Abstract:
Concentrating solar power (CSP) systems and methods are disclosed featuring the use of a solid-liquid phase change heat transfer material (HTM). The systems and methods include a solar receiver configured to receive concentrated solar flux to heat a quantity of the solid HTM and cause a portion of the solid HTM to melt to a liquid HTM. The systems and methods also include a heat exchanger in fluid communication with the solar receiver. The heat exchanger is configured to receive liquid HTM and provide for heat exchange between the liquid HTM and the working fluid of a power generation block. The heat exchanger further provides for the solidification of the liquid HTM. The systems and methods also include a material transport system providing for transportation of the solidified HTM from the heat exchanger to the solar receiver.

Description:
RELATED APPLICATIONS 
       [0001]    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. 
     
    
     TECHNICAL FIELD 
       [0002]    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. 
       BACKGROUND 
       [0003]    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. 
         [0004]    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. 
         [0005]    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. 
         [0006]    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. 
         [0007]    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. 
         [0008]    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&#39;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. 
         [0009]    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. 
         [0010]    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. 
         [0011]    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. 
         [0012]    The embodiments disclosed herein are directed toward overcoming one or more of the problems discussed above. 
       SUMMARY OF THE EMBODIMENTS 
       [0013]    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. 
         [0014]    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. 
         [0015]    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. 
         [0016]    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. 
         [0017]    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. 
         [0018]    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. 
         [0019]    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. 
         [0020]    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. 
         [0021]    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. 
         [0022]    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. 
         [0023]    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. 
         [0024]    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. 
         [0025]    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. 
         [0026]    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. 
         [0027]    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. 
         [0028]    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. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0029]      FIG. 1  is schematic diagram representation of a prior art concentrated solar power generation system operating in charge mode. 
           [0030]      FIG. 2  is schematic diagram representation of a prior art concentrated solar power generation system operating in discharge mode. 
           [0031]      FIG. 3  is schematic diagram representation of an improved concentrated solar power generation system operating to produce energy. 
           [0032]      FIG. 4  is schematic diagram representation of an improved concentrated solar power generation system during warm-up operations prior to charging and after discharge. 
           [0033]      FIG. 5  is schematic diagram representation of an improved concentrated solar power generation system operating in discharge mode. 
           [0034]      FIG. 6  is schematic diagram representation of an improved concentrated solar power generation system operating in a partial discharge mode. 
           [0035]      FIG. 7  is schematic diagram representation of a concentrated solar power generation system featuring a combination of improvements. 
       
    
    
     DETAILED DESCRIPTION 
       [0036]    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”. 
         [0037]    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. 
         [0038]    A conventional concentrated solar energy power generation system  100  is schematically illustrated in  FIGS. 1 and 2 . Various embodiments of solar powered generation systems having enhanced thermal energy storage control methods and apparatus are disclosed herein and illustrated in  FIGS. 3-7 . The enhanced thermal energy storage embodiments disclosed herein are improvements upon the basic design of  FIGS. 1 and 2   
         [0039]    The solar power generation system  100  of  FIGS. 1 and 2  may be considered to have multiple functional blocks including; one or more solar energy concentrators  102 , one or more thermal energy storage systems  104  and one or more power blocks  106 . A commercially implemented solar power generation system  100  will generally have many solar energy concentrators  102  in a solar field for each thermal energy storage system  104  or power block  106 . 
         [0040]    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. 
         [0041]    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  FIGS. 1 and 2  is referred to herein as the primary heat transfer fluid circuit. The heat transfer fluid circuit  108  has heat transfer fluid flowing within pipes, valves, pumps, heat exchange elements and other structures of the circuit  108 . The heat transfer fluid flowing in the circuit  108  is typically heat transfer oil or other liquid having appropriate chemical, thermal and physical qualities. 
         [0042]    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  FIG. 1 . 
