Thermal management system

Abstract

The present invention provides a thermal management system comprising a housing having an internal space, at least one heat-generating component disposed within the internal space, and a working fluid disposed within the internal space so as to be in direct contact with at least a part of the heat-generating component, wherein the working fluid includes a base fluid and at least one phase change material selected from a microencapsulated phase change material, a nanoencapsulated phase change material, and a mixture thereof. The present invention also provides a method for thermal management of a heat-generating component, the method comprising the steps of bringing at least a part of the heat-generating component into direct contact with a working fluid, and discharging heat from the heat-generating component using the working fluid, wherein the working fluid includes a base fluid and at least one encapsulated phase change material selected from a microencapsulated phase change material, a nanoencapsulated phase change material, and a mixture thereof.

Description

Technical Field

[0001] The present invention relates to a thermal management system and a thermal management method for heat generating components.

Background Art

[0002] In various industrial fields, new technologies are required to provide solutions for higher energy efficiency and CO 2 emission reduction. The present invention relates to some possible efficiencies that drive lower energy requirements and lower CO 2 emissions. First, the present invention relates to electric vehicle technology. Second, the present invention is further applicable to thermal management of IT devices such as servers. However, the present invention described herein is not essentially limited by the technology to which it can be applied. The present invention is applicable to any electrical technology that generates heat.

[0003] It is expected that by 2040, up to 50% of the total new car sales of passenger cars will be electric vehicles. This includes battery electric vehicles (BEVs), hybrid electric vehicles (HEVs), and plug-in hybrid vehicles (PHEVs).

[0004] One of the main components of an electric transport device is the battery. The battery requires effective thermal management. Current battery technology relies on lithium-ion batteries and is likely to remain the main battery technology for at least the next 15 years. Slow charging at home or at the destination is considered the main charging method, while on the other hand, high-performance fast-charging (HPC) during travel will be necessary for many customers who desire longer-distance driving. To improve and shorten the charging process, it is necessary to increase the voltage, the current, or both simultaneously. The more the current is increased, the more excess heat is generated. The amount of excess heat can be very large and can even reach over 20 kWh. Effective thermal management is required to control the temperature and temperature uniformity within the cell pack to prevent the battery cells from deteriorating irreversibly.

[0005] Within an electric transport device, thermal management of other components, especially cooling, is also necessary. Heat is generated in both the electric motor and the inverter during use. A thermal management method that can be applied to each of these components, preferably to the entire circuit incorporating all of these components (including the battery), is most desirable.

[0006] As another type of electric transport device, there is a fuel cell electric vehicle (FCEV). Similarly, in an electric system powered by a fuel cell, a significant amount of waste heat is generated during use (about 50% of the energy is generated as waste heat), and it is necessary to remove such waste heat during operation.

[0007] Thermal management of components is also a problem in other industrial fields. There are also many problems in the thermal management of IT components, especially servers. Air cooling of these components requires a large amount of energy and expensive cooling equipment. A simpler and more energy-efficient system for the thermal management of these electronic components is highly desirable.

[0008] Conventionally, in many cooling systems, air has been passed over a heat source for the management of excess heat. However, since the thermal conductivity, heat capacity, and density of air are relatively low, the cooling effect can be limited. Also, the equipment included in an air-cooling system can be complex and expensive, and maintenance work on many moving parts may be required.

[0009] More advanced thermal management systems that conventionally use a water / glycol mixture as a heat transfer fluid have been developed. Using a water / glycol mixture, a battery block containing a number of individual battery cells can be effectively cooled. Since this is more efficient than air cooling, it is rapidly spreading as a thermal management technology used in electric transportation equipment sold today. U.S. Patent Application Publication Nos. 20090023056, 20100025006, and 2011021356 in the name of Tesla Motors Inc. describe a pipe system in which approximately 11% of the cell surface is in contact with pipes that contain a coolant. In these indirect thermal management systems, heat has to pass through the pipe material and then be transferred to the glycol / water. Further, in this configuration, the contact surface area through which heat can be transferred may be limited. Due to these factors, the overall effectiveness of this heat transfer design is limited.

[0010] Under high-power charging (HPC) conditions, if these systems cannot effectively control the cell temperature and temperature uniformity, the current is limited by the vehicle's battery management system (BMS) to protect the battery. As a result, the charging process can be significantly delayed, and the high-speed charging ability is limited. In other systems, under high-temperature conditions, they may simply overheat or shut down for safety reasons. SUMMARY OF THE INVENTION PROBLEMS TO BE SOLVED BY THE INVENTION

[0011] In direct liquid cooling, which includes a novel thermal management system, generally the heat-generating components are designed to be in direct contact with a liquid coolant. Since the fluid is in direct contact with the surface of the components, temperature control of each component can be much more effectively achieved. In many direct cooling systems, the heat-generating components are fully or partially immersed in the cooling fluid. Therefore, direct liquid cooling is also referred to as immersion cooling or immersive cooling. However, not all direct cooling systems are immersion-based. For example, in some applications, the cooling fluid is sprayed onto the heat-generating components. Although the term "direct cooling" is used herein, this term is often used in the industry to mean the same as "immersion cooling". As used herein, the term "direct cooling" encompasses both immersion direct cooling systems and non-immersion direct cooling systems. Such direct cooling systems are described, for example, in U.S. Patent Application Publication No. 20170279172. In a direct cooling system, a fluid with very good dielectric properties is required to prevent short circuits, which means that the electrical conductivity of the fluid must be low. Suitable fluids are further required to have a low viscosity to facilitate pump transport, as well as high thermal conductivity and heat capacity. It has already been demonstrated that direct thermal management can contribute to an increase in battery output and energy density, as well as a significant improvement in cell durability. Water / glycol-based fluids have very high thermal conductivity and heat capacity but are not dielectrics. Therefore, in a direct thermal management system, an alternative working fluid needs to be used.

[0012] A better method for the direct thermal management of electrical systems and the development of suitable working fluids remain open issues. Such working fluids are required to have excellent material compatibility, thermodynamic properties, and low flammability. For practical purposes, cost and weight also need to be considered. To prevent short circuits and / or damage to the heat-generating components, it is important that the electrical conductivity of the fluid is low and that it can be maintained even as the fluid ages. It is also desirable to avoid electrostatic charging of the working fluid during use, for example, during pump transport at high flow rates.

