrude generally radially, with respect to the longitudinal
axis of the receiver, from the outer surface of the receiver to
prevent rotation of the receiver about the longitudinal axis and
keep the receiver chamber and aperture aligned to a high degree
with the focal point of the generally parabolic trough reflector.
The tabs accordingly fit into slots in the support bridge 28, which
are preferably shaped to allow expansion and contraction of the
receiver along the receiver's longitudinal axis while restricting
said rotation. The tabs are preferably of minimal length with
respect to the receiver, and are located near the bridge
mounting.
[0064] FIG. 9 is a cross-sectional view of an alternative receiver
configuration. The tube 30' of receiver 14' may be any shape from a
standard round tube or pipe to a tube with one or more flat, convex
and/or concave ray-receiving faces Alternative shapes may be easier
to manufacture and insulate, and may therefore be less expensive
albeit less efficient. However for certain operating conditions,
such as lower operating temperature of the system, these designs
may be very cost effective. Additionally, for example, a flat
bottom design may be more conducive for photovoltaic applications
such as those in which photovoltaic cells are attached to the
receiver surface at or near the focal point of a solar
concentrator. A cover 40' may be optionally included to form a
chamber that can be filled with flowing or entrapped air or other
fluid.
[0065] Although a preferred trough and receiver unit and the
various preferred and alternative components thereof, as well as
some of the advantages that can be derived from them, have been
described in detail, it should be understood that various changes,
substitutions and alterations can be made herein without departing
from the spirit and scope of the invention as defined solely by the
appended claims.
* * * * *