U.S. patent application number 13/736055 was filed with the patent office on 2013-08-01 for cavity receivers for parabolic solar troughs.
This patent application is currently assigned to Norwich Technologies, Inc.. The applicant listed for this patent is Oliver J. Brambles, Troy O. McBride, Joel Stettenheim. Invention is credited to Oliver J. Brambles, Troy O. McBride, Joel Stettenheim.
Application Number | 20130192226 13/736055 |
Document ID | / |
Family ID | 48869046 |
Filed Date | 2013-08-01 |
United States Patent
Application |
20130192226 |
Kind Code |
A1 |
Stettenheim; Joel ; et
al. |
August 1, 2013 |
Cavity Receivers for Parabolic Solar Troughs
Abstract
A tubular heat-absorbing element partly enclosed in an
insulating layer or jacket, has absorbing surface that is
accessible to solar radiation. The thermal insulation is designed
to provide entry to solar radiation by way of a cavity. The
absorbing surface can be substantially planar.
Inventors: |
Stettenheim; Joel; (Norwich,
VT) ; McBride; Troy O.; (Norwich, VT) ;
Brambles; Oliver J.; (Norwich, VT) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Stettenheim; Joel
McBride; Troy O.
Brambles; Oliver J. |
Norwich
Norwich
Norwich |
VT
VT
VT |
US
US
US |
|
|
Assignee: |
Norwich Technologies, Inc.
Norwich
VT
|
Family ID: |
48869046 |
Appl. No.: |
13/736055 |
Filed: |
January 7, 2013 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
61583585 |
Jan 5, 2012 |
|
|
|
Current U.S.
Class: |
60/641.15 ;
126/600; 126/652; 126/674; 126/696 |
Current CPC
Class: |
F24S 23/74 20180501;
F24S 30/40 20180501; F24S 10/70 20180501; F24S 10/75 20180501; Y02E
10/44 20130101; F24S 80/56 20180501; F24S 80/50 20180501; F24S
40/10 20180501; F24S 10/45 20180501; F24S 20/20 20180501; F24S
23/80 20180501; F24S 70/60 20180501; F24S 70/225 20180501; Y02E
10/46 20130101; Y02E 10/47 20130101; F24S 70/16 20180501; F24S
30/425 20180501; Y02E 10/40 20130101; F24S 10/40 20180501; F24S
23/30 20180501; F03G 6/06 20130101 |
Class at
Publication: |
60/641.15 ;
126/600; 126/674; 126/652; 126/696 |
International
Class: |
F03G 6/06 20060101
F03G006/06; F24J 2/12 20060101 F24J002/12; F24J 2/05 20060101
F24J002/05; F24J 2/54 20060101 F24J002/54; F24J 2/26 20060101
F24J002/26 |
Goverment Interests
STATEMENT REGARDING FEDERALLY FUNDED RESEARCH OR DEVELOPMENT
[0002] This invention was made with government support under
DE-EE0005803 awarded by the DOE. The government has certain rights
in the invention.
Claims
1. A system for generating energy from solar radiation as part of a
solar power system, said system comprising: a plurality of linear
receivers, each of said plurality of linear receivers including at
least a solar radiation absorbing element designed to absorb an
incident flux of solar radiation and transfer an absorbed flux of
energy to a heat transfer medium, said heat transfer medium
designed to receive and transport at least a portion of said
absorbed flux of energy, at least a portion of said radiation
absorbing element being covered with a solar selective absorber,
said solar selective absorber having a thermal emittance value and
an optical absorptance value, said optical absorptance value being
different from said thermal emittance value; a parabolic trough
mirror collector for concentrating solar radiation onto said
plurality of linear receivers; a control system for directing said
parabolic trough mirror at the sun, wherein said heat transfer
medium circulating in a first receiver in said plurality of linear
receivers is heated by solar radiation from a first elevated
temperature T1 to a second elevated temperature T2 over a first
distance corresponding to a length of said first receiver and said
heat transfer medium circulating in a second receiver in said
plurality of linear receivers is heated by solar radiation from a
third elevated temperature T3 to a fourth elevated temperature T4
over a second distance corresponding to a length of said second
receiver, where T4>T3.gtoreq.T2>T1, said first receiver and
said second receiver having structures designed for operation in
different temperature ranges.
2. The linear solar receiver of claim 1, wherein the portion of the
radiation absorbing element being covered with the solar selective
absorber for said first receiver is greater than the portion of the
radiation absorbing element being covered with the solar selective
absorber for said second receiver.
3. The linear solar receiver of claim 1, wherein said first portion
of said outer surface of said solar radiation absorbing element is
substantially planar.
4. The linear solar receiver of claim 1, wherein said first portion
of said outer surface of said solar radiation absorbing element
comprises a fraction in the range of 0.50 to 0.20 of an area of
said outer surface of said solar radiation absorbing element
determined on a per unit length basis.
5. The linear solar receiver of claim 1 further comprising a glass
cover enclosing the solar radiation admitting region.
6. The linear solar receiver of claim 5 wherein an inert gas is
introduced into the radiation admitting region.
7. The linear solar receiver of claim 1, wherein said interior
surface of said solar radiation admitting region forms a compound
parabolic collector.
8. The linear solar receiver of claim 1, wherein said interior
surface of said solar radiation admitting region is a reflective
surface.
9. The linear solar receiver of claim 1, wherein said heat transfer
medium is selected from the group consisting of a molten solar
salt, a molecular silicone based fluid, and steam.
10. The linear solar receiver of claim 1, wherein said linear
receiver has a thermal efficiency, said thermal efficiency selected
from the group consisting of 94 percent at 450 degrees Celsius and
92 percent at 500 degrees Celsius, 89 percent at 550 degrees
Celsius, 85 percent at 600 degrees Celsius, 80 percent at 650
degrees Celsius.
11. The linear solar receiver of claim 1 in combination with an
energy collection system configured to operate a Carnot cycle
energy recovery machine.
12. The linear solar receiver of claim 1 in combination with a
plurality linear solar receivers, said linear solar receivers each
including at least solar radiation absorbing elements, adjacent
solar radiation absorbing elements forming a nearly continuous
absorbing surface.
13. The linear solar receiver of claim 1 in combination with a
plurality linear solar receivers, a first one of said plurality of
receivers operating at a first temperature and a second one of said
plurality of receivers operating at a second temperature, said
first receiver and said second of said plurality of receivers
having different designs, said first and said second temperatures
being different.
14. The linear solar receiver of claim 1, further comprising a
symmetric parabolic trough collector mirror structure having a rim
angle of less than 75 degrees, said symmetric parabolic trough
collector mirror structure focusing said incident flux of solar
radiation on said solar radiation absorbing element, said linear
solar receiver disposed between said symmetric parabolic trough
collector mirror structure and the sun.
15. The linear solar receiver of claim 1, further comprising a
symmetric parabolic trough collector mirror structure, said
symmetric parabolic trough collector mirror structure being held in
a substantially rigid form with cable suspension.
16. The linear solar receiver of claim 1, wherein at least a
portion of said absorbed flux of absorbed energy is used to perform
an action chosen from the group consisting of generating
electricity and completing an industrial process.
17. The linear solar receiver of claim 1, wherein said solar
selective absorber is a plasmonic nanochain cermet structure.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application claims priority to and the benefit of
co-pending U.S. provisional patent application Ser. No. 61/583,585,
filed Jan. 5, 2012, which application is incorporated herein by
reference in its entirety.
FIELD OF THE INVENTION
[0003] The invention relates to concentrating solar power in
general and particularly to a system that employs reflective
troughs to focus solar radiation upon a linear receiver.
BACKGROUND OF THE INVENTION
[0004] Solar parabolic troughs, which focus sunlight on tubes
carrying a fluid that conveys heat for steam generation (e.g. to a
steam-driven electric generator, or for industrial process heat) or
to a body of material for energy storage, are a proven, reliable,
and relatively low-cost technology for collecting energy. The tubes
upon which the light is focused in such systems are typically
termed "receiver tubes," "receivers," or "heat collection
elements." At a typical electric generation plant employing solar
parabolic troughs, many receivers (e.g., thousands) are arrayed
with reflective troughs in parallel rows to form a "field" that can
collect sufficient energy for a generating system of economical
size. At present, receivers represent approximately 12% of the
capital cost of a concentrating solar power installation employing
solar parabolic troughs.
[0005] In a typical receiver constructed according to the prior
art, a central liquid-carrying tube with an outer optical
absorption coating is surrounded by a vacuum held within a
transparent concentric envelope. Light focused on the receiver by a
mirror (also known as a "collector") passes through the transparent
concentric envelope and through the vacuum and impinges on the
central liquid-carrying tube. The coating on the central tube
absorbs most (preferably all, although this cannot be realized in
practice) of the energy incident upon it and is thus heated. This
heat is transmitted by conduction through the wall of the central
tube and thence to the tube's liquid contents. The heated liquid is
pumped through the receiver and, in general, through additional
receivers, being thus raised to a high temperature (e.g.,
400.degree. C.) before being pumped to a boiler, or energy storage
device (e.g., reservoir of hot fluid). The function of the vacuum
between the inner, fluid-carrying tube and the outer, transparent
envelope is to prevent loss of heat from the receiver by convection
and conduction to the outer envelope and thence, by radiation and
conduction, to the environment.
[0006] A number of problems in the use of standard
vacuum-containing receiver tubes have been observed. These include,
but are not limited to, the following: (a) The absorption coatings
on the inner, fluid-carrying tube are expensive to manufacture. (b)
Degradation of a receiver's vacuum entails increased thermal losses
from the receiver and, if severe enough, requires replacement of
the receiver. In practice, vacuum degradation causes failure of
1-5% of receiver tubes per year. (c) The tubular outer glass
envelope of a conventional receiver must be thick enough to
withstand the stresses imposed by containing a vacuum as well as by
wind and its own weight. This strength requirement increases the
cost of the envelope. (d) The absorptive coating upon the inner,
fluid-containing tube of a receiver not only absorbs radiant energy
but emits it, particularly in the infrared part of the spectrum.
Emission losses increase with coating temperature T approximately
as the fourth power of T (i.e., as T.sup.4). Energy thus emitted is
for the most part lost to the environment, diminishing the
receiver's efficiency. Moreover, the absorptive coating may be
destroyed by sufficiently high T. Prohibitively large T.sup.4
radiation losses, coupled with high-temperature instability of the
absorber coating, today prevent practical operation of solar
parabolic-trough generating plants at elevated temperatures
(>500.degree. C.). Yet, for fundamental thermodynamic reasons it
is more efficient to operate any thermal generating plant at higher
peak T.
[0007] There is thus a need for receivers for parabolic-trough
solar power that are less costly to acquire and maintain than are
receivers constructed according to the prior art and that allow
operation at higher temperature.
SUMMARY OF THE INVENTION
[0008] In particular, the invention addresses the following issues
and provides advantages over the prior art. Receivers are provided
that, relative to state-of-the-art receivers, do not suffer from
vacuum degradation, whose absorptive coatings and other components
are less costly and simpler to manufacture, that operate durably
with undiminished or improved overall energetic efficiency at
today's typical operating temperatures (e.g., 350.degree. C.) or at
higher temperatures (e.g., at 600.degree. C. or above), and that
can be used either in the construction of new power plants or as
drop-replacements for defunct conventional receivers in existing
power plants. Such improved receivers are expected to lower the
levelized cost of energy for concentrating solar power produced
using solar parabolic troughs.
[0009] Moreover, for most thermal energy storage systems, the
operational temperature range has an effect on the cost of storage:
that is, increased temperature tends to decrease cost. The
increased operating temperature enabled by embodiments of the
invention will reduce the amount of thermal storage media required
for thermal energy storage compared to the current approaches known
in the art, which, for linear, trough-style collection systems,
typically operate in the range of 300.degree. C.-400.degree. C. For
example, for a system employing salt (or other thermal storage
medium storing thermal energy in sensible heat capacity, such as
gravel, ceramics, oils, and other solids and fluids) as the thermal
storage media, the mass of salt required to store a given quantity
of energy is inversely proportional to the temperature differential
in the storage system. Thus, a trough system operating from
300.degree. C.-600.degree. C. requires approximately three times
less storage salt for a given quantity of energy than does a trough
system operating from 300.degree. C.-400.degree. C. This reduction
in storage-material mass and the associated reduction in costs make
it possible to economically add higher thermal-energy storage
capacities.
[0010] As used hereinafter, a "tube" or object having a "tubular"
form is any elongated, two-ended, hollow body whose cross-sectional
form is a simple closed figure (e.g., circle, rectangle, rectangle
with rounded corners); a tube may be either closed or open at its
ends. The invention pertains to a tubular receiver or
heat-absorbing element for use in concentrating solar power
systems. Light is focused along the length of the receiver by a
trough-shaped collector having a reflective surface that is
typically parabolic in cross-section.
[0011] According to one aspect, the invention features a central
tube or pipe (herein also termed "the radiation-absorbing element")
through which a fluid heat-transfer medium flows. A portion of the
energy focused upon the receiver by the collector is ultimately
absorbed by the fluid medium (e.g., the fluid medium is heated
and/or undergoes a phase change). The fluid is then circulated
through the radiation-absorbing element and circulated through
piping in order to transport the heat energy to a boiler, to a
storage unit, or to another destination.
[0012] The invention also features a solar absorber coating on part
or all of the exterior surface of the radiation-absorbing element.
The portion of the radiation-absorbing element's surface that is
coated with the absorbent coating is herein termed the "absorbing
surface." The solar absorber coating is designed to absorb a large
portion of the light that impinges upon it, converting this energy
to the form of heat; is designed to be stable at temperatures up to
and in excess of 400.degree. C.; and is designed to have both high
optical absorptance and low thermal emissivity. That is, the solar
absorber coating effectively absorbs light, especially in the
visible part of the spectrum, but tends not to re-radiate the
energy thus absorbed as infrared light. Thus, energy collected by
the radiation-absorbing element tends to be retained rather than
dissipated to the environment in the form of infrared
radiation.
[0013] The invention also features a substantially opaque,
thermally insulating jacket around at least a portion of the
radiation-absorbing element.
[0014] The invention also features a second tube (herein termed the
"shell") that surrounds the radiation-absorbing element and
thermally insulating jacket. The shell admits light through a
non-opaque strip or segment (herein also termed the "aperture")
that runs lengthwise along the shell.
[0015] In various embodiments, the aperture may be covered partly
or wholly by one or more strips of a solid, transparent material
(e.g., glass) or may consist partly or wholly of an unobstructed
opening. The heat-absorbing element is located within the shell,
aligned with the shell, and separated from the shell by an
intervening space at most points. The space between the pipe and
the shell is herein also termed the "cavity." The absorbing surface
of the heat-absorbing element is exposed to light that enters
through the aperture. Light that has entered through the aperture
may impinge directly on the absorbing surface, or may undergo one
or more reflections or be absorbed and re-emitted one or more times
before impinging on the absorbing surface. The portion of the
cavity through which light can pass after entering the aperture or
being reflected or re-emitted within the cavity is herein termed
the "optical cavity"; the absorbing surface is exposed to (i.e.,
forms one surface or wall of) the optical cavity. The portion of
the cavity between the heat-absorbing element and the shell that is
not the optical cavity is filled with the thermally insulating
jacket and is herein termed the "insulation cavity." Portions of
the cavity not occupied by some solid material are occupied by a
gas (e.g., ordinary air) at or near ambient atmospheric
pressure.
[0016] In one embodiment, the optical cavity and the insulation
cavity are separated by two barriers that are herein termed the
"sidewalls." The sidewalls bound the sides of the optical cavity
and may consist essentially of strips of a relatively thin
material. The sidewalls prevent or impede the mixing of gas in the
insulation cavity with gas in the optical cavity. The aperture, the
absorbing surface, and the sidewalls are positioned and sized so
that when the receiver is approximately aligned with the focus of a
trough-shaped mirror of specific dimensions, light reflected from
any portion of the mirror enters the aperture along a path that
leads directly to the absorbing surface of the collector. Light
focused from the mirror does not impinge substantially upon the
sidewalls. The sidewalls may be opaquely absorbent, opaquely
reflective, or transparent.
[0017] In another embodiment, the optical cavity and the insulation
cavity are separated by sidewalls that are parabolic in
cross-section. Each sidewall is reflective on the side facing the
optical cavity. The aperture, absorbing surface, and parabolic
sidewalls are sized and shaped so that when the receiver is placed
approximately at the focus of a trough-shaped mirror of specific
dimensions, light reflected from every portion of the mirror either
(a) enters the aperture along a path that leads directly to the
absorbing surface or (b) after entering the aperture is reflected
from a parabolic sidewall and then impinges on the absorbing
surface.
[0018] In yet another embodiment, sidewalls bound the optical
cavity on two sides and the opening is covered wholly or partly by
a linear strip of transparent material (e.g., glass) having a
cross-section that causes the strip to act as a lens. The lens,
aperture, absorbing surface, and sidewalls are positioned and sized
so that when the receiver is approximately aligned with the focus
of a trough-shaped mirror of specific dimensions, light reflected
from every portion of the mirror impinges on the lens and is
refracted thereby into the optical cavity. Light reflected by any
portion of the mirror and passing through the lens does not impinge
substantially upon the sidewalls. The sidewalls may be opaquely
absorbent, opaquely reflective, or transparent.
[0019] In a further embodiment, the absorbing surface of the
heat-absorbing element is planar or approximately planar.
[0020] In still another embodiment, sidewalls bound the optical
cavity on two sides and the receiver features a concentric outer
tube (herein also termed "the cover") that encloses the shell. The
cover includes a transparent strip or segment (herein also termed
"the window") that runs along its length. The window may consist
substantially of a transparent solid material or an opening. The
cover tube also includes an opaque strip or segment (herein also
termed "the cap") that runs along its length. The cap may feature
one or more of a reflective inward-facing surface and an insulating
layer. Both the window and the cap are wide enough and long enough
to at least cover the aperture. The cover may be rotated around its
long axis so as to bring either the cover window or the cap into
alignment with the aperture. The function of the window is to admit
light into the optical cavity; the function of the cap is to
prevent or reduce radiative and/or convective losses of energy from
the interior of the receiver. To admit light into the receiver, the
cover is rotated so that the cover window is over the aperture. To
conserve heat within the receiver, the cover is rotated so that the
cap is over the aperture. The cover window (if solid) and/or the
cap may make a partially or wholly airtight seal with the shell of
the receiver.
[0021] In a further embodiment, the outer surface of the receiver
(e.g., in various embodiments, the shell; e.g., in various other
embodiments, the cover) is shaped aerodynamically, i.e., in a
manner that minimizes the average force exerted upon the receiver
by winds.
[0022] According to one aspect, the invention features a linear
solar receiver for use in a concentrating solar power system. The
linear solar receiver comprises a solar radiation absorbing element
having an outer surface configured to circumscribe an interior
volume, the interior volume designed to contain a heat transfer
medium, the solar radiation absorbing element designed to absorb an
incident flux of solar radiation and transfer an absorbed flux of
energy to the heat transfer medium, the heat transfer medium
designed to receive and transport at least a portion of the
absorbed flux of energy, the heat transfer medium when transporting
at least a portion of the absorbed flux of energy being primarily
in a fluid phase; a solar selective absorber located on a first
portion of the outer surface of the solar radiation absorbing
element, the solar selective absorber having a thermal emittance
value and an optical absorptance value, the optical absorptance
value being different from the thermal emittance value, being
exposed to ambient atmospheric pressure; an substantially opaque
thermally insulating jacket, the substantially opaque thermally
insulating jacket in contact with a second portion of the outer
surface of the solar radiation absorbing element; and a solar
radiation admitting region having an interior surface, at least a
portion of the solar radiation admitting region being surrounded by
at least a portion of the substantially opaque thermally insulating
jacket, the solar radiation admitting region designed to allow
transmission of at least a portion of the incident flux of solar
radiation to be incident on the solar selective absorber, the solar
radiation admitting region being symmetric with respect to a plane
parallel to a length dimension of the solar radiation absorbing
element, the plane oriented in a perpendicular direction to the
outer surface of the solar radiation absorbing element.
[0023] According to another aspect, the invention relates to a
linear solar receiver for use in a concentrating solar power
system. The linear solar receiver comprises a solar radiation
absorbing element having an outer surface configured to
circumscribe an interior volume, the interior volume designed to
contain a heat transfer medium, the solar radiation absorbing
element designed to absorb an incident flux of solar radiation and
transfer an absorbed flux of energy to the heat transfer medium,
the heat transfer medium designed to receive and transport at least
a portion of the absorbed flux of energy, the heat transfer medium
when transporting at least a portion of the absorbed flux of energy
being primarily in a fluid phase, the solar radiation; a solar
selective absorber located on a first portion of the outer surface
of the solar radiation absorbing element, the solar selective
absorber having a thermal emittance value and an optical
absorptance value, the optical absorptance value being different
from the thermal emittance value, the solar selective absorber
being exposed to ambient atmospheric pressure; an substantially
opaque thermally insulating jacket, the substantially opaque
thermally insulating jacket in contact with a second portion of the
outer surface of the solar radiation absorbing element; and a solar
radiation admitting region having an interior surface, at least a
portion of the solar radiation admitting region being surrounded by
at least a portion of the substantially opaque thermally insulating
jacket, the solar radiation admitting region designed to allow
transmission of at least a portion of the incident flux of solar
radiation to be incident on the solar selective absorber, wherein
the receiver is designed to be located symmetrically between outer
edges of a solar mirror collector system such that at least a
portion of the receiver shades at least a portion of the solar
mirror collector from incident solar radiation.
