Thermal management system

Abstract

The present invention provides a thermal management system comprising: a housing having an internal space; the at least one heating component is arranged in the internal space; the working fluid is arranged in the internal space, so that at least one part of the heating component is in direct contact with the working fluid; wherein the working fluid comprises a base fluid and at least one phase change material selected from the group consisting of microencapsulated phase change materials, nanoencapsulated phase change materials, and mixtures thereof. The invention also provides a method for carrying out thermal management on the heating assembly. The method comprises the following steps: enabling at least one part of the heating assembly to be in direct contact with working fluid; and transferring heat away from the heat generating component using the working fluid, wherein the working fluid comprises a base fluid and at least one encapsulated phase change material selected from the group consisting of microencapsulated phase change materials, nanoencapsulated phase change materials, and mixtures thereof.

Description

Technical Field

[0001] The invention relates to a thermal management system and a thermal management method for a heat generating component. Background Art

[0002] Across a range of industries, new technologies are being sought to provide more energy efficient, lower CO2 solutions. The present invention relates to many possible efficiencies that drive lower energy demand and lower CO2 emissions. Firstly, the present invention relates to electric vehicle technology. Secondly, the present invention is also applicable to thermal management of IT equipment (e.g., servers). However, the invention described herein is not inherently limited by the technology to which it can be applied. The present invention can be applied to any heat generating power technology.

[0003] By 2040, electric vehicles are expected to account for 50% of all new passenger vehicle sales. This includes battery electric vehicles (BEV), hybrid electric vehicles (HEV), and plug-in hybrid vehicles (PHEV).

[0004] One of the core components in an electric vehicle is the battery. It requires effective thermal management. Current battery technology relies on lithium-ion batteries, and lithium-ion batteries are likely to remain the dominant battery technology for at least the next 15 years. While slow charging at home or at the destination may be the main way to charge, many customers who want to drive long distances will require high-performance fast charging (HPC) on the go. To improve and shorten the charging process, it is necessary to increase the voltage, current, or both. Higher currents also increase the waste heat generated. The level of waste heat is very high and can reach 20kWh or more. Effective thermal management is required to control the temperature within the battery pack and the uniformity of the temperature to prevent irreversible degradation of the battery cells.

[0005] In electric vehicles, other components also require thermal management, especially cooling. In use, both the electric motor and the inverter generate heat. A thermal management method that can be applied to each of these components and preferably incorporates a circuit with all of these components (including the battery) would be most desirable.

[0006] Another type of electric vehicle is a fuel cell electric vehicle (FCEV). Fuel cell powered electric systems also generate a large amount of waste heat during use (approximately 50% of the energy is generated as waste heat) which needs to be removed during operation.

[0007] Thermal management of components also presents challenges to other industries. Thermal management of IT components, especially servers, also presents many challenges. Air cooling of these components requires high energy usage and expensive cooling infrastructure. A simpler, more energy-efficient system for thermal management of these electronic components would be highly desirable.

[0008] Many cooling systems have historically used air passed over a heat source to manage excess heat. However, due to the relatively small thermal conductivity, heat capacity, and density of air, the cooling effect can be limited. The infrastructure included in air cooling systems can also be complex, expensive, and involve maintenance of many moving components.

[0009] More advanced thermal management systems have been developed in which conventional water / glycol mixtures are used as heat transfer fluids. Battery blocks containing a large number of individual battery cells can be effectively cooled with a water / glycol mixture. This is rapidly becoming the primary thermal management technology used in electric vehicles sold today because it is more efficient than air cooling. US20090023056, US20100025006, and US2011021356 in the name of Tesla Motors Inc. describe a piping system in which approximately 11% of the battery surface is in contact with pipes containing coolant. In these indirect thermal management systems, heat needs to pass through the piping material, and then the heat is transferred to the glycol / water. In addition, this configuration can limit the contact surface area through which heat can be transferred. These factors limit the overall effectiveness of this heat transfer design.

[0010] Under high power charging (HPC) conditions, the battery management system (BMS) in the car will limit the current to protect the battery if these systems cannot effectively control the battery cell temperature and temperature uniformity. This can slow down the charging process very significantly and limit fast charging capabilities. Other systems may simply overheat or undergo safety shutdown under high temperature conditions.

[0011] Direct liquid cooling involving a new thermal management system can help control the temperature of each component more effectively because the fluid is in direct contact with the surface of the component, and the new thermal management system is generally designed to have a heat-generating component in direct contact with the liquid coolant. In many direct cooling systems, the heat-generating component is completely or partially immersed in the cooling fluid. Therefore, direct liquid cooling is also called immersion cooling or immersion cooling. However, not all direct cooling systems are immersion. For example, in some applications, the cooling fluid is sprayed onto the heat-generating component. The term "direct cooling" is used herein, although this term is often used interchangeably with "immersion cooling" or "immersion cooling" in industry. The term "direct cooling" used herein includes immersion direct cooling systems and non-immersion direct cooling systems. Such a direct cooling system is described in US20170279172, for example. In order to prevent short circuits, this requires the fluid to have very good dielectric properties, which means that the fluid must have low conductivity. Suitable fluids also require low viscosity to help pumping, as well as high thermal conductivity and heat capacity. It has been proven that direct thermal management can help increase the power and energy density of batteries, and also significantly enhance battery cell durability. While water / glycol based fluids have very high thermal conductivity and heat capacity, they are not dielectric. Therefore, there is a need to use alternative working fluids in direct thermal management systems.

