U.S. patent application number 12/757100 was filed with the patent office on 2010-10-14 for generation of steam from solar energy.
This patent application is currently assigned to Victory Energy Operations LLC. Invention is credited to Bochuan Lin, John C. Viskup, JR..
Application Number | 20100258112 12/757100 |
Document ID | / |
Family ID | 42933344 |
Filed Date | 2010-10-14 |
United States Patent
Application |
20100258112 |
Kind Code |
A1 |
Viskup, JR.; John C. ; et
al. |
October 14, 2010 |
GENERATION OF STEAM FROM SOLAR ENERGY
Abstract
A cavity-type solar energy receiver for generating high pressure
steam, which includes panels of tubes defining a cavity within an
outer enclosure. Concentrated solar energy provided by a heliostat
enters the cavity opening in the enclosure and evaporates water
within some of the tube panels. The evaporating tubes receive hot
water from a steam drum by natural circulation and return a mixture
of steam and hot water to the steam drum. Additional tube panels
are positioned to receive reflected solar energy, which is used to
preheat feed water and to superheat steam.
Inventors: |
Viskup, JR.; John C.;
(Owasso, OK) ; Lin; Bochuan; (Owasso, OK) |
Correspondence
Address: |
KELLEY DRYE & WARREN LLP;STEVEN J. MOORE
400 ALTLANTIC STREET , 13TH FLOOR
STAMFORD
CT
06901
US
|
Assignee: |
Victory Energy Operations
LLC
Collinsville
OK
|
Family ID: |
42933344 |
Appl. No.: |
12/757100 |
Filed: |
April 9, 2010 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
61212390 |
Apr 10, 2009 |
|
|
|
61217425 |
May 29, 2009 |
|
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Current U.S.
Class: |
126/645 |
Current CPC
Class: |
F22B 1/006 20130101;
Y02E 10/46 20130101; Y02E 10/40 20130101; F24S 2023/86 20180501;
F22B 35/02 20130101; F24S 20/20 20180501 |
Class at
Publication: |
126/645 |
International
Class: |
F24J 2/10 20060101
F24J002/10; F24J 2/24 20060101 F24J002/24 |
Claims
1. A solar energy receiver for generating high pressure steam with
concentrated solar energy comprising: (a) a steam drum for
receiving heated boiler feed water and discharging high pressure
steam; (b) at least one panel of evaporator tubes, said evaporator
tubes for receiving hot water by natural circulation from said
steam drum and returning steam and hot water into said steam drum;
and (c) at least one panel of superheater tubes, said superheater
tubes for receiving steam from said steam drum and discharging
superheated steam; wherein said steam drum is mounted adjacent to
an enclosure containing said tube panels of (b) and (c), said tube
panels defining a cavity surrounding an opening in said enclosure
for admitting concentrated solar energy.
2. A solar energy receiver of claim 1 further comprising at least
one panel of economizer tubes, said economizer tubes for receiving
boiler feed water and discharging said heated boiler feed water
into said steam drum;
3. A solar energy receiver of claim 1 wherein said at least one
panel of said evaporator tubes is positioned in said enclosure
opposite said opening to receive concentrated solar energy
directly.
4. A solar energy receiver of claim 3 further comprising additional
panels of evaporator tubes adjacent said at least one panel of
evaporator tubes of claim 2, said additional panels of evaporator
tubes being positioned on the side walls of said cavity to receive
reflected solar energy.
5. A solar energy receiver of claim 2 wherein panels of economizer
tubes are positioned on the side walls and/or floor of said cavity
adjacent said opening for admitting concentrated solar energy.
6. A solar energy receiver of claim 1 wherein said at least one
panel of superheater tubes is positioned at the roof of said
cavity.
7. A solar energy receiver of claim 1 further comprising a second
set of tube panels corresponding to tube panels (b) and (c) within
a second enclosure positioned in a mirror image of said at least
one tube panels of claim 1, said steam drum being positioned
between said first and second tube sets of tube panels.
8. A solar energy receiver of claim 1 wherein at least one of said
panel of tubes is disposed to present a continuous heat transfer
surface exposed to solar energy, each of said tubes being joined to
adjacent tubes by metal bars welded to each tube, thereby creating
a continuous heat transfer surface.
