Patent Application
20140299122
The embodiments disclosed herein are directed toward control optimization methods and apparatus for thermal energy storage. The disclosed embodiments are more particularly directed toward control optimization for thermal energy storage in a cascaded phase change material thermal energy storage system associated with a concentrated solar power generation system.
Many electrical power providers are incorporating concentrated solar power generation facilities into their mix of electricity sources. In these facilities concentrated solar energy provides the heat required to drive conventional steam turbines for power generation. Most existing concentrated solar power generation facilities are operated only when the sun is not obscured by cloud cover and is sufficiently positioned above the horizon to provide adequate light for plant operation. Thus, many existing concentrated solar power generation facilities can not operate in the evening or in periods of intermittent cloud cover.
Shifting plant operation away from strict solar dependence has many economic benefits, including a potentially extended operational period each day. To properly operate through periods of cloud cover or in the evening, a plant must have the ability to store energy in some form at a low cost. Thermal energy storage is the most economically feasible way for a plant to accomplish the required energy storage. To date, many forms of thermal energy storage have been investigated, including: two tank, thermocline, chemical, solid media, and phase change material storage. Presently, no one technology has emerged as a dominant storage strategy. On the contrary, each technology has recognized advantages and disadvantages. Phase Change Material (PCM) based thermal energy storage systems are of great interest for high temperature concentrated solar power applications because of the potential for enhanced performance at relatively low material cost.
The basic phase change material thermal energy storage concept features the use of a material with a melting temperature in between the hot and cold side temperatures of a solar field as a thermal energy storage medium. When the system is operated in a “charge” mode, heat transfer fluid from the solar field is cooled by melting the phase change material. In a “discharge” mode, relatively cool heat transfer fluid is heated by running it in reverse through the thermal energy storage system thus solidifying the phase change material. The benefit of a phase change material based system is the high energy density realized by exploiting the latent heat of a suitable material in addition to utilizing the sensible heat. The energy storage density of a suitable energy storage material can typically be doubled by adding latent heat storage over a 100° C. temperature range.
Phase change material based thermal energy storage systems must include multiple types of salts with different melt temperatures to effectively store and discharge energy over a temperature range of 100° C. or more. In a multiple-material design, the total amount of energy that can be stored for a given storage mass over the 100° C. temperature differential can be greatly increased. The forgoing arrangement of linearly arrayed phase change material groups, (with each group or container of a given phase change material being known as a “bucket”) is called a cascade and can be thought of like a cascading waterfall, with the highest melt temperature at the top followed by progressively lower melt temperatures to the bucket at the bottom.
A phase change material thermal energy storage system having a sufficient number of buckets provides for energy storage at the highest temperature possible. A theoretical best case phase change material system would have an exceptionally large number of phase change material buckets with different melt temperatures spread equally through the range of expected heat transfer fluid temperatures. Implementing an exceptionally large number of distinct phase change material buckets is not practical however, in part because there are a limited number of suitable phase change material choices. It is generally more feasible to utilize 3-5 phase change materials with melt temperatures spread as evenly as possible throughout the designed storage temperature range.
Nearly all thermal energy storage systems can be described as belonging to one of two categories: active and passive. An active system is classified as a system that actively engages its storage material with the system's heat transfer fluid, typically through mechanical interactions. For example, a two-tank molten salt system is classified as an active system because the molten salt is actively pumped. Passive systems do not have mechanical interaction. A common example of a passive system is concrete storage where the storage material encases heat transfer fluid pipes and passively accepts and gives thermal energy to the working fluid. A phase change material thermal energy storage system as described above is a type of passive storage system.
Certain physical limitations cause difficulty controlling a passive phase change material storage system for optimal transient performance. First, the salts used as phase change materials have very low heat transfer rates compared to the heat transfer fluid. The lower heat transfer rate of a phase change material occurs in part because the material is stationary and also because suitable phase change materials conduct heat poorly. Low heat transfer rates cause power output from the storage system to be lower even if the total energy storage is large. Second, phase change materials accept and release heat isothermally over the melting region whereas heat transfer fluid accepts and releases heat over a range of temperatures. Therefore, in a manageable system of three to five phase change material buckets in a cascade, the highest temperature bucket will have a substantially lower temperature than the maximum heat transfer fluid temperature.