[0012] Developing improved methods and suitable working fluids for direct thermal management of electrical systems remains an ongoing challenge. Such working fluids require excellent material compatibility, thermodynamic properties and low flammability. For practical purposes, cost and weight also need to be considered. It is important that the fluid has a low conductivity level that can be maintained as the fluid ages to prevent short circuits and / or damage to the heating components. It is also desirable to avoid electrostatic charging of the working fluid during use, such as when pumped at high flow rates.

[0013] Phase change materials (PCMs) are very attractive for thermal control applications. When the phase change is ongoing, a large amount of heat can be removed from the heat-generating component without significantly increasing the temperature due to the latent heat of the PCM. PCMs can be divided into liquid / gas PCMs and solid / liquid PCMs.

[0014] Hydrofluorinated ethers are an example of liquid / gas PCMs and are disclosed in WO2018 / 224908, where hydrofluorinated ethers are used for direct thermal management. Hydrofluorinated ethers reduce flammability risks but may lead to challenges in terms of material compatibility. An attractive feature of this class of fluids is the possibility of boiling. 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 boil during cooling operations and its latent heat can be used for cooling and temperature control. A fundamental problem associated with the application of liquid / gas PCMs is the drastic reduction in thermal conductivity and heat capacity after boiling, resulting in poor heat transfer performance. The possibility of pressurization due to the change from liquid to gas phase is another important issue.

[0015] For solid / liquid PCMs, there are already applications where heat generating components are surrounded by PCM for cooling. The main challenge is to maintain its shape after melting into a liquid. To address this challenge, PCMs have been used with a matrix material to form a composite material. The matrix material will maintain the shape of the PCM after melting. The main problem associated with such systems is the lack of a circulating flow of cooling material. After all available PCM is melted, there is no replenishment mechanism and effective cooling can no longer be maintained. Solid / liquid PCMs have also been mixed with liquid cooling fluids. In this way, a circulating flow can be introduced while the advantages of the PCM can be utilized. An example of this situation is disclosed in US2013 / 0004806, which discloses a microencapsulated phase change material used in combination with a cooling fluid as part of a thermal management system for an automotive battery pack assembly. However, this document only relates to indirect cooling of the battery.

[0016] Many common PCMs are organic materials that can have a lower density than water. On the other hand, cooling fluids used in indirect cooling systems are often water / glycol based. The difference in density can make it challenging to evenly suspend microencapsulated PCMs in the base fluid. They can float on the surface of the fluid.

[0017] Fischer-Tropsch derived base fluids have been shown to be 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 by a working fluid comprising a Fischer-Tropsch derived base fluid. The thermal conductivity and heat capacity of Fischer-Tropsch derived base fluids are generally higher than hydrofluorinated ethers, but still much lower than water / glycol based fluids. At the same time, Fischer-Tropsch derived base fluids do not boil during normal operation. No latent heat is available 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, but in which the cooling fluid provides more efficient cooling to the heat generating components that it is intended to cool. In particular, it is desirable to develop a thermal management system that utilizes direct cooling, but in which the heat capacity of the cooling fluid is increased. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 A simplified circuit for the thermal management system of the present invention is shown.

[0020] Figure 2 is a schematic diagram of thermal analysis of a directly cooled battery module, which is one embodiment of the thermal management system of the present invention.

[0021] Figure 3 Schematic diagram of the equilibrium position of encapsulated phase change material (PCM) in fluid flow through the gap between battery cells or between battery cells and module housing. Figure 2 The thermal analysis shown in is useful.

[0022] Figure 4 Shows the general temperature distribution on the surface of a battery cell resulting from a thermal analysis. Parameters are represented by symbols. No parameter values ​​are integrated in the general results.

[0023] Figure 5 The temperature distribution in a case study of typical materials and a typical battery cell is shown.

[0024] Figure 6 Test results of an exemplary embodiment are shown. Summary of the invention

[0025] The present invention provides a thermal management system, which comprises:

[0026] a housing having an interior space;

[0027] at least one heat generating component, the at least one heat generating component being disposed in the internal space;

[0028] and

[0029] a working fluid disposed in the internal space so that at least a portion of the heat generating component is in direct contact with the working fluid;

[0030] The fluid comprises a base fluid and at least one phase change material selected from microencapsulated phase change materials, nanoencapsulated phase change materials and mixtures thereof.

[0031] The present invention also provides a method for thermal management of a heat-generating component, the method comprising the following steps: bringing at least a portion of the heat-generating component into direct contact with a working fluid; and transferring the heat away from the heat-generating component using the working fluid, wherein the working fluid comprises a base fluid and an encapsulated phase change material selected from microencapsulated phase change materials, nanoencapsulated phase change materials, and mixtures thereof.

[0032] In a preferred embodiment, the base fluid is a Fischer-Tropsch derived base fluid.

[0033] It has been found that the present invention provides an improved direct thermal management system that allows heat generating components such as batteries to be more effectively maintained at their optimal operating temperature. Therefore, the thermal management system of the present invention can effectively improve the efficiency of the battery and extend the battery life.

[0034] In particular, it has been found that the thermal properties, in particular the heat capacity, of a direct cooling fluid, such as a direct cooling fluid comprising a Fischer-Tropsch derived fluid, are enhanced by the addition of said phase change material (PCM).

[0035] Furthermore, it has been discovered that the use of encapsulated phase change materials in the thermal management system of the present invention can slow the temperature rise of the battery and thus better prevent overheating.

[0036] Furthermore, it has been discovered that the use of encapsulated phase change materials in the thermal management system of the present invention can maintain a greater portion of the battery cells at an optimal operating temperature. DETAILED DESCRIPTION

[0037] It has been found that the present invention significantly enhances the effectiveness of direct thermal management systems, where the heat generating components are in direct contact with the working fluid. This has been achieved by introducing encapsulated phase change materials in the working fluid.