9. A solar energy receiver of claim 1 further comprising insulation
positioned between each of said tube panels and said enclosure.
10. A solar energy receiver of claim 1 wherein said at least one
panel of evaporator tubes is in fluid communication with said steam
drum via inlet and outlet manifolds.
11. A solar energy receiver of claim 10 wherein said outlet
manifold is in fluid communication with said steam drum through
multiple pipes, said multiple pipes having a bend for accommodating
thermal expansion and contraction.
12. A solar energy receiver of claim 10 wherein said inlet manifold
is in fluid communication with said steam drum via multiple down
corner pipes between said steam drum and said inlet manifold.
13. A solar energy receiver of claim 11 wherein said bends have an
angle of about 90.degree..
14. A solar energy receiver of claim 10 wherein the evaporator
tubes enter said inlet manifold and exit into said outlet manifold
through horizontal connections.
15. A solar energy receiver of claim 1 wherein a reflective coating
is located on each of said tube panels of (b)(c), and (d).
16. A solar energy receiver of claim 15 wherein said reflective
coating is a silicone-based material resisting temperatures up to
about 1100.degree. F.
17. A solar energy receiver of claim 16 wherein said reflective
coating reflects up to 50% of incident light.
18. A solar energy of claim 16 wherein said reflective coating is a
mixture of black and white coatings.
19. A solar energy receiver of claim 18 wherein said reflective
coating reflects about 20% of incident light.
20. A solar energy receiver of claim 9 wherein said insulation is
ceramic fiber in back of said superheater tube panels and mineral
wool in back of said evaporator and economizer tube panels.
21. A cavity-type solar energy receiver for generating high
pressure steam from concentrated solar energy comprising: (a) a
steam drum for receiving heated boiler feed water and discharging
high pressure steam; (b) two cavities having openings for receiving
concentrated solar energy and facing in opposite directions, said
cavities located on opposite sides of said steam drum. said
cavities defined by panels of tubes forming the sides of said
cavities, said panels of tubes for receiving solar energy and
generating high pressure steam therefrom; (c) at least one panel of
evaporator tubes in each of said cavities for receiving hot water
by natural circulation from said steam drum and for returning steam
and hot water into said steam drum; and (d) at least one panel of
superheater tubes in each of said cavities for receiving steam from
said drum and discharging superheated steam.
22. A cavity-type solar energy receiver of claim 21 further
comprising at least one panel of economizer tubes in each of said
cavities for preheating boiler feed water and discharging said
heated boiler feed water into said steam drum;
23. A cavity-type solar energy receiver of claim 21 wherein said at
least one panel of evaporator tubes in each of said cavities is
located opposite said openings for receiving concentrated solar
energy directly.
24. A cavity-type solar energy receiver of claim 22 further
comprising additional panels of evaporator tubes located adjacent
said at least one panel of evaporator tubes of claim 21, said
additional panels of evaporator tubes being positioned on the side
walls of said cavity to receive reflected energy.
25. A cavity-type solar energy receiver of claim 22 wherein said
panels of economizer tubes for each of said cavities are located on
the side walls of said cavities adjacent the openings for admitting
concentrated solar energy.
26. A cavity-type solar energy receiver of claim 21 wherein said at
least one panel of superheater tubes are located at the top of each
of said cavities.
27. A cavity-type solar energy receiver of claim 21 wherein at
least one of said panels of tubes is disposed to present a
continuous heat transfer surface exposed to solar energy, each of
said tubes being joined to adjacent tubes by metal bars welded to
each tube, thereby creating a continuous heat transfer surface.
28. A cavity-type solar energy receiver of claim 21 further
comprising insulation positioned between each of said cavities and
an enclosure surrounding both of said cavities and said steam
drum.
29. A cavity-type solar energy receiver of claim 21 wherein said at
least one panel of evaporator tubes in each cavity is in fluid
communication with said steam drum via inlet and outlet
manifolds.
30. A cavity-type solar energy receiver of claim 29 wherein said
outlet manifold is in fluid communication with said steam drum
through multiple pipes, said multiple pipes having bends for
accommodating thermal expansion and contraction.
