Abstract:
A method of storing and controlling a release of latent heat of transition of a phase-change material is disclosed. The method comprises trapping a crystallite of the material between two solid objects and retaining it there under high pressure by applying a force to press the two solid objects tightly together. A crystallite of the material is exposed to a quantity of the material that is in a supercooled condition to nucleate the crystallization of the supercooled material.

Description:
CONTRACTUAL ORIGIN OF THE INVENTION 
     The United States Government has rights in this invention under Contract No. DE-AC02-83CH10093 between the U.S. Department of Energy and the Solar Energy Research Institute, a Division of Midwest Research Institute. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to methods and apparatus for nucleating the crystallization of supercooled materials such as those used for phase-change thermal energy storage (e.g., salt hydrates such as sodium acetate, sodium thiosulfate, and trimethylol ethane hydrate). Such nucleation forces the supercooled material to change from its high-temperature phase to its low-temperature phase very rapidly and, in doing so, surrender its latent heat of transition. The invention relates more specifically to a method and apparatus for controlling and triggering nucleation at specific temperatures and times through manual, thermal, or electromechanical triggering. 
     2. Description of Prior Art 
     The general principles of thermal energy storage by means of phase-change materials are widely known. As materials are heated from a beginning state or phase, the solid phase for example, to the liquid phase, energy is absorbed. In the temperature range at which the material changes from one phase to another, more energy is required to raise the temperature an additional increment than to raise the temperature by the same increment when the material is not changing phase. This additional energy required at the phase change (or transition from one state to another) of the material is called the latent heat of transition. 
     More specifically, the heat required for the phase change from liquid to gas and is called latent heat of vaporization. The heat required for the phase change from solid to liquid and given up in the reverse phase change from liquid to solid is known as the latent heat of fusion. When a material cools, the energy absorbed at the phase-change point is normally given up. Some materials will cool well below the normal phase change temperature, but still retain the latent heat of transition and remain in the higher-temperature phase or state. For example, some materials under some circumstances may be cooled below the temperatures at which they normally change from liquid to crystalline, yet remain in the liquid state, thus still retain the latent heat of fusion. A material in this condition is said to be undercooled or supercooled. It is possible to create conditions in an undercooled material that will cause it to change very rapidly from the high-temperature phase to the low-temperature phase, thus giving up the energy stored as the latent heat of transition or fusion rapidly. The energy so surrendered may potentially be put to practical use in many ways. 
     A major drawback to effective use of the latent heat of transition or fusion of supercooled thermal storage materials has been the inability to control the conditions of release of the energy so that it can be accomplished reliably and predictably when desired by the user. There have been previous devices and methods developed for inducing the crystallization of undercooled or supercooled materials. For example, U.S. Patent No. 4,077,390 issued to J. Stanley, et al., utilizes a method of bending a ferrous metal strip positioned in a supercooled sodium acetate solution. Stanley et al, teach that crystallization of the supercooled solution may be caused by the flexing of the ferrous metal strip. Somewhat similarly, U.S. Patent No. 2,220,777, issued to D. Othmer, teaches the scratching or rubbing together of two pieces of metal or other materials in a sodium acetate solution in order to begin the crystallization of the solution. 
     U.S. Patent No. 1,384,747, issued to Eckelmann et al., crystallizes a sodium acetate solution by disturbing it with a rod or by exposing the solution to air. Likewise, U.S. Patent No. 1,385,074, issued to G. Ferguson, teaches a method of crystallizing a sodium acetate solution by piercing a seal to expose the solution to air and, then shaking the solution; and U.S. Patent No. 1,920,853, issued to R. Ferguson, also exposes sodium acetate solution to air in a valve mechanism to begin crystallization of the solution. 
     U.S. Patent No. 4,512,846, issued to Schlicta, teaches placing a solution of material to be crystallized in a pressure vessel and subjecting it to extremely high pressure to keep the solution in a supercritical state and then providing a seed crystal so that slow growth of a large crystal may proceed on the seed crystal. 
