Patent Publication Number: US-6698233-B2

Title: Active sorption thermal storage container

Description:
This application is a continuation-in-part of U.S. patent application Ser. No. 09/834,080, which was filed on Apr. 12, 2001, now U.S. Pat. No. 6,502,419, and which is hereby incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to a thermal storage device for maintaining the temperature of an article at a desired temperature for a period of time. More particularly, the invention relates to such a device which comprises a sorption compression refrigeration system that, when activated by an external power supply, generates a quantity of pressurized refrigerant that may later be controllably evaporated to produce a cooling effect and thereby maintain the temperature of the article at the desired temperature for a period of time. 
     The present invention is particularly useful as a shipping container for refrigerated articles, such as frozen foods. Frozen foods must usually be shipped in refrigerated trucks or individual shipping containers which are packed with ice. However, refrigerated trucks are generally not well insulated and therefore require one or more relatively large vapor compression refrigeration units to maintain the temperature of the cargo at a desired temperature. These refrigeration units are typically powered by the electrical system of the truck; consequently, they can significantly reduce the fuel efficiency of the truck. In addition, the use of a refrigerated truck is not economical when less than an entire truckload of articles is to be shipped. On the other hand, ice-packed shipping containers do not allow for precise temperature control, require a fresh source of ice each time they are used, and must be packed and shipped before the ice melts. Consequently, these types of shipping containers are usually only practical when shipping certain types of articles relatively short distances. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention, these and other limitations in the prior art are overcome by providing a thermal storage device for maintaining the temperature of an article at a desired temperature for a length of time. The thermal storage device comprises a compartment within which the article may be positioned, an evaporator which is disposed in heat exchange relation with respect to the compartment, a receiver which is fluidly connected to the evaporator, a sorber which is fluidly connected between the evaporator and the receiver and which includes a sorbent that is capable of adsorbing a refrigerant, means for desorbing the refrigerant from the sorbent, and means for releasably connecting an external power supply to the desorbing means. When the desorbing means is connected to the external power supply, the refrigerant is desorbed from the sorbent and communicated to the receiver. Furthermore, after the desorbing means is disconnected from the external power supply, the refrigerant within the receiver may be evaporated in the evaporator and adsorbed onto the sorbent to thereby produce a cooling effect in the compartment. Thus, the thermal storage device is capable of cooling the compartment after it has been disconnected from the external power supply. 
     In a preferred embodiment of the invention, the desorbing means comprises first and second electrical conductors between which the sorbent is positioned, and the connecting means includes a pair of electrical leads which are electrically connected to the first and second conductors. In addition, the sorbent and the refrigerant are selected such that, when an electrical current is conducted through the sorbent, the current will desorb the refrigerant from the sorbent. In this manner, the refrigerant may be desorbed from the sorbent by connecting the leads to the external power supply, which can be a conventional source of line voltage. This provides a convenient means for “charging” the thermal storage device prior to use. 
     In one embodiment of the invention, the thermal storage device includes means for controlling the flow of refrigerant into the evaporator. Such means could be, for example, an orifice valve, a capillary tube, or a manual, electrical or pressure actuated valve. Thus, when the temperature of the article rises above the desired temperature, the flow control means is operable to allow the refrigerant to evaporate and thereby cool the article. 
     In another embodiment of the invention, the thermal storage device comprises a valve which is fluidly connected between the receiver and the evaporator, a temperature sensor which is thermally connected to the compartment, and means for indicating whether the temperature of the compartment is above the desired temperature. Thus, when the temperature of the compartment rises above the desired temperature, the valve may be opened to allow the refrigerant to evaporate and thereby cool the compartment. 
     In yet another embodiment of the invention, the thermal storage device includes a controllable valve which is fluidly connected between the receiver and the evaporator, a temperature sensor which is thermally connected to the compartment, and a controller which is connected to both the temperature sensor and the valve. Thus, when the temperature of the compartment rises above the desired temperature, the controller will open the valve to allow the refrigerant to evaporate and thereby cool the compartment. More preferably, the controller also operates to close the valve when the temperature of the compartment drops below the desired temperature. In this manner, the thermal storage device can automatically maintain the temperature of the compartment, and thus the article, at the desired temperature for a length of time. 
     These and other objects and advantages of the present invention will be made apparent from the following detailed description, with reference to the accompanying drawings. In the drawings, the same reference numbers are used to denote similar elements in the various embodiments. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a cross sectional view of an embodiment of a thermal storage device of the present invention; 
     FIG. 2 is a longitudinal cross sectional view of the sorber component of the thermal storage device depicted in FIG. 1; 
     FIG. 3 is a longitudinal cross sectional view of an alternative sorber for the thermal storage device depicted in FIG. 1; 
     FIG. 4 is a longitudinal cross sectional view of another alternative sorber for the thermal storage device depicted in FIG. 1; 
     FIG. 5 is a longitudinal cross sectional view of yet another alternative sorber for the thermal storage device depicted in FIG. 1; 
     FIG. 6 is a diagrammatic representation of an exemplary control system for the thermal storage device depicted in FIG. 1; 
     FIG. 7 is a cross sectional view of second embodiment of a thermal storage device of the present invention; 
     FIG. 8 is a cross sectional view of another embodiment of a thermal storage device of the present invention; 
     FIG. 9 is a partial cross sectional, partial schematic view of a portion of the thermal storage device depicted in FIG. 8; 
     FIG. 10 is an exploded view of the sorber component of the thermal storage device depicted in FIG. 8; 
     FIG. 11 is an enlarged cross sectional view of a portion of the sorber component of the thermal storage device depicted in FIG. 8; 
     FIG. 12 is a cross sectional view of another embodiment of a thermal storage device of the present invention; and 
     FIG. 13 is a cross sectional view of an exemplary re-charging stand for the thermal storage device shown in FIG.  12 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention employs a sorption compression system to first generate a quantity of pressurized refrigerant and then evaporate the refrigerant at a later time to produce a cooling effect. In existing adsorption and absorption compression systems, which will be referred to herein simply as sorption compression systems, a first, typically gaseous substance called a sorbate is alternately adsorbed (or absorbed) onto and desorbed from a second, typically solid substance called a sorbent. Particular sorption compression systems utilize specific sorbates and sorbents to produce a desired effect which is dependent upon the affinity between the two substances. During the adsorption reaction, the sorbate is drawn onto and combines with the sorbent to produce a sorbate/sorbent compound. During the desorption reaction, energy is supplied to the sorbate/sorbent compound to break the bonds between the sorbate and sorbent molecules and thereby desorb the sorbate from the sorbent. In this reaction, the sorbate molecules are driven off of the sorbent molecules and into a relatively high pressure, high energy gaseous state. Substantial energy is imparted to the sorbate during the desorption reaction, and this energy can be harnessed for various uses. 
