Relation to Other Patent Application(s)

[0001] This patent application claims the benefit of commonly owned U.S. provisional Patent Application Serial Number 63/371,637, filed on August 16, 2022 in the name of Guy Leath Gettle. Where permitted by law, the entire contents of this provisional patent application is hereby incorporated by reference.

Statement regarding U.S. Federal Government Support

[0002] None.

Technical Field

[0003] This invention relates to coatings with enhanced convective heat transfer properties, and specifically to coatings designed to regulate temperatures within substantially enclosed spaces.

Background Art

[0004] For structures that form enclosed volumes, the interior spaces are often desirably kept within a temperature range that is tolerable to humans, which is roughly between 15 and 35 degrees Celsius. Occupied spaces in dwellings and vehicles, cases for electronic devices, cabinets for chemical and pharmaceuticals storage, and packaging for lithium-containing batteries are just of few of many

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SUBSTITUTE SHEET ( RULE 26) possible examples. Electronic controllers for powered equipment such as compressors and generators often break down when they become too hot. When temperatures around refrigeration equipment rise above human-tolerable levels, they become unable to maintain acceptable conditions inside spaces required to remain cool.

[0005] Air conditioning equipment is widely used to maintain desired temperature ranges for food and pharmaceuticals warehouses, vehicles, occupied structures, and many other enclosed spaces. Many other places would benefit from air conditioning but available spaces are too small to accommodate the equipment. Examples of this include storage cabinets, electronic equipment housings, and crates. Other enclosed spaces lack power for air conditioning or refrigeration, such as remote storage facilities. Hand-held power tools and measuring devices powered by lithium-containing batteries must rely solely on conduction and natural convection of heat to the surroundings.

[0006] The introduction of electric vehicles ("EVs"), with battery enclosures filled with lithium-containing batteries has created immense demand for regulating temperatures between 25 and 45 degrees Celsius in densely-packed spaces. Virtually all lithium-containing batteries available currently are desirably maintained within this temperature range. When cells operate above fifty degrees Celsius (50 C), their usable service life is shortened considerably. Such temperatures are easily reached during fast charging and high power demand.

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SUBSTITUTE SHEET ( RULE 26) [0007] During frequent fast charging and discharging rates, cells do not return to cooler conditions, thus leading to a series of irreversible chemical decomposition reactions that create a phenomenon generally called "thermal runaway". Thermal runaway ultimately results in violent venting of flaming electrolyte, ejection of molten electrode components, and generation of large volumes of flammable gases.

[0008] Electric vehicles require hundreds of lithium-containing cells installed in battery packs. Not only must these cells be kept at human-tolerable temperatures, temperature differences between these hundreds of cells must be kept within a few degrees Celsius of one another as large gradients lead to early cell breakdown. Keeping hundreds of lithium-containing cells in large battery packs at uniform temperatures below 50 degrees Celsius is a difficult engineering challenge.

[0009] With the current art, EV engineers use liquid cooling to handle the heat generated during rapid charge and discharge modes. Liquid cooling systems need radiators and flow channels in cooling plates not significantly different than those used for internal combustion engines. Yet even with liquid cooling, heat generated during abuse conditions can exceed safe levels and lead to thermal runaway. Cooling systems are presently the greatest source of parasitic energy drain from EV battery packs.

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SUBSTITUTE SHEET ( RULE 26) [0010] Air cooling would be ideal but its convective heat transfer properties over most surface materials cannot provide adequate cooling with practical airflow rates. When ambient air temperatures exceed 50 C, convective air cooling regardless of flow rate cannot keep lithium- containing cells within human-tolerable conditions since convection can only reduce temperature to that of the inlet air. Enhancing convective heat transfer with airflow over hot metal surfaces is essential for improving cooling systems.

Removing Heat Energy from Hot Devices and Systems

[0011] Regardless of the states of matter, heat energy can be transferred from a material at one temperature to another at a different temperature by conduction, radiation, convection, or a combination of any of these. Only convection can remove enough heat energy to keep ambient conditions tolerable to lithium-containing batteries, air conditioning, and electronic equipment.

[0012] Convection is a process involving three steps. The first step is conduction of heat to the surface of the material where it is in contact with a fluid. The second step is conduction of heat energy from the material surface into the fluid. The third step is mass transport, in which the fluid having received heat energy moves away.