[0013] Phase change materials (PCMs) are very attractive for thermal management applications. When a phase change occurs, the latent heat of the PCM can remove a significant amount of heat from the heat-generating components without causing a significant temperature rise. PCMs can be divided into liquid / vapor PCMs and solid / liquid PCMs.

[0014] Hydrofluorinated ethers are an example of liquid / vapor PCMs and are disclosed in International Publication No. WO 2018 / 224908, where they are used for direct thermal management. Hydrofluorinated ethers reduce the risk of flammability but can pose problems in terms of material compatibility. An attractive feature of this type of fluid is that it is boilable. The boiling points of some hydrofluorinated ethers are within the normal operating temperature range of lithium-ion batteries. By selecting a fluid with a specific boiling point, the fluid can be boiled during the cooling operation, and its latent heat can be utilized for cooling and temperature control. An essential concern regarding the use of liquid / vapor PCMs is that the thermal conductivity and heat capacity decrease dramatically after boiling, thereby reducing the heat transfer performance. Another major concern is the potential for pressurization due to the phase change from liquid to vapor.

[0015] Regarding solid / liquid PCMs, there is an application where the heat-generating component is surrounded by the PCM for cooling. The main problem is the maintenance of the shape after melting into a liquid. To solve this problem, the PCM has been used together with a matrix material to form a composite material. The matrix material retains the shape of the PCM after melting. The essential concern related to such a system is the absence of a circulating flow of the cooling material. After all the available PCM has melted, there is no replenishment mechanism, so no more effective cooling can be maintained. Also, mixing solid / liquid PCMs with a liquid cooling fluid has been done. This allows the introduction of a circulating flow and, at the same time, the utilization of the advantages of the PCM. An example of this is disclosed in U.S. Patent Application Publication No. 2013 / 0004806, which discloses a microencapsulated phase change material used with a cooling fluid as part of a thermal management system for an automotive battery pack assembly. However, this document only relates to the indirect cooling of the battery.

[0016] Many common PCMs are organic materials and may have a density lower than that of water. On the other hand, the cooling fluid for an indirect cooling system is usually water / glycol-based. Due to such density differences, it can be difficult to uniformly suspend the microencapsulated PCM in the base fluid. The microencapsulated PCM may float on the fluid surface.

[0017] It has been found that a Fischer-Tropsch-derived base fluid is promising for direct cooling. European Patent Application No. 20166789.6 discloses a thermal management system in which a heat-generating component such as a battery is directly cooled through a working fluid containing a Fischer-Tropsch-derived base fluid. The thermal conductivity and heat capacity of the Fischer-Tropsch-derived base fluid are generally higher than those of hydrofluorinated ethers but much lower compared to water / glycol-based fluids. On the other hand, the Fischer-Tropsch-derived base fluid does not boil under normal operation. The latent heat cannot be utilized for thermal control.

[0018] It is desirable to develop an improved thermal management system that overcomes the limitations of known thermal management systems as described above. It is also desirable to develop a thermal management system that utilizes direct cooling and provides more effective cooling to the heat-generating components to be cooled by the cooling fluid. In particular, it is desirable to develop a thermal management system that utilizes direct cooling and has an increased heat capacity of the cooling fluid.

Means for Solving the Problems

[0019] The present invention includes a housing having an internal space, at least one heat-generating component disposed within the internal space, and a working fluid disposed within the internal space so as to be in direct contact with at least a part of the heat-generating component, wherein the working fluid includes a base fluid and at least one phase change material selected from microencapsulated phase change materials, nanoencapsulated phase change materials, and mixtures thereof, and provides a thermal management system.

[0020] The present invention further provides a method for thermal management of a heat-generating component, the method including the steps of bringing at least a part of the heat-generating component into direct contact with a working fluid, and using the working fluid, which includes a base fluid and an encapsulated phase change material selected from microencapsulated phase change materials, nanoencapsulated phase change materials, and mixtures thereof, to dissipate heat from the heat-generating component.

[0021] In a preferred embodiment, the base fluid is a base fluid derived from Fischer-Tropsch. It has been clarified that the present invention provides an improved direct thermal management system that enables more effectively maintaining a heat-generating component such as a battery at an optimal operating temperature. Therefore, according to the thermal management system of the present invention, the efficiency of the battery can be effectively improved and the battery life can be extended.

[0022] In particular, it has been found that the thermal properties, especially the heat capacity, of a direct cooling fluid, such as a direct cooling fluid containing a fluid derived from Fischer-Tropsch, are improved by the addition of the above-mentioned phase change material (PCM).

[0023] Furthermore, in the heat management system of the present invention, it has been found that by using the encapsulated phase change material, the temperature rise of the battery can be delayed, thereby better preventing overheating.

[0024] Furthermore, in the heat management system of the present invention, it has been found that by using the encapsulated phase change material, most of the battery cells can be maintained at an optimal operating temperature.

Brief Description of the Drawings

[0025]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Modes for Carrying Out the Invention

[0026] It has been found that the effectiveness of a direct thermal management system in which a heat-generating component is in direct contact with a working fluid is substantially enhanced according to the present invention. This is achieved by introducing an encapsulated phase change material into the working fluid.

[0027] The heat-generating component is preferably an electrical component. Exemplary electrical components that can benefit from the systems and methods described herein include computer servers, batteries, inverters, electric motors, and fuel cells, or any combination thereof.

[0028] In the thermal management system of the present invention, one or more heat-generating components may be cooled. The thermal management system of the present invention comprises a housing having an internal space, a heat-generating component disposed within the internal space, and a working fluid disposed within the internal space so as to be in direct contact with at least a portion of the heat-generating component. As used herein, the phrase "direct contact" means that the heat-generating component is partially immersed, up to and including fully immersed, in the working fluid, or that a part or the whole of the heat-generating component is brought into contact with the working fluid by spraying or injection. In a preferred embodiment of the present specification, the heat-generating component is partially immersed, up to and including fully immersed, in the working fluid. In a preferred embodiment, at least 20%, more preferably at least 60%, even more preferably at least 90% of the surface area of the heat-generating component is in direct contact with the working fluid. In a particularly preferred embodiment, 100% of the surface area of the heat-generating component is in direct contact with the working fluid.

[0029] Preferably, the thermal management system is constructed such that a circulating flow of the working fluid that passes through one or more heat-generating components, travels towards a heat exchanger, and then returns to the heat-generating components can be maintained, as needed and under the control of a control system.