     Russian Patent No. 488,611, issued to Kurtzman, discloses apparatus that permits a crystal grower to drop a seed crystal into a supersaturated solution in order to initiate crystal growth. Besides application such as those described in the earlier patents above, where the undercooling of a material is a desired way of storing and then releasing on demand the latent heat of transition or fusion, there are also circumstances in which undercooling is an undesired effect. For example, in a process where crystallization of a material is desired to occur at a predictable temperature or at a controlled rate, the undercooling phenomenon can result in the material remaining in the liquid phase, even through it has been cooled to below its normal freezing or crystallization temperature, thus impeding or blocking the desired crystallization process. In such circumstances, an automatic, reliable means of preventing such undercooled condition would be very beneficial, but has not been available prior to this invention. 
     While all of the above-described prior developments were effective to various extents for particular purposes, there has remained a fundamental inability to trigger nucleation of the crystallization process at precise, predetermined times or temperatures with automatic, or at least easily controlled, trigger mechanisms. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is a general object of the present invention to provide a method and apparatus for releasing the latent heat of transition of a thermal storage (phase-change) material in a reliable and consistent manner. 
     Further, it is a general object of the present invention to provide a method and apparatus for releasing the latent heat of transition of a thermal storage material at a predetermined temperature that can be specified or selected by the user of the present invention. 
     Another general object of the invention is to provide a method and apparatus for releasing the latent heat of fusion of an undercooled material at any time selected by a user. 
     A more specific object of the present invention is to provide a method and apparatus for releasing the latent heat of transition of a thermal storage material by the process of nucleation of a supercooled phase of the material. 
     Another specific object of the present invention is to provide a method and apparatus for release of the stored energy of a supercooled thermal storge material that may be entirely automatic. 
     It is also a specific object of the present invention is to provide a method and apparatus for release of the stored energy of a supercooled thermal storage material that may be controlled manually by the user. 
     A further object of the present invention is to provide a method and apparatus for storage of thermal energy during periods of excess supply of that energy, for release at a time when the thermal energy is in short supply or not present at all. 
     Still another specific object of the present invention is to provide a method and apparatus for triggering the crystallization of undercooled materials by introduction of seen crystals into the materials. 
     Yet another specific object of the present invention is to provide a method and apparatus for storing a seed crystal in a container of material in both high-temperature phase and low-temperature, undercooled phase and of releasing the seed crystal into the undercooled phase material to trigger crystallization by nucleation at precisely the time or temperature desired. 
     Additional objects, advantages, and novel features of the present invention shall be set forth in part in the description that follows, and in part will become apparent to persons skilled in the art upon examination of the following or may be learned by the practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and in combinations particularly pointed out in the appended claims. 
     To achieve the foregoing and other objects and in accordance with the purpose of the present invention as embodied and broadly described herein, the method of the present invention may comprise the steps of preserving and isolating crystallites of a thermal storage (phase-change) material in its low-temperature phase by high-pressure clamping or squeezing the crystallite between two hard surfaces. The pressure should be intense enough to keep the crystallite from melting when heating the thermal storage material to its high-temperature phase. After allowing the material to cool again to a supercooled state, the pressure can be at least partially relieved, thereby presenting the crystallite of low-temperature phase material to the supercooled, high-temperature (liquid) phase of the material, thereby causing nucleation and chain reaction crystallization of the supercooled material with consequent surrender of its latent heat of transition. 
     The apparatus of this invention may comprise any hard bodies or surfaces that can be pressed together with enough pressure to trap and isolate at least one crystallite therebetween and with enough pressure to keep the crystallite from melting at temperatures higher than the phase-change temperature. For example, such apparatus can include a bistable, bimetallic trigger device wherein at temperatures above that selected by the user or designed into the device, the bimetallic portion configures itself to exert a force against a monolithic portion of the trigger, thereby trapping and preventing expansion and melting of crystals or crystallites of the thermal storage material as it is heated to a temperature above the phase-change temperature. At temperatures at or below that selected by the user, preferably at a temperature below the phase-change temperature, the bimetallic portion reconfigures itself to release enough of the pressure to present the crystallite to the supercooled material, causing nucleation and surrender of the stored energy (latent heat of transition) of the thermal storage material. 
