Patent Publication Number: US-2012037148-A1

Title: Articles and devices for thermal energy storage and methods thereof

Description:
CLAIM OF BENEFIT OF FILING DATE 
     The present invention claims the benefit of the filing date of U.S. Provisional Patent Application No. 61/373,008, filed on Aug. 12, 2010, the contents of which are hereby incorporated by reference in their entirety. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to thermal energy storage using a thermal energy storage material and to the packaging of the thermal energy storage material to allow for both efficient heat storage and efficient heat transfer. 
     BACKGROUND OF THE INVENTION 
     Industry in general has been actively seeking a novel approach to capture and store waste heat efficiently such that it can be utilized at a more opportune time. Further, the desire to achieve .energy storage in a compact space demands the development of novel materials that are capable of storing high energy content per unit weight and unit volume. Areas of potential application of breakthrough technology include transportation, solar energy, industrial manufacturing processes as well as municipal and/or commercial building heating. 
     Regarding the transportation industry, it is well known that internal combustion engines operate inefficiently. Sources of this inefficiency include heat lost via exhaust, cooling, radiant heat and mechanical losses from the system. It is estimated that more than 30% of the fuel energy supplied to an internal combustion engine (internal combustion engine) is lost to the environment via engine exhaust. 
     It is well known that during a “cold start” internal combustion engines operate at substantially lower efficiency, generate more emissions, or both, because combustion is occurring at a non-optimum temperature and the internal combustion engine needs to perform extra work against friction due to high viscosity of cold lubricant. This problem is even more important for hybrid electric vehicles in which the internal combustion engine operates intermittently thereby prolonging the cold start conditions, and/or causing a plurality of occurrences of cold start conditions during a single period of operating the vehicle. To help solve this problem, original equipment manufacturers are looking for a solution capable of efficient storage and release of waste heat. The basic idea is to recover and store waste heat during normal vehicle operation followed by controlled release of this heat at a later time thereby reducing or minimizing the duration and frequency of the cold start condition and ultimately improving internal combustion engine efficiency, reducing emissions, or both. 
     To be a practical solution, the energy density and the thermal power density requirements for a thermal energy storage system are extremely high. Applicants have previously filed 1) U.S. Patent Application Publication No. 2009-0211726 by Soukhojak et al., entitled “Thermal Energy Storage Materials” and published on Aug. 27, 2009; 2) U.S. Patent Application Publication No. 2009-0250189 by Bank et al., entitled “Heat Storage Devices” and published on Oct. 8, 2009, 3) PCT Application No. PCT/US09/67823 entitled “Heat Transfer Systems Utilizing Thermal Energy Storage Materials” and filed on Dec. 14, 2009, and 4) U.S. Provisional Application No. 61/299,565 entitled “Thermal Energy Storage” and filed on Jan. 29, 2010. These previous applications are incorporated herein by reference in their entirety. 
     There are known heat storage devices and exhaust heat recovery devices in the prior art. However, in order to provide a long term (e.g., greater than about 6 hour) heat storage capability, they generally occupy a large volume, require pumping of a large volume of heat transfer fluid, require a relatively large pump to overcome the hydraulic resistance, and the like. Therefore, there is a need for a heat storage system which can offer an unprecedented combination of high energy density, high power density, long heat retention time, light weight, low hydraulic resistance for heat transfer fluid flow, or any combination thereof. 
     The issue of packaging of the thermal energy storage materials for applications requiring systems that are light weight, such as in transportation, requires both strong packaging and packaging that is light weight. For example, the packaging should be durable so that it may contain a large concentration of thermal energy storage material, contain the thermal energy storage material over a wide range of temperatures (upon which it may undergo large changes in volume), contain the thermal energy storage material in a plurality of cells or capsules that are sealed from each other, or any combination thereof. The need for light weight thermal energy storage systems may require that the weight of the packaging be reduced. 
     For example, there is a need for thermal energy storage material that is encapsulated in capsules that are light weight, have a high energy density, or have a high power density; and are durable (e.g., so that the capsules do not leak upon heating to a temperature of about 400° C.; the capsules do not leak upon repeated heating between a temperature of about 25° C. and a temperature of about 240° C. for about 1,000 cycles or more; or both). 
     SUMMARY OF THE INVENTION 
     One aspect of the invention is an article comprising: a base sheet (preferably a metal base sheet); a cover sheet (preferably, a metal cover sheet) sealingly joined to the base sheet to form a capsular structure, wherein the capsular structure includes one, two or more sealed spaces; a thermal energy storage material, wherein the thermal energy storage material is contained within the sealed spaces, and wherein the thermal energy storage material has a liquidus temperature of about 150° C. or more; wherein the sealed spaces are substantially free of water or includes liquid water at a concentration of about 1 percent by volume or less at a temperature of about 25° C., based on the total volume of the sealed spaces; and wherein the article includes one or more of the following features: a) the pressure in a sealed space is a vacuum of about 700 Torr or less, when the temperature of the thermal energy storage material is about 25° C.; b) the cover sheet includes one or more one or more stiffening features, wherein the stiffening features include indents into the sealed space, protrusions out of the sealed space, or both, that are sufficient in size and number to reduce the maximum von Mises stress in the cover sheet during thermal cycling; c) the base sheet and/or the cover sheet includes one or more volume expansion features; or d) the cover sheet has a thickness, t c , and the base sheet has a thickness, t b , wherein t c  is greater than t b ; so that the article does not leak after thermal cycling between about 25° C. and about 240° C., for about 1,000 cycles. The stiffening feature may be any feature (such as an indention or a protrusion) that redistributes the stresses in the base sheet and the cover sheet so that when the pressure in the sealed space increases (e.g., due to thermal expansion, or melting of the thermal energy storage material) the maximum von Mises stress is reduced compared to a base sheet and/or a cover sheet without the stiffening feature and subjected to the same pressure. Without limitation, examples of stiffening features that may be employed in the metal cover sheet include dimples, chevrons, ribs, or any combination thereof. 
     In a particularly preferred aspect of the invention, the capsular structure has one or more fluid passages which are sufficiently large to allow a heat transfer fluid to flow through the one or more fluid passages; and when the capsular structure is in contact with a heat transfer fluid, the thermal energy storage material is isolated from the heat transfer fluid. 
     Another aspect of the invention is a device including a container and a stack of two or more articles within the container. For example, the device may contain a plurality of articles described herein. Preferably, each article includes a fluid passage and contains the thermal energy storage material. Preferably the articles having a fluid passage are stacked so that their fluid passages are generally aligned, preferably axially. 
     Another aspect of the invention is a process for preparing an article, such as an article described herein, including the steps of: forming one, two or more troughs in a base sheet; ii) at least partially filling one, two or more of the troughs with a thermal energy storage material; and at least partially joining (e.g., sealingly joining) a first metal foil (e.g., the base sheet) with a second metal foil (e.g., the cover sheet) to form a sealed space; wherein the thermal energy storage material includes a metal salt, and wherein the metal salt is in a molten state during the joining step. 
     Yet another aspect of the invention is directed at a process for storing heat comprising a step of: transferring a sufficient amount of thermal energy to an the article of the invention, so that the thermal energy storage material in the article is heated to a temperature of about 200° C. or more. 
     The articles, devices, systems and processes of the present invention advantageously are capable of containing a high concentration of thermal energy storage material so that a large amount of thermal energy can be stored (e.g., having a high energy density), are capable of having a high surface area between the heat transfer fluid and the article containing the thermal energy storage material so that heat can be quickly transferred into and/or out of the thermal energy storage material (e.g., having a high power density, preferably greater than about 8 kW/L), are capable of having multiple flow paths that have similar or equal hydraulic resistance so that heat is uniformly transferred to and/or transferred from different regions; have a rotational symmetry so that they may be arranged easily; have a structure that is strong and durable; have a high heat storage density so that they can be used in applications requiring compact designs, light weight components, or both; have lower hydraulic resistance for a heat transfer fluid flow (for example, a pressure drop of less than about 1.5 kPa at a heat transfer fluid pumping rate of about 10 liters/min) so that the pumping requirements for the heat transfer fluid are reduced, are sufficiently strong so that the thermal energy storage material does not leak from a sealed space after heating a capsular structure including the thermal energy storage material to a temperature of about 400° C., are sufficiently durable so that the thermal energy storage material does not leak from a sealed space after repeatedly heating the capsular structure with the thermal energy storage material between about 25° C. and about 240° C. for about 1,000 cycles or more, or an combination thereof. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
       The present invention is further described in the detailed description which follows, in reference to the noted plurality of drawings by way of non-limiting examples of embodiments of the present invention, in which like reference numerals represent similar parts throughout the several views of the drawings, and wherein: 
         FIG. 1  is a drawing of an illustrative article having a sealed compartment. 
         FIG. 2A  is a cross-sectional view of an illustrative article including a plurality of sealed spaces. 
         FIG. 2B  is a drawing of an illustrative cross-section of a single capsule having a sealed space that may be employed in the article. 
         FIG. 3  is a drawing of an illustrative base sheet that may be employed in the article. 
         FIG. 4A  is a drawing of an illustrative cover sheet having chevrons that may be employed in the article. 
         FIG. 4B  is a drawing of an illustrative capsule including a cover sheet having dimples that may be employed in an article having one or more capsules. 
         FIG. 4C  is a drawing of a section of an illustrative cover sheet including dimples that may be employed in the article. 
         FIG. 4D  is a drawing of an illustrative cover sheet having a plurality of stiffening features, such as plurality of protrusions and/or recesses that may be used in an article. 
         FIG. 5A  is a drawing of an illustrative cross-section of a sealed compartment at the temperature at which the compartment is sealed. 
         FIG. 5B  is a drawing of an illustrative cross-section of a sealed at a temperature below the sealing temperature. 
         FIG. 5C  is a drawing of an illustrative cross-section of a sealed compartment at a temperature above the sealing temperature. 
         FIGS. 6A ,  6 B, and  6 C are drawings of an illustrative sheet that has been formed to have ribs. 
         FIGS. 7A and 7B  are drawings illustrating a sheet having a volume expansion feature that allows the volume of the sealed space to increase (e.g., to accommodated the volume expansion of the thermal energy material as it is heated and/or melts). 
         FIG. 8  is an illustrative graph showing the relationship between the thickness of the cover sheet and the maximum von Mises stress in the cover sheet when the cover sheet is employed in an article containing a thermal energy storage material and the thermal energy storage material is heated. 
         FIG. 9  is an illustrative graph showing the relationship between the thickness of the cover sheet and the maximum expected von Mises stress in the cover sheet for a cover sheet that is flat and for a cover sheet that includes ribs. 
         FIG. 10  is an illustrative graph showing the relationship between the thickness of the cover sheet and the maximum expected von Mises stress in the cover sheet for a cover sheet that is flat, for a cover sheet that includes dimples, for a cover sheet that includes chevrons, and for a cover sheet that includes ribs. 
         FIG. 11  is an illustrative drawing of a portion of tooling that may be employed in manufacturing a sheet that includes ribs. 
         FIG. 12  shows an illustrative stack of articles. 
         FIG. 13  shows a surface of a base sheet of an article including one or more sealed compartment.  FIG. 13  illustrates that a sealed compartment may have a primary seal and one or more secondary seals. 
         FIG. 14  is a drawing of an illustrative heat storage device. 
         FIG. 15 . Is a drawing of an illustrative tooling for embossing a base sheet. 
         FIG. 16  shows a surface of a base sheet of an article including one or more spaces being filled with thermal energy storage material using a nozzle. 
     
    
    
     DETAILED DESCRIPTION OF THE PRESENT INVENTION 
     In the following detailed description, the specific embodiments of the present invention are described in connection with its preferred embodiments. However, to the extent that the following description is specific to a particular embodiment or a particular use of the present techniques, it is intended to be illustrative only and merely provides a concise description of the exemplary embodiments. Accordingly, the invention is not limited to the specific embodiments described below, but rather; the invention includes all alternatives, modifications, and equivalents falling within the true scope of the appended claims. 
     As will be seen from the teachings herein, the present invention provides unique articles, devices, systems, and process for storing thermal energy and/or transferring stored thermal energy to a fluid. For example, the articles and devices for storing thermal energy of the present invention are more efficient at storing thermal energy, allow for transferring thermal energy more uniformly, allow for transferring thermal energy with a smaller pressure drop of the heat transfer fluid, or any combination thereof. 
     Various aspects of the invention are predicated on an article including a capsular structure having one or more sealed spaces (i.e., capsules) and one or more thermal energy storage materials that are encapsulated in the one or more sealed spaces of the capsular structure so that the thermal energy storage material cannot flow out of the capsular structure or otherwise be removed from the capsular structure. When the thermal energy storage material is heated during operation, the volume may increase due to thermal expansion, due to difference in the densities of the liquid and solid phases of the thermal energy storage material, or both. The increase in volume of the thermal energy storage material may be about 5% or more, about 10% or more, about 15% or more, or even about 20% or more. For example, a metal salt, such as lithium nitrate, may increase in volume by more than 20% when heated from about 23° C. to about 300° C. It will be appreciated that as the thermal energy storage material is heated, the pressure in the sealed space may increase. The capsular structure should be sufficiently durable so that it does not leak or otherwise fail when the thermal energy storage material expands during use. The capsular structure preferably has a geometry that allows a heat transfer fluid to efficiently remove heat from the thermal energy storage material. Without limitation, examples, of preferred capsular structures include those described in U.S. Patent Application Publication No. 2009/0250189 by Soukhojak et al., published on Oct. 8, 2009, and U.S. Provisional Patent Application No. 61/299,565 filed on Jan. 29, 2010, incorporated herein by reference. For example, the capsular structure may have a geometry that includes one or more fluid passages that are sufficiently large so that the capsular structure is capable of allowing a fluid (e.g., a heat transfer fluid) to flow through the fluid passage. The thermal energy storage materials may be sufficiently encapsulated in one or more of the sealed spaces so that when the heat transfer fluid contacts the capsular structure, the thermal energy storage material is isolated from the fluid. 