[0030] This heat exchanger may be disposed inside or outside the housing. The thermal management system may comprise a liquid circuit having a pump and a heat exchanger. In this embodiment, due to the operation of the pump, the working fluid is moved from the heat-generating component to the heat exchanger and also from the heat exchanger. The pump is preferably controlled by a control system to generate an appropriate flow rate of the working fluid based on instantaneous operating conditions.

[0031] Heat is transferred from the heat-generating component to the working fluid. Then, the working fluid may be pumped away from the heat-generating component towards the heat exchanger. Then, heat may be transferred from the working fluid through the heat exchanger. Then, the working fluid may be returned to the heat-generating component.

[0032] The thermal management system not only functions to remove heat from the heat-generating components disposed therein, but may also be suitable for providing heat to the heat-generating components in certain situations during the operation of those components, for example, at startup or during operation in a cold environment. In this embodiment of the present invention, a heat source is included in the thermal management system. Such a heat source may include an internal heat source or an external heat source. The thermal management system will further comprise a control mechanism that enables switching between the cooling embodiment and the heating embodiment of the system.

[0033] Suitable internal heat sources may include a battery and a load forming a heat-generating circuit. Suitable external heat sources include heat pumps, phase change materials that can release heat during phase change, electric heaters, and heaters that burn ethanol, bioethanol or other fuels.

[0034] The working fluid includes a base fluid and at least one encapsulated phase change material (PCM). The encapsulated phase change material is composed of very small two-component particles or capsules containing a high latent heat inner core material. This inner core material is adjusted to undergo a phase change within the temperature range that typically occurs in one or more heat-generating components and is typically covered by an outer shell or capsule made of a polymer or related material.

[0035] To maintain the fluidity of the base fluid, e.g., a base fluid derived from Fischer-Tropsch, and to prevent aggregation of the PCM, it has been found useful to encapsulate the PCM at the micrometer and / or nanometer scale and disperse it in the base fluid. Thus, the encapsulated phase change material used herein is selected from microencapsulated phase change materials, nanoencapsulated phase change materials, and mixtures thereof. For clarity, this encapsulated phase material can include mixtures of two or more microencapsulated phase change materials, mixtures of two or more nanoencapsulated phase change materials, and mixtures of one or more microencapsulated phase change materials and one or more nanoencapsulated phase change materials.

[0036] The average particle size of the phase change material capsules for use herein is typically in the range of 5 nm to 200 μm, more preferably 10 nm to 50 μm. The particle size of the encapsulated phase change material is measured using well-known analytical techniques well known to those skilled in the art, such as SEM and TEM.

[0037] As used herein, the term "microencapsulation" typically means that the particle size is in the range of 1 μm to 1 mm, preferably 1 μm to 200 μm. The term "nanoencapsulation" means that the particle size is less than 1 μm.

[0038] Macroencapsulated phase change materials (i.e., typically having a particle size greater than 1 mm) are not suitable for use herein. Preferably, the actual melting point of the phase change material is selected based on the optimal operating temperature of the specific exothermic component. As an example, when cooling a lithium-ion battery, the core of the phase change material should have a melting point in the range of 20 °C to 60 °C, preferably 30 °C to 55 °C.

[0039] Although the melting points of some of the phase change materials listed below are higher than 60°C, if they are used to form a eutectic mixture of phase change materials together with other phase change materials and the eutectic mixture has a melting point below 60°C, such phase change materials can be used.

[0040] By carefully selecting a specific material for encapsulation, the desirable dielectric properties of the bulk fluid can be maintained. Further, by matching the melting point of the PCM with the optimal operating temperature of the battery, the battery can be more effectively maintained at the optimal operating temperature.

[0041] Theoretically, both solid-liquid phase change materials and liquid-vapor phase change materials can be used to form the inner core, but solid-liquid phase change materials are preferred in this specification because of their small volume change. Although not wishing to be limited by theory, it is considered that by adopting solid-liquid PCMs, the pressurization associated with liquid-vapor phase changes and the degradation of thermal properties can be avoided.

[0042] The inner core preferably contains one or more materials selected from paraffinic wax, n-alkanes, fatty acids, fatty alcohols, C4-C14 alkyl alcohols, fatty acid esters, polyglycols, chlorinated paraffins, inorganic salts, salt hydrates, sugar alcohols, carbohydrates and polyols, and eutectic mixtures thereof made from one or more of the aforementioned materials.

[0043] Suitable n-alkanes for use in this specification include those having 17 to 27 carbon atoms, such as heptadecane, octadecane, nonadecane, eicosane, heneicosane, docosane, tricosane, tetracosane, pentacosane, hexacosane, heptacosane, and mixtures thereof.

[0044] Also, n-alkanes having a larger number of carbon atoms may be used in eutectic mixtures with other phase change materials. Suitable fatty acids for use in this specification include capric acid, lauric acid, myristic acid, palmitic acid, stearic acid, arachidic acid, and mixtures thereof.

[0045] Suitable fatty alcohols for use in this specification include 1-dodecanol, 1-tridecanol, 1-tetradecanol, 1-pentadecanol, 1-hexadecanol, 1-heptadecanol, 1-octadecanol, and mixtures thereof.

[0046] Suitable C4 - C14 alkyl alcohols include 2-methyl-2-propanol, 2,2-dimethyl-1-propanol, cyclohexanol, diphenylmethanol, and mixtures thereof.

[0047] Suitable fatty acid esters include butyl stearate, propyl palmitate, vinyl stearate, methyl stearate, and mixtures thereof. Suitable polyglycols include polyalkylene glycols such as polyethylene glycol.