[0038] The heat generating component is preferably an electrical component. Typical electrical components that may benefit from the systems and methods described herein include computer servers, batteries, inverters, electric motors, and fuel cells, or any combination of these.

[0039] One or more heat generating components may be cooled within the thermal management system of the present invention.

[0040] The thermal management system of the present invention includes a shell having an internal space; a heat generating component arranged in the internal space; and a working fluid, wherein the working fluid is arranged in the internal space so that at least a portion of the heat generating component is in direct contact with the working fluid. As used herein, the phrase "direct contact" means that the heat generating component is partially immersed in the working fluid, and until it is completely immersed in the working fluid, or a portion of the heat generating component or the entire heat generating component is in contact with the working fluid via a spray or a jet. In a preferred embodiment of the present invention, the heat generating component is partially immersed in the working fluid, and until it is completely immersed in the working fluid. In a preferred embodiment, at least 20%, more preferably at least 60%, and 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.

[0041] The thermal management system is preferably configured so that, when necessary and as controlled by the control system, a circulating flow of the working fluid can be maintained through one or more heat generating components, to the heat exchanger and then back to the heat generating components.

[0042] The heat exchanger may be arranged inside the housing or outside the housing.

[0043] The thermal management system may include a liquid circuit having a pump and a heat exchanger. In this embodiment, the pump operates to move the working fluid from the heat generating component to the heat exchanger and from the heat exchanger. The pump is preferably controlled by a control system so that it produces an appropriate working fluid flow based on instantaneous operating conditions.

[0044] Heat is transferred from the heat generating component to the working fluid. The working fluid can then be pumped from the heat generating component to the heat exchanger. Heat can then be transferred from the working fluid via the heat exchanger. The working fluid can then be returned to the heat generating component.

[0045] In addition to being used to remove heat from a heat generating component disposed therein, the thermal management system may also be adapted to provide heat to the heat generating component at certain times during the operation of the component, such as at startup or during operation in a cold environment. In this embodiment of the invention, a heat source will be included in the thermal management system. Such a heat source may include an internal heat source or an external heat source. A control mechanism will also be included in the thermal management system to allow switching between a cooling embodiment and a heating embodiment of the system.

[0046] Suitable internal heat sources may include batteries with a load to form a heat generating loop. Suitable external heat sources include heat pumps, phase change materials that can release heat when changing phases, electric heaters, and heaters that burn ethanol, bioethanol, or other fuels.

[0047] The working fluid comprises a base fluid and at least one encapsulated phase change material (PCM).

[0048] Encapsulated phase change materials consist of very small bicomponent particles or capsules comprising a high latent heat core material adapted to undergo a phase change within the temperature range typically encountered in heat generating components, encapsulated in an outer shell or capsule typically made of a polymer or related material.

[0049] In order to maintain the fluidity of the base fluid (e.g., Fischer-Tropsch derived base fluid) and prevent agglomeration of the PCM, it has been found useful to encapsulate the PCM at the micron 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, the encapsulated phase material may include a mixture of two or more microencapsulated phase change materials and a mixture of two or more nanoencapsulated phase change materials and a mixture of one or more microencapsulated phase change materials and one or more nanoencapsulated phase change materials.

[0050] The average particle size of the phase change material capsules used herein is generally 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 analytical techniques well known to those skilled in the art, such as SEM and TEM.

[0051] The term "microencapsulation" as used herein generally means a particle size ranging from 1 μm to 1 mm, preferably 1 μm to 200 μm. The term "nanoencapsulation" means a particle size less than 1 μm.

[0052] Macroscopically encapsulated phase change materials (ie, typically having a particle size greater than 1 mm) are not suitable for use herein.

[0053] Preferably, the actual melting point of the phase change material is selected based on the optimal operating temperature of the particular heat generating component. As an example, for cooling of lithium ion batteries, 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.

[0054] Some of the phase change materials in the table below have melting points higher than 60°C, but they can still be used with other phase change materials to form a eutectic mixture of phase change materials, provided that the eutectic mixture has a melting point no higher than 60°C.

[0055] By carefully selecting certain materials for the encapsulation, the desired dielectric properties of the bulk fluid can be maintained. In addition, by matching the melting point of the PCM to the optimal operating temperature of the battery, the battery can be more efficiently maintained at its optimal operating temperature.

[0056] Although both solid-liquid PCM and liquid-gas PCM can be used to form the inner core in theory, solid-liquid PCM is preferred in this article because of their small volume change. Without being limited by theory, it is believed that by using solid-liquid PCM, the pressure and thermal properties associated with liquid-gas phase change can be avoided.

[0057] The inner core preferably comprises one or more materials selected from the group consisting of paraffin 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 above materials.

[0058] Suitable normal alkanes for use herein include normal alkanes having 17 to 27 carbon atoms, including heptadecane, octadecane, nonadecane, eicosane, heneicosane, docosane, tricosane, tetracosane, pentacosane, hexacosane, heptacosane, and mixtures thereof.

[0059] Normal alkanes with higher carbon numbers may also be used in the form of eutectic mixtures with other phase change materials.

[0060] Suitable fatty acids for use herein include capric acid, lauric acid, myristic acid, palmitic acid, stearic acid, arachidic acid, and mixtures thereof.

[0061] Suitable fatty alcohols for use herein include 1-dodecanol, 1-tridecanol, 1-tetradecanol, 1-pentadecanol, 1-hexadecanol, 1-heptadecanol, 1-octadecanol, and mixtures thereof.

[0062] Suitable C4-C14 alkyl alcohols include 2-methyl-2-propanol, 2,2-dimethyl-1-propanol, cyclohexanol, diphenylmethanol, and mixtures thereof.