31. A cavity-type solar energy receiver of claim 29 wherein said
inlet manifold is in fluid communication with said steam drum via
multiple downcomer pipes between said steam drum and said inlet
manifold.
32. A cavity-type solar energy receiver of claim 30 wherein said
bends have an angle of about 90.degree..
33. A cavity-type solar energy receiver of claim 29 wherein the
evaporator tubes enter said inlet manifold and exit into said
outlet manifold through horizontal connections.
34. A cavity-type solar energy receiver of claim 21 wherein a
reflective coating is located on each of said tube panels in (b),
(c), and (d).
35. A cavity-type solar energy of receiver of claim 34 wherein said
reflective coating is a silicone-based material resisting
temperatures up to about 1100.degree. F.
36. A cavity-type solar energy receiver of claim 35 wherein said
reflective coating reflects up to 50% of incident light.
37. A cavity-type solar energy receiver of claim 35 wherein said
reflective coating is a mixture of black and white coatings.
38. A cavity-type solar energy receiver of claim 37 wherein said
reflective coating reflects about 20% of incident light.
39. A cavity-type solar energy receiver of claim 28 wherein said
insulation is ceramic fiber in back of said superheater tube panels
and mineral wool in back of said evaporator and economizer tube
panels.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application claims the benefit of priority of U.S.
Provisional Applications 61/212,390 filed Apr. 10, 2009 and
61/217,425 filed May 29, 2009 and incorporated herein by
reference.
BACKGROUND OF THE INVENTION:
[0002] This invention relates to recovering solar energy in the
form of steam, which may be used to generate electricity or in
industrial processes. More particularly, the invention includes a
novel method and apparatus for converting solar energy to high
pressure steam; that is, a steam boiler that employs the sun's rays
to produce steam.
[0003] Many proposals have been made to focus the sun's rays to
concentrate solar energy. Electricity can be made directly by using
heat engines that receive heat from focusing the sun's rays with a
solar dish. Generating electricity with hot heat transfer liquids
has been demonstrated for some years. Parabolic trough systems
focus the sun's rays on receiving tubes running through the trough
that carry heat transfer fluids. In what have been referred to as
"power towers" an array of reflecting mirrors is used to focus the
sun's rays on a central receiver that recovers the energy. If the
concentrated solar energy can be applied to water at a high enough
temperature, steam can be produced. While low pressure steam can be
used for some purposes, such as heating buildings, high pressure
steam is needed if it is to be used in a steam turbine to generate
electricity. Thus, the problem addressed by the present inventors
was how one could efficiently transfer concentrated solar energy to
water at temperature of about 800-900.degree. F. and efficiently
generate high pressure steam, for example at about 900-1000
psig.
[0004] Conventional steam boilers receive heat from two sources,
radiant heat from the flames of fossil fuels and convective heat
from hot combustion gases, both of which will vary with the amount
and type of the fuel being burned and the air to fuel ratio.
However, in applying solar energy only radiant heat is available,
which must be transferred to the boiler tubes to generate steam.
Furthermore, the energy density varies as the sun moves across the
sky or with changing atmospheric conditions. The solar boiler
design that will be described in detail below accommodates these
conditions and can produce high pressure steam when solar energy is
available.
[0005] The present invention is within the class of solar energy
power systems referred to as power towers. As contrasted with the
use of parabolic reflectors at ground level that focus the sun's
rays on tubes running through the reflectors, a power tower uses an
array of reflecting mirrors at ground level to focus the sun's rays
onto a central receiver mounted on a tall tower. The solar energy
is absorbed by a heat transfer medium, such as water, liquid
sodium, molten salts, and organic liquids. Steam can be generated
directly, or indirectly using another heat transfer medium, and
used to drive a turbine generating electricity or in other uses. In
order to maintain the electricity supply when the sun's energy is
not available, a means for storing heat may be provided and used to
produce steam and electricity. An example of such a solar energy
system is Solar One, built and tested at Barstow, Calif. in the
1980's, which successfully generated steam directly with the sun's
rays. Heat was stored in oil and rock for later use. Cylindrical
central receivers of the type used at Solar One to generate steam
are described in U.S. Pat. Nos. 4,485,803; 4,245,618; and
4,789,114. Water was pumped through tube panels that formed a
cylindrical receiver to preheat and evaporate feed water and then
to superheat the steam produced. Solar Two converted the Solar One
system to operate with molten salt as the heat transfer fluid,
which had the advantage of more efficiently storing energy for
generating steam when solar energy was not available.