     A variety of approaches have been identified that advantageously improve the durability of the capsular structure including having a pressure in a sealed space that is a vacuum of about 700 Torr or less when the temperature of the thermal energy storage material is about 25° C., using a metal base sheet that includes one or more stiffening features (e.g., one or more ribs), using a metal cover sheet that includes one or more stiffening features (e.g., one or more ribs, one or more dimples, one or more chevrons, or any combination thereof), using a cover sheet having a thickness greater than the thickness of the base sheet, or any combination thereof. 
     By reducing the pressure of a sealed space at a temperature of about 25° C., the pressure of the sealed space when the thermal energy storage material is heated may also be reduced. The pressure of a sealed space at a temperature of about 25° C. may be reduced to about 700 Torr or less using any convenient means. By way of example, the cover sheet and the base sheet may be joined to formed the sealed space when the thermal energy storage material is at a joining temperature, T j , that is sufficiently high so that when the thermal energy storage material in the sealed space is cooled, the thermal energy storage material contracts and the pressure in the sealed space drops below about 700 Torr. The joining temperature may be greater than the liquidus temperature, T L, TESM , of the thermal energy storage material. Preferably the joining temperature, T j , is about T L, TESM +10° C. or more, even more preferably about T L, TESM +20° C. or more, even more preferably about T L, TESM +30° C. or more, even more preferably about T L, TESM +40° C. or more, even more preferably about T L, TESM +50° C. or more, and most preferably about T L, TESM +60° C. or more. By way of example, T j , may be about 200° C. or more, preferably about 230° C. or more, more preferably about 250° C. or more, even more preferably about 270° C. or more, and most preferably about 290° C. or more. The temperature of the thermal energy storage when the base sheet and the cover sheet are joined may be about 700° C. or less, preferably about 500° C. or less, and more preferably about 400° C. or less. 
     In another example, the cover sheet and the base sheet may be joined while a vacuum is applied to the region that becomes the sealed space. If employed, the vacuum should have a pressure sufficiently low so that upon sealingly joining the base sheet and the cover sheet, the sealed space is a vacuum. For example, a vacuum may be applied having a pressure of about 700 Torr or less, about 660 Torr or less, about 600 Torr or less, about 550 Torr or less, about 500 Torr or less, about 400 Torr or less, or about 300 Torr or less. As such, the entire joining process may be done in a vacuum environment. The cover sheet and the base sheet may be joined while the pressure in the region that becomes the sealed space is about 0.1 Torr or more, preferably about 1.0 Torr or more, and more preferably about 10 Torr or more. Lower pressures may also be employed. Preferably the thermal energy storage material is at a predetermined sealing temperature that is at an elevated temperature when the base sheet and the cover sheet are sealingly joined, so that upon cooling the article to about 25° C. a vacuum is formed in the sealed space. The predetermined sealing temperature may be any temperature at which the density of the thermal energy storage material is lower than its density at 25° C. Preferably the predetermined sealing temperature is greater than the liquidus temperature of the thermal energy storage material (e.g., greater than the liquidus temperature by about 10° C. or more, by about 30° C. or more, or about 60° C. or more). The predetermined sealing temperature may be about 50° C. or more, about 100° C. or more, about 150° C. or more, about 200° C. or more, about 250° C. or more, or about 300° C. or more. The predetermined sealing temperature preferably is sufficiently low so that the thermal energy storage material does not degrade during the sealing process. The predetermined sealing temperature may be about 500° C. or less, about 400° C. or less, or about 350° C. or less. 
     When the thermal energy storage material in the sealed space is at a temperature of about 25° C., the sealed space preferably is a vacuum of about 600 Torr or less, more preferably about 500 Torr or less, even more preferably about 400 Torr or less, and most preferably about 300 Torr or less. The lower limit on the pressure in the sealed space when the thermal energy storage material is at 25° C. is based on manufacturability and is preferably about 0.1 Torr or more, more preferably about 1 Torr and most preferably about 10 Torr or more. 
     The durability of the capsular structure may be increased by adding one or more stiffening features to the base sheet, the cover sheet, or both. The stiffening feature may be any feature that redistributes the stresses in the base sheet and the cover sheet so that when the pressure in the sealed space increases (e.g., due to thermal expansion, or melting of the thermal energy storage material) the maximum von Mises stress is reduced compared to a base sheet and/or cover sheet without the stiffening feature and subjected to the same pressure. The stiffening features may be an indention or protrusion formed in a sheet. As such, the stiffening feature may be a change in the profile of the sheet. The stiffening feature may function by redistributing the stresses in a sealed space (e.g., the stresses obtained when heating the sealed space) so that the maximum von Mises stress is reduced. A stiffening feature may be described by its depth (i.e., the amount of change in the profile, such as compared to the region away from the stiffening feature), its length, its width, or any combination thereof. The stiffening feature preferably has a depth of about 0.1 mm or more, more preferably about 0.2 mm or more, even more preferably about 0.3 mm or more, even more preferably about 0.4 mm or more, even more preferably about 0.5 mm or more, and most preferably about 0.6 mm or more. The stiffening feature preferably has a sufficiently small depth so that the effect of packing or stacking multiple capsular structures, as described herein, is not greatly affected. As such, the stiffening feature preferably has a depth of about 10 mm or less, more preferably about 5 mm or less, even more preferably about 3 mm or less, and most preferably about 2 mm or less. Without limitation, examples of stiffening features that may be employed include ribs, dimples, chevrons, and the like. The stiffening features may include a protrusion or indention that has a length to width that is greater than about 1, preferably about 2 or more, even more preferably about 4 or more, and most preferably about 10 or more, such as a rib. If employed, two ribs in a base sheet or cover sheet in the region of a sealed space may be parallel, perpendicular, or at an acute angle. The stiffening feature may have a generally circular cross-section, such as a dimple. The stiffening features may be arranged in a repeating pattern that includes a plurality of stiffening features aligned in one direction and a plurality of stiffening features aligned in a different direction, such as a chevron pattern. The stiffening features are preferably located on the regions of the sheet that contain a sealed space. There may also be stiffening features located in the region of a sheet that does not containing a sealed space. 
     The stiffening features (e.g., the stiffening features in a cover sheet) may have a sufficient size and number so that the maximum von Mises stress of the capsular structure containing the thermal energy storage material and heated to 250° C. is lower than the von Mises stress of an identical capsular structure, with the exception that the stiffening feature is eliminated (e.g., a sheet having a generally smooth surface, is generally flat, or both, such as a generally flat, smooth cover sheet). The stiffening features are preferably present in a sufficient size and number so that the von Mises stress in the capsular structure containing the thermal energy storage material at 250° C. is reduced by about 5% or more, more preferably about 10% or more, even more preferably about 15% or more, even more preferably about 20% or more, even more preferably about 30% or more, and most preferably about 40% or more, compared with the von Mises stress of an identical capsular structure, with the exception that the stiffening feature are removed (e.g., the sheet has a generally smooth surface, is generally flat, or both). 
     The stiffening feature (optionally along with one or more other features disclosed herein) may be used to reduce the maximum von Mises stress in a sealed space containing thermal energy storage material so that the at a temperature of 250° C., the ratio of the maximum von Mises stress in the base sheet and the cover sheet, S Max, 250 , to the yield stress of the metal at 250° C. (e.g., the metal of the cover sheet, or the lower of the base sheet and the cover sheet), S Y, 250 , is preferably about 0.95 or less, more preferably about 0.90 or less, even more preferably about 0.85 or less, even more preferably about 0.80 or less, and most preferably about 0.70 or less. 
     The durability of the capsular structure may be increased by adding one or more ribs to the base sheet, the cover sheet or both. Preferably the base sheet, the cover sheet include a rib structure (e.g., a sufficient number or ribs and/or ribs of sufficient size) that provides a sufficient rigidity to the capsular structure so that the capsular structure does not bend enough to yield and does not otherwise distort enough to yield. 
     It will be appreciated according to the teachings herein that the base sheet may have a structure, such as a structure including one or more troughs, that is generally more rigid than the cover sheet. Advantageously, the thickness of the base sheet may be sufficiently reduced so that the rigidity of the base sheet more closely matches the rigidity of the cover sheet. By reducing the thickness of the base sheet, the volume of the base sheet and/or the packaging material may be reduced, the weight of the base sheet and/or packaging material may be reduced, or both. Thus, a higher percentage of the weight of the capsular structure may be the weight of the thermal energy storage material. As such, the base sheet may have a thickness (e.g., an average thickness) of t b , and the cover sheet may have a thickness (e.g., an average thickness) of about t c , where t c  is greater than t b . The ratio of t c /t b  preferably is about 1.05 or more, more preferably about 1.10 or more, even more preferably about 1.15 or more, even more preferably about 1.20 or more, even more preferably about 1.25 or more, even more preferably about 1.30 or more and most preferably about 1.35 or more. The difference between t c  and t b  preferably is about 0.01 mm or more, more preferably about 0.02 mm or more, even more preferably about 0.03 mm or more, even more preferably about 0.035 mm or more, even more preferably about 0.04 mm or more, and most preferably about 0.05 mm or more. The difference between t c  and t b  preferably is about 1 mm or less, more preferably about 0.5 mm or less, and most preferably about 0.25 mm or less. By way of example, i) the ratio of t c /t b  may be about 1.05 or more, about 1.10 or more, about 1.20 or more, or about 1.30 or more; ii) the difference between t c /t b  may be about 0.01 mm or more, about 0.02 mm or more, about 0.03 mm or more, or about 0.05 mm or more; or both (i) and (ii). 
     The base sheet, the cover sheet or both may have one or more volume expansion features so that the volume of the sealed space may reversibly increase as the thermal energy storage material expands during heating and/or melting. Examples of volume expansion features include wrinkles, pleats, convolutions, folds, oscillations, and the like. By way of example, the volume expansion feature may include one, two, or more convolutions, folds, or pleats. Preferable volume expansion feature may have a generally bellow, or accordion shape (albeit, typically without an orifice). A stiffening feature, such as one or more dimples, one or more chevrons, or one or more ribs, may also function as a volume expansion feature. It will be appreciated that the size, shape of a volume expansion feature and number of volume expansion features will affect the amount by which the volume of the sealed space is capable of expanding. If employed, the volume expansion features may be sufficient to increase the volume of the sealed space by about 5% or more, preferably by about 10% or more, more preferably by about 13% or more, and most preferably by about 15% or more. The volume expansion feature may allow the sealed space to sufficiently expand so that the internal pressure in the sealed space changes increases by about 35 kPa or less (preferably by about 20 kPa or less, and more preferably by about 10 kPa or less) when the thermal energy storage material is heated from about 25° C. to a temperature at which the thermal energy storage material is a liquid (e.g., to about 200° C., to about 240° C., or to about 250° C.). 
     Other aspects of the invention include novel arrangements including a plurality of the articles, novel devices including one or more of the articles, novel processes for manufacturing the article, and novel processes for using one or more of the articles. By employing the novel article, it is possible to assemble devices capable of storing a large quantity of thermal energy, capable of rapidly transferring thermal energy into or out of the thermal energy storage material, capable of being compact, capable of being light weight, capable of having a low pressure drop of a heat transfer fluid, or any combination thereof. 
     The capsular structure generally has a dimension in one direction (i.e., a thickness) that is smaller than the dimensions in the other directions. Without limitation, examples of capsular structures include those disclosed in U.S. patent application Ser. No. 12/389,598 entitled “Heat Storage Devices” and filed on Feb. 20, 2009, and U.S. Provisional Application No. 61/299,565 entitled “Thermal Energy Storage” and filed on Jan. 29, 2010, both incorporated herein by reference. 
     The shape of the capsular structure and/or article may be defined by the packaging space and may be oddly shaped. The article may include a cover sheet (i.e., a cover sheet) having a top surface and a generally opposing base sheet having a bottom surface. The cover sheet (e.g., the top surface of the cover sheet), the base sheet (e.g., the bottom surface of the base sheet), or both, may have a portion that is (or may be) generally flat (e.g., have a generally planar surface), generally arcuate, or any combination thereof. Preferably the base sheet and/or the bottom surface of the base sheet includes a generally arcuate portion or is generally arcuate, the top surface of the article is generally planar (e.g., the cover sheet is generally flat), or both. In various embodiments of the invention a generally planar cover sheet may include one or more stiffening features (such one or more ribs, dimples, chevrons, or other protrusions or recesses as described herein) and/or one or more volume expansion features. As described herein, a cover sheet may also be replaced by a second base sheet. As such, the capsular structure may be define by two base sheets that are the same or different. 
     The capsular structure may include one or more openings, such as a fluid passages. For example, the capsular structure may include one or more fluid passages so that a fluid, such as a heat transfer fluid, can flow through the article without contacting the thermal energy storage material. Without limitation, the capsular structure may include a fluid passage having one or more features described in paragraphs 7-12, 28-43, and 54-67, and FIGS. 1, 2, 3, 4, 5, 6, and 7 of U.S. Provisional Application No. 61/299,565 entitled “Thermal Energy Storage” and filed on Jan. 29, 2010, incorporated herein by reference. 
     The cover sheet and the base sheet may include one or more openings. The cover sheet and the base sheet may be arranged so that at least one opening of the cover sheet overlaps at least one opening of the base sheet. As such, the cover sheet and the base sheet may have one or more corresponding openings. The cover sheet has an outer periphery in the regions furthest from the center of the cover sheet. The cover sheet may have one or more opening peripheries in the region of the cover sheet near an opening (preferably near the center) of the cover sheet. The base sheet has an outer periphery in a region far from the center of the base sheet and may also have an opening periphery near the opening (preferably near the center) of the base sheet. Each of the cover sheet and the base sheet may be sealingly attached to each other or to one or more other optional sub-structures (such as an outer ring) along the respective outer peripheries of the sheets, for forming one or more sealed spaces therebetween. Each of the cover sheet and the base sheet may be sealingly attached to each other or to one or more other optional sub-structures (such as an inner ring) along the respective opening peripheries of the sheets, for forming one or more sealed spaces therebetween. Preferably the cover sheet and the base sheet are sealingly attached to each other along their respective outer peripheries, along at least one of their respective corresponding opening peripheries, or both. Most preferably the cover sheet and the base sheet are sealingly attached to each other both along their respective outer peripheries and along at least one of their respective corresponding opening peripheries. It will be appreciated that the cover sheet and the base sheet may also be sealingly attached to each other or to one or more other optional sub-structures along one or more additional regions (other than their peripheries) so that a plurality of sealed spaces are formed. 