[0048] Suitable inorganic salts and salt hydrates include LiNO 3 ·3H 2 O, Na 2 SO 4 ·10H 2 O, Na 2 CO 3 ·10H 2 O, NaCH 3 COO·3H 2 O, Na 2 HPO 4 ·12H 2 O, Na 2 HPO 4 ·7H 2 O, Na 2 S 2 O 3 ·5H 2 O, Na 2 CrO 4 ·10H 2 O, NaOH·H 2 O, KF·4H2 O, KF·2H 2 O, K(CH 3 COO)·1.5H 2 O, K 3 PO 4 ·7H 2 O, CaCl 2 ·6H 2 O, CaBr 2 ·6H 2 O, Ca(NO 3 ) 2 ·4H 2 O, Zn(NO 3 ) 2 ·6H 2 O, Zn(NO 3 ) 2 ·4H 2 O, Zn(NO 3 ) 2 ·2H 2 O, Cd(NO 3 ) 2 ·4H 2 O, Mn(NO 3 ) 2 ·6H 2 O, Fe(NO 3 ) 2 ·6H 2 O, Ba(OH) 2 ·8H 2 O, Na 2 B 4 O 7 ·10H 2 O, Na 3 PO 4 ·12H 2 O, Na 2 P 2 O 7 ·10H 2 O, MgCl 2 ·6H 2 O, Mg(NO 3 ) 2 ·6H 2 O, Ba(OH) 2 ·8H 2 O, (NH 4 )Al(SO 4 ) 2 ·12H 2 O, and their mixtures are included.

[0049] Suitable sugar alcohols include, for example, erythritol, mannitol, galactitol, and mixtures thereof. A suitable carbohydrate is, for example, ribose.

[0050] A suitable polyol is, for example, pentaerythritol. Preferably, the inner core contains an n-alkane having 17 to 27 carbon atoms. In one embodiment, the inner core is eicosane. Eicosane is a paraffin-based mixture of alkanes that exhibits a high latent heat of fusion (e.g., about 240 kJ / kg). Further, in electric transportation equipment and related automotive applications, in a normal battery cell, when the temperature exceeds 40 °C, acceleration of deterioration and / or an increase in the risk of thermal runaway may occur. Therefore, the melting point of eicosane (about 37 °C) is ideal for electric transportation equipment and related automotive applications. Such paraffin-based PCMs are excellent electrical insulators with a high electrical resistivity of 10 13 ~10 17 Since it has a high electrical resistivity of ohm·meter, safe and reliable electrical operation is promoted.

[0051] The outer shell preferably includes one or more materials selected from polymers, resins, inorganic oxides, multi-walled carbon nanotubes, nanocellulose, and mixtures thereof. Suitable polymers for use in the outer shell include, for example, polymethyl methacrylate (PMMA), polystyrene, polyurea, polyurethane, polyethylene, polysiloxane, gelatin, and copolymers thereof, and mixtures thereof. The polymer may or may not be crosslinked.

[0052] Suitable resins for use in the outer shell include, for example, melamine-formaldehyde resins, urea-formaldehyde resins, and epoxy resins, and mixtures thereof. The resin may or may not be crosslinked.

[0053] Suitable inorganic oxides for use in the outer shell include, for example, SiO 2 , boehmite, CaCO 3 , TiO 2 , ZrO 2 , and mixtures thereof. Preferably, the outer shell is a polymer material.

[0054] Examples of commercially available encapsulated phase change materials suitable for use herein include those available from Microtek under the trade names Nextek®, Vivtek®, Apaptek®, Fibratek®, Micronal®, MPCM, and PCMBlend, and those available from Croda under the trade name CrodaTherm®.

[0055] The encapsulated phase change material is preferably present in the working fluid in a total amount of 0.5 wt% to 35 wt%, more preferably 5 wt% to 20 wt%, even more preferably 5 wt% to 15 wt%, particularly 8 wt% to 12 wt% of the weight of the working fluid.

[0056] Two or more encapsulated phase change materials can also be incorporated into the working fluid herein. For example, a first encapsulated phase change material configured to exhibit a phase change at a first (e.g., lower) temperature may be included, and a second encapsulated phase change material configured to exhibit a phase change at a second (e.g., higher) temperature may be included. When two or more encapsulated phase change materials are used, the amounts mentioned above relate to the total amount of encapsulated phase change material present in the working fluid.

[0057] A second essential component of the working fluid is the base fluid. The base fluid is preferably a hydrocarbon-based base fluid and may be petroleum-based, bio-based, or synthetic. Preferably, the base fluid is present in the working fluid in an amount of 40.0 to 99.4 wt%, more preferably 60 wt% to 90 wt%, even more preferably 70 wt% to 85 wt% of the weight of the working fluid.

[0058] A preferred base fluid for use in this specification is a base fluid derived from Fischer-Tropsch. The combination of a base fluid derived from Fischer-Tropsch and a phase change material is particularly useful in the present invention from the viewpoint of achieving better cooling and thermal control of heat-generating components as compared to the case of using a working fluid based on a base fluid derived from Fischer-Tropsch but not containing a phase change material. The latent heat of the phase change material can absorb additional heat, and thus better cooling and thermal control can be achieved.

[0059] Base fluids derived from Fischer-Tropsch are known in the art. The term "derived from Fischer-Tropsch" means that the base fluid is a synthetic product of the Fischer-Tropsch process or is derived from such a synthetic product.

[0060] Base fluids derived from Fischer-Tropsch are often classified by the starting materials in the Fischer-Tropsch process (i.e., "X-to-liquids" or "XTL" (where X represents the starting material)). Some examples of the Fischer-Tropsch process for producing base fluids include the Biomass-to-liquid (BTL) process, the coal to liquids (CTL) process, the gas-to-liquid (GTL) process, and the power-to-liquid (PTL) process. Preferably, the base fluid derived from Fischer-Tropsch is a GTL (gas-to-liquid) base fluid.

[0061] Suitable Fischer-Tropsch-derived base fluids containing oils that can be advantageously used in working fluids are, for example, those disclosed in European Patent No. 0776959, No. 0668342, International Publication No. 97021788, No. 0015736, No. 0014188, No. 0014187, No. 0014183, No. 0014179, No. 0008115, No. 9941332, European Patent No. 1029029, International Publication No. 0118156, and No. 0157166.

[0062] Particularly preferred Fischer-Tropsch-derived base fluids for use in the working fluids of this specification have a kinematic viscosity at 100 °C of up to 4 mm 2 / s, particularly in the range of 2 - 4 mm 2 / s, such as Fischer-Tropsch-derived base oils having a kinematic viscosity of about 3 mm 2 / s at 100 °C, such as GTL 3.