[0006] The present invention relates to an improved method and
apparatus for directly generating high pressure steam at the top of
a power tower. In contrast with the cylindrical receiver mentioned
above, in which water is pumped through the tubes, the present
application includes a cavity type receiver which employs natural
circulation of water. The source of the solar energy may be an
array of reflecting mirrors such as described in U.S. Pat. Nos.
6,959,993 and 7,192,146. The mirrors direct the sun's rays onto
openings in the side of the steam generating apparatus, which is
enclosed in a structure that confines the solar energy and limits
heat losses from reflected solar radiation. The entire steam
generating apparatus or steam boiler is completely enclosed. Only
the entrance ports for the focused solar energy are open.
SUMMARY OF THE INVENTION
[0007] The invention includes a cavity-type solar energy receiver
for generating high pressure superheated steam in which panels of
tubes are positioned inside an enclosure to receive concentrated
solar energy through an opening in the enclosure. Water from a
steam drum is passed by natural circulation through evaporator
tubes exposed to concentrated solar energy to produce steam. The
steam is separated from the steam/water mixture in the steam drum
and then superheated before being supplied to a turbine driven
electrical generator or used for other purposes.
[0008] In a preferred embodiment, two of the cavity-type solar
energy receivers are positioned as mirror images, one facing north
and the other south, both receiving concentrated solar energy from
mirrors at ground level (a heliostat) reflecting light to the solar
energy receivers mounted on a tower. A steam drum serving both
receivers is mounted between them and within an enclosure for both
receivers. In combination, the panels of tubes form a cavity within
which concentrated solar energy is used to generate high pressure
steam. Between the cavity formed by panels of tubes and the
surrounding enclosure refractory insulation is provided to limit
heat losses and to protect the outer enclosure. The solar energy
receiver(s) has evaporator tubes mounted opposite the solar energy
opening and against adjacent side walls. Economizer tubes for
preheating boiler feed water may be included which, if used, could
be positioned on the side walls and/or floor of the receiver
adjacent the solar energy opening. Superheater tubes are positioned
as a roof at the top of the cavity adjacent the evaporator tubes.
Auxiliary superheater tubes may be added on the side walls as
desired in some embodiments.
BRIEF DESCRIPTION OF THE DRAWINGS
[0009] FIG. 1 is a process diagram.
[0010] FIG. 2 is a perspective view of a duplex steam boiler of the
invention.
[0011] FIG. 3 is a sectional elevation view of the steam boiler of
FIG. 2.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0012] Design Problems
[0013] There are problems unique to the generation of high pressure
steam from concentrated solar energy. They begin with the location
of the steam generating equipment. Since the steam boiler and
associated piping will be located atop a tower of one hundred feet
or so, the weight of the equipment and environmental (e.g. wind and
earthquake) forces on it must be accounted for in the design. Also,
because access to the boiler equipment will be limited, especially
when it is receiving the sun's rays reflected from mirrors at
ground level, the design must be reliable and operated from a
remote location. Some other problems are unique to the steam
generating equipment itself.
[0014] Of first importance is the availability of solar energy.
Since the amount of the sun's energy that falls on an array of
mirrors will vary during the day and with the time of year, the
amount of the sun's energy that can be reflected to the solar
boiler equipment will change from minute to minute and day to day.
In addition, the changing weather conditions will impact the
available solar energy. This means that the amount of high pressure
steam produced by a solar boiler can vary greatly and,
consequently, control of the solar boiler operating conditions and
of the reflecting mirrors is essential if optimum performance is to
be attained. Thus, in contrast to most conventional boilers, the
availability of the supply of energy (the sun) will be constantly
varying, but the quality of the steam must be maintained to satisfy
the turbine generator requirements or of other users of steam.
