Abstract:
This invention provides a thermal energy battery having an insulated tank contains a multitude of densely packed plastic tubes filled with a phase-change material (PCM, such as ice) that changes from solid to liquid and vice-versa. Energy is stored when the PCM transitions from liquid to solid form, and released when the PCM transitions back from solid to liquid form. The tubes are arranged vertically, span the height of a well-insulated tank, and are immersed in heat transfer fluid (HTF) contained within the tank. The HTF is an aqueous solution with a freezing point temperature below the freezing point temperature of the chosen PCM. The HTF remains in liquid form at all times during the operation of the battery. Diffusers located allow the HTF to be extracted uniformly from the tank, pumped and cooled by a liquid chiller situated outside the tank and then and inserted back into the tank.

Description:
RELATED APPLICATION 
       [0001]    This application claims the benefit of U.S. Provisional Application Ser. No. 61/711,833, filed Oct. 10, 2012, and entitled THERMAL ENERGY BATTERY WITH ENHANCED HEAT EXCHANGE CAPABILITY AND MODULARITY, by Sorin Grama, et al, the teachings of which are expressly incorporated herein by reference. 
     
    
     FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
       [0002]    This invention was made with United States Government support under Grant #1113206 awarded by the National Science Foundation. The U.S. Government has certain rights in this invention. 
     
    
     FIELD OF INVENTION 
       [0003]    The present invention relates to thermal energy storage systems, and more particularly to a phase-change energy storage system for refrigeration and air conditioning applications. 
       BACKGROUND OF THE INVENTION 
       [0004]    During a 24-hour period, the electrical grid experiences a large variation in demand. Electrical power consumption peaks during the day and dips significantly at night. Energy storage is often employed to mitigate and smooth out these large fluctuations in power demand. There are many ways of storing energy including electrical, chemical, thermal and mechanical means. Of these, thermal energy is an effective method to store energy in the form of heat or cold for use in heating, refrigeration and air conditioning applications. Considering the fact that a majority of the peak demand is generated by power-hungry heating and refrigeration appliances, thermal energy storage stands to become a leading contender in grid storage applications. 
         [0005]    In countries such as India, thermal energy storage can also be used to mitigate the unreliable grid. By way of example, in many rural areas of India grid electricity is only available for a limited time during the day or night. In these situations, a thermal energy storage system can be charged when the grid is on and discharged when the grid is off to provide constant power for critical applications. 
         [0006]    One such application is a village-based milk chiller incorporating a thermal energy storage system. The chiller can be operated in remote villages, requires only 5-6 hours of grid electricity to charge and, most significantly, does not require a regular backup diesel (or other) generator. Once charged, the system can quickly cool large amounts of raw milk to preserve its freshness and eliminate spoilage. In these situations, thermal energy storage is not only used to increase energy efficiency, but is essential to mitigate the unreliable grid supply while avoiding the use of expensive and polluting fossil fuels. 
         [0007]    Thermal energy storage systems are typically designed for specific applications and are exactly matched for those applications. An example of a cold thermal energy storage system is an ice-bank tank which is typically designed for and fully integrated into an end-user application such as milk chilling. The prior art includes many examples of ice-based milk cooling systems dating back as far as the 1950s, such as U.S. Pat. No. 2,713,251. In this disclosure, the ice-based energy storage system is built into and is an integral part of the milk cooling tank, therefore cannot be easily separated and used for other refrigeration applications. Another example of a monolithic thermal energy storage system for cooling applications is the ice-on-coil storage system used in commercial HVAC applications such as disclosed by Gilbertson et al. in U.S. Pat. No. 5,090,207. The storage system described by Gilbertson et al. is a large monolithic system that requires custom designs and significant civil engineering effort to adapt to other applications. 