In addition, day-to-day repeatability presents a significant difficulty in the operation of a passive thermal energy storage system. Problems arise from driving temperature differences during charge and discharge in combination with variable solar field outlet temperature and variation in heat transfer fluid flow rates. For example, in a cascading phase change material system with four phase change material buckets, a third bucket may have a driving temperature difference of nearly 30° C. during charge compared to only a 10° C. temperature difference during discharge. These temperature differences are constrained by the availability of materials with desired melt temperatures. Furthermore, a bucket sees a varying mass heat transfer fluid flow rate that may fluctuate between 0 kg/s and a maximum rate during charge operation compared to a constant mass flow rate at or near the maximum during discharge. In addition, the heat transfer characteristics for a given phase change material salt are different for charge and discharge. These and potentially other factors combine to make the charging transient response of any system quite different from that of the discharge transient response.
The foregoing considerations become a problem when attempting to design a thermal energy storage system that will properly exploit the beneficial energy characteristics of phase change in nearly 100% of the phase change material provided. For example, a system that is able to melt 100% of the phase change material during charge may only be able to solidify 50% of the phase change material during discharge. The next day, this system might melt the remaining 50% solid during charge and continue to superheat the phase change material sensibly during the remainder of the charge. Now, during discharge the system will only be able to solidify 25% of the phase change material. This process will continue until only a small portion of the phase change material is going through a phase change every day. Thus, the storage system has lost a significant portion of its energy storage density.
The embodiments disclosed herein are directed toward overcoming one or more of the problems discussed above.
One embodiment is a solar power generation system including a heat transfer fluid circuit, a solar energy concentrator and a thermal energy storage system. The thermal energy storage system comprises a cascaded series of multiple buckets of phase change material all in thermal communication with the heat transfer fluid circuit. In this embodiment an outlet from one of the buckets of the thermal energy storage system is in direct communication through a secondary branch of the heat transfer fluid circuit with an inlet into a power block steam train component. The secondary branch provides for the routing of some or all of the heat transfer fluid flowing from the solar field to the power block through a storage bucket during active energy production.
In this embodiment, the bucket in direct thermal communication with the power block may be a high temperature bucket containing a phase change material that has a melting temperature greater than the phase change materials contained in other buckets of the cascaded series.
A related embodiment is a cascaded thermal energy storage system having multiple buckets of phase change material connected in series by a heat transfer fluid circuit. This embodiment further includes a secondary branch of the heat transfer fluid circuit connecting an outlet of one or more buckets directly to a power block inlet. The foregoing connection is made through the secondary branch while producing energy within the power block.
Another related embodiment is a method of utilizing solar energy comprising the following steps; providing a heat transfer fluid circuit, a solar energy concentrator, a cascaded thermal energy storage system and a power block all connected by a primary heat transfer fluid circuit. The method further includes flowing heat transfer fluid from the solar energy concentrator outlet through a bucket of phase change material and then flowing heat transfer fluid from the bucket through a secondary branch of the heat transfer fluid circuit to the power block while producing energy with the power block. In the foregoing embodiment, the bucket may be a high temperature bucket containing a phase change material that has a melting temperature greater than the phase change materials contained in other buckets of the cascaded series.
Another embodiment is a solar power generation system generally as described above but further comprising an inlet to one or more selected buckets of the thermal energy storage system in direct communication through a secondary branch of the heat transfer fluid circuit with an outlet from a power block component. In addition, an outlet from the one or more selected buckets is in direct communication through the heat transfer fluid circuit with the solar energy concentrator or the power block. This configuration provides for heat transfer fluid flow to be preheated after at least one bucket of the thermal energy storage system has been substantially discharged but before the thermal energy system is recharged. In this embodiment, the buckets of the thermal energy storage system in communication with the power block outlet may be colder temperature buckets containing a phase change material that has a lower melting temperature than the phase change materials contained in at least one other bucket of the cascaded series.
A related embodiment includes a cascaded thermal energy storage system as generally described above but further comprising at least one secondary branch of the heat transfer fluid circuit connecting an outlet from the power block to the inlet to one or more buckets of the cascaded thermal energy storage system.
A related embodiment includes a method of preheating a solar energy system comprising the step of flowing heat transfer fluid from a power block outlet through one or more partially discharged buckets of the thermal energy storage system prior to charging the thermal energy storage system. The method thus provides for preheating heat transfer fluid which may then be flowed to the solar energy concentrator and the power block before active power generation commences.
An alternative embodiment includes a solar power generation system as generally described above but further comprising multiple secondary heat transfer fluid circuit branches directly connecting at least two buckets of the thermal energy storage system to at least two corresponding steam train components. The secondary heat transfer fluid branches provide for direct heat transfer fluid injection between individual phase change material buckets and corresponding steam train components during the discharge phase of power generation. In this embodiment, the melting temperature of the phase change material in each bucket may correspond to the designed operating temperature of the corresponding steam train component.