[0063] Suitable fatty acid esters include butyl stearate, propyl palmitate, vinyl stearate, methyl stearate and mixtures thereof.

[0064] Suitable polyglycols include polyalkylene glycols, such as polyethylene glycol.

[0065] Suitable inorganic salts and salt hydrates include LiNO3·3H2O, Na2SO4·10H2O, Na2CO3·10H2O, NaCH3COO·3H2O, Na2HPO4·12H2O, Na2HPO4·7H2O, Na2S2O3·5H2O, Na2CrO4·10H2O, NaOH·H2O, KF·4H2O, KF·2H2O, K(CH3COO)·1.5H2O, K3PO4·7H2O, CaCl2·6H2O, CaBr2·6H2O, Ca(NO3)2·4H2 O, Zn(NO3)2·6H2O, Zn(NO3)2·4H2O, Zn(NO3)2·2H2O, Cd(NO3)2·4H2O, Mn(NO3)2·6H2O, Fe(NO3)2·6H2O, Ba(OH)2·8H2O, Na2B 4O7·10H2O, Na3PO4·12H2O, Na2P2O7·10H2O, MgCl2·6H2O, Mg(NO3)2·6H2O, Ba(OH)2·8H2O, (NH4)Al(SO4)2·12H2O and their mixtures.

[0066] Suitable sugar alcohols include, for example, erythritol, mannitol, dulcitol, and mixtures thereof.

[0067] A suitable carbohydrate is, for example, ribose.

[0068] A suitable polyol is, for example, pentaerythritol.

[0069] Preferably, the inner core comprises 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). In addition, the melting point of eicosane (about 37°C) makes it ideal for use in electric vehicles and related automotive applications, where typical battery cells may experience accelerated degradation and / or increased risk of thermal runaway when temperatures exceed 40°C. Such paraffin-based PCMs are excellent electrical insulators with a thermal conductivity of 10 13 with 10 17 The high resistivity between the ohmmeters thus promotes safe, reliable electrical operation.

[0070] The shell preferably comprises one or more materials selected from the group consisting of polymers, resins, inorganic oxides, multi-walled carbon nanotubes, nanocellulose, and mixtures thereof.

[0071] Suitable polymers for the shell include, for example, polymethyl methacrylate (PMMA), polystyrene, polyurea, polyurethane, polyethylene, polysiloxane, gelatin and copolymers thereof, and mixtures thereof. The polymer may be cross-linked or non-cross-linked.

[0072] Suitable resins for the shell include, for example, melamine-formaldehyde resins, urea-formaldehyde resins and epoxy resins, and mixtures thereof. The resins may be cross-linked or non-cross-linked.

[0073] Inorganic oxides suitable for use in the outer shell include, for example, SiO2, boehmite, CaCO3, TiO2, ZrO2, and mixtures thereof.

[0074] Preferably, the housing is a polymer material.

[0075] Commercially available examples of encapsulated phase change materials suitable for use herein include those available from Microtek under the trade names Nextek (RTM), Vivtek (RTM), Apaptek (RTM), Fibratek (RTM), Micronal (RTM), MPCM, and PCMBlend, and those available from Croda under the trade name CrodaTherm (RTM).

[0076] The encapsulated phase change material is preferably present in the working fluid in a total content of 0.5 to 35 wt %, more preferably 5 to 20 wt %, even more preferably 5 to 15 wt %, and especially 8 to 12 wt % based on the weight of the working fluid.

[0077] More than one encapsulated phase change material may 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, while a second encapsulated phase change material configured to exhibit a phase change at a second (e.g., higher) temperature may be included. In the case of using more than one encapsulated phase change material, the above content relates to the total content of the encapsulated phase change material present in the working fluid.

[0078] The second essential component of the working fluid is the base fluid. The base fluid is preferably a hydrocarbon-based base fluid, which may be petroleum-based, bio-based or synthetic. Preferably, the base fluid is present in the working fluid at a level of 40.0 to 99.4 wt%, more preferably 60 to 90 wt%, and more preferably 70 to 85 wt%, based on the weight of the working fluid.

[0079] The preferred base fluid for use herein is a Fischer-Tropsch derived base fluid. It has been found that the combination of a Fischer-Tropsch derived base fluid and a phase change material is particularly useful in the present invention from the perspective of achieving better cooling and thermal control of heat generating components, especially compared to using a working fluid based on a Fischer-Tropsch derived base fluid but without a phase change material. The latent heat of the phase change material can absorb additional heat, thereby achieving better cooling and thermal control.

[0080] Fischer-Tropsch derived base fluids are known in the art.The term "Fischer-Tropsch derived" means that the base fluid is, or is derived from, a synthesis product of a Fischer-Tropsch process.

[0081] Fischer-Tropsch derived base fluids are generally classified according to the starting material in the Fischer-Tropsch process, i.e. "X to Liquids" or "XTL", where X represents the starting material. Biomass to Liquids (BTL), Coal to Liquids (CTL), Gas to Liquids (GTL) and Power to Liquids (PTL) processes are some examples of Fischer-Tropsch processes for producing base fluids. Preferably, the Fischer-Tropsch derived base fluid is a GTL (gas to liquids) base fluid.

[0082] Suitable Fischer-Tropsch derived base fluids (including oils) that may conveniently be used in the working fluid are, for example, those disclosed in EP0776959, EP0668342, WO97021788, WO0015736, WO0014188, WO0014187, WO0014183, WO0014179, WO0008115, WO9941332, EP1029029, WO0118156 and WO0157166.

[0083] Particularly preferred Fischer-Tropsch derived base fluids for use as working fluids herein are those having a kinematic viscosity at 100°C of at most 4 mm 2 / s, especially in 2-4mm 2 Fischer-Tropsch derived base oils in the range of 1.2 to 1.5 g / s, such as GTL 3 (kinematic viscosity at 100°C is about 3 mm 2 / s).