[0063] Another particularly preferred Fischer-Tropsch-derived base fluid for use in the working fluids of this specification is a Fischer-Tropsch-derived base fluid produced from a gas oil stream from the GTL process, preferably a dewaxed gas oil stream, which fluid has a kinematic viscosity in the range of 2.0 - 22 mm 2 / s, preferably 2.0 - 11 mm 2 / s. Preferably, this Fischer-Tropsch-derived base fluid produced from a gas oil stream has a kinematic viscosity at 40 °C of at least 2.1 mm 2 / second, more preferably at least 2.2 mm 2 / second. Also preferably, this fluid has a kinematic viscosity at 40 °C of up to 10.0 mm 2 / second, more preferably up to 7.0 mm 2 / second, most preferably up to 6.0 mm 2 / second.

[0064] In one embodiment of the present invention, the working fluid comprises a mixture of two or more Fischer-Tropsch-derived base fluids. For example, the working fluid has a kinematic viscosity at 100 °C of 2 - 4 mm2 It may include both a Fischer-Tropsch-derived base oil having a kinematic viscosity within the range of / s and a Fischer-Tropsch-derived base fluid produced from a gas oil stream.

[0065] Since many typical PCMs have a density similar to that of GTL or organic base fluids, it has been shown that mixing encapsulated PCMs into GTL or organic base fluids is more practical than mixing them into water / glycol-based fluids. This allows the encapsulated PCMs to be made suitable for direct cooling rather than indirect cooling.

[0066] Other components in the working fluid may include one or more additional base oils such as mineral oils and synthetic oils. Mineral oils include paraffinic, naphthenic, or mixed paraffinic / naphthenic liquid petroleum that can be further refined by a hydrofinishing process and / or dewaxing, and solvent-treated or acid-treated mineral oil lubricants. Synthetic oils include hydrocarbon oils such as olefin oligomers (polyalphaolefin base oil (PAO)), dibasic acid esters, polyol esters, polyalkylene glycol (PAG), alkylbenzenes, alkylnaphthalenes, and dewaxed waxy isomerates.

[0067] In one preferred embodiment, the working fluid includes one or more additional base oils selected from alkylbenzenes, alkylnaphthalenes, and mixtures thereof. When present, the one or more additional base oils selected from alkylbenzenes, alkylnaphthalenes, and mixtures thereof are present at 35 wt% or less based on the total weight of the working fluid. Preferably, the one or more additional base oils selected from alkylbenzenes, alkylnaphthalenes, and mixtures thereof are present in an amount in the range of 1 to 30 wt% based on the total weight of the working fluid.

[0068] The working fluid has a pour point of preferably -40°C or lower, more preferably -50°C or lower, as measured in accordance with ISO 3016. The working fluid also has a flash point of preferably at least 100°C, more preferably at least 110°C, most preferably at least 120°C, and preferably at most 240°C, as measured in accordance with ASTM D93.

[0069] The thermal conductivity of the working fluid at 20°C is preferably at least 0.135 W / mK, as measured in accordance with ASTM D7896. The specific heat capacity of the working fluid at 20°C according to ASTM D 1269 is preferably at least 1.9 kJ / kg·K, more preferably at least 2.0 kJ / kg·K.

[0070] Preferably, the working fluid further contains an antioxidant additive and an antistatic additive. The antioxidant additive is preferably a hindered phenolic antioxidant additive, a sterically hindered monovalent, divalent, and trivalent phenol, a sterically hindered dinuclear, trinuclear, and polynuclear phenol. Optionally, an additional amine antioxidant, such as an alkylated or styrenated diphenylamine, may be added to the working fluid.

[0071] The total amount of one or more antioxidant additives present in the working fluid is preferably at least 0.1% by weight, more preferably at least 0.15% by weight, and preferably at most 3.0% by weight, more preferably at most 2.0% by weight, based on the total weight of the working fluid.

[0072] The antistatic additive used in the present invention is preferably selected from those containing an alkyl-substituted naphthalenesulfonic acid, benzotriazole, and a substituted benzotriazole. The content of the antistatic additive is preferably more than 0.5 mg / kg, more preferably more than 1 mg / kg, based on the total weight of the working fluid. The actual upper limit may vary depending on the specific use of the lubricating composition. This concentration may be up to 3% by weight at most based on the total weight of the working fluid, but is preferably in the range of 1 mg / kg to 1% by weight. However, such compounds can be advantageously used at a concentration of less than 1000 mg / kg, more preferably less than 300 mg / kg, based on the total weight of the working fluid.

[0073] Detailed Description of the Drawings FIG. 1 shows an embodiment of a liquid circuit suitable for the thermal management system of the present invention. In FIG. 1, a heat-generating component (1) is arranged in a housing (2). The working fluid (3) flows through the internal space of the housing (1) and then through a connecting pipe (4) to one or more heat exchangers (5). In this embodiment, this flow is maintained by one or more pumps (6).

[0074] FIG. 2 is a schematic diagram of a thermal analysis of a direct-cooled battery module which is an embodiment of the system described in the present invention. FIG. 3 is a schematic diagram of the equilibrium position of an encapsulated phase change material (PCM) in a fluid flow through a gap between battery cells or between a battery cell and a module housing. This is useful for the thermal analysis shown in FIG. 2.

[0075] FIG. 4 shows a general temperature distribution on the cell surface of a battery derived from a thermal analysis. The parameters are indicated by symbols. The values of the parameters are not incorporated into this general result. FIG. 5 shows the temperature distribution in a case study using typical materials and a typical battery cell.

[0076] FIG. 6 is a graph showing measured values of the specific heat of two embodiments of the working fluid described herein in the temperature range of 10°C to 80°C. Here, the present invention will be further illustrated by the following non-limiting examples.

[0077] Examples For battery thermal management applications, a theoretical analysis was conducted to demonstrate the advantages of using phase change materials encapsulated in a working fluid. To theoretically analyze the temperature distribution on the surface of a directly cooled battery cell, a one-dimensional convection model was established. By comparing the temperature distributions of the base fluid and the fluid mixed with one or more encapsulated phase change materials, it can be shown that the latter can achieve better cooling and more precise thermal management.

[0078] As shown in FIG. 2, the convective heat transfer in a battery module composed of prismatic cells or pouch cells was utilized to analyze the fluid flow between cells and on the sides of the cells. The fluid flow between cells and on the sides of the cells was analyzed. In the left figure of FIG. 2, the horizontal gap shown in light gray is an example of the fluid flow between cells, and the vertical gap shown in dark gray is an example of the fluid flow on the sides of the cells. One-dimensional convection models for these two flows are shown in the central and right figures, respectively.