A related embodiment includes a cascaded thermal energy storage system as generally described above but further comprising multiple secondary branches of the heat transfer fluid circuit connecting outlets from at least two buckets to inlets to at least two steam train components during discharge of the thermal energy storage system. In this embodiment, the melting temperature of the phase change material in each bucket may correspond to the designed operating temperature of the corresponding steam train component.
A related embodiment includes a method of utilizing solar energy comprising the step of flowing heat transfer fluid from at least two selected buckets of phase change material to the inlet of at least two corresponding steam train components while discharging the thermal energy storage system. This method provides for direct heat transfer fluid injection between individual phase change material buckets and corresponding steam train components during the discharge phase of power generation.
Another embodiment includes a solar power generation system generally as described above but further comprising secondary branches of the heat transfer fluid circuit connecting one or more buckets of the thermal energy storage system with an outlet from the power block and further connecting the one or more buckets with the solar energy concentrator. This embodiment provides for heat transfer fluid flowing in the heat transfer fluid circuit to be heated by partial discharge of the thermal energy storage system during periods of insufficient insolation to charge the thermal energy storage system.
In the foregoing embodiment, the one or more buckets of the thermal energy storage system in communication with the power block outlet may be colder temperature buckets containing a phase change material that has a lower melting temperature than the phase change materials contained in other buckets of the cascaded series.
A related embodiment includes a cascaded thermal energy storage system generally as described above but further comprising one or multiple secondary branches of the heat transfer fluid circuit connecting the power block to an inlet to one or more buckets during periods of insufficient insolation to charge the thermal energy storage system.
A related embodiment is a method of utilizing solar energy comprising the step of partially discharging the thermal energy storage system during periods of insolation too low to charge the thermal energy storage system by flowing heat transfer fluid from the power block outlet through one or more buckets of the thermal energy storage system.
Another embodiment includes a solar power generation system as generally described herein comprising any combination of secondary branches of the heat transfer fluid circuit extending between selected phase change material buckets and selected steam train components and/or any combination of secondary branches of the heat transfer fluid circuit extending between selected phase change material buckets and the solar field.
A related embodiment is a cascaded thermal energy storage system generally as described above further comprising any combination of secondary branches of the heat transfer fluid circuit extending between selected phase change material buckets and selected steam train components and/or any combination of secondary branches of the heat transfer fluid circuit extending between selected phase change material buckets and the solar field.
Unless otherwise indicated, all numbers expressing quantities of ingredients, dimensions, reaction conditions and so forth used in the specification and claims are to be understood as being modified in all instances by the term “about”.
In this application and the claims, the use of the singular includes the plural unless specifically stated otherwise. In addition, use of “or” means “and/or” unless stated otherwise. Moreover, the use of the term “including”, as well as other forms, such as “includes” and “included”, is not limiting. Also, terms such as “element” or “component” encompass both elements and components comprising one unit and elements and components that comprise more than one unit unless specifically stated otherwise.
A conventional concentrated solar energy power generation system 100 is schematically illustrated in
The solar power generation system 100 of
The solar energy concentrator elements 102 may be of any known type, including but not limited to, parabolic trough reflectors, heliostat based solar energy towers or similar apparatus. In all cases the solar concentrator element 102 concentrates reflected sunlight upon the surface of a tube or other receiver structure within which heat transfer fluid is circulated. The heat transfer fluid is thus heated by the concentrated sunlight to a temperature sufficient to drive a steam turbine generator as described below.