         [0008]    To increase adoption of thermal energy storage and facilitate ease-of-use, it is desirable that the thermal energy storage be designed as a modular and compact component that can be added to (or subtracted from) any cooling or heating application according to the expected load on the system. Furthermore, thermal energy storage systems can be provided with well-defined specifications such that designers or users can incorporate them easily into their cooling or heating applications. An analogy to this is the electrical battery storage system. Designers can easily connect one or more electrical batteries in series or parallel to build a battery bank for a wide range of applications, from simple off-grid lighting systems to complex electrical vehicle storage systems and large solar power storage systems. This wide variety of electrical storage applications is facilitated by the modularity, compactness and well-defined specifications of the common electrical battery, such as the car battery. 
         [0009]    Likewise, it is desirable to provide thermal energy storage systems that are generally as flexible, and as easy to build, as electrical storage systems. To achieve this, it is desirable to provide a compact and modular thermal energy battery with appropriate features to store and release thermal energy at a constant temperature and at a constant rate of discharge. These two specifications (temperature and rate of discharge) can become part of a standard set of specifications of a thermal energy battery which can be adopted by any manufacturer of such batteries. 
         [0010]    U.S. Pat. No. 7,225,860 discloses a compact heat battery comprising of a cylinder containing encapsulating tubes filled with a phase-changing material (PCM) that absorbs and releases thermal energy. This battery uses maximally-packed PCM tubes to provide sufficient surface area to achieve a desired discharge rate. Disadvantageously, no provision is made for maintaining a constant discharge rate, other than having sufficient surface area for heat transfer. This is a common and well known method described in prior art, but it makes the battery less compact than it can otherwise be. Furthermore, by relying only on surface area for heat transfer, the battery will not be able to maintain a constant output during discharge because the heat exchange surface area becomes smaller as the PCM begins to melt. 
         [0011]    U.S. Pat. No. 4,403,645 describes a high performance thermal storage apparatus which stores and releases its energy more efficiently using one long spiral tube rather than a plurality of PCM-filled encapsulants. However, the system is not modular, is difficult to build and cannot be easily sized for other applications. In another attempt at increasing performance, U.S. Pat. No. 7,503,185 describes a method for enhancing heat exchanging capability using ice-based thermal storage system. However this is a very expensive method of forming ice on copper tube coils. 
         [0012]    Various methods of making thermal energy systems more modular are found in prior art, such as U.S. Pat. Nos. 5,239,839 and 4,827,735 which describe methods of encapsulating the PCM into expandable plastic tubes or quilts that can be modularly arranged and therefore used to build compact batteries of any size. These devices suffer from the same limitation as they can not maintain a constant output and discharge rate for long periods of time. 
         [0013]    U.S. Pat. No. 4,524,756 describes a thermal energy storage system using modular batteries. This system does not use phase-change materials and thus cannot be very compact. Furthermore, the system described in this disclosure is limited to heat storage and cannot be easily adapted to refrigeration and air conditioning applications. A modular approach suited to refrigeration applications is described in US Patent 2002/0007637, but this method is expensive and relies on fixed path-ways that can only be changed manually using expensive quick-disconnects. 
         [0014]    It would be desirable to provide a system that combines the dual demands of compactness and modularity to build efficient thermal energy storage banks that can be easily adapted to a variety of heating and cooling applications. 
       SUMMARY OF THE INVENTION 
       [0015]    This invention overcomes the disadvantages of the prior art by combining the compactness and modularity of encapsulated phase-change materials with the benefits and simplicity of gravity as a motive force to increase heat exchange capability and provide a more constant rate of discharge. 
         [0016]    In an illustrative embodiment, an insulated tank contains a plurality of densely packed plastic tubes filled with a phase-change material that changes from solid to liquid and vice-versa. An example of usable PCM is water and ice. Energy is stored when the PCM transitions from liquid to solid form and is released when the PCM transitions back from solid to liquid form. Since PCMs will expand during freezing, the tubes must allow for this expansion to occur without bursting. Therefore, at the top of each tube a portion of air is left such that the PCM can expand during freezing and not spill outside the tube. 
