U.S. patent application number 12/780783 was filed with the patent office on 2011-06-02 for systems and methods for producing steam using solar radiation.
This patent application is currently assigned to AREVA Solar, Inc.. Invention is credited to William M. Conlon, Robert J. Hanson, Peter M. Tanner, Milton Venetos.
Application Number | 20110126824 12/780783 |
Document ID | / |
Family ID | 43085614 |
Filed Date | 2011-06-02 |
United States Patent
Application |
20110126824 |
Kind Code |
A1 |
Conlon; William M. ; et
al. |
June 2, 2011 |
SYSTEMS AND METHODS FOR PRODUCING STEAM USING SOLAR RADIATION
Abstract
Methods and systems for generating steam using solar energy are
provided here. The methods and systems can be used to generate
steam of a desired quality, e.g. about 70%, or superheated steam.
Some methods for producing steam of a desired quality comprise
flowing water into an inlet of receiver in a linear Fresnel
reflector system, wherein the receiver comprises multiple parallel
tubes t.sub.i connected in parallel, and i=1,k, and irradiating
each tube t.sub.i along its respective length L.sub.i with solar
radiation so that solar radiation absorbed at each tube generates
thermal input along its length and so that water begins to boil in
at least one of the tubes at a point .lamda..sub.i along its
length. The methods comprise using one or more temperatures T.sub.i
in an economizer region of a tube t.sub.i or one or more changes in
length of the tubes as input to a controller that controls mass
flow of water into each of the multiple tubes, thereby controlling
quality of steam exiting the receiver.
Inventors: |
Conlon; William M.; (Palo
Alto, CA) ; Tanner; Peter M.; (Menlo Park, CA)
; Venetos; Milton; (Los Altos, CA) ; Hanson;
Robert J.; (Palo Alto, CA) |
Assignee: |
AREVA Solar, Inc.
Mountain View
CA
|
Family ID: |
43085614 |
Appl. No.: |
12/780783 |
Filed: |
May 14, 2010 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
61216253 |
May 15, 2009 |
|
|
|
61216878 |
May 22, 2009 |
|
|
|
Current U.S.
Class: |
126/601 ;
126/569 |
Current CPC
Class: |
F24S 20/20 20180501;
Y02E 10/40 20130101; F24S 50/00 20180501; F24S 2023/872 20180501;
F24S 2020/16 20180501; F24S 30/425 20180501; F22B 35/16 20130101;
Y02E 10/47 20130101; F22B 1/006 20130101; F24S 50/40 20180501 |
Class at
Publication: |
126/601 ;
126/569 |
International
Class: |
F24J 2/38 20060101
F24J002/38 |
Claims
1. A method for producing steam, the method comprising: flowing
water through an inlet to enter a tube of length L under pressure;
irradiating the tube along its length with solar radiation so that
solar radiation absorbed at the tube generates thermal input to the
tube along its length and so that steam exits the tube; and
providing a control variable as input to a controller that controls
mass flow of water through the inlet, thereby controlling quality
of steam exiting the tube.
2. The method of claim 1, wherein the control variable comprises a
change in tube length.
3. The method of claim 1, wherein the control variable comprises a
temperature in an economizer region of the tube.
4. The method of claim 3, wherein a temperature setpoint of the
control system depends on the position of the temperature
measurement relative to the tube inlet, tube length L, and a
desired output steam quality.
5. The method of claim 1, wherein: the tube has a transverse
dimension W orthogonal to length L; irradiating the tube comprises
rotating a reflector to direct solar radiation to irradiate the
tube along its length L; and the method further comprises adjusting
a thermal input to the tube by rotating a position of the reflector
to control the quality of steam exiting the tube.
6. The method of claim 1, wherein the control variable comprises
predictive information associated with thermal input.
7. The method of claim 6, further comprising separating water that
exits the tube from the steam using a separator, and wherein the
predictive information comprises thermal input that is based on
steam flow out of the separator.
8. The method of claim 7, wherein the separator comprises a steam
drum.
9. The method of claim 1, wherein the desired steam quality is 70%
or higher.
10. The method of claim 1, adapted for producing superheated
steam.
11. A method for producing steam, the method comprising: flowing
water into an inlet of a solar receiver in a linear Fresnel
reflector system, wherein the receiver comprises multiple tubes
connected in parallel; irradiating each tube along its respective
length with solar radiation so that solar radiation absorbed by
each tube generates thermal input along its length and so that
steam exits the tube; and using one or more control variables
associated with one or more tubes as input to a controller that
controls mass flow of water into each of the multiple tubes,
thereby controlling steam quality exiting the receiver.
12. The method of claim 11, wherein the one or more control
variables comprise one or more temperatures measured in an
economizer region of the one or more tubes.
13. The method of claim 11, wherein the one or more control
variables comprise a change in tube length of the one or more
tubes.
14. The method of claim 11, wherein the one or more control
variables comprise predictive information associated with thermal
input.
15. The method of claim 11, wherein: the receiver has a length L
and a transverse dimension W orthogonal to L; irradiating each tube
along its respective length with solar radiation comprises rotating
one or more rows of linear Fresnel reflectors in a field of
reflectors about an axis to direct solar radiation to irradiate the
tubes along length L; and the method further comprises adjusting
thermal input to the multiple parallel tubes along the transverse
dimension W by rotating one or more of the reflector rows about the
axis to control the steam quality.
16. A solar boiler comprising: a tube having an inlet for receiving
water and an outlet; a control valve capable of regulating flow of
water into the inlet; and a controller for controlling a position
of the control valve to control flow of water into the inlet based
at least in part on a control variable to control a steam quality
at the outlet.
17. The solar boiler of claim 16, wherein the control variable
comprises a temperature in an economizer region of the tube.
18. The solar boiler of claim 16, wherein the control variable
comprises predictive information associated with thermal input.
19. The solar boiler of claim 16, wherein: the tube is anchored at
a position P between the inlet and the outlet, the position P
extending further from the inlet than a boiling boundary in the
tube; the tube is relatively free to expand at the inlet; and the
control variable comprises a measurement of a change in length of
the tube between the inlet and position P.
20. A solar boiler comprising: an elevated receiver comprising
multiple parallel tubes extending along the length of the receiver;
a plurality of linear Fresnel reflectors configured to rotate about
an axis to track diurnal motion of the sun; a control valve
associated with each of the tubes to regulate mass flow of water
into the tubes; and a controller for adjusting a position of the
control valve associated with each tube based at least in part on
one or more control variables associated with one or more tubes so
as to control mass flow of water into each tube and to control
steam quality output from the receiver.
21. The solar boiler of claim 20, further comprising one or more
temperature sensors positioned to sense fluid temperature in the
economizer region of the one or more tubes, wherein the one or more
control variables comprise output from the one or more temperature
sensors.
22. The solar boiler of claim 20, wherein the one or more control
variables comprise predictive information associated with thermal
input.
23. The solar boiler of claim 20, wherein the one or more control
variables comprise a change in length of the one or more tubes.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application claims the benefit of priority from U.S.
provisional patent application entitled "Systems and Methods for
Producing Steam Using Solar Radiation", application Ser. No.
61/216,253, inventors William M. Conlon, Peter M. Tanner, Milton
Venetos, and Robert J. Hanson, filed on May 15, 2009, and U.S.
provisional patent application entitled "Systems and Methods for
Producing Steam Using Solar Radiation", application Ser. No.
61/216,878, inventors William M. Conlon, Peter M. Tanner, Milton
Venetos, and Robert J. Hanson, filed on May 22, 2009, each of which
is hereby incorporated by reference in its entirety for all
purposes as if put forth in full below.
FIELD
[0002] This application relates to methods, systems and apparatus
for producing steam, in particular for producing steam using solar
radiation. The methods, systems and apparatus include control
schemes to control output steam quality, especially during
variations in or interruptions to thermal input (e.g. thermal input
generated by solar radiation).
BACKGROUND
[0003] Solar thermal power plants generate electricity by using
solar radiation to heat a working fluid to power a turbine, such as
a steam turbine, that is coupled to an electrical generator.
Various solar energy collector systems for generating steam have
been developed. Solar energy collector systems may comprise, for
example, parabolic trough systems, central receiver systems with
2-axis heliostats, or Linear Fresnel Reflector (LFR) systems.
[0004] In some cases, it may be desired to use solar-generated
steam directly, e.g. as process steam that can be used for a
variety of applications, including process heat, enhanced oil
recovery, food processing, agricultural processing, refrigeration,
pulp and paper processing. In many applications for steam, e.g. oil
field steam injection for enhanced oil recovery, it is important to
know steam mass flow rate and steam quality. Examples of control
methods that can be used in solar thermal power plants are
described in U.S. Patent Publication No. 20090101138, published
Apr. 23, 2009 and U.S. Patent Publication No. 20080184789,
published Aug. 7, 2008.
[0005] A need exists for improved methods, systems, and apparatus
for producing steam with a determined mass flow rate and steam
quality, especially in instances where there may be variability or
interruptions in thermal input, such as there may be in the case
where solar radiation is used to provide all or part of the thermal
input to a steam generator.
SUMMARY
[0006] The invention in one instance provides a method of operating
a steam boiler which utilizes solar energy to generate the steam.
This method involves using information on a system variable that
affects steam quality in a control strategy predictive of the steam
quality to be output from the boiler to adjust the flow rate (e.g.
mass flow rate) of water passing through a tube of the boiler to
attain that steam quality.
[0007] In one instance, the amount of heat in water is assessed at
a known location along the tube, and this information on amount of
heat in water is used to adjust a flow control valve for water
entering this tube or another tube to provide steam of the desired
quality. Alternatively, a flow controlling orifice may be used
alone or in conjunction with the flow control valve to control the
amount of water entering this tube or another tube to provide steam
of the desired quality. The flow controlling orifice may include a
device that restricts flow (e.g. by having a reduced inner
diameter) and/or modifies flow, e.g. to reduce turbulence, bubbles,
rotational flow, or the like.
[0008] In another instance, the elongation of a tube is used to
predict the quality of steam that will emerge from the solar
boiler, and the flow rate of water into the boiler tube is adjusted
to produce the desired quality of steam. Elongation may be measured
in a region of the tube in which water is heated prior to
generating steam.
[0009] In another instance, the elongation of a portion of a tube
in a superheat region of the tube is used to calculate or represent
the quality of superheated steam in the superheat region of the
tube.
[0010] Flow rates may, for instance, deliberately differ through
different tubes of a boiler or of a receiver to provide steam of a
desired quality from each of the tubes. The control of flow-rates
may be based on a change of length of an individual tube, or a
deviation in change of length from an average or mean of the change
in length of all tubes. The change in length of each tube of a
multi-tube array or a multi-pass array having two or more absorber
tubes may be the same, and water flow rate in tubes of the array
may be controlled to provide the same tube elongation. As noted
above, the elongation of fewer than all tubes may be used to
control water flow rate in each tube and/or illumination of each of
the tubes of the array.
[0011] In another instance, one or more characteristics indicative
of total heat in steam and any condensate emerging from a tube of a
solar boiler is used to predict the amount of heat in steam and any
condensate that will be produced in a second boiler tube of the
solar boiler. For instance, a receiver of a linear Fresnel
reflector array may have multiple parallel boiler tubes in an array
in the receiver, such as a planar array. The heat in steam emerging
from a tube at or near the center of the array of tubes, which
tends to be illuminated better than tubes at ends of the (e.g.
planar) array of tubes, may be used to predict the heat that will
emerge from other tubes of the array (such as the end tubes), and
water flow rate and/or heat input to the other tubes may be
adjusted by adjusting a water flow control valve and/or a flow
controlling orifice for the tube and/or moving reflectors to
illuminate end tubes more or less. The elongation of a tube in the
portion of the tube in which steam is superheated may be one of the
characteristics that indicates total heat in steam emerging from a
tube.
[0012] The invention also provides steam boilers and control
systems that are configured to operate as described above. In one
instance, a solar boiler has an elongation measurement device in an
economizer section of a tube. The elongation measurement device may
be coupled to a control system that utilizes information
representative of mass flow rate of water through the tube and
elongation to assess the amount of heat contained in water entering
the tube. The control system actuates a water flow control valve
for the tube that opens and closes to regulate the flow rate of
water through the tube based on a correlation of the tube
elongation with heat in the combined steam and condensate, if any,
emerging from the pipe. Alternatively, a flow controlling orifice
may be used alone or in conjunction with the water flow control
valve to regulate the flow rate of water through the tube based on
a correlation of the tube elongation with heat in the combined
steam and condensate, if any, emerging from the pipe.
[0013] In another instance, a solar boiler has a measurement
instrument such as one or more pressure and/or temperature sensors
that interacts with a control system to assess quality of steam at
an end of a boiler tube of a multi-tube receiver or multi-pass
receiver having two or more absorber tubes upon which solar energy
is focused. The control system is configured to change a flow rate
of water into a second tube of the multi-tube or multi-pass
receiver, a position of one or more reflectors that illuminate the
second tube of the receiver, or both as a result of deviation of
quality of the steam from expected or target quality absent an
upset such as a shadow or cloud passing across the reflectors of
the solar boiler.
[0014] The control systems incorporated into a solar boiler and
method as described above may be configured to accept inputs from
one or more temperature, pressure, steam quality, flow rate,
photodetector, reflector position, tube elongation, and other
detectors or instruments that measure these values and control
water flow rate and/or reflector position. The control system may
incorporate a proportional controller, a proportional-integral (PI)
controller, a proportional-derivative (PD) controller, a
proportional-integral-derivative (PID) controller in analog or
digital form, or another form of control or modification of one of
these control schemes. The control system of any of the solar
boilers disclosed herein may also have two or more cascaded
controllers, where an output of one controller is an input to a
second controller.
[0015] The method, apparatuses, and control systems as discussed
herein may be reactive to an input such as change of length of
receiver tube. For instance, the control system may contain look-up
data representative of desired change in length of each tube of a
receiver. The set-point may represent a steady-state operation for
the particular receiver tube. The control system may compare the
instrument input representative of the value of change in length
with the set-point and adjust one or more of the water flow rate
through the tube and reflector position to provide the desired
change in length. An instrument input representative of e.g. steam
quality may be used to adjust the set-point.
[0016] The above methods, apparatuses, and control systems may be
used or configured to generate saturated steam. Alternatively, the
methods, apparatuses, and control systems may be used or configured
to generate superheated steam.
[0017] Tube elongation may be measured a number of ways. A
reference point may be selected, such as a point at which the tube
is secured to a support. Alternatively, a reference point may be
provided at a movable position on a tube, and elongation may be
measured from that reference point to another point on the
tube.
[0018] An amount of heat lost from a tube or the apparatus overall
may be used to refine the operation of an apparatus and control
system as described herein. The heat lost may be modeled, measured,
or calculated from measured values.
[0019] Data from a first tube of a multi-tube receiver, multi-pass
receiver having two or more absorber tubes, or multi-tube solar
boiler through which water and steam pass quickly relative to other
tubes of the receiver or boiler can be used to adjust flow rate of
water through a second tube, rate at which heat is input, or both.
Thus, data obtained on e.g. steam quality emerging from the first
tube can be used to adjust flow rate and/or heat input to affect
steam quality in the second tube to compensate for any deviations
from a desired steam quality encountered in steam from the first
tube.
[0020] One benefit of this type of configuration and method is that
data need only be obtained for some but not all of the tubes of the
multi-tube solar boiler, multi-tube receiver, or multi-pass
receiver having two or more absorber tubes in order to control
steam quality emerging from each of the tubes of the receiver or
boiler. This reduces the number of parts required to operate a
system, making the system more reliable and less costly. Transit
times of fluid through elongated tubes can be on the order of
several minutes or even hours; utilizing data from those tubes
exhibiting fastest transit times of fluid to control steam quality
emerging from each of the tubes in the receiver can improve time
response in a system employing a multi-tube receiver or multi-pass
receiver with two or more absorber tubes, which can result in
faster stabilization and faster response to transient changes in
thermal input (e.g. due to clouds or shadows).
[0021] In another instance, the method and boiler are configured to
provide a position of an initial boiling point from an input end of
a solar boiler tube that is the same for some or all tubes in a
multi-pass receiver having two or more absorber tubes or a
multi-tube receiver or some or all tubes in the solar boiler.
[0022] In another instance, the method and boiler are configured to
provide a position of where superheat begins from an input end of a
solar boiler tube that is the same for some or all tubes in a
multi-pass receiver having two or more absorber tubes or a
multi-tube receiver or for some or all tubes in the solar
boiler.