Furthermore, since in many installations there will be multiple
towers, each with its own steam boiler, changes in the solar energy
available may be accommodated by starting up or shutting down
individual solar boilers. Similarly, the electrical demand may
vary, depending on customer use, which will affect demand for high
pressure steam independently of the solar energy availability.
[0015] For each solar boiler, the steam quality will have to be
maintained as conditions change. For example, a solar boiler will
be designed to produce a given amount of high pressure steam at a
given temperature using solar energy of a given maximum
concentration. If the steam is to maintain its design quality (i.e.
temperature and pressure) the ground-based mirrors can be adjusted
to optimize the amount of the solar energy reaching the solar
boiler. To a lesser degree, the steam temperature can be
continuously adjusted by injecting boiler feed water into the
superheated steam.
[0016] The tubes receiving solar energy will frequently expand and
contract as the temperature of the tube walls varies with changes
in the concentrated solar energy. It will be appreciated that
excessive temperatures can cause tube failures so that adjusting
the mirrors and maintaining water and steam flows is important. The
tubes also must prevent solar energy from overheating the
insulation and the external structure.
[0017] The solar receiver tubes may be subject to cyclic fatigue
failure. This is a unique problem that results from the frequent
heating up and cooling down due to unstable solar energy heat
input. The worst operating conditions are expected to be in the
superheater tubes, where the tube temperature may become very high,
making the tubes more vulnerable to cyclic damage.
[0018] An important feature of this invention is that the
superheater tubes are warmed with reflected heat from the
evaporator panels, that is, an indirect method, rather than a
direct method. The maximum heat flux in the reflected sun's rays is
substantially less than that from direct rays. This reduces the
maximum tube wall temperature and increases the reliability and
life of the superheater panels.
[0019] Another important feature of this invention is natural water
circulation. That is, water leaving the steam drum flows downward
and enters the tubes. In the tubes the water it receives solar
energy and a portion of the water becomes steam, rising up to the
steam drum. The density difference between the water and the
steam/water mixture creates a natural flow or circulation, which
does not require the complication and expense of pumping and
improves reliability of boiler operation.
[0020] General Arrangement of the Solar Energy Receiver
[0021] The solar energy receiver of the invention is referred to as
a cavity-type solar energy receiver. It is deployed at the top of a
"power tower" that receives concentrated solar energy from a
heliostat, i.e. a set of ground-based mirrors. The solar energy
receiver includes an external enclosure that protects the steam
boiler and limits energy losses. When viewed from the outside, the
steam boiler and other internals are not generally visible.
[0022] Inside the outer enclosure, a steam drum and its associated
tubes are positioned so as to receive concentrated solar energy
reflected from fields of ground-based mirrors. The steam drum and
its associated tubes are generally referred to as a steam boiler.
The tubes that receive solar energy are backed by insulation, which
contains and helps to reflect the solar energy received from the
heliostat and also protects the external structure
[0023] A cavity-type solar receiver by definition has an opening
sized to receive energy focused on it by minors, which are usually
located at ground level. The size of the cavity opening will be
determined by the arrangement and positioning of the mirrors. The
intensity of the focused solar energy will be determined by the
number and size of the mirrors used and their reflective
properties. Consequently, a cavity-type solar receiver would be
designed to have a specific opening size and shape and to receive a
specified quantity of solar energy. Within those basic parameters
one must determine the arrangement of tubes to preheat and
evaporate boiler feed water and to superheat the steam produced.
Since the heat to evaporate water will be greater than the heat
needed to preheat feed water and to superheat steam, the
arrangement of the evaporator tubes will be of particular
importance.
[0024] In the presently preferred embodiment shown herein, two
cavity-type receivers are paired with a single steam drum. The
evaporator tubes receive the focused solar energy directly by being
placed opposite the cavity opening. Additional evaporator tube
panels may be located on the side walls of the cavity adjacent the
main panels, where they receive some reflected solar energy.