     The capsular structure may optionally include one or more sub-structures that when sealingly attached to a base sheet and a cover sheet forms one or more sealed spaces. Without limitation, the sub-substructure, if employed, may include one or any combination of the features described in U.S. Provisional Application No. 61/299,565 entitled “Thermal Energy Storage” and filed on Jan. 29, 2010, incorporated herein by reference. For example, the base sheet, the cover sheet, or both may be attached to one or more rings, such as one or more inner rings, one or more outer rings, or both. The substructure, if employed, may include a honeycomb or other open cell structure, such as described in paragraph 83 of U.S. Patent Application Publication No. 2009-0250189 by Bank et al., published on Oct. 8, 2009, incorporated herein by reference. 
     The thickness of the capsular structure is defined by the average separation between the top surface of the article (e.g., the top surface of the cover sheet) and the bottom surface of the article (e.g., the bottom surface of the base sheet). The article may have a geometry sufficiently thin so that heat can be rapidly provided from a fluid to thermal energy storage material and/or rapidly removed from the thermal energy storage material to a fluid. The article may have a thickness that is less than the length or diameter of the article. 
     For example, ratio of the length or diameter of the article to the thickness of the article may be about 2 or more, about 5 or more, about 10 or more, or about 20 or more. Without limitation, the ratio of the length or diameter of the article to the thickness of the article may be about 1,000 or less, preferably about 300 or less, and more preferably about 150 or less. Preferably, the thickness of the article is 80 mm or less, more preferably about 20 mm or less, even more preferably about 10 mm or less, and most preferably about 8 mm or less. The thickness of the article preferably is greater than about 0.5 mm, more preferably greater than about 1 mm. 
     The longest dimension of the article (e.g., the length or diameter of the article) is typically much greater than the thickness of the article so that the article can both have a large volume (e.g., for containing a large volume of thermal energy storage material), and a large surface area (e.g., for rapid transfer of thermal energy). The longest dimension of the article preferably is greater than about 30, more preferably greater than about 50 mm and most preferably greater than about 100 mm. The longest dimension is defined by the use, and can be any length that meets the need for heat storage, heat transfer, or both, in a particular use. The longest dimension of the article typically is less than about 2 m (i.e., 2,000 mm), however articles having a longest dimension greater than about 2 m may also be employed. 
     The article may have one or more side surfaces. For example the article may have one or more side surfaces that are nonplanar. The article may have a single side surface that is generally arcuate, generally nonplanar, generally continuous, or any combination thereof. Preferably the one or more side surfaces are generally equidistant from a center of the article so that the article can be placed in a container having a generally cylindrical cavity with a cavity diameter that is only slightly larger than the average diameter of the article. When the ratio of the cavity diameter to the average diameter of the article is low, a large amount of the cavity is occupied by the article. For example, the ratio of the maximum diameter of the article to the average diameter of the article may be less than about 1.8, preferably less than about 1.2, more preferably less than about 1.1, and most preferably less than about 1.05. It will be appreciated that the ratio of the maximum diameter to the average diameter of the article is about 1.0 or more (e.g., about 1.001 or more). 
     A large portion of the volume of the capsular structure is the encapsulated volume (i.e., the volume of the one or more sealed spaces) so that the article can contain a relatively large amount of the thermal energy storage material. The total volume of the one or more sealed spaces of the article is preferably at least about 50 volume percent, more preferably at least about 80 volume percent, even more preferably at least about 85 volume percent and most preferably about 90 volume percent based on the total volume of the article. The total volume of the one or more sealed spaces of the article is typically less than about 99.9 volume percent based on the total volume of the article. The remaining volume, not occupied by the thermal energy storage material, may include or consist substantially entirely of the capsular structure, void spaces (e.g., containing one or more gases), one or more structures for improving the heat transfer between the thermal energy storage material and the capsular structure, or any combination thereof. Structures for improving heat transfer between the thermal energy storage material and the capsular structure include any structure formed of a material having a relatively high thermal conductivity (e.g., relative to the thermal energy storage material) that is capable of increases the rate of heat flow from the thermal energy storage material to a heat transfer fluid. Preferable structures for improving the rate of heat flow include fins, wire mesh, protrusions into the sealed space, and the like. 
     The article preferably is easy to stack with other identical shaped articles, or other articles having a generally mating surface. For example, two articles to be stacked may have opposing surfaces that are generally mating surfaces so that when stacked, the two articles nest together. It will be appreciated that one approach for stacking articles so that they easily nest together is to select a shape (e.g., a shape of an arcuate surface, a shape of the sealed spaces, or both) having a rotational symmetry of a high order. The rotational symmetry may be about an axis in the stacking direction (e.g., an axis through the fluid passage of the capsular structure). The order of the rotational symmetry typically describes the number of distinct rotations between the two surfaces being stacked together in which they will nest together. The order of the rotational symmetry of the article, the base sheet (e.g., the arcuate surface of the base sheet), or both, preferably is at least 2, more preferably at least 3, even more preferably at least 5, and most preferably at least 7. 
     The article preferably has a capsular structure that is difficult to bend. For example, the capsular structure may be free of a cross-section in which a cover sheet and a base sheet are in contact throughout most or even all of a length of the cross-section (such as a diameter of the capsular structure). There are various approaches that may be employed for assuring that the capsular structure will be difficult to bend, including selecting an arrangement of the capsules so that the order of rotational symmetry is not an even number, selecting an arrangement of the capsules so that there is no rotational symmetry, selecting an arrangement of capsules including two or more rings of capsules (such as concentric rings) that are rotated relative to each other so that every radial section includes at least one sealed space, or any combination thereof. It will be appreciated that other geometries and other means may be employed to make the capsular article resistant to bending. For example, the materials for the capsular structure may be chosen to be generally stiff, the structure may include one or more ribs (e.g., in a tangential direction), and the like. 
     All of the thermal energy storage material of the article may be in a single sealed space. The thermal energy storage material of the article may optionally be divided between a plurality of sealed spaces so that if a sealed space is punctured or otherwise leaks, despite the improvements described herein, only a portion of the thermal energy storage material can be removed. As such, the number of sealed spaces in the article (e.g., sealed spaces that contain thermal energy storage material) may be 1 or more, 2 or more, 3 or more, 4 or more, or 5 or more. The upper limit on the number of sealed spaces is practicality and for a particular application is defined by the need of the application. Nevertheless, the number of sealed spaces in the article typically is less than 1,000. However, it will be appreciated that very large articles may have 1,000 or more sealed spaces. For the same reasons, the volume fraction of the thermal energy storage material that is found in any single sealed compartment may be about 100%, less than about 55%, less than about 38%, than about 29%, or less than about 21%, based on the total volume of the thermal energy storage material in the article. Typically a sealed space includes at least 0.1 volume % of the thermal energy storage material in the article. However, it will be appreciated that the article may include one or more sealed spaces that are substantially or even entirely free of the thermal energy storage material. 
     The sealed spaces may optionally be arranged in a pattern which facilitates efficient stacking of the articles of the invention and efficient energy transfer to and/or from the capsules, such as a plurality of concentric rings, including an innermost ring (e.g., a ring closest to an opening periphery) and an outermost ring (e.g., a ring closest to the outer periphery), each containing one or more sealed spaces. The sealed spaces in one ring may have a generally repeating pattern. For example, each sealed space or each groups of 2, 3, 4 or more sealed spaces in a ring may have generally the same shape and size. The number of sealed spaces in each ring may be the same or different. Preferably the outermost ring has more sealed spaces than the innermost ring, the average length of the sealed spaces of the outermost ring is less than the average length of the sealed spaces of the innermost ring (where the average length is measured in the radial direction from the opening to the outer periphery), or both, so that the volume variation between the sealed spaces of the outermost ring and the innermost ring is reduced. 
     As discussed hereinafter, the article may be placed in a container, such as a container having a generally cylindrical shaped cavity. Preferably, the cavity may be only slightly greater in dimension than the longest dimension of the article. For example, the diameter of the cavity of the container may be only slightly larger than the diameter of the capsular structure of the article. The diameter of the cavity should be sufficiently large so that the article can be inserted into the cavity. When the article (or a stack of the articles) is placed in the container, it may be desirable for a fluid to be capable of flowing between the outer periphery of the article and an interior wall of the container. This can be achieved by designing the relationship of the interior of the container and the shape of the article to create and maintain fluid flow paths. Any means of creating such fluid flow paths may be used. As such, the article may optionally have one or more indents along its periphery (e.g., the cover sheet and the base sheet may have one or more corresponding indents along their respective outer peripheries) so that a space is formed for flowing a heat transfer fluid. Alternatively, or in addition, the cavity of the container may have a surface with one or more grooves for flowing a fluid between the outer periphery of the article and the surface of the container. As another example, the diameter of the article may be sufficiently small in relation to the diameter of the interior of the cavity so that a fluid can flow along the entire outer periphery of the article. For example, the article may have one or more indents or the container may have one or more grooves, for each sealed space in the outermost ring of sealed spaces. An indent or a groove may have any shape, such as a polygonal shape, an arcuate shape, a wedge shape, and the like, provided it has a sufficient size to allow for the heat transfer fluid to flow. If employed, the smallest dimension of the indents and/or grooves is typically at least about 0.1 mm). It will be appreciated that a combination of two or more means of creating a fluid flow path may be used. For example, the article may have one or more indents along its outer periphery and the article may have a sufficiently small diameter so that fluid can flow along its entire outer periphery when placed in a cavity. 
     The article for containing thermal energy storage material includes a base sheet that is formed so that it includes one or more troughs suitable for holding a liquid. The base sheet may used in a process in which one or more troughs are filled with a thermal energy storage material, covered with a generally flat cover sheet and then joined to the cover sheet (preferably while the thermal energy storage material is at least partially in a molten state). The base sheet may optionally have one or more protrusions, so that when the article is stacked with another article having a surface that generally mates with the base sheet, the two articles only partially nest. As such the one or more protrusions may function as a spacer to separate the generally mating surfaces so that a fluid (e.g., a heat transfer fluid) can flow between the mating surfaces. If employed, the protrusions preferably cover only a small portion of the surface are of the base sheet so that the one or more protrusions do not substantially interfere with the flow of the fluid. The height of the protrusions may be selected to define the height (e.g., the average height) of the flow path between the two generally mating surfaces. 
     The base sheet may additionally include one or more volume expansion features and/or one or more stiffening features, such as one or more ribs, one or more dimples, one or more chevrons, or any combination thereof, that function to stiffen the base sheet. It has been surprisingly observed that the troughs, the stiffening features, the volume expansion features, or any combination thereof of the base sheet may reduce the maximum stress on the base sheet when an article containing a thermal energy storage material is heated. For example the maximum stress, such as the maximum von Mises stress, on a base sheet may be less than the maximum von Mises stress of a flat cover sheet made of the same material (e.g., the same metal) and having the same thickness as the base sheet when the thermal energy storage material in a sealed space is heated. The relatively low stress in the base sheet may provide an opportunity to reduce the weight of the article and/or increase the amount of thermal energy storage material in the article by using a base sheet having a lower thickness (e.g., a thickness that is less than the thickness of the cover sheet). 
     It has also been surprisingly observed that the thickness of the base sheet, the cover sheet, or both may be further reduced by adding the optional stiffening features such as ribs, dimples, or chevrons to the base sheet. If employed, the stiffening features preferably have a sufficient size, shape and number so that the maximum von Mises stress of the sheet (e.g., the base sheet or the cover sheet) is reduced, preferably by about 2% or more, more preferably by about 5% or more, and most preferably by about 10% or more. The stiffening features may include features that are protrusions (i.e., that go away from the sealed space), features that are recesses (i.e., that go into the sealed space), or both. Preferred stiffening features protrude or recess by a depth/height of about 0.2 mm or more, more preferably about 0.4 mm or more, and most preferably about 0.6 mm or more. Preferred stiffening features protrude or recess by a depth/height of about 5 mm or less, more preferably about 3 mm or less, and most preferably about 1 mm or less. 
     When stacking articles, they may be arranged so that cover sheets from two adjacent articles are at least partially in contact with each other. It may be desirable for the two cover sheets to have a large contact area so that they are in good thermal communication and/or for there to be little gaps or spaces between the two cover sheets so that space is not wasted. As such, the cover sheets may only include recesses. Good contact between two cover sheets may also be achieved by having cover sheets that are generally mating surfaces. For example, a first cover sheet may have one or more recesses that mate with one or more protrusions of a second cover sheet and/or vice versa. 
     The base sheet and the cover sheet are attached so that a sealed space is formed that includes a thermal energy storage material. The attachment of the base sheet and the cover sheet may include a primary seal around one or more (e.g., all) of the sealed paces so that a sealed space is isolated from any other sealed space and/or from regions outside of the article. The attachment of the base sheet and the cover sheet may include one or more secondary seals, such as a seal that isolates a sealed space from a region outside of the article and/or from other sealed spaces in the event that a primary seal fails. 
     Without limitation, preferable thermal energy storage materials for the heat storage device include materials that are capable of exhibiting a relatively high density of thermal energy as sensible heat, latent heat, or preferably both. The thermal energy storage material is preferably compatible with the operating temperature range of the heat storage device. For example the thermal energy storage material is preferably a solid at the lower operating temperature of the heat storage device, is at least partially a liquid (e.g., entirely a liquid) at the maximum operating temperature of the heat storage device, does not significantly degrade or decompose at the maximum operating temperature of the device, or any combination thereof. The thermal energy storage material preferably does not significantly degrade or decompose when heated to the maximum operating temperature of the device for about 1,000 hours or more, or even for about 10,000 hours or more. 