     For example, in a sorption compression refrigeration system the sorbate is typically a refrigerant. During the desorption reaction, which occurs in an enclosure that is sometimes called a sorber, the refrigerant is driven off of the sorbent and into a relatively high pressure gaseous state. The refrigerant gas is subsequently condensed and then, during the adsorption reaction, evaporated to produce a cooling effect. The refrigerant is specifically selected in conjunction with the sorbent to evaporate at a desired temperature when exposed to the sorbent. Therefore, as soon as the refrigerant is evaporated it is once again adsorbed onto the sorbent. The desorption and adsorption reactions may be repeated numerous times depending on the cooling requirements of the refrigeration system. 
     The thermal storage device of the present invention takes advantage of the fact that, in a sorption compression refrigeration system, the pressurized refrigerant is produced during the desorption reaction but the cooling effect is produced during the adsorption reaction. Therefore, energy from an external power supply need only be applied to the system during the desorption reaction. Moreover, the pressurized refrigerant does not have to be evaporated and re-adsorbed onto the sorbent immediately after the desorption reaction. Therefore, the thermal storage device can be temporarily connected to an external power supply to desorb the refrigerant from the sorbent, and then disconnected from the power supply and transported as desired. Furthermore, the sorbent can be evaporated to produce a cooling effect after the thermal storage device is disconnected from the power supply, for example while the device is in transit. Thus, the thermal storage device of the present invention is in effect a “thermal battery” which is capable of storing a “cooling potential” and then releasing this cooling potential at a later time. 
     Referring to FIG. 1, an embodiment of the present invention is shown which is particularly useful as a shipping container. The thermal storage device of this embodiment, which is indicated generally by reference number  10 , is shown to comprise an outer housing  12  that encloses a compartment  14  into which one or more articles to be refrigerated may be placed. The compartment  14  is surrounded by a container  16  which includes an upper opening  18  that is sealed by a removable lid  20 . The container  16  may comprise any suitable, preferably insulating device, such as a vacuum vessel. The lid  20  optimally includes an outwardly extending flange  22  which may be secured to the housing  12  to maintain the lid in position over the opening  18 . The insulating characteristics of the container  16  may be improved by surrounding the container with a layer of, for example, foam insulation  24 . 
     The thermal storage device  10  also comprises a sorption compression refrigeration system that includes an evaporator  26 , a receiver  28  and a number of sorbers  30  which are connected together in a closed refrigeration loop. Thus, an inlet  32  of the evaporator  26  is fluidly connected to an outlet  34  of the receiver  28  by a conduit  36 . Also, an outlet  38  of the evaporator  26  is in communication with an inlet  40  of each sorber  30  via a conduit  42 , which may be connected to each sorber either directly or via a first manifold  44 , as shown in FIG.  1 . Furthermore, an outlet  46  of each sorber  30  is in communication with an inlet  48  of the receiver  28  via a conduit  50 , which may be connected to the receiver either directly or, as shown in FIG. 1, through a second manifold  52 . 
     The thermal storage device ideally also comprises suitable means for controlling the flow of the refrigerant through the refrigeration loop. For example, a first check valve  54  may be positioned in the conduit  42  to ensure that the refrigerant flows from the evaporator  26  only to the sorbers  30 . Similarly, a second check valve  56  may be positioned in the conduit  50  to ensure that the refrigerant flows from the sorbers  30  only to the receiver  28 . Also, a flow control device  58  may be positioned in the conduit  36  to control the release of the refrigerant into the evaporator  26  for the cooling of the compartment  14 . The operation of the device  58 , which could be, for example, an orifice valve, a capillary tube, or a manual, electrical or pressure actuated valve, will be described more fully below. 
     The evaporator  26  can be any conventional heat exchange device which is capable of absorbing heat from the compartment  14 . The evaporator should therefore be disposed in heat exchange relation with respect to the compartment  14 . Accordingly, the evaporator  26  may be positioned in the compartment  14 , attached to the lid  20  or incorporated into the container  16 . However, in the embodiment of the invention shown in FIG. 1 the evaporator  26  is positioned above the opening  18  in a support ring  60  that is secured to the housing  12 . The support ring  60  includes a through hole  62  which is aligned with the opening  18 . In addition, the lid  20  ideally comprises a central plug portion  64  which engages the through hole  62  to thereby seal the opening  18  from the outside environment. 
     The purpose of the receiver  28  is to contain the refrigerant after it has been desorbed from the sorbers  30  and before it is evaporated in the evaporator  26 . Therefore, it should be understood that, depending on the volume of refrigerant which is desorbed from the sorbers  30 , the receiver  28  need not necessarily comprise a separate container. Rather, the conduit  36  and/or the second manifold  46 , if present, may have a sufficient volume to function as a suitable receiver. 