However, the solar receiver of the invention is not limited by or
require such adjacent panels. An embodiment that is shown in the
drawings places preheat tube panels and superheater tube panels so
that they receive solar energy reflected back from the evaporator
tube panels opposite the cavity opening. While this arrangement of
these panels is considered to provide a useful configuration, the
location of such panels may be rearranged if desired. In a
preferred embodiment the preheat panels are eliminated. Overall,
the interior of the cavity receiver contains tube panels sized and
arranged to receive a specified solar heat flux and to provide the
specified amount of heat to water and steam.
[0025] Process Description
[0026] FIG. 1 is a process flow diagram that will assist the reader
in understanding the description of the steam boiler which will
follow. In this design, two sets of heat exchange panels forming
two cavities are shown, one which typically would face north and
the second one typically south, with ground-based mirrors
reflecting solar energy into the cavity on each side. The position
of the panels is also indicated, that is, whether the panels are on
the east side, the west side, the roof, or on the floor of the
cavity. Alternatively, a single cavity-type receiver having only
one set of tube panels could be used if positioned at the edge of a
field of mirrors. Electricity is generated at ground level by a
turbine-generator (not shown) driven by the high pressure steam
produced by the solar receivers. After leaving the
turbine-generator, the steam is condensed and then pumped as hot
water up the tower into the steam boiler 10 via the four economizer
panels, shown as ECON 1-4. In a specific example, this water, which
has a temperature of about 425.degree. F. and a pressure of about
998 psig, is reheated in the economizer panels ECON 1-4 to a
temperature just below that of the steam drum 10, about 502.degree.
F. The steam drum 10 supplies the hot water via natural circulation
to six evaporating panels, shown as EVAP 1-6, which generate steam
at about 544.degree. F., which returns to the steam drum 10. The
saturated steam from steam drum 10 is then passed through the
superheating panels, shown as SH 1-4, where the temperature is
raised to about 825.degree. F. for use in the turbine-generator. A
desuperheater is provided between SH 3 and 4, to adjust the
temperature of the steam as required. The temperature also may be
adjusted by changes to the position of the ground-based reflecting
mirrors (not shown).
[0027] Arrangement of the Steam Boiler
[0028] FIG. 2 is a perspective view of a duplex steam boiler of the
invention. In this design there are two mirror image receivers, one
facing north and the other south, so that two sets of ground level
mirrors can direct the sun's rays into the south opening 18 and
north opening (not seen). Alternatively only one receiver could be
used, positioned at the edge of one set of mirrors. The steam drum
10 is located between the north and south receivers and supplies
water by natural circulation to the evaporating tube panels (EVAP
1-6), which return steam and water to the steam drum 10. After the
saturated steam is separated from water returning to drum 10, it is
superheated before being sent down the tower to be used. The heat
exchange tube panels are backed by ceramic type insulation, which
is adjacent to an outer enclosure (not shown).
[0029] Referring back to FIG. 1, focused solar energy enters the
north and south openings and is partially reflected from the back
wall to the side walls and the roof of the north and south units
where the radiant energy is absorbed by three types of tube panels,
which preheat boiler feed water, produce steam, and superheat the
steam. The location of some of the tube panels are identified in
FIG. 2. It should be understood that, except for superheater panels
SH 1 and 2 which are on opposite sides of their respective units,
the panels are mirror images. As was shown in FIG. 1, the preheater
and superheater tube panels operate in series, water or steam
passing through tube panels in both of the north and south units.
The evaporator panels operate in parallel, three sets in each
unit.
[0030] In the north unit economizer tube panels ECON 1-2 heat
boiler feed water pumped from ground level and then send the heated
water to ECON 3-4 in the south unit for additional heating before
passing to the steam drum 10. These economizer tube panels are not
required and may be omitted if desired. The evaporator tube panels
EVAP 3 and 6 (not shown), which face the steam drum and are
opposite the solar energy openings, are located where the solar
energy is most concentrated, since they are directly exposed to the
focused sun's rays. Evaporator tube panels EVAP 1-2 (north section)
and EVAP 4-5 (south section) are located on the side walls of their
cavities. The north unit has the same arrangement of tube panels
except for superheater panel SH 2; the corresponding superheat
panel SH1 is located on the east side of the south unit.