     The thermal energy storage material may be a phase change material having a solid to liquid transition temperature. The solid to liquid transition temperature of the thermal energy storage material may be a liquidus temperature, a melting temperature, or a eutectic temperature. The solid to liquid transition temperature should be sufficiently high so that when the thermal energy storage material is at least partially or even substantially entirely in a liquid state enough energy is stored to heat the one or more objects to be heated to a desired temperature. The solid to liquid transition temperature should be sufficiently low so that the heat transfer fluid, the one or more objects to be heated, or both, are not heated to a temperature at which it may degrade. As such the desired temperature of the solid to liquid transition temperature may depend on the object to be heated and the method of transferring the heat. For example, in an application that transfers the stored heat to an engine (e.g., an internal combustion engine) using a glycol/water heat transfer fluid, the maximum solid to liquid transition temperature may be the temperature at which the heat transfer fluid degrades. As another example, the stored heat may be transferred to an electrochemical cell of a battery using a heat transfer fluid where the heat transfer fluid has a high degradation temperature, and the maximum solid to liquid temperature may be determined by the temperature at which the electrochemical cell degrades or otherwise fail. The solid to liquid transition temperature may be greater than about 30° C., preferably greater than about 35° C., more preferably greater than about 40° C., even more preferably greater than about 45° C., and most preferably greater than about 50° C. The thermal energy storage material may have a solid to liquid transition temperature less than about 400° C., preferably less than about 350° C., more preferably less than about 290° C., even more preferably less than about 250° C., and most preferably less than about 200° C. It will be appreciated that depending on the application, the solid to liquid transition temperature may be from about 30° C. to about 100° C., from about 50° C. to about 150° C. from about 100° C. to about 200° C., from about 150° C. to about 250° C., from about 175° C. to about 400° C., from about 200° C. to about 375° C., from about 225° C. to about 400° C., or from about 200° C. to about 300° C. 
     For some applications, such as transportation related applications, it may desirable for the thermal energy material to efficiently store energy in a small space. As such, the thermal energy storage material may have a high heat of fusion density (expressed in units of megajoules per liter), defined by the product of the heat of fusion (expressed in megajoules per kilogram) and the density (measured at about 25° C. and expressed in units of kilograms per liter). The thermal energy storage material may have a heat of fusion density greater than about 0.1 MJ/liter, preferably greater than about 0.2 MJ/liter, more preferably greater than about 0.4 MJ/liter, and most preferably greater than about 0.6 MJ/liter. Typically, the thermal energy storage material has a heat of fusion density less than about 5 MJ/liter. However, thermal energy storage materials having a higher heat of fusion density may also be employed. 
     For some applications, such as transportation related applications, it may be desirable for the thermal energy storage material to be light weight. For example, the thermal energy storage material may have a density (measured at about 25° C.) less than about 5 g/cm 3 , preferably less than about 4 g/cm 3 , more preferably less than about 3.5 g/cm 3 , and most preferably less than about 3 g/cm 3 . The lower limit on density is practicality. The thermal energy storage material may have a density (measured at about 25° C.) greater than about 0.6 g/cm 3 , preferably greater than about 1.2 g/cm 3 , and more preferably greater than about 1.7 g/cm 3 . 
     The sealed spaces may contain any art known thermal energy storage material. Examples of thermal energy storage materials that may be employed in the thermal energy storage material compartments include the materials described in Atul Sharma, V. V. Tyagi, C. R. Chen, D. Buddhi, “Review on thermal energy storage with phase change materials and applications”, Renewable and Sustainable Energy Reviews 13 (2009) 318-345, and in Belen Zalba, Jose Ma Marin, Luisa F. Cabeza, Harald Mehling, “Review on thermal energy storage with phase change: materials, heat transfer analysis and applications”, Applied Thermal Engineering 23 (2003) 251-283, both incorporated herein by reference in their entirety. Other examples of preferred thermal energy storage materials that may be employed in the heat transfer device include the thermal energy storage materials described in U.S. Patent Application Publication Nos: 2009/0211726 (entitled “Thermal Energy Storage Materials” and published on Aug. 27, 2009) and 2009/0250189 (entitled “Heat Storage Devices” and published on Oct. 8, 2009), and paragraphs 54-63 of U.S. Provisional Patent Application No. 61/299,565 (entitled “Thermal Energy Storage” and filed on Jan. 29, 2010). 
     The thermal energy storage material may include an organic material, an inorganic material or a mixture of an organic and an inorganic material that exhibits the solid to liquid transition temperature, the heat of fusion density, or both, described hereinbefore. Organic compounds that may be employed include paraffins and non-paraffinic organic materials, such as a fatty acid. Inorganic materials that may be employed include salt hydrates and metallics. The thermal energy storage material may be a compound or a mixture (e.g., a eutectic mixture) having a solid to liquid transition at generally a single temperature. The thermal energy storage material may be a compound or a mixture having a solid to liquid transition over a range of temperatures (e.g., a range of greater than about 3° C., or greater than about 5° C.). 
     Without limitation, the thermal energy storage material may include one or more inorganic salts selected from the group consisting of nitrates, nitrites, bromides, chlorides, other halides, sulfates, sulfides, phosphates, phosphites, hydroxides, carboxides, bromates, mixtures thereof, and combinations thereof. 
     The thermal energy storage material may include (or may even consist essentially of or consist of) at least one first metal containing material, and more preferably a combination of the at least one first metal containing material and at least one second metal containing material. The first metal containing material, the second metal containing material, or both, may be a substantially pure metal, an alloy such as one including a substantially pure metal and one or more additional alloying ingredients (e.g., one or more other metals), an intermetallic, a metal compound (e.g., a salt, an oxide or otherwise), or any combination thereof. One preferred approach is to employ one or more metal containing materials as part of a metal compound; a more preferred approach is to employ a mixture of at least two metal compounds. By way of example, a preferred metal compound may be selected from oxides, hydroxides, compounds including nitrogen and oxygen (e.g., nitrates, nitrites or both), halides, or any combination thereof. It is possible that ternary, quaternary or other multiple component material systems may be employed also. The thermal energy storage materials herein may be mixtures of two or more materials that exhibit a eutectic. 
     The volume of the thermal energy storage material in the one or more sealed spaces of the article is sufficiently high so that the article can store a large amount of thermal energy. The ratio of the volume of the thermal energy storage material contained in the article to the total volume of the one or more sealed spaces, the ratio of the volume of the thermal energy storage material to the total volume of the article, or both (the volumes measured at a temperature of about 25° C., or at a temperature at which the thermal energy storage material is a liquid) preferably is greater than about 0.5, more preferably greater than about 0.7, and most preferably greater than about 0.9. The ratio of the volume of the thermal energy storage material contained in the article to the total volume of the one or more sealed spaces, the ratio of the volume of the thermal energy storage material to the total volume of the article, or both (the volumes measured at a temperature of about 25° C., or at a temperature at which the thermal energy storage material is a liquid) is typically less than about 1.0, and more typically less than about 0.995. 
     The sealed space may include a volume that contains a gas, such as air, N 2 , or an inert gas such as He, Ar, and the like, so that the thermal energy storage material can expand when heated. For example, the sealed space may have a region that is free of thermal energy storage material at a temperature of about 25° C., so that upon heating the thermal energy storage material above its liquidus temperature, the thermal energy storage material can expand without forming a hole in the cover sheet or base sheet or causing one or more sheets to delaminate. The volume of a sealed space that is free of thermal energy storage material (e.g., the volume of the sealed space that contains a gas) at 25° C., may be at least about 0.5%, preferably at least about 1%, and most preferably at least about 1.5%, based on the total interior volume of the sealed space. 
     The thermal energy storage material, the sealed space, or both may be substantially free of, or entirely free of materials that undergo vaporization or sublimation when the article is used to store heat so that the pressure in the sealed space is not greatly increased. For example, the thermal energy storage material, the sealed space, or both may be substantially free of materials that undergo vaporization or sublimation at a temperature from about 25° C. to about 100° C., preferably from about 25° C. to about 150° C., more preferably about 25° C. to about 200° C., and most preferably from about 25° C. to about 300° C. As such, the thermal energy storage material, the sealed space, or both may be substantially free of water. In applications that employ temperatures of about 100° C. or more for storing thermal energy, it may be desirable for the sealed space to be substantially free of, or even entirely free of water. If present the concentration of water in the sealed space may be about 5 wt. % or less, more preferably about 1 wt. % or less, even more preferably about 0.2 wt. % or less, and most preferably about 0.1 wt. % or less. 
       FIG. 1  is a drawing that illustrates a portion of an article  2  having a capsular structure. A portion of the outer surface (bottom surface) of the base sheet  12  of the article is shown in  FIG. 1 . The article includes a plurality of capsules  10 . As shown in  FIG. 1 , the capsules  10  may be positioned in a periodic arrangement  11 . The base sheet may have one or more trough  8 , such as a trough that may be employed to hold the thermal energy storage material before and/or during the attachment of a base sheet to a cover sheet. The base sheet also includes a lip region  6 . The lip region may be employed to attach the base sheet to a cover sheet. As such, the lip region may be a region that does is not covered with thermal energy storage material when the base sheet and the cover sheet are attached. 
       FIG. 2A  is a cross-sectional view of an illustrative capsular structure  2  that includes one or more sealed spaces  18 .  FIG. 2B  is a cross-sectional view of a single capsule , such as a capsular structure having one capsule  10  or a portion of a capsular structure  2  having a plurality of capsules  10 . As illustrated in  FIGS. 2A and 2B , the capsule  10  may be a sealed space  18  that contains a thermal energy storage material  16 , and optionally a gap  20  or other space that is generally free of thermal energy storage material. The article may include one or more primary seals  22 , such as a seal that isolates the sealed space  18  from a region outside of the article  24 , from other sealed spaces, or both. The article may include one or more secondary seals  22 ′ that may isolate the sealed space in the event a primary seal  22  fails. As illustrated in  FIGS. 2A and 2B , the sealed space  18  may be formed by attaching a base sheet  12  about a lip region  6  to a cover sheet  14  (e.g., a cover sheet that is generally flat). The base sheet may also includes a trough region  8 . The thickness  13  of the base sheet may be reduced (e.g., relative to the thickness  15  of the cover sheet) due to the stiffening of the base sheet by the troughs  8 ′. The seals (e.g., the primary seal  11 , the secondary seal  22 ′, or both) may be formed by welding the base sheet and cover sheet, such as by laser welding. 
     The one or more sealed spaces may be prepared by joining a metal cover sheet and a metal base sheet with one, two, or more welds, wherein the welds completely encloses the one or more sealed spaces. An individual sealed space may be prepared using a single continuous weld, or a plurality of welds. The plurality of welds may form a continuous perimeter. The plurality of welds may be discontinuous. For example, an individual sealed space may have one weld along an outer perimeter and a second weld along an inner perimeter. 
       FIG. 3  is a schematic drawing of a portion of a formed sheet  40  (e.g., a base sheet  12 ) that may be employed in an article  2  having a plurality of sealed spaces. The formed sheet may have an opening  46  (e.g., a generally circular opening) near the center of the sheet.  FIG. 3  shows only about ¼ of the formed sheet  40  and thus only ¼ of the opening  46  is shown.  FIG. 3  shows the bottom surface  41  of the formed sheet  40 . The formed sheet has a plurality of trough regions  8  and a plurality of lip regions  6 . The trough regions preferably provides troughs  55  that are capable of containing thermal energy storage material. The trough regions  8  may be arranged in a plurality of rings of troughs  50 ,  50 ′,  50 ″. As illustrated, the formed sheet may have an innermost ring of troughs  50  and an outermost ring of troughs  50 ′. The formed sheet may also have one or more additional rings of troughs  50 ″, between the innermost and the outermost rings of troughs  50 ,  50 ′. As illustrated in  FIG. 3 , some or all of the troughs in a ring, or even some or all of the troughs in different rings may have about the same shape, about the same volume, be substantially congruent, or any combination thereof. It will be appreciated that the number of troughs in the innermost ring of troughs may be more than, less than or the same as the number of troughs in the outermost ring of troughs. Preferably, the number of troughs in the innermost ring of the formed sheet  40  is less than the number of troughs in the outermost ring, as illustrated in  FIG. 6 . Some, or preferably all of the troughs regions  8  have a lip region  6  around the trough region. As such, a trough region  8  may be separated by the other trough regions by a lip region  6 . The formed sheet  40  may have an outer periphery  45 , an inner periphery  47 , or both. As illustrated in  FIG. 3 , the formed sheet may have one or more indents  51  near the outer periphery  45 . The one or more indents may be used for flow channels or flow paths along the outer periphery  45 . Preferably the outer perimeter of the bottom surface of the formed sheet  40  has a generally circular shape (excluding the optional one or more indents  51 ). As illustrated in  FIG. 3 , the outer periphery  45 , the inner periphery  47 , and preferably both, may be lip regions  6 . 
       FIG. 4A  illustrates a portion of a cover sheet  14  having chevrons  30 . The dimensions (e.g., x, y, z, or any combination thereof) in  FIG. 4A  may be in units of mm. The chevrons  30  may have a periodicity (in one or more directions) of about 3 mm or more, a periodicity of about 50 mm, or less, or both. The periodicity of the chevrons may be sufficiently small so that the portion of a cover sheet over an individual sealed space has a plurality of chevrons  30 . For example, the number of chevrons  30  over the region of a cover sheet  14  over a single sealed space may be about 2 or more, about 5 or more, about 10 or more about 20 or more, or about 30 or more. The chevrons  30  may have a periodicity of about 5 mm. The chevrons  30  may have a depth of about 0.2 mm or more, a depth of about 4 mm or less, or both. For example, the chevrons  30  may have a depth of about 1 mm. It will be appreciated that chevrons having a higher or lower periodicity and/or a higher or lower depth may also be employed. The dimensions (e.g., x, y, z, or any combination thereof) in  FIG. 4A  may be in any arbitrary units that are the same or different, or may be in units of mm. The chevrons may be in a section of the cover sheet that covers a sealed space, in a region of a cover sheet that is away from sealed spaces, or both. It will be appreciated that chevrons may be employed in a base sheet, a cover sheet or both. 