         [0043]    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  FIGS. 1-7 , phase change material buckets  122 ,  124 , and  126  respectively are illustrated in a cascaded series. It is important to note however, that a commercially implemented thermal energy storage system may have more than or less than  3  buckets. In addition, a commercially implemented system may have multiple containers of any shape or size holding a specific type of phase change material. In this case, each collection of interlinked containers holding the same phase material constitutes one functional bucket. 
         [0044]    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  FIG. 1 , the phase change material bucket  122  nearest the outlet from the solar field is designated as a “hot” phase change material bucket. This bucket contains a phase change material having a melting point temperature higher than the phase change materials contained in other buckets in the series. A heat transfer fluid circuit outlet from the hot phase change material bucket  122  leads to an inlet to a medium phase change material bucket  124 . An outlet from the medium temperature phase change material bucket  124  leads to a cold bucket  126  and so on until a complete cascade from the highest temperature bucket to lowest temperature bucket is complete. In each bucket, relatively simple or more complex heat exchange apparatus provides for heat exchange between the heat transfer fluid flowing in the heat transfer fluid circuit  108  and the phase change material contained within each bucket. 
         [0045]    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  FIG. 1 . In the charge mode, incident solar radiation falling upon a solar energy concentrator  102  is concentrated by reflection upon a portion of the heat transfer fluid circuit  108  flowing through or near the concentrator. Thus, in charge mode relatively cooler heat transfer fluid enters a solar field inlet  128  in the heat transfer fluid circuit  108  and flows to a solar field outlet  130  on the opposite side of a solar energy concentrator  102  while being heated by concentrated sunlight. Upon exit from the solar energy concentrator  102  the heated heat transfer fluid is routed to the power block  106  and/or the thermal energy storage system  104 . In the power block  106 , the heat transfer fluid flows through various steam train components to create super-heated steam for power generation as described above. The cooled heat transfer fluid is then returned in the heat transfer fluid circuit  108  to the solar field for additional heating. 
         [0046]    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. 
         [0047]    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. 
         [0048]    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  FIG. 2 . As shown in  FIG. 2 , heat transfer fluid flowing in the heat transfer fluid circuit  108  flows through the steam train components  110  in the same direction to accomplish the same steam and power production steps described above. In discharge mode however, the high temperature heat transfer fluid is obtained by flowing cooled heat transfer fluid in reverse order through the cascaded buckets of phase change material. In particular, cooled heat transfer fluid is flowed through the cold phase change material bucket  126 , the medium temperature phase change bucket  124  and the hot phase change material bucket  122  in that order. As the phase change material in each bucket solidifies, heat is transferred to the heat transfer fluid. When all phase change materials in all buckets have solidified, the thermal energy storage system may be described as fully “discharged” and typically it is inefficient or impossible to further extract sensible heat from the system for additional power generation. 
         [0049]    As noted above, day-to-day repeatability presents a significant difficulty in the operation of a thermal energy storage system such as shown in  FIGS. 1 and 2 . In particular, the charging transient response of the system is quite different from that of the discharge transient response. This become a problem when attempting to design a system such as that illustrated in  FIG. 1  and  FIG. 2  that will properly exploit the beneficial energy characteristics of phase change in nearly 100% of the phase change material provided. 
         [0050]      FIG. 3  schematically illustrates an improved method and apparatus for optimizing the control of a thermal energy storage system  104  to improve transient performance. The enhanced method illustrated in  FIG. 3  includes routing some or all of the heat transfer fluid flow from the solar field through a phase change material bucket before the heat transfer fluid is routed to the power block  106 . This re-routing occurs during active energy production. In particular, some or all of the heat transfer fluid taken from the solar field outlet  130  over a selected period of time may be routed through the hot phase change material bucket  122  before sending it to the power block. When this strategy is employed, the hot bucket  122  can charge fully even when the charge driving temperature difference is much lower than the discharge driving temperature difference. Implementation of the control improvement strategy illustrated in  FIG. 1  requires at least one secondary branch to the heat transfer fluid circuit, for example an additional pipe  132  or other conduit and associated valves be added to the heat transfer fluid circuit between an outlet from a bucket, for example, hot bucket  122  and the inlet of the steam train  110 , for example before the super-heater element  118 . 