         [0017]    The tubes are arranged vertically and span most of the height of the tank. In an embodiment, the tubes are sealed and submersed in a heat transfer fluid (HTF) contained within the walls of the well-insulated tank. Other embodiments allow for the tubes to be open at the top and only partially immersed in HTF such that the HTF and PCM do not mix. The HTF is an aqueous solution with a freezing point temperature below the freezing point temperature of the chosen PCM. The HTF remains in liquid form at all times during the operation of the battery. 
         [0018]    One or more diffusers located at the top of the tank allow the HTF to be extracted uniformly from the top of the tank, pumped and cooled by a liquid chiller situated outside the tank and then and inserted back into the bottom of the tank. As the HTF medium cools below the freezing point of the PCM, ice begins to form inside and at the bottom of the PCM-filled tubes. As the HTF surrounding the PCM-filled tubes continues to cool, the PCM inside the tubes begins to freeze progressively from bottom to top. In this manner, ice inside the tubes progressively builds from bottom to top until the PCM-filled tubes are completely frozen. This is defined as the “constant charge cycle.” Freezing the tubes from bottom to top is desirable to enable ice to grow progressively upwards while filling the expansion air pocket at the top thereby minimizing the chance of tube bursting if the tube is hermetically sealed. 
         [0019]    A second set of diffusers, one located at the bottom of the tank and one located approximately at the top of the tank, allows the HTF to be extracted and returned gently into the tank for the purpose of transferring the latent energy stored in the PCM to a load located outside the tank. Cold HTF is extracted from the bottom of the tank, circulated through a load heat exchanger located outside the tank and returned hot at the top of the tank. It is desirable that the cold HTF at the bottom of the tank does not mix with the hot HTF returning at the top of the tank. This is achieved by tightly packing the PCM-filled tubes in the battery such that the hot HTF at the top exchanges heat with the ice-filled tubes first and does not mix with the cold HTF at the bottom. 
         [0020]    As the top of the tank experiences the highest temperature differential between PCM and HTF, ice inside the tubes melts quickly and the stored thermal energy in the PCM is transferred to the HTF. As the PCM inside the tubes begins to melt, the solid form of PCM (i.e. the ice) floats freely to the top of the tubes while the liquid form settles at the bottom of the tubes. Ice floats up to the top because its specific gravity is lower than the liquid form of PCM. As ice floats up it always makes thermal contact with the hottest HTF returning from the load. In this manner, ice progressively melts from bottom to top at a constant and fast rate. This is called the “constant discharge cycle.” 
         [0021]    Discharge and charge cycles can be run simultaneously or independently, and the output of the thermal energy battery, basically a cold stream of fluid, remains at constant temperature for the longest possible time. The constant temperature output profile and the consequent constant rate of discharge output profile can be defined as a key specification of the thermal energy battery. Because the thermal performance of the thermal energy battery is predictable, multiple batteries can be connected together to form thermal energy storage bank with a well-defined thermal energy transfer characteristic. Multiple batteries can be connected in parallel or in series to build a thermal energy storage bank which can be adapted to any heating or cooling application. Although the current embodiment was designed for a cooling application, the same device can be used for heating applications by simply changing the phase-change material inside the tubes. If the PCM solidifies at a higher temperature, it stores and releases energy at that temperature. 