[0079] In the analysis, referring to FIG. 2, the cooling fluid flows through a gap of width w. x is the longitudinal coordinate along the central axis of the gap, and y is the transverse coordinate with the position of the central axis being 0. The volume flow rate (per unit length perpendicular to the page) of the coolant is Q, and the heat transfer rate per unit area to the fluid is

[0080]

Number

[0081] is. As shown in the central figure of FIG. 2, between cells,

[0082]

Number

[0083] is the heat generation rate per unit area of the side surface of one cell, and half of it enters the fluid from each side of the gap. On the side of the cell,

[0084] [Number]

[0085] is the total heat generated by all the cells included, and the heat comes only from one side of the gap. Typically, the width w of the gap is on the order of millimeters, while the dimension of the side surface of the cell is on the order of 100 millimeters. The width w of the gap is typically much smaller than the width in the z direction perpendicular to the page. Therefore, the variation in the z direction is ignored. Furthermore, the width w is usually much smaller than the length of the gap. Therefore, for simplicity, the developing region (inlet region) is ignored. The one-dimensional fully developed flow in the gap is analyzed.

[0086] Flow field The basic equation for the flow velocity in one-dimensional fully developed flow is as follows.

[0087] [Number]

[0088] where p is the pressure, μ is the fluid viscosity, and u is the flow velocity in the x direction. For the two side walls, the no-slip boundary condition is applied.

[0089] [Number]

[0090] In fully developed flow, the pressure gradient

[0091] [Number]

[0092] is independent of y. Therefore, by integrating Equation 1 twice in the y direction, the following velocity profile can be obtained.

[0093]

Number

[0094] (The volumetric flow rate Q per unit length perpendicular to the page) can be obtained by integrating Equation 3.

[0095]

Number

[0096] By substituting Equation 4 into Equation 3, the velocity profile can be written in terms of the flow rate instead of the pressure gradient.

[0097]

Number

[0098] The flow velocity profile does not change due to the thermal boundary conditions. That is, Equation 5 applies to both conditions in Figure 2. Further, for the working fluid containing one or more encapsulated phase change materials, it is assumed that the capsules are small enough, the concentration of the capsules is small enough, and the capsules are homogeneously suspended in the working fluid. Due to these assumptions, most of the fluid is still treated as a homogeneous Newtonian fluid, and Equation 5 adequately models the flow field. Therefore, Equation 5 is applicable to all scenarios analyzed in these examples.

[0099] Temperature distribution The basic equation for the temperature distribution in a one-dimensional fully developed flow is as follows.

[0100]

Number

[0101] Wherein, T is the temperature, α is the thermal diffusivity of the fluid, and u is the flow velocity in the x direction described by Equation 5. This basic equation is applicable to all scenarios to be analyzed, but the boundary conditions vary depending on the thermal boundary conditions and the fluid.

[0102] In the case without encapsulated PCM For the working fluid without PCM capsules, Equation 6 is applicable throughout the gap. In a fully developed flow,

[0103]

Number

[0104] is independent of y. Integrating Equation 6 across the gap gives the following equation.

[0105]

Number

[0106] From here, differences due to the thermal boundary conditions occur. When there is heat transfer from both side walls This scenario applies to the gap between cells as shown in the central figure of Figure 2. The heat transfer rate to the fluid at the side walls should match the heat transfer from the cells. Therefore, the following holds.

[0107]

Number

[0108] Wherein, k is the thermal conductivity of the fluid. Substituting Equation 8 into Equation 7 gives the following equation.

[0109]

Number

[0110] In the formula, ρ is the fluid density, and c p is the specific heat of the fluid. Note that

[0111]

Number

[0112] is as follows. Here, substituting Equation 5 and Equation 9 into Equation 6, the following basic equation for T can be obtained.

[0113]

Number

[0114] By integrating Equation 10 twice with the boundary condition 7, the following temperature distribution can be obtained.

[0115]

Number

[0116] In the formula, T w is the temperature of the cell surface, that is, as follows.

[0117]

Number

[0118] T w is the ultimate goal of this analysis. Thereby, the effectiveness of thermal control for the battery cell when there is a given heat generation from the battery cell under specific operating conditions is shown. To make it easier to compare the surface temperature of the battery cell with the fluid temperature that varies across the gap, the bulk (average) temperature T b of the fluid is defined as follows.

[0119]

Number

[0120] Calculating gives the following.

[0121]

Number

[0122] Up to here, the cell surface temperature T w has been shown to increase along the flow direction with the slope described by Equation 9 and to be higher than the bulk fluid temperature by the amount described by Equation 14. When there is heat transfer from only one wall This scenario applies to the side gap of the cell as shown in the right figure of Figure 2. In this analysis, it was assumed that the other wall was adiabatic so that all the heat generated from the cell was carried away by the convection of the working fluid. That is, there was no mechanism of heat transfer through the module housing. With this assumption, the thermal boundary conditions are as follows.

[0123]

Number

[0124] Substituting Equation 15 into Equation 7 gives the same equation as Equation 9, and thus Equation 10 is obtained. By integrating Equation 10 twice with the boundary condition 15, the following can be obtained.

[0125]

Number

[0126] When there is encapsulated PCM When the working fluid flows through the gap, the temperature of the working fluid rises. For the fluid without encapsulated PCM, the above analysis holds true before the encapsulated phase change material reaches its melting point and after it is completely melted. However, during the melting process, a different temperature distribution occurs. This is the focus of this section.

[0127] Studies have shown that in fluid flow through a gap, suspended particles are not uniformly distributed across the short direction [see Koh, C., Hookham, P., & Leal, L. (1994). “An experimental investigation of concentrated suspension flows in a rectangular channel”, Journal of Fluid Mechanics, 266, 1 - 32]. Suspended particles tend to move to an equilibrium position, which is reported to be 0.4 - 0.6 times half of the gap width from the central axis [see Feng, J., Hu, H., & Joseph, D. (1994), “Direct simulation of initial value problems for the motion of solid bodies in a Newtonian fluid. Part 2. Couette and Poiseuille flows, Journal of Fluid Mechanics, 277, 271 - 301. Also see Schonberg, J., & Hinch, E. (1989). Inertial migration of a sphere in Poiseuille flow, Journal of Fluid Mechanics, 203, 517 - 524, and Di Carlo, D., Edd, J.F., Humphry, K.J., Stone, H.A., & Toner, M. (2009), Particle Segregation and Dynamics in Confined Flows. Physical Review Letters, 102, 094503]. The particle concentration is highest at the equilibrium position and decreases away from the equilibrium position but does not reach zero. For simplicity, in this analysis, as shown in Figure 3, it is assumed that all PCM capsules have moved to an equilibrium position where the distance from the side wall is d, and it is also assumed that the PCM capsules form a band with negligible thickness. This is a simple model to roughly demonstrate the effect of the encapsulated phase change material.