[0024] According to another aspect, the invention relates to a
system for generating energy from solar radiation as part of a
solar power system. The system comprises a plurality of linear
receivers, each of the plurality of linear receivers including at
least a solar radiation absorbing element designed to absorb an
incident flux of solar radiation and transfer an absorbed flux of
energy to a heat transfer medium, the heat transfer medium designed
to receive and transport at least a portion of the absorbed flux of
energy, at least a portion of the radiation absorbing element being
covered with a solar selective absorber, the solar selective
absorber having a thermal emittance value and an optical
absorptance value, the optical absorptance value being different
from the thermal emittance value; a parabolic trough mirror
collector for concentrating solar radiation onto the plurality of
linear receivers; a control system for directing the parabolic
trough mirror at the sun, wherein the heat transfer medium
circulating in a first receiver in the plurality of linear
receivers is heated by solar radiation from a first elevated
temperature T1 to a second elevated temperature T2 over a first
distance corresponding to a length of the first receiver and the
heat transfer medium circulating in a second receiver in the
plurality of linear receivers is heated by solar radiation from a
third elevated temperature T3 to a fourth elevated temperature T4
over a second distance corresponding to a length of the second
receiver, where T4>T3.gtoreq.T2>T1, the first receiver and
the second receiver having structures designed for operation in
different temperature ranges.
[0025] According to another aspect, the invention relates to a
concentrating solar power system. The system comprises a plurality
of linear solar receivers connected in series, the plurality of
linear solar receivers connected in series arranged such that a
heat transfer medium flowing therethrough exhibits an increase in
temperature as it passes from a first end to a second end of the
plurality of linear receivers connected in series, at least one of
the plurality of linear receivers comprising: a solar radiation
absorbing element having an outer surface configured to
circumscribe an interior volume, the interior volume designed to
contain a heat transfer medium, the solar radiation absorbing
element designed to absorb an incident flux of solar radiation and
transfer an absorbed flux of energy to the heat transfer medium,
the heat transfer medium designed to receive and transport at least
a portion of the absorbed flux of energy, the heat transfer medium
when transporting at least a portion of the absorbed flux of energy
being primarily in a fluid phase; a solar selective absorber
located on a first portion of the outer surface of the solar
radiation absorbing element, the solar selective absorber having a
thermal emittance value and an optical absorptance value, the
optical absorptance value being different from the thermal
emittance value, the solar selective absorber being exposed to
ambient atmospheric pressure; an substantially opaque thermally
insulating jacket, the substantially opaque thermally insulating
jacket in contact with a second portion of the outer surface of the
solar radiation absorbing element; and a solar radiation admitting
region having an interior surface, at least a portion of the solar
radiation admitting region being surrounded by at least a portion
of the substantially opaque thermally insulating jacket, the solar
radiation admitting region designed to allow transmission of at
least a portion of the incident flux of solar radiation to be
incident on the solar selective absorber, the solar radiation
admitting region being symmetric with respect to a plane parallel
to a length dimension of the solar radiation absorbing element, the
plane oriented in a perpendicular direction to the outer surface of
the solar radiation absorbing element; a parabolic trough reflector
in optical registry with the at least one of the plurality of
linear receivers such that when incident solar radiation falls on
the parabolic trough reflector, the incident solar radiation is
directed to the solar radiation admitting region; and device
configured to extract thermal energy from the heat transfer medium
that exits the second end of the plurality of linear receivers
connected in series, thereby cooling the heat transfer medium, and
configured to return cooled heat transfer medium to the first end
of the plurality of linear receivers connected in series.
[0026] According to another aspect, the invention relates to a
linear solar receiver for use in a concentrating solar power
system. The receiver comprises a solar radiation absorbing element
having an outer surface configured to circumscribe an interior
volume, the interior volume designed to contain a heat transfer
medium, the solar radiation absorbing element designed to absorb an
incident flux of solar radiation and transfer an absorbed flux of
energy to the heat transfer medium, the heat transfer medium
designed to receive and transport at least a portion of the
absorbed flux of energy, the heat transfer medium when transporting
at least a portion of the absorbed flux of energy being primarily
in a fluid phase; a solar selective absorber located on a first
substantially planar portion of the outer surface of the solar
radiation absorbing element, the solar selective absorber having a
thermal emittance value and an optical absorptance value, the
optical absorptance value being different from the thermal
emittance value, the solar selective absorber being exposed to
ambient atmospheric pressure; an substantially opaque thermally
insulating jacket, the substantially opaque thermally insulating
jacket in contact with a second portion of the outer surface of the
solar radiation absorbing element, the second portion of the outer
surface of the solar radiation absorbing element comprising at
least 50% of an area of the outer surface of the solar radiation
absorbing element determined on a per unit length basis; a solar
radiation admitting region having an interior surface defined in an
external region, at least a portion of the external region in
contact with at least a portion of the substantially opaque
thermally insulating jacket, the solar radiation admitting region
designed to allow transmission of at least a portion of the
incident flux of solar radiation to be incident on the solar
selective absorber; and a parabolic mirror collector having a rim
angle of less than 75 degrees, the parabolic mirror collector
configured to reflect solar radiation on to the solar radiation
admitting region.
[0027] According to another aspect, the invention relates to a
linear solar receiver for use in a concentrating solar power
system. The receiver comprises a solar radiation absorbing element
having an outer surface configured to circumscribe an interior
volume, the interior volume designed to contain a heat transfer
medium, the solar radiation absorbing element designed to absorb an
incident flux of solar radiation and transfer an absorbed flux of
energy to the heat transfer medium, the heat transfer medium
designed to receive and transport at least a portion of the
absorbed flux of energy, the heat transfer medium when transporting
at least a portion of the absorbed flux of energy being primarily
in a fluid phase; a solar selective absorber located on the solar
radiation absorbing element, the solar selective absorber having a
thermal emittance value and an optical absorptance value, the
optical absorptance value being different from the thermal
emittance value; and wherein the receiver has a thermal efficiency
defined as one minus a heat loss divided by the absorbed flux of
energy, the thermal efficiency equal to or greater than at least
one of 94 percent at 450 degrees Celsius and 92 percent at 500
degrees Celsius.
[0028] In one embodiment of the apparatus as previously described,
the first portion of the outer surface of the solar radiation
absorbing element is substantially planar.
[0029] In another embodiment of the apparatus as previously
described, the first portion of the outer surface of the solar
radiation absorbing element comprises a fraction in the range of
0.50 to 0.20 of an area of the outer surface of the solar radiation
absorbing element determined on a per unit length basis.
[0030] In another embodiment of the apparatus as previously
described, the apparatus further comprises a symmetric parabolic
trough collector mirror having a rim angle of less than 75
degrees.
[0031] In another embodiment of the apparatus as previously
described, the interior surface of the solar radiation admitting
region is substantially parabolic in cross section as viewed
parallel to the length dimension of the solar radiation absorbing
element.
[0032] In another embodiment of the apparatus as previously
described, the interior surface of the solar radiation admitting
region is a reflective surface.
[0033] In another embodiment of the apparatus as previously
described, the heat transfer medium is selected from the group
consisting of a heat transfer salt, a low-melting-point inorganic
nitrate salt fluid, a hybrid organic siloxane-based fluid, a
molecular silicone-based fluid, an oil and steam.
[0034] In another embodiment of the apparatus as previously
described, a thermal efficiency of the linear solar receiver as a
function of temperature is at least as high as given in any of the
four rightmost columns of Table 1.
[0035] In another embodiment of the apparatus as previously
described, the apparatus appears in combination with an energy
collection system configured to operate an energy recovery machine
that relies upon the Carnot cycle for recovery of energy from the
heat transfer fluid.
[0036] In another embodiment of the apparatus as previously
described, the apparatus appears in combination with a machine that
generates steam.
[0037] In another embodiment of the apparatus as previously
described, the apparatus appears in combination with a machine that
generates electricity.
[0038] In another embodiment of the apparatus as previously
described, the apparatus appears in combination with a thermal
energy storage device.
[0039] In another embodiment of the apparatus as previously
described, the apparatus appears in combination with a controller
that controls the rate of generation of energy.
[0040] In another embodiment of the apparatus as previously
described, the apparatus has a thermal efficiency and an optical
efficiency such that increasing the thermal efficiency by
increasing a thickness of the substantially opaque thermally
insulating jacket decreases the optical efficiency due to increased
shading of the collector by the receiver.
[0041] In another embodiment of the apparatus as previously
described, the solar radiation absorbing element, substantially
opaque thermally insulating jacket, and the solar radiation
admitting region are symmetric with respect to a bisecting plane
that is parallel to the axis of the linear solar receiver.
[0042] In another embodiment of the apparatus as previously
described, the apparatus further comprises a glass cover enclosing
the solar radiation admitting region.
[0043] In another embodiment of the apparatus as previously
described, an inert gas is introduced into the radiation admitting
region.
[0044] In another embodiment of the apparatus as previously
described, the apparatus appears in combination with a plurality
linear solar receivers, the linear solar receivers each including
at least solar radiation absorbing elements, adjacent solar
radiation absorbing elements forming a nearly continuous absorbing
surface.
[0045] In another embodiment of the apparatus as previously
described, the apparatus appears in combination with a plurality
linear solar receivers, a first one of the plurality of receivers
operating at a first temperature and a second one of the plurality
of receivers operating at a second temperature, the first receiver
and the second of the plurality of receivers having different
designs, the first and the second temperatures being different.
[0046] In another embodiment of the apparatus as previously
described, the apparatus further comprises a symmetric parabolic
trough collector mirror structure, the symmetric parabolic trough
collector mirror structure being held in a substantially rigid form
with cable suspension.
[0047] In another embodiment of the apparatus as previously
described, the solar selective absorber is a plasmonic nanochain
cermet structure.
[0048] According to another aspect, the invention relates to a
method of generating energy from solar radiation. The method
comprises the steps of concentrating a flux of solar radiation
incident on a parabolic trough collector mirror onto a linear
receiver, the linear receiver being symmetric with respect to a
plane bisecting the linear receiver and parallel to a linear axis
of the linear receiver, the linear receiver including at least a
heat transfer conduit; absorbing a portion of the flux of solar
radiation with a solar selective absorber disposed on at least a
portion of the heat transfer conduit, the solar selective absorber
having a thermal emittance value and an optical absorptance value,
the optical absorptance value being different from the thermal
emittance value, the portion of the flux of solar radiation
constituting absorbed solar radiation; heating a heat transfer
medium circulating with the heat transfer conduit to a temperature
exceeding 350 degrees Celsius by transferring a first portion of
the absorbed solar radiation to the heat transfer medium, the first
portion of the absorbed solar radiation constituting transferred
solar radiation; controlling the parabolic trough collector mirror
with a control system to maintain directional focus at the sun,
wherein the receiver has a thermal efficiency defined as one minus
a heat loss divided by the absorbed flux of energy, the thermal
efficiency equal to or greater than at least one of 94 percent at
450 degrees Celsius and 92 percent at 500 degrees Celsius.
[0049] According to another aspect, the invention relates to a
method of generating energy from solar radiation. The method
comprises the steps of concentrating a flux of solar radiation
incident on a parabolic trough collector mirror onto a linear
receiver; controlling the parabolic trough collector mirror with a
control system to maintain directional focus at the sun; and
maintaining the parabolic trough collector mirror in a rigid shape
with a cable suspension system, the cable suspension system
attached to an upper suspension element above the parabolic trough
collector mirror and attached to a lower suspension element below
the parabolic trough collector mirror, the upper suspension element
and the lower suspension element attached to a plurality of support
elements, the plurality of support elements attached to a frame of
the trough collector mirror.
[0050] In another embodiment of the method as previously described,
a weight of the cable suspension system is less than a support
structure of a traditional parabolic trough mirror collector.
[0051] In another embodiment of the method as previously described,
at least a portion of the linear receiver functions as the upper
suspension element.
[0052] In another embodiment of the method as previously described,
the linear receiver includes at least an absorber conduit recessed
within substantially opaque insulation.
[0053] In another embodiment of the method as previously described,
the thermal efficiency is greater than or equal to at least one of
89 percent at 550 degrees Celsius, 85 percent at 600 degrees
Celsius, and 80 percent at 650 degrees Celsius.
[0054] In another embodiment of the method as previously described,
the at least a portion of the heat transfer conduit having the
solar selective absorber disposed is substantially planar.
[0055] In another embodiment of the method as previously described,
a portion of the heat transfer conduit is covered by substantially
opaque thermal insulation.
[0056] In another embodiment of the method as previously described,
at least a portion of the substantially opaque thermal insulation
has a thermal conductivity of less than 40 milliWatts per meter per
degree Kelvin.
[0057] In another embodiment of the method as previously described,
the substantially opaque thermal insulation is pyrogenic
silica.
[0058] In another embodiment of the method as previously described,
the solar selective absorber is a plasmonic nanochain cermet
structure.
[0059] In another embodiment of the method as previously described,
the plasmonic nanochain cermet structure is a Ni
nanochain-Al.sub.2O.sub.3 cermet.
[0060] In another embodiment of the method as previously described,
the parabolic trough collector mirror has a rim angle less than 75
degrees.
[0061] These and other objects, along with the advantages and
features of the present invention herein disclosed, will become
apparent through reference to the following description, the
accompanying drawings, and the claims. Furthermore, it is to be
understood that the features of the various embodiments described
herein are not mutually exclusive and may exist in various
combinations and permutations.
BRIEF DESCRIPTION OF THE DRAWINGS
[0062] The objects and features of the invention can be better
understood with reference to the drawings described below, and the
claims. The drawings are not necessarily to scale, emphasis instead
generally being placed upon illustrating the principles of the
invention. In the drawings, like numerals are used to indicate like
parts throughout the various views.
[0063] FIG. 1 is a schematic diagram of a concentrating solar-power
electrical generation facility.
[0064] FIG. 2 illustrates exemplary embodiments of a system
supporting a parabolic trough mirror and receiver tube.
[0065] FIG. 3A and FIG. 3B illustrates aspects of the prior art for
vacuum-containing receiver tubes.
[0066] FIG. 4 is a cross sectional view of a receiver according to
principles of the invention.
[0067] FIG. 5A, PLOT 5A is a graph that shows the receiver thermal
efficiency as a function of temperature for a prior art receiver
and a receiver constructed according to the principles of the
present invention.
[0068] FIG. 5A, PLOT 5B is a graph that shows the receiver total
efficiency as a function of temperature for a prior art receiver
and a receiver constructed according to the principles of the
present invention.
[0069] FIG. 5B is a cross-sectional plot of simulated contours of
constant temperature in a state-of-the-art receiver.
[0070] FIG. 5C is a graph of heat losses for a receiver built
according to the prior art under three different conditions of
operation.
[0071] FIG. 6 is a graph comparing experimental and extrapolated
data to simulated data, showing that the simulation produces
realistic results.
[0072] FIG. 7 is a schematic diagram of an illustrative embodiment
of an advanced cavity receiver according to the invention.
[0073] FIG. 8 is a graph of lines of constant temperature in a
simulation of three illustrative embodiments of the invention.
[0074] FIG. 9 is a graph of the heat loss per unit length of a
receiver employing the prior art as compared to that of
illustrative receivers embodying aspects of the invention.
[0075] FIG. 10 illustrates terms used herein to describe the
geometry of a receiver mounted above a collector.
[0076] FIG. 11 is a schematic diagram of an illustrative embodiment
of an advanced cavity receiver according to the invention,
including a lenticular aperture cover.
[0077] FIG. 12 is a schematic diagram of an illustrative embodiment
of an advanced cavity receiver according to the invention,
including a different lenticular aperture cover.
[0078] FIG. 13A through FIG. 13F show ray tracings for light
focused upon a concentric-tube receiver geometry.
[0079] FIG. 14A through FIG. 14C show ray tracings for light
focused upon a concentric-tube receiver geometry, incorporating
collector focusing errors.
[0080] FIG. 15A and FIG. 15B are graphs of the optical efficiency
of a system as a function of collector focal length and the
diameter of the receiver's absorber absorber for specified
collector errors.
[0081] FIG. 16 is a graph of the relationship between system
concentration ratio (focal spot size) and collector rim angle for a
receiver.
[0082] FIG. 17A and FIG. 17B graph the optical official of a
receiver having a planar absorbing surface for a range of
absorbing-surface widths, collector focal lengths and glass-covered
or uncovered apertures.
[0083] FIG. 18 illustrates the geometry of a compound parabolic
concentrator.
[0084] FIG. 19A1 and FIG. 19A2 graph the optical efficiencies of
receiver having a planar absorbing surface, 100% or 95% reflective
compound parabolic concentrators, with an uncovered aperture.
[0085] FIG. 19B1 and FIG. 19B2 graph the optical efficiencies of
receiver having a planar absorbing surface, 100% or 95% reflective
compound parabolic concentrators, and a glass-covered aperture.
[0086] FIG. 20A and FIG. 20B display illustrative graphs of energy
gained and lost by a receiver as a function of absorbing-surface
width.
[0087] FIG. 21A is a schematic diagram of an illustrative
receiver.
[0088] FIG. 21B is a graph of heat loss as a function of
temperature for this receiver and others.
[0089] FIG. 22A1, FIG. 22A2, FIG. 22A3 and FIG. 22A4 graph the
circulation of heat within, a receiver partially covered by
insulation and a receiver not covered by insulation.
[0090] FIG. 22B1 and FIG. 22B2 show categories of heat loss from a
receiver partially covered by insulation.
[0091] FIG. 23A graphs the circulation of heat within a receiver
having internal dividers.
[0092] FIG. 23B graphs the convection heat loss from, a receiver
having internal dividers.
[0093] FIG. 24A1 is a schematic diagram of a receiver having
internal insulation.
[0094] FIG. 24A2 graphs the circulation of heat within a receiver
having internal insulation.
[0095] FIG. 24B1 shows categories of heat loss for a receiver with
internal insulation.
[0096] FIG. 24B2 graphs heat loss as a function of temperature for
a receiver with internal insulation and others.
[0097] FIG. 25A, FIG. 25B, and FIG. 25C graph the circulation of
heat within the optical cavity of, and heat loss from, a cavity
receiver at various angles of tilt.
[0098] FIG. 26A is a graph of lines of constant temperature for a
cavity receiver both without an external cover over its cavity.
[0099] FIG. 26B is a graph of lines of constant temperature for a
cavity receiver with an external cover over its cavity.
[0100] FIG. 27 is a schematic diagram of an illustrative cavity
receiver with provisions for moving an external cover into place
over its cavity and removing said cover at will.
[0101] FIG. 28A, FIG. 28B, FIG. 28C and FIG. 28D are graphs of
contours of constant velocities from an illustrative receiver when
blown upon by wind at various angles.
[0102] FIG. 29 is a table of heat losses from various receivers
when blown upon by wind at various angles.
[0103] FIG. 30A and FIG. 30B are schematic diagrams of illustrative
cavity receivers having partially covered cavities.
[0104] FIG. 31 is a schematic diagram of a solar field in which a
graded series of cavity receivers is included in each row of the
field.
[0105] FIG. 32 is a schematic diagram of a solar field in which a
graded series of cavity receivers is included in each row of the
field, in addition to receivers employing prior art.
[0106] FIG. 33 is a schematic diagram of a method for supporting a
receiver above a collector.
[0107] FIG. 34A is a schematic diagram of an optical absorbing
layer applied to a substrate.
[0108] FIG. 34B is a schematic diagram of an optical absorbing
layer applied to a substrate.
[0109] FIG. 35 is a graph of extinction, absorption, and scattering
for optical absorbing layers that include nickel nanochains of
various lengths.
[0110] FIG. 36 is a pair of photomicrographs showing exemplary
nickel nanochain compounds.
[0111] FIG. 37 is a graph of the reflectance of a plasmonic
nanochain cermet structure.
[0112] FIG. 38A and FIG. 38B are, respectively, a schematic drawing
of the molecular structure of hydrogen silsesquioxane and a graph
of X-ray diffraction intensity of hydrogen silsesquioxane as a
function of time.
[0113] FIG. 39 is a graph of estimated component costs of some
embodiments of the invention compared to costs for the state of the
art.