     Although the present invention preferably does not include a separate condenser, in the event thermal energy is used to desorb the refrigerant from the sorbent, suitable means should be provided to allow the refrigerant to condense and the heat of condensation to be removed from the refrigerant. In many instances, the receiver  28  may be sufficient for this purpose. Otherwise, a suitable condenser could be inserted in the refrigeration loop between the sorbers  30  and the receiver  28  or the evaporator  26 . 
     As mentioned above, each sorber  30  is the enclosure within which the desorption and adsorption reactions take place. Therefore, the sorber  30  must function to contain the sorbent and provide for the communication of the refrigerant to and from the sorbent. In addition, depending on the particular mechanism which is employed to desorb the refrigerant from the sorbent, the sorber  30  must at least accommodate and in some cases even facilitate the desorption reaction. In one embodiment of the invention, the sorption compression system utilizes an electrical current as the desorption mechanism. Therefore, the sorber  30  is optimally designed to conduct the electrical current to the sorbent in order to effect the desorption of the refrigerant from the sorbent. 
     Referring to FIG. 2, the sorber  30  is thus shown to include a tubular, preferably cylindrical housing  66  which comprises a first electrical conductor  68 , a cylindrical sleeve  70  which comprises a second electrical conductor  72 , and a sorbent  74  which is positioned between the first and second conductors. The housing  66  comprises a first end  76  through which the sorber inlet  40  extends and a second end  78  through which the sorber outlet  46  extends. The sleeve  70  is supported on a shaft  80  which includes two enlarged diameter end portions  82  that engage the inner diameter of the housing  66  to maintain the second conductor  72  properly positioned relative to the first conductor  68 . One of the end portions  82  may be detachable from the shaft  80  to allow the sleeve  70  to be assembled onto the shaft. In addition, one of the ends  76 ,  78  of the housing  68  may be a separate piece which is attached to the housing by suitable means, such as welding, after the sorbent  74 , the sleeve  70  and the shaft  80  have been inserted into the housing. 
     As will be discussed more fully below, during each desorption reaction the first and second conductors  68 ,  72  conduct a current through the sorbent  74  to desorb the refrigerant from the sorbent. Accordingly, the first and second conductors are made from a suitable electrically conducting material, such as an aluminum alloy, and are each connected to corresponding leads  84 ,  86  by suitable means. For example, the lead  84  may be soldered to the first conductor  68 , and the lead  86  may be connected to a slip ring  88  which is mounted on the shaft  80  and engages the inner diameter of the second conductor  72  when the sorber  30  is assembled. The shaft  80  is optimally made of a lightweight, non-conducting material, such as polyethylene. Consequently, the shaft  80  will serve to insulate the first conductor  68  from the second conductor  72 . 
     In order to provide for the communication of the refrigerant to and from the sorbent  74 , the shaft  80  is provided with a longitudinal bore  90  which extends between the inlet  40  and the outlet  46 , and a number of radial holes  92  which pass through the shaft and intersect the longitudinal bore. The radial holes  92  in turn communicate with a corresponding number of apertures  94  in the sleeve  70 . Thus, during each adsorption reaction the refrigerant will enter the sorber  30  through the inlet  40  and be communicated to the sorbent  74  through the longitudinal bore  90 , the radial holes  92  and the apertures  94 . Similarly, during each desorption reaction the refrigerant will pass through the apertures  94 , the radial holes  92  and the longitudinal bore  90  and exit the sorber  30  through the outlet  46 . 
     FIG. 3 illustrates an alternative sorber which may be used to facilitate the desorption of the refrigerant from the sorbent using an electrical current. The sorber of this embodiment, which is indicated generally by reference number  30 ′, is similar to the sorber  30  in that it includes a housing  66  which comprises a first electrical conductor  68 , a sleeve  70  which comprises a second electrical conductor  72  and a sorbent  74  which is disposed between the first and second conductors. However, in this embodiment each end of the sleeve  70  is supported in a corresponding receptacle  96  which is formed in each of the first and second ends  76 ,  78  of the housing  66 . Thus, a shaft such as  80  is not required to support the sleeve  70  in the housing  66 . In order to provide for the communication of the refrigerant between the refrigeration loop and the sorbent  74 , the sleeve  70  includes a longitudinal bore  98  which extends between the inlet  40  and the outlet  46 , and a number of radial bores  100  which extend between the longitudinal bore and the outer diameter of the sleeve. 
     As in the sorber  30 , the housing  66  and the sleeve  70  are preferably made of a suitable electrically conducting material. Thus, the housing  66  must be electrically insulated from the sleeve  70 . This may be accomplished by positioning an insulating gasket (not shown) between the ends of the sleeve  70  and the receptacles  96 . Alternatively, either the ends of the sleeve  70  or the receptacles  96 , or both, may be treated, such as by anodizing, to create an electrically insulating coating between theses components. In one embodiment of the invention, the entire ends  76 ,  78  of the housing  66  are so treated to limit the flow of current to the cylindrical portion of the housing which is located between the ends. 
     Another embodiment of a sorber which can be used to facilitate the desorption of the refrigerant from the sorbent using an electrical current is shown in FIG.  4 . The sorber of this embodiment, which is indicated generally by reference number  30 ″, is shown to include a housing  66  which comprises a first electrical conductor  68 , a sleeve  70  which comprises a second electrical conductor  72 , and a sorbent  74  which is disposed between the first and second conductors. In this embodiment the sleeve  70  is supported between a pair of plug members  102 . In addition, the inlet  40  and the outlet  46  both extend through the portion of the housing  66  which is located between the ends  76 ,  78 . Therefore, the refrigerant may be communicated between the sorbent  74  and the refrigeration loop directly through the inlet  40  and the outlet  46 . 