     The apparatus of the present invention may also include an electromagnetically actuated, spring-loaded solenoid device to apply the force to a ball mounted on an anvil, with said ball and anvil being placed within a chamber containing the thermal storage (phase-change) medium. At temperatures above that selected by the user, the spring-loaded actuator of the solenoid exerts a force against the ball, which in turn exerts a force against the anvil, trapping and preventing expansion of crystals of the thermal storage material as it is heated from a low-temperature phase to a high-temperature phase, preserving said crystallite in the low-temperature phase. Then, at temperatures at or below that selected by the user, power is applied to the solenoid, causing retraction of the actuator, or at least release of enough of the force exerted against the ball to present the crystallite to the supercooled material, causing nucleation and surrender of the stored energy (latent heat of transition) of the thermal storage material within the chamber. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings, which are incorporated in and form a part of the specification, illustrate the preferred embodiments of the present invention and together with the description serve to explain the principles of the invention. 
     In the drawings: 
     FIG. 1 is a cross-sectional diagram of the entrapment of a crystallite of phase-change material by asperites of surfaces of hard materials as they are forced together in a phase-change material medium; 
     FIG. 2 is an isometric view of a bistable, bimetallic triggering device according to the first preferred embodiment of the present invention; 
     FIGS. 3-4 are cross-sectional representations of the bistable, bimetalic triggering device of FIG. 2, taken about line X--X&#39;. 
     FIG. 5 is a diagrammatic representation of the forces exerted on the ball and anvil portion of the second preferred embodiment according to the present invention. 
     FIG. 6 is a cross-sectional view of the solenoid-actuated ball and anvil triggering device of the second preferred embodiment of the present invention. 
     FIG. 7 is a cutaway view of the triggering device of the second preferred embodiment installed in a thermal storage tank. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The method and the apparatus of this invention are directed toward inducing or triggering a supercooled, phase-change thermal storage material (e.g., salt hydrates, such as sodium acetate, sodium thiosulfate, trimethylol ethane hydrates, and the like) to crystallize and thus give up its latent heat of transition by sequential formation, preservation, isolation, and introduction of a low-temperature phase seed crystal of the material at a chosen time. 
     Some materials, such as the salt hydrates mentioned above, that are useful for storing thermal energy also exhibit strong tendencies to remain in their high-temperature, i.e., liquid, phases when cooled below their normal phase-change, i.e., freezing or crystallizing, temperatures. Often, such undercooling (supercooling) is considered undesirable, because the crystallization and consequent release of thermal energy stored in the high-temperature (liquid) phase may occur at unpredictable times. However, the phenomenon of a very strong tendency to supercool, as exhibited by such materials as those listed above, may also be used to advantage by controlling the release of the stored energy so that it occurs on demand. 
     FIG. 1 is helpful in illustrating the general principles of this invention as they apply to both its method and apparatus. In general, the invention provides a convenient means of preserving small seed crystals or crystallites of the low-temperature (solid or crystalline) phase of the thermal storage material 20 and a reliable means of introducing these seed crystals into the supercooled, but high-temperature (liquid) phase of the thermal storage material 20 so as to nucleate its transformation to its low-temperature (crystalline) phase and thereby release the stored latent heat of transition on demand. 
     According to FIG. 1, opposing forces F1, F2 may be applied to two opposing hard, solid bodies 13, 14. The respective opposing surfaces 23, 24 of these bodies 13, 14 are, on a microscopic level, comprised of a plurality of asperites 19, which are simply roughnesses of the surfaces 23, 24. The magnitudes ofthe forces F1, F2 as applied to the two opposing bodies 13, 14 produce very large pressures on the minute surface areas at the points of contact of the asperites 19, easily resulting in thousands of atmospheres of pressure. 