[0023] In some instances, the quality of steam output from the
solar boiler and/or individual tubes of the solar boiler is no more
than 70% (0.70). In other instances, the quality of steam is
greater than 1.
[0024] Pressure of the steam output from the system may be
controlled separately by a controller that senses steam pressure at
or from a steam drum or other steam accumulator and adjusts a valve
at the drum or in a steam line to or from the drum to increase or
decrease pressure. Alternatively, a flow controlling orifice may be
used alone or in conjunction with the valve to adjust the
pressure.
[0025] The systems and methods above may be configured in a linear
Fresnel reflector array or in an array of trough collectors, as
desired.
[0026] Also included herein are various methods of start-up for a
solar boiler. The invention is not limited to the apparatuses,
methods, and control systems described in this summary but is
additionally described in various portions of the text, figures,
and claims below.
[0027] Solar input along a length of tube may or may not be
uniform. For instance, solar input may be uniform on a cloudless
day, where one or more reflectors focuses sunlight along a length
of the tube. Solar input may not be uniform where, for instance,
clouds block light from reaching portions of the length of a tube
but not the entire length of tube that is illuminated on a
cloudless day. Solar input may not be uniform where, for instance,
light from various structures of a solar array block light from
reaching portions of the length of a tube but not the entire length
of the tube.
[0028] Thus, methods and systems for generating steam using solar
energy are provided here. The methods and systems can be used to
generate steam of a desired quality, e.g. about 70%, or superheated
steam. The steam generated by the methods and systems described
herein can be used directly, e.g. as process steam for applications
such as food processing, enhanced oil recovery, agricultural
processing, pulp and paper processing, industrial processes,
heating and cooling, and the like, or to power a turbine to
generate electrical power.
[0029] Variations of the methods and systems for controlling output
steam quality described herein are applicable to solar thermal
systems employing a single absorber tube, those employing multiple
absorber tubes, which may be connected in parallel, and those
employing multi-pass absorber tubes. The methods and systems allow
improved production of a desired steam quality or superheated
steam, where steam quality and steam output can be controlled
within a desired range even in the event of systematic or transient
variations in insolation that results in systematic or transient
variations in thermal input to the absorber tubes.
[0030] Variations of the methods and systems for controlling output
steam quality described herein are applicable to solar thermal
systems employing a single absorber tube, those employing multiple
parallel-connected absorber tubes, and those employing multi-pass
absorber tubes. The methods and systems allow improved production
of a desired steam quality or superheated steam, where steam
quality and steam output can be controlled within a desired range
even in the event of systematic or transient variations in
insolation that results in systematic or transient variations in
thermal input to the absorber tubes. In certain variations, the
methods and systems may allow for decreased requirements for water
inventory, and/or reduced start up losses.
[0031] Some methods for producing steam of a desired quality
comprise flowing water through an inlet to enter an elongated tube
under pressure, and irradiating the tube along its length with
solar radiation so that solar radiation absorbed by the tube
generates thermal input along its length, water begins to boil at a
boundary along the tube, and steam exits the tube. The methods
further comprise using a change in tube length as input to a
controller that controls mass flow of water into the tube inlet,
thereby controlling quality of steam exiting the tube. For
instance, the tube can be mounted such that it is relatively free
to expand at the inlet. In some variations, the tube is anchored at
a position P between the tube inlet and a tube outlet, where
position P extends further from the inlet than the boiling
boundary, and the change in tube length between position P and the
inlet can be used to control mass flow of water into that tube.
[0032] Some methods for producing steam of a desired quality
comprise flowing water into an inlet of receiver in a linear
Fresnel reflector system, wherein the receiver comprises multiple
parallel tubes t.sub.i connected in parallel, and i=1, . . . , k,
and irradiating each tube t.sub.i along it respective length
L.sub.i with solar radiation so that solar radiation absorbed at
each tube generates thermal input along its length and so that
water begins to boil in at least one of the tubes at a point
.lamda..sub.i along its length. The methods comprising using one or
more temperature measurements T.sub.i in an economizer region of a
tube t.sub.i as input to a controller that controls mass flow of
water into each of the multiple tubes, thereby controlling quality
of steam exiting the receiver.
[0033] Some methods for producing steam of a desired quality
comprise flowing water into an inlet to enter an elongated tube of
length L under pressure, irradiating the tube along its length L so
that steam exits the tube, and controlling water flow into the tube
with a control system that utilizes a temperature measurement in
the economizer region of the tube as a control variable. A set
point for the control system depends from the position of the
temperature measurement relative to the inlet, tube length L, and a
desired output steam quality.
[0034] Some methods for producing steam of a desired quality
comprise flowing water through an inlet to enter an elongated tube
under pressure, the tube having a length L and a transverse
dimension W that is orthogonal to L and rotating a reflector about
a single axis parallel to the tube to direct solar radiation to
irradiate the tube along its length L to provide thermal input to
the tube along its length L and so that steam exits the tube. The
methods comprise i) controlling mass flow of water into the tube
inlet; and ii) adjusting a thermal input to the tube by rotating a
position of the reflector to control quality of steam that exits
the tube.
[0035] Some methods for producing steam of a desired quality
comprise flowing water through an inlet to enter an elongated
elevated receiver comprising multiple parallel tubes under pressure
or one or more multi-pass tubes under pressure, the receiver having
a length L and a transverse dimension W that is orthogonal to L,
and rotating one or more linear Fresnel reflectors about an axis
parallel to the receiver in a field comprising multiple rows of
linear Fresnel reflectors to direct solar radiation to irradiate
the tubes along length L to provide thermal input to the tubes
along length L and so that steam exits the receiver. The methods
further comprise adjusting a thermal input to the multiple parallel
tubes, multiple segments of a multi-pass tube, or multiple
multi-pass tubes along the transverse dimension W of the receiver
by rotating one or more reflector rows about an axis that is
parallel to the elongated receiver, and controlling steam quality
exiting the receiver by i) controlling water flow into the multiple
parallel tubes, single multi-pass tube, or multiple multi-pass
tubes; and ii) adjusting thermal input into the multiple parallel
tubes, multiple segments of a multi-pass tube, or multiple
multi-pass tubes along the transverse dimension W.
[0036] Some methods for producing steam of a desired quality
comprise flowing water through an inlet to enter a tube of length L
under pressure and irradiating the tube along its length to provide
thermal input to the tube so that steam exits the tube. The methods
comprise using a predicted thermal input as input to a control
scheme to control quality of steam that exits the tube. The methods
in some variations comprise adjusting a mass flow of water into the
inlet using a control system (e.g. feedforward control) that
utilizes the estimated thermal input to control quality of steam
that exits the tube. In some variations, the predicted thermal
input may comprise a modeled, tabulated, measured, or estimated
time-dependent thermal input. For example, any one of or any
combination of daily variations in thermal input due to diurnal
motion of the sun, seasonal variations in insolation, or shadows
moving across a solar array over the course of a day, can be looked
up (e.g. in a lookup table) or measured and provided as input to a
control scheme. In another example, the predicted thermal input may
incorporate an estimate of thermal losses based on measured process
temperatures and a thermal loss model that can be either
analytically or empirically derived. In a multi-tube solar boiler,
a multi-tube receiver, or a multi-pass receiver having two or more
absorber tubes, thermal output (e.g. a temperature measurement or
steam output) from one tube can be used as predicted thermal input
to a second tube. In some variations, the methods comprise
separating water from a steam/water mixture that exits the tube
using a separator (e.g. a steam drum or a steam accumulator) and
estimating a thermal input to the tube using steam flow out of the
separator. In some variations, pressure in a steam drum, liquid
level in a steam drum, steam mass flow from the steam drum, and
liquid mass flow from the steam drum may be used to estimate
thermal input. In some variations, the methods comprise using a
predicted thermal input and input from one or more additional
control variables (e.g. temperature in an economizer region,
temperature at an inlet, temperature at or near a tube exit,
pressure, optical input such as DNI, change in tube length, or
estimated or measured steam quality) as input to a control system
to control steam quality. For example, some methods employ a
control scheme wherein temperature in the economizer region and a
predicted thermal input are used as control variables to adjust
mass flow of water into the tube to control steam quality. Some
methods employ a control scheme wherein a change in length of the
tube (e.g., change in length between the inlet and an anchored
position P that extends further from the inlet than a boiling
boundary) and a predicted thermal input are used as control
variables to adjust mass flow of water into the tube.
[0037] Variations of solar boilers and systems for producing steam
are described here. Some variations of solar boilers comprise a
tube having an inlet for receiving water and an outlet, a control
valve capable of regulating mass flow of water into the inlet, and
a controller for controlling a position of the control valve.
Alternatively, a flow controlling orifice may be used alone or in
conjunction with the control valve to regulate mass flow of water
into the inlet of the tube. In some variations, the tube is
anchored at a position P between the inlet and the outlet, where
the position P extends further from the inlet along the tube than a
boiling boundary that occurs in use. In the solar boilers, a
measurement of a change in tube length (e.g., between the inlet and
position P) is provided as input to the controller, and the
controller controls mass flow of water into the inlet to control
quality of steam exiting the tube.
[0038] Variations of solar boilers comprise a receiver comprising
multiple parallel tubes or multiple multi-pass tubes t.sub.i
extending along the length of the receiver, where i=1, . . . , k,
one or more linear Fresnel reflectors configured to rotate about a
single axis that is parallel to the receiver to track diurnal
motion of the sun, one or more temperature sensors TC.sub.i
positioned to sense fluid temperature in an economizer region of
each of the tubes t.sub.i and a controller, wherein output from
each of the temperature sensors TC.sub.i is provided as input to
the controller and used by the controller to adjust a position of
the control valve associated with tube t.sub.i so as to control
mass flow of water into the tube t.sub.i and to control steam
quality that exits the receiver. Alternatively, a flow controlling
orifice may be used alone or in conjunction with the valve to
control mass flow of water into the tube t.sub.i and to control
steam quality that exits the receiver.
[0039] Any of the methods, systems, or solar boilers described
herein can be used to produce steam having a quality of at most
about 70%, or about 70% or higher, or for producing superheated
steam.
[0040] Any of the methods for controlling steam quality can be used
in supplying process steam, or in supplying superheated steam. In
some variations, the steam generated (e.g. superheated steam) by
the methods, systems, and solar boilers described herein can be
used to generate electric power.
[0041] Any of the methods, systems, or solar boilers described
herein can be used to produce steam having a quality of about 70%
or higher (70%+/-10%, or 70%+/-5%), or for producing superheated
steam (e.g. about 10, about 20, about 30, about 49, about 50, about
60, about 70, about 80, about 90, or about 100 degrees of
superheat).
[0042] Any of the methods for controlling steam quality described
herein can be used in supplying process steam, or in supplying
superheated steam. In some variations, the steam generated (e.g.
superheated steam) by the methods, systems, and solar boilers
described herein can be used to generate electric power.
[0043] Any of the methods for controlling steam quality described
herein can be used in stand-alone steam generators or stand-alone
power generation, or in steam generators that are used in
combination with other steam sources or other power sources. For
example, any of the methods described herein can be adapted for use
with solar booster steam generation, or with hybrid solar/coal or
hybrid solar/natural gas plants.
[0044] The methods for controlling steam quality described herein
can be adapted to a variety of solar boilers having a variety of
configurations. For example, variations of the methods for
controlling steam quality can be used in single tube solar boilers
(e.g. parabolic troughs or a single tube receiver in an LFR array),
in multi-tube systems (e.g. multi-line parabolic troughs, or solar
arrays with multi-tube receivers), or multi-pass absorber tube
systems. Variations of the methods for controlling steam quality
can be adapted for solar boilers comprising recirculation systems.
Variations of the methods for controlling steam quality can be
adapted for once-through steam generators that employ no
recirculation systems.
BRIEF DESCRIPTION OF THE FIGURES
[0045] FIG. 1 illustrates a solar boiler tube that includes an
economizer section, an evaporator section, and a superheated steam
section.
[0046] FIG. 2 illustrates an example of a steam generating system
that includes a solar boiler tube.
[0047] FIGS. 3A-3D illustrate examples of LFR systems in which
transverse thermal input across a width of a receiver and/or
longitudinal thermal input along a length of a receiver can by
varied by controlling reflectors.
[0048] FIG. 4A illustrates an example of a control system for
controlling mass flow into a solar boiler tube using any suitable
process control variable; FIG. 4B illustrates an example of a
control system for controlling mass flow into a solar boiler tube
using temperature in the economizer region as a process control
variable; FIG. 4C illustrates an example of a control system for a
multi-tube system such as a multi-tube receiver; FIG. 4D
illustrates an example of a control system for controlling steam
quality by controlling mass flow into a solar boiler tube using a
change in tube length as a process control variable.
[0049] FIGS. 5A-5C illustrates an example of a control system for a
multi-tube receiver using temperature as a process control
variable.
[0050] FIGS. 6A-6C illustrate various configurations for control
valve manifolds.
[0051] FIG. 7 illustrates an example of a control system that
incorporates a predictor, such as a Smith predictor.
[0052] FIG. 8 illustrates an example of a control system that uses
a predicted thermal input as input to a control scheme. In this
particular example, the control scheme is configured so that the
predicted thermal input is provided as feedforward input.
[0053] FIG. 9A illustrates an example of a control system for a
solar array in which multiple receivers are arranged in a parallel
configuration, where each receiver contains a single boiler tube;
FIG. 9B illustrates an example of a control system for a solar
array in which multiple receivers are arranged in a parallel
configuration, where each receiver contains multiple
parallel-connected boiler tubes.
[0054] FIGS. 10A-10B illustrate examples of control systems for a
solar boiler tube that can be used during warm up.
[0055] FIG. 11 illustrates an example of a LFR system configured
for utilizing superheated steam that includes a heating system
comprising an LFR system and a reflector system having one or more
solar boiler tubes, a steam turbine for receiving superheated
steam, and an electrical generator associated with the steam
turbine and from which electricity is generated.
[0056] FIG. 12 illustrates various aspects that may be included in
a power plant configured for utilizing superheated steam, including
e.g. a condenser, a thermal energy storage system, and a
recirculation system.
[0057] FIGS. 13A-13B illustrate examples of a system for generating
superheated steam using at least two series of receivers, each
containing one or more boiler tubes, with a first receiver in
series generating saturated steam that is fed into the second
receiver system in the series from which superheated steam is
generated, e.g. to power a steam turbine. A separator and
recirculation system may be configured to the outlet of the first
receiver and optionally to the second receiver. The second receiver
may in one variation be replaced by an external heat source, such
as a coal-fired or natural gas fired boiler to generate superheated
steam.
[0058] FIG. 14 illustrates an example of a system for generating
superheated steam using a single receiver containing one or more
boiler tubes, where water is introduced into the inlet of the
receiver system, which is converted to saturated steam and then to
superheated steam before the outlet of the receiver. Optionally,
superheated steam is utilized by a steam turbine.
[0059] FIGS. 15A-15D illustrate exemplary sensor positions in a
receiver comprising multiple absorber tubes.
[0060] FIG. 16 shows an example of an LFR that comprises a field of
ground-mounted reflectors that are arrayed in parallel rows, and
elevated receivers positioned to receive and absorb reflected
radiation from the reflectors.
[0061] FIG. 17 shows an aerial view of a terminal end of an LFR
system.
[0062] FIG. 18 illustrates the reflection of solar radiation from
four reflectors to two receivers within an LFR system.
[0063] FIG. 19 illustrates an example of an absorber tube piping
configuration, including riser and downcomer designs that allow for
thermal expansion.
[0064] FIG. 20 shows an example of a receiver containing one or
more boiler tubes where, for illustration purposes only, the number
of parallel-connected tubes in the receiver is 5. The boiler tubes
are housed in the receiver. Optionally, as shown at the terminus of
the receiver, the boiler tubes can be supported on rollers that
allow the tubes to expand and contract with thermal change without
causing damage to themselves or other portions of the receiver.
[0065] FIG. 21 illustrates an array of reflectors where the angled
position of each row of linearly coupled reflectors is controlled
by a drive located at a terminus of the row.
[0066] FIG. 22 illustrates an array of reflectors where the angled
position of each row of linearly coupled reflectors is controlled
by a drive located at a central region of the row.
[0067] FIG. 23 illustrates a field of single reflectors that are
each individually controlled by a drive located at a terminus.