[0084] Another particularly preferred Fischer-Tropsch derived base fluid for use as a working fluid herein is a Fischer-Tropsch derived base fluid produced from a gas oil stream, preferably a dewaxed gas oil stream, from a GTL process, wherein the fluid has a 2 / s, preferably 2.0-11mm 2 Preferably, the Fischer-Tropsch derived base fluid produced from the gas oil stream has a kinematic viscosity of at least 2.1 mm 2 / second, more preferably at least 2.2 mm 2 / sec kinematic viscosity at 40°C. Also preferably, the kinematic viscosity of the fluid at 40°C is at most 10.0 mm 2 / second, more preferably at most 7.0 mm 2 / second, most preferably at most 6.0 mm 2 / Second.

[0085] In one embodiment of the invention, the working fluid comprises a mixture of more than one Fischer-Tropsch derived base fluid. For example, the working fluid may comprise a base fluid having a kinematic viscosity of 2-4 mm at 100°C. 2 / s of Fischer-Tropsch derived base oils and Fischer-Tropsch derived base oils produced from gas oil streams.

[0086] It has been found that mixing encapsulated PCMs in GTL or organic base fluids is more practical than mixing with water / glycol base fluids because many typical PCMs have similar densities to GTL or organic base fluids. By doing so, the encapsulated PCMs may be more suitable for direct cooling than indirect cooling.

[0087] Other components in the working fluid may include one or more additional base oils, including mineral oils and synthetic oils. Mineral oils include liquid petroleum and solvent-treated or acid-treated paraffin, cycloparaffin or mixed paraffin / cycloparaffin type mineral lubricating oils, which may be further refined by hydrofining process and / or dewaxing. Synthetic oils include hydrocarbon oils, such as olefin oligomers (including polyalphaolefin base oils; PAO), dibasic acid esters, polyol esters, polyalkylene glycols (PAG), alkylbenzenes, alkylnaphthalenes and dewaxed waxy isomers.

[0088] In a preferred embodiment, the working fluid comprises one or more additional base oils selected from alkylbenzenes, alkylnaphthalenes and mixtures thereof. If present, the one or more additional base oils selected from alkylbenzenes, alkylnaphthalenes and mixtures thereof are present in an amount not exceeding 35 wt % relative to 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 % relative to the total weight of the working fluid.

[0089] The working fluid preferably has a pour point measured according to ISO 3016 of less than or equal to -40°C, more preferably less than or equal to -50°C.

[0090] The working fluid also preferably has a flash point according to ASTM D93 of at least 100°C, more preferably at least 110°C, most preferably at least 120°C, and preferably at most 240°C.

[0091] The thermal conductivity of the working fluid at 20°C is preferably at least 0.135 w / mK as measured according to ASTM D7896.

[0092] 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.

[0093] Preferably, the working fluid further comprises an antioxidant additive and an antistatic additive.

[0094] The antioxidant additive is suitably a hindered phenol antioxidant additive, sterically hindered mono-, di- and tri-phenols, sterically hindered di-, tri- and poly-nuclear phenols. Optionally, additional amine antioxidants may be added to the working fluid, such as alkylated or styrenated diphenylamines.

[0095] The total amount of one or more antioxidant additives present in the working fluid is preferably at least 0.1 wt%, more preferably at least 0.15 wt% and preferably at most 3.0 wt%, more preferably at most 2.0 wt%, relative to the total weight of the working fluid.

[0096] The antistatic additives for use herein are preferably selected from those containing alkyl-substituted naphthalenesulfonic acid, benzotriazole and substituted benzotriazole.

[0097] The content of antistatic additive is preferably higher than 0,5 mg / kg, more preferably higher than 1 mg / kg, relative to the total weight of the working fluid. The practical upper limit may vary depending on the specific application of the lubricating composition. This concentration may be up to 3 wt%, however it is preferably in the range of 1 mg / kg to 1 wt% relative to the total weight of the working fluid. However, such compounds may advantageously be used in a concentration lower than 1000 mg / kg and more preferably lower than 300 mg / kg, relative to the total weight of the working fluid.

[0098] Detailed description of the drawings

[0099] Figure 1 One embodiment of a suitable liquid circuit for use in the thermal management system of the present invention is shown. Figure 1 In the embodiment, the heat generating assembly (1) is arranged in a housing (2). The working fluid (3) flows through the inner space of the housing (1) and then flows to one or more heat exchangers (5) through a connecting pipe (4). In this embodiment, the flow is maintained by one or more pumps (6).

[0100] Figure 2 is a schematic diagram of a thermal analysis of a directly cooled battery module, which is one embodiment of the system described in the present invention.

[0101] Figure 3 Schematic diagram of the equilibrium position of encapsulated phase change material (PCM) in fluid flow through the gap between battery cells or between battery cells and module housing. Figure 2 The thermal analysis shown in is useful.

[0102] Figure 4 Shows the general temperature distribution on the surface of a battery cell obtained from a thermal analysis. Parameters are represented by symbols. No parameter values ​​are integrated in the general results.

[0103] Figure 5 The temperature distribution in a case study of typical materials and a typical battery cell is shown.

[0104] Figure 6 is a graph showing the results of measurements of specific heat over the temperature range of 10°C to 80°C for two embodiments of the working fluids described herein.

[0105] The invention will now be further illustrated by the following non-limiting examples.