         [0022]    A compact thermal electric battery is comprised of a tank having insulated walls and containing heat transfer fluid (HTF), a plurality or tubes being substantially filled with a phase change material (PCM) and diffusers operatively connected to the tank constructed and arranged to enable flow of the HTF through a chiller and a heat exchanger. The PCM contains a mixture of water and a nucleating agent, which can include at one least one of Borax and/or IceMax® powder in solution or another equivalent compound or combination of compounds. The PCM can include a freezing point depression agent. The freezing point suppressant can include MKP, NaCl, KCl or other salts, among other equivalent compounds or combinations of compounds. The tubes are arranged vertically between a bottom and a top of the tank and float relative to the gravity in the tube. The tubes include an open space when the PCM is in a liquid phase for expansion of the PCM from liquid to solid. The diffusers are located so that a diffuser in which the HTF enters the tank is located at the top of the tank and a diffuser in which the HTF exits the tank is located on a bottom of the tank. A temperature sensor is located at the bottom of the battery where it can provide an accurate indication of the battery state of charge. An estimate of the battery state of charge can be made by analyzing a single temperature trend. At least one entry diffuser located remote from the bottom at a distance that causes entering HTF to be substantially free of thermal interference with the coldest HTF. The HTF comprises a mixture of water and Isopropyl alcohol having a concentration adapted to a predetermined freezing point. A multiple thermal battery system is comprised of a plurality of thermal batteries constructed and arranged to interconnect in parallel or series to increase storage capacity, wherein each of the batteries includes connectors constructed and arranged to enable addition or subtraction of batteries to match the predetermined storage capacity. Each battery is provided with a diffuser connected to another diffuser using an interconnection system. A method for controlling the freeze melt cycle of a thermal battery providing vertically oriented tubes containing PCM; and initiating a freeze cycle from the bottom of each of the tubes towards the top and/or initiating a melt cycle from the top of each of the tube toward the bottom. The method for controlling the freeze melt cycle further comprising a display of a state of charge and for obtaining the state of charge from a single sensor. The storage capacity can vary by selectively connecting and disconnecting a plurality of batteries together via an interconnection system. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0023]    The invention description below refers to the accompanying drawings, of which: 
           [0024]      FIG. 1  is a block diagram showing all major components of a complete system including one thermal energy battery, shown in side cross section, connected in a charge and discharge loop; 
           [0025]      FIG. 2  is a more detailed exposed perspective view of the inside of the illustrative battery of  FIG. 1 ; 
           [0026]      FIG. 3  is a schematic diagram showing the theory of operation of the system of  FIG. 1 ; 
           [0027]      FIG. 4  is a view of the PCM-filled tubes within the illustrative battery of  FIG. 1 , shown in different operating states; 
           [0028]      FIG. 5  is a graph of the expected output of the illustrative battery of  FIG. 1 ; 
           [0029]      FIG. 6  is a block diagram showing how two batteries in accordance with embodiments herein can be operatively connected in parallel to provide a thermal battery bank; and 
           [0030]      FIG. 7  is a diagram of interconnections used to connect multiple batteries such as those shown in  FIG. 6 . 
       
    
    
     DETAILED DESCRIPTION 
       [0031]    According to an illustrative embodiment, a compact and modular thermal energy storage (TES) battery is shown and described herein. Also shown and described are systems and methods for charging and discharging the illustrative battery and systems and methods for connecting multiple batteries to form a thermal energy storage bank. 
         [0032]    A thermal energy battery  10  and all associated components to charge and discharge the battery are shown in  FIG. 1 . The TES battery  10  comprises of an insulated container  100  oriented vertically about axis  101 . Container  100  is filled with heat transfer fluid (HTF)  102 . In an illustrative embodiment, HTF  102  is a mixture of water and 30% isopropyl alcohol, the freezing point of which is −15 degrees C. (5 degrees F.). The mixture is based on a predetermined freezing point. Other HTF mixtures can also be used, such as water and propylene glycol (PPG), in proper proportion to ensure a depressed freezing point below the freezing point of the chosen phase-change material (PCM). 