[0068] FIG. 24 illustrates an example of a control system for a
recirculation pump in a solar energy collector system.
[0069] FIG. 25 illustrates an example of a control scheme for a
system in which saturated steam is generated in a first solar
boiler and the saturated steam is supplied to a second solar boiler
in series with the first to generate superheated steam.
[0070] FIG. 26 illustrates an example of a control scheme for
generating superheated steam in a solar boiler.
DETAILED DESCRIPTION
[0071] Methods and systems for generating steam using solar energy
are described herein. The methods and systems can be used to
generate steam of a desired quality at a delivery pressure, e.g.
saturated steam having about 30%, about 40%, about 50%, about 60%,
about 70%, about 80%, or about 90% quality, or superheated steam.
The steam generated by the methods and systems described herein can
be used directly, e.g. as process steam for applications such as
food processing, enhanced oil recovery, agricultural processing,
pulp and paper processing, industrial processes, heating and
cooling, and the like, or to power a turbine to generate electrical
power. The delivery pressure of the generated steam can be selected
for a particular application, e.g. about 600-2800 psi. However, it
should be appreciated that other applications may require other
delivery pressures.
[0072] Variations of the methods and systems for controlling output
steam quality described herein are applicable to solar thermal
systems employing a single absorber tube, those employing multiple
parallel-connected absorber tubes, and those employing multi-pass
absorber tubes. The methods and systems allow improved production
of a desired steam quality or superheated steam, where steam
quality and steam output can be controlled within a desired range
even in the event of systematic or transient variations in
insolation that results in systematic or transient variations in
thermal input to the absorber tubes. The methods and systems may
allow sufficient control to produce steam with a target exit
quality (e.g. about 30%, about 40%, about 50%, about 60%, about
70%, about 80%, or about 90%, such as a steam quality of 70%+/-10%
or about 70%+/-5%) while operating with sufficient flow to avoid
dry out in any tube in the system, and to avoid situations where
the solar boiler reaches a trip condition, e.g. due to low flow
when thermal input is low.
[0073] Any of the systems and methods described herein may be used
in conjunction with a solar energy collector system that is used as
a stand-alone steam generator or electric power generator, or that
is used in combination with another steam generating plant or
electric power generator. For example, the methods and systems may
be used in conjunction with a solar energy collector system that is
used during relatively high insolation periods to augment output of
an existing steam plant or power plant (e.g. one that uses coal,
natural gas, biomass, oil, or nuclear energy as a fuel source). In
some variations, the methods and systems described herein may be
used in a configuration where natural gas, coal, nuclear energy, or
another type of fuel is used to augment output of a solar thermal
steam plant or power plant. In some situations, the methods and
systems described herein may be used in conjunction with hybrid
steam plants or power plants that are designed so that output
(power or steam) is switchable so that output is entirely generated
by solar energy, entirely generated by another fuel source such as
coal, natural gas, or nuclear energy, or generated by a combination
of solar energy and a non-solar fuel source.
[0074] Variations of the methods and systems described herein
comprise controlling steam exit quality from an absorber tube by
controlling mass flow into the absorber tube using an open or
closed loop control system in which a measurement of one or more
process variables is provided as feedback and/or feedforward input
into a controller that controls mass flow into an absorber tube,
e.g. by controlling a valve position and/or using various
fixed-size orifices. For example, a measurement of any one of or
any combination of the process variables including feedwater
temperature, temperature in the economizer region of an absorber
tube, temperature at or near a tube exit, measured or estimated
steam quality exiting an absorber tube, a change in length of a
tube or of a segment of an absorber tube, solar input such as
direct normal irradiance (DNI), thermal input (e.g. predicted,
measured, modeled, tabulated or estimated thermal input) for an
absorber tube and pressure can be used as input into a controller
that controls mass flow into an absorber tube. In one example, the
predicted thermal input may incorporate an estimate of thermal
losses based on measured process temperatures and a thermal loss
model that can be either analytically or empirically derived. Also
described herein are methods and systems for controlling steam
quality that involve i) controlling mass flow of water into an
absorber tube using feedback and/or feedforward input from a
process variable as described above; and ii) controlling thermal
input to one or more absorber tubes by adjusting one or more
reflectors directing solar radiation to the absorber tube by
defocusing, dithering and/or redirecting radiation at the receiver.
Variations of the methods and systems described herein include
predictive control, where a detected or anticipated change in
thermal input is provided as input to a feedforward or feedback
control loop. Such predictive control can be used in a multi-tube
receiver or multi-pass receiver having two or more absorber tubes,
where information about thermal input gleaned from one tube is
provided as input to control another tube, and in multi-line
systems, where information about thermal input gleaned from one
line is used in the control of another line. For example, a change
in thermal input can be indicated by a change in temperature that
has occurred at or near an exit of a tube, and that information can
be provided to adjust a mass flow into that tube or another tube.
In a multi-tube receiver or multi-pass receiver having two or more
absorber tubes, a change in thermal input indicated in one tube
(e.g., the tube with the fastest transit time down the length of
the tube due to highest thermal input) can be provided as
information to adjust a mass flow in another tube with a slower
transit time due to lower thermal input. Variations of the methods
and control systems as described herein may be adapted to a single
tube system (e.g. a single tube receiver in an LFR system or a
single line parabolic trough system), a system comprising multiple
parallel-connected tubes in a single receiver (e.g. an LFR solar
array comprising a line in which the receiver comprises multiple
parallel tubes), a system comprising multiple single tube lines
(e.g. an LFR solar array comprising multiple single-tube receivers
or a multi-line parabolic trough system), a LFR system comprising
multiple receivers, each receiver comprising multiple
parallel-connected tubes, or multi-pass systems having one or more
absorber tubes.
[0075] In any of the examples described herein, mass flow rates and
pressure into a tube may be controlled with one or more flow
control devices (e.g. valves and/or flow controlling orifices), and
flow out of a tube may be controlled with one or more flow control
devices (e.g. valves and/or flow controlling orifices). A flow
controlling orifice may be a device that restricts flow (e.g. by
having a reduced inner diameter) and/or modifies flow, e.g. to
reduce turbulence, bubbles, rotational flow, or the like. A flow
control device may be active (e.g. a valve that can be adjusted) or
passive (a fixed diameter orifice or a valve that is fixed). In
some cases, a valve may be used to determine a desired orifice size
or during setup of a system, and subsequently the valve may be
replaced by the orifice.
[0076] The methods and systems described herein can be used in any
solar thermal system in which steam is generated in an elongated
tube, e.g. linear Fresnel reflector (LFR) solar arrays or parabolic
trough systems. LFR systems employ a field of reflectors that
direct incident solar radiation to one or more elevated, elongated
receivers. An elevated receiver comprises one or more absorber
tubes to carry a heat exchange fluid, such as water and/or steam.
The one or more absorber tubes absorb incident solar radiation so
as to transfer thermal energy to the heat exchange fluid. In some
variations, a receiver in an LFR system may comprise a plurality of
parallel absorber tubes extending along a length of a receiver.
Examples of multi-tube receivers are described in International
Patent Application No. PCT/AU2005/000208, filed 17 Feb. 2005 and in
U.S. patent application Ser. No. 12/012,829 filed 5 Feb. 2008, each
of which is incorporated by reference herein in its entirety. In
some variations, a receiver in an LFR system may comprise one or
more absorber tubes arranged in a multi-pass configuration.
Multi-pass solar thermal systems are described in U.S. provisional
patent application entitled "Multi-Tube Solar Thermal Receiver",
application Ser. No. 61/303,615, inventors Peter L. Johnson, Robert
J. Hanson, and William M. Conlon, and filed on Feb. 11, 2010, which
is incorporated herein by reference in its entirety. Examples of
suitable reflectors and reflector systems that rotate about a
single axis to track motion of the sun for LFR systems utilizing
either single absorber tube receivers, multi-tube receivers, or
multi-pass absorber tube systems are provided in U.S. Patent No.
International Patent Application Nos. PCT/AU2004/000883 filed 1
Jul. 2004, International Patent Application No. PCT/AU2004/000884
filed 1 Jul. 2004, and U.S. patent application Ser. No. 12/012,829
filed 5 Feb. 2008, each of which is incorporated by reference
herein in its entirety.
[0077] In some variations, a solar selective coating may be
disposed on an absorber tube, for example a solar selective coating
that has been designed to increase absorptivity over the received
solar spectrum (e.g. DNI at Air Mass 1.5), while reducing loss of
heat through thermal emission. Examples of suitable solar selective
coatings are described in U.S. Pat. No. 6,632,542 to Maloney et
al., and U.S. Pat. No. 5,523,132 to Zhang et al., each of which is
incorporated by reference in its entirety.
[0078] In one aspect, a solar thermal steam generator that is
capable of generating superheated steam (that may, in turn, be used
to drive a turbine to generate electric power) or saturated steam
of a desired steam quality and that comprises a field of linear
Fresnel reflectors directing solar radiation to an elevated
receiver comprising one or more absorber tubes (e.g. multiple
parallel-connected absorber tubes housed in a single elevated
receiver) or one or more absorber tubes arranged in a multi-pass
configuration is provided. The LFR system preferably allows control
of the amount and/or quality of saturated or superheated steam.
Such control may include an adjustment to optimize output (steam
quality and/or quantity) in response to a measurement of one or
more system parameters that indicates that optimization is desired
and/or possible, or in anticipation that optimization will be
desired and/or will be possible.
[0079] In one aspect, systems comprising a field of linear Fresnel
reflectors configured to direct solar radiation to an elevated
receiver comprising multiple parallel-connected absorber tubes or
multi-pass configured absorber tubes are provided, and a control
system configured to decrease a temperature difference between at
least two of the absorber tubes in the receiver are described. In
one variation, the control system is configured to decrease a
temperature difference and/or a length difference between at least
two absorber tubes by modifying the mass flow rate of water into an
absorber tube and/or causing incremental reflector movements in one
or more reflectors in the reflector field and/or by introducing an
attemperating spray into at least one absorber tube. In one
variation, the control system is configured to respond to a
measurement of any one or more of: feedwater temperature, absorber
tube temperature in the economizer region, absorber tube
temperature at or near the tube exit, mass flow rate, pressure,
measured or estimated steam quality, thermal input (predicted,
measured, estimated, modeled, or tabulated), and solar input (e.g.
DNI).
[0080] In the case where a tubing arrangement comprises multiple
parallel outbound tubes and/or multiple parallel return tubes, a
single flow control device may be used to control mass flow rates
into the multiple parallel tubes, and/or a single flow control
device may be used to control flow out of multiple parallel return
tubes. In other variations, a separate flow control device (e.g. a
valve or an orifice) can be used on each outbound tube and/or on
each return tube. In some cases, more than one flow control device
may be used in combination, e.g. a flow controlling orifice may be
used in series with a valve. In tubing arrangements in which
multiple tubes in an upstream loop are branched into multiple tubes
in a down stream loop, a flow control device can be used between
the upstream loop and the downstream loop (e.g. at a turnaround
region) to reduce or prevent flow imbalance from developing in the
downstream loop or to control the amount and/or quality of steam
produced. In some cases, a flow control device on a tube in an
upstream loop (e.g. at an inlet to an upstream loop) can be used to
control flow in a downstream loop, e.g. where that tube is
channeled into a single tube so that the potential for flow
imbalance to develop is reduced or to control the amount and/or
quality of steam produced. Valves may be selected to modulate
control of medium to low flow rates at system pressures up to about
5000 psig. Any suitable valve may be used, e.g. a standard globe
control valve sized for 1/2'', 3/4'', or 1'' sizes. However, it
should be appreciated by one of ordinary skill that valves of other
types and sizes may be used.
[0081] Steam quality x is x=(h-h.sub.f)/h.sub.fg, where h is the
enthalpy of the fluid produced, h.sub.f is the enthalpy of
saturated liquid, and h.sub.fg=h.sub.g-h.sub.f, the difference
between the enthalpy of the saturated vapor h.sub.g and h.sub.f.
For saturated steam, steam quality is the mass fraction of vapor in
a two-phase mixture of water and vapor. For saturated steam, a
steam quality of unity indicates no liquid, and a steam quality of
zero indicates no vapor. For superheated steam, x will be greater
than or equal to one. The control of steam quality is important to
any type of boiler. For example, steam quality may determine in
part certain grades of boiler pipes that are required for certain
applications, expected operating conditions, and equipment
lifetime. Steam quality control may be important for intended uses
of steam such as driving a turbine or for use in enhanced oil
recovery. Steam quality can be affected by any one of or any
combination of flow rate through a boiler tube, pressure drop along
a boiler tube, and heat flux to a boiler tube. Steam quality can be
difficult to measure, especially in high pressure steam systems. In
some instances, a separator is used to separate vapor from water to
determine steam quality. In some instances, imaging techniques may
be used such as X-ray computed tomography. In some instances, steam
quality can be determined or estimated by comparing heat output to
heat input. In some situations, the concentration of dissolved
solids between the inlet and the outlet can be used to estimate
steam quality. While specific methods for determining steam quality
are discussed above, it should be appreciated by one of ordinary
skill that any method or apparatus for determining or measuring
steam quality may be used.
[0082] In a solar boiler, one or more elongated boiler tubes can be
disposed above one or more mirrors. Each boiler tube is fed with
feedwater, which typically enters the tube as a subcooled liquid.
As sunlight is reflected onto a boiler tube, heat generated by
absorption of solar radiation at the tube is transferred into the
fluid. Three distinct sections can be identified within a boiler
tube, with reference to FIG. 1 below for a boiler tube of length L:
A) an economizer section; B) an evaporator section; and C) a
superheated steam section. All steam generators include A) and B);
only some steam generators will include C). In any of these
sections, the exterior temperature of the boiler wall T.sub.wall
can be determined by {dot over
(Q)}.sub.in=HTC*(T.sub.wall-T.sub.fluid), where {dot over
(Q)}.sub.in is the heat flux in, and HTC is the heat transfer
coefficient.
[0083] The economizer or sensible heat section (A) occurs just
beyond an inlet in which feedwater is fed into a tube. In the
economizer section, temperature of the fluid increases from the
temperature of the feedwater (T.sub.fw) until it reaches a
saturation temperature T.sub.sat, corresponding to the pressure in
the tube. Although subcooled nucleate boiling may occur in the
economizer region, the average enthalpy of the fluid at any cross
section within the economizer region is still subcooled. The
economizer region ends at a position .lamda., which occurs when the
bulk fluid is saturated liquid, where it contains the maximum
amount of thermal energy that is possible before boiling.
[0084] The evaporator section (labeled B) begins after position
.lamda.. There, additional thermal energy causes the fluid to boil,
increasing the steam quality x of the mixture. The temperature may
stay relatively constant in the evaporator section as shown, or may
decrease somewhat as energy is absorbed by the heat of evaporation.
In some variations, the thermal input, tube pressure, flow rate,
and tube length may be such that essentially full evaporation
occurs so that the steam quality approaches unity at the dry point
.gamma. within the tube. In some cases, any one of or any
combination of the preceding factors (thermal input, tube pressure,
flow rate, and tube length) may be such that a steam exit quality
is less than one. In the latter situations, it is desirable to
control the exit steam quality, e.g. to about 0.3, about 0.4, about
0.5, about 0.6, about 0.7, about 0.8, or about 0.9. For example, it
may be desirable to control steam quality to be about 0.7, e.g.
0.7+/-10% or 0.7+/-5% for certain applications such as enhanced oil
recovery.
[0085] In some variations, the thermal input, tube pressure, flow
rate and tube length may be such that a superheated steam region
(labeled C) is reached, starting at a point .gamma.. In the
superheat region, additional thermal input causes sensible heating
of the vapor phase. While the regions of a boiler tube are
described above with respect to a single-pass tube, it should be
appreciated that in a multi-pass tube having total length L, the
regions may be located on any segment (corresponding to each pass)
of the multi-pass tube and the distances .lamda., .gamma., and L
may be measured from the inlet of the tube and along each
segment.