DETAILED DESCRIPTION
[0114] FIG. 1 depicts an illustrative system 100 for the collection
of solar energy in the form of heat and the conversion of the
collected heat to electricity. Subsequent figures will clarify the
application of embodiments of the invention to such a system and
similar systems which use the collected heat in other manners, such
as to generate steam for industrial processes. The system 100
depicted in FIG. 1 comprises a solar field 102 for the collection
of solar energy. The solar field 102 comprises a plurality N rows
(e.g., row 104, row 106), depicted in FIG. 1 as viewed from above,
where each row comprises a plurality M receivers (e.g., receivers
108a, 108b; receivers 110a, 110b, etc.) upon which solar radiation
is focused by parabolic collectors (not depicted). The plurality of
the N rows is indicated in FIG. 1 by horizontal ellipses (e.g.,
ellipsis 112); the plurality of M receivers in each of the N rows
is indicated in FIG. 1 by vertical ellipses (e.g., ellipsis 114).
The number of receivers K in the solar field of the illustrative
system 100 is therefore K=NM. In other embodiments, the number of
receivers may vary from row to row.
[0115] Each receiver (e.g., receiver 108a) may be either a
conventional receiver constructed according to the prior art or a
novel cavity receiver according to embodiments of the invention as
described herein. In other words, it is possible to build a system
100 using receivers of the present invention as new construction,
or to retrofit some or all of an existing system 100 with receivers
of the present invention.
[0116] In one state of operation, a heat-transfer fluid (e.g.,
molten salt, refractory oil, a gas) is admitted to an entry
manifold 116 by a valve 118. In FIG. 1, any piping through which
the heat-transfer fluid may flow in some state of operation of the
system 100 is denoted by a thick dark line. The heat-transfer fluid
is distributed by the entry manifold to the N rows and, in each
row, passes sequentially through the M receivers in that row. In
general, an approximately equal amount of solar radiation Q.sub.c
is gathered by the collector associated with each receiver, and
absorbed by the fluid in each receiver with a collector efficiency
.eta..sub.r. Thus, an approximately equal quantity of solar energy
Q.sub.r=Q.sub.c.times..eta..sub.r is added to the heat-transfer
fluid during its passage through each receiver. In passing through
the M receivers in each row, the heat-transfer fluid therefore
gains an amount of energy approximately equal to Q.sub.r.times.M
before entering an exit manifold 120. The total amount of thermal
energy Q.sub.total added to the heat-transfer fluid that enters
through manifold 116, passes through the solar field 102, and exits
through manifold 120 is therefore, to a first approximation,
Q.sub.t=Q.sub.r.times.M.times.N. However, in practice the amount of
energy will be a lesser quantity Q'.sub.t<Q.sub.t because of
losses from collectors and other components. Where the receivers
are vacuum-containing tubes according to the prior art, receiver
losses are dominated by, but not restricted to, radiation losses
from collectors, i.e., infrared radiation emitted by the hot
coating on each collectors' heat-absorbing element. Because
emissive losses tend to increase by approximately the fourth power
of T, losses are greater from hotter tubes, i.e., those closer to
the exit manifold 120. Energy losses are therefore not uniform
throughout the solar field 102. The overall efficiency of the solar
field, .eta..sub.f, depends complexly on direction and intensity of
insolation (solar radiation impinging on the solar field 102),
collector efficiency, absorber coating absorptivity and emissivity
as a function of temperature, receiver geometry, convection and
conduction losses from receivers, heat-transfer fluid
characteristics, and other factors.
[0117] Heated heat-transfer fluid leaving the solar field 102
through the exit manifold 120 is gated by a valve 122. Two
additional valves 124, 126 direct the heat-transfer fluid from the
exit manifold 120 to at least one of a hot-fluid reservoir 128,
piping 130, and piping 132. Fluid may also be extracted from hot
reservoir 128 by a pump (not depicted).
[0118] Heat-transfer fluid passing through piping 132 encounters
three devices in which the fluid exchanges heat with liquid water
or steam, i.e., a superheater 134, a heater 136, and a preheater
138. In each of the three heat-exchange devices 134, 136, 138, the
heat-transfer fluid gives up some of its thermal energy to liquid
water or steam passing through the same three devices 134, 136, 138
in the opposite direction. In FIG. 1, any piping through which
water and/or steam may flow in some state of operation of the
system 100 is denoted by a thick dashed line. Heat-transfer fluid
passing through piping 130 passes through a fourth heat-exchange
device (reheater) 140, giving up some of its thermal energy to
steam passing through the reheater 140 in the opposite direction.
Heat-transfer fluid that has flown through the heat-exchange
devices 134, 136, 1348, and 140 is re-united in piping 142. The
fluid may be directed through a valve 144 to a "cool" storage
reservoir 146. Fluid may also be extracted from cool reservoir 146
by a pump (not depicted). Heat-transfer fluid not directed to the
cool reservoir 146 passes through a pump 148 and is pressurized
sufficiently to enable circulation of the fluid through the solar
field 102. Booster pumps for heat-transfer fluid and water and/or
steam that are not depicted in FIG. 1 may be present in the system
100. Valves 118 and 150 direct the heat-transfer fluid pressurized
by the pump 148 to at least one of the solar field 102 and a boiler
154. The boiler 154 may burn a fossil fuel (e.g., natural gas) or
derive thermal energy from some other source in order to heat the
heat-transfer fluid passing therethrough. When the solar field 102
is not capable of maintaining the temperature of the heat-transfer
fluid throughout system 100 at or above an acceptable minimum (due
to, e.g., lack of insolation), the boiler 152 may be employed to
heat the heat-transfer fluid. Alternatively or additionally, the
boiler 152 may be used as a backup source of heat to drive energy
generation by the system 100.
[0119] Water heated in the preheater 138 is boiled to steam in the
heater 136 and the steam is further heated in the superheater 134.
Steam from the superheater 134 is directed through piping 154 to a
first turbine stage 156. After expanding in the first turbine stage
156, imparting energy thereto and declining in temperature, the
steam is directed through piping 158 to the reheater 140, where it
is heated by maximum-temperature heat-transfer fluid direct from
the solar field 102 or from the hot-fluid reservoir 128. The steam
is then directed through piping 160 to a second turbine stage 162.
After expanding in the second turbine stage 162, imparting energy
thereto and declining in temperature, the steam is directed through
piping 164 to a condenser 166, where it gives up further energy
(to, e.g., a preheater flow of heat-transfer fluid not depicted in
FIG. 1) and is condensed to water. The water from condensor 166 is
directed through piping 168 to a water reservoir 170. Water exiting
the reservoir 170 passes through a pump 172. Water pressurized by
the pump 172 enters the initial heating loop (devices 138, 136,
134) and continues as already described.
[0120] The turbine stages 156, 162 turn a generator 174 that
produces electricity. In other embodiments, the heat or generated
steam may be used for other processes such as industrial
processes.
[0121] A controller (e.g., a general purpose programmable computer
that operates under the control of one or more instructions
recorded on a machine readable memory) 180 receives data from the
solar field 102 (on, e.g., fluid temperature and pressure in
different parts of the field; collector orientation; insolation
intensity and direction) and sends control signals to various parts
of the solar field 102 to optimize its importance. For example, the
controller may progressively tilt collectors as the sun moves
across the sky in order to collect maximum energy.
[0122] In an ordinary state of energy-producing operation, the sole
source of energy input to the system 100 is sunlight falling on the
collectors of the solar field 102. The efficiency with which this
solar energy is converted to heat by absorption in the receivers
(108a, 110a, . . . , 108b, 110b, . . . ), and retention of the
energy thus absorbed by the heat-transfer fluid as it passes
through the solar field 102, are therefore influence the cost of
electricity produced by the system 100.
[0123] FIG. 2 depicts an illustrative assembly 200 for the
collection of solar energy in the form of heat that may be part of
a larger system for the conversion of solar energy to thermal
and/or electrical energy. Assembly 200 is typical of
receiver-and-trough assemblies, according to the prior art, that
could be used in the solar field 102 of a power-generating system
similar to that illustrated in FIG. 1. Receivers embodying the
invention could also be employed, advantageously, in assembly 200
and in solar field 102 in FIG. 1.
[0124] Illustrative assembly 200 comprises a receiver tube 202.
Solar radiation 204 is reflected from a trough-shape collector 206
having a parabolic cross-section and impinges on the receiver 202.
The receiver 202 is held in place at the focus of the parabolic
trough 206 by struts 208. The approximately rigid assembly
comprising receiver 202, struts 208, and trough 206 is held in
place by a truss-type support structure 210 which is in turn
mounted upon supports 212 and can be rotated on a joint or bearing
mechanism 214 so that the trough 206 faces the sun (i.e., is tilted
at an angle equal to the elevation of the sun above the horizon at
a given moment). Movement of the assembly 200 for sun-track, or
operation of other controllable features of the assembly 200, may
be controlled by a controller (e.g., a general purpose programmable
computer that operates under the control of one or more
instructions recorded on a machine readable memory) 216.
[0125] FIG. 3A (side view) and FIG. 3B (cross-sectional view)
depict some of the features of a typical receiver tube 300
according to the prior art. The tube 300 is typical of tubes that
would presently be used in the solar field 102 of a
power-generating system similar to that illustrated in FIG. 1. In
receiver 300, a transparent glass envelope 302 contains a steel
absorber tube (heat-absorbing element) 304. Between the envelope
302 and the tube 304 is a vacuum (evacuated annulus) 306. An
evacuation nozzle 308 permits removal of air from the space between
the envelope 302 and tube 304 during manufacture of the receiver
300. At each end of the receiver 300, a metal bellows 310 flexibly
accommodates changing forces due to thermal expansion and
contraction and to wind, and provides a point of attachment for the
struts (e.g., struts 208 in FIG. 2) that support the receiver 300.
A glass-to-metal seal 312 preserves the vacuum 306. A "getter"
insert 314 absorbs some of the hydrogen that infiltrates the vacuum
306.
[0126] FIG. 3B makes clear that the tube 304 is coaxial with the
envelope 302, with the vacuum 306 between them. In various
embodiments according to the prior art, coatings to selectively
absorb and reflect radiation, conduct heat, resist corrosion, or
achieve other purposes are applied to the outer surface 318 and
inner surface 320 of the envelope 302 and to the outer surface 322
and inner surface 324 of the tube 304. Heat-transfer fluid 316
circulates within the tube 304.
[0127] FIG. 4 pertains to one embodiment of the invention. Before
describing FIG. 4 in detail, we note the following general
considerations, which pertain to various embodiments of the
invention: Various embodiments, including that depicted in FIG. 4,
comprise a tubular heat-absorbing element partly enclosed in an
insulating layer or jacket, with a recess or cavity (also herein
termed the "optical cavity"), transparent to light on one side
along an aperture, below an exposed surface (also termed "the
absorbing surface") of the heat-absorbing element. In
cross-section, the insulation jacket has approximately the form of
a letter "C" that can be turned, or is permanently turned, so that
the opening of the "C" faces toward a parabolic collector that
focuses light thereon. In this analogy, the gap in the "C"
corresponds to the optical cavity, and the width of the gap in the
"C" corresponds roughly to the width of the receiver aperture. A
portion of the heat-absorbing element, herein termed the "absorbing
surface," is exposed to light entering the optical cavity through
the aperture. Making the absorbing surface planar (i.e., flat)
rather than another shape (e.g., an arc) reduces the absorbing
surface's area for a given aperture width; the advantages realized
by doing so are discussed further below.
[0128] An additional design consideration in various embodiments is
the inclusion or omission of a transparent aperture cover (e.g., of
glass) enclosing the recessed optical cavity. Omitting the aperture
cover tends to improve optical performance but hinder thermal
performance in some circumstances (e.g., wind directed along
cavity) and tends to expose the absorber surface to potential
contamination (e.g., by dust). A transparent aperture cover may be
shaped to act as a lens that concentrates light that is incident on
the aperture (especially light incident on the aperture at a low
angle) into a narrower cavity. A lensing aperture cover can enable
further reduction in absorbing-surface size, as clarified in later
Figures and descriptions.
[0129] An additional design consideration in various embodiments is
the inclusion or omission of reflective parabolic sidewalls that
bound the lateral faces of the recessed optical cavity. Such
reflectors, also herein termed "compound parabolic concentrators"
because they constitute a stage of parabolic concentration
additional to that of the collector trough, can enable further
reduction in absorbing-surface size, aperture width, and overall
receiver width, as clarified in later Figures and descriptions. If
compound parabolic concentrators are omitted, the sidewalls of the
optical cavity may be oriented and shaped so as to intercept
essentially none of the light-rays directed into the optical cavity
by the collector, with or without passage through a lensing
aperture cover. Such non-intercepting sidewalls are also herein
termed "passive" sidewalls. Reflective sidewalls having
non-parabolic geometries may also be employed.
[0130] Receiver designs incorporating (a) a tubular heat-absorbing
element partially surrounded by an insulating jacket, (b) a
recessed optical cavity with sidewalls (either passive sidewalls or
compound parabolic concentrators), and (c) a portion of the
heat-absorbing element that is exposed to light in at least some
states of operation of the receiver and which, when exposed to
light, acts as absorbing surface (either planar or curved), are
herein also referred to as illustrative of the "advanced cavity
receiver geometry." Various embodiments of the advanced cavity
receiver geometry may also include one or more of the following
features, among others: (a) a tubular shell around the insulation
layer, (b) an outermost tubular cover that may be moveable, or with
respect to which the other components of the receiver may be
moveable, and which may incorporate transparent or insulating
covers that can cover the aperture in various states of operation,
(c) gas (e.g., air) within the optical cavity and/or other portions
of the receiver, which gas may be at atmospheric pressure, (d) a
vacuum within the optical cavity and/or other portions of the
receiver, and (e) a transparent cover over part or all of the
aperture, which cover may be shaped to act as a lens, (f) various
specialized optical and thermal coatings over the surfaces of any
of the components (e.g., over a transparent aperture cover), (g)
the capability to flow/insert an inert gas into the aperture.
[0131] The geometric details of various embodiments of the advanced
cavity receiver, particular the depth of the optical cavity and the
angles and shapes of the sidewalls of the optical cavity, are
influenced by the collector rim angle (this term is clarified in
FIG. 10). Variations on the advanced cavity receiver design seek to
minimize absorbing-surface width and overall cavity width, to
maintain sufficient cavity depth for insulating purposes, and to
operate efficiently in conjunction either with existing trough
collectors or new collectors having similar key dimensions.
[0132] FIG. 4 is a cross-sectional diagram that depicts features of
an illustrative novel receiver that incorporates aspects of the
invention and shows one possible response to the range of design
considerations discussed in the foregoing paragraphs. In this novel
design, an advanced cavity receiver 400, is a dramatic departure
from the construction of prior art receivers (e.g., as depicted in
FIG. 3A and FIG. 3B). This illustrative advanced cavity receiver is
simpler to manufacture and maintain than the conventional receiver
of FIG. 3B because it does not contain a vacuum and reduces the
exposed glass component. Elimination of the vacuum confers a number
of advantages, including obviation of the airtight seal 312 in FIG.
3 and the "getter" insert 314, elimination of vacuum failure
(responsible for failure of .about.1-5% of conventional
vacuum-containing receivers per year in real-world usage), and
possible easing of design constraints on the bellows 310 or
equivalent contrivances for accommodating structural stresses and
movements (since there is no airtight seal that needs to be
protected against mechanical failure). In additional, the removal
of vacuum baffles allows for adjacent receivers to be nearly
continuous.
[0133] The illustrative advanced cavity receiver 400 comprises a
tubular shell 402, a heat-absorbing element 404 through which a
heat-transfer fluid 406 can flow, reflective parabolic sidewalls
408, 410, an aperture 412, a transparent aperture cover 414
spanning the aperture 412, an insulating jacket 416 partially
surrounding the heat-absorbing element 404, a planar (flat)
absorbing surface 418 coated with an absorbent coating and exposed
to light entering the aperture 412, and an optical cavity 420
bounded by the absorbing surface 418, sidewalls 408, 410, and
aperture cover 414. The shell 402 and insulation 416 may be
substantially opaque or be optical transmitting. Substantially
opaque insulation blocks most but not necessarily all of the
radiation. Optical transmitting insulation has a transmittance of
above 80%. The receiver 400 is approximately uniform in
cross-section along its entire length, apart from mounting and
other hardware (not shown) at each end. The optical cavity or other
portions of the collector 400 may be evacuated or filled with a gas
(e.g., air) at approximately ambient atmospheric pressure. If the
gas is air, the advantage is gained over the prior art that no
provision need be made to exclude ambient air from any portion of
the receiver 400 except the fluid-containing heat-absorbing element
404.
[0134] The specific dimensions of the components of a receiver
having a schematic cross-section like receiver 400 are chosen to
optimize performance (i.e., solar energy collected) when the
receiver 400 is mounted at the focus of a trough-shaped collector
of specific dimensions whose reflective surface is parabolic in
cross-section. That is, the design of a receiver cannot be
considered entirely apart from the dimensions of the collector with
which it is intended to work. Geometric and other considerations
pertaining to the design of a receiver resembling receiver 400 in
FIG. 4 are described and clarified further in FIG. 18 and other
Figures below.
[0135] In various embodiments, the selective absorber coating on
the absorbing surface 418 is a spectrally selective plasmonic
nickel nanochain cermet (ceramic-metal blend) material that may be
created using solution-chemical fabrication. The advantages offered
by advanced coating materials of this type include, but are not
limited to, simpler fabrication than conventional cermet absorber
coatings, tolerance of higher temperatures, and desirable
absorbance and emissivity properties. Such materials have been
demonstrated in the laboratory by Prof Jifeng Liu and colleagues at
Dartmouth College, NH and disclosed in, e.g., Wang X, Li H, Yu X,
Shi X, and Liu J, "High-performance solution-processed plasmonic Ni
nanochain-Al.sub.2O.sub.3 selective solar thermal absorbers," Appl.
Phys. Lett. 101, 203109 (2012), but have not hitherto been
disclosed as part of any novel receiver design such as the advanced
cavity receiver 400.
[0136] In brief, receiver 400 operates as follows: light rays 422,
424 are focused across a range of angles upon the aperture 412 by a
parabolic collector (not shown). All light rays passing through the
transparent aperture cover 414 strike the absorber surface 418
after either (a) traversing the optical cavity 420 directly, as
does ray 424, or (b) reflecting off a parabolic sidewall 408 or
410, as does ray 422. The absorber surface 418 absorbs a large
portion of the solar radiation incident upon it and is heated
thereby. Because the absorber surface 418 is planar, in a departure
from the prior art, it minimizes the surface area over which
infrared light is emitted by the heated absorber coating. Any
non-planar absorber surface in a receiver otherwise identical to
receiver 400 would, by straightforward geometry, have a larger
surface area than the planar absorber surface 418. Consequently,
for a given coating composition at a given temperature, the planar
absorber surface of FIG. 4 will radiate less energy than a
non-planar absorber surface simply because its area is smaller. In
an extreme case, the absorber coating of a receiver constructed
according to the prior art is 100% of the lateral surface of the
tubular heat-absorbing element. By contrast, in various embodiments
incorporating aspects of the invention, the absorbing surface
(e.g., absorbing surface 418 in FIG. 4) may occupy only 20% of the
lateral surface area of the heat-absorbing element, and at most 50%
of the lateral surface of the heat-absorbing element.
[0137] Moreover, the receiver geometry and advanced coatings
proposed for the absorber surface 418 will allow operation of the
receiver 400 at higher temperatures than can be tolerated by the
absorber coatings of vacuum-tube receivers constructed according to
the prior art. Operation at higher temperature (i.e., temperature
of absorber surface 418, heat-absorbing element 404, and
heat-transfer fluid 406) in energy-generating facility such as
system 100 in FIG. 1 will, in general, operate more efficiently if
the temperature of the heat-transfer fluid exiting the solar field
102 is higher: preferably, as high as the material construction of
the system 100 permits.
[0138] These and other advantages of the invention will be made
clear in subsequent Figures and descriptions.
[0139] In order to operate a solar field 102 at elevated
temperatures, it is desirable that heat-transfer fluid be available
which can tolerate such elevated temperatures and have other
advantageous properties (e.g., does not freeze ambient
environmental temperatures). Candidate heat-transfer fluids
include, e.g., molten solar salt, steam, superheated steam,
high-pressure steam, Coastal Chemical's HITEC.RTM. heat transfer
salt ("HITEC Heat Transfer Salt," Coast Chemical Co., LLC,
www.coal2nuclear.com/MSR%20-%20HITEC%20Heat%20Transfer%20Salt.pdf,
accessed Jan. 6, 2012), Sandia National Laboratory's
low-melting-point inorganic nitrate salt fluid ("Low-melting point
inorganic nitrate salt heat transfer fluid," U.S. Pat. No.