         [0033]    In various embodiments, the PCM can compromise a mixture of water with additives that reduce the super-cooling effect of water. One such additive is the commercially available SnoMax® snow inducer, available from York International Corporation of Norwood, Mass. Another PCM can be a mixture of water and Mono-Potassium Phosphate (KH 2 PO 4  also abbreviated as MKP) with an appropriate nucleating agent such as SnoMax® to reduce super-cooling effects or IceMax®. IceMax® powder is an ice machine cleaner containing sulfamic acid and is manufactured by Highside Chemicals, Inc. of Gulfport, Miss. Another PCM can be a mixture of water and Borax. In an embodiment we mix Mono-Potassium Phosphate (MKP) with water in concentrations of 12% to 14% which is near the eutectic point of the mixture to ensure direct transition from liquid to solid and vice-versa without partial solid-liquid formation. The freezing point of this MKP mixture ranges from −3 to −6 degrees C. (21.2 to 26.6 degrees F.) and the latent heat of fusion this MKP mixture is approximately 290 kJ/kg. The MKP mixture is ideal for food refrigeration applications where the target cooling temperature of the food is approximately 3 to 5 degrees C. (37.4 to 41 degrees F.). Because the MKP mixture melts at approximately −2 degrees C. (28.4 degrees F.) it provides a sufficient temperature differential (relative to the food&#39;s target temperature) to make the load heat exchangers more efficient. At the same time, because the lowest temperature for freezing this mixture is approximately −6 degrees C. (21.2 degrees F.), the refrigeration system required to freeze the mixture can be operated at temperatures that ensure high coefficient of performance and therefore good energy efficiency. The latent heat of fusion of the MKP mixture is high compared with other PCM mixtures generally used in the industry. Finally, MKP is a non-toxic, affordable and easily obtainable material. 
         [0034]    Inside container  100 , and immersed in HTF  102 , are placed in a plurality of tubes  109  filled with the PCM  110 . In an illustrative embodiment, the tubes hermetically sealed and completely submersed in HTF  102  and are held together vertically by closely packing them inside insulated container  100 . As the tubes float upwards, a restricting mesh  107  is used to keep tubes  109  submersed in HTF  102  at all times. Two diffusers,  106  at the top and  104  at the bottom, are located horizontally inside container  100  to extract and return HTF  102  for achieving heat transfer with outside mediums. In an illustrative embodiment, diffusers are circular tubes with a plurality of holes to diffuse flow for the purpose of achieving a gentle discharge or a uniform suction. 
         [0035]    Bottom diffuser  104  is connected via cold suction pipe  116  to pump  111  which circulates cold HTF  102  through load heat exchanger  113 . After exchanging heat with the load, hot HTF  102  returns via pipe  117  and through diffuser  106  back to the top of the battery  10 . 
         [0036]    In an embodiment, diffuser  106  is shared between the hot discharge and hot suction. Diffuser  106  is connected via suction pipe  119  to pump  112  which circulates warm HTF  102  through liquid chiller  114 . After it is cooled by liquid chiller  114 , cold HTF  102  returns through pipe  118  via diffuser  105  into battery  10 . In the illustrative embodiment diffuser  105  is located at the bottom of the container  100  to initiate freezing cycle from the bottom of battery  10 . 
         [0037]    Liquid chiller is defined as a type of heat exchanger that removes heat from the liquid as it passes from the inlet to the outlet thereof. This can include fluid mechanical systems, thermoelectrics, etc. 
         [0038]    Other locations for diffuser  105  are also acceptable. Diffuser  105  can also be located at the top if a freezing direction from top to bottom is desired. Diffusers  104 ,  105  and  106  are submerged at all times in HTF  102 . Diffusers  104 ,  105 , and  106  are arranged such that the HTF  102  being extracted or returned through diffusers does not mix significantly in a vertical direction. 
         [0039]    In  FIG. 1 , two methods of operation are shown. Charge loop  130  is a closed loop which cools HTF  102  and charges battery  10  by freezing PCM  110  inside tubes  109 . Because coldest HTF is returned near the bottom of battery  10 , ice insides tubes  109  begins to form near the bottom first. Tubes  109  progressively freeze upwards as cold HTF  102  rises from the bottom of battery  10 . When PCM  110  inside all tubes  109  is completely frozen, battery  10  can be considered fully charged. To achieve a fully charged status, charge loop  130  must be operated for a minimum amount of time, depending on the cooling power of the liquid chiller  114 . If loop  130  is operated for less than the minimum time, battery  10  can be partially charged without loss of operational capability. 