       FIG. 4B  is schematic view of an illustrative capsule  10  having a cover sheet  14  that includes dimples  32 .  FIG. 4C  is a top view of the portion of a cover sheet  14  over a single capsule, such as the capsule illustrated in  FIG. 4B . As illustrated in FIGS.  4 Ba and  4 C, the cover sheet  14  may include one or more dimples  32 , and particularly one or more recessed dimples  33 . The dimples  32  may be in any arrangement. For example, the dimples may have a brick wall pattern, so that adjacent rows of dimples are shifted. The dimples preferably have a periodicity of about 1 mm or more, more preferably about 2 mm or more, and most preferably about 3 mm or more. The periodicity of the dimples preferably is about 30 mm or less, more preferably about 15 mm or less, and most preferably about 10 mm or less. The periodicity of the dimples may be sufficiently small so that the portion of a cover sheet over an individual sealed space has a plurality of dimples  32 . For example, the number of dimples  32  over the region of a cover sheet  14  over a single sealed space may be about 2 or more, about 5 or more, about 10 or more about 20 or more, or about 30 or more. The dimples preferably have a depth of about 0.1 mm or more, more preferably about 0.2 mm or more, even more preferably about 0.3 mm or more, even more preferably about 0.5 mm or more, and most preferably about 0.5 mm or more. The dimples preferably have a depth of about 3 mm or less, more preferably about 2 mm or less, and most preferably about 1 mm or less. It will be appreciated that dimples having a higher or lower periodicity and/or a higher or lower depth may also be employed. The dimples  32  may be in a section of the cover sheet that covers a sealed space, in a region of a cover sheet that is away from sealed spaces, or both. It will be appreciated that dimples may be employed in a base sheet, a cover sheet or both. 
       FIG. 4D  shows an illustrative to view of a portion of a sheet (e.g., a cover sheet  14 ) that includes stiffening features. The stiffening features  34  may be arranged in a generally random pattern. As illustrated in  FIG. 4D , the stiffening features  34  may have different shapes, different sizes, or both. The periodicity of the stiffening features may be sufficiently small so that the portion of a cover sheet  14  over an individual sealed space has a plurality of stiffening features  34 . For example, the number of stiffening features  34  over the region of a cover sheet  14  over a single sealed space may be about 2 or more, about 5 or more, about 10 or more about 20 or more, or about 30 or more. 
       FIGS. 5A ,  5 B, and  5 C illustrate the pressure  36 ,  36 ′,  36 ″ in a sealed space  18 ′,  18 ′″ at different temperatures and sealed at different sealing temperatures. As illustrated in  FIG. 5A , at the sealing temperature, the pressure inside  36  and the pressure outside  38  of the sealed space  18 ′,  18 ″ may be about the same. As illustrated in  FIG. 5B , the pressure inside  36 ′ the sealed space  18 ′ may be higher than the external pressure  38  when the sealed space  18 ′ is sealed at a low temperature and the article  2  is heated to a higher temperature. As illustrated in  FIG. 5B , there may be a net outward force  35  on the cover sheet  14  due to the pressure difference when the temperature is greater than the sealing temperature.  FIG. 5C  illustrates that the pressure inside  36 ″ the sealed space  18 ″ may be less than the external pressure  38  when sealing at a high temperature and then reducing the temperature. As illustrated in  FIG. 58 , there may be a net inward force  35  on the cover sheet  14  due to the pressure difference when the temperature is below the sealing temperature. 
       FIGS. 6A ,  6 B, and  6 C shows an illustrative example of a sheet (e.g., a cover sheet) that includes a rib structure  39  that includes protrusions and recesses.  FIG. 6A  is a topographical drawing of the region of the sheet for a single capsule.  FIG. 6B  is a photograph of a portion of a sheet including a plurality of the features of  FIG. 6A .  FIG. 6C  illustrates the effect of the structural features on the forces on the sheet when a sealed space including the sheet is heated. As illustrated in  FIG. 6 , the ribs may be confined to the region of the top sheet that is around the sealed space. For example, the cover sheet may have a lip region that is free of the ribs or other stiffening features in the region where the cover sheet is attached to a base sheet. 
       FIGS. 7A and 7B  illustrate a sheet  60  (e.g., a base sheet  12 ) that includes volume expansion features  62  so that the sealed space is capable of increasing in volume as the thermal energy storage material in the sealed space expands, and decreasing in volume as the thermal energy storage material contracts.  FIG. 8A  is a schematic of a section of a base sheet  12  having a volume expansion feature  62 . As illustrated in  FIG. 7A , the volume expansion feature  62  may include one or more wrinkles, such as one more convolutions  64 . The volume expansion feature may include a bellow.  FIG. 7A  illustrates a volume expansion feature  62  in a base sheet  12 . However, it will be appreciated that the base sheet  12 , the cover sheet  14  or both may include one or more volume expansion features  62 . The volume expansion feature may function by allowing a sheet to reversibly contract into the sealed space.  FIG. 7B  is an illustrative cross-section of a portion of a capsule  10  including the sheet  60  having the volume expansion feature  62 , a cover sheet  14 , and the thermal energy storage material  16 . The thermal energy storage material  16  may be in a sealed space  18 , which is sealed by a primary seal  22 , and preferably a secondary seal  22 ′. 
       FIG. 8  shows an illustrative relationship between the expected peak Von Mises stress in a cover sheet when the capsule  10  is heated to about 250° C. as a function of the thickness of the cover sheet  15 .  FIG. 8  also shows the yield stress of the metal employed in forming the cover sheet. At low thickness, the peak von Mises stress is higher than the yield stress of the metal and the cover sheet may yield or crack. At higher thicknesses, the peak von Mises stress is lower than the yield stress of the metal and the cover sheet does not yield or fail.  FIG. 9  shows an illustrative relationship between the expected peak Von Mises stress in a cover sheet when the capsule is heated to about 250° C. as a function of the thickness of the cover sheet  15  for cover sheets that are generally flat and for cover sheets that have the rib structure shown in  FIG. 6 . The thickness of the cover sheet required to prevent yielding of the cover sheet may be reduced when a ribs structure is employed. As such, the rib structure of  FIG. 6  may allow for articles and heat storage devices that are light weight, contain a greater amount of thermal energy storage material, or both. 
       FIG. 10  shows an illustrative relationship between the expected peak Von Mises stress in a cover sheet when the capsule is heated to about 250° C. as a function of the thickness of the cover sheet for cover sheets that are generally flat and for cover sheets that have the rib structure shown in  FIG. 6 , the chevron structure of  FIG. 4A , and the dimple pattern of  FIG. 4B . The thickness of the cover sheet required to prevent yielding of the cover sheet may be reduced when the different stiffening structure are employed. As such, the structure of  FIGS. 4A ,  4 B, and  6  may allow for articles and heat storage devices that are light weight, contain a greater amount of thermal energy storage material, or both. 
       FIG. 11  illustrates a portion of tooling  61  that may be employed in preparing a sheet (e.g., a cover sheet  14 ) having one or more stiffening features, such as one or more ribs. Such tooling may be employed in an embossing process. It will be appreciated that an embossing process may be a continuous process or a batch process. 
     The articles containing the thermal energy storage material preferably are capable of being stacked either with other identical articles or with a second article having a generally mating surface (such as a generally mating base sheet). The articles may be stacked in axial layers with a space between adjacent axial layers so that a heat transfer fluid can flow between the axial layers. An axial layer will generally contain one, two or more article. An axial layer (e.g., each axial layer) preferably contains one or two articles. For example, an axial layer may have two articles that are in contact on a surface, such as a base surface or a cover surface, so that a fluid cannot generally flow between the two articles). As such, some of the articles (e.g., each article except the articles at an end of a stack) may have a first surface (e.g., a base surface) that is generally in complete contact with a surface of a first adjacent article so that a fluid cannot flow along the first surface, and a second surface that is separated from a second adjacent article (e.g., having an opposing surface that is generally a mating surface with the second surface) so that a fluid can flow along some, most, or even all of the second surface. The separation between two adjacent axial layers may be due to any art known spacing means. By way of example, preferable spacing means include one or more protrusions on a surface of at least one of the articles, a spacer material between, the two layers, a capillary structure between two layers, or any combination thereof. Preferably the second surface of the article has a generally arcuate shape and the article partially nests with the second adjacent article. The spacing between two articles that partially nest preferably is generally constant (except for the protrusions or other spacers that cause the adjacent articles to be separated). It will be appreciated that the stacking of the articles may include a step of rotating an axial layer (e.g., rotating an article), or otherwise arranging it so that the axial layer at least partially nests with an adjacent axial layer. The flow of a fluid between two opposing surfaces of two adjacent axial layers will generally be in a radial direction and may be described as a generally radial flow. Each pair of axial layers that are spaced apart will have a radial flow path. The stack of articles will typically have a plurality of radial flow paths (e.g., 2, 3, or more). Two or more (e.g., each radial flow path may have the same flow length, the same thickness, the same cross-sectional shape, or any combination thereof. For example, two or more (e.g., all) of the radial flow paths may be congruent. It will be appreciated that if the opening (i.e., fluid passage) is at the center of the article, the radial flow path may be generally symmetric, irrespective of the flow direction. 
     When stacked (e.g., in a stack containing 3, 4 or more articles), the articles preferably each have at least one opening that corresponds with an opening from each of the other articles (except possibly an article at one end of the stack), so that a portion of a fluid can flow from a first article in the stack to a last article in the stack by flowing through each of the corresponding openings of the articles interposed between the first and last article without flowing between adjacent articles (i.e., without a generally radial flow). The flow through the openings will generally be in an axial direction and may be described as a generally axial flow. 
     As described above, the stack of articles may define a central axial flow path (e.g., through the central axis formed by the openings of the articles) and one or more radial flow paths that are generally perpendicular to the central axial flow path. 
     The stack of articles will generally be tightly packed (e.g., except for the radial flow paths) so that the stack of articles is compact and contains a large amount of thermal energy storage material. As such, the radial flow path has a height (in the direction between the adjacent articles), e.g., an average height, that is generally small. The height of the radial flow path preferably is less than about 15 mm, more preferably less than about 5 mm, even more preferably less than about 2 mm, even more preferably less than about 1 mm, and most preferably less than about 0.5 mm. The height of the radial flow path typically is large enough so that the fluid can flow through the path. Typically the height (e.g., the average height) of the radial flow path is greater than about 0.001 mm (e.g., greater than about 0.01 mm). 
       FIG. 12  illustrates an aspect of the invention that includes a plurality of articles  2 , each having one or more sealed spaces  18  for containing a thermal energy storage material  16  arranged to form a stack of articles  70 . The articles  2  may include a formed sheet, such as a base sheet  12  having a generally arcuate surface  41 . The surface  41  of one article may generally mate with the surface of a second article. The articles may be arranged so that adjacent articles partially nest together. The articles illustrated in  FIG. 12  have an inner ring of  9  generally identical capsules and an outer ring of  17  generally identical capsules. The articles illustrated in  FIG. 12  have a rotational symmetry of order  1  and thus have only 1 position in which two facing articles will partially nest. To facilitate in the stacking of articles, each article may have one or more locating features. It will be appreciated that articles with a higher order of symmetry may be employed. For example, the article may have a single ring of capsules (or even a single capsule), or the article may have a first ring of capsules having an integer multiple of capsules (e.g., 1, 2, 3, or more) for each capsule of a second ring of capsules. As illustrated in  FIG. 12 , the articles  2  may have a generally circular cross-section (e.g., in a direction perpendicular to the stacking direction). The outer periphery of each article may have a plurality of indents  51  that are large enough to allow for a fluid flow. The articles  2  may have sealed spaces  74  arranged in one or more concentric rings of sealed spaces. Each article  2  may have a fluid passage  46 . The fluid passage  46  may be generally near the center of the articles  2  so that when the articles are stacked (e.g., stacked in an axial direction), an axial flow path  84  is formed. The axial flow path  84  preferably includes a fluid passage  46  of each article  2 . 
       FIG. 13  illustrates an article  2  that includes an odd number of capsules  10 . The article may have an odd rotational symmetry, so that it cannot be easily deformed about a diameter. The article  2  may have a generally circular cross-section with one or more openings  46  in the center of the article. The opening  46  may be generally circular. As illustrated in  FIG. 13 , one or more, or even each of the capsules or sealed spaces may have a primary seal  22  that isolates the sealed space from the outside of the article. The article may also have one or more secondary seals  22 ′. The secondary seal may include a seal near an inner periphery  47  (i.e., an opening periphery) of the article, a seal near an outer periphery  45  of the article, or both. The secondary seal  22 ′ may be sufficient to prevent leaking of a thermal energy storage  16  material from a sealed space  18  if a primary seal fails  22 . 
     The articles (e.g., a stack of articles) described herein may be employed in a heat storage device. The heat storage device may include a container or other housing having one or more orifices for flowing a heat transfer fluid into the container and one or more orifices for flowing a heat transfer fluid out of the container. The heat storage device has one or more heat transfer fluid compartments. Preferably, the heat storage device includes a single heat transfer fluid compartment. A heat transfer fluid compartment may include or consist substantially of a contiguous space in the container between the inlet and the outlet, where the heat transfer fluid can flow. The container preferably is at least partially insulated so that heat losses from the container to the ambient may be reduced or minimized. 
     The heat storage device may be designed so that it contains a large concentration of thermal energy storage material, so that it can transfer thermal energy between a heat transfer fluid and the thermal energy storage material rapidly and/or uniformly, so that it is generally compact, so that it can store heat for a long time, or any combination thereof. 