     The operation of the sorbers  30 ,  30 ′ and  30 ″ will now be described. During the adsorption reaction, refrigerant from the evaporator  26  is communicated to the sorbent through the inlet  40 . The refrigerant combines with the sorbent in this reaction to form a refrigerant/sorbent compound. The refrigerant thus remains trapped within the sorber until a desorption reaction is initiated. During the desorption reaction, an electrical current from an external power supply, which will be discussed below, is conducted by the first and second conductors  68 ,  72  across the refrigerant/sorbent compound to desorb the refrigerant from the sorbent. The electrical current liberates the refrigerant molecules from the sorbent molecules, and the resulting high pressure, high energy refrigerant expands through the outlet  46  and into the receiver  28 . During this reaction, the check valve  54  prevents the refrigerant from expanding back into the evaporator  26 . 
     The exact mechanism by which the electrical current effects the desorption of the refrigerant molecules from the sorbent molecules varies depending on the type of sorbent employed. Moreover, while the exact mechanism is not known, the inventors believe that, when the current is conducted through the refrigerant/sorbent compound, electrons are channeled into each refrigerant—sorbent bond until the bond is broken and the refrigerant molecule is liberated from the sorbent molecule. With respect to the carbon based sorbents which will be discussed below, one theory is that the electrons from the power supply displace the electrons of the refrigerant molecule in the conduction band of the sorbent molecule, thereby freeing the refrigerant molecule from the sorbent molecule. Another theory is that the electrons impart sufficient energy to the refrigerant molecule to allow it to escape the electrical potential binding it to its associated sorbent molecule. 
     The selection of the particular refrigerant and sorbent materials for the thermal storage device  10  depends in part on the desired electrical and thermal conductivities of these materials. Since in one embodiment of the invention the desorption reaction is driven by an electric current, the refrigerant/sorbent compound should be a good electrical conductor. In addition, in the event that the refrigerant molecules bind only to the surface of the sorbent during the adsorption reaction, the sorbent should also be a good electrical conductor. Moreover, if the external power supply is an AC power supply, the refrigerant and sorbent materials should ideally be selected so that the combined impedance of the sorber and the refrigerant/sorbent compound matches that of the power supply to ensure that the maximum amount of power is transferred from the power supply to the refrigerant/sorbent compound. If on the other hand the external power supply is a DC power supply, the refrigerant and sorbent materials should optimally be selected so that the combined resistance of the sorber and the refrigerant/sorbent compound, or the combined resistance of the sorber and the sorbent alone, is sufficient to avoid overloading the power supply. 
     Furthermore, during each adsorption reaction the kinetic energy of the refrigerant molecules is converted to heat as the refrigerant molecules combine with the sorbent molecules. This heat, which is often referred to as the heat of adsorption, inhibits the further adsorption of the refrigerant onto the sorbent and should therefore be dissipated from the sorbent. Therefore, both the refrigerant/sorbent compound and the sorbent are ideally good thermal conductors. In a preferred embodiment of the invention, the sorbent comprises a thermal conductivity at least as great as that of aluminum or copper. It has been found that using a sorbent with such a thermal conductivity and a refrigerant that meets the other requirements of the sorption compression system will result in a refrigerant/sorbent compound that has a sufficient thermal conductivity for purposes of the present invention. 
     The selection of the refrigerant and sorbent materials also depends on the desired nature of the desorption reaction. In accordance with one embodiment of the invention, the refrigerant and sorbent materials are selected such that, when the electrical current is conducted through the refrigerant/sorbent compound to effect the desorption reaction, the refrigerant/sorbent compound is not heated appreciably. Thus, the desorption reaction is substantially non-thermal. In the context of the present invention, “non-thermal desorption” refers to a mechanism of desorption that does not rely on thermal energy to stochastically heat the refrigerant/sorbent compound to the degree sufficient to break the bonds between the refrigerant and sorbent molecules. Thus, while some isolated, localized heating of the refrigerant/sorbent compound may occur during the desorption reaction, the temperature of the refrigerant/sorbent compound should remain statistically below the threshold temperature for thermal desorption to take place. 
     One method for determining whether a particular desorption reaction is either thermal or substantially non-thermal is to measure the bulk temperature of the refrigerant/sorbent compound during the desorption reaction. If the bulk temperature of the compound during the desorption reaction is greater than the known temperature which is required to effect a thermal or heat-activated desorption, then the reaction is thermal. However, if the bulk temperature of the refrigerant/sorbent compound during the desorption reaction is less than the temperature required to effect the thermal desorption, the reaction may or may not be thermal. 
     In this event, the velocity distribution of the desorbed refrigerant molecules may be analyzed to determine whether the desorption reaction is substantially non-thermal. The molecular velocity distribution can be determined by, for example, using time-of-flight spectroscopy to produce a time-resolved distribution of the florescence intensities of a characteristic molecular beam. Then, using a Fourier transform, the molecular velocity distribution can be extracted from the florescence data. Since it is known that in a non-thermal process the velocity distribution of the desorbed refrigerant molecules should be primarily non-Maxwellian, by analyzing the time-of-flight spectroscopy data, the thermal/non-thermal nature of the desorption process can be determined. 
     The sorbent should also comprise certain physical properties to enable it to be effectively utilized in the thermal storage device  10 . For example, the sorbent is preferably sufficiently strong to withstand repeated adsorption and desorption reactions without fracturing or decomposing. In addition, the sorbent is ideally comprised of a material that can be soldered, brazed or otherwise attached to the sorber to enhance the transfer of thermal and electrical energy through the junction between the sorbent and the sorber. Furthermore, the sorbent is optimally configured or constructed to comprise suitable mass transfer paths to facilitate the passage of a maximum amount of refrigerant through the sorbent in a minimum amount of time during the adsorption and desorption reactions. Also, since the total amount of refrigerant that can be adsorbed on a sorbent is proportional to the total surface area of the sorbent, the sorbent preferably comprises a relatively large surface area per unit volume of material. 