[0013] The second step, conduction of heat energy from a solid surface to a fluid, depends upon the properties of both solid surface and the fluid at their interface. For the solid surface, properties are

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SUBSTITUTE SHEET ( RULE 26) combined in what is generally called the convective heat transfer coefficient, often designated by the letter h in English-language technical literature. The convective heat transfer coefficient is a characteristic of the solid material, although surface cleanliness, roughness and profiles such as dimpled or corrugated do affect h.

[0014] For the fluid, important properties are its velocity, degree of turbulence, heat capacity and density. When the fluid is not moving, only natural convection caused by density and temperature differences within the fluid can occur.

[0015] Changes possible with the present art to parameters significant to the convective heat transfer coefficient are limited. Fluid velocity is limited by motive force and flow restrictions, which also affect the degree of turbulence. For ambient conditions tolerable to humans, fluid density changes are limited by temperature. The ranges of heat capacity and density between 10 and 50 degrees Celsius at atmospheric pressure are very small for air, thus manipulation of these parameters cannot make significant improvements.

[0016] For solid materials having similar surface profiles, the convective heat transfer coefficient varies most with fluid velocity. Thus technical literature often provides separate values of h for natural and for forced convection, forced meaning that the fluid is moved by mechanical means such as pumps and fans. Aluminum has a high convective heat transfer coefficient range, being greater than 200 watts

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SUBSTITUTE SHEET ( RULE 26) per square meter per degree Celsius. Graphitic materials have comparatively high coefficients, in the range of 150 to 200 W/m ² - C. Other metals, such as tin, bismuth, and lead, have low convective heat transfer coefficients. Steel and copper alloys have coefficients between those of bismuth and graphitic materials.

[0017] Increasing values of convective heat transfer coefficients is crucial for improving the performance of electronic equipment, lithium- containing cells and battery packs, air conditioning as well as reducing energy demand for these systems. The current art, unfortunately, constrains the enhancements of convective heat transfer coefficients needed by designers of systems and devices for removing heat.

Passive Means for Cooling

[0018] Many substances absorb large amounts of heat energy through changes of phase. These substances are generally called phase change materials, or "PCMs".

[0019] Heat transfer into or out from PCMs can occur by means of conduction, convection, or a combination of these processes. Many solid PCMs exist that melt or decompose between 20 and 50 C while absorbing more than 100 kilojoules of heat energy per kilogram during the transition. This characteristic can be exploited for maintaining temperatures of fluids and solids within a range desirable in many applications.

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SUBSTITUTE SHEET ( RULE 26) [0020] Heat absorption and rejection in PCMs occur at or within a narrow range around a specific temperature characteristic for the particular substance. The heat energy transferred is generally referred to as "latent heat" . The term enthalpy of fusion applies to latent heat absorbed when a PCM changes from solid to liquid, enthalpy of decomposition if the compound breaks apart, and enthalpy of vaporization if the phase change is from liquid to a gas.

[0021] Some PCMs change from one state to another without changing composition. One example is dihydrogen oxide, which does not dissociate into hydrogen and oxygen when it vaporizes and returns to liquid water when cooled. Another PCM, paraffin, retains its chemical composition when it melts and conversely when it becomes solid again. Many fatty acids such stearic acid, capric acid, and lauric acid behave similarly.

[0022] Other PCMs do change. A category of inorganic compounds called fusible salts have molecules of water bonded to anhydrous salt molecules which are released at specific temperatures. For these compounds, the released water molecules do not necessarily re-attach when temperatures drop below the fusion temperature. Examples of such fusible salts include calcium chloride hexahydrate (CaCh-6H2O), sodium sulfate decahydrate ("Glauber's salt, Na2SO4- IOH2O), and sodium tetraborate decahydrate ("borax", Na2B4O7-10H2O).

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SUBSTITUTE SHEET ( RULE 26) [0023] Unfortunately, neither fusible inorganic salt, fatty acid, nor paraffinic wax PCMs possess good thermal conductivity properties. Despite their excellent energy absorption properties, heat energy from hot regions diffuses slowly inside these PCMs. In thick layers they trap heat in the nature of insulations, delaying transfer of heat energy to surroundings. Low thermal conductivity reduces the convective heat transfer coefficient.

[0024] To get around the problem of low thermal conductivity, a number of researchers have evaluated fibrous, mesh, and open-celled foam materials having high thermal conductivities filled with PCMs. Aluminum and several forms of graphite have been the most widely used materials for this purpose. Boron nitride and silicon carbide possess high thermal conductivities as well and can be used in the same way as graphitic materials.