     The inside of the container of the heat storage device may have any shape capable of holding a stack of articles. Preferably, the shape of the inside of the container is such that the stack of articles occupies a large portion of the interior volume of the container. The ratio of the total volume of thermal energy storage material (e.g., measured at about 25° C.) contained in the sealed spaces of the articles in the container to the total interior volume of the container (e.g., at a temperature of about 25° C.) may be greater than about 0.3, preferably greater than about 0.5, more preferably greater than about 0.6, even more preferably greater than about 0.7, and most preferably greater than about 0.8. The upper limit on the volume of thermal energy storage material in the container is the need for space for a heat transfer fluid that contacts the articles for transferring thermal energy. The ratio of the total volume of thermal energy storage material (e.g., measured at about 25° C.) contained in the sealed spaces of the articles in the container to the total interior volume of the container (e.g., at a temperature of about 25° C.) may be less than about 0.99, preferably less than about 0.95. 
     The heat storage device has a heat transfer fluid compartment for flowing a capable of containing a heat transfer fluid as it circulates through the device. The heat transfer fluid compartment preferably is connected to one or more orifices (e.g. one or more inlets) for flowing a heat transfer into the heat transfer fluid compartment. The heat transfer fluid compartment preferably is connected to one or more orifices (e.g., one or more outlets) for flowing a heat transfer out of the heat transfer fluid compartment. The heat transfer fluid compartment may be a space at least partially defined by one or more heat transfer fluid compartment walls, a space at least partially defined by one or more articles, a space at least partially defined by a housing or container of the heat storage device, or any combination thereof. 
     The heat transfer fluid compartment defines the flow path of a heat transfer fluid through the heat storage device. The heat transfer fluid compartment includes a generally axial flow path through the openings of the stack of articles. The heat transfer fluid compartment includes a generally radial flow path between two adjacent articles. It will be appreciated that the radial flow may be an inward flow from an outer periphery to the opening of an article, or an outward flow from an opening to the outer periphery of an article. The heat transfer fluid compartment includes a flow path having a generally axial component (and optionally a tangential component) between an outer periphery of the article and a wall of the container. Preferably the combined radial flow paths have a relatively high hydraulic resistance. For example, the combined radial flow paths has a hydraulic resistance that is greater than (more preferably at least two times greater than) the hydraulic resistance of the central axial flow path, the outer axial flow path, or both. 
     The heat transfer fluid compartment preferably has sufficient thermal communication with the sealed spaces containing the thermal energy storage material so that it can remove heat or provide heat to the thermal energy storage material. The heat transfer fluid compartment preferably is in direct thermal communication with one or more (or more preferably all) of the sealed spaces. A direct thermal communication can be any path of shortest distance between a sealed space and a portion of the heat transfer fluid compartment that is free of a material having low thermal conductivity. Low thermal conductivity materials include materials having a thermal conductivity less than about 100 W/(m·K), preferably less than about 10 W/(m·K), and more preferably less than about 3 W/(m·K). For example, the heat transfer fluid or the heat transfer fluid compartment may contact a wall of one or more (or preferably all) of the sealed. or be separated from the sealed spaces substantially or entirely by materials having high thermal conductivity (e.g., greater than about 5 W/(m·K), greater than about 12 W/(m·K), or greater than about 110 W/(m·K). 
     The heat transfer fluid compartment preferably is in direct thermal communication with one or more (or more preferably all) of the sealed spaces in the heat storage device. A direct thermal communication can be any path of shortest distance between a thermal energy storage compartment and a portion of the heat transfer fluid compartment that is free of a material having low thermal conductivity. For example, the heat transfer fluid or the heat transfer fluid compartment may contact a wall of one or more (or preferably all) of the sealed paces (such as a base sheet or a cover sheet), or be separated from the sealed spaces substantially or entirely by materials having high thermal conductivity (e.g., greater than about 5 W/(m·K), greater than about 12 W/(m·K), or greater than about 110 W/(m·K). It will be appreciated that a very thin layer (e.g., less than about 0.1 mm, preferably less than about 0.01 mm, and more preferably less than about 0.001 mm) of a material having a low thermal conductivity may be between the heat transfer fluid compartment and a thermal energy storage material compartment without appreciably affecting the heat transfer. 
     The size and shape of the sealed spaces and/or articles may be chosen to maximize the transfer of heat to and from the phase change material contained in the capsules. The average thickness of the article may be relatively short so that the heat can quickly escape from the center of the sealed space, The average thickness of the article, sealed space, or both may be less than about 100 mm, preferably less than about 30 mm, more preferably less than about 10 mm, even more preferably less than about 5 mm, and most preferably less than about 3 mm. The average thickness of the article, the sealed space, or both, may be greater than about 0.1 mm, preferably greater than about 0.5 mm, more preferably greater than about 0.8 mm, and most preferably greater than 1.0 mm. 
     The articles preferably have a relatively high surface area to volume ratio so that the area of contact with the heat transfer fluid is relatively high. For example, the article may have a surface that maximizes the contact with a heat transfer fluid compartment, the article may have a geometry that maximizes the transfer of heat between the capsule and the heat transfer fluid compartment, or both. The ratio of the total surface area of the interface between the heat transfer fluid compartment and the articles in the heat storage device to the total volume of the thermal energy storage material in the heat storage device may be greater than about 0.02 mm −1 , preferably greater than about 0.05 mm −1 , more preferably greater than about 0.1 mm −1 , even more preferably greater than about 0.2 mm −1 , and most preferably greater than about 0.3 mm −1 . 
     The heat storage device has a container for containing the stack of articles. The stack of articles may be contained in one or more cavities of the container. Without limitation, examples of containers that may be employed include those described in U.S. Patent Application Publication No. 2009-0211726 (published on Aug. 27, 2009), PCT Application No. PCT/US09/67823 (filed on Dec. 14, 2009), and U.S. Provisional Application No. 61/299,565 (filed on Jan. 29, 2010). Preferable containers may have one or more orifices (e.g., one or more inlets) for flowing a heat transfer fluid into the cavity of the container and one or more orifices (e.g., one or more outlets) for flowing a heat transfer fluid out of the cavity of the container. The inlet and the outlet may be on the same side or on different sides (e.g., opposing sides) of the heat storage device. Other than the orifices, the container preferably is sealed or constructed so that a fluid flowing through the container does not leak out of the container, so that a fluid flowing through the container may have a pressure greater than ambient pressure, or both. 
     The heat storage device may be used in applications that require storing heat for long periods of time, storing heat in a generally cold environment (e.g., an environment having a temperature less than about 0° C., or even less than about −30° C.), or both. Preferably the heat stored in the heat storage device is slowly lost to the environment. Therefore some form of insulation is preferably used in the present invention. The better the insulation of the system is, the longer is the storage time. 
     Any known form of insulation which reduces the rate of heat loss by the heat storage device may be utilized. For example, any insulation as disclosed in U.S. Pat. No. 6,889,751, incorporated herein of its entirety by reference, may be employed. The heat storage device preferably is (thermally) insulated container, such that it is insulated on one or more surfaces. Preferably, some or all surfaces that are exposed to ambient or exterior will have an adjoining insulator. The insulating material may function by reducing the convection heat loss, reducing the radiant heat loss, reducing the conductive heat loss, or any combination. Preferably, the insulation may be through the use of an insulator material or structure that preferably has relatively low thermal conduction. The insulation may be obtained through the use of a gap between opposing spaced walls. The gap may be occupied by a gaseous medium, such as an air space, or possibly may even be an evacuated space (e.g., by use of a Dewar vessel), a material or structure having low thermal conductivity, a material or structure having low heat emissivity, a material or structure having low convection, or any combination thereof. Without limitation, the insulation may contain ceramic insulation (such as quartz or glass insulation), polymeric insulation, or any combination thereof. The insulation may be in a fibrous form, a foam form, a densified layer, a coating or any combination thereof. The insulation may be in the form of a woven material, a knit material, a nonwoven material, or a combination thereof. The heat transfer device may be insulated using a Dewar vessel, and more specifically a vessel that includes generally opposing walls configured for defining an internal storage cavity, and a wall cavity between the opposing walls, which wall cavity is evacuated below atmospheric pressure. The walls may further utilize a reflective surface coating (e.g., a mirror surface) to minimize radiant heat losses. 
     Preferably, a vacuum insulation around the heat storage device and or the heat storage system is provided. More preferably, a vacuum insulation as disclosed in U.S. Pat. No. 6,889,751, incorporated by reference herein in its entirety, is provided. 
     The heat storage device may optionally include one or more compaction means to a stack of articles so that the spacing between layers is generally maintained. The compaction means may be any means capable of applying a compressive force to the stack of articles. The compressive force should be sufficiently high so that two articles do not rotate relative to each other, do not move axially relative to each other, or both. The compressive force may be sufficiently low so that an article is not permanently deformed, cracked, or both. Preferred means of compaction will allow for some changes in the thickness of the articles as the temperature of the thermal energy storage material changes, as the thermal energy storage material changes between a solid and a liquid phase, or both. By way of example, the one or more means of compaction may include one or more springs above the stack of articles, one or more springs below the stack of articles, or both. Without limitation, a means of compaction such as a spring, may be employed to reduce or minimize the change in the thickness of a radial flow path between two adjacent articles when the thermal energy storage material is heated, undergoes a phase transition (such as a solid to liquid transition) or both. 
     The heat storage device may have a plurality of flow paths for the flow of a heat transfer fluid through the device. Each flow path may include at least one radial flow between two adjacent articles. Preferably two or more (e.g., each) of the flow paths through the heat storage device has a similar total length, a similar total hydraulic resistance, or both. Without limitation, the heat storage device may include one or more seals, one or more plates, one or more connectors, or one or more flow paths for a heat transfer fluid as described in U.S. Patent Application Publication No. 2009-0211726 (published on Aug. 27, 2009), PCT Application No. PCT/US09/67823 (filed on Dec. 14, 2009), and U.S. Provisional Application No. 61/299,565 (filed on Jan. 29, 2010). 
     The capsular structure and the articles containing the thermal energy storage material may be formed using any method that provides for the encapsulation of the thermal energy storage material. Without limitation, the process may employ one or any combination of the following: cutting or punching an opening (e.g., a hole) through a cover sheet, cutting or punching an opening (e.g., a hole) through a base sheet (e.g., a thin sheet such as a foil), forming (e.g., thermoforming, stamping, embossing or otherwise deforming) a base sheet to define a pattern in the sheet including at least one depression or trough region, forming a base sheet to define a pattern in the sheet including one or more lip regions and one or more trough regions, cutting or punching an outer periphery (e.g., a generally circular outer periphery) on a base sheet, cutting or punching an outer periphery (e.g., a generally circular outer periphery) on a cover sheet, filling a trough (e.g., a trough formed from the base sheet) with a thermal energy storage material, covering a trough (e.g., a filled trough) with a cover sheet, sealingly attaching a cover sheet (e.g., to a base sheet) so that one or more sealed spaces containing thermal energy storage material are formed, sealingly attaching a base sheet along an outer periphery, sealingly attaching a base sheet along an opening periphery, sealingly attaching a cover sheet (e.g., to a base sheet) along an opening periphery, or sealingly attaching a cover sheet (e.g., to a base sheet) along an outer periphery. The process of forming the article preferably includes a step of stamping, embossing, or thermoforming a base sheet. The process of forming the article may employ one or more of the process steps for producing a capsule described in U.S. patent application Ser. No. 12/389,598 entitled “Heat Storage Devices” and filed on Feb. 20, 2009. The method for forming the article may optionally include one or any combination of the following: sealingly attaching a base sheet to one or more substructures such as an inner ring, an outer ring, or both; sealingly attaching a cover sheet to one or more substructures such as an inner ring, an outer ring, or both; or cutting, stamping or punching one or more indents along the outer periphery of a base sheet and/or a cover sheet.  FIG. 15  is a photograph of an illustrative tooling that may be employed for embossing a sheet (e.g., a base sheet).  FIG. 15  illustrates a sheet  5  placed in the tooling  61  prior to forming the sheet  5  into a base sheet  12 . 
     The process for preparing a cover sheet, a base sheet, or both may include one or more steps of embossing or otherwise forming the sheet so that it includes one or more stiffening features such as one or more dimples, one or more ribs, one or more chevrons, or any combination thereof. 
     The process for preparing an article may include a step of filling a base sheet with a thermal energy storage material. The base sheet may be filled when the thermal energy storage material is in a solid state or in a molten state. Preferably the base sheet is filled when the thermal energy storage material is in a molten state. As such, the process may include a step of heating and/or melting a thermal energy storage material. 
     The process for preparing an article may include a step of joining a top sheet and a base sheet so that one or more sealed spaces is formed. The step of joining may include a step of forming a primary seal. Preferably the step of joining includes both a step of forming a primary seal and a step of forming a secondary seal. The step of joining preferably occurs while the thermal energy storage material is in a molten state. For example, the step of joining may occur when the thermal energy storage material is at a temperature of about 100° C. or more, about 150° C. or more, about 200° C. or more, about 250° C. or more, or about 300° C. or more. 
     The process for preparing an article may include a step of partially joining a base sheet and a cover sheet to form a partially sealed space that can contain a liquid when the base sheet and the cover sheet are not in a generally horizontal orientation. A space between the base sheet and the cover sheet may be at least partially filled by inserting an end of a nozzle into the space to be filled and pumping thermal energy storage material (preferably in a molten state) through the nozzle and into the partially sealed space. Thus, the space between a base sheet and a cover sheet may be at least partially filled while the sheets are in a troughs in the base sheet may be filled while the base sheet is generally vertical. It will be appreciated that such an approach for filling a space with thermal energy storage material may result in a higher volume of thermal energy storage material. For example, the percent of the volume of a sealed space that is occupied by air or other gas may be about 8% or less, about 6% or less, about 5% or less, about 4% or less, about 3% or less, about 2% or less, or about 1% or less. This approach for filling a space may also be used for filing a space between two base sheets. As such, the cover sheet may be a second base sheet. After filling a space with thermal energy storage material, the remainder of the primary seal may be formed so that the filled space is a sealed space. With respect to a secondary seal, if any, at least a portion of the secondary seal (e.g., in the region where a nozzle is inserted) is not formed until after the thermal energy storage material is inserted into the space. An article having a plurality of sealed spaces may be filled by a process including one or more of the following steps: partially sealing one or more spaces (e.g., by joining a base sheet and a cover sheet, or by joining two base sheets) by forming a portion of a primary seal, inserting thermal energy storage material into the one or more spaces, forming the remainder of the primary seal (e.g., so that the space is sealed), rotating the article being filled, inserting thermal energy storage material into one or more additional spaces that have a portion of a primary seal, and forming the remainder of the primary seal of the one or more additional spaces. 