     Consistent with the above discussion, suitable sorbent materials for use in the present invention include pitch-based carbon and graphitic foams, examples of which are disclosed in U.S. Pat. Nos. 5,961,814 and 6,033,506, which are hereby incorporated herein by reference. Another suitable sorbent material is a graphitic foam product which is available from Poco Graphite, Inc. of Decatur, Tex. under the brand name PocoFoam™. In order to improve the adsorption capacity of these foams, they may be activated using any suitable activation technique. Alternatively, the sorbent could comprise a pre-activated graphitic foam product, such as is described in applicants&#39; co-pending U.S. patent application Ser. No. [Docket No. SUNM-P006 US], which is hereby incorporated herein by reference. 
     Simple carbon and graphite pellets, granules, powders and fibers may also be used as the sorbent material. These materials are preferably activated using a suitable activation method in order to improve their adsorption capacity. Also, any of the sorbent materials disclosed in applicants&#39; U.S. patent application Ser. No. 09/834,080 may be used in the present invention. It should be understood that this list of possible sorbent materials is not complete, and that other materials which meet some or all of the above-listed requirements, including liquid materials, may also make suitable sorbents. The present invention should therefore not be limited by the particular sorbent materials listed above. 
     The refrigerant which is employed in the thermal storage device  10  depends in large part on the sorbent selected and the temperature differential desired to be achieved between the compartment  14  and the ambient atmosphere. Generally, once a suitable sorbent is chosen an appropriate refrigerant may be selected by examining the vapor pressure curves for various refrigerant/sorbent compounds. The inventors have discovered that suitable refrigerants for use with the carbon and graphitic foam sorbents discussed above include R134, Ammonia, Carbon Dioxide, Nitrous Oxide, Nitrogen, Krypton, Hydrogen and Methane, among others. 
     In accordance with another embodiment of the present invention, thermal energy may be used to effect the desorption of the refrigerant from the sorbent. Referring to FIG. 5, a sorber which may be used with such a desorption mechanism is indicated generally by reference number  30 ′″. Similar to the sorbers discussed above, the sorber  30 ′″ comprises a tubular, preferably cylindrical housing  66 , an optional heater mounting sleeve  70  which is secured with the housing such as by welding, and a sorbent  74  which is positioned between the housing and the sleeve. In this embodiment, thermal energy is provided by a suitable heater  104 , such as an electrical resistance or gas combustion heater, which is supported in a longitudinal bore  106  in the mounting sleeve  70 . In the event the heater  104  is an electrical resistance heater, it is ideally connected to an external power source via a pair of electrical leads  84 ,  86 . The refrigerant is communicated between the sorbent  74  and the refrigeration loop through an inlet  40  and an outlet  46 . Suitable refrigerant and sorbent materials for this embodiment include the refrigerant and sorbent materials discussed above. 
     In operation of the sorber  30 ′″, the desorption reaction is initiated by activating the heater  104 . When activated, the heater  104  will generate thermal energy in the form of heat, and this heat will be conducted through the sleeve  70  to the refrigerant/sorbent compound. Accordingly, the sleeve  70  is preferably made of a thermally conductive material, such as aluminum. When the refrigerant/sorbent compound is stochastically heated to a degree sufficient to break the bonds between the refrigerant and sorbent molecules, the refrigerant will begin to desorb from the sorbent. Continued heating of the refrigerant/sorbent compound will ensure that a desired amount of refrigerant is desorbed from the sorbent. 
     In accordance with the present invention, the power required to drive the desorption reactions in each of the sorber embodiments described above is supplied by an external power supply, such as a source of conventional line voltage. Referring again to FIG. 1, the thermal storage device  10  comprises a power jack  108  which is connected to the external power supply by a suitable cable (not shown). If the power from the external power supply is not in a form that is usable by the sorbers  30 , the thermal storage device  10  may also comprise an internal power supply  110 . The internal power supply  110  is connected to the jack  108  and may include, for example, conventional transformer, rectifier, filter and regulator devices for converting the voltage from the external power supply into a current which is required to power the sorbers  30 . Each of the sorbers  30  is electrically connected to the internal power supply  110  by a pair of leads  84 ,  86 . 
     During the assembly of the thermal storage device  10 , the refrigeration loop is evacuated and the sorbers  30  are each charged with a predetermined amount of refrigerant by adsorbing the refrigerant onto the sorbent. In order to prepare the thermal storage device  10  for transport, the valve  58  is closed and the external power supply is connected to the jack  108 . The power supply is then activated to desorb the refrigerant from the sorbent. As the refrigerant is desorbed from the sorbent, it will expand into the receiver  28  and the conduit  36 , where it will condense and remain until the compartment  14  requires cooling. 
     The amount of power and the approximate length of time required to complete the desorption reaction are dependent on the amounts and types of refrigerant and sorbent materials used in the sorption compression refrigeration system. For example, if the system requires X sorbate  grams of refrigerant and it is known that E desorb  joules of energy are required to desorb one gram of refrigerant from the sorbent, then a total of E desorb  joules/gram times X sorbate  grams=E total  joules of energy will be required to completely desorb the refrigerant from the sorbent. The total desorption time, t desorb , is obtained by dividing E total  by the applied power level, P supply . 