[0086] Referring now to FIG. 2, a variation of a steam generator is
illustrated. Steam generator 100 comprises an elongated tube 101
having an end-to-end physical length L. In some variations, the
steam generator 100 is any type of steam generator in which the
heat flux applied to the tube 101 is relatively uniform along an
illuminated length L.sub.illum, where L.sub.illum comprises a
substantial portion of the physical length L of the tube 101. For
example, the steam generator 100 may comprise a single absorber
tube or multi-tube receiver in a LFR solar array, where L.sub.illum
may be essentially equal to L, or L.sub.illum may be somewhat less
than L due to potential shading effects at the entrance 121 and/or
the exit end 123 of tube 101. In other variations, the steam
generator 100 may comprise an elongated tube formed from a series
of end-to-end connected parabolic trough sections, where
L.sub.illum may be essentially equal to L. In yet another
variation, the steam generator 100 may comprise one or more
multi-pass absorber tubes in a LFR solar array, where each has an
end-to-end physical length L and where each segment of the tube
passing through the concentrated region of solar radiation has a
length L.sub.segment. In this variation, L.sub.illum may be
essentially equal to L.sub.segment, or L.sub.illum may be somewhat
less than L.sub.segment due to potential shading effects at the
entrance 121 and/or the exit end 123 of tube 101. The steam
generator 100 can be a stand-alone steam generator, can be used to
augment steam generated by another source, or can be used in
parallel with steam from another source, as described above. In
some variations the ratio L.sub.illum/L may be about 70%, about
75%, about 80%, about 85%, about 90%, or about 95%.
[0087] The physical length L of a tube may be any suitable length.
For example, L may be determined by any one of or any combination
of two or more of the following factors: tube diameter, operating
pressure/temperature, tube composition (e.g. stainless steel or
carbon steel), ease of handling during manufacture or installation,
size of solar field, diameter of tube, desired steam quality, and
the like. In some variations, a tube may comprise multiple tube
sections connected together in series in an end-to-end fashion. For
example, in an LFR solar array, an absorber tube in a receiver may
comprise standard commercially-available lengths of tubing
connected together to reach a physical length of about 300 to about
400 meters, e.g. about 384 meters. The tube materials and
construction may be selected to meet local or industry standards or
codes for the particular operating conditions (e.g. temperature and
pressure) of the steam generator, e.g. local or national boiler
codes.
[0088] The illuminated length L.sub.illum of an absorber tube can
be measured, calculated, or estimated. One example of a calculation
for L.sub.illum is as follows. The solar position can be determined
for the location of the tube, comprising the azimuthal angle az and
the zenith ze. The rotation of the tube rot in degrees relative to
straight north can be determined. The height of the tube h.sub.tube
relative to the focal point of one or more reflectors directing
solar energy to the tube may be determined. For example, in a LFR
system, h.sub.tube may be about 10 meters, about 12 meters, about
15 meters, about 18 meters, about 20 meters, or about 25 meters.
The length of a shaded section l.sub.dark for a reflector
positioned directly beneath a tube is approximated by
l.sub.dark=h.sub.tube tan(ze) cos(az-rot+180). The illuminated
length can be approximated as L.sub.illum=L-l.sub.dark. Reflectors
that are positioned in a reflector field at farther distances from
a receiver may have longer shaded sections. The effects for such
longer shaded lengths may be calculated, the same shaded length may
be used for all reflectors regardless of distance from the
receiver, or the actual shaded length may be calculated for some
reflectors in a field (e.g. the outermost reflectors positioned
furthest from the receiver) and approximated shaded length using
l.sub.dark for a reflector positioned directly beneath the receiver
may be used for some reflectors in the field (e.g. those positioned
closest to the receiver).
[0089] Referring again to FIG. 2, within the tube 101 an economizer
section 103 and a saturated steam section 105 are included. Thus,
in operation, there is a boiling point boundary 117 that occurs at
a length .lamda. from inlet 121. In certain variations where
superheated steam is formed within the tube 101, there is a dryout
point 128 that occurs at a length .gamma. from inlet 121. As
indicated by arrow 119, thermal input {dot over (Q)}.sub.in is
provided along the illumination length L.sub.illum of the tube 101.
Again, the illumination length L.sub.illum may or may not be the
same as the physical length L, depending on whether there are dark
regions such as shading effects as described above. Solar radiation
can be directed to the tube 101 using any suitable reflector
configuration, e.g. parabolic troughs, heliostat reflectors, or
linear Fresnel reflectors such as illustrated herein or otherwise
known.
[0090] In some variations, the thermal input {dot over (Q)}.sub.in
may be relatively uniformly distributed along the length
L.sub.illum; that is, L.sub.illum may for example represent a
relatively uniformly irradiated portion of the tube 101, e.g. where
tube 101 is installed in a parabolic trough system or in a receiver
in a linear Fresnel reflector solar array. The thermal input {dot
over (Q)}.sub.in may vary over time. For example, in a solar array,
motion of the sun relative to the earth may cause systematic
intra-day and day-to-day variations in irradiation, and therefore
in thermal input {dot over (Q)}. In some cases, one or more
transient factors such as cloud cover, shadows (e.g. shadows from
the solar array itself) or other events such as mirror alignment
issues may cause intermittent or non-systematic variability in
thermal input.
[0091] Water supplied into the inlet 121 of tube 101 has a
temperature T.sub.in, enthalpy h.sub.in and mass flow {dot over
(m)}.sub.in. Mass flow into the tube 101 can be regulated with
control valve 115. Alternatively, a flow controlling orifice (not
shown) may be used alone or in conjunction with flow control valve
115 to control the mass flow entering tube 101. Steam that exits
the tube 101 may optionally enter a separator 113 (such as a steam
accumulator or a steam drum at pressure P.sub.drum), from which a
dry steam flow 125 having a mass flow {dot over (m)}.sub.steam and
enthalpy h.sub.g can be extracted. Other types of separators may be
used, such as baffles or cyclone separators. In instances where
superheated steam is generated in tube 101, a separator may not be
necessary. Water recovered from the separator 113 may optionally be
used in a recirculation system. For example, if a steam drum is
used as a separator, water recovered may have a liquid level
L.sub.drum in the drum. Recirculated water flow 107 may be
extracted from the separator with a mass flow {dot over
(m)}.sub.recirc and enthalpy h.sub.f. Feedwater flow 109 with a
mass flow {dot over (m)}.sub.feed and enthalpy h.sub.feed may be
mixed with the recirculated water flow 107 to provide input into
the tube 101.
[0092] As stated above, in an LFR solar array, an elevated receiver
can be a single tube receiver, multi-tube receiver, or a multi-pass
receiver. For single tube receivers, a tube diameter can be in a
range from about 1 inch to about 12 inches, or in a range from
about 12 inches to about 24 inches, where a tube diameter selection
may depend on factors such as the size of the reflector field being
used, the pressure during operation, the temperature during
operation, the material and composition of the tube, the amount of
steam, and the quality of steam desired. For multi-tube receivers,
a tube diameter can be in a range from about 0.5 inches to about 6
inches (e.g. about 0.5 inch, about 1 inch, about 1.25 inches, about
1.5 inches, about 1.75 inches, about 2 inches, about 2.5 inches,
about 3 inches, about 3.5 inches, about 4 inches, about 4.5 inches,
about 5 inches, about 5.5 inches, or about 6 inches), again
depending on such factors as the size of the reflector field being
used, the pressure and temperature during operation, the material
composition and structure of the tubes, the steam flow required,
and the steam quality desired. Any suitable number of tubes may be
used for a receiver, e.g. 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, or 12
tubes, or even more. For multi-pass receivers, the diameter of the
return tube can be selected to be larger than that of the outbound
tubes, e.g. the outbound tubes can have an inner or outer diameter
ranging from 1'' to 6'', for example, 1.5'', 1.66'', 2.0'', or
2.5'', and the return tube can have an inner or outer diameter
ranging from 1'' to 9'', for example, 0.5'', 1.0'', or 1.5'' larger
than that of the outbound tubes. In some variations, 2'' inner or
outer diameter outbound tubes are used with a 3'' inner or outer
diameter return tube, and in some variations, 1.66'' inner or outer
diameter outbound tubes are used with a 3.5'' inner or outer
diameter return tube. While example diameters have been provided,
it should be appreciated by one of ordinary skill that tubes having
other diameters may be used. Single, multi-tube, or multi-pass
receivers may have various attributes for improving efficiency or
performance, such as solar selective coatings applied to the tubes
and/or cavities for trapping thermal energy such as an inverted
trough cavity for housing one or more tubes as described in U.S.
patent application Ser. No. 12/012,829 which is incorporated by
reference herein in its entirety, insulation placed near or around
nonirradiated sections of the tube to reduce thermal losses, and
roofs and the like to reduce thermal losses and/or protect tubes
from environmental effects. Tubes in a multi-tube or multi-pass
receiver may be arranged side-by-side in a single row (e.g. a
planar array), or in more than one row (a nonplanar array). Tubes
may be supported below by one or more rollers to accommodate
thermal expansion in the length of the tube, e.g. as described in
International Patent Application No. PCT/AU2005/000208 and U.S.
patent application Ser. No. 12/012,829, each of which is
incorporated by reference herein in its entirety.
[0093] The amount of solar radiation incident on an absorber tube,
and hence thermal input, can be varied. In some variations it may
be desired to adjust the total system thermal input while
maintaining the distribution of thermal input along the illuminated
length of the tube. An example of an LFR solar array utilizing a
multi-tube tube solar receiver is illustrated in FIG. 3A. The
receiver 300 comprises multiple elongated tubes 301 extending along
the length of the receiver. A field of elongated reflectors 306 in
rows that can be rotated about a single axis parallel to the
elongated receiver 300 directs solar radiation to the tubes 301. By
adjusting the angle of one or more reflectors so that reflected
solar radiation is defocused, displaced, dithered, or partially or
completely misses the receiver, the thermal input across a
transverse dimension 305 of the receiver 300 can be varied. In some
variations, the thermal input across a transverse dimension 305 can
be varied while maintaining a relatively constant thermal input
along the illumination length of each receiver tube 301. Similarly,
for a multi-pass absorber tube system, by adjusting the angle of
one or more reflectors so that reflected solar radiation is
defocused, displaced, dithered, or partially or completely misses
the receiver, the thermal input across a transverse dimension of
the multi-pass receiver can be varied. In some variations, the
thermal input across the transverse dimension can be varied while
maintaining a relatively constant thermal input along the
illumination length of each receiver segment of each multi-pass
tube. Similarly, for a single tube system such as a single tube LFR
solar collector system, variation of thermal input across a
transverse dimension of the single tube (e.g. while maintaining a
relatively constant longitudinal thermal input) can be achieved by
rotating one or more elongated reflectors directing solar radiation
to the single tube about a single axis parallel to the tube. An
example of a situation in which thermal input is translated across
a transverse dimension of a tube by rotating linear Fresnel
reflectors is illustrated in FIG. 3B. There, receiver 320 comprises
multiple parallel-connected side-by-side tubes 321. One or more
reflector rows (not shown) reflect solar radiation to provide an
illuminated band 322 incident on the tubes 321. As indicated by
arrows 323, the illuminated band 322 can be translated back and
forth across a transverse dimension 324, e.g. so that the
illuminated band is centered relative to the tubes 321 or is offset
relative to the tubes 321. An example of a situation in which
illumination is defocused or focused to change thermal input is
illustrated in FIG. 3C. There, a receiver 340 comprises a bank of
multiple parallel-connected tubes 341. One or more reflector rows
(not shown) reflects solar radiation to provide an illumination
band 342 incident on the tubes 341. By defocusing the illumination
band to form a broadened band indicated by dashed lines 343, the
thermal input to the tubes 341 can be varied. Defocusing can, for
example, be achieved in a LFR array having multiple parallel rows
of reflectors by rotating one reflector row about a single axis
parallel to the tubes 341 to direct light to a somewhat different
location along the transverse direction 334 of the receiver 340
than other rows of reflectors. In some cases, a reflector position
can be dithered on a relatively rapid timescale e.g. a frequency
selected to accommodate mass and structure of reflector structures
but sufficiently fast so that irradiation of the pipes is blurred
to avoid local heating, which may be about 0.01 to about 50 Hz
(e.g. about 0.1 Hz, about 0.5 Hz, about 1 Hz, or about 10 Hz) to
adjust transverse heat flux. That is, a reflector can be adjusted
back and forth between incremental first and second locations.
Although the receivers in FIGS. 3A-3C are illustrated to include
multiple tubes, it is to be understood that the concepts
illustrated in FIGS. 3A-3C and related discussion and description
apply to receivers having single tubes and one or more multi-pass
tubes.
[0094] Alternatively to or in addition to the adjustment of thermal
input across a transverse direction of a receiver (e.g. while
maintaining a relatively constant thermal input longitudinally as
illustrated in FIGS. 3A-3C), longitudinal adjustments of thermal
input are possible using reflectors. An example is illustrated in
FIG. 3D. There, an elongated reflector row 360 directs solar
radiation to an elevated elongated receiver 363 that comprises one
or multiple tubes. Reflectors in the reflector row 360 are
supported by supports 361. One or more drive mechanisms (not shown)
allow the reflectors in the reflector row 360 to be rotated about a
single axis 362 that is parallel to the tubes in the elevated
receiver. One or more segments 364 of the reflector row 360 can be
rotated independently of other segments in the reflector row 360.
For example, the reflectors in the segment 364 can be rotated to
direct radiation to the receiver, whereas other reflector segments
are inverted or otherwise rotated so as to result in a selectively
irradiated length 365 of the one or more absorber tubes in the
receiver 363, thereby creating a corresponding selectively heated
longitudinal section of the absorber tubes. Thermal input can be
varied transversely across a receiver (multi-tube, multi-pass
tube(s), or single tube), e.g. as illustrated in FIGS. 3A-3C in
conjunction with (in parallel with or alternating with) variation
of thermal input longitudinally along a length of a receiver, e.g.
as illustrated in FIG. 3D. Thermal input can be varied by use of an
attemperating spray. An attemperating spray to adjust thermal input
can be used in conjunction with (e.g. in parallel with or
alternated with) adjustment of reflector position, or an
attemperating spray can be used without adjustment of reflector
position.
[0095] Steam quality (e.g. about 30% about 40%, about 50%, about
60%, about 70%, about 80%, about 90%, or superheated steam)
produced by a multi-tube solar receiver, multi-pass solar receiver,
or a single tube solar receiver (e.g. linear Fresnel solar receiver
or parabolic trough) can be controlled by regulating mass flow of
water into the one or more tubes. The process control variable used
in a control system that regulates mass flow of water into one or
more tubes can be temperature in an economizer region of the tube,
feedwater temperature, temperature at or near the exit end of a
tube, solar input (e.g. DNI), change in length of a tube or a
section of a tube, measured or estimated steam quality, thermal
input (e.g. predicted, measured, tabulated or estimated thermal
input), pressure, or a combination of two or more of the preceding
variables. A control system can include any suitable control
scheme, such as a control scheme that includes only feedback
control, includes only feedforward control, or includes a
combination of feedback and feedforward control. A control system
may be set up to control using information from only one process
variable, or from multiple process variables. In some variations,
cascaded control systems can be used, where an output of one
controller is an input to a second controller. The control system
may incorporate a proportional controller, a proportional-integral
(PI) controller, a proportional-derivative (PD) controller, a
proportional-integral-derivative (PID) controller in analog or
digital form, or another form of control or modification of one of
these control schemes. Some control systems include a feedback
control in combination with a feedforward control.
[0096] In some variations, predictive control can be used so that
an estimate or indication of an upcoming change in a variable (such
as a change in thermal input due to transients or other changes in
insolation) can be taken into account to improve a response time to
that change. Such predictive control can improve control of steam
quality in a system with relatively long tubes, e.g. where a
transit time from a tube inlet to a tube outlet is on the order of
a minute, or several minutes or longer, e.g. an hour or more. In
some variations, predictive control can be accomplished by sensing
a change in a process variable near the end of the tube, and using
that information as a predictor of what is happening further
upstream in the tube, and providing that predicted information as
input to a control system. In some variations, predictive control
can be accomplished by utilizing information gleaned from one tube
with a relatively fast transit time, and using that information in
a control system controlling a tube having a relatively slow
transit time. In some variations, a predictor (e.g. a Smith
predictor) may be used to compensate for time delay between an
inlet of a tube and a downstream point in the tube at which a
process variable is measured. In some variations, temperature in an
economizer section of a tube, a change in length of a tube or a
section of a tube, estimated or measured steam quality, or thermal
input (e.g. measured, estimated, tabulated, or calculated) can be
used as a process variable or to provide predictive information to
a control scheme that regulates valve position to control mass flow
of water into one or more tubes. In some variations, thermal input
(e.g. an estimated, measured, tabulated or calculated change in
thermal input) is used to provide predictive input (e.g.
feedforward input) to a controller operating a valve to control
mass flow of water into one or more tubes. In some variations, a
detected or anticipated change in thermal input is used to provide
predictive input (e.g. in a feedforward control), and one or more
of the other process variables (e.g. temperature in an economizer
region, feedwater temperature, solar input (e.g. DNI), pressure,
temperature near the tube outlet, a change in length of a tube or a
section of a tube, or estimated or measured steam quality) is used
to provide input (e.g. feedback) into a controller operating a
valve to control mass flow of water into one or more tubes. In some
variations, a fixed diameter flow controlling orifice may be used
alone or in conjunction with the valve. In some variations, a
control system that controls mass flow into one or more tubes is
coupled to a system that controls reflector position, so that
reflector position can be used to adjust transverse and/or
longitudinal thermal input into the one or more tubes, e.g. as
described above and in connection with FIGS. 3A-3D.