       FIG. 16  illustrates an example of an article  2  having one or more (e.g., four) capsules  10  that have been filled with thermal energy storage material and one or more (e.g., a fifth space) that is being filled. The space being filled may have a partial primary seal  74 . The space being filled may optionally include a partial secondary seal  75 . The space being filled preferably has a fill region  76  that does not have a complete primary seal  22  and does not have a complete secondary seal  22 ′. The sealed space may be filled using a nozzle  77  that is inserted or otherwise placed in the fill region  76 . It will be appreciated that the nozzle may be placed in the fill region before, during, or after forming the partial primary seal  74 . As illustrated in  FIG. 16 , a nozzle  77  may be may be inserted into the top of the space being filled, so that the thermal energy storage material  16  does not leak out of fill region  76 . After inserting the thermal energy material  16  into the space being filled, the process may include one or more of the following steps: removing the nozzle (preferably before rotating the article), forming the remainder of the primary seal (preferably before rotating the article), or forming a secondary seal or the remainder of a secondary seal. As such, the article may be prepared using a machine that is capable of 1) forming a partial primary seal between two sheets (e.g., by laser welding the two sheets), 2) injecting thermal energy storage material (e.g., in a molten state) into a space between the two sheets, 3) completing the primary seal. If a plurality of sealed spaces are desired, the process may include one or more steps of rotating the sheets so that another space between the sheets can be filled. 
     Suitable sheets for encapsulating the thermal energy storage material include any thin metal sheets (e.g., metal foil) that are durable, corrosion resistant, or both, so that the sheet is capable of containing the thermal energy storage material, preferably without leakage. The metal sheets may be capable of functioning in a vehicle environment with repeated thermal cycling for more than 1 year and preferably more than 5 years. The metal sheet may otherwise have a substantially inert outer surface that contacts the thermal energy storage material in operation. The outer surface of the metal sheet that contacts the thermal energy storage material should include or consist essentially of one or more materials that do not significantly react with, corrode, or both, when contacted with the thermal energy storage material. Without limitation, exemplary metal sheets that may be employed include metal sheets having at least one layer of brass, copper, aluminum, nickel-iron alloy, bronze, titanium, stainless steel or the like. The sheet may be a generally noble metal or it may be one that includes a metal which has an oxide layer (e.g. a native oxide layer or an oxide layer which may be formed on a surface). One exemplary metal sheet is an aluminum foil which comprises a layer of aluminum or an aluminum containing alloy (e.g. an aluminum alloy containing greater than 50 weight percent aluminum, preferably greater than 90 weight percent aluminum). Another exemplary metal sheet is stainless steel. Preferable stainless steels include austenitic stainless steel, ferritic stainless steel or martensitic stainless steel. Without limitation, the stainless steel may include chromium at a concentration greater than about 10 weight percent, preferably greater than about 13 weight percent, more preferably greater than about 15 weight percent, and most preferably greater than about 17 weight percent. The stainless steel may include carbon at a concentration less than about 0.30 weight percent, preferably less than about 0.15 weight percent, more preferably less than about 0.12 weight percent, and most preferably less than about 0.10 weight percent. For example, stainless steel 304 (SAE designation) containing 19 weight percent chromium and about 0.08 weight percent carbon. Preferable stainless steels also include molybdenum containing stainless steels such as 316 (SAE designation). The metal sheet may have any art known coating that may reduce or eliminate the corrosion of the metal sheet. 
     The metal sheet has a thickness sufficiently high so that holes or cracks are not formed when forming the sheet, when filling the capsules with thermal energy storage material, during use of the capsules, or any combination thereof. For applications such as transportation, the metal sheet preferably is relatively thin so that the weight of the heat storage device is not greatly increased by the metal sheet. The thickness of the metal sheet may be greater than about 10 μm, preferably greater than about 20 μm, and more preferably greater than about 50 μm. The metal sheet may have a thickness less than about 3 mm, preferably less than 1 mm, and more preferably less than 0.5 mm (e.g., less than about 0.25 mm). 
       FIG. 14  illustrates a cross-section of an exemplary heat storage device  80  having a plurality of articles  2 ″, and  2 ′″ each having thermal energy storage material  16  encapsulated in a plurality of sealed spaces  18 . The articles are arranged in an insulated container  82  which may have a generally cylindrical shape. The device includes an article  2 ″ having a first adjacent article  2 ′″( a ) and a second adjacent article  2 ′″( b ). The article  2 ″ and its first adjacent article  2 ′″( a ) may be arranged with the top surfaces (i.e., exterior surfaces) of their respective flat cover sheets generally in contact. The article  2 ″ and the second adjacent article  2 ′″( b ) may have generally mating surfaces (e.g., the exterior surfaces of their respective base sheets may be generally mating surfaces) and may be arranged so that they partially nest together. A spacer (not shown) may be used to maintain a distance between the article  2 ″ and its second adjacent article  2 ′″( b ) so that a heat transfer fluid can flow through a radial flow path  83  in a generally radial direction between the two articles,  2 ″ and  2 ′″( b ). The space between the article  2 ″ and the second adjacent article  2 ′″( b ) may be formed from one of the sheets of the article  2 . As illustrated in  FIG. 14 , each article may have a surface (e.g., a surface of the base sheet) that is capable of contacting a heat transfer fluid so that the heat transfer fluid can be in direct contact with each article and preferably each sealed space. As illustrated in  FIG. 14 , each radial flow path  83  may have the same length, the same cross-section, or even may be congruent. Each article  2  may have an opening  46  near its center. The openings may be part of a compartment that allows a heat transfer fluid to flow through the device. The articles  2 ″ and  2 ′″ may be arranged so that their openings form a central axial flow path  84 . The space between the outer periphery of the articles  2 ″ and  2 ′″ and the interior surface of the container  85  is also part of the heat transfer fluid compartment and forms an outer axial flow path  86 . The heat storage device may have a first orifice  87  that is in fluid connection with a central axial flow path  84 . The heat storage device  80  may have a first seal or plate  88  that separates a first orifice  87  from the outer axial flow path  86 . The container  82  may have a second orifice  89  which may be on the same side of the container as the first orifice  87 , or on a different side of the container, as illustrated in  FIG. 14 . The heat storage device may have a second seal  90  that separates the second orifice  89  from the central axial flow path. The first seal, the second seal, or both may prevent a fluid from flowing between the two axial flow paths  84  and  86 , without flowing through a radial flow path  83 . The container  82  preferably is insulated. For example, the container may have an inner wall  91  and an outer wall  92 . The space between the two walls  93  may be evacuated or filled with an insulating material having low thermal conductivity. The device may also have one or more springs, such as one or more compression springs  94 , that exert a compressive force on the stack of articles. 
       FIG. 14  illustrates a heat storage device  80  having two orifices  87  and  89  on one side of the container. Such a device may employ a tube  95  that is connected to the first orifice  87  for flowing the fluid between the first orifice and a region  96  of the central axial flow path  84  furthest from the first orifice. With reference to  FIG. 14 , the first seal  88  and the second seal  90  may be employed to prevent a fluid from flowing from the first orifice  87  to the second orifice  89  without first flowing through a radial flow path  83 . By selecting the sizes for the two axial flow paths  84  and  86 , the heat storage device  80  may be characterized as a Tichelmann system. 
     The pressure in one or more, or even all of the sealed spaces may be less than atmospheric pressure, e.g. under a vacuum, when the temperature is about 25° C. For example, the pressure in a sealed space at 25° C. may be preferably about 600 Torr or less, about 500 Torr or less, about 400 Torr or less, about 300 Torr or less, or about 100 Torr or less. A vacuum in a sealed space may be a result of applying a vacuum when sealingly joining the cover sheet and the base sheet, a result of sealingly joining the cover sheet and the base sheet when the thermal energy storage material is at an elevated temperature, or both. For example, the process of sealingly joining the base sheet and the cover sheet may include a step of applying a vacuum of about 600 Torr or less, about 500 Torr or less, about 400 Torr or less, about 300 Torr or less, about 200 Torr or less, about 100 Torr or less, or about 50 Torr or less to a trough region of a base sheet. 
     The heat storage device may be used in a heat storage system that employs one or more heat transfer fluids for transferring heat into the heat storage device, for transferring heat out of the heat storage device, or both. 
     The heat transfer fluid used to transfer heat into and/or out of the thermal energy storage material may be any liquid or gas so that the fluid flows (e.g., without solidifying) through the heat storage device and the other components (e.g., a heat providing component, one or more connecting tubes or lines, a heat removing component, or any combination thereof) through which it circulates when it is cold. The heat transfer fluid may be any art known heat transfer fluid or coolant that is capable of transferring heat at the temperatures employed in the heat storage device. The heat transfer fluid may be a liquid or a gas. Preferably, the heat transfer fluid is capable of flowing at the lowest operating temperature that it may be exposed to during use (e.g., the lowest expected ambient temperature). For example, the heat transfer fluid may be a liquid or gas at a pressure of about 1 atmosphere pressure and a temperature of about 25° C., preferably about 0° C., more preferably −20° C., and most preferably at about −40° C. Without limitation, a preferred heat transfer fluid for heating and/or cooling the one or more electrochemical cells is a liquid at about 40° C. 
     The heat transfer fluid should be capable of transporting a large quantity of thermal energy, typically as sensible heat. The heat transfer fluid may have a specific heat (measured for example at about 25° C.) of at least about 1 J/g·K, preferably at least about 2 J/g·K, even more preferably at least about 2.5 J/g·K, and most preferably at least about 3 J/g·K. Preferably the heat transfer fluid is a liquid. 
     Heat transfer fluids and working fluids that may be employed include those described in U.S. Patent Application Publication 2009-0250189 (published on Oct. 8, 2009) and PCT Application No. PCT/US09/67823 (filed on Dec. 14, 2009. For example, any art known engine coolant may be employed as the heat transfer fluid. The system preferably employ a single heat transfer fluid for transferring heat into the thermal energy storage material in the heat storage device and for removing heat from the thermal energy storage material in the heat storage device. Alternatively, the system may employ a first heat transfer fluid for transferring heat to the thermal energy storage material and a second heat transfer fluid for removing heat from the thermal energy storage material. 
     Without limitation, heat transfer fluids which may be used alone or as a mixture include heat transfer fluids known to those skilled in the art and preferably includes fluids containing water, one or more alkylene glycols, one or more polyalkylene glycols, one or more oils, one or more refrigerants, one or more alcohols, one or more betaines, or any combination thereof. 
     The heat storage system may optionally include one or more heaters. The heater may be any heater that is capable of increasing the temperature of the thermal energy storage material in the heat storage device to a temperature above its transition temperature. The heater may be any heater that converts energy (e.g., electrical energy, mechanical energy, chemical energy, or any combination thereof) into heat (i.e., thermal energy). The one or more heaters may be one or more electric heaters. The one or more heaters may be employed to heat some or all of the thermal energy storage in the heat storage device. Preferably the system includes one or more heaters that are in thermal communication with a heat storage device. For example, the system may include one or more heaters within the insulation of a heat storage device. An electric heater may employ electricity from one or more electrochemical cells, from an external source, or both. For example, when a vehicle is plugged into an outlet connected to a stationary object, the heat storage device may be maintained at a temperature above the liquidus temperature of the thermal energy storage material in the heat storage device using the electricity form an external source. When the vehicle is not plugged into an outlet connected to a stationary object, the heat storage device may be maintained at a temperature above the liquidus temperature of the thermal energy storage material in the heat storage device using electricity generated from an electrochemical cell. 
     The heat storage device may be used in a process for heating one or more components. The process may include flowing a heat transfer fluid through the heat transfer device. The step of flowing a heat transfer fluid through the heat storage device may include flowing a heat transfer fluid having an initial temperature through an inlet of the device; flowing the heat transfer fluid through an axial flow path so the heat transfer fluid can be divided into a plurality of radial flow paths; flowing the heat transfer fluid through a radial flow path so that it can remove heat from the thermal energy storage material, wherein the thermal energy storage material has a temperature greater than the initial temperature of the heat transfer fluid; flowing the heat transfer fluid through a different axial flow path so that a plurality of radial flow paths can recombine; flowing the heat transfer fluid having an exit temperature through an outlet of the device; or any combination thereof. Preferably the heat transfer fluid exit temperature is greater than the initial temperature of the heat transfer fluid. The process for heating one or more components may employ a flow path through the heat storage device including one of a selection of radial flow path and two axial flow paths, the flow path having a total flow length, wherein the total flow length is generally constant for the different radial flow paths. 
     The heat storage device and/or the heat storage system may characterized as having a relatively high power (e.g., as measured during the initial 30 or 60 seconds of heating) so that it can rapidly heat a component, such as an internal combustion engine. The heat storage device and/or the heat storage system may be characterized by an average power greater than about 5 watts, preferably greater than about 10 watts, more preferably greater than about 15 watts, and most preferably greater than about 20 watts. 
     The heat storage device and/or the heat storage system may be characterized as having a relatively high power density, so that it can hold a large quantity of thermal energy in a relatively small compartment. For example, the heat storage device and/or the heat storage system may be characterized as having a power density greater than about 4 kW/L, preferably greater than about 8 kW/L, more preferably greater than about 10 kW/L, and most preferably greater than about 12 kW/L. 