7,588,694, issued Sep. 15, 2009), the hybrid organic siloxane-based
fluid being developed by Los Alamos National Laboratory ("Hybrid
Organic Silicone HTF Utilizing Endothermic Chemical Reactions for
Latent Heat Storage,"
http://www1.eere.energy.gov/solar/sunshot/csp_recoveryact_lan1.html,
accessed Jan. 6, 2012), and CX500, a molecular silicone-based fluid
with exceptional thermal stability (to at least 500.degree. C.) and
low freezing point (-40.degree. C.) developed at Los Alamos
National Laboratory by Dr. Stephen Obrey and colleagues (P. C.
DeBurgomaster, S. J. Obrey, L. N. Lopez, "Rational Tailoring of
Functional Siloxanes for Enhanced Thermal Performance,"
Presentation at the American Chemical Society National Meeting in
Anaheim, Calif., 2011, at
http://permalink.lan1.gov/object/tr?what=info:lan1-repo/lareport/LA-UR-11-
-01885, accessed Jan. 6, 2012).
[0140] FIG. 5A includes two data plots, Plot 5A and Plot 5B, which
compare aspects of performance of a state-of-the-art conventional
receiver (i.e., the Schott PTR70) and of an illustrative advanced
cavity receiver that resembles receiver 400 in FIG. 4 and
incorporates aspects of the invention. Figures for Schott PTR70
conventional receiver performance are derived from Burkholder F,
Kutscher C, "Heat Loss Testing of Schott's 2008 PTR70 Parabolic
Trough Receiver," National Renewable Energy Laboratory (NREL)
Technical Report NREL/TP-550-45633, May 2009. Testing by Burkholder
and Kutscher did not exceed 500.degree. C., and the cermet absorber
coating of the Schott PTR70 cannot endure prolonged exposure to a
temperature of 650.degree. C.; therefore, the efficiency figures
for "Current Receivers" in Plot 5A and Plot 5B are based on
extrapolation of curves presented by Burkholder and Kutchscher
(2009) (whose results are collectively henceforward referred to as
"NREL data"). The figures represent the approximate performance
that a state-of-the-art conventional receiver would achieve if it
were endowed with an absorber coating capable of operating at
temperatures above approximately 500.degree. C.
[0141] To predict performance of advanced cavity receiver designs,
a computer model was constructed for both the conventional receiver
design and a range of advanced cavity designs. Validation of the
model, and additional results therefrom, will be further explained
in later Figures.
[0142] The Zemax software tool was used to predict the optical
efficiency of both the conventional and advanced cavity receiver.
Zemax is available from Radiant Zemax, 22908 NE Alder Crest Drive,
Suite 100, Redmond, Wash. 98053. The ANSYS Fluent software package
was used to predict the thermal efficiency of both the conventional
receiver and a range of advanced cavity receivers. ANSYS Fluent is
available from ANSYS, Inc., 275 Technology Drive, Canonsburg, Pa.
15317.
[0143] As used herein, the "optical efficiency" of a receiver is
defined as the fraction of the radiant energy incident on the
collector that is collected as heat by the heat-transfer fluid
within the receiver. Simulation of the conventional receiver was
compared to NREL data to validate the model: conventional-receiver
results matched both measured and extrapolated results within 1-3%,
lending confidence to the advanced-cavity receiver results
presented herein.
[0144] As used herein, the "total efficiency" of a receiver is the
fraction of radiant energy focused upon the receiver that is
delivered as heat energy by the heat-transfer fluid upon exit from
the receiver, and is defined as the product of the receiver's
optical efficiency and thermal efficiency.
[0145] As used herein, the "thermal efficiency" of the receiver is
defined as one minus the heat loss from the receiver divided by the
energy absorbed by the receiver. In computer simulations of
receiver behavior described herein, to specify the thermal
efficiency independently of the optical efficiency, the absorbed
energy is kept constant at 4500 W/m. This value assumes an optical
efficiency of 90% for an incident solar energy source of 1000
W/m.sup.2 on a collector with a 5 m diameter. Heat-loss data for
the state-of-the-art Schott PTR-70 receiver have been
experimentally determined (the NREL data) or are extrapolated from
such data, while heat loss for other receivers is calculated herein
using computational fluid dynamics models. Table 1 shows that
planar receivers (e.g., receiver 400 of FIG. 4) offer increased
thermal efficiency performance relative to a state-of-the-art
receiver. The numbers given in Table 1 are clarified below. As used
herein planar is used to generally distinguish a structure of the
present invention from a strictly circular structure and as such
includes variations that include some degree of curvature. For
example in some embodiments to facilitate the efficient transport
of fluid the central fluid transport channel can include rounded
corners and a generally bowed cross-sectional profile
TABLE-US-00001 TABLE 1 Thermal Efficiency of Schott PTR70 and
Planar Receiver Schott- Modeled PTR70 Planar Operating Receiver
Efficiency Efficiency Efficiency Receiver Temperature Prior Art
Estimate 1 Estimate 2 Estimate 3 Efficiency 400 94.9% 95.5% 96%
96.1% 96.8% 450 92.6% 93.8% 94% 94.4% 96.2% 500 89.4% 91.4% 92%
92.4% 95.3% 550 85.1% 88.2% 89% 90.2% 94.3% 600 79.3% 83.9% 85%
86.8% 93.1% 650 71.7% 78.4% 80% 81.7% 91.7%
[0146] While efforts have been made to include relevant important
physical factors in the CFD models, it is likely that various
embodiments of the invention will have efficiencies less than those
calculated by the ideal model value. The thermal efficiencies of
these embodiments may fall between those of the modeled planar
receiver and the measured PTR70 receiver due to additional heat
loss mechanisms and unaccounted for physical factors. Three
estimates for these efficiencies are the following:
[0147] The estimate 1 percentage is defined as:
Estimate 1 % = SOA % + Opimized Planar Receiver % - SOA % 3
##EQU00001##
[0148] Estimate 2 is the average of estimate 1 and estimate 3,
expressed as the nearest whole number.
[0149] The estimate 3 percentage is defined as:
Estimate 3 % = Opimized Planar Receiver % + SOA % 2
##EQU00002##
[0150] Plot 5A in FIG. 5A shows that in at least one embodiment,
the vacuum-free advanced cavity receiver maintains >90% receiver
thermal efficiency (plotted using black squares and solid lines) up
to 650.degree. C. operating temperature, substantially
outperforming the current vacuum-based state-of-the-art receiver
(plotted using black diamonds and dotted lines), whose thermal
efficiency drops below 75% efficiency at 650.degree. C.
[0151] Plot 5B in FIG. 5A combines the thermal-efficiency curves of
Plot 5A with optical-efficiency curves (not shown) to derive total
receiver efficiency curves for the same current vacuum-based
receiver and advanced cavity receiver as in Plot 5A. Even with
operation in air and the complete removal of receiver vacuum,
performance of the advanced cavity receiver at current operating
temperatures (.about.375.degree. C.) is approximately equal to that
of the current receiver; at higher temperatures (i.e., above
approximately 475.degree. C.), the total receiver of the efficiency
of the advanced cavity receiver exceeds that of the current
receiver.
[0152] Moreover, the estimated materials and manufacturing cost of
the embodiment whose simulated performance is depicted in FIG. 5A
are approximately 20% lower than those of the state-of-the-art
receiver whose performance is depicted in FIG. 5A. Therefore,
advantages to be realized by at least some embodiments of the
invention include higher efficiency at high operating temperatures
and lower capital cost per receiver. Lower rates of replacement for
receivers in the field is another advantage likely to be realized
by at least some embodiments of the invention, given the complete
absence of vacuum degradation as a concern in the advanced cavity
receiver.
[0153] FIG. 5B is a cross-sectional plot of simulated contours of
constant temperature (degrees Centigrade) in a state-of-the-art
receiver 500 constructed according to the prior art and operating
at a fixed temperature of the absorber coating of 400.degree. C.
Simulation was performed using the ANSYS Fluent software tool,
which was also used to create all other plots of simulated contours
of constant temperature and of velocity vectors in circulating
gases in subsequent Figures. In this instance, the vacuum normally
present in a state-of-the-art receiver 500 between its envelope 502
and heat-absorbing element 504 is presumed to have been lost, i.e.,
the space between the envelope 502 and heat-absorbing element 504
is filled with air. This simulation also assumes a heat-absorbing
coating having an emissivity of approximately 0.1 that does not
degrade when exposed to air i.e., does not increase in emissivity.
As used herein, the emissivity of an object is the dimensionless
ratio of the energy radiated by a the material to the energy
radiated by an ideal black body at the same temperature. Assuming a
nondegraded coating is a conservative assumption, because actual
coatings according to the previous art do degrade when exposed to
air and increase in emissivity. Increased emissivity entails
increased energy loss through infrared radiation from the
heat-absorbing surface.
[0154] FIG. 5C is a plot of simulated radiative heat loss in units
of watts per meter for the same simulated state-of-the-art receiver
500 depicted in cross-section in FIG. 5B. Simulated heat loss for a
range of assumed conditions and operating temperatures is plotted
in FIG. 5C. For each of three operating temperatures (i.e.,
400.degree. C., 550.degree. C., 650.degree. C.), the three assumed
operating conditions are as follows: (1) The black bars correspond
to a receiver 500 with intact vacuum; (2) the horizontal-striped
bars correspond to a receiver 500 with vacuum lost (as in FIG. 5B)
but no degradation of absorber coating (i.e., no increase in
emissivity above 0.1); and (3) the diagonal-striped bars correspond
to a receiver 500 with vacuum lost (as in FIG. 5B) and with
realistic degradation of absorber coating (i.e., emissivity
increased to 0.65). The operating condition and temperature of the
contour plot of FIG. 5B corresponds to the horizontal striped bar
in FIG. 5C for an operating temperature of 400.degree. C. FIG. 5C
shows that radiative heat loss increases both with temperature
increase and with loss of vacuum; this clarifies (a) the
disadvantage conferred on the prior art by reliance on vacuum and
(b) the even more drastic disadvantage conferred on the prior art
by coatings that increase in emissivity when exposed to air and to
higher temperatures (e.g., 550.degree. C. and 600.degree. C.).
[0155] FIG. 6 pertains to the validation of the ANSYS Fluent
computational fluid dynamics numerical (computer) model used to
predict the performance of a range of novel, advanced cavity
receiver designs, including that whose performance is depicted in
FIG. 5A and FIG. 5B. Thermal performance of receiver configurations
was determined by analyzing the results of radiative-loss,
convective-loss, and conductive-loss models in ANSYS Fluent, a
comprehensive finite-volume computational fluid dynamics program.
Radiative heat transfer losses were solved for using the
discrete-ordinates radiation model, in which the spatial domain is
discretized into a finite number of directions and the model solves
the radiative transfer equation for each discretized direction. The
radiation, conduction, and convective heat transfer models are
coupled in the Fluent solver through the energy equation and the
core Navier-Stokes equations. Fluent can capture the effects of
material properties such as surface emissivity and
temperature-dependent gas behavior.
[0156] Models to study natural convection effects within air-filled
receiver designs were tested by comparing to NREL data. The
receiver thermal models are based on a two-dimensional cross
section of each receiver with a constant-temperature boundary
condition at the external surface of the heat-absorbing element.
This boundary condition setup reflects parabolic-trough plant
operation for a target heat-transfer fluid loop outlet
temperature.
[0157] The computational fluid dynamics model for the current
state-of-the-art receiver is dominated by heat losses due to
radiation from the outer surface of the heat-absorbing element;
convective and conductive losses through the vacuum to the envelope
(and thence to the environment) are negligible. To test the model's
realism, the model was first used to predict the heat loss
(characterized in watts per meter of receiver) of a conventional
vacuum-based receiver (i.e., the Schott PTR70) independently of
that receiver's performance as measured and extrapolated in the
NREL data. Model values (plotted as black squares) in FIG. 6 match
experimental and extrapolated NREL data (plotted as black diamonds
in FIG. 6) within 1% for 400.degree. C. and 550.degree. C., and
within 3% for 650.degree. C. A convection-only model (omitting
radiation) matched heat losses predicted from the Raithby and
Holland annular flow heat-transfer correlation within 10% (see
Raithby and Holland, "Natural Convection," Ch. 4, Handbook of Heat
Transfer, Rohsenow W M, Hartnett J P, Cho Y I eds., McGraw-Hill,
1998), which is acceptable given the uncertainty in experimental
correlations. As further assurance, our model of a conventional
receiver with lost vacuum matched NREL data within 4%.
[0158] FIG. 7 cross-sectionally depicts an illustrative receiver
that incorporates features of the invention. The illustrative
advanced cavity receiver 700 comprises a tubular shell 702, a
heat-absorbing element 704 through which a heat-transfer fluid 706
can flow, planar sidewalls 708, 710, an aperture 712, a transparent
aperture cover 714 made of glass or another transparent material
and spanning the aperture 712, an insulating jacket 716 partially
surrounding the heat-absorbing element 704, a substantially planar
(flat) absorbing surface 718 coated with an radiation-absorbent
coating and exposed to light that enters the aperture 712, and an
optical cavity 720 bounded by the absorbing surface 718, sidewalls
708, 710, and aperture cover 714. In other embodiments, the
aperture cover 714 could be a partial covering to reduce
transmission losses (e.g., see FIG. 30), or could be absent. The
receiver 700 is approximately uniform in cross-section along its
entire length, apart from mounting and other hardware (not shown)
at each end. The optical cavity 720 and other portions of the
collector 700 are filled with a gas (e.g., air) at approximately
ambient atmospheric pressure.
[0159] The receiver cavity sidewalls 708, 710 are sloped at an
angle chosen to maximize admission of radiation from a reflective
parabolic trough collector (not depicted). Light rays concentrated
by the collector (e.g., rays 722, 724) enter the aperture 712 and
impinge directly on absorbing surface 718. The insulation may be
substantially opaque or be optically transmitting.
[0160] Thickness and properties of the insulation 716, along with
the dimensions of the shell 702, are chosen as a compromise between
minimizing thermal losses and minimizing shadowing of the collector
by the receiver. The insulating material maintains low thermal
conductivity at high temperatures (>600.degree. C.). The
radiation-absorbent coating on the heat-absorbing element 704 has
high (e.g., >90%) absorptance and low (e.g., <15%) thermal
emittance and is stable at receiver operating temperatures and
under indefinite exposure to ambient conditions (air, humidity).
The transparent aperture cover 714 may include a coating on its
external surface 726 to mitigate reflection of light from the cover
714.
[0161] FIG. 8 is a cross-sectional plot of simulated contours of
constant temperature (degrees Centigrade) in three advanced cavity
receivers 800, 802, 804 that embody aspects of the invention.
Receivers 800, 802, and 804 all geometrically resemble the
illustrative advanced cavity receiver depicted in FIG. 7: i.e., all
three receivers 800, 802, 804 comprise a tubular shell 806, 808,
810, a heat-absorbing element 812, 814, 816 through which a
heat-transfer fluid 816, 820, 822 can flow, substantially planar
sidewalls 824, 826, 828, 830, 832, 834, an aperture 836, 838, 840,
a transparent aperture cover 842, 844, 846 spanning the aperture
836, 838, 840, an insulating jacket 848, 850, 852 partially
surrounding the heat-absorbing element 812, 814, 816, a
substantially planar (flat) absorbing surface (absorber) 854, 856,
858 coated with an radiation-absorbent coating and exposed to light
that enters the aperture 836, 838, 840, and an optical cavity 860,
862, 864 bounded by the absorbing surface 854, 856, 858, sidewalls
824, 826, 828, 830, 832, 834, and aperture cover 842, 844, 846 made
of glass. Each receiver 800, 802, 804 is approximately uniform in
cross-section along its entire length, apart from mounting and
other hardware (not shown) at each end. The optical cavity 860,
862, 864 of each collector 800, 802, 804 is filled with air at
atmospheric pressure.
[0162] The three receivers 800, 802, and 804 differ in the
dimensions of certain components and are not necessarily drawn to a
common scale in FIG. 8. In particular, receiver 800 has an absorber
854 that is 6 cm wide and an insulating jacket 848 that is 2.25 cm
thick (measured at right angles from any point on the lateral or
upper surfaces of the heat-absorbing element 818); receiver 802 has
an absorber 856 that is 5 cm wide and an insulating jacket 850 that
is 3.0 cm thick (measured as for receiver 800); and receiver 804
has an absorber 858 that is 5 cm wide and an insulating jacket 852
that is 3.5 cm thick (measured at right angles from any point on
the lateral or upper surfaces of the heat-absorbing element
818).
[0163] Receiver 800 is simulated as operating at 400.degree. C.
(fixed temperature of the absorber coating), receiver 802 is
simulated at operating at 550.degree. C., and receiver 804 is
simulated as operating at 650.degree. C. FIG. 8 shows results from
a ANSYS Fluent computational model of receiver thermal properties
as described above in reference to FIG. 5A and FIG. 5B.
[0164] The constant-temperature contours in FIG. 8 show that, as
would be expected from elementary physics, in all three receivers
800, 802, 804, the surface temperature of each receiver is lower
over the surface of the insulating jacket 848, 850, 852 than at the
aperture cover 842, 844, 846, the latter being separated from the
heat-absorbing surface 854, 856, 858 only by air. Although the
surface area of the non-aperture surface of each receiver 800, 802,
804 is greater than that of the aperture cover 842, 844, 846, the
temperature at the aperture cover 842, 844, 846, is so much greater
than that over the surface of the insulating jacket 848, 850, 852
that most loss occurs in all designs primarily through the
aperture.
[0165] FIG. 9 is a plot of measured and simulated heat loss (watts
per meter) for 10 receiver designs, namely a state-of-the-art (SOA)
receiver constructed according to the prior art (i.e., the Schott
PTR70) and nine illustrative advanced cavity receivers resembling
the receiver 700 depicted in FIG. 7 and the receivers 800, 802, and
804 in FIG. 8. The nine illustrative advanced cavity receivers
differ in width of absorbing surface and thickness of insulating
jacket, as follows:
[0166] Design 1 has absorber width 5 cm and insulating jacket
thickness 2.25 cm.
[0167] Design 2 has absorber width 6 cm and insulating jacket
thickness 2.25 cm.
[0168] Design 3 has absorber width 7 cm and insulating jacket
thickness 2.25 cm.
[0169] Designs 1-3 are simulated at 400.degree. C. for FIG. 9.
[0170] Design 4 has absorber width 5 cm and insulating jacket
thickness 3.0 cm.
[0171] Design 5 has absorber width 6 cm and insulating jacket
thickness 3.0 cm.
[0172] Design 6 has absorber width 7 cm and insulating jacket
thickness 3.0 cm.
[0173] Designs 4-6 are simulated at 550.degree. C. for FIG. 9.
[0174] Design 7 has absorber width 5 cm and insulating jacket
thickness 3.5 cm.
[0175] Design 8 has absorber width 6 cm and insulating jacket
thickness 3.5 cm.
[0176] Design 9 has absorber width 7 cm and insulating jacket
thickness 3.5 cm.
[0177] Designs 7-9 are simulated at 650.degree. C. for FIG. 9.
[0178] The simulation condition for receiver 800 in FIG. 8
corresponds to the simulation of Design 2 at 400.degree. C. for
FIG. 9; the simulation condition for receiver 802 in FIG. 8
corresponds to the simulation of Design 4 at 550.degree. C. for
FIG. 9; and the simulation condition for receiver 804 in FIG. 8
corresponds to the simulation of Design 7 at 650.degree. C. for
FIG. 9.
[0179] NREL data (i.e., Burkholder and Kutscher 2008, described
above for FIG. 5A) for the Schott PTR70 did not exceed 500.degree.
C., and the cermet absorber coating of the Schott PTR70 cannot
endure prolonged exposure to a temperature of 650.degree. C.;
therefore, the heat-loss figures for "SOA" receiver in FIG. 9 for
temperatures above 500.degree. C. are based on extrapolation of the
NREL data, making the assumption that the cermet absorber coating
of the SOA receiver remains stable at such high temperatures. In
other words, actual SOA performance would be worse at 550.degree.
C. and 650.degree. C. than is depicted in FIG. 9. Figures for the
three advanced cavity receivers at all temperatures are based on
the ANSYS Fluent simulation tool described above for FIG. 5A and
FIG. 6.
[0180] FIG. 9 shows that the three illustrative advanced cavity
receivers experience lower thermal losses than the SOA receiver at
all operating temperatures examined, and that the cavity receiver
losses increase much more slowly with temperature than those of the
SOA receiver. Also, increasing the width of the heat-absorbing
surface (absorber) increases thermal losses (albeit modestly)
across the three advanced-cavity receiver designs (5 cm, 6 cm, 7 cm
absorber width). This is as expected, since increased absorber area
increases the area for energy loss through infrared radiation. FIG.
9 thus also illustrates the advantage, in general, of decreasing
absorber area (e.g., through use of a planar rather than a curved
heat-absorbing surface of given width).