[0097] As stated above, variations of the methods and systems
described herein include predictive control, where a detected or
anticipated change in thermal input or other process variable as
described herein is provided as input to a feedforward or feedback
control loop. Such predictive control can be used in a multi-tube
receiver or a multi-pass receiver having multiple tubes, where
information about thermal input or another process variable that is
gleaned from one tube is provided as input to control another tube,
and in multi-line systems, where information about thermal input or
another process variable gleaned from one line is used in the
control of another line. For example, a change in thermal input can
be indicated by a change in temperature that has occurred at or
near an exit of a tube, and that information can be provided to
adjust a mass flow into that tube. In a multi-tube receiver or a
multi-pass receiver having multiple tubes, a change in thermal
input indicated in one tube (e.g., the tube with the fastest
transit time down the length of the tube due to highest thermal
input) can be provided as predictive information to adjust a mass
flow in another tube with a slower transit time due to lower
thermal input.
[0098] An example of a control system is illustrated in FIG. 4A.
There, a steam generating system 400 includes a steam plant 401,
which includes at least one or more solar boiler tubes as
illustrated in FIG. 2, and may optionally include a separator and a
recirculation loop whereby warm water recovered from the separator
is mixed with feedwater for subsequent introduction into and
reheating in a boiler tube. Associated with the one or more boiler
tubes is a control valve manifold (indicated by CV.sub.k) and/or
fixed diameter flow controlling orifice that regulates mass flow of
water into the tubes. Each boiler tube may have a dedicated control
valve and/or fixed diameter flow controlling orifice, or a control
valve and/or fixed diameter flow controlling orifice may control
mass flow of water into more than one tube. A process variable
associated with a k.sup.th tube PR.sub.k (represented by box 403)
is measured within the plant. Examples of process variables that
can be measured or estimated include thermal input, feedwater
temperature, a temperature of water within the economizer region of
the tube, temperature of fluid (water, saturated steam, or
superheated steam) near the outlet of the tube, an external
temperature of the tube surface in its economizer region or near
the outlet, measured or estimated steam quality at exit, pressure,
solar input (e.g. DNI), or a change in physical length of a tube or
a section of a tube, e.g. a section of the tube between the inlet
and the boiling boundary .lamda., or between a floating inlet end
of the tube and a position at which the tube is anchored in place,
which may be selected to extend further from the inlet than the
boiling boundary .lamda.. The process parameter PR.sub.k 403 for
the k.sup.th tube is provided to an operator portion 405 of a
controller, e.g. an operator such as a summer. A set point for the
physical parameter for the k.sup.th tube PR.sub.set,k is also
provided to the operator portion 405. A set point may, for example,
be a temperature set point for a temperature measurement within the
economizer region of a tube, a temperature measurement near the
tube exit, or a target change in length. A qualitative or
quantitative comparison between the set point PR.sub.set,k and the
measured physical process variable PR.sub.k 403 is made by the
operator portion 405 of the controller (e.g. by a summer), and the
results of that comparison are fed into a main portion 407 of the
controller employing any suitable control algorithm, e.g. so as to
provide proportional integral (PI) control, proportional-integral
derivative control (PID), proportional-derivative control (PD) and
the like. Output from the controller 407 is provided to the one or
more control valves CV.sub.k in the manifold to adjust the valve
position to control mass flow of water into the tubes. Although the
example illustrated in FIG. 4A shows the control system as a
feedback control loop, it is to be understood that other control
configurations are contemplated. In some variations employing
multi-tube receivers or a multi-pass receivers having multiple
tubes, control may be accomplished so that all tubes in the
receiver reach approximately the same length and/or temperature at
or near the tube exit in steady state operation.
[0099] For any of the multi-tube or multi-pass receiver steam
generating systems described herein, it is contemplated that a
process variable measured for a k.sup.th tube may be used as
control input for a different tube (not the k.sup.th tube). For
example, a process variable such as temperature in an economizer
region, fluid temperature at or near the end of the tube, estimated
or measured steam quality, estimated or measured thermal input, or
change in tube length for a k.sup.th tube may be used in a control
system for a different tube. In some variations, if a first tube
has a faster transit time than a second transit tube, it may be
desired to provide information about one or more process variables
from the first tube as input into a control system for the second
tube, e.g. as part of a predictive control algorithm. Control
systems employing such cross-tube information may be useful in
multi-tube or multi-pass receivers, where a centrally positioned
tube may receive a higher level of irradiance and hence exhibit a
faster transit time than a tube positioned near the edge.
[0100] An example of a control system that can be used to control
steam quality in an LFR solar array comprising an elevated receiver
that, in turn, comprises multiple parallel-connected absorber tubes
is illustrated in FIG. 4B. A reflector field (not shown) provides
thermal input to the solar receiver 450. Temperature of one or more
of the solar boiler tubes in the economizer region 451 of the
receiver is provided as input to an operator portion 452 of a
controller. A temperature set point for the k.sup.th tube
T.sub.set,k is provided to the operator 452. Output from the
operator portion is provided to a main portion of a controller 455,
where as described above, based on a qualitative or quantitative
comparison between the temperature set point and measured
economizer temperature (e.g. a calculated difference between the
measured value and set point), the controller 455 effects an
adjustment to a control valve 453 for the k.sup.th tube to control
mass flow water in the k.sup.th tube. Temperature measurement in an
economizer section can be used alone or in combination with any
other process variable, e.g. in combination with one or more of
temperature at or near the tube outlet, estimated or measured
thermal input, estimated or measured steam quality, solar input
(e.g. DNI), feedwater temperature, change in tube length, and
pressure. In some variations, a temperature measurement in an
economizer region of a k.sup.th tube can be used as a process
control variable for a different tube (not the k.sup.th tube). For
example, if a transit time in a first tube is faster than in a
second tube, it may be desired to use the temperature in the
economizer region or a change in length of the first tube with a
relatively fast transit time as input into a control system for the
second tube with a relatively slow transit time, for example where
the measurement of the temperature or length change in the first
tube is providing predictive information for the control system for
the second tube. Although the example illustrated in FIG. 4B shows
a feedback control loop, it is to be understood that that other
control configurations for controlling mass flow into a tube using
temperature in an economizer region of a tube are contemplated,
e.g. a feedforward control system or cascaded control.
Additionally, while the example illustrated in FIG. 4B comprises a
receiver having multiple parallel-connected absorber tubes, it
should be appreciated that the control system described above may
similarly be applied to a multi-pass receiver having two or more
absorber tubes.
[0101] Temperature measurement in the economizer region or at or
near the exit of the tube can be made using any suitable method,
e.g. using a thermocouple or other thermal sensor welded or
otherwise thermally coupled to a metal exterior of the tube, an
infrared temperature sensing device, a temperature sensor such as a
thermocouple inserted into the tube via a well (a thermowell), and
the like. The temperature set point T.sub.set can be determined
using any suitable method. (Note that T.sub.set refers to the
temperature set point used by a controller, and in some variations
different set points may be used for individual tubes, so that the
set point for the k.sup.th tube is referred to as T.sub.set,k). In
some situations, the temperature set point can be determined based
on the position of the temperature measurement (e.g. position of a
thermocouple) relative to the tube inlet, target heat enthalpy
h.sub.target of the fluid that exits the tube, and the illumination
length of the tube L.sub.illum (which may in certain variations be
essentially the same as the physical length L of the tube as
described above). The temperature set point T.sub.set can be such
that
c p ( T set - T in ) l TC = ( h target - c p T in ) L illum ,
##EQU00001##
where h.sub.t arg et=h.sub.f+x.sub.t arg eth.sub.fg, x.sub.t arg
et=x+x_bias, and h.sub.fg refers to the enthalpy required to change
from a saturated liquid to a saturated vapor (h.sub.g-h.sub.f),
h.sub.g refers to the enthalpy of saturated vapor, c.sub.p refers
to the heat capacity of fluid under the operating conditions,
T.sub.in is the temperature of the water at the tube inlet,
l.sub.TC refers to the position of the temperature sensor relative
to the tube inlet, and x_bias refers to an auxiliary offset (manual
or automatic). It should be noted that in some variations,
temperature can be measured at two or more locations within an
economizer region (l.sub.1 and l.sub.2) and the change in
temperature between the locations l.sub.1 and l.sub.2 can be used
as a process control variable and/or in setting the temperature set
point. As stated above, a temperature set point can be set for an
individual tube (T.sub.set,k), the same set point can be used for
multiple tubes (e.g. neighboring tubes, or tubes symmetrically
placed in the receiver relative to each other such as two tubes on
the ends), or the same set point can be used for all tubes. Thus,
the auxiliary offset can be set for an individual tube in some
variations, in which case x_bias.sub.k for that individual tube
could be used in determining a set point.
[0102] An example of a control system for a multi-tube solar array
(e.g. one comprising a multi-tube receiver, or multiple single tube
receivers) or multi-pass solar array comprising a receiver having
two or more absorber tubes is provided in FIG. 4C. The steam
generating system 430 includes a plant 424 that includes multiple
solar boiler tubes, and may optionally include a separator (e.g. a
steam drum) and a recirculation system as described herein. There,
a process control variable PR.sub.k 425 for the k.sup.th tube
PR.sub.k is provided as input to an operator (e.g. a summer)
portion 420 of a controller, which is provided to a main portion of
a controller 421. The process control variable PR.sub.k can be any
suitable variable, such as any one of or any combination of
feedwater temperature, temperature in an economizer region of a
tube, temperature at or near the tube outlet, change in length of
tube or length of a section of a tube, measured or estimated steam
quality, measured or estimated solar input (e.g. DNI), measured or
estimated thermal input, and pressure. The controller uses any
suitable algorithm (e.g. PI, PD, or PID control) to determine a
proportionality constant for the k.sup.th tube, .alpha..sub.k. A
mass flow into an individual tube {dot over (m)}.sub.in,k can then
be determined: {dot over (m)}.sub.in,k=a.sub.k{dot over (m)}.sub.in
using a multiplier 422, which may be integral with the controller
or may be a separate device. The output from the multiplier can
then be used to adjust a valve position and/or diameter of a flow
controlling orifice, using a function to correlate mass flow to
valve position and/or diameter of a flow controlling orifice. In
some variations, information about one or more process control
variables from one tube may be used as input to a controller or
control channel that is controlling mass flow into another tube.
Such cross-tube control may, for example, be desirable in a
multi-tube receiver or multi-pass receiver having two or more
absorber tubes. Although the control system in FIG. 4C is depicted
as feedback control, any suitable control configuration can be
used, e.g. feedforward control or cascaded control.
[0103] Another example of a control system that can be used to
control steam quality in a single, multi-tube, or multi-pass solar
array (e.g. parabolic trough or single or multi-tube or multiple
pass LFR array) is illustrated in FIG. 4D. As a boiler tube is
heated, it undergoes thermal expansion as a function of
temperature: dL/dT=L.alpha..sub.TE, where dL/dT is the change in
tube length per temperature change, and .alpha..sub.TE is the
coefficient of linear thermal expansion for the pipe material,
which may be a type of steel selected for the particular operating
conditions of the boiler (pressures, temperatures, environmental,
and the like), e.g. carbon steel or stainless steel. A change in
tube length for a section of a tube can be used to represent an
integrated change in temperature over that section. A change in
tube length can be measured, or a change in length of a section of
a tube can be measured, and used as a process control variable for
a control system that controls quality of steam output, and in some
variations a change in length of one tube can be used as predictive
control information for a second tube (e.g. in a multi-tube
receiver). For example, if a boiler tube is fixed in position in a
central region, and allowed to expand at each end, a measurement of
the change in length of the tube relative to the fixed position can
be used as a process control variable. For example, if a boiler
tube is fixed or anchored in position in the evaporator section,
the length of the tube between the inlet and the anchored position
can be measured over time. By neglecting any change in temperature
that occurs in the relatively isothermal evaporator section, a
change in the tube length can be attributed to integrated change in
temperature in the economizer section. By using tube length between
an inlet and a point beyond the boiling boundary .lamda., as a
measure, integration over all temperature points in the economizer
section is accomplished. Referring back to FIG. 4D, an absorber
tube 470 is relatively free floating at its inlet end 471, but is
fixed in position at a point 473 between the inlet 471 and the
outlet 472. The tube, when cold, has a physical end-to-end length
L.sub.cold. A change in length .DELTA.L of the length L.sub.segment
of the tube between the inlet 471 and the anchored position 473 can
be measured. Change in length can be measured using any suitable
technique, e.g. using an optical detector, a ruler or scale, stress
or strain indicator, or any kind of device that measures physical
displacement such as a caliper, transducer (e.g. linear variable
displacement transducer), a compressible spring, and the like. In
some variations, a limit indicator may be included so that if the
length reaches a certain limit, the limit indicator is activated to
reduce thermal input. Such a limit indicator may improve safety.
The measured change in length .DELTA.L may be provided as a process
variable into a controller input 480, where a qualitative or
quantitative comparison (e.g. a difference calculation between
measured and set values) between the measured value of change in
length and the set point for the k.sup.th tube .DELTA.L.sub.set,k
is provided to an appropriate algorithm in a controller 482 that
effects a change in a control valve 483 position to regulate mass
flow of water into the k.sup.th tube. Change in tube length can be
used alone or in combination with one or more other variables in a
control system that controls steam quality, e.g. in combination
with one or more of: a temperature in an economizer region,
temperature at an outlet, estimated or measured thermal input,
estimated or measured steam quality, pressure, solar input such as
DNI, and feedwater temperature. For example, in some variations, a
measured change in length of a tube or a section of tube can be
used in combination with a temperature measurement of that tube in
its economizer region as input to a controller that controls mass
flow of water to that tube. In some variations, a measured change
in length of a tube or section of a tube can be used in combination
with a steam quality estimate as input to a controller that
controls mass flow of water to that tube. In some variations, a
measured change in length of a tube or a section of a tube, a
temperature measurement of that tube in its economizer region, and
an estimated steam quality can be used as input to a controller
that controls mass flow of water to that tube. In some variations
of multi-tube systems and multi-pass systems comprising a receiver
having two or more absorber tubes, a change in length in one tube
may be used as input to a control system or channel for another
tube in the system (e.g. where a multi-tube receiver or multi-pass
receiver with two or more tubes is being used). For example, change
in length in a tube with a relatively fast response time may be
used as predictive information provided as input to a control
system to a tube with a relatively slow response time. Although the
control system in FIG. 4D is illustrated as a feedback control
system, any suitable control configurations using change in tube
length as a process variable can be used, e.g. feedforward control,
or cascaded control.
[0104] In some variations, it may be desired to estimate steam
quality
x ( e . g . x .apprxeq. m . steam m . in ) , ##EQU00002##
where {dot over (m)}.sub.steam is the mass flow of steam from a
steam drum or accumulator and {dot over (m)}.sub.in is the mass
flow of water into the steam generator, to compare such estimated
steam quality with target steam quality x.sub.target, and to use
the comparison between target and estimated steam quality as input
to a controller in a control system (e.g. a feedback control loop
or a feedforward control system) to adjust mass flow into one or
more of the tubes. Estimated steam quality can be used alone or in
combination with one or more other process variables such as
feedwater temperature, temperature in an economizer region,
temperature at or near a tube exit, solar input such as DNI,
estimated or measured thermal input, change in tube length, or
pressure.
[0105] An example of a control system used in conjunction with a
multi-tube receiver or multi-pass receiver comprising two or more
absorber tubes, in which k multiple tubes are arranged in parallel
is provided in FIGS. 5A-5C. The number of tubes in a receiver may
be 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, or even more, e.g. 15 or 20.