[0106] Example

[0107] A theoretical analysis was performed to demonstrate the benefits of using encapsulated phase change materials in the working fluid for battery thermal management applications. A one-dimensional convection model was built to theoretically analyze the temperature distribution on the surface of a directly cooled battery cell. By comparing the temperature distribution of a base fluid and a fluid mixed with an encapsulated phase change material, it can be shown that the latter can produce better cooling and more precise thermal management.

[0108] like Figure 2 As shown, convective heat transfer in a battery module composed of prismatic cells or pouch cells is used to analyze fluid flow between cells and on the sides of cells. Figure 2 In the left figure, the horizontal gaps shaded in light grey are examples of fluid flow between battery cells; the vertical gaps shaded in dark grey are examples of fluid flow on the sides of battery cells. One-dimensional convection models of the two flows are depicted in the middle and right figures, respectively.

[0109] In the analysis, and see Figure 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 zero located at the central axis. The volume flow rate of the coolant (per unit length perpendicular to the page) is Q, and the heat transfer rate per unit area into the fluid is Between battery cells, is the heat generation rate per unit lateral surface area of ​​a battery cell, half of which enters the fluid from each side of the gap, such as Figure 2 On the side of the battery cell, is the total heat generated by all covered battery cells, and the heat comes from only one side of the gap.

[0110] Typically, the width w of the gap is in the order of millimeters, while the size of the cell side surface is in 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. In addition, the width w is typically much smaller than the length of the gap. Therefore, for simplicity, the development region (entry region) is ignored. A one-dimensional fully developed flow in the gap is analyzed.

[0111] Flow Field

[0112] The governing equation for flow velocity in a one-dimensional fully developed flow is:

[0113]

[0114] Where p is the pressure, μ is the fluid viscosity, and u is the flow velocity in the x-direction. No-slip boundary conditions apply to both side walls:

[0115]

[0116] In fully developed flow, the pressure gradient dp / dx is independent of y. Therefore, Equation 1 can be integrated twice in the y direction to obtain the velocity profile:

[0117]

[0118] The volume flow rate (per unit length perpendicular to the page) Q can be obtained by integrating Equation 3:

[0119]

[0120] By substituting Equation 4 into Equation 3, the velocity profile can be expressed in terms of flow rate rather than pressure gradient:

[0121]

[0122] The velocity distribution does not vary with the thermal boundary conditions. That is, Equation 5 applies to Figure 2 In addition, regarding the working fluid with encapsulated phase change material, it is assumed that the capsules are small enough, the concentration of the capsules is small enough, and the capsules are uniformly suspended in the working fluid. Under these assumptions, most of the fluid is still considered as a homogeneous Newtonian fluid, and Equation 5 adequately simulates the flow field. Nevertheless, Equation 5 is applicable to all scenarios to be analyzed in these examples.

[0123] Temperature distribution

[0124] The governing equation for temperature distribution in a one-dimensional fully developed flow is:

[0125]

[0126] Where T is the temperature, α is the thermal diffusivity of the fluid, and u is the flow velocity in the x direction, as described in Equation 5. While the governing equations apply to all cases analyzed, the boundary conditions vary with the thermal boundary conditions and the fluid.

[0127] PCM without encapsulation

[0128] For a working fluid without PCM capsules, Equation 6 applies to the entire gap. In fully developed flow, dT / dx does not depend on y. Integrating Equation 6 across the gap, we find:

[0129]

[0130] From here on, the thermal boundary conditions will come into play.

[0131] Heat transfer from both side walls

[0132] This applies to the gaps between battery cells, such as Figure 2 The heat transfer rate to the incoming fluid on the sidewall should match the heat transfer from the battery cell. Therefore:

[0133]

[0134] where k is the thermal conductivity of the fluid. Substituting Equation 8 into Equation 7 yields:

[0135]

[0136] where ρ is the fluid density, and c p is the specific heat of the fluid.

[0137] Now, Equations 5 and 9 can be substituted into Equation 6 to obtain the governing equation for T:

[0138]

[0139] By integrating Equation 10 twice with boundary condition 7, the temperature distribution can be obtained:

[0140]

[0141] Where T w is the temperature on the surface of the battery cell, that is:

[0142]

[0143] T wis the ultimate goal of the current analysis. It indicates the effectiveness of thermal control of the battery cell given the heat generation under the specific operating conditions of the battery cell. In order to more easily compare the battery cell surface temperature and the fluid temperature that varies across the gap, the overall (average) temperature of the fluid T is defined b :

[0144]

[0145] After some algebra:

[0146]

[0147] So far, it has been shown that the battery cell surface temperature T w It increases along the flow direction at the slope described in Equation 9 and is higher than the bulk fluid temperature by the amount described in Equation 14.

[0148] Heat transfer from one wall only

[0149] This applies to gaps on the sides of the battery, such as in Figure 2 In this analysis, it is assumed that the other wall is insulated so that all heat generated from the battery cell is carried away by convection of the working fluid. That is, there is no mechanism for heat transfer through the module's housing. Under this assumption, the thermal boundary conditions are:

[0150]

[0151] Substituting equation 15 into equation 7 yields the same equation as equation 9, and thus equation 10. By integrating equation 10 twice with boundary condition 15, we obtain:

[0152]

[0153] as well as

[0154]

[0155] PCM with Encapsulation

[0156] As the working fluid flows through the gap, its temperature increases. The previous analysis of the fluid without the encapsulated PCM still holds true, before the encapsulated phase change material reaches its melting point and after it is completely melted. However, the temperature distribution during the melting process is different. This is the focus of this section.