[0128] When the phase change material is melting, the temperature within the band of the PCM capsule is maintained at the melting point. On the other hand, in an ideal fully developed flow,

[0129]

Number

[0130] does not change in the y - direction. As a result, at any position

[0131]

Number

[0132] holds, and Equation 6 becomes as follows.

[0133]

Number

[0134] Therefore, the following holds.

[0135]

Number

[0136] where C 1 and C 2 are two constants determined from the boundary conditions. Note that Equation 19 cannot be applied across the entire band of the phase change material. These constants will vary across the band. In reality, the transition from the fourth - order temperature distribution (Equations 11 and 16) to the linear temperature distribution is a smooth transition through the transition region. However, in this theoretical analysis, the transition region was ignored.

[0137] When there is heat transfer from both side walls between the wall and one of the PCM bands, i.e.,

[0138]

Number

[0139] For the fluid between them, the boundary conditions are as follows.

[0140]

Number

[0141] In the formula, T sl is the melting point of the encapsulated phase change material. Calculating, the following is obtained.

[0142]

Number

[0143] From symmetry,

[0144]

Number

[0145] The same result can be obtained for the fluid inside. Equation 21 shows that when the phase change material is melting, the temperature of the wall is higher than the melting point of the phase change material by a certain amount. In this scenario, the temperature of the wall does not rise along the flow direction, which is a great advantage of introducing the phase change material into the working fluid.

[0146] Applying Equation 19 to the fluid between the two PCM bands, if the transition region is ignored, it is shown that the fluid there is at a uniform temperature T sl This uniform temperature distribution indicates that the heat generated in the battery cell is sufficiently absorbed by the phase change material through latent heat. Therefore, the length of the gap segment where the phase change material melts can be calculated.

[0147]

Number

[0148] In the formula, l sf is the length of the gap segment where the phase change material melts, ρ PCM is the density of the phase change material, φ is the volume concentration of the phase change material in the fluid, and h sl is the latent heat of fusion of the phase change material. Note that it is assumed that the phase change material accumulates in two bands. Therefore, it is necessary to use the velocity u at the position of the band instead of the bulk / average velocity.

[0149] When there is heat transfer from only one wall

[0150]

Equation

[0151] When heat is transferred only from the lower wall at [], the temperature distribution between the lower wall and the lower PCM band can be described similarly by Equations 20 and 21, except that the heat transfer coefficient

[0152]

Equation

[0153] needs to be doubled.

[0154]

Equation

[0155] However, in the fluid between the upper wall and the upper PCM band, the temperature is uniform at T sl . The heat from the battery is absorbed only by the phase change material in the lower band, and the upper band does not function to absorb heat. Therefore, l sf needs to be halved.

[0156]

Equation

[0157] Outline of analysis To facilitate quantitative evaluation,

[0158]

Number

[0159] it was assumed that. That is, it was assumed that the band of the encapsulated PCM is located in the middle between the central axis and the side wall. This is consistent with the range of the equilibrium position reported in the literature [see Feng, J., Hu, H., & Joseph, D. (1994), "Direct simulation of initial value problems for the motion of solid bodies in a Newtonian fluid", Part 2, Couette and Poiseuille flows, Journal of Fluid Mechanics, 277, 271 - 301. Also see Schonberg, J., & Hinch, E. (1989), "Inertial migration of a sphere in Poiseuille flow", Journal of Fluid Mechanics, 203, 517 - 524, and Di Carlo, D., Edd, J. F., Humphry, K. J., Stone, H. A., & Toner, M. (2009), "Particle Segregation and Dynamics in Confined Flows", Physical Review Letters, 102, 094503]. Next, the change in the temperature of the wall (battery cell surface) along the flow direction can be summarized in Fig. 4.

[0160] In Fig. 4, for simplicity of illustration

[0161]

Number

[0162] it is assumed that

[0163]

Number

[0164] is the inlet (bulk) fluid temperature. When the working fluid does not contain a phase change material encapsulated therein, the temperature of the wall rises linearly with the longitudinal position, and the slope is the same in both scenarios

[0165]

Number

[0166] is. When the fluid contains phase change material encapsulated therein, there can be up to three segments. Assuming that the influence on the properties of the bulk fluid can be ignored if the PCM capsules are at low concentration, the wall temperature rises as it would in the absence of the phase change material until the phase change material reaches its melting point. If the heat generation rate is sufficiently large and / or the fluid flow rate is sufficiently small, the phase change material reaches its melting point and can keep the wall temperature constant in the second segment. For an even higher heat generation rate and / or an even smaller flow rate, all of the encapsulated phase change material can melt. Thereafter, the wall temperature rises linearly again with the longitudinal position with the same slope as in the first segment, as if there were no phase change material in the fluid. In Figure 4, as is clearly visible, the transitions between the three segments are not gradual. This is due to the assumption of fully developed flow in the analysis. In reality, there should be a relatively small development region, which would make the transitions smooth. As shown in the case study, the step in the small transition is actually very small. Therefore, even if the development region is ignored, it will not practically interfere with the analysis of the temperature distribution. Also, it is valuable to clarify the possible misleading implications resulting from the assumption of fully developed flow. Judging from Figure 4, the constant temperature in the second segment can be higher than the temperature of the PCM-free base fluid at the right end of the second segment. In this case, the phase change material seems to weaken the cooling effect. This can happen in the idealized setting of the analysis but is not possible in reality. This occurs when the second segment is very small in the analysis. In reality, it would be even smaller than the small transition region. The transition region smooths out the small temperature rise and delays the temperature rise downstream. As a result, the actual temperature rise should be smaller than the temperature rise predicted by the analysis, and the PCM should still result in a lower temperature compared to the case of the base fluid only.