[0025] Chen et al (2020) studied the use of PCM-infused graphitic foams with air cooling for hybrid EV battery packs. Greco et al (2015) showed that thermal conductivity outward from lithium-containing cells is increased by using PCM-infused graphitic foams, whereas normally radial thermal conductivity is much lower than along the surface of the cell. One company, AllCell Technologies LLC of Broadview, Illinois has developed and commercialized products using this concept, including forms that hold and separate batteries in arrays. Heat generated by batteries during use is absorbed in these products, helping to regulate temperatures while allowing for convective airflow. With sufficient

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SUBSTITUTE SHEET ( RULE 26) masses of PCMs filling these forms, adequate cooling of lithium- containing battery packs for as long as 1 hour have been achieved, such as demonstrated by Javani et al and described in more detail in Dincer et al (2017).

[0026] Helpful effects of using PCMs for cooling and thermal management of lithium-containing cells have been proven by a significant number of researchers since 2002. Examples include Verma et al (2019), who studied different thicknesses of fatty acid PCMs surrounding lithium-containing pouch cells and Javani et al (2015) who studied the use of paraffinic wax separators between lithium-containing pouch cells, including the effect of different PCM thickness. Ramandi et al (2011) evaluated the use of an encapsulated paraffinic PCM, comparing a jacket containing one PCM with cooling obtained with one jacket enclosed another, each with a PCM having a different melting temperature. Kaul obtained a patent (US 6,939,610) for coatings comprised of microencapsulated PCMs in thermosetting resins that were developed to protect Space Shuttle components. All of these researchers showed that temperatures in the heat-generating cells could be kept at relatively constant levels as long as solid PCM material remained.

Background Art References

Chen, F, Huang, R, Wang, C, Yu, X, et al: Air and PCM cooling for thermal management considering battery cycle life; Applied Thermal Engineering 173 (2020): 115154

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SUBSTITUTE SHEET ( RULE 26) Dincer, I, Hamut, H S & Javani, N; Thermal Management of Electric Vehicle Battery Systems; John Wiley & Sons Ltd, 2017

Greco, A, Jiang, X & Cao, D; An investigation of lithium-ion battery thermal management using paraffin/porous graphite-matrix composite; J Power Sources 278 (2015): 50 - 68

Javani, N, Dincer, I, Naterer, G F & Rohrauer, G L; Modeling of passive thermal management for electric vehicle battery packs with PCM between cells; Applied Thermal Engineering 73 (2014): 307 - 316

Kaul, R K; US Patent 6,939,610; Issue date September 6, 2005

Ramandi, M, Dincer, I & Naterer, G F; Heat transfer and thermal management of electric vehicle batteries with phase change materials; Heat Mass Transfer 47 (2011): 777 - 788

Verma, A, Shashidhara, S & Rakshir, D; A comparative study on battery thermal management using phase material (PCM); Thermal Science & Engineering Progress 11 (2019): 74- 83

Xie, Y, Tang, J, Shi, S et al; Experimental and numerical investigation on integrated thermal management for lithium-ion battery pack with composite phase change materials; Energy Conversion & Management 154 (2017): 562 - 575

Limitations of the Present Art

[0027] PCMs must be confined when melted, otherwise they fall onto other surfaces. Unconfined bulk PCMs must be replaced, which is generally impractical.

[0028] Use of blocks, slabs, or thick components substantially comprising fatty acid and paraffinic wax PCMs can provide only limited

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SUBSTITUTE SHEET ( RULE 26) heat energy absorption. Melting of the PCM can only absorb energy amounting to the mathematical product of the PCM mass and fusion enthalpy. The total energy absorption possible is thus limited to the mass of PCM that melts.

[0029] Kaul showed that coatings with microencapsulated PCMs were effective in retarding heat penetration. His invention preached the use of solvent-based resins such as epoxies, polyesters, polyurethanes, and silicones. None of these are considered recyclable. Solvent-based resins are increasingly being constrained by environmental regulations.

[0030] Fusible salt PCMs can absorb heat energy sufficient to dissociate water molecules only once. After the water molecules have been detached, fusible salts become thermal insulators in the nature of ceramic materials. For fire suppression or cooling inside an enclosure in which thermal runaway events are underway, the inability to deal with a second exposure to intense heat is acceptable but this is not the case for applications in which frequent or cyclic high-temperature excursions occur.