     The compartment  14  may then be loaded with articles to be shipped or refrigerated. If the articles have been pre-refrigerated, the flow control device  58  will remain closed until the temperature of the articles rises above a predetermined desired temperature. In this event, the flow control device  58  will operate to allow some of the refrigerant to pass into the evaporator, where it will evaporate and thereby cool the articles to the desired temperature. If the articles have not been pre-refrigerated, the flow control device  58  will operate to allow the refrigerant to evaporate until the temperature of the articles drops to the desired temperature. The flow control device  58  will then operate to retain the remaining portion of the refrigerant within the receiver  28 . If the temperature of the articles subsequently rises above the desired temperature, the flow control device  58  will again operate to allow more refrigerant to evaporate and further cool the articles. 
     A particularly advantageous feature of the present invention is the ability of the sorber to control the flow of the refrigerant into the evaporator  26  without a separate flow control device  58  positioned upstream of the evaporator. Therefore, the sorber itself may be used as the flow control device. As mentioned above, the heat of adsorption will inhibit the ability of the sorbent to adsorb additional refrigerant. Thus, once the heat of adsorption raises the temperature of the sorbent above a certain temperature, the sorbent will substantially stop adsorbing additional refrigerant. Moreover, the sorbent will not be able to absorb additional refrigerant until the sorbent is cooled by removing the heat of adsorption. 
     Therefore, for a given range of ambient conditions, the sorber can be designed so that the heat of adsorption will be dissipated from the sorbent at the same rate at which the ambient heat is transferred to the compartment  14 . In this manner, as the ambient heat is transferred to the compartment  14 , the heat of adsorption will be dissipated from the sorbent. Moreover, as the heat of adsorption is dissipated from the sorbent, the sorbent will be able to adsorb additional refrigerant. Consequently, the refrigerant will be drawn through the evaporator  26 , where it will evaporate and thereby cool the compartment  14 . 
     In another embodiment of the invention, the flow control device  58  comprises a capillary tube, an orifice valve, or a similar flow control device. Moreover, as is understood by those of skill in the art, the flow control device  58  is designed to maintain the desired temperature in the compartment  14 . Thus, as the temperature in the compartment  14  rises above the desired temperature, the flow control device  58  will release the refrigerant into the evaporator, where it will evaporate and thereby cool the compartment  14 . The evaporated refrigerant will then be re-adsorbed onto the sorbent. 
     In yet another embodiment of the invention, the flow control device  58  comprises a thermostatic expansion valve (TEV), a thermal expansion valve (TXV), or a similar flow control device. As is understood by those of skill in the art, such valves may be adjusted to maintain the desired temperature in the compartment  14 . Thus, as the temperature in the compartment  14  rises above the desired temperature, the flow control device  58  will release the refrigerant into the evaporator, where it will evaporate and thereby cool the compartment  14 . The evaporated refrigerant will then be re-adsorbed onto the sorbent. 
     Referring also to FIG. 6, in one embodiment of the invention the thermal storage device  10  comprises a temperature sensor  112  which is thermally coupled to the compartment  14 , and a programmable controller  114  which is designed to manage certain operations of the thermal storage device in response to preprogrammed instructions that are stored in an associated memory device. The power required to operate the controller  114  and the devices which are attached to the controller is optimally provided by a portable power supply, such as a battery (not shown). The temperature sensor  112 , which can be, for example, a conventional thermistor, generates a signal that is indicative of the temperature in the compartment  14  and sends this signal to the controller  114 . The controller  114  then either displays this temperature on an associated display  116 , or compares this temperature to a desired temperature for the compartment  14 , which may be input into the controller using a simplified keypad  118 . If the temperature in the compartment  14  rises above the desired temperature, the controller can provide a visual indication of such on the display  116  or an audible indication of such on a speaker (not shown). In the event the flow control device  58  is a manually operable valve, the operator of the thermal storage device can then open the valve to cool the compartment  14  as described above until the temperature of the compartment reaches the desired temperature. 
     In another embodiment of the invention, the flow control device  58  is a solenoid type valve which can be opened or closed with an appropriate electrical signal. In addition, the controller  114  includes a switch  120  which is connected to the valve  58 . In this manner, the operator may open or close the valve  58  using the switch  120 . 
     In yet another embodiment of the invention, the flow control device  58  is a solenoid type valve which can be opened or closed with an appropriate electrical signal, and the controller  60  actuates the valve to initiate and terminate each adsorption cycle. Thus, when the controller  114  determines that the temperature in the compartment  14  has risen above a desired temperature, it will generate an appropriate signal to open the valve  58 . When the controller  114  then determines that the temperature of the compartment has dropped to a predetermined temperature below the desired temperature, it will generate an appropriate signal to close the valve  58 . In this manner, the controller  114  can automatically maintain the temperature of the compartment  14 , and thus the articles within the compartment, at the desired temperature. 
     The controller  114  may also be programmed to manage each desorption reaction. In this example, a switch similar to the switch  120  is connected to the internal power supply  110 . In addition, the time required to complete the desorption reaction is determined, for example as described above, and input into the controller  114 . The operator may initiate the desorption reaction by pressing the switch to activate the internal power supply  110 . The controller will then provide a visual or audible indication when the time required to complete the desorption reaction has elapsed, whereupon the operator may terminate the desorption reaction by pressing the switch again to deactivate the internal power supply  110 . 