[0167] Figure 4 shows the benefits of introducing encapsulated phase change material into the working fluid for direct cooling. The phase change material delays the temperature rise of the battery and enables the battery temperature to be maintained at a certain level within a specific range. By matching the melting point of the phase change material to the optimal operating temperature of the battery, the working fluid can significantly improve efficiency and delay the aging of the battery over time.

[0168] Case Study In this section, the advantages of encapsulated phase change material are demonstrated more directly by incorporating some typical values of the parameters into the analysis.

[0169] For the base fluid, the properties of Shell Thermal Fluid E5 TM 410 (commercially available from Shell) at 40°C were used. All the properties included in this analysis are approximately constant between 20°C and 60°C, which is the normal operating temperature range of the battery cells.

[0170] For the phase change material, the properties of eicosane were adopted. Eicosane has a melting point of approximately 36.2°C, which is close to the optimal rapid charging temperature of many battery cells. For the parameters related to the battery, the typical parameters of a module with 10 - 20 prismatic cells or pouch cells were adopted.

[0171] All the relevant numerical values are shown in Table 1 below.

[0172]

Table 1

[0173] Figure 5 shows the battery cell surface temperature predicted by this analysis. Under both heat boundary conditions, although the encapsulated phase change material has almost melted but not completely melted, there are only two segments in the temperature distribution. In fact, this indicates that the flow rate used in the analysis is well designed. When using a phase change material, it is desirable to almost completely melt the phase change material while preventing the occurrence of a long third segment.

[0174] Under the bilateral boundary condition (the middle figure in Figure 2), the surface temperature of the battery cell is kept constant at more than 40% of the cell surface. At the outlet of the gap, due to the phase change material, the temperature of the cell surface is more than 8 °C lower than that in the case of the base fluid. Under the unilateral boundary condition (the right figure in Figure 2), the battery cell surface temperature is constant at approximately one-third of the cell surface. At the outlet of the gap, due to the phase change material, the temperature of the cell surface is approximately 4 °C lower than that in the case of the base fluid.

[0175] Discussion It has been clearly demonstrated from the case studies of the examples that the encapsulated phase change material can delay the temperature rise of the battery, thereby better preventing overheating. Furthermore, by appropriately selecting the phase change material to have an appropriate melting point, the encapsulated phase change material can maintain most of the battery cell at the optimal operating temperature.

[0176] Examples 1 and 2 Two types of embodiments of the working fluid described in this specification were blended. The blending compositions of these two types of fluids are shown in Table 2.

[0177] [Table 2]

[0178] Both PCM capsules were commercially available at the time of blending. The specific heat of the blended working fluid was measured according to the ASTM E1269 standard, which is generally called the Differential Scanning Calorimeter (DSC) method. Each measurement was performed twice. The results are shown in Figure 6 and Table 3. The specific heat of the working fluid clearly increases near the melting point of the PCM core. The reproducibility of the two tests was not good for unknown reasons, but the increase near the melting point was evident in both measurements. This means that when the working fluid is heated, even if the amount of heat absorbed is the same in that temperature range, the temperature rise becomes smaller. As expected, since the melting point of the PCM in Fluid 2 is higher, the peak of the specific heat of Fluid 2 is located at a higher temperature compared to Fluid 1. By appropriately selecting the PCM material and adjusting the melting point to match the optimal operating temperature of the system, the system can be maintained at the optimal operating temperature for a longer time.

[0179]

Table 3

Claims

1. a housing having an interior space; at least one heat generating component disposed within the interior space; a working fluid disposed within the interior space in direct contact with at least a portion of the heat-generating component; The thermal management system, wherein the working fluid comprises a base fluid and at least one phase change material selected from a microencapsulated phase change material, a nanoencapsulated phase change material, and mixtures thereof.

2. 10. The thermal management system of claim 1, wherein the microencapsulated phase change material and / or the nanoencapsulated phase change material comprises an outer shell and an inner core of a high latent heat material surrounded by the outer shell.

3. 3. The thermal management system of claim 2, wherein the inner core comprises one or more materials selected from paraffin waxes, n-alkanes, fatty acids, fatty alcohols, C4 to C14 alkyl alcohols, fatty acid esters, polyglycols, chlorinated paraffins, inorganic salts, salt hydrates, sugar alcohols, carbohydrates, and polyols, and mixtures thereof.

4. The thermal management system of claim 2 or 3, wherein the outer shell comprises one or more materials selected from polymers, resins, inorganic oxides, multi-walled carbon nanotubes, nanocellulose, and mixtures thereof.

5. The thermal management system of any one of claims 1 to 3, wherein the thermal management system includes a heat exchanger.

6. The thermal management system of claim 5 , constructed such that a circulating flow of working fluid can be generated that passes through the one or more heat-generating components, toward the heat exchanger, and then back to the one or more heat-generating components.

7. A thermal management system according to any one of claims 1 to 3, wherein the base fluid is a hydrocarbon-based base fluid.

8. A thermal management system according to any one of claims 1 to 3, wherein the base fluid is a Fischer-Tropsch derived base fluid.

9. The thermal management system of any one of claims 1 to 3, further comprising a pump, the pump configured to move the working fluid to and from the heat exchanger.

10. The thermal management system of any one of claims 1 to 3, wherein the heat-generating component comprises a server.

11. The thermal management system of any one of claims 1 to 3, wherein the heat-generating component is a battery.

12. The thermal management system of any one of claims 1 to 3, wherein the heat-generating component is one or more of a battery, an electric motor, and an inverter in an electric transport device.

13. 1. A method of thermal management of a heat-generating component, the method comprising: bringing at least a portion of the heat-generating component into direct contact with a working fluid; and using the working fluid to transfer heat away from the heat-generating component, the working fluid comprising a base fluid and at least one encapsulated phase change material selected from a microencapsulated phase change material, a nanoencapsulated phase change material, and mixtures thereof.

14. 14. The method of claim 13, wherein the heat is removed from the heat-generating component using the working fluid in a circulating flow of working fluid that passes through the heat-generating component, to a heat exchanger, and then back to the heat-generating component.

15. 15. The method of claim 13 or 14, wherein the method includes pumping the working fluid through a heat exchanger, transferring heat from the working fluid, and returning the working fluid to the heat-generating component.

16. 15. The method of claim 13 or 14, wherein the base fluid is a hydrocarbon-based base fluid.

17. 15. The method of claim 13 or 14, wherein the base fluid is a Fischer-Tropsch derived base fluid.

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