     If the thermal storage (phase-change) material 20 is positioned in its low-temperature (solid or crystalline) phase between the opposing surfaces 23, 24 of bodies 13, 14 when the forces F1, F2 are applied, then crystals 25 (also referred to as crystallites) may be trapped between the asperites 19. Application of forces F1, F2 to the opposing bodies 13, 14 will perform two functions: (1) The pressure created at the points of contact of the microscopic asperites 19 will prevent the crystal 25 from expanding as the thermal storage material is heated to and above its phase-change temperature, thereby maintaining the crystal 25 in the low-temperature phase state (according to Clapeyron&#39;s relationship from classical thermodynamics); and (2) The pressure will cause a deformation of the surface of the microscopic asperites 19, so that they encapsulate the crystals 25, isolating them from the thermal storage material 20. Thus, as the thermal storage material 20 is heated above its phase-change temperature, it will transform and exist in its high-temperature (liquid) phase in the area ouside the contact points of asperites 19. However, the crystals of the material 20 within the encapsulated area of contact of asperites 19 remain in the low-temperature (crystalline) phase. While the phase-change material 20 that is not trapped by asperites 19 is not subjected to pressure greater than ambient, the encapsulated crystals 25 are maintained under tremendous pressure. 
     Later, when the selected thermal storage material 20 is again cooled below its phase-change temperature, i.e., undercooled or supercooled, it maintains its high-temperature (liquid) phase, because it is slow to give up its latent heat of transition. However, when this supercooled material 20 is exposed to the seed crystal 25 of material 20 in the low-temperature phase, the seed crystal 25 nucleates the crystallization of the thermal storage material 20 by causing a chain crystallization reaction that rapidly changes all of the material 20 to its low-temperature phase, thereby giving up its latent heat of transition. The energy represented by this latent heat of transition can be applied to many useful purposes. 
     The exposure of the crystal 25 to the supercooled thermal storage (phase-change) material 20 can be accomplished simply by releasing the forces F1, F2 that hold the asperites 19 together, so that crystal 25 is no longer isolated. The forces F1, F2 need not be released entirely, but only enough so that the surfaces of asperites 19 separate slightly, thus allowing the supercooled thermal storage material 20 to contact the crystals 25. 
     Bodies 13, 14 may be selected for their hardness as measured in a variety of ways, such as Knoop or Brinnell hardness. In general, the material must be hard enough to withstand the thousands of atmospheres of pressures necessary to prevent the crystals 25 of phase-change material 20 from reverting to the high-temperature (liquid) phase as the temperature is raised, but soft enough to deform and seal around, and encapsulate, the crystal 25. Many commonly available kinds of materials, such as metals, glasses, and the like, meet this requirement. 
     A first preferred embodiment of a nucleating trigger device 10, according to the principles of, and to facilitate the practice of, the present invention, is shown in FIGS. 2 through 4. The general principles of the operation of the device 10 have been described above. However, the first embodiment 10 is preferably a bistable, bimetallic trigger mechanism comprised of a bimetallic element 11 and a monolithic element 14 enclosing a chamber 17 therebetween, with an aperture 16 opening the chamber 17 to the exterior of the device 10. 
     Bimetallic element 11, as shown in FIGS. 3 and 4, is comprised of an upper metallic layer 12 and a lower metallic layer 13, having another coefficient of thermal expansion laminated together in a circular disk having a hollow, central depression 15. Metallic layer 12 and metallic layer 13 are selected so that the element 11 is bistable; that is, at a temperature above a design temperature T*, the element 11 stable shape is convex downward, as shown in FIG. 3. At ambient temperatures below T*, the element 11 shape is convex upward, as shown in FIG. 4. The transition in shape of element 11 from the shape shown in FIG. 3 to that shown in FIG. 4 occurs spontaneously as a &#34;snap action&#34; at or near design temperature T* as the ambient temperature decreases toward T*. Similarly, the transition in shape of element 11 from that shown in FIG. 4 to that shown in FIG. 3 occurs spontaneously as a &#34;snap action&#34; at or near design temperature T* as the ambient temperature increases toward T*. 