     In another embodiment of the invention, the controller  114  can be programmed to automatically terminate the desorption reaction. For example, the controller can deactivate the internal power supply  110  when the time required to complete the desorption reaction has elapsed. Alternatively, the thermal storage device  10  may comprise a transducer  122  to measure a condition of the sorption compression refrigeration system which is indicative of the end of the desorption cycle, and the controller  114  can deactivate the internal power supply  110  once it receives the appropriate signal from the transducer. For example, when an electrical current is used to effect the desorption reaction and the sorbent comprises a carbon based material, the current will tend to resistively heat the sorbent after the refrigerant has been desorbed. Therefore, the transducer  122  could comprise a temperature sensor, which would enable the controller  114  to monitor the temperature of the sorber  30  and deactivate the internal power supply  110  when a predetermined increase in the temperature is detected. Also, as the refrigerant is desorbed from the refrigerant/sorbent compound, the impedance of the refrigerant/sorbent compound will decrease. Thus, the transducer  122  could comprise an impedance sensor, which would allow the controller  114  to sense a change in the impedance of the sorber  30  and deactivate the internal power supply  110  when the refrigerant has been desorbed. In addition, since the pressure within the receiver  28  will reach a maximum level once the entire amount of refrigerant is desorbed from the sorber  30 , the transducer  122  could comprise a pressure sensor which is connected to the receiver. In this case, the controller  114  would sense the pressure in the receiver  28  and deactivate the internal power supply  110  once the maximum pressure is reached. 
     Referring now to FIG. 7, an embodiment of a thermal storage device is shown which is similar in many respects to the thermal storage device  10  just described. However, in this embodiment the thermal storage device, which is indicated generally by reference number  200 , comprises a single sorber  124  which is helically wound around the container  16 . The sorber  124  may be similar to any of the sorber embodiments described above, except that it is elongated to a degree sufficient to contain the entire amount of sorbent which may be required by the sorption compression refrigeration system. The advantage of this sorber design is that it may be connected directly between the outlet  38  of the evaporator  26  and the inlet  48  of the receiver  28 . In addition, only a single pair of leads  84 ,  86  is required to connect the internal power supply  110  to the sorber  124 . In all other respects, the construction and operation of the thermal storage device  200  is similar to those of the thermal storage device  10 . 
     Yet another embodiment of a thermal storage device according to the present invention is shown in FIG.  8 . The thermal storage device of this embodiment, which is indicated generally by reference number  300 , is particularly suited for use as a cooler for transporting and storing refrigerated food items and the like. The thermal storage device  300  thus comprises a preferably insulated housing  12  which encloses a compartment  14 , a lid  20  which is attachable to the housing to seal the compartment from the environment, and a sorption compression refrigeration system for maintaining the temperature in the compartment at a desired temperature. As in the previous embodiments, the sorption compression refrigeration system includes an evaporator  26  which is positioned in heat exchange relation with respect to the compartment  14 , a receiver  28  which is fluidly connected to the evaporator, and a sorber  126  which is fluidly connected between the evaporator and the receiver. 
     The sorber  126  is especially useful in effecting the desorption of the refrigerant from the sorbent using an electrical current. Referring to FIGS. 9 and 10, the sorber  126  comprises a housing  128  which includes a top plate  130  that is attached to a bottom plate  132 , and a sorbent  134  which is positioned between the top and bottom plates. In this example, the top plate  130  comprises a first electrical conductor and the bottom plate comprises a second electrical conductor. Accordingly, the top and bottom plates  130 ,  132  are made of a suitable electrically conductive material, such as an aluminum alloy. In addition, the top and bottom plates  130 ,  132  are electrically insulated from each other, such as by a suitable gasket  136  or any of the means previously described. Furthermore, the top and bottom plates  130 ,  132  are secured together with a number of suitable fasteners  138 , such as high strength steel bolts. Also, as shown most clearly in FIG. 11, an insulating grommet  140 , which is made of an appropriate electrically insulating and heat resistant material, such as Teflon®, is positioned between each bolt  138  and the top plate  130  to electrically insulate the bolt, and thus the bottom plate  132 , from the top plate. The refrigerant is communicated between the sorbent  134  and the refrigeration loop through an inlet port  46  and an outlet port  46 , which may both be formed in the top plate  130 . 
     The sorber  126  is preferably designed to help dissipate the heat of adsorption from the refrigerant/sorbent compound. Thus, in addition to being electrically conductive, the top and bottom plates  130 ,  132  are preferably constructed of a material having a good thermal conductivity. In addition, if as shown in FIGS. 9 and 10 the sorbent  134  comprises relatively large top and bottom surfaces  142 ,  144 , respectively, compared to its thickness “t”, the top and bottom plates  130 ,  132  preferably each include a respective inner surface  142 ′,  144 ′ which engages substantially the entire corresponding top or bottom surface  142 ,  144 . In this manner, the thermal diffusion path length for the refrigerant/sorbent compound will be minimized (in effect one-half the thickness “t”), and the rate of heat transfer from the refrigerant/sorbent compound will consequently be maximized. In addition, the top plate  130  or the bottom plate  132 , or both, may be provided with cooling fins  146  to assist in the dissipation of the heat of adsorption from the refrigerant/sorbent compound. As shown in FIG. 8, the cooling fins  146  are ideally exposed to the external environment through an opening  148  in the housing  12 . 
     The transfer of thermal and electrical energy through the junction between the refrigerant/sorbent compound and the sorber  126  is preferably optimized by enhancing the contact between the sorbent  134  and the top and bottom plates  130 ,  132 . Depending on the sorbent  134  which is employed in the thermal storage device  300 , this may be accomplished by soldering or brazing the sorbent to the top and/or bottom plates  130 ,  132 . Alternatively, the sorbent  134  may be affixed to the top and/or bottom plates  130 ,  132  using a suitable thermally and electrically conductive adhesive. Where brazing, soldering or gluing are not appropriate, the sorbent  134  and the sorber  126  may be designed with a slight interference fit to produce a suitable contact pressure between the sorbent and the top and bottom plates  130 ,  132 . The contact between the sorbent  134  and the sorber  126  may also be enhanced by positioning a foil of soft metal, such as indium, between the sorbent and each of the top and bottom plates  130 ,  132 . 