[0181] Another design consideration relevant to the nine advanced
cavity receiver designs simulated for FIG. 9 is that there is a
tradeoff between increasing insulation thickness and decreasing
overall efficiency. Increasing insulation thickness decreases
thermal losses, but increases shadowing of the collector by the
receiver: e.g., for a collector having a 5 m aperture, each
addition cm of insulation thickness on each side of the
heat-absorbing element will increase shadowing of the collector by
approximately 0.2%, and so reduce system optical efficiency by
approximately 0.2%. In general, the optimum insulation thickness
will be that which maximizes overall energetic efficiency of the
collection system.
[0182] FIG. 10 illustrates terms used herein as they pertain to a
receiver 1000 mounted above a trough mirror 1002 having a uniform,
parabolic cross-section and parallel to the receiver 1000. The
collector "aperture length" is the length of a line segment
perpendicular to the collector 1000 and connecting the rims of the
mirror 1002. The collector "focal length" is the length of a line
segment 1006 connecting the vertex 1008 of the parabola to the
focus 1010 of the parabola. The receiver 1000 is located at or near
the focus 1010. The "rim angle" 1012 of the collector is the angle
between line segment 1006 and a second line segment 1014
perpendicular to the axis of the receiver 1000 and connecting the
focus 1010 to a point 1016 on the rim of the collector 1002. The
rim angle extent is the same on either side of bisecting line
1006.
[0183] Gossamer's extremely low angular error derives from the
novel space-frame technology that the company has perfected. Per
NREL/Sandia testing, Gossamer's standard 7.3-m trough, which is
commercially ready, offers up to 99.3% intercept factor on a PTR70
(70 mm receiver tube), where standard designs are usually <96%),
and an unprecedented 104.times. concentration factor.
[0184] In addition to the support offered by angular accuracy to a
smaller cavity opening, a flatter reflector profile may enable
designs that are more rigid and less susceptible to torsional
errors and wind disturbances. Some studies suggest that
wind-induced imperfections are a significant factor, and that a
more planar structure may be of benefit 17. In general, the design
of our cavity opening will exploit recent impressive advances in
collector optical error (i.e., Gossamer at <2 mrad).
[0185] FIG. 11 cross-sectionally illustrates an illustrative
advanced cavity receiver 1100 embodying aspects of the invention.
The illustrative advanced cavity receiver 1100 comprises a tubular
shell 1102, a heat-absorbing element 1104 through which a
heat-transfer fluid 1106 can flow, planar sidewalls 1108, 1110, an
aperture 1112, a transparent aperture cover 1114 spanning the
aperture 1112, an insulating jacket 1116 partially surrounding the
heat-absorbing element 1104, a substantially planar (flat)
absorbing surface 1118 coated with an radiation-absorbent coating
and exposed to light that enters the aperture 1112, and an optical
cavity 1120 bounded by the absorbing surface 1118, sidewalls 1108,
1110, and aperture cover 1114. The receiver 1100 is approximately
uniform in cross-section along its entire length, apart from
mounting and other hardware (not shown) at each end. The optical
cavity 1120 and other portions of the collector 1100 are filled
with a gas (e.g., air) at approximately ambient atmospheric
pressure.
[0186] With a planar aperture cover (e.g., cover 714 in FIG. 7)
having no tendency to refract incident light, the receiver 1100
would be compatible with a first rim angle 1124: that is, light
rays incident at a rim angle greater than that defined by the
sidewalls 1108, 1110 would not travel directly to the absorbing
surface 1118 (although they might, in various embodiments, be
absorbed and re-radiated by a sidewall 1108, 1110 or be reflected
therefrom to the absorbing surface 1118). The aperture cover 1114
is lenticular in cross-section, having a planar upper surface and a
convex lower surface. Light directed upon on the aperture cover
1114 by a parabolic collector (not shown) is refracted by the
aperture cover 114 toward the absorbing surface 1118. An exemplary
light ray 1122 shows how the lenticular aperture 1114 directs light
toward the absorbing surface 1118. With a lenticular aperture cover
1114, the receiver 1100 is compatible with a second rim angle 1126
that is larger than the rim angle 1124 that would be compatible
with a similar receiver equipped, instead, with a planar aperture
cover. That is, light rays incident at a rim angle 1126 greater
than that defined by the sidewalls 1108, 1110 are collected by the
lenticular aperture cover 1114 and directed to the absorbing
surface 1118. One advantage conferred by the inclusion of a
lenticular aperture cover 1114 is that for a given rim angle (i.e.,
collector geometry), a collector 1100 having a narrower, deeper
optical cavity 1120 and thus, in general, lower thermal losses will
be feasible. In various embodiments, to reduce thermal losses it is
in general advantageous to minimize absorber width and optical
cavity width.
[0187] FIG. 12 cross-sectionally illustrates an illustrative
advanced cavity receiver 1200 embodying features of the invention.
Receiver 1200 resembles receiver 1100 in FIG. 11, but includes a
lenticular aperture cover 1202 having a different cross-sectional
shape. The lenticular aperture cover 1202 has a concave upper
surface and a convex lower surface. If equipped with a planar
aperture cover, receiver 1200 would accept light from a maximum rim
angle 1204; if equipped with a lenticular aperture cover resembling
that in FIG. 11, receiver 1200 would accept light from a greater
maximum rim angle 1206; with the lenticular aperture cover shape
depicted in FIG. 12, receiver 1200 would accept light from a yet
greater maximum rim angle 1208. FIG. 11 and FIG. 12 illustrate how
lenticular aperture-cover geometry can influence overall collector
geometry and thus collector efficiency.
[0188] FIG. 13 depicts Zemax simulations of a receiver/collector
geometry typical of a state-of-the-art receiver (e.g., the Schott
PTR70). For the reader's convenience, the upper portion of FIG. 13
(FIG. 13A, FIG. 13B and FIG. 13C) shows the geometry without
numerical labels. The lower portion of FIG. 13 (FIG. 13D, FIG. 13E
and FIG. 13F) shows the geometry with numerical labels. Reference
is now made to the lower portion of FIG. 13.
[0189] The left-hand panel (FIG. 13D) shows how parallel light rays
1302 from a solar source are focused by a parabolic collector 1304
onto a receiver tube 1306. In this figure, the source is modeled as
having a spatially homogeneous distribution and a uniform angular
distribution within a 4.7-mrad-radius disk, corresponding to the
angular extent of the Sun. In a magnified view of the receiver
(FIG. 13E), reflected rays from the collector 1304 are shown to
strike the receiver envelope directly 1308, be refracted by the
envelope glass 1310 and strike the absorbing tube of the receiver
1312. One notable ray 1314 strikes the receiver envelope directly
from the solar source, is refracted by the envelope glass and
escapes the receiver. The furthest right panel (FIG. 13F) depicts
the complex interaction between a single light ray and the receiver
materials. Here, a single ray 1316 incident from the collector
results in many subsequent rays due to surface interactions due to
refraction, partial reflection 1320 and scattering 1322 (e.g., by
the absorbing surface 1318. In this model, each glass receiver
surface 1324, 1326 is coated with an idealized material that
transmits approximately 98% of the incident light (independent of
incidence angle) and specularly reflects the remaining 2%. The
transmittance of the glass envelope tube is approximately 96% and
is consistent with figures from literature for the state-of-the-art
receiver. The absorber tube surface 1318 is modeled such that 95%
of incident light is absorbed and the remaining 5% is scattered
according to a Lambertian model. Because we are currently focused
on receiver optical performance numbers, the collector is modeled
as being 100% reflective and free of dirt.
[0190] The Zemax software tool rigorously accounts for energy in
the simulated system and as a result, can be used to evaluate
critical parameters such as the optical efficiency. For example, a
power of 1 W was assigned to the solar source used in the simulated
system shown in FIG. 13. To measure the collected light, a series
of cylindrical detectors were used to determine the fraction of
incident light that ultimately reached the central absorber
surface. A first detector placed across the entire width of the
collector aperture (1304) determined the total amount of radiation
incident on the collector. This detector measured 0.926 W (for a
100,000 ray Monte Carlo simulation)--this value is consistent with
the source size, which was selected to extend somewhat beyond the
collector. A second cylindrical detector was positioned just
outside the receiver glass envelope (1310)--this source also
measured 0.926 W, indicating that in the simulation the all light
from the parabolic collector strikes the receiver, despite the
angular extent of the sun. A third cylindrical detector, positioned
immediately inside the glass envelope, determined the amount of
radiation lost due to reflection at the glass interfaces. This
detector measured 0.888 W. This 4% loss relative to the previous
detector is consistent with stated values in the literature. The
final detector recorded the amount of radiation that was absorbed
by the central tube. This detector measured 0.844 W, corresponding
to a 95% which is consistent with the value of the tubes'
absorptivity. The overall efficiency of the receiver is 0.844
W/0.926 W=91%. With the inclusion of 2% for shading and optical
errors, this value closely matches that stated in the
literature.
[0191] FIG. 14 demonstrates the ability of Zemax to incorporate
collector errors into the model. In this method a prescribed amount
of scattering is implemented at the collector surface allowing for
uncertainty in the ray paths to be introduced in a controllable
manner. This Gaussian error distribution combines with the angular
extent of the sun to determine the focal volume. Another method of
introducing collector error would be to model specific mirror
deformations and calculate the resulting focus degradations
directly. FIG. 14 shows the percentage of radiation from the
collector that strikes a 7-cm-diameter receiving tube (1402, 1404,
1406) for a 10 mrad (FIG. 14A), 5 mrad (FIG. 14B) and 0 mrad (FIG.
14C) collector errors. In FIG. 14C the rays exhibits a finite focal
volume (i.e., the rays do not intersect at a single point but in a
volume sized such that all rays strike the receiver) due to the sun
only. When the collector error is increased to 5 mrad (FIG. 14B),
0.6% of the light rays (e.g., light ray 1408) now miss the circular
tube, while for a collector error of 10 mrad (FIG. 14A), the mirror
errors have expanded the focal volume such that a significant
fraction of energy (3.8%) misses a 7-cm-diameter receiver tube
(e.g., light ray 1110).
[0192] Current collector designs have a typical error of 5.4 mrad
resulting in a simulated collector optical efficiency of
approximately 99% for a typical state-of-the-art vacuum tube
system. When combined with the 91% simulated receiver efficiency
for a perfect mirror, the total optical efficiency of the simulated
collector/receiver system is 90% excluding shading of the collector
by to the metal bellows. With inclusion of bellows shading, the
result is comparable to the 89% efficiency of the SOA system found
in the literature. The very close correspondence of these numbers
serves to validate our optical modeling of the radiation
concentration, transmittance, and absorption processes of a SOA
collection-receiver system.
[0193] Investigations into reducing collector optical errors set a
total system error target of 2.5 mrad. These reductions are
regarded as being attainable given the .about.1.5 mrad errors
achieved by dish-Stirling prototypes. Typical system errors are
currently 5.4 mrad. The target value was based on reducing, the
slope, alignment, tracking and wind loading errors to 1, 0.5, 1,
and 2 respectively, giving an overall system error of 2.5 mrad.
Industry has made progress towards these goals with Gossamer Space
Frames recently reporting that their slope error is close to 1.1
mrad. In our analysis of an optimized system for the planar
receiver a more conservative target of 3 mrad is used.
[0194] For a collector error of 5.4 mrad, 99% of incident light
hits a 7 cm diameter absorber tube. Therefore, the majority of
optical losses in the state-of-the-art design do not occur from
rays missing the absorber. In the model, optical losses are
primarily due to the fact that the transmittance of the glass
envelope and absorptivity of the absorbing surface are not 100%. In
addition, optical performance will be reduced by bellows shading
(.about.1-2%), and from non-ideal mirror reflectivity and
cleanliness. These losses will occur in all systems that involve a
mirror, glass tube and absorber tube. Various embodiments of this
invention may have an optical advantage relative to the
state-of-the-art design through the elimination of a glass cover
and by eliminating bellows shading.
[0195] FIG. 15 depicts the optical efficiency of embodiments of the
invention that varies the focal length and size of receiver tube as
means of reducing thermal losses without a loss in optical
efficiency. In this figure, the optical efficiency assumes that the
absorbing tube has 100% absorptivity as opposed to 95% (as in FIG.
13), to focus on the geometric factors that affect optical
efficiency. In these simulations, the geometry of the receiver
envelope is kept constant and consistent with the Schott PTR70
receiver. The inner and outer glass surfaces have a transmittance
of 98% and the collector aperture is fixed at 5 m. FIG. 15 shows
the effects of modifying the absorber diameter and focal length on
the optical efficiency of the system for collector errors of 3.0
mrad (FIG. 15A) and 5.4 mrad (FIG. 15B). FIG. 15 demonstrates that
the Schott PTR70 is already well-optimized for optical efficiency.
Further increases to the absorber diameter do not significantly
increase the optical efficiency but will act to reduce thermal
efficiency. Likewise, for a collector error of 5.4 mrad, the
optimal performance deteriorates rapidly as the absorber diameter
decreases from 6 cm to 3 cm. The analysis demonstrates that a focal
length of 1.5 m (rim angle of 80.degree.) as used in the Schott
PTR70 is optimal for a cylindrical receiver. At larger focal
lengths, more of the incident light misses the receiver to due to
the increased spreading of the light as a result of collector
errors and the angular distribution of the sun.
[0196] FIG. 16 depicts a theoretical geometric analysis of the
focal-spot size of a planar receiver and a cylindrical receiver for
a linear trough. The acceptance angle (typically the sum in
quadrature of the solar angular extent and the collector errors) is
projected from the collector to the receiver in order to determine
a focus extent. In this analysis, the calculated focal size is
chosen so that approximately 95% of the reflected light from the
collector strikes the receiver. The focal size is affected by the
collector error and the angular distribution of the sun. In this
calculation the collector error is 5.4 mrad and the angular
distribution of the sun is 4.7 mrad. The concentration ratio is
defined as the collector aperture diameter divided by the absorber
width for a planar receiver and collector aperture diameter divided
by absorber diameter for a cylindrical receiver.
[0197] For a given collector error, the theoretical planar receiver
has a lower maximum concentration ratio than a cylindrical
receiver. The higher concentration ratio of the cylindrical
receiver does not necessarily correspond to an improved thermal
performance since the exposed area of the cylindrical absorber is
the circumference and not the diameter. The rim angle that produces
the maximum concentration ratio is smaller for the planar receiver
than the cylindrical receiver. For a planar receiver the optimum
rim angle is 55-65.degree. (1602), as compared to 90.degree. (1604)
for the cylindrical receiver. This analysis does not include the
effects of shadowing of collector by the planar receiver or the
reduction in optical efficiency from a glass envelope or covering.
The maximum concentration ratio of the planar receiver can be
improved through the use of secondary non-imaging optics such as a
compound parabolic concentrator, a lens or other non-imaging
optical devices.
[0198] FIG. 17 shows the optical efficiency of a planar receiver
calculated using Zemax for a range of focal lengths and absorber
widths. In this analysis the collector diameter was fixed at 5 m
and the collector error set to be 3 mrad. The optical efficiencies
of a receiver without a glass cover included at the base of the
cavity are shown in FIG. 17A and the optical efficiencies of a
receiver with a glass cover are shown in FIG. 17B. When glass is
used in the receiver design the maximum optical efficiency is
approximately 1% lower than the state-of-the-art design. The
improvement in optical efficiency is approximately 3-4% in the
embodiment where a glass covering is not included which is
consistent with the transmittance percentage of the glass. The
optimal focal length (rim angle) for a planar design is longer
(smaller) than for the state-of-the-art which is consistent with
theoretical analyses shown in FIG. 16.
[0199] FIG. 18 shows the geometry for the compound parabolic
concentrator (CPC), which is a non-imaging optical device used to
concentrate light incident on a planar entrance aperture. A CPC is
used to concentrate light incident on a planar entrance aperture of
length d.sub.2 (1802), within an acceptance angle 1804, to a
smaller exit aperture of length d.sub.1 (1806). The exit aperture
1806 may be occluded by, e.g., the absorbing surface of a
heat-absorbing element. The light at the exit aperture will
necessarily occupy a larger propagation angle, as dictated by
etendue considerations. The CPC consists of two parabolic mirrors
(1808, 1810), which form the entrance aperture (1802) and receiver
opening (1806). The truncated portions of parabolic mirrors 1808
and 1810 are shown by sections 1820 and 1582 respectively. Provided
that light enters the CPC within the acceptance angle (1804), it
will exit the receiver opening either directly or after one
reflection from the CPC walls. The focal point of parabola 1808 is
on parabola 1810 and is located at 1812. The focal point of
parabola 1810 is on parabola 1808 and is located at 1814. If the
incoming light is parallel to the parabola axis (i.e., first axis
1816 for parabola 1808, second axis 1818 for parabola 1810), then
the light will be focused on the relevant focal point. If the light
enters at an angle of less than half of the acceptance angle the
light will be focused onto the receiver opening (1806). If the
light enters an angle greater than half of the acceptance angle
then the light may be reflected back out of the aperture
opening.
[0200] Assuming the exit aperture (dictated by the desired receiver
size) and the acceptance angle (.theta.), which will depend on the
collector rim angle, are defined, the entrance aperture is given
by
d 2 = d 1 sin ( .theta. / 2 ) ##EQU00003##
and the CPC length (L) is given by
L = d 1 [ 1 + sin ( .theta. / 2 ) ] 2 tan ( .theta. / 2 ) sin (
.theta. / 2 ) . ##EQU00004##
These equations are sufficient to fully specify the CPC. The
concentrating power of the CPC increases as the acceptance angle
(and rim angle decreases). However as the rim angle decreases the
length of the CPC increases. In addition, the increased width of
the CPC at lower rim angles increases the shadowing of the
collector by the receiver reducing optical efficiency. The length
and width of the CPC becomes prohibitive to the use of lower rim
angles (longer focal lengths).
[0201] FIG. 19A and FIG. 19B compare the optical efficiencies of a
planar receiver with a 100% reflective CPC (FIG. 19A1, FIG. 19B1)
and 95% reflective CPC (FIG. 19A2, FIG. 19B2) for a range of
absorber sizes and focal lengths.
[0202] Reference is now made to FIG. 19A, which pertains to a
receiver without a glass aperture cover. When the CPC is 100%
reflective (FIG. 19A1) the optical efficiency is improved for
smaller absorber widths compared to the embodiment without a CPC
(FIG. 18). This improvement corresponds to an increase in the
concentration ratio for a given optical efficiency. However, the
CPC increases the shadowing of the collector as evidenced by the
decrease in optical efficiency for larger absorber size. Since the
concentration ratio (and therefore width) of the CPC increases at
larger focal lengths (smaller rim angles), this effect is more
apparent when the focal length is longer.
[0203] When the reflectivity of the CPC is reduced to 95%, the
optical efficiency and concentration ratios are reduced. At a focal
length of 2.5 m and an absorber width of 7 cm the optical
efficiency is decreased approximately 2-3% compared to the 100%
reflective CPC. The optimum focal length for a given absorber size
is decreased when the CPC reflectivity is reduced from 100% to 95%.
The optical efficiency of the planar receiver with 95% reflective
CPC is lower than the planar receiver without a CPC over much of
the range of absorber widths and focal lengths. However, the
optical advantage of the CPC is still apparent at lower absorber
widths and longer focal lengths.
[0204] Reference is now made to FIG. 19B, which pertains to a
receiver with a glass aperture cover. The addition of glass to the
bottom of the receiver decreases the optical performance
universally by approximately 4%. A glass covering may be included
to reduce thermal losses at the base of the receiver due to forced
convection from the wind. When glass is included, the peak optical
efficiency of the receiver with a 95% reflective CPC (FIG. 19B2) is
3% lower than the Schott PTR70. Without glass, the optical
performance of this receiver exceeds that of the Schott PTR70 by
1%.
[0205] FIG. 20A and FIG. 20B display illustrative plots relevant to
the invention. Specifically, the illustrative plots may be employed
in a method whereby the optimum receiver size (e.g., width of the
heat-absorbing surface 418 in FIG. 4) is determined for a given
temperature of operation of the heat-transfer-fluid (e.g., fluid
406 in FIG. 4) by maximizing the performance of the receiver at
that operating temperature.
[0206] FIG. 20A displays a plot of energy gained by the receiver
versus receiver size. The energy gained by the receiver is plotted
on the y-axis and is based on optical and thermal analyses. The x-
and y-axes are not given numeric values, as the general shape of
the three curves are universal and demonstrate the procedure for
determining the optimal point.
[0207] The optical energy intercepted 2002 is calculated from
ray-tracing software (Zemax) and based on parameters such as
optical error, receiver size, mirror size, and mirror shape. The
energy gained is calculated from the power per unit length (e.g.,
Watts/meter) intercepted by the receiver in the ray-tracing
software. The characteristic shape of the optical energy
intercepted by the receiver as a function of receiver size is
displayed in FIG. 20A. where optical energy intercepted increases
rapidly with receiver size until all the light is intercepted (just
about the point 1708, i.e., optimum receiver size). For a given
size mirror (e.g., 5 meters), optical error (e.g., 5 mrad), nearly
all (e.g., over 99%) of the optical radiation is intercepted for a
planar a given receiver size (e.g., 10 cm). After that, the optical
energy intercepted stays almost constant, as indicated by the long
nearly flat part of the curve, with a slight decrease due to
shading caused by the larger receiver size.