[0031] For energetic lithium-containing cells, thick slabs would be needed to provide cooling for thousands of heating cycles, as is required for EVs. Importantly, thick sheets or blocks force unacceptable separations between cells in multi-cell battery packs. The same limitations apply to graphitic and aluminum foams impregnated with waxy materials. All of these shortcomings limit or prevent their

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SUBSTITUTE SHEET ( RULE 26) practical employment inside computers, electronic devices, packaging, air conditioning systems, and storage cabinets.

Disclosure of the Invention

[0032] The desirable absorption of substantial heat energy by PCMs can be achieved more efficiently and effectively by the present invention of thin coatings of compositions containing encapsulated PCM powder particles. Even faster heat transfer rates are achieved through the present invention by using encapsulated PCMs that also have characteristic dimensions less than 100 microns. Such materials are commercially available at present. Heat transfer rates can be further increased with dispersions of graphitic powders and other powders offering high thermal conductivities that have characteristic dimensions less than 100 microns within thin coatings.

[0033] By increasing heat absorption with PCMs while simultaneously increasing thermal conductivity, the convective heat transfer coefficient of the coated surface can be greatly enhanced compared with uncoated surfaces. Unlike thick PCM layers or PCM- filled metal foams, heat transfer in thin coatings occurs rapidly. The present invention contemplates total coating thicknesses less than 1 millimeter.

[0034] Encapsulants prevent contact between surrounding liquids or resins, thus melted PCMs do not leak into the coating matrix.

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SUBSTITUTE SHEET ( RULE 26) Encapsulated PCM powders are bound within the continuous resin matrix when dispersed therewithin, thus no confinement is needed for PCM-loaded coatings.

[0035] When powders having high thermal conductivities are dispersed in coatings, the path between encapsulated PCMs having low thermal conductivity and small particles having high thermal conductivity is short. The path to the surface is also short, since coating thicknesses are less than one millimeter. In one millimeter there can be 20 to 50 particles of encapsulated PCMs and high thermal conductivity materials.

[0036] For the abovementioned reasons, the use of aqueous-based coating compositions that incorporate dispersions of encapsulated PCMs having small characteristic dimensions such as microencapsulated powders would provide enhanced heat transfer coefficients that meet needs not available through the current art.

Summary of the Invention

[0037] In view of the shortcomings of existing means of regulating temperatures between 25 and 45 degrees Celsius for lithium-containing electrochemical cells, inside electronic devices, and within containers, novel means are required. The present invention accordingly offers a means for maintaining temperatures within the desired temperature range through the use of coatings alone.

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SUBSTITUTE SHEET ( RULE 26) Brief Description of the Drawings

[0038] Figure 1 depicts the basic embodiment of the coating for enhancing convective heat transfer.

[0039] Figure 2 illustrates the coating applied to the surface of a lithium-containing electrochemical cell.

Reference Numerals in Drawings

10 coating , or first coating

12 aqueous resin

16 encapsulated phase change material particles

18 optional high thermal conductivity powder particles

20 substrate

30 second coating

40 electrochemical cell

Detailed Description of Embodiments of the Invention

[0040] The drawing figures accordingly depict a number of embodiments according to the present invention. Those embodiments are summarized below followed by a more detailed description of the respective figures.

[0041] Figure 1 shows an embodiment of the coating for enhancing convective heat transfer. The coating or first coating 10 comprises an

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SUBSTITUTE SHEET ( RULE 26) aqueous resin 12 with a plurality of encapsulated phase change material particles 16 dispersed within the coating. A plurality of optional high thermal conductivity powder particles 18 are also shown dispersed within the coating. The coating illustrated has been applied to a substrate 20. A second coating 30 is shown applied to the first coating 10.

[0042] Figure 2 depicts a coating 10 applied to a substrate 20 featuring an electrochemical cell 40.

Operation

[0043] The coating adheres to the substrate without gaps and without use of adhesive intermediate layers. Heat from the substrate is conducted into and throughout the coating. When the temperature of the encapsulated phase change material reaches its fusion temperature, heat energy is absorbed as the phase change material melts. In one embodiment, the encapsulated phase change material includes a fatty acid such as stearic acid, capric acid or lauric acid. In another embodiment, the encapsulated phase change substance includes an alkane hydrocarbon such as paraffin, octadecane or icosane.