     Monolithic element 14 is also disk-shaped, and it is preferably a spring steel or similar material having a shallow central dome 18, as shown in FIGS. 3 and 4. Element 11 and element 14 are preferably attached at their respective perimeters 21 and 22 establishing a prestressed condition such that the central depression 15 of bimetallic element 11 and the central dome 18 of monolithic element 14 are always in contact. At ambient temperatures greater than design temperature T*, elements 11 and 14 are more forcefully in contact with each other at 18, as shown in FIG. 3. Conversely, at ambient temperatures below design temperature T*, elements 11 and 14 are less forcefully in contact with each other at 18, as shown in FIG. 4. 
     Aperture 16 is provided in bimetallic element 11 to allow the phase-change material 20 into chamber 17. It should be understood that the aperture 16 could also be provided in monolithic element 14 as well. 
     In operation, a bimetallic trigger mechanism 10 that is above temperature T* (and is therefore in the shape shown in FIG. 3), containing crystals of phase-change material 20, is placed in a container (not shown) having phase-change thermal storage material 20 that is in its high-temperature (liquid) phase heated above its phase change temperature. The container may be of any shape and may be adapted to a wide variety of purposes requiring the application or transfer of heat. Design temperature T* of the bimetallic trigger 10 is selected to be below the phase-change temperature of material 20. The high-temperature (liquid) phase thermal storage material 20 in the container enters chamber 17 of trigger 10 through apertures 16. The phase-change material 20 is then allowed to cool quiescently below its phase-change temperature, where it becomes undercooled or supercooled. As the ambient temperature of the material 20 decreases further and approaches design temperature T*, trigger mechanism 10 snaps to the shape shown in FIG. 4, thereby releasing some of the pressure on crystals 25 and exposing crystals 25 to the surrounding supercooled phase-change material 20. Crystals 25 serve to nucleate the supercooled phase-change material 20, thereby triggering a chain crystallization reaction and causing the material 20 to change abruptly to the low-temperature phase and thereby to surrender its latent heat of transition. When the phase-change material 20 is once again heated, before material 20 reaches its phase change temperature, trigger device 10 (at T*) will again snap into the shape of FIG. 3, sealing crystals 25 from phase-change material 20 at the pressure point 18. Phase-change material 20 can then be heated above its phase-change temperature and assume the high-temperature (liquid) phase while the crystals 25 are encapsulated and preserved at pressure point 18. When the heat source is removed, the material 20 will once again supercool, and the cycle will repeat indefinitely. 
     Operation of trigger device 10 may be fully automatic, based upon ambient temperature change. Where it is used to prevent unwanted undercooling, the design temperature T* can be just below the phase-change temperature of the liquid. Then, every time the material 20 is cooled to that temperature, the trigger device 10 will actuate automatically and nucleate the crystallization. On the other hand, where storage and release of heat is the objective, the design temperature T* can be even lower to trigger crystallization and consequent heat of transition release at the desired ambient temperature. 
     A second preferred embodiment of the invention is shown in FIGS. 5-7. A hard ball 60 and a hard anvil 61 are provided for immersion in a phase-change, thermal storage material 20 according to FIG. 5. When a force F is exerted on ball 60, ball 60 is driven into anvil 61. Crystals 25 are trapped and sealed under pressure where ball 60 contacts anvil 61. When phase-change material 20 is heated past its phase change temperature, it assumes a high-temperature phase (liquid, for example). The trapped crystals 25 are maintained in the low-temperature phase, because the expansion necessary for changing to the high-temperature phase is prevented by the pressure of ball 60 against anvil 61. A chart of the stress as reflected on the surface 24 of anvil 61 is provided in FIG. 5. 
     When the heat source is removed, phase-change material 20 will supercool, maintaining its high-temperature phase although cooling below its phase-change temperature. When the force F is partially released from ball 60, crystals 25 of the phase-change material 20 are exposed to the high-temperature phase of material 20. This exposure nucleates the high-temperature (liquid) phase of material 20, causing chain reaction crystallization and shift to the low-temperature phase immediately, thereby causing phase-change material 20 to surrender its latent heat of transition. 