[0208] The thermal energy lost from the receiver 2004 is calculated
from CFD software (ANSYS Fluent) and based on parameters such as
receiver size, heat transfer fluid temperature, absorber
emissivity, and heat transfer properties. The energy lost is
calculated from the power (heat) per unit length (e.g.,
Watts/meter) transferred to the environment in the CFD software.
The characteristic shape of the thermal energy loss by the receiver
as a function of receiver size is displayed in FIG. 20A, where
thermal energy loss increases nearly linearly with receiver size
(exposed absorber size).
[0209] The total energy gained by the receiver 2006 is calculated
as the sum of the optical energy gained 2002 and thermal energy
lost 2004. To maximize the total energy gained by receiver, the
receiver size would be chosen as the peak 2008 of the curve 2006.
This optimum receiver size 1708 is dependent on operating
parameters such as operating heat transfer fluid temperature,
absorber emissivity, receiver heat transfer properties, optical
error, mirror size, and mirror shape. In this manner, optimum
receiver size can be determined, improving overall performance of a
solar trough-based concentrating solar plant.
[0210] FIG. 20B displays three plots of energy gained by the
receiver versus receiver size for different heat transfer fluid
temperatures. The energy gained by the receiver is plotted on the
y-axis and is based on the same optical and thermal analyses
described in FIG. 20A. The x and y axes are not given numeric
values, as the general shape of the three curves are universal and
demonstrate the procedure for determining the optimal point.
[0211] The y axis displays the total energy gained by the receiver
and, as in FIG. 20A, is calculated as the sum of the optical energy
gained and thermal energy lost. Three curves are shown for
different heat transfer fluid temperatures within the receiver:
curve 2052 displays the total energy gained by the receiver for a
heat transfer fluid temperature of 400.degree. C., curve 2054
displays the total energy gained by the receiver for a heat
transfer fluid temperature of 550.degree. C., and curve 2056
displays the total energy gained by the receiver for a heat
transfer fluid temperature of 650.degree. C. Other operating
parameters such as absorber coating, receiver heat transfer
properties, optical error, mirror size, and mirror shape are kept
the same for the three analyses. The optimum receiver size can be
determined for each operating temperature: point 2062 represents
the optimum receiver size for 400.degree. C., point 2064 represents
the optimum receiver size for 550.degree. C., and point 2066
represents the optimum receiver size for 650.degree. C. Receiver
size can be varied within a solar trough-based concentrating solar
plant based on temperature to improve overall plant performance.
Likewise, if for manufacturability, cost, or other reasons, a
single receiver size is preferred for a solar trough-based
concentrating solar plan, a single optimum receiver size can be
determined averaging over temperatures and parameters, improving
overall performance of a solar trough-based concentrating solar
plant.
[0212] FIG. 21 cross-sectionally illustrates an illustrative
embodiment (FIG. 21A) of aspects of the invention and a comparison
(FIG. 21B) of the simulated heat loss of the embodiment 2100 with
the measured and extrapolated heat loss of a state-of-the-art (SOA)
receiver constructed according to the prior art (i.e., the Schott
PTR70). The receiver 2100 comprises a tubular envelope 2102, a
tubular heat-absorbing element 2104 through which a heat-transfer
fluid 2106 can flow, a heat-absorbing coating 2108 upon the outer
surface of the heat-absorbing element 2104, an outer insulating
jacket 2110 that covers a part of the outer surface of the envelope
2102, and an aperture 2112, not covered by insulation 2110, through
which light can pass through the envelope 2102. The annulus or
space 2114 between the heat-absorbing element 2104 and the envelope
2102 is vacuum-filled. The receiver 2100 may or may not include a
reflective coating 2116 on the inner surface of the envelope 2102
except over the aperture 2112.
[0213] FIG. 21B compares measured and extrapolated heat-loss
performance of the SOA receiver to the simulated performance of the
receiver 2100, where the latter is simulated both with and without
a reflective coating 2116. As the plot makes clear, the receiver
2100 has lower heat loss at all temperatures than the SOA receiver,
with relatively greater improvement at higher temperatures. The
plot also shows that the presence or absence of a reflective
coating 2116 on the inner surface of the envelope 2102 ("Coated
Cavity" points vs. "Uncoated Cavity" points) makes relatively
little difference to heat loss. Compared to the structurally
similar non-cavity SOA receiver, the addition of insulation to the
cavity receiver 2100 makes most of the difference.
[0214] NREL data (i.e., Burkholder and Kutscher 2008, described
above for FIG. 5A) for the Schott PTR70 did not exceed 500.degree.
C., and the cermet absorber coating of the Schott PTR70 cannot
endure prolonged exposure to temperatures significantly above
500.degree. C.; therefore, the heat-loss figures for "SOA" receiver
in FIG. 21 for temperatures above 500.degree. C. are based on
extrapolation of the NREL data, making the assumption that the
cermet absorber coating of the SOA receiver remains stable at such
high temperatures. In other words, actual SOA performance would be
worse at 550.degree. C. and 650.degree. C. than is depicted in FIG.
21B. Figures for the receiver 2100 at all temperatures are based on
the ANSYS Fluent simulation tool described above for FIG. 5A and
FIG. 6. This simulation assumes in the receiver 2100 an absorbent
coating with the same temperature-emissivity curve as the coating
in the Schott PTR70 receiver--as opposed to a more advanced
coating--but with the additional property of being stable up to at
least 650.degree. C. The external insulation 2110 was modeled based
on the properties of Microtherm Super G type insulation, which has
a thermal conductivity of 0.02 W/m*K at 300.degree. C. For
Microtherm Super G insulation, 2 cm of insulation has a thermal
resistance of 1 K*m.sup.2/W.
[0215] FIG. 22A1 and FIG. 22A2 are cross-sectional plots of
simulated contours of constant temperature within (a) (FIG. 22A1)
an illustrative state-of-the-art receiver 2200 similar to the
Schott PTR70 receiver and constructed according to the prior art,
but with air rather than vacuum in the annulus 2202, and also
within (b) (FIG. 22A2) an illustrative receiver 2204 similar to
receiver 2100 in FIG. 21 and also containing air in the annulus.
Receiver 2204 incorporates aspects of the invention. Both receivers
2200, 2204 include a transparent tubular envelope 2208, 2210 and a
tubular heat-absorbing element 2212, 2214. The heat-absorbing
elements 2212, 2214 contain a heat-transfer fluid 2216, 2218. The
insulated receiver 2204 includes an insulating jacket 2220 partly
covering the outer surface of the envelope 2210; the envelope 2210
includes a reflective coating 2222 on its inner surface. Both
receivers 2200, 2204 are simulated at an operating at temperature
of 400.degree. C. (fixed temperature of the absorber coating). An
absorber coating composition for receivers 2200, 2204 and an
insulation composition for the cavity receiver 2204 are both
assumed, for this analysis, as for the analysis providing the
results plotted in the lower portion of FIG. 21.
[0216] FIG. 22A3 and FIG. 22A4 are plots of velocity vectors for
convecting air for the receivers 2200, 2204, again at an operating
temperature of 400.degree. C.
[0217] FIG. 22A shows that conviction is more vigorous in an
air-filled, non-insulated receiver 2200 than in an air-filled,
insulated receiver 2204.
[0218] FIG. 22B1 is a cross-sectional illustration defining energy
loss fluxes for the air-filled, insulated receiver 2204 from FIG.
22A4. "Qinsulation" (arrow 2224) is the heat flux through the
insulation-covered portion of the envelope 2210. "Q_Radiation"
(arrow 2226) is the radiation flux through the
non-insulation-covered portion of the envelope 2210. "Q_Cond_Conv"
(arrow 2228) is the heat flux via conduction through the
non-insulation-covered portion of the envelope 2210. A Q-Radiation
flux and Q_Cond_Conv flux may be similarly defined for the
uninsulated receiver 2200 in FIG. 22A1, also termed herein the
"bare receiver"; there is no Q_insulation flux for the bare
receiver 2200.
[0219] FIG. 22B2 is a chart of the three fluxes for both the bare
receiver and the insulated receiver for operating temperatures of
400.degree. C., 550.degree. C. and 650.degree. C. The chart shows
clearly that the Q-Radiation flux and Q_Cond_Conv fluxes are
significantly smaller for the insulated receiver 2204 than for the
bare receiver 2200. The strong loss reductions seen with adding
insulation to the receiver exterior suggest that gas-filled
receivers are feasible in terms of efficiency when combined with an
insulated covering.
[0220] FIG. 23A is a cross-sectional plot of simulated contours of
constant temperature in an illustrative receiver 2300 incorporating
aspects of the invention. Receiver 2300 resembles the bare receiver
2200 in FIG. 22A1, but with the addition of six barriers (e.g.,
barrier 2302) connecting the envelope 2304 to the heat-absorbing
element 2306 in a gas-proof manner and so dividing the air-filled
annulus (space between the envelope 2304 and heat-absorbing element
2306) into six cells (e.g., cell 2308) running the length of the
receiver 2300. Addition of dividers to segment the annulus into
cells to reduce convection might be attempted as an alternative or
supplement to the addition of an insulating jacket over a bare
receiver, with the goal of reducing heat losses from the receiver;
however, as FIG. 23B shows, in simulations, that the addition of
two or six dividers to an otherwise identical bare receiver actual
increases convection heat loss from the receiver. It appears the
divider strategy fails because it does not alter surface
temperatures or cylinder diameters, which are the driving
parameters of annular-flow heat transfer. However, the plot of
lines of constant temperature in the upper portion of FIG. 23A does
reveal one benefit of dividers: isolating a section of air below
the hot absorber creates a stratified (horizontally layered) zone
with minimal convection currents. Stratification tends to retain
heat energy hear the heat-absorbing element (top of the cavity)
rather than transporting it to the envelope (bottom of the cavity),
and thus is advantageous in reducing thermal losses from the
receiver.
[0221] FIG. 24A1 cross-sectionally illustrates an illustrative
receiver 2400 incorporating aspects of the invention. Receiver 2400
resembles the bare receiver 2200 in FIG. 22A1, but with the
addition of an insulating jacket 2402 partly filling the annulus
between the heat-absorbing element 2404 and the transparent tubular
envelope 2406. The portion of the annulus not filled by the
insulating jacket 2402 constitutes an air-filled optical cavity
2408 that light may enter by passing through a portion 2410 of the
envelope 2106; this portion 2010 of the envelope constitutes an
optical aperture.
[0222] FIG. 24A2 is a cross-sectional plot of simulated contours of
constant temperature within the insulating jacket 2402 and
air-filled optical cavity 2408 of receiver 2400 at an operating
temperature of 400.degree. C. An absorber coating composition for
receivers the heat-absorbing element 2104 and an insulation
composition for the insulating jacket 2402 are both assumed, for
this analysis, as for the analysis providing the results plotted in
FIG. 24A2.
[0223] In FIG. 24A2, the lines of constant temperature in the
optical cavity 2408 reveal a stratified (horizontally layered) zone
with minimal convection currents. As noted above in the description
of FIG. 23A, stratification tends to advantageously reduce thermal
losses.
[0224] FIG. 24B1 is a cross-sectional illustration defining energy
loss fluxes for the receiver 2400 from FIG. 24A1, also herein
termed the "wedge" receiver. "Qinsulation" (arrow 2410) is the heat
flux through the portion of the envelope 2406 overlying the
insulating jacket 2402. "Q_Radiation" (arrow 2412) is the radiation
flux through the portion of the envelope 2406 covering the cavity
2408. "Q_Cond_Conv" (arrow 2414) is the heat flux via conduction
through the portion of the envelope 2106 covering the cavity
2408.
[0225] FIG. 24B2 is a chart of the three fluxes for both the bare
receiver 2200 of FIG. 22A1, the externally insulated receiver 2204
of FIG. 22A2, and the internally insulated receiver 2400 of FIG.
24A1, for operating temperatures of 400.degree. C., 550.degree. C.
and 650.degree. C. The chart shows clearly that the Q-Radiation
flux and Q_Cond_Conv fluxes are significantly smaller for the
internally insulated receiver 2400 than for either the bare
receiver 2200 or the externally insulated receiver 2204. By
directly covering the majority of the surface of the heat-absorbing
element 2404, reducing the size of the air pocket, and inducing
stratification, the wedge receiver 2400 substantially reduces both
radiative and convective losses.
[0226] FIG. 25A demonstrates the effect of receiver tilt angle on
the thermal performance of the receiver. As the position of the sun
moves throughout the day, the receiver/collector system must tilt
in order to maximize the incident solar radiation. If the sun is
directly overhead then the tilt angle is defined as being 0.degree.
and the receiver cavity points vertically down. If the sun is at
the horizon then the tilt angle is 90.degree..
[0227] FIG. 25A shows the effect of the receiver tilt on heat loss
for the current receiver design at an operating temperature of
650.degree. C. This embodiment consists of an absorber with a width
of 5 cm, a compound parabolic concentrator, a focal length of 2.5
m, 3.5 cm of Microtherm insulation, and a glass cover. The heat
loss at a tilt angle of 80.degree. is approximately 17% higher than
at 0.degree. (2502). The increase in heat loss as the receiver
tilts from 0.degree. to 80.degree. results in a decrease of total
receiver efficiency from 78.5% to 77.3%. At a tilt angle of
40.degree. the heat loss increases by 7% and the total receiver
efficiency is decreased to 78.0%. The radiation heat loss remains
relatively constant as the tilt angle changes (2404). This initial
analysis demonstrates that the enhanced heat loss at larger
receiver tilt angles is not prohibitive to the high total
efficiency of the receiver. Upcoming analysis will predict an
average efficiency based on the angular position of the collector
over the course of a day.
[0228] FIG. 25B shows velocity vectors for three tilt angles:
0.degree. (2306), 40.degree. (2308) and 80.degree. (2510) for the
current leading receiver design at 650.degree. C. FIG. 25B
demonstrates that as the tilt angle increases from 0.degree. to
80.degree., the stratified layer of air in the receiver (2512)
breaks down and a one-cell convection pattern forms (2514). The
magnitude of the convective velocity increases as the receiver is
tilted from 0.degree. to 80.degree.. The enhanced convection
facilitates the transfer of energy from the absorber (2516) to the
glass covering (2518) where it may be lost through the glass by
conduction.
[0229] FIG. 25C shows contours of constant temperature for three
tilt angles: 0.degree. (2520), 40.degree. (2522), and 80.degree.
(2524) for the current leading receiver design at 650.degree. C. As
the tilt angle increases the temperature of the air in the vicinity
of the glass aperture is increased due to the enhanced convective
transfer of energy from the absorber to the glass covering. The
increased temperature of the fluid at the glass covering will
result in larger heat transfer across the glass window due to
conduction. In other embodiments of the invention, the material
used for the cover is changed in order to reduce thermal losses
while maintaining a high optical efficiency. Optimization of the
position of the covering and length of the receiver legs is
possible to reduce heat loss due to a tilted receiver.
[0230] FIG. 26 displays plots of constant temperature contours for
two illustrative embodiments of the invention. Specifically, FIG.
26 cross-sectionally depicts results of a CFD analysis of an
illustrative novel receiver that incorporates aspects of the
invention, including a selective aperture cover as described in
FIG. 27.
[0231] The CFD analysis displayed is for an illustrative advanced
cavity receiver in two states, i.e., (1) without a selective
insulating aperture cover (receiver 2602) and (2) with a selective
insulating aperture cover 2604 (receiver 2604). Receivers 2602,
2604 comprise an insulating jacket 2606 partially surrounding a
heat-absorbing element 2408 above a gas-filled (e.g., atmospheric
air) cavity, and an aperture 2612 with a transparent aperture cover
2612 through which light is concentrated by mirror collector (not
shown) onto heat-absorbing element 2608. Receivers 2602, 2604 are
approximately uniform in cross-section along its entire length,
apart from mounting and other hardware (not shown) at each end.
Receivers 2602 and 2604 are identical except that receiver 2604 has
been closed by a selective insulating aperture cover 2614.
[0232] Displayed in FIG. 26 are dashed lines of constant
temperature based on a CFD analysis where the heat-absorbing
element 2608 is fixed at operating temperature 650.degree. C.
Contour lines are not shown in the selective insulating cover 2614.
Heat loss by the receiver 2604 is reduced by the addition of the
cover as compared with heat loss by receiver 2602. For the case
shown, with operating temperature of 650.degree. C., the heat loss
decreases from 374 W/m in the receiver 2602 without the selective
insulating cover, to 159 W/m for the receiver 2604 with the
selective insulating cover 2614. This reduction by over half in
heat loss by addition of selective insulating cover 2614, indicates
that performance of a trough-based concentrating solar plant may be
substantially improved by reducing thermal losses during idle
times. CFD analyses for other operating temperatures were completed
(not shown), showing that when operating at 550.degree. C., the
heat loss decreases from 256 W/m in the receiver 2602 without the
selective insulating cover, to 147 W/m for the receiver 2604 with
the selective insulating cover 2614. Additionally, when operating
at 400.degree. C., the heat loss decreases from 144 W/m in the
receiver 2402 without the selective insulating cover, to 112 W/m
for the receiver 2604 with the selective insulating cover 2614.
[0233] FIG. 27 displays another illustrative embodiment of the
invention. Specifically, FIG. 27 cross-sectionally depicts features
of an illustrative novel receiver that incorporates aspects of the
invention.
[0234] The illustrative advanced cavity receiver with selective
aperture cover 2700 comprises a tubular shell 2702, a
heat-absorbing element 2704 through which a heat-transfer fluid
2706 can flow, an aperture 2712, (with optional transparent
aperture cover (not shown)), an insulating jacket 2716 partially
surrounding the heat-absorbing element 2704, a planar (flat)
absorbing surface 2718 coated with an absorbent coating and exposed
to light entering the aperture 412. The receiver 400 is
approximately uniform in cross-section along its entire length,
apart from mounting and other hardware (not shown) at each end.
[0235] Additionally, the receiver may include a moveable aperture
cover 2720. The cover 2720 may, for example, be a transparent
material such as glass, a structural material such as plastic or
metal, or an insulating material such as opaque insulation. The
cover 2720 may be rotated or other moved to cover the receiver
aperture opening 2712. In this illustrative example, the cover 2720
may be mounted to a circular track 2730 at both the lengthwise
(coming out of the page) front and back end of the receiver. A
motor (not shown) or other rotary device (e.g., manual crank) may
be used to rotate the aperture cover 2720 from a full open position
as shown in 2700 to a full closed position as shown in 2770. A
mechanical stop (e.g., a fixed piece of metal at the lengthwise
front and back end of the receiver) 2734 may be used to assure
proper seating and location of the aperture cover when rotated full
open 2700 or full closed 2770. Arrow 2732 indicates the direction
of rotation to close the aperture cover 2720.
[0236] Performance (e.g., overall efficiency) may be improved
during different operating conditions by the selective covering of
the aperture opening 2712 with the cover 2720. For example, during
windy conditions, a glass aperture cover may be rotated into place
to reduce convective thermal losses. The glass will result in
reduced optical efficiency, but in conditions of high wind, this
loss may be more than compensated by gains in thermal efficiency.
In another embodiment, additional insulation 2722 may be added to
the cover 2720. This insulating covering 2720 2722 may be rotated
into place to further reduce thermal losses during night time and
other times of little or no sunlight to reduce thermal losses.
[0237] This illustrative cavity receiver 2700 can be the same as
shown in FIG. 7 with the addition of the selective insulated
aperture cover. In other embodiments, other receiver geometries
(e.g., that of receiver 400 in FIG. 4) may include the selective
insulated aperture cover.
[0238] 2750 shows the same illustrative cavity receiver 2700, but
with the aperture cover 2720 and insulating covering 2722 rotated
halfway towards closing against stop 2734 on track 2730. Arrow 2752
indicates the direction of rotation to close the aperture cover
2720.
[0239] 2770 shows the same illustrative cavity receiver 2700, but
with the aperture cover 2720 and insulating covering 2722 rotated
completely towards closing against stop 2734 on track 2730.
[0240] In other embodiments, not shown, the selective aperture
cover 2720, 2722 could be internal to the stationary insulation
2716.
[0241] FIG. 28 and FIG. 29 show the effect of forced convection
(wind) on the thermal performance of an embodiment of the receiver
with no glass cover at the cavity aperture. In this embodiment, the
cavity and absorbing surface are open to the environment. FIG. 28A,
FIG. 28B, FIG. 28C and FIG. 28D show the convective velocity in and
around the open receiver for a wind speed of 2.2 m/s incoming at a
direction of 0.degree., 15.degree. downwards, 15.degree. upwards
and 30.degree. upwards respectively. The simulations were performed
for an illustrative receiver with a compound parabolic
concentrator, a 2.5 m focal length, 2 cm external Microtherm
insulation, 7 cm absorber width and an operating temperature of
550.degree. C. For the receiver which has a glass cover and a
sealed air-filled cavity at atmospheric pressure, the heat loss is
342 W/m. In an embodiment of the invention, the evacuation of the
cavity results in a reduction of the heat loss to 293 W/m.