[0044] Optional high thermal conductivity powder particles distributed throughout the coating promote rapid transmission of heat for absorption the phase change material particles in the form of latent heat. Graphite, powdered activated carbon, boron nitride and silicon carbide can fill this role. Short distances between high thermal

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SUBSTITUTE SHEET ( RULE 26) conductivity powder particles and encapsulated phase change materials ensure that heat energy is conducted quickly throughout the coating as well as to the coating surface exposed to the surroundings.

[0045] Where applications require that coatings must be electrically non-conductive, graphitic powder and powdered activated carbon particles would not be used. Powders comprising boron nitride or silicon carbide would be preferable.

[0046] The combination rapid heat transfer from hot surfaces to latent heat absorption in multitudinous particles of encapsulated phase change materials in thin coating layers enhances the convective heat transfer coefficient between the coating surface and surrounding fluid. The convective heat transfer coefficient is thereby greater than that of either the aqueous resin or the substrate. Dispersing a plurality of high thermal conductivity powder particles throughout the resin increases thermal conductivity, which further enhances the convective heat transfer coefficient with respect to that of the substrate.

[0047] With sufficient mass flow of a fluid having a mean temperature below the fusion temperature of the phase change material, the phase change material will return to the solid state, thereby being available to absorb more heat energy as latent heat. Incorporating two or more phase change materials, each with a fusion temperature different than the other, then more latent heat energy can be absorbed

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SUBSTITUTE SHEET ( RULE 26) and released at different temperatures if the mean temperature of the cooling fluid fluctuates.

[0048] Heat energy released as the PCM having a lower fusion temperature melts will accelerate melting of the other PCMs. Coatings according to the present invention that use two different encapsulated phase change materials will provide substantially more uniform substrate temperatures over large surface areas as well as in containers and battery packs filled with numerous lithium-containing electrochemical cells. Embodiments of the present invention contemplate electrochemical cells having different forms such as cylindrical, pouch and prismatic, and the embodiments further contemplate covering at least 30% of the external surface of such electrochemical cell.

Advantages

[0049] The invention offers numerous alternatives for a person skilled in the arts of air conditioning systems, refrigeration, thermal management of electronic equipment, and lithium-containing battery safety. Packages and containers can easily utilize the present invention by existing means and methods for coating. The present invention makes all of this possible with environmentally friendly coatings only needed in thin layers.

[0050] Exemplary packages, enclosures and containers may contain combustible materials such as explosives, propellants, and

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SUBSTITUTE SHEET ( RULE 26) explosive devices. In such an embodiment, the coating composition may be applied to at least a portion of such enclosures and containers.

[0051] By means of the present invention, thermal management of lithium-containing electrochemical cells is made possible regardless of their size or format. The present invention also makes possible substantial improvements over the present art in cooling environments within enclosures, ducts, wall surfaces, and inside containers. In an embodiment, these cooling environments could include tubular forms, such as those that enable transport of fluids internal to the tubular forms, with the coating composition being applied to at least one surface of the tubular form. In another embodiment, the cooling environment could include a radiator that is a component in a system used to cool fluids, wherein the coating composition is applied to the radiator.

[0052] New materials and fabrication processes may be developed in the future that could further enhance capabilities within preferred embodiments. All embodiments would make possible substantially increased thermal management capabilities for numerous electronic devices and air conditioning systems, as well as provide protection against lithium-containing electrochemical cell thermal runaway events. The advantages of the present invention over any means available in the present art for a specified weight and a specified thickness are considerable.

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SUBSTITUTE SHEET ( RULE 26) Conclusion, Ramifications and Scope

[0053] Accordingly, the reader will observe that coatings comprising substantial loadings of encapsulated phase change materials applied to lithium-containing battery cells, electronic device enclosure surfaces, and cooling systems that use air as the cooling medium would offer substantial protection of electrochemical cells containing lithium from fire, prevent thermal runaway conditions from developing in any one cell, and substantially reduce the need for air conditioning in many applications and provide thermal management even when no air conditioning is present. The reader will furthermore appreciate that the electronic device enclosed by the container/enclosure of the present invention could include a device used for communication, a computer, or a laser, with the coating being applied to the enclosure. Still further, the reader will appreciate that the coating composition further being applied to a surface may enable that surface to modify its infrared radiative emanations in the manner of camouflage.

SUBSTITUTE SHEET ( RULE 26)