     The heat storage device and/or the heat storage system may be characterized as having a relatively low pressure drop of the heat transfer fluid (measured at a heat transfer fluid flow rate of about 10 L/min). For example, the heat storage device and/or the heat storage system may be characterized as having a heat transfer fluid pressure drop less than about 2.0 kPa, preferably less than about 1.5 kPa, more preferably less than about 1.2 kPa, and most preferably less than about 1.0 kPa. 
     By way of example, the thermal energy storage system may be employed in a transportation vehicle (e.g., an automotive vehicle) for storing energy from an engine exhaust gas. When the engine produces exhaust gas, a bypass valve may either direct the flow of the gas through the heat storage device so that the heat storage device is charged, or through a bypass line to prevent the heat storage device from overheating. When the engine is shut down, e.g. during a period when the vehicle is parked, a substantial portion of the heat stored in the heat storage device may be retained for a long time (e.g., due to vacuum insulation surrounding the heat storage device). Preferably at least 50% of the thermal energy storage material in the heat storage device remains in a liquid state after the vehicle has been parked for 16 hours at an ambient temperature of about −40° C. If the vehicle is parked for a long enough time (e.g., at least two or three hours) for the engine to cool down substantially (e.g., so that the difference in temperature between the engine and the ambient is less than about 20° C.), the heat stored in the heat storage device may be discharged into the cold engine or other heat recipient indirectly by flowing a heat transfer fluid (such as the engine coolant) through the heat exchanger that includes the condenser for the working fluid. The working fluid is circulated in a capillary pumped loop using the capillary structure inside the heat storage device where the working fluid is vaporized. The heat from the working fluid is transferred to the engine coolant in the heat exchanger. By employing the heat storage device, heat that otherwise would be wasted may be captured during a previous trip to mitigate cold start and/or provide instant cockpit heating. 
     The transfer of heat using the working fluid may begin by opening the working fluid valve (i.e., the discharge valve). The sealed working fluid reservoir connected to the loop via an additional liquid line serves to accommodate changes in the working fluid liquid volume inside the loop without substantial pressure changes. Once sufficient or all useful heat is transferred from the heat storage device, the discharge valve may close. The remaining working fluid in the heat storage device may evaporate (e.g., from heat remaining in the heat storage device or when the heat storage device begins to charge) and then condenses in the condenser. As the heat storage device becomes evacuated of the working fluid, the liquid level of the working fluid level may change (e.g., rise). 
     The heat storage device may optionally be a cross-flow heat exchanger (i.e., having a flow direction for the working fluid and a perpendicular flow direction for the flow of the exhaust gas). For example, during operation, the heat storage device may include three chambers occupied by 1) exhaust gas; 2) stagnant phase change material (e.g., inside capsules, such as a blisters pack); and 3) working fluid. All three chambers are kept separate by thin walls made of an appropriate material, preferably stainless steel. Exhaust gas may flow between the surfaces (e.g., the curved surfaces) of the capsules of phase change material inside blisters, and the working fluid may flow between different surfaces (e.g., flat surfaces) of the capsules of phase change material inside blisters in a direction that is generally perpendicular to the exhaust gas flow direction. The liquid working fluid entering its chamber preferably wets a capillary structure (e.g., a metal wick) and gets transported up against the combined forces of gravity and vapor pressure by the capillary forces acting upon the working fluid liquid menisci formed inside the capillaries. This flow is sustained by continuous evaporation of the liquid using the heat drawn from the phase change material inside blisters. The vapor of working fluid leaves the capillary structure and escapes to the top of the device via vapor channels which may be interdigitated between columns of the capillary structure squeezed between the surfaces (e.g., the flat surfaces) of the capsules of phase change material inside blisters. The vapor of working fluid flows into the condenser where it transfers its heat of vaporization and sensible heat to the cold coolant and becomes liquid again to return to the heat storage device and continue its circulation in the loop, being pumped only by the capillary forces existing inside the capillary structure (e.g., metal wick) that is partially impregnated by liquid working fluid. All columns of the capillary structure may be connected to a common porous base. Such a porous base may be employed to distribute the liquid working fluid entering from the bottom of the device to the different columns. 
     Furthermore, the present invention may be used in combination with additional elements/components/steps. For example, absorption or adsorption cycle refrigeration system for air conditioning may be used as the heat recipient instead of or in addition to the cold coolant (e.g., the condenser may serve also as an evaporator for the refrigerant circulating inside an air conditioner&#39;s fluid loop). In another application, a steady-state waste heat recovery system using a heat engine, e.g. a Rankine cycle, can be constructed so that it uses the same or different capillary pumped loop working fluid and adds a mechanical power generating turbine to the vapor line between the heat storage device and the condenser, (e.g., to overcome high vapor pressure upstream from the turbine), and/or adds a liquid pump to the liquid line between the condenser and heat storage device. The above turbine can convert a part of the captured from the exhaust gas waste heat into useful mechanical or electrical work and thus improve the overall fuel efficiency of the vehicle. 
     EXAMPLES 
     Example 1 is an article including 7 sealed spaces containing thermal energy storage material and suitable for heat storage. The packs are formed by filling a base sheet having 7 troughs with a thermal energy storage material. Each trough is capable of containing about 7 cm 3  of a liquid. The base sheet is covered with a flat cover sheet. The base sheet and the cover sheet are made of stainless steel 304 and have a thickness of about 0.102 mm. The thermal energy storage material is a metal salt and has a liquidus temperature of about 195° C. The thermal energy storage material is anhydrous or has a moisture concentration of about 0.01 wt. % or less. The two sheets are joined while the thermal energy storage material is in a solid state (about 23° C.). A primary seal is provided by laser welding together the base sheet and the cover sheet about the periphery of each sealed space. When heated to a temperature of about 250° C., the sealed space has an internal pressure of about 69 kPa (about 10 psi). 
     Thermal Cyclic Testing 
     About 10 articles of Example 1 are stacked and placed in a container having an inlet and an outlet. The inlet is connected to a hot reservoir of a heat transfer fluid at a temperature of about 250° C. and a cold reservoir of a heat transfer fluid at a temperature of about 15° C. The heat transfer fluid is allowed to flow through the container until the temperature of the thermal energy storage material is about 240° C., then the cold heat transfer fluid is allowed to flow through the container until the temperature of the thermal energy storage material is about 25° C. The temperature of the thermal energy storage material is cycled about every 5 minutes for about 1,000 cycles. 
     One or more sealed spaces of Example 1 rupture and/or develop a leak in the primary seal during the thermal cyclic testing prior to reaching 1,000 cycles. Thermal energy storage material leaks out of one or more sealed spaces and Example 1 fails the thermal cycling test. 
     Heat Test 
     An article of Example 1 is placed in an oven at a temperature of about 400° C. for about 30 minutes. The article is then evaluated to determine if there are any leaks or ruptures that would allow thermal energy storage material to leak out of the article. One or more leaks and/or ruptures are observed and Example 1 fails the heat test. 
     Example 2 is an article including 7 sealed spaces prepared using the method of Example 1, except that a secondary seal is prepared by laser welding the base sheet and the cover sheet near their outer peripheries and near their opening peripheries. The articles are tested using the same method as described for Example 1. The maximum von Mises stress is above the yield stress of the foil. During the thermal cycling, the primary seal fails around one or more sealed spaces. The secondary seal does not fail after 1,000 thermal cycles and the thermal energy storage material does not leak out of the article. 
     Another article of Example 2 is tested by heating to 400° C. At 400° C., one or more primary seals fail. However, the secondary seal does not fail and thermal energy storage material does not leak. 
     Example 3 is an article including 7 sealed spaces prepared using the method of Example 1, except the base sheet and the cover sheet both use foils having a thickness of about 0.204 mm. The articles are tested using the same method as described for Example 1. The maximum von Mises stress is below the yield stress of foil. The primary seal does not fail after 1,000 thermal cycles and the thermal energy storage material does not leak out of the article. 
     Another article of Example 3 is tested by heating to 400° C. for 20 minutes. At 400° C., none of the seals fail and the thermal energy storage material does not leak. 
     Example 4 is an article including 7 sealed spaces prepared using the method of Example 1, except the cover sheet uses a foils having a thickness of about 0.204 mm. The articles are tested using the same method as described for Example 1. The maximum von Mises stress is reduced to about 180 MPa, below the yield stress of foil. The primary seal does not fail after 1,000 thermal cycles and the thermal energy storage material does not leak out of the article. 
     Another article of Example 4 is tested by heating to 400° C. for 20 minutes. At 400° C., none of the seals fail and the thermal energy storage material does not leak. 
     Example 5 is an article including 7 sealed spaces prepared using the method of Example 1, except the cover sheet is embossed so that the cover sheet over each sealed space containing thermal energy storage material has about 15 ribs including both indentions and protrusions each having a depth of 0.1 to 0.5 mm. The articles are tested using the same method as described for Example 1. The maximum von Mises stress is about 233 MPa, below the yield stress of foil. The primary seal does not fail after 1,000 thermal cycles and the thermal energy storage material does not leak out of the article. It will be appreciated that one, two, or more ribs may be employed and that ribs may be indentions, protrusions, or both. 
     Another article of Example 5 is tested by heating to 400° C. for 20 minutes. At 400° C., none of the seals fail and the thermal energy storage material does not leak. 
     Example 6 is an article including 7 sealed spaces prepared using the method of Example 1, except the cover sheet is embossed so that a sealed space including thermal energy storage material has about 34 dimples that recess about 0.6 mm into the sealed space. The dimples in the cover sheet are in a brickwall pattern as shown schematically in  FIG. 4B . The articles are tested using the same method as described for Example 1. The maximum von Mises stress is about 590 MPa and is above the yield stress of foil. The primary seal fails during the thermal cycling and the thermal energy storage material leaks out of the article. It will be appreciated that fewer or more dimples may be employed and that the dimples may be deeper or shallower. 
     Another article of Example 6 is tested by heating to 400° C. for 20 minutes. At 400° C., the seals fail and the thermal energy storage material leaks. 
     Example 7 is an article including 7 sealed spaces prepared using the method of Example 6, except the cover sheet is made of a foil having a thickness of about 0.153 mm. The articles are tested using the same method as described for Example 1. The maximum von Mises stress is about 282 MPa and is below the yield stress of foil. The primary seal does not fail during the thermal cycling and the thermal energy storage material does not leak out of the article after 1,000 thermal cycles. 
     Another article of Example 7 is tested by heating to 400° C. for 20 minutes. At 400° C., the seals do not fail and the thermal energy storage material does not leak. 
     Example 8 is an article including 7 sealed spaces prepared using the method of Example 1, except the cover sheet is embossed with a plurality of chevrons that include recesses and protrusions of about 0.5 mm. The chevrons in the cover sheet are in a repeating pattern as shown schematically in  FIG. 4A . The articles are tested using the same method as described for Example 1. The maximum von Mises stress is about 600 MPa and is above the yield stress of foil. 
     Example 9 is an article including 7 sealed spaces prepared using the method of Example 1, except a vacuum of about 200 Torr is applied when the cover sheet and the base sheet are welded together. When the thermal energy storage material is at a temperature of about 25° C., the pressure in the sealed space is less than about 400 Torr. The articles are tested using the same method as described for Example 1. The maximum von Mises stress is less than the yield stress of foil. The primary seal does not fail during the thermal cycling and the thermal energy storage material does not leak out of the article after 1,000 thermal cycles. 
     Another article of Example 9 is tested by heating to 400° C. for 20 minutes. At 400° C., the seals fail and the thermal energy storage material leaks. 
     Example 10 is an article including 7 sealed spaces prepared using the method of Example 1, except the cover sheet and base sheet are welded together when the thermal energy storage material is at a temperature of about 250° C. When the thermal energy storage material is at a temperature of about 25° C., the pressure in the sealed space is less than about 400 Torr. The articles are tested using the same method as described for Example 1. The maximum von Mises stress is less than the yield stress of the foil. The primary seal does not fail during the thermal cycling and the thermal energy storage material does not leak out of the article after 1,000 thermal cycles. 
     Another article of Example 10 is tested by heating to 400° C. for 20 minutes. At 400° C., the seals fail and the thermal energy storage material leaks. 
     The preferred embodiment of the present invention has been disclosed. A person of ordinary skill in the art would realize however, that certain modifications would come within the teachings of this invention. Therefore, the following claims should be studied to determine the true scope and content of the invention. 
     Any numerical values recited in the above application include all values from the lower value to the upper value in increments of one unit provided that there is a separation of at least 2 units between any lower value and any higher value. As an example, if it is stated that the amount of a component or a value of a process variable such as, for example, temperature, pressure, time and the like is, for example, from 1 to 90, preferably from 20 to 80, more preferably from 30 to 70, it is intended that values such as 15 to 85, 22 to 68, 43 to 51, 30 to 32 etc. are expressly enumerated in this specification. For values which are less than one, one unit is considered to be 0.0001, 0.001, 0.01 or 0.1 as appropriate. These are only examples of what is specifically intended and all possible combinations of numerical values between the lowest value and the highest value enumerated are to be considered to be expressly stated in this application in a similar manner. Unless otherwise stated, all ranges include both endpoints and all numbers between the endpoints. The use of “about” or “approximately” in connection with a range applies to both ends of the range. Thus, “about 20 to 30” is intended to cover “about 20 to about 30”, inclusive of at least the specified endpoints. Parts by weight as used herein refers to compositions containing 100 parts by weight. The disclosures of all articles and references, including patent applications and publications, are incorporated by reference for all purposes. The term “consisting essentially of” to describe a combination shall include the elements, ingredients, components or steps identified, and such other elements ingredients, components or steps that do not materially affect the basic and novel characteristics of the combination. The use of the terms “comprising” or “including” to describe combinations of elements, ingredients, components or steps herein also contemplates embodiments that consist essentially of the elements, ingredients, components or steps. Plural elements, ingredients, components or steps can be provided by a single integrated element, ingredient, component or step. Alternatively, a single integrated element, ingredient, component or step might be divided into separate plural elements, ingredients, components or steps. The disclosure of “a” or “one” to describe an element, ingredient, component or step is not intended to foreclose additional elements, ingredients, components or steps.