[0157] Studies have shown that in a fluid flowing through a gap, suspended particles are not uniformly distributed in the lateral direction [see Koh, C., Hookham, P. and Leal, L. (1994). 'An experimental investigation of concentrated suspension flows in a rectangular channel', Journal of Fluid Mechanics, 266, 1-32.]. They tend to move to an equilibrium position, which is reported to be between 0.4 and 0.6 times the half-gap width from the central axis [see Feng, J., Hu, H. and 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. See also Schonberg, J. and Hinch, E. (1989). Inertial migration of a sphere in Poiseuilleflow, Journal of Fluid Mechanics, 203, 517-524 and Di Carlo, D., Edd, JF, Humphry, KJ, Stone, HA and Toner, M. (2009), Particle Segregation and Dynamics in Confined Flows. Physical Review Letters, 102, 094503]. The particle concentration peaks at the equilibrium position and decays away from the equilibrium position, although it is not zero. For simplicity, in this current analysis, it is assumed that all PCM capsules move to the equilibrium position, the equilibrium position is a distance d from the side wall, and the PCM capsules form a band of negligible thickness, such as Figure 3 This is a simple model that generally demonstrates the effect of encapsulated phase change materials.

[0158] As the phase change material melts, the temperature in the band of PCM capsules remains at the melting point. At the same time, in an ideal fully developed flow, dT / dx does not change in the y direction. As a result, dT / dx = 0 in all cases and Equation 6 becomes:

[0159]

[0160] And therefore:

[0161] T=C1y+C2 19

[0162] Where C1 and C2 are two constants determined by the boundary conditions. Note that Equation 19 cannot be applied across the phase change material band. The constants will vary across the band. In practice, the transition from the fourth-order temperature distribution (Equations 11 and 16) to the linear temperature distribution is gradual in the transition region. However, in this theoretical analysis, the transition region is ignored.

[0163] Heat transfer from both side walls

[0164] For the space between the wall and one of the PCM strips (e.g. The boundary conditions are:

[0165]

[0166] Where T sl is the melting point of the encapsulated phase change material. After some algebra:

[0167]

[0168] Due to symmetry, for

[0169] The same results can be obtained for the fluid in the working fluid. Equation 21 shows that when the phase change material melts, the wall temperature is a certain amount higher than the melting point of the phase change material. In this case, the wall temperature does not increase in the direction of flow, which is the main benefit of introducing phase change materials into the working fluid.

[0170] Applying Eq. 19 to the fluid between the two PCM zones shows that, assuming the transition region is neglected, the fluid there is at a uniform temperature T sl The uniform temperature distribution indicates that the heat generated by the battery cell is completely 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.

[0171]

[0172] Among them 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 melting of the phase change material. Note that the phase change material is assumed to accumulate in two bands. Therefore, the velocity u at the band location should be used instead of the volume / average velocity.

[0173] Heat transfer from one wall only

[0174] When heat is only transferred from the lower wall During the heat transfer, the temperature distribution between the lower wall and the lower PCM belt can still be described by Eqs. 20 and 21, except for the heat transfer rate Need to double.

[0175]

[0176] However, in the fluid between the upper wall and the upper PCM zone, the temperature is T sl The heat from the battery is only absorbed by the phase change material in the lower band, while the upper band has no function of absorbing heat. sf Needs to be halved.

[0177]

[0178] Summary of analysis

[0179] In order to facilitate quantitative evaluation, it is assumed That is, the band of encapsulated PCM is located midway between the central axis and the sidewalls. This is consistent with the range of equilibrium positions reported in the literature [see Feng, J., Hu, H. and Joseph, D. (1994), "Direct simulation of initial value problems for the motion of solidbodies in a Newtonian fluid", Part 2, Couette and Poiseuille flows, Journal of Fluid Mechanics, 277, 271-301. See also Schonberg, J. and Hinch, E. (1989), "Inertial migration of a sphere in Poiseuille flow", Journal of Fluid Mechanics, 203, 517-524 and Di Carlo, D., Edd, JF, Humphry, KJ, Stone, HA and Toner, M. (2009), "Particle Segregation and Dynamics in Confined Flows", Physical Review Letters, 102, 094503]. Then, the variation of the wall (cell surface) temperature along the flow direction can be summarized as Figure 4 middle.

[0180] exist Figure 4 middle, For a simple demonstration,

[0181] T b,in =T b (x=0) is the inlet (bulk) fluid temperature.

[0182] Without encapsulated PCM in the working fluid, the wall temperature increases linearly with longitudinal position, with a slope of

[0183] With encapsulated phase change material in the fluid, there can be up to 3 segments. Before the phase change material reaches its melting point, the wall temperature rises, just as if there was no phase change material, assuming that the diluted PCM capsules have a negligible effect on the bulk fluid properties. If the heat generation rate is large enough, and / or if the fluid flow rate is small enough, the phase change material can reach its melting point and keep the wall temperature constant in the second segment. At an even more sufficient heat generation rate and / or a smaller enough flow rate, all of the encapsulated phase change material can be melted. Thereafter, the wall temperature will again rise linearly with longitudinal position at the same slope as in the first segment, just as if there was no phase change material in the fluid. It can be noted that in Figure 4 In the example, 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 short development region that smooths the transitions. As will be shown in the case study, the steps in the subtle transitions are actually very small. Therefore, neglecting the development region does not effectively hinder the analysis of the temperature distribution. It is also worth clarifying the misleading indications caused by the assumption of fully developed flow. According to Figure 4 Judging from the above, the constant temperature in the second segment may be higher than the temperature of the base fluid without PCM at the right end of the second segment. In this case, the phase change material appears to mitigate the cooling effect. While this is possible under the idealized settings in the analysis, this is not possible in practice. In the analysis, this happens when the second segment is very short. In practice, it is even shorter than the short transition region. The transition region will have a smooth, slight temperature increase and a delayed temperature increase towards the downstream. As a result, the actual temperature increase should be lower than predicted by the analysis, and the PCM will still produce lower temperatures compared to the pure base fluid.