[0031] Using natural circulation through the evaporator tube
panels, hot water from the steam drum 10 is vaporized and a mixture
of steam and water is returned to the steam drum. After separating
steam from hot water in the steam drum, the steam passes through
superheater tubes SH 1-4 before being sent down the tower to the
users. A desuperheater (not shown in FIG. 2) is provided to adjust
the steam outlet temperature by spraying water into the superheated
steam.
[0032] Steam Drum
[0033] The steam drum 10 is positioned between the north and south
units as seen in FIG. 2. It receives heated feed water leaving
economizer tube panel ECON 4 and entering the steam drum 10 though
holes in a pipe extending into the drum (not shown). Water leaves
the bottom of the steam drum 10 and enters the lower manifolds 12 N
and 12 S, which each serve three evaporator tube panels, two which
are seen in the north unit as EVAP 1 and 2 on the east and west
walls. Evaporator tube panel EVAP 3 is seen in part in the north
unit, where it is exposed to the most direct solar energy. The
corresponding evaporator panels are EVAP 4-6 are in the south unit.
The evaporator tube panels discharge into the upper manifolds 14 N
and 14 S, which are generally U-shaped. From the upper manifolds
multiple return pipes (16 N and S in FIG. 2), are used to provide
uniform recirculation. The steam drum internals (not shown) include
steam-water separators and demisters to remove water droplets from
the saturated steam before leaving the drum and entering the
superheat panels SH 1-4.
[0034] Economizer Tubes
[0035] Economizer tubes are not required but, when included, may be
placed in locations not suited for evaporator or super heater tubes
in order to complete the cavity. In the embodiment shown, the
economizer tubes are positioned inside the solar energy openings
(e.g. 18 in the south unit) as tube panels ECON 1-2 and ECON 3-4 on
each side of each of the north and south units respectively.
Economizer tube panels could be added at the floor of the cavity,
although not preferred. The tubes in each panel receive boiler feed
water from a manifold at one end of the tubes and deliver heated
water to an outlet manifold at the other end of the tubes. Thehot
feed water enters panel ECON 1 in the north unit, then leaves and
proceeds to panels ECON 2-4 for further heating. The tubes enter
horizontally with bends to each manifold as shown in FIG. 2 in
order to accommodate thermal expansion and contraction, which will
occur during operation of the receiver as the concentration of
solar energy or electrical load varies.
[0036] In one embodiment, the economizer tubes are expected to
receive a maximum heat density of about 90,000 BTU/ft.sup.2 per
hour. Tubes have an outside diameter of 1.25 inches with a 0.165
inch thick wall (0.120 min) and are made of ASME SA-178A low carbon
steel. Membrane bars, which are welded to the tubes to bind them
together as a continuous heat transfer surface, are 0.25 inches
thick and 0.825 inches wide. The cavity side of the economizer
tubes are coated with a high emissivity coating to be discussed
below.
[0037] Evaporator Tubes
[0038] Each of the north and south units has three sets of
evaporator tube panels, operating in parallel in connection with
the steam drum. Two panels EVAP 4 and 5 (south) and EVAP 1 and 2
(north) are located adjacent the wall opposite the solar energy
opening. These panels principally receive reflected light, while
panels EVAP 3 and 6 are located on the back wall of each section
that receives direct exposure to the concentrated solar energy.
EVAP 3 is shown in part of FIG. 2, but EVAP 6 is not visible due to
the orientation of the two units. Each of the evaporator panels
consists of a series of tubes receiving hot water by natural
circulation from the steam drum through manifolds at the lower end
of the tubes (12 N and 12 S). Steam is generated in the tubes and a
mixture of steam and water passes up through each tube and into
upper manifolds (14 N and 14 S). The steam/water mixture returns to
the steam drum 10 from the upper manifolds through a series of
tubes (16 N and 16 S) having a bend of about 90.degree. , which
will be described in more detail below. As with the economizer
tubes, the evaporator tubes bend at both top and bottom to enter
the manifolds horizontally.