In the receiver 500, each tube 501(1), . . . , 501(k) comprises a
thermal sensor TC.sub.k that is positioned on or in the tube (e.g.
using a thermowell) at a distance l.sub.TC,k from each respective
inlet 512(1) . . . 512(k). The distances l.sub.TC,k are selected to
be within the economizer region of each tube, i.e. before the
boiling point boundary (as described in FIG. 2 above) for each
respective tube. The temperature measurement device may be any
suitable device, e.g. a thermocouple welded or otherwise thermally
coupled to a metal exterior of the tube, an infrared temperature
sensing device, a thermal sensor (such as a thermocouple) inserted
into the tube via a well (a thermowell), and the like. In some
variations, the devices TC.sub.k may be positioned at identical
distances from the respective inlets 512(k). For example, if all
tubes have an identical length L, the positions l.sub.TC,k may be
at about 1/10 L, about 1/8 L, or about 1/6 L (all distances
relative to the tube inlet). In other variations, the devices
TC.sub.k may be positioned at different distances from the
respective inlets 512(k). For example, if tubes near the center of
a multi-tube receiver or multi-pass receiver comprising two or more
absorber tubes tend to receive more thermal input due to a heat
flux profile transversely across the receiver, then the boiling
boundary in those tubes may occur closer to the inlet, and the
position of the temperature measurement device may be adjusted
accordingly to be relatively closer to the inlet. As stated above,
in some variations, multiple temperature sensors may be placed in
an economizer region, and change in temperature between two of
these temperature sensors in an economizer region can be used as a
process variable, and/or in setting the temperature set point.
[0106] In some variations, thermal input for each tube {dot over
(Q)}.sub.in,k is provided so as to be relatively uniform along the
length of the tube. As shown, each of the tubes 501(k) has a
physical length L.sub.k, and an illumination length L.sub.illum,k
which may or may not be the same as the physical length L.sub.k due
to effects such as shading at the tube ends. In some variations,
all tubes within a single receiver may have the identical physical
length L.sub.k. However, as described above, the illumination
length is generally at least about 80% at least about 90%, at least
about 95% of the physical length L.sub.k. The total thermal input
to a multi-tube receiver is given by {dot over
(Q)}.sub.in=.SIGMA..sub.1.sup.k{dot over (Q)}.sub.in,k.
[0107] A mass flow of water {dot over (m)}.sub.in at a temperature
T.sub.in (and having a heat enthalpy h.sub.in) is provided into a
manifold, where it is split into k branches for feeding into each
of the k multiple tubes. Flow into each of the tubes is controlled
with a control valve CV.sub.k and/or a fixed diameter flow
controlling orifice, leading to mass flows into the inlets 512(1) .
. . 512(k) of each of the respective individual tubes represented
by {dot over (m)}.sub.in,k, such that {dot over
(m)}.sub.in=.SIGMA..sub.1.sup.k{dot over (m)}.sub.in,k. In the
systems and methods described herein, any suitable type of control
valve can be used, e.g. linear, equal percentage, electric,
pneumatic, electropneumatic, or manual.
[0108] Although the particular example illustrated in FIGS. 5A-5C
shows a control valve and/or a fixed diameter flow controlling
orifice regulating mass flow into each of the k tubes, embodiments
are contemplated in which a single control valve and/or a fixed
diameter flow controlling orifice may control mass flow of water
into more than one tube. Examples of such variations are shown in
FIGS. 6A-6C. For example, In FIG. 6A, a variation is illustrated in
which water mass flow into all tubes is controlled by a single
control valve and/or fixed diameter flow controlling orifice. In
FIG. 6B, a variation is illustrated in which water mass flow into
two neighboring tubes is controlled by a single valve and/or a
fixed diameter flow controlling orifice, one variation of which is
a configuration in which mass flow into a first half of the tubes
is controlled by a first control valve and/or a first fixed
diameter flow controlling orifice and water mass flow into a second
half of the tubes is controlled by a second control valve and/or a
second fixed diameter flow controlling orifice. In FIG. 6C, a
variation is illustrated in which water flow into two outermost
tubes is controlled by a single valve and/or a fixed diameter flow
controlling orifice, and water mass flow into an inner group of
three tubes is controlled by a single valve and/or a fixed diameter
flow controlling orifice.
[0109] Referring back to FIG. 5C, output from the tubes 501(k) is
combined, where the combined output of the tubes has a heat
enthalpy h.sub.out. The combined output of the tubes can optionally
be fed into a separator 515, which may in some variations comprise
a steam drum at a pressure P.sub.drum. Other types of separators
may be used such as baffles and cyclone separators. If superheated
steam is being generated in all tube 501(k), a separator may not be
necessary. Saturated steam in the separator 515 has a heat enthalpy
h.sub.g and a mass flow {dot over (m)}.sub.steam. Steam output from
the separator 515 is controlled by valve and/or fixed diameter flow
controlling orifice 516. Heated water (saturated liquid) collected
by the separator 515 (e.g. water collected in a lower portion of
steam drum) has a heat enthalpy of h.sub.f. Optionally a
recirculation system can be used in which water is drawn from the
separator 515 and is mixed with a feedwater supply having a mass
flow {dot over (m)}.sub.feed and enthalpy h.sub.feed to provide
water input into the tubes 501(k). As illustrated in FIG. 5A, the
enthalpy increase of the fluid in a tube between the inlet
(h.sub.in), the combined outlet h.sub.out can be modeled to
increase linearly along the length of the tube L in steady state
operation. As shown in FIG. 5B (and referring back to FIG. 1),
temperature increase of the fluid is a nonlinear function of the
tube length, because of the phase change that occurs at the boiling
point .lamda.. However, within the economizer region, the
temperature increases linearly with length. Thus, a correlation can
be made between the linear increase in temperature in the
economizer region and the desired increase in enthalpy required to
achieve the target enthalpy h.sub.target, and correspondingly, the
target steam quality, as h.sub.t arg et=h.sub.f+x.sub.t arg
eth.sub.fg. The temperature rise in the economizer region can be
used as an indicator for whether the target enthalpy is being
reached, for example by measuring a temperature difference between
two spaced apart thermal sensors, or if one or more temperature
sensors TC.sub.k is placed at a distance l.sub.TC from the inlet
within the economizer region, by measuring the temperature at
TC.sub.k relative to T.sub.in. The steady state boiling boundary
can be estimated:
.lamda. = l TC h f - h i h TC - h in . ##EQU00003##
Temperature measured in the economizer region by TC.sub.k and a
temperature set point (e.g. a temperature set point as described
above) can be provided as input into a controller that uses a
qualitative or quantitative comparison and appropriate control
algorithm (e.g. PI or PID) to adjust a control valve position for
that tube. The temperature set point can be set for an individual
tube within a multi-tube receiver or multi-pass receiver comprising
two or more absorber tubes, or the temperature set point can be
identical for a subset of tubes within a multi-tube receiver or
multi-pass receiver comprising two or more absorber tubes, or the
temperature set point can be identical for all tubes within a
multi-tube receiver or multi-pass receiver comprising two or more
absorber tubes.
[0110] Alternatively to or in addition to measuring temperature in
an economizer region, a change in tube length or a change in length
of a section of a tube as described above (e.g. in connection with
FIG. 4D) may be used as a process control variable for a control
system that regulates mass flow into tubes in a multi-tube receiver
or multi-pass receiver. Using a change in tube length may provide
an integrated measure of temperature in an economizer region which
may in some circumstances reduce or average out experimental error
associated with a temperature measurement at one or more discrete
locations and/or improve a time response to the control system.
[0111] In some cases, predictive control may be used to improve a
control system, e.g. by improving time response of the control
system, accuracy or precision of control, and/or reducing
oscillations during control. For example, a predictive control
scheme that accounts for time delay between a point in time at
which measurement of a process variable (such as feedwater
temperature, temperature in an economizer region, temperature at or
near the tube exit, change in tube length, estimated or measured
steam quality, estimated or measured thermal input, pressure, solar
input such as DNI, and the like) takes place and a point in time at
which an adjustment is made to affect such system parameter used.
In a single tube, multi-tube, or multi-pass solar boiler, a
predictive control scheme that accounts for time delay within the
tube and/or a recirculation system can be used. In the case of the
control systems illustrated herein (e.g. in FIGS. 4A-4D and 5A-5C),
the time delay experienced by water between entry into the inlet
and reaching the position temperature measurement l.sub.TC can be
accommodated using any suitable predictive control method. For
example, a Smith predictor may be used to compensate for time
delay. An example of a suitable predictor that can be used in solar
boilers is illustrated in FIG. 7. There, a control system 700
comprises an outer control system 702, in which an output 716 from
the plant (e.g. temperature, length, estimated steam quality, etc.)
from the plant 721 is fed back into an input portion of the
controller 714 for qualitative or quantitative comparison with a
process set point PR.sub.set, which is, in turn, used to adjust a
valve position 722 or diameter of a flow controlling orifice of a
tube to adjust mass flow of water into that tube. The control
system 700 also comprises an inner control system 704. For the
inner control loop 704, the plant output 717 of the desired process
parameter or parameters (e.g. temperature in economizer region,
tube length, steam quality) is modeled, and such modeled output
value is input into a compensator 708, where the modeled output
from box 717 is modified according to a time dependent function to
account for any change in the output parameter that occurs during
the time delay between an actual measurement of the parameter and a
control to affect that parameter. The output from the compensator
708 is then provided as input into controller 718 for the inner
control system 704. The compensator can use any appropriate time
dependent function; in some instances the effect on the output
parameter can be modeled as a first order time dependent effect
that varies as e.sup.-t/.tau.. For example, if a tube temperature
measurement at a location t.sub.TC is used as a control variable to
feedback to control mass flow input at the k.sup.th tube inlet {dot
over (m)}.sub.in,k, then the compensator 708 can use a time
dependent compensating factor e.sup.-(t-.tau.)/.tau. to account for
the time delay in the measured mass flow (indicated by control
valve position) at the inlet and mass flow at l.sub.TC, where
.tau.=.rho.Al.sub.TC/{dot over (m)}.sub.in,k, A represents a
cross-section inner dimension of the tube, and .rho. represents
fluid density in the tube. That is, mass flow of fluid in the pipe
at location l.sub.TC may be estimated by {dot over
(m)}.sub.in,k+Be.sup.-(t-.tau.)/.tau., where B is an appropriate
proportionality constant. Accordingly, a steam exit quality
estimator may be used to predict steam quality as a function of
time x(t):
x ( t ) = m . steam ( t ) m . in ( t - .tau. ) . ##EQU00004##
This instantaneous steam quality estimate may then be used to as
input for a predictive controller, e.g. predictive control loop 704
as illustrated in FIG. 7. Predictive control can be used with
systems comprising single absorber tubes. multiple
parallel-connected absorber tubes, or multi-pass absorber tubes. As
stated above, process variable information from one tube may be
used as input to a predictive control system for another tube in a
multi-tube system or multi-pass absorber system having two or more
absorber tubes.
[0112] In some variations, information that can be supplied to a
control scheme can include changes in a process parameter with time
that have been previously modeled, measured, tabulated, or
calculated, so that it is possible to provide advance information
about that process variable to a control system. Such information
can be used as predictive control information, or can be used to
correct output from a controller. Such advance information can
improve a time response of a control scheme. For example,
previously modeled, measured, tabulated, or calculated information
about solar input may be supplied to a control system as predictive
information. Thus, expected changes in insolation due to diurnal
motion of the sun or seasonal variations in insolation can be
provided in a lookup table or otherwise to a control system.
Similarly, expected changes in feedwater temperature can be
provided as information to a control system. In some cases, known
or expected shadowing patterns, where the sun moves past one or
more structures in a solar array, can be provided as information to
a control system. Thus, such information about shadowing patterns
that change over time can be used to adjust estimated thermal input
that may be used in a predictive manner, e.g. as feedforward
control.
[0113] In some variations, it may be desired to use a feedforward
control system in addition to or in lieu of a feedback control
system. For example, it may be desirable to provide feedforward
information to a control system regarding changes in insolation, as
systematic or non-systematic changes in insolation translate to
corresponding changes in thermal input. It may be especially useful
to provide feedforward information regarding thermal input due to
fluctuations in insolation. Solar input (e.g. DNI) and/or a thermal
input estimator or thermal input measurement may also be used in a
start-up procedure to provide guidance for rotating reflectors to
direct solar radiation to begin warming up a receiver, e.g. to
indicate when thermal input is exceeding thermal losses. A thermal
input estimator that is coupled to a control system for regulating
water mass flow into an absorber tube can protect that absorber
tube from overheating or dry out by ensuring mass flow when
significant thermal input is present, and can stabilize performance
by providing stable operation during transients such as occur
during shadowing or cloud cover. In one example, the predicted
thermal input may incorporate an estimate of thermal losses based
on measured process temperatures and a thermal loss model that can
be either analytically or empirically derived.
[0114] A thermal estimator for a tube can be used that in steady
state depends on the boiling point boundary in that tube, the mass
flow of steam produced, and the enthalpy of the steam produced.
Energy is balanced in the total volume in a control system such as
illustrated in FIG. 2, including a boiler tube, a steam separator,
and a recirculation system, so that
m . in + L - .lamda. L Q . in - [ m . steam h g + m . recirc h f ]
- E stored dt = 0. ##EQU00005##
The energy stored in the volume is
E.sub.stored=.mu..sub.fm.sub.water+.mu..sub.gm.sub.steam+C.sub.p,steelm.s-
ub.steelT.sub.steel, where .mu..sub.f=specific internal energy of
water under the operating conditions, m.sub.water=mass water,
.mu..sub.g=specific internal energy of steam under the operating
conditions, m.sub.steam=mass steam, c.sub.p,steel=heat capacity of
the drum material (e.g. steel), m.sub.steel=mass drum, and
T.sub.steel=drum temperature. The change in stored energy with
respect to time is
E stored t = .mu. f P drum P drum t m water + m water t .mu. f +
.mu. g P drum P drum t m steam + m steam t .mu. f + c p , steel m
steel T sat P drum P drum t . ##EQU00006##
Accordingly, the thermal input for a tube of length L can be
estimated during steam production with the steam valve open as
Q . in , est = L - .lamda. L [ m . steam h g + m . recirc h f - m .
in h f + E stored t . ##EQU00007##
At steady state, when operating at constant pressure in the steam
drum, the estimated thermal input is
Q . in , est = L L - .lamda. m . in h fg , where .lamda. = l TC h f
- h i h TC - h in . ##EQU00008##
During warm-up with the steam valve closed
Q . in , est = L L - .lamda. [ E stored t ] = L L - .lamda. [ P
drum t { .mu. f P drum m water + .mu. g P drum m steam + c p ,
steel m steel T sat P drum } + m water t .mu. f + m steam t .mu. g
] ##EQU00009##
Thus, it is possible to estimate thermal input based on the length
of the tube and the boiling point boundary and rate of change of
pressure in a steam drum, and to use that estimated thermal input
in a feedforward control.
[0115] An example of a control system incorporating feedforward
information regarding thermal input and feedback information
regarding a process variable such as temperature in an economizer
region, length of a tube, or estimated steam quality is provided in
FIG. 8. There, control system 800 comprises a plant 801 that
includes one or more solar boiler tubes, and optionally a steam
drum or the equivalent and a recirculation system. A measured
process variable PR 802 from the plant 801 such as temperature in
the economizer region, tube length, or measured or estimated steam
quality, pressure, feedwater temperature, temperature at or near
the exit, or solar input (e.g. DNI) is provided as input to an
operator portion 802 of a controller, where it is qualitatively or
quantitatively compared with a set point for that process variable
PR.sub.set. The result of this comparison is used in an appropriate
algorithm (e.g. PI, PD or PID) in a main portion of a controller
804 to provide an output signal 805 to eventually control a valve
that, in turn, controls mass flow alone or in conjunction with a
fixed diameter flow control orifice in to the one or more solar
boiler tubes in the plant 801. Before reaching the control valve,
the output signal 805 from the feedback controller 804 is provided
as input into an operator 806. Feedforward information 807 (which
may for example be derived from the estimated thermal input) is
used to adjust the control signal being sent to the control valve
so that a mass flow {dot over (m)}.sub.in results that takes into
account the change in thermal input. Although the particular
embodiment illustrated in FIG. 8 shows estimated thermal input
being provided as feedforward information, other types of
information can be used as input in such a feedforward control
scheme. For example, thermal input changes that have been modeled,
measured, tabulated or calculated (e.g. for expected shadowing as a
function of time) can be used as feedforward control. In some
variations, information from one tube (e.g. temperature at the
exit, or in the economizer region, or change in length) can be used
as feedforward information for a control system controlling another
tube. In some variations, calculated or modeled thermal losses can
be provided as feedforward information.