         [0040]    Discharge loop  131  is a closed loop which circulates coldest HTF  102  from the bottom of battery  10  so it can transfer energy with a load. In the illustrative embodiment the load is warm milk entering heat exchanger  113  via port  121  and exiting cold via port  123 . After transferring heat with the load, hot HTF  102  exits load heat exchanger  113  and returns to top of battery  10  via diffuser  106 . If battery  10  is fully or partially charged, PCM  110  in solid form (ice) will be present at the top of tubes  109 . Warm HTF  102  will transfer heat with frozen PCM  110  through the walls of tubes  109 . As a result PCM  110  melts. As PCM  110  melts the liquid form settles at the bottom of tube  109  while the solid form floats freely to the top due to gravity. In this manner, PCM  110  in solid form is continually present at the top in constant thermal contact with warm HTF  102  returning from the load. As PCM  110  melts it transfers energy to HTF  102  which cools and settles to the bottom of battery  10 . In this manner the coldest HTF  102  is available at the bottom of battery  10  for the longest period of time, depending on the quantity of PCM in the battery. Discharge loop  131  can be operated for as long as solid PCM  110  remains in tubes  109 . When all PCM in tubes  109  are melted, battery  10  can be considered fully discharged. If, after running discharge loop  131  for some time, some PCM in solid forms still remains inside tubes  109 , battery  10  can be considered partially discharged without loss of operational capability. Successive operations of loop  131  will progressively discharge battery  10  until battery is fully discharged. 
         [0041]    Charge loop  130  and discharge loop  131  can be operated simultaneously or independently. If operated simultaneously, output of discharge loop  131  will not be disturbed by the performance of charge loop  130 . Charge loop  130  can be operated manually or automatically based on a timer or a temperature sensor  125  placed inside the battery. In the illustrative embodiment a temperature controller  126  starts and stops discharge loop  130  based on a pre-set temperature. Independent of charge loop  130 , discharge loop  131  can be manually or automatically operated as long as battery  10  is partially or fully charged. In the illustrative embodiment, loop  131  is operated manually when needed to cool milk. 
         [0042]    The battery is defined by all and/or at least one of a plurality of parameters. A first parameter is that the battery is provided with a capacity at a determined load power. A second parameter is that the battery is provided with a capacity of a determined number of hours at a desired load power. A third parameter is that the depth of discharge is at least a desired percentage rate. A fourth parameter is that the battery is provided with at least a desired number of rated cycles. A fifth parameter is that the battery is provided with a desired output temperature. Other parameters can also be defined in determining standard sizes by one of ordinary skill. These differences can be used to determine standardized rating size. Such rating sizes can be defined in a manner similar to commercial consumer batteries (for example, A, AA, AAA, C and D). The nomenclature of the sizes is highly variable. For example, the nomenclature can be numeric (1, 2, 3, etc.), alphabetic (A, B, C, etc.), symbolic or by another system. 
         [0043]    A more detailed view of the present construction of battery  10  is shown in  FIG. 2 . In the illustrative embodiment, insulated container  100  is a plastic (polymer, composite, etc.) tank that is generally free of any chemical reaction with HTF  102 . Tubes  109  are sealed and submersed in heat transfer fluid (HTF)  102  and held in vertical position by the restricting mesh  107 . HTF  102  extends to level  201 . In another embodiment, tubes  109  can float freely in HTF  102  and are not restricted by mesh  107 . General orientation  200  is desirably maintained to ensure free floating of ice to the top of tubes  109 . 
         [0044]    Diffusers  104  and  106  are illustratively formed by bending a plastic (polymer) tube into a circular shape and drilling (or otherwise forming) a multiplicity of holes  202  that server to diffuse the flow during operation. In an embodiment, holes  202  in diffuser  104  point downwards while holes  202  in diffuser  106  point upwards. Holes  202  can also be oriented radially from the vertical axis to minimize vertical mixing of HTF  102  layers. 