[0039] FIG. 3 is an sectional elevation view of the east wall of
the duplex steam solar boiler in FIG. 2, as viewed from the inside
of the cavities. The north unit is at the left and the south unit
at the right. Two of the economizer panels are shown (ECON 1 and 3
and the position of superheater panels (SH 3 and 4) atop the two
cavities can be seen. Solar energy enters the north and south
openings as indicated by the arrows. The natural circulation of
water from the steam drum 10 through evaporator panels EVAP 1
(north side) and 4 (south side) is illustrated by arrows. (The main
evaporator tube panels EVAP 3 and 6 are not visible in this
sectional view). Water leaves the steam drum 10 through multiple
nozzles at the bottom to join manifolds 12 N and 12 S and flows
upward through the evaporator tube panels where solar energy
evaporates a portion of the water. The mixture of steam and water
enters outlet manifolds 14 N and 14 S, then passes back into the
steam drum 10 again through multiple return pipes 16 N and 16 S.
The return pipes have a bend of about 90 degrees, which helps to
absorb thermal expansion and contraction during operation.
[0040] In a preferred embodiment, the evaporator tubes are expected
to receive a maximum heat density of about 100,000 BTU/ft.sup.2 per
hour.. The tubes have an outside diameter of 1.75 inches, with a
0.135 inch thick (0.120 min) wall and are made of ASME SA-178A low
carbon steel. The tubes are joined by membrane bars, which are 0.25
inches thick and 0.5 inches wide. The cavity side of the evaporator
tubes is also coated with a high emissivity coating.
[0041] Superheater Tubes
[0042] Each of the north and south sections has three sets of
superheater tube panels SH 1-4, operating in series. The first
panels, SH 1 and 2, receive the higher heat density but, since they
receive saturated steam leaving the steam drum, operate at a lower
temperature. Tube panel SH 2 can be seen on the west wall of the
north section in FIG. 2. Tube panel SH 1 can be seen on the east
wall of the south section in FIG. 3. Superheater tube Panels SH 1
and 2 are positioned adjacent evaporator tube panel EVAP 4 (south)
and evaporator panel EVAP 2 (north). Superheater panels SH 3 and 4
are located at the top or roof of their respective cavities.
[0043] In a preferred embodiment, the first superheater panels SH 1
and 2 are expected to receive a maximum heat density of about
70,000 BTU/ft.sup.2 per hour. The tubes are 1.25 inches in outside
diameter, with a 0.15 inch thick wall, made of ASME SA-213T22 2 1/4
chrome, 1% molybdenum steel. They do not have membrane bars, but
are positioned to abut adjacent tubes to limit passage of solar
energy. The second superheater tube panels SH 3 and 4 are expected
to receive a maximum heat density of about 60,000 BTU/ft.sup.2 per
hour. The tubes are 1.25 inches in outside diameter, with a 0.165
inch thick (0.165 min) wall and made of ASME SA-213T22 2 1/4%
chrome, 1% molybdenum steel. The cavity side of the superheater
tubes are coated with a high emissivity coating. Each of the
superheater tubes will bend at one or both ends to absorb thermal
expansion.
[0044] High Emissivity Coating
[0045] The cavity side of the boiler tubes are coated to both
improve heat transfer to the tubes and to reflect solar energy
towards the other tubes, Since the heat density is high and steam
temperatures reach as high as 825.degree. F., a very durable
coating is required. In a preferred embodiment, the coating is
CORR-PAINT CP-40XX Series (AREMCO PRODUCTS, Valley Cottage, NY),
which is a silicone-based material resisting temperatures up to
1100.degree. F. In general, the coating should absorb between 50
and 99% of the solar energy received and have a reflectivity rating
of 1 to 50%. The preferred coating is a mixture of 80% white and
20% black paint to produce a gray shade that reflects about 20% of
the incident light. The coating used required high temperature
curing.
[0046] Insulation
[0047] In order to contain the solar energy and limit losses, which
are calculated to be about 3% of the energy received, the tube
panels are backed by insulation inside the outer structure. The
insulation is shielded from direct exposure to solar radiation by
the tubes joined by the attached bars that make a continuous
surface. Water passing through the tubes also limits the
temperature at the surface of the insulation. The insulation
thickness varies between 1 to 6 inches, depending on the
temperature expected at each area of the solar receiver. In a
preferred embodiment the insulation may be mineral wool on the back
of the evaporator or economizer panels and ceramic fiber in back of
the superheater panels. Claims
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