     FIG. 6 illustrates a solenoid-actuated nucleating trigger 70 that uses the ball 60 and anvil 61 principle as explained above. A conventional, solenoid 50 is provided. Solenoid 50 comprises an actuator 51, magnet core 52, windings 53, insulators 63, loading spring 54. Load plate 57 is attached to actuator 51. An adjustment nut 59 is provided. A tube 55, having as its bottom portion anvil 61, provides a chamber 62 for containing phase-change material 20, ball 60, and insertion of actuator rod 51. Top plate 56 of tube 55 has an aperture 64 through which actuator 51 is inserted and allowed to pass freely. Load spring 54 presses against top plate 56 and load plate 57, exerting sufficient force on ball 60 by means of actuator 51 to provide pressure for maintaining and sealing crystals 25 as explained above. Adjustment nut 59 may be used to achieve the proper pressure exerted on ball 60. 
     When power is applied to windings 53, the electromagnet 52 decreases the force applied by load spring 54 on actuator 51, unsealing crystals 25 and exposing them to the supercooled phase-change material 20, as described above. The phase-change material then assumes its low-temperature phase, surrendering its latent heat of transition. The solenoid-actuated nucleating trigger device 70 may be actuated manually, thermally, by time, or other means, depending on the choice of conventional controls (not shown) used to apply or interrupt the application of power to the solenoid 50. Chamber 62 may be used to contain all of the phase-change material 20 in a given design, or may be equipped with apertures (not shown) to admit phase-change material 20 from a larger container. 
     FIG. 7 is illustrative of one of many applications for the release of the latent heat of transition as explained in the description of the embodiments and method of the nucleating thermal storage device of the present invention. Thermal storage tank 40 contains chambers 41 and 42, and a thermal storage medium 43, which might be (for example) water. In this application, thermal storage tank 40 may be the hot water tank component of a solar hot water system. Chambers 41 and 42 each contain a phase-change thermal storage material 20, such as sodium acetate, sodium thiosulfate, trimethylol ethane hydrate, or the like. For purposes of illustration, chamber 41 would correspond to the tube portion 55 and chamber portion 62 of the solenoid actuated nucleating trigger device 70 of FIG. 6. Solenoid mechanism 50 of FIG. 6 is mounted atop chamber 41 and preferably external to storage tank 40 of FIG. 7. Again for purposes of illustration, chamber 42 of FIG. 7 also contains a trigger mechanism 10 such as described as the first preferred embodiment. 
     A thermal transfer medium 45 collects energy from a solar collector or other source (not shown) and transfers that energy to the storage medium 43 by means of a heat exchanger 44. As the storage medium 43 heats while the outside energy source is available, it also heats the phase-change material 20 in chambers 41 and 42 so that the high-temperature phase of material 20 is reached. Trigger mechanism 10 or 70 functions as described earlier to preserve crystals for nucleating the phase-change material 20. When the outside energy source is removed (for example, by the sun setting), the storage medium 43 and phase-change material 20 cool. The phase-change material will supercool as described above. Upon arrival at the desired temperature T*, trigger 10 or 70 will actuate, causing the supercooled phase-change material 20 in chambers 41 and 42 to give up its latent heat of transition as previously described. Storage medium 43 absorbs the energy thus given up by the phase-change material, increasing its temperature. 
     In this fashion, the energy stored during periods when an external energy source is plentiful is released to maintain the temperature of water, for example, during periods when the external energy source is not available. 
     It should be understood that storage tanks 40 may be built with multiple chambers 41 or chambers 42. The activation of the triggering devices 10 or 70 may be initiated at different times for each individual chamber, so that only one chamber at a time releases its stored energy. In this way, the desired temperature of the storage medium 43 may be maintained over a longer period of time. 
     The triggering devices described in the first and second preferred embodiments may also be utilized in a wide range of applications in addition to that described in FIG. 7. Examples include hand-warmers, warming pads for medical applications, food warmers, automotive battery or engine warmers, automotive seat warmers, hot tub heat recovery systems, and the like. 
     The foregoing is considered as illustrative only of the principles of the invention. Further, since numerous modifications and changes will readily occur to those skilled in the art, it is not desired to limit the invention to the exact construction and operation shown and described, and accordingly all suitable modifications and equivalence may be resorted to falling within the scope of the invention as defined by the claims which follow.