[0116] As stated above, any of the control systems and methods
described herein can be used to generate steam of a desired
quality, or superheated steam of a desired number of degrees of
superheat. FIG. 25 shows an example of a control scheme that can be
used in a system where saturated steam is generated in a first
solar boiler segment of a solar array, and such saturated steam is
fed into a second solar segment that is in series with the first
boiler segment, and superheated steam exits the second boiler
segment. An example of such a system is illustrated in FIG. 13A. In
FIG. 25, the first boiler segment 2510 comprises one or more
individual solar boilers 2501, connected in parallel. Although the
example illustrated in FIG. 25 shows three individual solar boilers
2501 connected in parallel, any suitable number can be used, e.g.
1, 2, 3, 4, 5, 6, 7, 8, 9, or 10, depending on the balance between
the first and second boiler segments. Feedwater is supplied to each
infidel boiler through control valves and/or fixed diameter flow
control orifices 2502. The combined output of the boilers 2501 is
delivered to a separator 2503 such as a steam drum or steam
accumulator, where it is held under pressure. Liquid recovered from
the separator 2503 can be recirculated, as indicated by line 2513.
Steam extracted from the separator 2504 is fed through a control
valve and/or fixed diameter flow control orifices 2504 into the
second boiler segment 2512. Although the particular example
illustrated in FIG. 25 shows only a single solar boiler 2505, the
second segment can comprise any suitable number of individual
boilers, e.g. 1, 2, 3, 4, or 5 depending on the balance between the
first and second segments. Superheated steam can be released from
the second segment through control valve 2506 to provide
superheated steam as indicated by line 2507. The superheated steam
generated in the second boiler segment 2512 has a pressure and a
temperature, thus, if output pressure is adjusted using valve 2506,
temperature can be downward adjusted by using an attemperating
spray. Any of the methods described herein can be used to adjust
thermal input to the second segment 2512 to attain the desired
superheated steam, e.g. by changing the aiming or focus of one or
more reflectors such as described in connection with FIGS. 3A-3D.
An output temperature from the second segment can for example be a
process control variable in a control scheme that positions one or
more reflectors in the second segment. It may or may not be desired
to control the steam quality exiting the first segment 2510. In
those variations where it is desired to control the steam quality
exiting the first segment 2510, any of the methods and control
schemes as described herein can be used. The boilers of the first
and second segments can comprise single tube absorbers, multi-tube
receivers, or multi-pass receivers. In some variations, a mix of
single tube absorbers, multi-tube receivers, and multi-pass
receivers is used in a single system. In other variations, second
segment 2512 may be a tower, linear solar (trough or CLFR), or an
external heat source such as a coal fired or natural gas fired
burner.
[0117] In some variations, it may be desired to use a staged
control scheme, where one control scheme is used until the boiler
reaches a certain predetermined status, and then a second control
scheme is activated. Such a scenario can occur, for example, in the
instance where superheated steam is being generated in a once
through configuration, e.g. as illustrated in FIG. 2 and FIG. 14.
An example of a staged control scheme is provided in FIG. 26.
There, control scheme 2600 comprises a first control system 2610
and a second control system 2612. The first control system 2610 is
operational when the plant is producing saturated steam. The first
control system 2610 comprises a plant 2611 that comprises one or
more boiler tubes, and optionally a separator and a recirculation
system as described herein. A process control parameter 2614
PR.sub.sat as described herein is provided to an operator portion
2630 of a controller. The measured value PR.sub.sat is
quantitatively or qualitatively compared with the set value for
that parameter PR.sub.set,sat. The operator portion 2630 feeds the
results of that comparison to a main portion of a controller 2631,
which provides as output a proportionality constant .alpha..sub.sat
which determines a mass flow per tube as described above in
connection with FIG. 4C, and is then fed into a converter function
2633 for controlling a position of a valve to control mass flow
into that tube alone or in conjunction with a fixed diameter flow
controlling orifice. In this particular example, the second control
system 2612 is identical in all respects, except for the process
control variable that is used. A process variable from the output
of the plant PR.sub.exit 2618 is used to detect whether or not the
system is close to producing superheated steam, e.g. temperature at
or near the exit of the tube, estimated or measured steam quality,
tube length, or pressure. A value of the output process variable
PR.sub.exit is compared to a set point for that variable
PR.sub.set,exit. A comparison between the output process variable
and the set point for that variable and a determination of whether
or not a threshold value for that parameter PR.sub.SH has been
exceeded or not determines which control system is operational.
Thus, if PR.sub.set,exit-PR.sub.exit>PR.sub.SH, then the control
scheme uses the first control system, and hence the values
.alpha..sub.sat to control valve positions. For example, if the
output process control variable is temperature, then if the
temperature at the exit of the tube is low relative to a threshold
(e.g. about 10 degrees, or about 20 degrees below) then a control
scheme for saturated steam is used, whereas if the temperature at
the exit of the tube is close to a point at which superheat will be
made (e.g. within about 10 degrees or about 20 degrees of a set
point), then a control system for superheated steam can be used. In
one example of a control system for superheated steam, a
temperature at or near the exit of the tube is used as a process
control variable to supply to a controller, e.g. as shown in FIG.
26.
[0118] Any one of the control systems (e.g. feedback or feedforward
control systems) described herein may comprise a clip function, so
that any control signal sent to a control valve will not cause a
control valve position to be adjusted below a certain minimum
setting and or above a certain maximum setting. In some variations,
a valve position may be provided as an input to a feedback control
system that controls operation of a recirculation pump, e.g. a
variable frequency drive for a recirculation pump. For example, if
one or more control valves is almost closed so that mass flow is
very low, a controller can reduce the pump frequency to avoid a low
flow trip condition in the plant. An example of such a control
system is illustrated in FIG. 24. There, a system 2400 comprises a
solar thermal plant 2401 that comprises a single absorber tube,
multiple absorber tubes in parallel (multiple parallel-connected
tubes within a receiver or multiple single tube or multi-tube lines
connected in parallel), or one or more multi-pass absorber tubes, a
steam separator such as a steam drum, a recirculation system for
recirculating recovered water from the separator, and a feedwater
input. Mass flow into one or more of the absorber tubes is
regulated using a control valve or manifold of k control valves
CV.sub.k and/or fixed diameter flow controlling orifices as
described herein. Control valve position 2402 is provided as input
to a controller 2403 (e.g. in a feedback loop). A valve position is
qualitatively or quantitatively compared with a valve position set
point CV.sub.set (e.g. a maximum value for closing a valve is 60%
closed by mass or by volume). If a maximum valve position for any
one of k control valves in the system exceeds the set point, then
the frequency that the recirculation pump is running at can be
reduced. For example, the operation frequency of a variable
frequency drive for the pump can be reduced by a factor
1 - trip_m arg in min_m arg in ##EQU00010##
where min_margin is an empirically determined number such as about
0.1 kg/sec, 0.2 kg/sec or 0.3 kg/sec, and trip_margin represents
the lowest mass flow value that will cause a low flow trip in the
system. A clipping function can be included in such a control
system so that the pump frequency does not drop below a minimum
frequency. Such a control system for adjusting recirculation pump
frequency can be used in conjunction with any one of the control
systems for controlling mass flow and/or thermal input into one or
more absorber tubes as described herein.
[0119] As stated above, the systems and methods described herein
for controlling steam quality may be adapted to single tube solar
thermal systems, such as a single line, single tube LFR system or a
single line of parabolic trough sections, solar thermal systems
that include multiple parallel-connected absorber tubes, such as a
multi-line single tube LFR system, a single line multi-tube LFR
system, a multi-line multi-tube LFR system, or a multi-line
parabolic trough system, or multi-pass solar thermal systems having
one or more multi-pass absorber tubes. Further, it may be desirable
to provide individual process variable input for each tube to a
controller, or it may be desirable to combine process variable
input from multiple tubes to provide an aggregated input to a
controller. FIG. 9A illustrates an example of a solar thermal steam
generator in which m tubes 901(1) . . . 901(m) (m>1) are
arranged in parallel. In this particular example, process variable
input (e.g. temperature within an economizer section, length of a
tube section, or steam quality estimate) from each of the m tubes
can be used to provide an individual set point PR.sub.set,1 . . .
PR.sub.set,m for each of the m tubes to a controller 904 capable of
controlling m channels individually, or to multiple controllers.
Alternatively, an aggregate set point PRset can be used for each of
the m tubes. FIG. 9B illustrates an example of a solar thermal
steam generator comprising m multi-tube receivers (m>1), each
receiver comprising k tubes, so that each tube 950(1,1) . . .
950(k,m) within the system can be individually controlled, or
controlled as part of an aggregate group. For example, all k tubes
within a receiver may be controlled as part of an aggregate group,
but each of the m receivers may be controlled separately from one
another.
[0120] In some variations, it may be desired to control both mass
flow and thermal input into one or more tubes. Referring back to
FIGS. 3A-3D, thermal input may be varied transversely (e.g. across
multiple tubes) while being maintained as relatively constant
longitudinal, thermal input may be varied longitudinally while
being maintained as relatively constant transversely, or thermal
input may be varied both transversely and longitudinally.
Longitudinal thermal input may be varied, for example, by
selectively rotating a segment of an elongated reflector in an LFR
array, or by selectively rotating a section of a row of end-to-end
coupled parabolic troughs, as illustrated FIG. 3D. Other methods
for varying thermal input along a length of a tube include
selectively shading a segment of a receiver, or selectively shading
a segment of an elongated reflector. Transverse adjustment of
thermal input while maintaining a relatively constant longitudinal
thermal input can be achieved by rotating an entire length of an
elongated reflector as illustrated in FIGS. 3B-3C. It may, for
example, be desired to reduce thermal input to a tube if a position
of a control valve that controls mass flow to that tube is near a
limit such that mass flow cannot be increased, if a control valve
position indicates that mass flow is below a minimum level, or if a
temperature measurement indicates that overheating in a tube is
occurring.
[0121] Examples of drive systems for rotating reflector rows or
segments of reflector rows are provided in FIGS. 21-23. FIG. 21
illustrates a configuration in which the angle of a row of linearly
coupled reflectors is controlled by a drive located at a single
terminus of the row. Although FIG. 21 illustrates the reflectors
each turning the same direction, it is understood that each row of
reflectors can be individually activated or not, and when
activated, may move in a clockwise or counterclockwise direction.
In this configuration, if the angle at which a row of reflectors is
changed by moving the reflectors via the drive form a first to a
second position, the reflectors in the row that are distant from
the location of the drive may experience lag.
[0122] FIG. 22 illustrates an alternative configuration, in which
any lag that may be experienced with the configuration of FIG. 21
is reduced by placement of the drive at a more central position
along the row of the linearly positioned reflectors. In this way,
when the drive is activated to modify the angle at which the
reflectors are positioned, there is less distance between the drive
and any one portion of the rows of reflectors as compared to the
configuration of FIG. 21.
[0123] FIG. 23 illustrates a further alternative configuration, in
which single reflectors are individually controlled by a drive, as
compared to the linearly conjoined row of reflectors of FIGS. 21
and 22. In the configuration of FIG. 23, lag is reduced because
each reflector is individually controlled by a separate drive.
Similar embodiments are envisioned in which small groups of
reflectors within a row are controlled by a separate drive, where
the length of the group is limited so as to reduce lag to an
acceptable level. Each reflector drive may turn its reflector
clockwise or counterclockwise or remain in a set position. As
illustrated in the figure to the right, the reflectors may also
contain a supporting beam coupled to the drive and extending
linearly along the underside of the reflector to assist with
uniform rotation along the length of the reflector. Supporting
beams may also be used in other reflector configurations, such as
those in FIGS. 21 and 22, e.g. to reduce lag.
[0124] In one variation, a control system for controlling reflector
position activates a drive causing reflector movements in an amount
of about 1 to about 5 degrees or about 1 to about 10 degrees or
about 5 to about 15 degrees in a clockwise direction to a set
point, followed by reverse movement of the same amount in the
counterclockwise direction. The control system continues to
oscillate the reflectors by causing incremental reflector movements
in a first and then in an opposite direction at a desired frequency
for a period of time, e.g. at a frequency in a range from about
0.01 Hz to about 50 Hz, e.g. about 0.1 Hz, about 1 Hz, or about 10
Hz.
[0125] In some variations, it may be desired to provide a warm
start up for a solar boiler. In one variation, a warm start up for
a solar boiler can be accomplished by providing steam from an
auxiliary source into an exit of a boiler tube. Any suitable
auxiliary steam source can be used, e.g. from a steam accumulator,
a coal-fired or natural gas-fired steam source, or from another
solar boiler. In some variation, steam can be taken from a steam
accumulator for the solar boiler being started up. In FIG. 10B, a
temperature profile as a function of length along a boiler tube of
a cold system 1050 is shown, along with a desired operating
temperature profile 1052. By providing steam input into an exit end
of the boiler tube, the temperature profile in the tube can be
gradually built up (as indicated by curves 1053) to be similar to
that of the desired profile 1052. After the tube is warmed up in
this manner, water can be flowed into the inlet at a low flow, and
thermal input can be supplied by rotating reflectors. After steam
is observed exiting the tube, then full operation can begin by
increasing water flow and increasing thermal input.
[0126] A control system that controls both mass flow into one or
more tubes and reflector position may be used to adjust start up
conditions in a solar receiver. It may be desirable to adjust start
up conditions so that initial boiling occurs near the exit of the
tube, and then the boiling point moves along the pipe toward the
inlet as warm up progresses. By controlling startup conditions so
that initial boiling occurs near the exit of the tube, scenarios
can be avoided in which boiling occurs in an interior region of the
tube removed from the exit, so that the boiling displaces water
beyond the boiling point which is dumped into a recirculation
system, causing water level overflow. In a solar receiver, some
reservoir of warm water may exist from previous day's operation. As
illustrated in FIG. 10A, an example of a solar boiler system 1000
that includes a boiler section 1004 comprising one or more tubes
and a recirculation section 1002 is illustrated. During operation,
heat is directed to the boiler section 1004 using one or more
reflectors (e.g. linear Fresnel reflectors or parabolic trough
reflectors). Water is fed into the boiler section at the inlet
through control valve manifold 1014 (which may contain one control
valve and/or fixed diameter flow controlling orifice for all tubes,
one control valve and/or fixed diameter flow controlling orifice
for each tube, or multiple control valves and/or fixed diameter
flow controlling orifices, where each control valve and/or fixed
diameter flow controlling orifice controls mass flow into multiple
tubes, as described above). Boiling occurs at some position
.lamda., along the tube, so that steam exits the boiler section
1004. Output from the boiler section 1004 enters a steam drum or
equivalent 1006, where steam is extracted, e.g. through valve 1040.
During operation, a liquid level in the drum 1006 can be correlated
to a boiling point .lamda., in a tube, taking into account a flow
rate in that tube. After a period of nonoperation (e.g. shutdown or
darkness), the boiler section 1004 contains relatively cold water.
The recirculation section, including the steam drum 1006 contains
fluid at a temperature T.sub.recirc and pressure P.sub.drum. A
valve 1008 can be installed between the recirculation system 1002
and the boiler section 1004. The valve or valves CV.sub.k at the
boiler inlet and the valve 1008 at the boiler exit can be closed so
as to isolate the boiler section from the recirculation section
during periods of nonoperation. Pressure in the boiler section
drops as temperature drops; optionally, cold water can be drained
from the boiler section using valve 1010 during nonoperation, e.g.
using a dump condenser 1020. At startup, the valve 1008 can be
opened so that relatively warm water is sucked back into the
colder, lower pressure boiler section, which reduces the liquid
level in the drum 1006. After pressure equalization between the
boiler section 1004 and the recirculation section 1002, one or more
control valves CV.sub.k 1014 and recirculation valve 1012 can be
opened so that fluid begins to flow through all of the tubes (one
tube for a single tube receiver or multiple tubes) in the boiler
section 1004. After water begins to flow through the receiver, heat
can be preferentially applied to an end section 1015 of the boiler
section. The end section 1015 may be any suitable section of the
boiler adjacent the exit, but in some variations, the end section
may be about 1/4 or about 1/3 the length of the tube that is
adjacent the exit. Preferential heating can occur in a LFR solar
array by tuning only a section of reflectors in a line of
reflectors to selectively illuminate the end section, while the
other reflectors in the line are inverted or otherwise arranged so
as to not direct solar radiation to the boiler section.