         [0051]    An alternative control improvement method and apparatus is schematically illustrated in  FIG. 4 . This embodiment includes preheating the system  100  in the morning or when the system is otherwise cold by more fully discharging one or more relatively colder temperature buckets, for example bucket  126 . Because the melt temperatures of the one or more cold buckets are too low to heat the heat transfer fluid sufficiently to run the power block after the hot bucket  122  has fully discharged, the thermal energy storage system  104  and power block  106  must typically be shut down when there is still some latent energy available in the colder buckets. This energy can be used to improve overall plant performance by discharging it to preheat the solar field and power block  106  before the commencement of power generation operations. 
         [0052]    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  FIG. 4 , this embodiment could be implemented with any combination of pipes that exit the colder temperature buckets and bypass the hot bucket  122 . 
         [0053]    An alternative control improvement method and apparatus is schematically illustrated in  FIG. 5 . This embodiment includes direct heat transfer fluid injection between individual phase change material buckets and corresponding steam train components during the discharge phase of power generation. The heat transfer fluid temperature at the outlet of the phase change material cascade during discharge is required to be slightly less than the designed maximum power block inlet temperature; otherwise the heat transfer fluid at the maximum power block inlet temperature could not effectively charge the hot bucket  122 . Therefore, the steam train  110  can not receive the designed maximum heat input during discharge operation. At this off-design condition, it is may be favorable to overload some steam train heat exchangers on the heat transfer fluid side to increase the heat flow to the steam train and in addition to more accurately balance the phase change material discharge rates. 
         [0054]    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. 
         [0055]    As illustrated in  FIG. 5 , this embodiment may be implemented with one or more secondary branches to the heat transfer fluid circuit, for example, pipes  136  and  138  leading from intermediate buckets to corresponding steam train components. In addition, secondary heat transfer fluid pipes  140  and  142  may be required leading from the steam train back to the next warmest bucket 
         [0056]    An alternative control improvement method and apparatus is schematically illustrated in  FIG. 6 . This embodiment features partial discharge of the thermal energy storage system  104  during periods of low insolation. Thus, as the sun goes down but still provides some light to the solar concentrators or as clouds partially obscure the sun, heat transfer fluid flow from the solar field to the power block  106  is supplemented with heat transfer fluid flow from the thermal energy storage system  104  to maintain the optimum power block inlet flow rate. Because the colder buckets in a thermal energy storage system  104  (for example buckets  124  and  126 ) typically have excess stored energy when compared to the hot bucket  122 , it is possible to discharge the colder buckets  124 ,  126  first while maintaining full charge in the hot bucket. Thus, this embodiment features the routing of heat transfer fluid flow from the outlet of the steam train through one or two cold buckets to preheat it before sending it to the solar field for final solar heating to an operational temperature. The implementation of this improvement requires one or more secondary branches to the heat transfer fluid circuit, for example pipes  144  and  146  as shown on  FIG. 6 . Pipe  144  leads from the outlet of bucket  124  to the solar field inlet  128  and pipe  146  leads from the outlet of bucket  126  to the solar field inlet  128 . 
         [0057]    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,  FIG. 7  schematically illustrates a system  100  featuring each of the control enhancements described herein in combination. The  FIG. 7  embodiment includes, but is not limited to a solar power generation system or thermal energy storage system, that comprises both the primary heat transfer fluid circuit of  FIGS. 1 and 2 , in particular heat transfer fluid circuit element  108  and various secondary heat transfer fluid circuit branches. The secondary heat transfer fluid circuit branches can be implemented in any combination and include but are not limited to pipes  132 ,  134 ,  136 ,  138 ,  140 ,  142 ,  144  and  146 . The implementation of each improvement disclosed herein in any combination provides a concentrated solar power generation system operator with a great deal of flexibility over the charge and discharge management of a cascaded phase change material thermal energy storage system. 
         [0058]    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. 
         [0059]    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.