[0184] Figure 4 The benefits of introducing encapsulated phase change materials into the working fluid for direct cooling are demonstrated. The phase change material can slow down the temperature rise in the battery and maintain the battery temperature at a constant level within a certain area. By matching the melting point of the phase change material with the optimal operating temperature of the battery, the working fluid can significantly improve efficiency and slow down the aging of the battery.

[0185] Case Studies

[0186] In this section, some typical values ​​of the parameters are inserted into the analysis to demonstrate the benefits of the encapsulated phase change material in a more direct way.

[0187] For the base fluid, the properties of Shell Thermal Fluid E5™ 410 (commercially available from Shell) at 40° C. were used. All properties involved in this analysis are nearly constant between 20° C. and 60° C., which is the typical operating temperature range of a battery cell.

[0188] For the phase change material, the properties of eicosane are used. The melting point of eicosane is about 36.2°C, which is close to the optimal fast charging temperature of many battery cells.

[0189] For the battery-related parameters, typical parameters for a module with 10 to 20 prismatic cells or pouch cells were used.

[0190] All relevant values ​​are shown in Table 1 below.

[0191] Table 1

[0192]

[0193]

[0194] Figure 5 The cell surface temperatures predicted by this analysis are shown. Under both thermal boundary conditions, the encapsulated phase change material is not completely melted, although it is almost melted, resulting in only two segments in the temperature distribution. In fact, this shows that the flow design used in the analysis is good. For the phase change material, (almost) complete melting of the phase change material is expected, but without the generation of a long third segment.

[0195] Under the double-sided boundary conditions ( Figure 2 The cell surface temperature remains constant over more than 40% of the cell surface. At the exit of the gap, the phase change material reduces the temperature on the cell surface by more than 8°C compared to the base fluid. Under the one-sided boundary condition ( Figure 2 The cell surface temperature is constant in almost one third of the cell surface. At the exit of the gap, the phase change material reduces the cell surface temperature by almost 4°C compared to the base fluid.

[0196] discuss

[0197] The case studies of the embodiments clearly show that encapsulated phase change materials can delay the temperature rise of the battery and thus better prevent overheating. In addition, if the phase change material is properly selected to withstand the appropriate melting point, the encapsulated phase change material can maintain a larger portion of the battery cell at the optimal operating temperature.

[0198] Examples 1 and 2

[0199] The two embodiments of the working fluids described herein were mixed. The formulations of the two fluids are shown in Table 2.

[0200] Table 2

[0201]

[0202] Both PCM capsules were commercially available at the time of blending. The specific heat of the blended working fluids was measured according to the ASTM E1269 standard, or more commonly known as 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 increases significantly near the melting point of the PCM core. Although the repeatability of the two tests was poor for unknown reasons, the increase near the melting point was evident in both measurements. This means that the working fluid will be heated with a smaller temperature rise when absorbing the same amount of heat in the temperature range. As expected, the specific heat of fluid 2 peaks at a higher temperature than fluid 1 because the melting point of the PCM in fluid 2 is higher. By properly selecting the PCM material and matching the melting point to the optimal operating temperature of the system, the system can be maintained at the optimal operating temperature for a longer period of time.

[0203] Table 3

[0204]

Claims

1. A thermal management system, comprising: a housing having an inner space; at least one heat generating component, wherein the at least one heat generating component is disposed in the internal space; and a working fluid, the working fluid being disposed in the internal space so that at least a portion of the heat generating component is in direct contact with the working fluid; The working fluid comprises a base fluid and at least one phase change material selected from microencapsulated phase change materials, nanoencapsulated phase change materials and mixtures thereof.

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

3. The thermal management system of claim 2, wherein the core comprises one or more materials selected from the group consisting of paraffin wax, normal 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 mixtures thereof.

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

5. The thermal management system according to any one of claims 1 to 4, wherein the thermal management system comprises a heat exchanger.

6. The thermal management system of claim 5, configured to enable generation of a circulating flow of a working fluid that passes through the one or more heat generating components, reaches the heat exchanger and then returns to the one or more heat generating components.

7. The thermal management system of any one of claims 1 to 6, wherein the base fluid is a hydrocarbon based base fluid.

8. A thermal management system according to any one of claims 1 to 7, wherein the base fluid is a Fischer-Tropsch derived base fluid.

9. The thermal management system of any one of claims 1 to 8, wherein the thermal management system further comprises a pump, wherein the pump is configured to move the working fluid to and from the heat exchanger.

10. The thermal management system of any one of claims 1 to 9, wherein the heat generating component comprises a server.

11. The thermal management system of any one of claims 1 to 10, wherein the heat generating component is a battery.

12. The thermal management system according to any one of claims 1 to 11, wherein the heat generating component is one or more of: a battery, a motor and an inverter in an electric vehicle.

13. A method for thermal management of a heat generating component, the method comprising the following steps: Placing at least a portion of the heat generating component in direct contact with a working fluid; and transferring heat away from the heat generating component using the working fluid, wherein the working fluid comprises a base fluid and at least one encapsulated phase change material selected from microencapsulated phase change materials, nanoencapsulated phase change materials, and mixtures thereof.

14. The method of claim 13, wherein the working fluid is used to transfer heat away from the heat generating component 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. The method according to claim 13 or 14, wherein the method comprises the following steps: The working fluid is pumped to a heat exchanger; heat is transferred from the working fluid; and the working fluid is returned to the heat generating component.

16. The method of any one of claims 13 to 15, wherein the base fluid is a hydrocarbon based base fluid.

17. A method according to any one of claims 13 to 16, wherein the base fluid is a Fischer-Tropsch derived base fluid.

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