Preferential heating of an end section in a parabolic trough array
can occur by orienting only those parabolic trough sections near
the end of a line so as to receive solar radiation, whereas other
sections of a line of parabolic troughs remain inverted or
otherwise dark to the sun. As selective heating begins near the end
1015 of the boiler, the liquid swell level in the drum 1006
increases. Monitoring the liquid swell level in the drum 1006,
along with mass flow rate into the boiler section 1004 (indicated
by control valve position) can provide an indication of the
position of the boiling point .lamda.. After the initial boiling
point is established near the end of the boiler section by
selective irradiation of the end of the boiler section, the boiling
point can be systematically moved to a desired upstream position by
systematic increase of mass flow of water with control valve
manifold 1014 and systematic increase of heat flux to the boiler by
directing solar radiation to regions of the boiler upstream
relative to the end section 1015, while monitoring level swell in
drum 1006.
[0127] As stated above, saturated steam or superheated steam
produced using the systems and methods described herein may be used
to drive a turbine to generate electric power. Referring now to
FIG. 11, steam generated by a steam generator 1 which may comprise
any steam generator configuration described herein, including those
employing multi-tube receivers, those employing multi-pass
receivers, and those employing single tube receivers, is delivered
to a turbine 2, which drives an electric generator 3. The turbine 2
is driven by dry steam, so steam generated by steam generator 1 can
be dry by virtue of being superheated steam, or saturated steam can
be passed through a separator (not shown). FIG. 12 illustrates
another embodiment of an electric power plant in which a steam
generator 1 as described in FIG. 11 produces dry steam to drive a
turbine 2 that, in turn, drives an electric generator 3. Condensate
from the turbine 2 can be captured in condenser 5, and stored in a
reservoir 6. Pumps 7 can circulate the condensate to provide water
input to the steam generator 1. In some variations, a thermal
energy storage system 4 is utilized to store thermal energy
generated by the steam generator so that such stored thermal energy
can be tapped at a later time and used to drive turbine 2.
[0128] It is understood that the systems and methods described
herein can be used in conjunction with a variety of solar thermal
plants, including a variety of LFR solar arrays. For example, and
with reference to FIG. 13A, a LFR system 1300 comprises a first LFR
stage 1301 in series with a second LFR stage 1302. The first LFR
stage comprises a field of linear Fresnel reflectors 1304 arranged
in use to track diurnal motion of the sun and to direct reflected
solar radiation to one or more elevated receivers 1305. An elevated
receiver 1305 may comprise a single absorber tube, a plurality of
parallel-connected absorber tubes, or one or more multi-pass
absorber tubes. Saturated steam is generated in the first LFR
stage. The saturated steam output from the first stage is passed
through a separator 1306 (e.g. a steam drum, a steam accumulator,
one or more baffles, or a cyclone separator). Water collected in
the separator is circulated back to the inlet of the first stage.
Steam collected from the separator is provided as input into the
input of an elevated receiver 1307 in the second LFR stage 1302. A
field of linear Fresnel reflectors 1307 direct reflected solar
radiation to the elevated receiver 1309 to generate superheated
steam in the second stage. The elevated receiver 1309 can comprise
a single absorber tube, multiple absorber tubes, or one or more
multi-pass absorber tubes. The superheated steam can optionally be
passed through a separator 1308 to generate a higher quality of
steam. In some variations, the second LFR stage can be replaced in
whole or in part by an external heat source such as a coal fired or
natural gas fired burner. It may be possible to bypass the second
stage if superheated steam is not desired as indicated in the
figure by line 1310. After the superheated steam is used (e.g. to
drive a turbine 1311 to generate electric power), the turbine
exhaust may be sent to a condenser (not shown), from which
condensate may be collected and fed back into the first stage. As
the second stage is configured for generating superheated steam, a
different number, different diameter, or different composition of
tubes may be used in the second stage as compared to the first
stage. The second LFR system may have fewer, larger diameter tubes
that may also be shorter in length, if desired, than tubes of the
first LFR stage. A ratio of the diameter of the tubes in the second
LFR stage generating superheated steam to the diameter of tubes in
the first LFR stage may be greater than one, e.g. the ratio may be
at least 1.5, at least 2, at least 3, at least 4, at least 5, or
even larger, e.g. about 10. If the first stage LFR system utilizes
10 parallel-connected 2-inch diameter carbon steel absorber tubes,
the second stage LFR system may utilize 5 4-inch diameter absorber
tubes. Any one of or any combination of the control systems and
methods described herein can be used with the first LFR stage
and/or the second LFR stage.
[0129] FIG. 13B illustrates a multi-pass configuration of LFR
system 1300 shown in FIG. 13A. In particular, LFR system 1320 of
FIG. 13B comprises a first multi-pass LFR stage 1321 in series with
a second multi-pass LFR stage 1324. First multi-pass LFR stage 1321
may comprise one or more multi-pass absorber tubes 1322. Saturated
steam is generated in the first multi-pass LFR stage 1321 and
output through a separator 1326 (e.g. a steam drum, a steam
accumulator, one or more baffles, or a cyclone separator). Water
collected in the separator is circulated back to the inlet of the
first stage using circulation pump 1327. A blowdown valve 1330 may
be included to allow for draining and/or purging of contaminates
(e.g., particulates, scum, and the like) from the system. Steam
collected from the separator is provided as input to second
multi-pass LFR stage 1324. Additionally, feed-water may be input
into second multi-pass LFR stage 1324 at 1328. Second multi-pass
LFR stage 1324 may comprise one or more multi-pass absorber tubes
1325. In some variations, the second multi-pass LFR stage 1324 may
be replaced in whole or in part by an external heat source such as
a coal fired or natural gas fired burner. Second LFR stage 1324 may
output superheated steam at 1329 and provide heated water to
circulation pump 1327 to be fed back into first multi-pass LFR
stage 1321. The superheated steam can optionally be passed through
a separator to generate a higher quality of steam. As the second
stage is configured for generating superheated steam, a different
number, different diameter, or different composition of tubes may
be used in the second stage as compared to the first stage. The
second multi-pass LFR system may have fewer, larger diameter tubes
that may also be shorter in length, if desired, than tubes of the
first LFR stage. It may be possible to bypass the second stage if
superheated steam is not desired. Any one of or any combination of
the control systems and methods described herein can be used with
the first multi-pass LFR stage and/or the second multi-pass LFR
stage.
[0130] With reference to FIG. 14, an LFR system is detailed in
which superheated steam is generated in a single stage LFR
receiver. In such a configuration, as detailed here and elsewhere
throughout, the absorber tube or tubes or at least the portion
carrying superheated steam, will be configured for use with
superheated steam. As illustrated in FIG. 14, system 1400 comprises
a field 1401 of linear Fresnel reflectors directing solar radiation
to an elevated receiver 1402, and each row of reflectors rotating
about a single axis to track diurnal motion of the sun. In one
variation, the elevated receiver 1402 comprises multiple
parallel-connected absorber tubes, and each absorber tube is of
sufficient length, and receives sufficient thermal input to
generate superheated steam therein at a desired temperature and
pressure. In another variation, the elevated receiver 1402
comprises one or more multi-pass absorber tubes, and each absorber
tube is of sufficient length, and receives sufficient thermal input
to generate superheated steam therein at a desired temperature and
pressure. Superheated steam from the receiver 1402 can be used
directly as process steam, or can be used to drive a turbine 1403
to generate electric power. In one such variation, the portion of
the absorber tubes near the inlet may be of a different number,
different diameter, different composition, different type, and/or
different wall thickness than those at the more distant end from
the inlet, where the temperature and pressure are that required to
produce superheated steam. A skilled artisan would recognize that
adapters or other piping configurations may be assembled as
transition section between differing absorber tubes. Any one of or
any combination of the control systems and methods described herein
can be used with a single stage superheated steam generator, e.g.
as illustrated and described in connection with FIG. 14.
[0131] Any of the control systems described herein may employ
additional sensors. For example, multiple temperature sensors may
be positioned at spaced apart locations along a length of an
absorber tube. One or more flow rate sensors may be used to measure
flow rate of liquid and/or vapor within an absorber tube. One or
more pressure sensors may be used to monitor pressure along the
length of a tube, in a steam drum or accumulator, or in a
recirculation system.
[0132] In some situations, it may be desirable to measure
temperature, pressure, or flow rate differences between adjacent
ones of absorber tubes in a multi-tube receiver, adjacent ones of
absorber tubes in a multi-pass receiver having two or more absorber
tubes, or adjacent ones of segments in a multi-pass receiver. For
example, it may be desired to position temperature sensors at the
same or approximately the same location along multiple tubes in a
multi-tube receiver or multi-pass receiver having two or more
absorber tubes or along multiple segments in a multi-pass receiver
to map out a temperature profile transversely and longitudinally.
Various arrangements of sensors in a multi-tube receiver are
provided in FIGS. 15A-15D. With reference to FIG. 15A, each
absorber tube 1501 contains sensors 1502 positioned at both termini
of each absorber tube and at approximately equal distances
throughout the length of the absorber tube. As such, a measurement
of temperature, flow rate, and/or pressure may be taken at the
inlet and outlet and throughout the length of the receiver. In FIG.
15B, an arrangement is provided which is similar to that in FIG.
15A, except that no sensors at the absorber tube termini are
included. In FIG. 15C, a configuration is provided in which
adjacent pairs of absorber tubes have sensors at approximately the
same longitudinal positions. In FIG. 15D, a configuration is
provided in which the innermost absorber tubes in the receiver,
which may be susceptible to receiving more reflected incident solar
radiation than those positioned at the outer edges of the receiver,
are fitted with more sensors than the outer absorber tubes. While
FIGS. 15A-15D illustrate the placement of contact sensors on
multi-tube receivers, one of ordinary skill will appreciate that
principals described with respect to FIGS. 15A-15D may similarly be
applied to absorber tubes of a multi-pass receiver.
[0133] As stated above, any of the control systems described herein
can be employed in conjunction with a LFR system employing one or
more multi-tube receivers or multi-pass receivers. A reflector
field that may be employed includes the LFR array described in U.S.
patent application Ser. No. 10/597,966 entitled "Multi-tube solar
collector structure," filed Feb. 17, 2005 and U.S. patent
application Ser. No. 12/012,829 entitled "Linear Fresnel Solar
Arrays and Receivers Therefor," filed Feb. 5, 2008, each of which
is incorporated by reference herein in its entirety, specifically
with respect to the LFR reflector fields detailed therein. For
example, referring now to FIGS. 16-17, a linear Fresnel reflector
1600 may comprise a space frame 1601 to which multiple mirror
panels 1602 are adhered. The mirrors may be flat, or may have a
parabolic cross-section. In some variations, multiple support hoops
1603 connect to the space frame and engage mounted wheels that
allow the hoops and therefore the reflector to rotate about a
longitudinal axis parallel to an elevated receiver 1604. In other
variations, other types of rotatable supports are used to rotate
and position a reflector, e.g. supports that do not extend
substantially above the reflective surface of the mirror, such as
those described in U.S. Pat. No. 5,899,199 to Mills, which is
incorporated by reference herein in its entirety. A reflector may
be rotated and positioned using a single motor and drive that
drives one or more of the reflector supports, e.g. hoops. For
example, a single motor and drive may rotate and position a group
of ganged together reflectors, e.g. as described and illustrated in
connection with FIGS. 21-23. In some variations, a relatively
horizontal reflector may be positioned directly beneath the
receiver 1604 to provide an image on the receiver that is parallel
to the reflector and has relative constant transverse illuminance
across the receiver. FIG. 18 illustrates an example of positioning
of reflectors in an LFR array to illuminate a receiver distant from
the receiver closest to the LFR array.
[0134] Examples of multi-tube receivers that may be used in an LFR
array are described in U.S. patent application Ser. No. 10/597,966
entitled "Multi-tube Solar Collector Structure," and filed Feb. 17,
2005 and in U.S. patent application Ser. No. 12/012,829 entitled
"Linear Fresnel Solar Arrays and Receivers Therefor," and filed
Feb. 5, 2008, each of which is incorporated by reference herein in
its entirety.
[0135] Another example of a receiver that can be used, e.g. for
generating superheated steam, is provided in FIG. 20. There,
receiver 2000 comprises 5 tubes 2001 parallel to one another along
the length of the receiver. The tubes may, for example, have a 4''
diameter. Other variations are contemplated in which 10 parallel
tubes are used in a receiver, where each tube has a 2'' diameter or
a 1.5'' diameter. In the example illustrated in FIG. 20, the tubes
2001 are supported by a series of rollers 2002, spaced apart along
the length of the tubes. Other variations are contemplated in which
a single roller 2002 is replaced with a set of coaxial,
independently rotating rollers, where each roller in the set
supports a single tube to accommodate differential thermal
expansion of the multiple tubes. The tubes 2001 are housed in a
trapezoidal cavity 2003, with the bottom surface of the cavity
provided by a window 2006 selected to transmit solar radiation. The
trapezoidal cavity, tubes, and rollers 2002 are supported by a
frame 2004. A protective roofing 2005 is provided over the
frame.
[0136] The tubes in the receiver undergo thermal expansion that
must be accommodated. Rollers on which the tubes reside may allow
the tubes to expand and contract without damaging a coating on the
tubes. Rollers may assume a contoured shape, a "V" shape, or a "U"
shape to help guide individual tubes along a linear path and avoid
unwanted lateral deflection that might cause adjacent tubes to
damage each other. Tubes may be clamped e.g. to the receiver
housing or to another fixed support structure at or near the tubes'
midpoints, while ends of the tube remain free to move. This
configuration allows both ends of the tubes to move and limits the
extent of thermal expansion to half that which would have to be
accommodated if an end of a tube was anchored.
[0137] Piping connecting to ends of tubes, and downcomer piping
sections that transport steam from the receiver to ground level may
be designed to accommodate thermal expansion. For example, as
illustrated in FIG. 19, tubes 1900 may be connected to a piping
section 1902 that comprises one or more hairpin thermal expansion
sections 1901. The curved portions of the hairpin section may
increase and decrease in radius as needed to accommodate the linear
expansion and contraction of the tubes 1900. The hairpin may have
one large curved region generally shaped in the form of a question
mark, or the hairpin may comprise multiple curved sections and one
or more straight sections. Movement of one or more portions of
piping (e.g. as illustrated in FIG. 19) can be used to determine a
change in length, which can be, in turn, used as a process control
variable for any of the control schemes described herein.
[0138] Using any of the control systems and methods described
herein, the steam quality may be controlled to any desired quality,
e.g. about 0.3, about 0.4, about 0.5, about 0.6, about 0.7, about
0.8 or about 0.9. In some cases, superheated steam may be produced.
The steam quality within an individual tube may be controlled to
+/-about 20%, about 15%, about 10%, about 5%, or even better, e.g.
about 2%. The steam quality for a multi-tube receiver may be
controlled to about +/-20%, about 15%, about 10%, about 5%, or even
better, e.g. about 2%. For example, in some systems steam may be
controlled within the individual tubes to a desired steam quality
(e.g. 70%) within +/-10% in operation, and to within about +/-5%
for an overall multi-tube receiver, e.g. a multi-tube receiver
comprising 10 parallel 1.5 inch diameter carbon steel tubes.
[0139] Superheated steam may be produced by variations of systems
and methods described herein at a temperature of at least about
370.degree. C., at least about 371.degree. C., at least about
372.degree. C., at least about 373.degree. C., at least about
374.degree. C., at least about 375.degree. C., at least about
380.degree. C., or about 390.degree. C. or higher, or a temperature
in a range from about 370.degree. C. to about 380.degree. C., or
about 370.degree. C. to about 390.degree. C., or about 370.degree.
C. to about 400.degree. C. In some variations, superheated steam
may be produced at somewhat lower temperatures, e.g. in a range
from about 350.degree. C. to about 370.degree. C., or in a range
from about 350.degree. C. to about 360.degree. C., or in a range
from about 360.degree. C. to about 370.degree. C. such as about
369.degree. C. or lower, or about 365.degree. C. or lower. In yet
other variations, superheated steam may be produced up to
temperatures of about 580.degree. C. While specific temperature
ranges are described, it should be appreciated that steam having
any temperature may be produced depending on the desired
application.
* * * * *