Embodiments of the invention where a portion of the cavity is at
vacuum will result in improvements to the thermal efficiency and
may improve absorber coating stability. When the glass cover is
removed and an external wind speed applied at 2.2 m/s the heat loss
is increased between 11% and 80% depending upon the direction of
the incoming wind.
[0242] The removal of the glass covering increases the optical
performance by approximately 4% (the transmittance percentage of
the glass). In order to achieve gains in total efficiency when the
glass cover is removed in this example the increase in heat loss
must be less than 58% (absolute heat loss less than .about.530
W/m). Hence, these initial results suggest that the removal of the
glass cover may improve the total efficiency of the receiver over a
range of wind velocities.
[0243] These initial simulations were performed for a receiver tilt
angle of 0.degree.. For an embodiment of the receiver with no glass
covering the tilt angle will further affect the heat loss of the
receiver due to natural and forced convection. Embodiments of the
invention include a receiver with no glass cover that is optimized
to reduce heat loss for a variety of tilt angles.
[0244] FIG. 30 cross-sectionally illustrates two illustrative
advanced cavity receivers, 3000 and 3002, embodying aspects of the
invention. Receiver 3000 (FIG. 30A) resembles receiver 400 in FIG.
4, but includes glass sections (3004 and 3006) at the cavity
entrance that form a partial glass cover rather than a full glass
cover. Receiver 3002 (FIG. 30B) resembles receiver 700 in FIG. 7
but includes glass sections (3008 and 3010) at the cavity entrance
that form a partial glass cover rather than a full glass cover. The
middle section (3012 in receiver 3000 and 3014 in receiver 3002) of
the cavity aperture is left open.
[0245] The percentage of the cavity opening that is covered by
glass is chosen to maximize the total receiver efficiency.
Increasing the percentage of the cavity opening that is covered by
glass will increase the thermal performance at a cost to the
optical performance. The optimum percentage will depend upon the
profile of the irradiant flux on the cavity opening and the extent
to which the width of the partial glass cover affects the thermal
performance.
[0246] In one embodiment of the invention, the percentage of glass
covering the cavity opening may be variable and can be altered
depending upon the current weather conditions.
[0247] FIG. 31 displays another illustrative embodiment of the
invention. Specifically, FIG. 31 shows solar power system 3100
which includes an alternative embodiment of a solar field 3102 to
replace or supplement solar field 102 in FIG. 1 for a trough-based
concentrating solar power system (100 in FIG. 1). The solar field
is controlled to track the sun via controller 3180.
[0248] The solar field 3102 comprises N rows (e.g., row 3104, row
3106), depicted in FIG. 31 as viewed from above, where each row
comprises M receivers (e.g., receivers 3108a, 3108b; receivers
3110a, 3110b, receivers 3112a, 3112b; etc.) upon which solar
radiation is focused by parabolic collectors (not depicted). The
plurality of the N rows is indicated in FIG. 1 by horizontal
ellipses (e.g., ellipsis 3116); the plurality of M receivers in
each of the N rows is indicated in FIG. 1 by vertical ellipses
(e.g., ellipses 3114a, 3114b). The number of receivers K in the
solar field of the illustrative system 100 is therefore K=NM. In
other embodiments, the number of receivers may vary from row to
row.
[0249] In one state of operation, a heat-transfer fluid (e.g.,
molten salt, refractory oil, gas, pressurized water or steam) is
admitted to an entry manifold 3118 at a first temperature, T.sub.1
(e.g., 200.degree. C.). (In FIG. 31, any piping through which the
heat-transfer fluid may flow in some state of operation of the
system 3100 is denoted by a thick dark line.) The heat-transfer
fluid is distributed by the entry manifold to the N rows and, in
each row, passes sequentially through the M receivers comprised by
that row. In general, an approximately equal amount of solar
radiation Q.sub.c is gathered by the collector associated with each
receiver, and absorbed by the fluid in each receiver with an
efficiency .eta..sub.r. Thus, an approximately equal quantity of
solar energy Q.sub.r=Q.sub.c.eta..sub.r is added to the
heat-transfer fluid during its passage through each receiver. Each
incremental set of the M receivers will thus raise the thermal
energy and thus, for non-phase change heat transfer fluids, raise
the temperature of the heat-transfer fluid. The first set of
receivers (3108a and 3108b), which consists of a given length of
receivers (e.g., 40 meters), will raise the heat transfer fluid
from T.sub.1 to a second elevated temperature T.sub.2 (e.g.,
250.degree. C.) higher than the T.sub.1. For simplicity, the
average temperature T.sub.12, (e.g., 225.degree. C.) of the first
set of receivers 3108 may be considered the average of T.sub.1 and
T.sub.2 (T.sub.12=(T.sub.1+T.sub.2)/2). The second set of receivers
(3110a and 3110b), which consists of a second given length of
receivers (e.g., another 40 meters or some other length not
necessarily equal to the first length), will raise the heat
transfer fluid from T.sub.2 to a third elevated temperature T.sub.3
(e.g., 300.degree. C.). The second set of receivers 3110 operates
at average temperature T.sub.23 (e.g., 275.degree. C.) higher than
the average operating temperature, T.sub.12, of the first set of
receivers 3108. In the same manner, the third set of receivers 3112
has a given length, inlet temperature T.sub.3, elevated outlet
temperature T.sub.4 (e.g., 350.degree. C.), and average operating
temperature T.sub.34 (e.g., 325.degree. C.). Additional sets of
receivers 3114 may raise the temperature further to the outlet
operating temperature where it is admitted to an output manifold
3120 at a final temperature, T.sub.f (e.g., 550.degree. C.).
[0250] Detail 3130 shows an exemplary cross-section of the receiver
in the first receiver sections 3108a and 3108b which typically
operates at elevated operating temperature T.sub.12. The receiver
is similar to the receiver 700 in FIG. 7. The receiver 3130 has an
exposed opening of width 3140. In various other embodiments,
receivers may be used that comprise planar transparent aperture
covers, lenticular aperture covers, partial aperture covers,
selective insulated aperture covers, compound parabolic
concentration, and other aspects of the invention.
[0251] Detail 3132 shows an exemplary cross-section of the receiver
in the second receiver sections 3110a and 3110b which typically
operates at elevated operating temperature T.sub.23 which is higher
than T.sub.12. The receiver is similar to the receiver 700 in FIG.
7. The receiver 3132 has an exposed opening of width 3142. The
width 3142 is less than the width of 3140 in order to improve
overall efficiency of the receiver. The first 3108 and second 3110
receiver sections are connected via a coupler (not shown) that
connects the two receiver absorber tubes together. The receiver
absorber tubes may be the same dimensions with different exposed
areas (e.g., different extent of insulation) or different
dimensions (e.g., smaller tube for 3110 as compared with 3108).
[0252] As shown in FIG. 20B, for a given mirror size and geometry,
optimal receiver performance at a given operating temperature
occurs for different receiver geometries. Receiver efficiency is a
multiplication of optical and thermal efficiency. Smaller exposed
absorber surface area tends to result in higher thermal efficiency
(lower thermal losses due to smaller exposed area) but lower
optical efficiency (high optical losses due to smaller exposed
area).
[0253] Additional receivers 3134, 3136 corresponding to additional
sections 3114, 3116 will typically operate at further elevated
operating temperatures, each successively higher. The exposed width
of each successive receiver 3134, 3136 may be less than the
previous receivers (3132, 3134) in order to improve overall
efficiency of the receiver. Each section may be connected via a
coupler (not shown) that connects the two receiver absorber tubes
together. The receiver absorber tubes may be the same dimensions
with different exposed areas (e.g., different extent of insulation)
or different dimensions (e.g., smaller tube). Overall solar field
efficiency can be increased through this method.
[0254] Additionally, as shown in FIG. 32, receivers employing
different shapes and technologies can be used in different receiver
sections.
[0255] FIG. 32 displays another illustrative embodiment of the
invention. Specifically, FIG. 32 shows solar power system 3200
which includes an alternative embodiment of a solar field 3202 to
replace or supplement solar field 102 in FIG. 1 for a trough-based
concentrating solar power system (100 in FIG. 1). The solar field
is controlled to track the sun via controller 3280.
[0256] The solar field 3202 comprises N rows (e.g., row 3204, row
3206), depicted in FIG. 32 as viewed from above, where each row
comprises M receivers (e.g., receivers 3208a, 3208b; receivers
3210a, 3210b, receivers 3212a, 3212b; etc.) upon which solar
radiation is focused by parabolic collectors (not depicted). The
plurality of the N rows is indicated in FIG. 1 by horizontal
ellipses (e.g., ellipsis 3216); the plurality of M receivers in
each of the N rows is indicated in FIG. 1 by vertical ellipses
(e.g., ellipses 3214a, 3214b). The number of receivers K in the
solar field of the illustrative system 100 is therefore K=NM. In
other embodiments, the number of receivers may vary from row to
row.
[0257] In one state of operation, a heat-transfer fluid (e.g.,
molten salt, refractory oil, gas, pressurized water or steam) is
admitted to an entry manifold 3218 at a first temperature, T.sub.1
(e.g., 200.degree. C.). (In FIG. 32, any piping through which the
heat-transfer fluid may flow in some state of operation of the
system 3200 is denoted by a thick dark line.) The heat-transfer
fluid is distributed by the entry manifold to the N rows and, in
each row, passes sequentially through the M receivers comprised by
that row. In general, an approximately equal amount of solar
radiation Q.sub.c is gathered by the collector associated with each
receiver, and absorbed by the fluid in each receiver with an
efficiency .eta..sub.r. Thus, an approximately equal quantity of
solar energy Q.sub.r=Q.sub.c.times..eta..sub.r is added to the
heat-transfer fluid during its passage through each receiver. Each
incremental set of the M receivers will thus raise the thermal
energy and thus, for non-phase change heat transfer fluids, raise
the temperature of the heat-transfer fluid. The first set of
receivers (3208a and 3208b), which consists of a given length of
receivers (e.g., 40 meters), will raise the heat transfer fluid
from T.sub.1 to a second elevated temperature T.sub.2 (e.g.,
250.degree. C.) higher than the T.sub.1. For simplicity, the
average temperature T.sub.12, (e.g., 225.degree. C.) of the first
set of receivers 3208 may be considered the average of T.sub.1 and
T.sub.2 (T.sub.12=(T.sub.1+T.sub.2)/2). The second set of receivers
(3210a and 3210b), which consists of a second given length of
receivers (e.g., another 40 meters or some other length not
necessarily equal to the first length), will raise the heat
transfer fluid from T.sub.2 to a third elevated temperature T.sub.3
(e.g., 300.degree. C.). The second set of receivers 3210 operates
at average temperature T.sub.23 (e.g., 275.degree. C.) higher than
the average operating temperature, T.sub.12, of the first set of
receivers 3208. In the same manner, the third set of receivers 3212
has a given length, inlet temperature T.sub.3, elevated outlet
temperature T.sub.4 (e.g., 350.degree. C.), and average operating
temperature T.sub.34 (e.g., 325.degree. C.). Additional sets of
receivers 3214 may raise the temperature further to the outlet
operating temperature where it is admitted to an output manifold
3220 at a final temperature, T.sub.f (e.g., 550.degree. C.).
[0258] Detail 3230 shows an exemplary cross-section of the receiver
in the first receiver sections 3208a and 3208b which typically
operates at elevated operating temperature T.sub.12. The receiver
may be similar to the commercially available vacuum tube receiver
shown in FIG. 3. The receiver 3230 has a diameter of 3240 and an
exposed circumference of pi times the diameter 3240.
[0259] Detail 3232 shows an exemplary cross-section of the receiver
in the second receiver sections 3210a and 3210b which typically
operates at elevated operating temperature T.sub.23 which is higher
than T.sub.12. The receiver may be a vacuum tube type receiver (not
shown) as in 3230, but of different diameter, or may be, as shown,
similar to the receiver 700 in FIG. 7. The receiver 3232 has an
exposed opening of width 3242. The exposed width 3242 is less than
the exposed circumference (pi times 3240) in order to improve
overall efficiency of the receiver. The first 3208 and second 3210
receiver sections are connected via a coupler (not shown) that
connects the two receiver absorber tubes together. The receiver
absorber tubes may be the same dimensions with different exposed
areas (e.g., different extent of insulation) or different
dimensions (e.g., smaller tube for 3210 as compared with 3208). In
various other embodiments, receivers may be used that comprise
planar transparent aperture covers, lenticular aperture covers,
partial aperture covers, selective insulated aperture covers,
compound parabolic concentration, and other aspects of the
invention.
[0260] Additional receivers 3234, 3236 corresponding to additional
sections 3214, 3216 will typically operate at further elevated
operating temperatures, each successively higher. The exposed width
of each successive receiver 3234, 3236 may be less than the
previous receivers (3232, 3234) in order to improve overall
efficiency of the receiver. Each section may be connected via a
coupler (not shown) that connects the two receiver absorber tubes
together. The receiver absorber tubes may be the same dimensions
with different exposed areas (e.g., different extent of insulation)
or different dimensions (e.g., smaller tube). Overall solar field
efficiency can be increased through this method.
[0261] FIG. 33 depicts an illustrative alternative assembly 3300
for the collection of solar energy in the form of heat that may be
part of a larger system for the conversion of solar energy to
thermal and/or electrical energy. Assembly 3300 may use less
materials and be lower cost and/or higher performance than the
standard assembly 200 displayed in FIG. 2. Assembly 3300 may be
used in the solar field 102 of a power-generating system similar to
that illustrated in FIG. 1 and may use receiver and mirror
assemblies as described in FIGS. 4, 7, 11, 12, and other
embodiments shown herein.
[0262] Illustrative assembly 3300 comprises a receiver tube 3302.
Solar radiation (not shown, but similar to solar radiation depicted
in FIG. 2) is reflected from a trough-shape collector 3306 having a
parabolic cross-section and impinges on the receiver 3302. The
receiver 3302 is held in place at the focus of the parabolic trough
3306 by struts 3308. A supporting rod 3330 is also extended and
supported by struts 3318 on the reverse side of the mirror. The
assembly comprising receiver 3302, rod 3330, struts 3308 3318, and
trough 3306 is connected to a central rod and joint mechanism 3314,
which in turn is mounted upon supports 3312, and can be rotated on
a joint or bearing mechanism 3314 so that the trough 3306 faces the
sun (i.e., is tilted at an angle equal to the elevation of the sun
above the horizon at a given moment). Movement of the assembly 3300
for sun-track, or operation of other controllable features of the
assembly 3300, may be controlled by a controller (e.g., computer)
3316.
[0263] Mirror 3306 is mounted on a thin structure which is not
structurally stable without a truss-type support (210 as shown in
FIG. 2) or other stabilizing method. As illustrated in FIG. 33, a
suspension cable (solid lines 3322, 3324, 3332, 3334) stabilizing
structure can be used to stabilize and keep the mirror 3306 in its
proper shape under the forces of gravity, wind, and other forces. A
lower suspension structure may consist of cables connected to the
underside of the mirror 3306 and thin structure at multiple points,
including as illustrated cables 3322 connected between the
underside of the mirror structure to rod 3320, cables 3324
connected between the underside of the mirror structure to rod
3320. This lower suspension arrangement including struts 3318
maintains the shape of the mirror from flexing primarily in the
upward direction as illustrated. Additional such cables (not shown)
may extend down the length of the mirror. A second upper suspension
structure may comprise cables connected to the topside of mirror
3306 and thin structure at multiple points, including as
illustrated cables 3332a connected between the underside of the
mirror structure to receiver 3302, cables 3334a connected between
the underside of the mirror structure to receiver 3302. This upper
suspension arrangement including struts 3308 maintains the shape of
the mirror from flexing primarily in the downward direction as
illustrated. Additional such cables, including 3334b and 3334b, may
extend down the length of the mirror.
[0264] The cavity receivers described in FIG. 4, FIG. 7, and other
Figures herein are ideal for connection to a suspension type
structure, since they have a rigid non-glass top surface. For other
receivers that do not have a rigid or otherwise strong enough
structure to support a suspension type cable connection, an
additional rod (not shown) may be installed above or below the
receiver.
[0265] Suspension-type support and stabilizing mechanisms are well
known in bridge construction to use less material than truss-type
support and stabilizing mechanisms in many applications. For mirror
structures for solar power towers, suspension-type support
mechanisms developed by Solaflect (e.g., European Patent
Application "Solar collector stabilized by cables and a compression
element," EP 2215712 A1) show potential for reducing materials and
cost, while maintaining or improving performance.
[0266] Various embodiments of the invention including a glass cover
can employ borosilicate crown glass, which typically has low
dispersion, low coefficient of thermal expansion (CTE), and low
refractive indices. Pyrex glass is such a material, with a specific
example being Pyrex 7740, manufactured by Corning Inc., having an
optical transmittance of 90-94% in the visible spectrum and a
refractive index of 1.473 at 589 nm (peak solar). As is known in
the art, the glass can be coated with anti-reflective (AR)
coatings, such as silica coatings, which increases the optical
transmission of its glass, for example from .about.92% to 96%. Such
coatings are described in more detail the reference N. Benz, Next
Generation Receivers, NREL Trough Work Shop, March 7-8, Golden
Colorado, 2007, hereby incorporated by reference in its
entirety.
[0267] The central absorber tube for various embodiments can be
manufactured from a variety of high temperature metals or ceramics
with an illustrative example being stabilized austenitic steel,
which has good high temperature performance and is able to
withstand repeated thermal cycling.
[0268] In various embodiments that eliminate vacuum, high
temperature thermal insulation is a required component. Various
candidate materials exist with considerations including cost and
low thermal conductivity, k, at both intermediate and high
temperatures. Some materials are silica-based, including Microtherm
MPS by Microtherm Group, a pyrogenic silica, and Pyrogel XT by
Aspen Aerogel, a silica aerogel. These insulators have conductivity
values on the order of 0.034 W/mK at 800.degree. C. and 0.089 W/mK
at 600.degree. C. and are stable up to 1000.degree. C. and
650.degree. C., respectively. As well, less expensive insulation
(e.g., fiberglass, polyurethane) may be added outside the high
temperature insulation or in other embodiments of the invention.
Additional options for low and high temperature insulations and
their approximate performance characteristics are noted in Table
2.
TABLE-US-00002 TABLE 2 Examples of Thermal Insulation Options for
Various Embodiments Thermal Insulation Maximum Thermal
Conductivity, k Stable Figure of at 400.degree. C. at 650.degree.
C. at 650.degree. C. Density Temperature Price/ Merit, 400.degree.
C. Description Form Factor W/mK W/mK W/mK kg/m.sup.2 .degree. C.
m.sup.2 1000/(K price/m.sup.3) Alumina silica ceramic fiber
flexible or solid 0.09 128 >650 $680 10.3 Calcium silicate solid
0.095 0.123 232 850 $2 10 5.0 Mineral flexible 0.12 0.175 0.22 126
850 $290 28.7 Milboard rock + clay solid 0.11 946 >850 $8120 1.1
Fiberglass flexible or solid 0.074 N/A N/A 48 up to 500 $080 20.5
Cellular Glass solid 0.13 N/A N/A 120 480 $4650 1.7 Pedia
particulate 0.12 0.15 Microtherm 3.5* OD MPS solid 0.02 0.026 0.03
320 1000 $6770 7.4 Pyrogel XT flexible 0.046 0.070 0.1 650 Pyrogel
6050 flexible 0.03 650 indicates data missing or illegible when
filed
[0269] Various embodiments may require reflective materials such as
the compound parabolic collector design described above. Specular
surfaces may be advantageous for such embodiments where as other
embodiments may benefit from diffuse reflection. A selection of
example candidate materials for reflectance applications are
summarized in Table 3. Such and other coatings and materials may
also be added in various embodiments to prevent radiation losses
and reduce emissivity to near zero.
[0270] For various embodiments, the absorber tube is covered with a
solar-selective coating. The coatings can require high solar (for
some embodiments the wavelengths are approximately .lamda..ltoreq.3
mm) absorptivity, .alpha., and low thermal (for some embodiments
the wavelengths are approximately .lamda..gtoreq.3 mm) emissivity,
.epsilon.. In some embodiments of the invention employing a vacuum,
coatings otherwise susceptible to oxidation may be used. Stability
at high temperature is a consideration for absorber coatings.
Furthermore, the coefficient of thermal expansion of the coating
and substrate must be considered with respect to thermal
cycles.