In the various embodiments disclosed herein, the solar energy concentrator 102, thermal energy storage system 104 and power block 106 are each maintained in thermal communication through a heat transfer fluid circuit 108. The heat transfer fluid circuit 108 as shown on
The power block 106 includes various steam train components 110 which provide for heat exchange between heat transfer fluid flowing in the heat transfer fluid circuit 108 and water flowing in a steam circuit 112. Typically, the power block 106 includes at least the following steam train components; a pre-heater 114, an evaporator 116 and a super-heater 118, arranged in order from lesser to greater operational temperature. In the various steam train components 110, heat is exchanged between the heat transfer fluid circuit 108 and the steam circuit 112 resulting in the production of super heated steam which may be used to drive a steam turbine 120 for power generation. It is important to note that a commercially implemented power block is substantially more complex than schematically illustrated in
The thermal energy storage system 104 includes a series of multiple buckets, each containing a phase change material having a selected melting temperature. In the schematic illustrations of
The buckets 122, 124, 126 are arranged in a cascade. As defined herein, a “cascade” or “cascaded” phase change material thermal energy storage system is one where the various phase change material buckets are arranged in a thermally decreasing series. For example, as shown on
The solar power generation system 100 may be operated in two modes with respect to the thermal energy storage system 104; charge mode and discharge mode. Operation in the charge mode is schematically represented in
Simultaneously, or alternatively, a portion of the heat transfer fluid in the heat transfer fluid circuit 108 may be routed through the thermal energy storage system 104. In the charge mode, heat transfer fluid flows first into the phase change material hot bucket 122, then into the medium temperature bucket 124 and finally into the coldest temperature bucket 126. In each bucket, heat exchange with the phase change material causes heat energy to be transferred to the phase change material. Ideally, heat is transferred to the phase change material until the phase change material becomes fully molten. Since the melting point of the material in the “hot” bucket 122 is higher than the melting point in the “medium” bucket 124 the somewhat cooled heat transfer fluid exiting bucket 122 still is sufficiently hot to melt the material in bucket 124 and so on. As noted above, it is typically not desirable to add a significant amount of additional sensible heat to a given bucket of phase change material after the phase change material contained therein has fully melted. Thus, when all phase change materials in all buckets have melted, the thermal energy storage system may be described as “charged” or fully charged. Upon exiting the coldest phase change material bucket 126, the then cooled heat transfer fluid may be routed back to the solar energy concentrator 102 for reheating by solar energy.
A solar energy power generation system 100 may be operated in charge mode at the discretion of the system operator provided sufficient insolation is available to heat the heat transfer fluid flowing through the solar energy concentrators 102 to a sufficiently high temperature to melt the phase change material in each bucket.
The thermal energy storage system 104 provides the system 100 with the ability to generate power for a period of time after the sun has set or when the sun is obscured by cloud cover. When the solar power generation system 100 is operated without solar input, the system is defined herein as being operated in a “discharge” mode. Operation of the basic system in the discharge mode is schematically illustrated in
As noted above, day-to-day repeatability presents a significant difficulty in the operation of a thermal energy storage system such as shown in
An alternative control improvement method and apparatus is schematically illustrated in
The startup period for a concentrated solar power plant is traditionally long, on the order of an hour. This period is required to warm up the turbines and the heat transfer fluid in the heat transfer fluid circuit pipes. Using the cold buckets to do some portion or all of the required preheating will allow the system 100 to begin power production earlier in the day, thus increasing total power output. In addition, this method and apparatus causes the cold buckets to become fully discharged so the thermal energy storage system 104 can more efficiently be charged during the day. Implementation of the method of pre-heating the colder buckets requires the addition of one or more secondary branches to the heat transfer fluid circuit, for example pipe 134 leading from the power block to bucket 126 and then on to the inlet 128 of the solar field or back to the power block. Although only one additional pipe 134 is shown on
An alternative control improvement method and apparatus is schematically illustrated in
Although the various buckets in the thermal energy storage system may be linked through heat transfer circuit inlet and outlet pipes with any of the steam train components, it is desirable to match the discharge temperature of a selected bucket with the optimum operating temperature of the corresponding steam train component. For example, the melting temperature of the phase change material in a given bucket may be approximately equal to the designed operating temperature of the corresponding steam train component.
As illustrated in
An alternative control improvement method and apparatus is schematically illustrated in
Each of the embodiments for enhanced thermal storage system control described above could be implemented alone, or in combination with other alternative embodiments. For example,
Various embodiments of the disclosure could also include permutations of the various elements recited in the claims as if each dependent claim was a multiple dependent claim incorporating the limitations of each of the preceding dependent claims as well as the independent claims. Such permutations are expressly within the scope of this disclosure.
While the embodiments disclosed herein have been particularly shown and described with reference to a number of alternatives, it would be understood by those skilled in the art that changes in the form and details may be made to the various configurations disclosed herein without departing from the spirit and scope of the disclosure. The various embodiments disclosed herein are not intended to act as limitations on the scope of the claims. All references cited herein are incorporated in their entirety by reference.
The instant application claims the benefit of U.S. Provisional Patent Application No. 61/558,275, filed Nov. 10, 2011, which application is incorporated herein by reference in its entirety.
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/US2012/060375 | 10/16/2012 | WO | 00 | 5/9/2014 |
| Number | Date | Country | |
|---|---|---|---|
| 61558275 | Nov 2011 | US |
