Patent Application
20110126824
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).
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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).
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.
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.
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.
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.
The systems and methods above may be configured in a linear Fresnel reflector array or in an array of trough collectors, as desired.
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.
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.
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.
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.
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.
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.
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 ti connected in parallel, and i=1, . . . , k, and irradiating each tube ti along it respective length Li 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 λi along its length. The methods comprising using one or more temperature measurements Ti in an economizer region of a tube ti 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.
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.
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.
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.
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.
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.
Variations of solar boilers comprise a receiver comprising multiple parallel tubes or multiple multi-pass tubes ti 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 TCi positioned to sense fluid temperature in an economizer region of each of the tubes ti and a controller, wherein output from each of the temperature sensors TCi is provided as input to the controller and used by the controller to adjust a position of the control valve associated with tube ti so as to control mass flow of water into the tube ti 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 ti and to control steam quality that exits the receiver.
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.
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.
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).
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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).
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 ½″, ¾″, or 1″ sizes. However, it should be appreciated by one of ordinary skill that valves of other types and sizes may be used.
Steam quality x is x=(h−hf)/hfg, where h is the enthalpy of the fluid produced, hf is the enthalpy of saturated liquid, and hfg=hg−hf, the difference between the enthalpy of the saturated vapor hg and hf. 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.
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
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 (Tfw) until it reaches a saturation temperature Tsat, 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 λ, which occurs when the bulk fluid is saturated liquid, where it contains the maximum amount of thermal energy that is possible before boiling.
The evaporator section (labeled B) begins after position λ. 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 γ 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.
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 γ. 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 λ, γ, and L may be measured from the inlet of the tube and along each segment.
Referring now to
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.
The illuminated length Lillum of an absorber tube can be measured, calculated, or estimated. One example of a calculation for Lillum 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 htube 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, htube 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 ldark for a reflector positioned directly beneath a tube is approximated by ldark=htube tan(ze) cos(az−rot+180). The illuminated length can be approximated as Lillum=L−ldark. 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 ldark 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).
Referring again to
In some variations, the thermal input {dot over (Q)}in may be relatively uniformly distributed along the length Lillum; that is, Lillum 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)}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.
Water supplied into the inlet 121 of tube 101 has a temperature Tin, enthalpy hin and mass flow {dot over (m)}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 Pdrum), from which a dry steam flow 125 having a mass flow {dot over (m)}steam and enthalpy hg 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 Ldrum in the drum. Recirculated water flow 107 may be extracted from the separator with a mass flow {dot over (m)}recirc and enthalpy hf. Feedwater flow 109 with a mass flow {dot over (m)}feed and enthalpy hfeed may be mixed with the recirculated water flow 107 to provide input into the tube 101.
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.
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
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
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.
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
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.
An example of a control system is illustrated in
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 kth tube may be used as control input for a different tube (not the kth 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 kth 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.
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
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 Tset can be determined using any suitable method. (Note that Tset 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 kth tube is referred to as Tset,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 htarget of the fluid that exits the tube, and the illumination length of the tube Lillum (which may in certain variations be essentially the same as the physical length L of the tube as described above). The temperature set point Tset can be such that
where ht arg et=hf+xt arg ethfg, xt arg et=x+x_bias, and hfg refers to the enthalpy required to change from a saturated liquid to a saturated vapor (hg−hf), hg refers to the enthalpy of saturated vapor, cp refers to the heat capacity of fluid under the operating conditions, Tin is the temperature of the water at the tube inlet, lTC 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 (l1 and l2) and the change in temperature between the locations l1 and l2 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 (Tset,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_biask for that individual tube could be used in determining a set point.
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
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
In some variations, it may be desired to estimate steam quality
where {dot over (m)}steam is the mass flow of steam from a steam drum or accumulator and {dot over (m)}in is the mass flow of water into the steam generator, to compare such estimated steam quality with target steam quality xtarget, 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.
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
In some variations, thermal input for each tube {dot over (Q)}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 Lk, and an illumination length Lillum,k which may or may not be the same as the physical length Lk 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 Lk. 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 Lk. The total thermal input to a multi-tube receiver is given by {dot over (Q)}in=Σ1k{dot over (Q)}in,k.
A mass flow of water {dot over (m)}in at a temperature Tin (and having a heat enthalpy hin) 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 CVk 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)}in,k, such that {dot over (m)}in=Σ1k{dot over (m)}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.
Although the particular example illustrated in
Referring back to
Temperature measured in the economizer region by TCk 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.
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
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
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
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.
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.
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
The energy stored in the volume is Estored=μfmwater+μgmsteam+Cp,steelmsteelTsteel, where μf=specific internal energy of water under the operating conditions, mwater=mass water, μg=specific internal energy of steam under the operating conditions, msteam=mass steam, cp,steel=heat capacity of the drum material (e.g. steel), msteel=mass drum, and Tsteel=drum temperature. The change in stored energy with respect to time is
Accordingly, the thermal input for a tube of length L can be estimated during steam production with the steam valve open as
At steady state, when operating at constant pressure in the steam drum, the estimated thermal input is
During warm-up with the steam valve closed
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.
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
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.
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
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
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.
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.
In some variations, it may be desired to control both mass flow and thermal input into one or more tubes. Referring back to
Examples of drive systems for rotating reflector rows or segments of reflector rows are provided in
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.
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
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
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
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
With reference to
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.
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
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
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.
Another example of a receiver that can be used, e.g. for generating superheated steam, is provided in
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.
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
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.
Superheated steam may be produced by variations of systems and methods described herein at a temperature of at least about 370° C., at least about 371° C., at least about 372° C., at least about 373° C., at least about 374° C., at least about 375° C., at least about 380° C., or about 390° C. or higher, or a temperature in a range from about 370° C. to about 380° C., or about 370° C. to about 390° C., or about 370° C. to about 400° C. In some variations, superheated steam may be produced at somewhat lower temperatures, e.g. in a range from about 350° C. to about 370° C., or in a range from about 350° C. to about 360° C., or in a range from about 360° C. to about 370° C. such as about 369° C. or lower, or about 365° C. or lower. In yet other variations, superheated steam may be produced up to temperatures of about 580° C. While specific temperature ranges are described, it should be appreciated that steam having any temperature may be produced depending on the desired application.
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.
| Number | Date | Country | |
|---|---|---|---|
| 61216253 | May 2009 | US | |
| 61216878 | May 2009 | US |