[0271] Table 4 lists current and candidate absorber coatings and
their properties. The ratio of solar absorptivity to thermal
emissivity at various temperatures, when available, gives a common
figure of merit to compare solar-selective coatings. Accurate
spectral and temperature-dependent physical properties for
solar-selective coatings are difficult to obtain partly due to
limitations of instruments capable of measurement at high
temperatures.
TABLE-US-00003 TABLE 3 Examples of Reflective Material Options for
Various Embodiments Reflective Materials Coefficient Maximum
Reflectance at of Thermal Stable Typical Solar Weighted Thermal
Temp. Expansion Temperature Thickness Price/ Material Hemispherical
% Specular % Emissivity .epsilon. .degree. C. 10.sup.-6/.degree. C
.degree. C. mils ft.sup.2 Silver coating polished 93 90 0.01 40 720
0.004 0.03 540 Silver metallized polymer 3M Solar Mirror Film 1100
94 95 70 65 4.6 $3 ReflecTech Mirror Film 93 94 55 60 4 $3 Aluminum
highly polished plate 88 85 0.038 200 22.2 600 32 $15 0.06 600
bright foil 85 0.04 20 6 to 62 0.05 40 polished plate 80 0.095 100
Stainless Steel bright foil 63 0.05 30 17 870 25 $72 Chrome
electro-plating polished 60 0.08 40 6.2 427 0.5 $430 0.4 1100
[0272] However, even from single values for .epsilon. (commonly
available up to 100.degree. C.), we can extrapolate emissivity at
higher Tabs to a first approximation for the next round of optical
and thermal modeling. Costs are estimated in Table 3 relative to
the baseline PTR70 coating, ranging from (-) slightly less
expensive, to (++) substantially more expensive. Additional
materials can be found in the reference by E. Kennedy and H. Price
entitled "Progress in Development of High-Temperature
Solar-Selective Coating," found in Solar Energy, vol. 2005, pp.
749-755, 2005 and hereby incorporated by reference in its
entirety.
[0273] FIG. 34A and FIG. 34B display two illustrations of selective
solar absorbers.
[0274] FIG. 34A displays an illustration 3402 of a conventional
cermet selective solar absorbers, consisting of metal nanoparticles
dispersed in a ceramic matrix, which have been developed for CSP.
It is well known in the art that metals have a high absorption
coefficient in the solar spectrum, yet due to refractive-index
mismatch with air a continuous layer of metal reflects most
incident light instead of absorbing it. Traditional cermet
structures address this issue by incorporating graded volume
fractions of metal nanoparticles (.about.5-10 nm diameter) into
ceramic layers in order to tune the refractive-index profile of the
coating for matching of optical impedance in the solar spectrum. A
graded-index cermet 3404 (also termed a graded cermet 3404)
comprises multiple ceramic layers, with increasing volume fractions
of small metal nanoparticles from top to bottom. This graded cermet
3404 is deposited upon a substrate 3408 and an infrared reflector
3406 ("IR Reflector"). Design
TABLE-US-00004 TABLE 4 Materials table for Absorber Coatings
Absorber Coatings Figure at of Solar Thermal Temp. Merit Relative
Description Absorptivity .alpha. Emissivity .epsilon. .degree. C.
.alpha./.epsilon. Deposition Cost SOA materials Luz Black Chrome
0.94 0.11 100 8.55 electro-deposition - 0.27 400 3.48 Luz Cermet
0.92 0.08 100 11.50 sputtering 0.15 400 6.13 Solel UVAC Cermet
0.955 0.076 100 12.57 Sub-mm metalic dendrons in transparent matrix
0.14 400 6.82 Siemens UVAC 2010 0.96 0.09 400 10.67 Siemens Al2O3
based cermet 0.96 0.1 400 9.60 + Siemens Mo--Al2O3 or W--Al2O3 0.96
0.16 350 6.00 planar magnetron + ENEA-Italy HEMS08 coatings for
moiten-sait HTF Mo/Mo--SiO2(HMVF)/Mo--SiO2(LMVF)/SiO2 0.94 0.13 580
7.23 sputtering + graded W/W--Al2O3/Al2O3 0.93 0.1 400 9.30
sputtering + 0.14 580 6.64 SCHOTT PTR70 absorber coating 0.95 0.1
400 9.50 unknown = SCHOTT multilayer selective 7-layer coating
0.955 0.1 100 9.55 2-source PVD Candidate materials black chrome
0.916 0.109 100 8.40 - 0.22 400 4.16 0.239 450 3.53 0.257 500 3.58
semiconductor-pigmented paint Ge in silicone binder, on stainless
steel substrate 0.91 0.7 100 1.30 painted - AMA multilayer coating,
Al2O3--Mo--Al2O3 0.85 0.11 500 7.73 vacuum evaporation - Titanium
Nitride TiNOx on Cu substrate 0.92 0.06 100 15.33 evaporation (ARE)
Double Mo--Al2O3 Cermet layer on Cu substrate 0.96 0.08 350 12.00
vacuum co-evaporation + Mo--SiO2 double layer cermet 0.94 0.13 580
7.23 sputtering + graded Pt--Al2O3 cermet on Pt coated fused quartz
substrate 0.94 0.08 150 11.75 vacuum co-evaporation ++ graded
Pt--Al2O3 cermet on quartz + SiOx AR top layer 0.98 0.2 150 4.90
vacuum co-evaporation ++ ZrOxNy single layer/tandem thin film
absorber 0.9 0.08 327 11.25 sputtering Ti-based tandem absorber,
TiAlN/TiAlON/Si3N4 0.93 0.15 82 6.20 Nanoparticles embedded in
dielectric matrix 0.94 0.07 750 13.43 = Sol-gel nano-structured
absorber coating Low-e surface coated with a high
solar-absorptance, 0.96 0.15 100 0.40 sol-gel ++ nano-pinnacle
structured selective layer, Ni2Sn3 alloy Nickel nanochains in Al2O3
cermet 0.93 0.09 100 10.33 solution-chemicals-coating -
optimization entails engineering the effective refractive index and
film thickness of each layer. In these structures, film thicknesses
are critical for optical performance, leading to stringent
requirements on thickness control. Therefore, existing cermet
fabrication techniques, such as sputtering, evaporation, and
chemical vapor deposition, typically rely on vacuum. The relatively
high cost and low throughput of these techniques limits cost
reduction of CSP systems. Additionally, mid- to high-temperature
selective solar absorbers that are stable in air remain a
significant challenge because metal nanoparticles are typically
easily oxidized in air. Currently, most CSP systems utilize vacuum
tubes to avoid high-temperature oxidation; vacuum failure is a
bottleneck for the lifetime of CSP systems.
[0275] FIG. 34B displays an illustration of a coating using a
plasmonic nanochain cermet structure which can, in various
embodiments, achieve high solar absorptance and low thermal
emittance at low fabrication cost. The plasmonic nanochain cermet
structure 3422 can be fabricated by cost-effective solution
chemical methods. As an example, a new Ni nanochain-Al.sub.2O.sub.3
cermet is schematically shown in FIG. 34B. The Ni nanochains 3424
consist of Ni nanoparticles .about.100 nm in diameter, an order of
magnitude larger than nanoparticles in traditional cermets. The Ni
nanochains 3424 are dispersed in a ceramic matrix 3426 such as
SiO.sub.x upon a substrate 3428. This nanoparticle size is chosen
such that absorption and scattering in the solar spectrum can be
significantly enhanced by optical excitation of surface plasma
polaritons in metal nanostructures. Experimentally this radius can
be well controlled by the molar ratio of Ni.sup.2+ to
N.sub.2H.sub.4. Strong surface plasma polariton scattering from Au
nanoparticles (50.about.100 nm in diameter) towards Si has been
utilized to increase the photocurrent in Si photodiodes and improve
the energy conversion efficiency in thin-film photovoltaic devices.
A challenge for solar absorber applications, however, is that the
plasmonic resonances in noble metals (like Au, Ag) are too narrow
to cover the entire solar spectrum. For broadband plasmonic
absorbers, Ag crossed gratings have been designed and fabricated by
e-beam lithography. In contrast to noble metals, ferromagnetic
metals such as Ni and Fe have a higher damping coefficient so that
the surface plasma polariton resonance appears broader in spectrum.
This feature is advantageous for solar thermal absorbers.
[0276] FIG. 35 displays an illustrative plot for extinction,
absorption, and scattering for various length nanochains using a
plasmonic nanochain cermet structure which can, in various
embodiments, achieve high solar absorptance and low thermal
emittance at low fabrication cost. As illustrated in FIG. 35, the
optical response (wavelength dependent extinction, absorption, and
scattering) can be optimized by tailoring the length of Ni
nanochains. A plot of efficiency factor versus optical wavelength
is displayed for a single nanosphere 3502, a 2-nanosphere chain
3504, a 6-nanosphere chain35, and a 10-nanosphere chain 3508.
Nanosphere diameter is 80 nm. Efficiency factors presented here are
absorption/scattering/extinction cross-sections normalized by
geometric area of the nanostructures. Incident light is polarized
along nanochain. For incident light polarized along the nanochain
(x-polarization), the optical response spectrum can be extended
from .lamda..about.1000 nm to .lamda..about.2500 nm by varying the
length of the nanochain, covering >99% of optical energy in the
solar spectrum. The optical field is found to be strongly enhanced
at the narrow gaps between nanospheres due to near-field plasmonic
effect, an advantage over nanorod structures in terms of surface
plasma polariton enhancement. A 6-nanosphere chain 3506 shows a
stronger field enhancement than the 2-nanosphere one 3504 at
.lamda.=2500 nm, confirming that the increase in absorption at
longer wavelengths (.lamda.>2000 nm) is indeed related to the
surface plasma polariton effect. On the other hand, the .about.100
nm diameter Ni nanoparticles are small enough that long-wavelength
MIR photons from thermal radiation cannot resolve them.
Consequently, these MIR photons see the Ni nanochain network as a
continuous metal sheet and are reflected back to the CSP system,
minimizing thermal emittance losses. Optical performance is
inherently determined by the structure of plasmonic Ni nanochains
instead of layer thicknesses, greatly facilitating low-cost,
solution-chemical processing.
[0277] Antioxidation behavior may be achieved at >500.degree. C.
by the use of a Si-rich ceramic matrix such as SiO.sub.x
(x=1.5-1.9) for two reasons: (1) it is possible to form
metal-silicon bonds similar to those in silicide materials that
shows much better thermal stability than metals. (2) the SiO.sub.x
matrix provides a high refractive index contrast to the Ni
nanoparticles, which enhances the plasmonic scattering effect for
better optical performance. Antioxidation behavior at 500 C has
been demonstrated in air.
[0278] FIG. 36 shows scanning electron microscopy images of example
Ni nanochains as synthesized 3602 and in a coating JJ04. The Ni
nanochains are coated and annealed as a Ni--Al.sub.2O.sub.3
composite at an annealing temperature of 400.degree. C. in N.sub.2.
In their preliminary studies, the Jifeng Liu Group at Dartmouth has
demonstrated spectrally selective plasmonic Ni nanochain cermet
structures created using solution-chemical fabrication. The Ni
nanochains form a 3D network as shown in FIG. 36, which offers
stronger overall solar absorption due to multiple scatterings.
Since the nanochains are randomly oriented, the spectral response
becomes polarization independent.
[0279] FIG. 37 displays an illustrative plot for reflectance for a
plasmonic nanochain cermet structure which can, in various
embodiments, achieve high solar absorptance and low thermal
emittance at low fabrication cost. FIG. 37 shows the reflectance as
a function of wavelength for a plasmonic nanochain cermet structure
3702. Also plotted are the normalized AM 1.5 solar spectrum 3704
and normalized 400 C blackbody radiation spectrum. Based on the
reflectance spectrum 3702, the Ni nanochain-Al.sub.2O.sub.3 cermet
structure spin-coated on stainless steel substrate demonstrates a
high overall solar absorptance of 93% and an overall thermal
emittance of 9%. This optical performance is comparable to vacuum
deposited multilayer cermets while the fabrication process is much
less expensive. The performance can be further optimized by fine
tuning the nanoparticle sizes to better match the solar spectrum
and/or using lower refractive index material such as SiO.sub.2
instead of Al.sub.2O.sub.3.
[0280] FIG. 38A shows the molecular structure of hydrogen
silsesquioxane; FIG. 38B shows a plot showing x-ray diffraction
peak intensity ratio data. The Liu Group has also studied the
passivation of metal nanochains using new ceramic matrix materials
to prevent oxidation of these nanostructures at high temperatures
(>500.degree. C.) in air. Preliminary studies tested hydrogen
silsesquioxane (HSQ). The molecular structure 3802 of HSQ is shown
FIG. 38A. HSQ has also been applied as a molecular precursor to
form Si--SiO.sub.2 nanocomposite at an annealing temperature of
>1000.degree. C. to achieve interesting light-emission
properties. A Si-rich matrix could lead to the formation of
metal-silicon bonds similar to those in silicide materials that
shows much better thermal stability than metals. The Ni nanochains
were ultrasonically dispersed in to HSQ solutions, spin-coated on
stainless steel substrates, and annealed at 700.degree. C. in
N.sub.2 to form the cermet coating. In sharp contrast to Ni
nanochain in Al.sub.2O.sub.3 matrix, which significantly oxidizes
after annealing at 450.degree. C. for 15 min in air, X-ray
diffraction analysis show that the Ni nanochains in HSQ matrix had
little oxidation even after annealing at 450.degree. C. for 2 h in
air. As shown in FIG. 38B, the ratio of NiO(200) to Ni(111) peak
intensity in X-ray diffraction analysis is an order of magnitude
lower in HSQ (curve 3812, circular markers) compared to
Al.sub.2O.sub.3 matrix (curve 3814, square markers). The Ni
nanochain-HSQ cermet coating was further annealed at 500.degree. C.
for an additional hour, and this ratio remained the same,
indicating no further oxidation even at 500.degree. C. This is a
significant improvement compared to Ni nanochains embedded in
Al.sub.2O.sub.3 matrix, and offers the promise that oxidation
resistance at >550.degree. C. can be achieved with process
optimization. In an embodiment of the invention, oxidation
resistance may be further improved through the insertion (or flow)
of an inert gas through the aperture into the cavity to improve
environmental conditions (i.e. displace water).
[0281] Embodiments of the invention that use such a solar selective
coating that does not degrade or oxidize during high temperature
operation within allows substantial improvements in performance and
cost of trough-based concentrating solar plants.
[0282] It is expected that Norwich Technologies Inc. receiver will
be not only more reliable and higher performing than existing
receivers, but also less expensive. The objective of lower cost is
realized by the Norwich Technologies designs shown herein (e.g.,
FIG. 4, FIG. 7, FIG. 11) as they eliminates the vacuum requirement,
removes the thick outer glass shell, and employs simpler coatings.
The elimination of the vacuum reduces not only manufacturing costs
(e.g., vacuum, bellows, and getters) but also maintenance and
operation costs (e.g., identifying and replacing vacuum failures,
avoiding most cases of glass breakage, and routine getter
maintenance).
[0283] FIG. 39 illustrates in graphical form the results of an
analysis relative to the SOA. The estimated cost breakdowns are
broken into categories and prices normalized to the total cost of
the SOA Schott PTR70 receiver. Cost breakdown categories include
absorber tube 3902, absorber coating 3904, vacuum accessories 3906,
reflective coating 3908, glass envelope 3910, anti-reflective
coating 3912, and insulation 3914. NT designs are shown based on
designs shown in early figures including FIG. numbers (FIG. 4, FIG.
7, FIG. 11).
DEFINITIONS
[0284] As used herein, the term "light" includes but is not
restricted to the visible portion of the electromagnetic
spectrum.
[0285] As used herein, the terms "pipe," "piping" and the like
refer to one or more conduits that are rated to carry gas or liquid
between two points. Thus, the singular term should be taken to
include a plurality of parallel conduits where appropriate.
[0286] Recording the results from an operation or data acquisition,
such as for example, recording results at a particular frequency or
wavelength, is understood to mean and is defined herein as writing
output data in a non-transitory manner to a storage element, to a
machine-readable storage medium, or to a storage device.
Non-transitory machine-readable storage media that can be used in
the invention include electronic, magnetic and/or optical storage
media, such as magnetic floppy disks and hard disks; a DVD drive, a
CD drive that in some embodiments can employ DVD disks, any of
CD-ROM disks (i.e., read-only optical storage disks), CD-R disks
(i.e., write-once, read-many optical storage disks), and CD-RW
disks (i.e., rewriteable optical storage disks); and electronic
storage media, such as RAM, ROM, EPROM, Compact Flash cards, PCMCIA
cards, or alternatively SD or SDIO memory; and the electronic
components (e.g., floppy disk drive, DVD drive, CD/CD-R/CD-RW
drive, or Compact Flash/PCMCIA/SD adapter) that accommodate and
read from and/or write to the storage media. Unless otherwise
explicitly recited, any reference herein to "record" or "recording"
is understood to refer to a non-transitory record or a
non-transitory recording.
[0287] As is known to those of skill in the machine-readable
storage media arts, new media and formats for data storage are
continually being devised, and any convenient, commercially
available storage medium and corresponding read/write device that
may become available in the future is likely to be appropriate for
use, especially if it provides any of a greater storage capacity, a
higher access speed, a smaller size, and a lower cost per bit of
stored information. Well known older machine-readable media are
also available for use under certain conditions, such as punched
paper tape or cards, magnetic recording on tape or wire, optical or
magnetic reading of printed characters (e.g., OCR and magnetically
encoded symbols) and machine-readable symbols such as one and two
dimensional bar codes. Recording image data for later use (e.g.,
writing an image to memory or to digital memory) can be performed
to enable the use of the recorded information as output, as data
for display to a user, or as data to be made available for later
use. Such digital memory elements or chips can be standalone memory
devices, or can be incorporated within a device of interest.
"Writing output data" or "writing an image to memory" is defined
herein as including writing transformed data to registers within a
microcomputer.
[0288] "Microcomputer" is defined herein as synonymous with
microprocessor, microcontroller, and digital signal processor
("DSP"). It is understood that memory used by the microcomputer,
including for example instructions for data processing coded as
"firmware" can reside in memory physically inside of a
microcomputer chip or in memory external to the microcomputer or in
a combination of internal and external memory. Similarly, analog
signals can be digitized by a standalone analog to digital
converter ("ADC") or one or more ADCs or multiplexed ADC channels
can reside within a microcomputer package. It is also understood
that field programmable array ("FPGA") chips or application
specific integrated circuits ("ASIC") chips can perform
microcomputer functions, either in hardware logic, software
emulation of a microcomputer, or by a combination of the two.
Apparatus having any of the inventive features described herein can
operate entirely on one microcomputer or can include more than one
microcomputer.
[0289] General purpose programmable computers useful for
controlling instrumentation, recording signals and analyzing
signals or data according to the present description can be any of
a personal computer (PC), a microprocessor based computer, a
portable computer, or other type of processing device. The general
purpose programmable computer typically comprises a central
processing unit, a storage or memory unit that can record and read
information and programs using machine-readable storage media, a
communication terminal such as a wired communication device or a
wireless communication device, an output device such as a display
terminal, and an input device such as a keyboard. The display
terminal can be a touch screen display, in which case it can
function as both a display device and an input device. Different
and/or additional input devices can be present such as a pointing
device, such as a mouse or a joystick, and different or additional
output devices can be present such as an enunciator, for example a
speaker, a second display, or a printer. The computer can run any
one of a variety of operating systems, such as for example, any one
of several versions of Windows, or of MacOS, or of UNIX, or of
Linux. Computational results obtained in the operation of the
general purpose computer can be stored for later use, and/or can be
displayed to a user. At the very least, each microprocessor-based
general purpose computer has registers that store the results of
each computational step within the microprocessor, which results
are then commonly stored in cache memory for later use, so that the
result can be displayed, recorded to a non-volatile memory, or used
in further data processing or analysis.
THEORETICAL DISCUSSION
[0290] Although the theoretical description given herein is thought
to be correct, the operation of the devices described and claimed
herein does not depend upon the accuracy or validity of the
theoretical description. That is, later theoretical developments
that may explain the observed results on a basis different from the
theory presented herein will not detract from the inventions
described herein.
[0291] Any patent, patent application, patent application
publication, journal article, book, published paper, or other
publicly available material identified in the specification is
hereby incorporated by reference herein in its entirety. Any
material, or portion thereof, that is said to be incorporated by
reference herein, but which conflicts with existing definitions,
statements, or other disclosure material explicitly set forth
herein is only incorporated to the extent that no conflict arises
between that incorporated material and the present disclosure
material. In the event of a conflict, the conflict is to be
resolved in favor of the present disclosure as the preferred
disclosure.
[0292] While the present invention has been particularly shown and
described with reference to the preferred mode as illustrated in
the drawing, it will be understood by one skilled in the art that
various changes in detail may be affected therein without departing
from the spirit and scope of the invention as defined by the
claims.
* * * * *
References