         [0045]    To facilitate connection to charge loop  130 , input port  205  and output port  207  are provided. Port  205  is connected to diffuser  105  and port  207  is connected to diffuser  106 . To facilitate connection to discharge loop  131 , input port  206  and output port  204  are provided. Port  206  is connected to diffuser  106  and port  204  is connected to diffuser  104 . It should be clear to those of skill in the art that ports  204 ,  205 ,  206  and  207  can be located at any height to facilitate easy connection with elements outside the battery so long as diffusers  104 ,  105  and  106  are maintained at the approximate locations shown in  FIG. 2   
         [0046]      FIG. 3  illustrates the different processes occurring inside the battery. The solid form (ice)  110   a  of PCM  110  in tubes  109  rises at the top because it is less dense than the liquid form  110   b  of PCM  110 . As hot HTF  102  is returned to the top and cold HTF  102  settles at the bottom, two regions are formed inside battery  10 . These regions are further maintained by the vertical arrangement and the connections to discharge loop  131  and charge loop  130 . Regions  301  and  302  are critical to maintaining optimal heat transfer and output from battery  10 . Region  301  located approximately at the top of the battery, is where most of the heat exchanging between hot HTF  102  and the solid form  110 a of PCM  110  occurs. Region  302  is where the coldest HTF  102  will be maintained and extracted at nearly constant temperature for the longest period of time. 
         [0047]      FIG. 4  further illustrates the PCM-filled tubes  109  and their operational state. Tube  109  can be constructed of plastic or any other material that facilitates optimal heat transfer. Tube  109  can be hermetically sealed or not sealed. Item a of  FIG. 4  illustrates a tube filled with PCM  110  in liquid form  110   b.  This is the discharged state of the tube  109 . A small amount of empty space  403  remains at the top of the tube  109  to provide room for expansion of PCM liquid  110   b  as it transitions from liquid to solid form. It is desirable that tube  109  is free of significant expansion during freezing process. Instead, PCM material will expand into empty space  403  at the top. Item B of  FIG. 4  illustrates a tube in fully charged state with PCM  110  in solid state  110   a.  PCM  110  in solid form  110   a  extends to the top of the tube. Item C of  FIG. 4  illustrates a tube in partially charged state with PCM  110  in both solid  110   a  form at the top and liquid  110   b  form at the bottom. 
         [0048]      FIG. 5  shows the expected output of the battery when the above construction is implemented as illustrated by curve  504 . The useful output of battery  10  is defined by two parameters:
       a) the temperature of HTF  102  extracted at the bottom through diffuser  104  and measured by temperature sensor  125 ; and   b) the duration of time at which a relatively constant output temperature can be maintained.       
 
         [0051]    A desired constant temperature output level can be centered within a narrow range  501  centered around PCM  110  melting point  501   a.  Temperature bandwidth  501  can be maintained for a period of time  502  which depends on the load presented to the battery by the load heat exchanger. It is desirable to provide a temperature output bandwidth  501  as narrow as possible for the longest period of time  502  as possible. Three regions of operation are observed. In Region  1 , the output power of battery  10  is mainly provided by the sensible heat of HTF  102 . The time duration of this region depends on the amount of HTF in the battery. Once PCM  110  in tubes  109  begins to melt, the battery enters Region  2  of operation. This is the main and optimal region of operation during which output HTF  102  temperature remains relatively constant within a narrow bandwidth  501  centered about melting point  501   a.  When all PCM  110  has melted, the battery enters Region  3 . In this region, the battery has exhausted its charge and less useful energy is delivered to the load. From starting point  506  to end point  505 , the battery provides a useful output for fast cooling or heating a large variety of loads. 
         [0052]    Placing the temperature sensor  125  at the bottom of the battery and monitoring its change over time gives an accurate indication of the state of charge of the battery. As the battery transitions from starting point  506  to end point  505 , its state of charge can be easily estimated by analyzing the temperature trend. 
         [0053]    This predictive performance and accurate display of the state of charge is essential in designing thermal battery banks and systems that use thermal battery backup. 