     In the embodiment of the invention shown in FIGS. 9 and 10, the sorbent  134  is formed into a monolithic member having a thickness “t” and generally parallel top and bottom surfaces  142 ,  144  which each have a length “l” and a width “w”. Although the surfaces  142 ,  144  are depicted as being rectangular, they could have any practical shape. Since in this embodiment the top and bottom plates  130 ,  132  of the sorber  126  function to both conduct the electrical current across and dissipate the heat of adsorption from the refrigerant/sorbent compound, the electrical conduction and thermal diffusion paths of the sorbent  134  are both aligned in the direction of the thickness “t”. As mentioned above, in order to maximize the amount of power which is transferred to the refrigerant/sorbent compound from an AC power supply, the combined impedance of the sorber  126  and the refrigerant/sorbent compound should match that of the external power supply. Thus, for given refrigerant and sorbent materials, the thickness “t” of the sorbent may be increased or decreased to adjust the impedance accordingly. 
     However, in order to minimize the thermal diffusion path length through the sorbent  134 , the thickness “t” should be kept as small as possible. In the event the heat of adsorption is dissipated through both the top and bottom surfaces  142 ,  144 , the thickness “t” is preferably less than the smallest linear dimension of the top or bottom surface, which, for example, is the length of the minor side of a rectangle, the length of any side of a square, or the length of the diameter of a circle. If the heat of adsorption is dissipated through only one of the top and bottom surfaces  142 ,  144 , the thickness “t” is preferably less than one-half the smallest linear dimension of the top or bottom surface. More preferably, the thickness “t” is less than one-tenth the smallest linear dimension of the top or bottom surface. By sizing the sorbent  134  accordingly, the minimum thermal diffusion path length will be transverse to the top and bottom surfaces  142 ,  144 , and the heat of adsorption will consequently be readily dissipated through either or both of these surfaces. 
     The thermal storage device  300  optimally includes a power cord  150  to connect the sorber  126  to an external power supply (not shown). The power cord  150  may be connected to the sorber  126  either directly, in the event the external power source generates a current which is useable by the sorber, or otherwise through an internal power supply  110 . If desired, the power cord  150  may be removably connected to a power jack which in turn is connected to the sorber  126  or the internal power supply  110 . This will enable the thermal storage device  300  to be transported more easily. The operation of the thermal storage device  300  is similar to that of the thermal storage device  10  discussed above. 
     Referring now to FIG. 12, a further embodiment of a thermal storage device is shown which is especially useful in refrigerating individual items such as bottles and cans. The thermal storage device of this embodiment, generally  400 , is shown to comprise a housing  12  which surrounds a compartment  14 , an evaporator  26  which is positioned in heat exchange relation with respect to the compartment, a receiver  28  which is fluidly connected to the evaporator, and a sorber  126  which is fluidly connected between the evaporator and the receiver. In this embodiment, the top of the housing  12  may be left open to allow elongated articles to at least be partially received within the compartment  14 . In addition, the evaporator  26  may include a cooling cylinder  152  which comprises an inner diameter that is sized to engage the outer diameter of a standard sized article and an outer diameter that is attached to an evaporator tube  154 , such as by brazing. The cooling cylinder  152  is ideally comprised of a material having a relatively high thermal conductivity, such as aluminum. 
     The thermal storage device  400  is preferably inexpensive to produce, sufficiently lightweight to be hand carried with an article to be cooled inserted therein, and simple to operate. Therefore, the flow control device  58  is optimally a manually operable valve which can be actuated via a push button  156 . In addition, the thermal storage device  400  ideally does not include an internal power supply, a temperature sensor or a controller. Instead, certain of these components may be located in a separate recharging stand. 
     Referring also to FIG. 13, such a recharging stand, which is indicated generally by reference number  158 , is shown to comprise a base  160  which includes a receptacle  162  that is sized to receive the lower portion of the housing  12  of the thermal storage device  400 . The recharging stand  158  also includes a pair of preferably retractable leads  184 ,  186  which are movably mounted in the base  160  and which will engage the top and bottom plates  130 ,  132  of the sorber  126  through corresponding holes  164  in the housing  12  when the thermal storage device  400  is inserted in the receptacle  162 . The recharging stand  158  may also comprise a power cord  150  to connect the leads  184 ,  186  to an external power source (not shown), an internal power source  110  to convert the power from the external power source into a form which is usable by the sorber  126 , and a manual power switch  120  to enable the leads to be selectively engaged and disengaged from the external power source. The recharging stand  158  preferably also includes a conventional thermal switch  166  which engages the sorber  126  when the thermal storage device  400  is inserted into the receptacle  162  and operates to electrically disconnect the power cord  150  from the leads  184 ,  186  once it detects a predetermined temperature. 
     In operation, the sorber  126  is charged by inserting the thermal storage device  400  into the recharging stand  158  and pressing the switch  120 . Power from the external power supply will consequently be conducted through the leads  184 ,  186  to the sorber  126  to desorb the refrigerant from the sorbent. The refrigerant will accordingly expand through the check valve  56  and into the receiver  28 , where it will stay as long as the valve  58  remains closed. When the refrigerant has been substantially fully desorbed from the sorbent, the temperature of the sorber  126  will increase, and the thermal switch  166  will cut the power to the leads  184 ,  186 . An article to be refrigerated, such as a beverage can, may then be placed in the compartment  14  and carried about. Once the temperature of the article rises above a desired temperature, the valve  58  may be manually actuated to release the refrigerant into the evaporator to thereby cool the article. If sufficient refrigerant remains in the receiver  28 , the article may be repeatedly cooled in this fashion. 
     It should be recognized that, while the present invention has been described in relation to the preferred embodiments thereof, those skilled in the art may develop a wide variation of structural and operational details without departing from the principles of the invention. For example, the various elements shown in the different embodiments may be combined in a manner not illustrated above. Therefore, the appended claims are to be construed to cover all equivalents falling within the true scope and spirit of the invention.