         [0054]      FIG. 6  shows a block diagram of an arrangement of multiple batteries  10  connected together to form a thermal energy battery bank that can be sized according to any specified heat or cooling load. In an embodiment, two batteries  10   a  and  10   b  are connected in parallel using manifolds  601   a,    601   b,    601   c  and  601   d  (collectively, “Manifolds  601 ”). Manifolds  601  eliminate the need for expensive valves. When two batteries  10  are connected in parallel, HTF  102  is extracted simultaneously from both batteries. Manifolds  601  act as junctions that merge the HTF flows to and from the load or the liquid chiller. Such a construction can be susceptible to an imbalance of flows in the two batteries. For example, if there is small restriction in one of the lines exiting or entering one of the batteries in the bank, less HTF will flow through that battery and more HTF will flow through the other battery. Over a period of time, one battery will empty out while the other will overflow potentially resulting in HTF spilling out of battery  10 . However, in the illustrative arrangement, the flow between batteries in the bank is balanced naturally by the way the batteries are connected. Manifolds  601  and especially manifold  601   a  contains a sufficiently large cross section to ensure that the HTF  102  in both batteries remains substantially balanced at the operating flow rates. In addition, all manifolds are submerged in HTF. Thus, the liquid pressure remains constant throughout the two batteries. It is contemplated that a multiple thermal battery system comprises a plurality of thermal batteries constructed and arranged to interconnect in a parallel or series to increase storage capacity, wherein each of the batteries includes connectors constructed and arranged to enable addition and/or subtraction of batteries to match a predetermined storage capacity. 
         [0055]    Manifolds  601  can be sized according to the number of batteries that can be connected to form a battery bank.  FIG. 7  shows for comparison a 2-way manifold  701 , a 3-way manifold  702  and a 4-way manifold  703 . Those skilled in the art can recognize and find other ways to connect multiple batteries together such as series connection or using multiple manual or automatically actuated valves. 
         [0056]    It is contemplated that the materials of the tubes can be a polymer, such as LDPE (low-density polyethylene) or HDPE (high-density polyethylene). The tank can be constructed of LDPE. It is contemplated that the volume of the tank is 700 liters (approximately between 500 to 1000 liters). The illustrative thermal battery storage system is practical for use in small installations, easy to load and unload and can be assembled by a couple of technicians. 
         [0057]    The foregoing has been a detailed description of illustrative embodiments of the invention. Various modifications and additions can be made without departing from the spirit and scope if this invention. More generally, as used herein the directional terms, such as, but not limited to, “up” and “down”, “upward” and “downward”, “rearward” and “forward”, “top” and “bottom”, “inside” and “outer”, “front” and “back”, “inner ” and “outer”, “interior” and “exterior”, “downward” and “upward”, “horizontal” and “vertical” should be taken as relative conventions only, rather than absolute indications of orientation or direction with respect to a direction of the force of gravity. Each of the various embodiments described above can be combined with other described embodiments in order to provide a variety of combinations of multiple features. Furthermore, while the foregoing describes a number of separate embodiments of the apparatus and method of the present invention, what has been described herein is merely illustrative of the application of the principles of the present invention. It is further contemplated that diffusers  104 ,  105  and  106  can be constructed in any manner that slows down the flow to minimize mixing between different vertical layers of HTF  102 . In other embodiments diffuser  105  can be located near the top, the middle or the bottom of battery  10  depending on the cooling power of liquid chiller  114  and/or how the battery is operated. Alternatively, diffuser  106  can be eliminated completely and the hot HTF  102  can be released without turbulence at the top of the battery where it circulates and mixes freely with only the top layer of HTF above tubes  109 . In another embodiment, insulated container  100  can be of any size, shape or aspect ratio as long as tubes  109  containing PCM  110  remain oriented in a vertical direction. In further embodiments, HTF  102  can consist of any liquid mixture that is free of freeze-over within the operating temperature range of the battery. The output temperature of the battery can vary, depending on the PCM used. It is further contemplated that the tubes can be of any shape, size and profile as long as the tubes avoid constriction of the free flow of ice to the area of the tube where maximum heat transfer with HTF will occur. 
         [0058]    Accordingly, this description is meant to be taken only by way of example, and not to otherwise limit the scope of this invention.