Heat transfer fluids

Abstract

Electrical apparatus comprises at least one electrical component in thermal contact with a heat transfer fluid, comprising by weight percent of the fluid >95% at least one ester of a C5-C9 monocarboxylic acid and a C5-C9 monoalcohol having a carbon number of less than 17.

Description

[0001] Heat transfer fluidsThis invention relates to heat transfer fluids for use in transferring heat from one location toanother, to electrical apparatus comprising such heat fluids, and to methods of using such heattransfer fluids in electrical apparatus.The invention is particularly, although not exclusively, applicable to heat transfer fluids for usein environments where exposure to electricity may occur, and where low temperatures mayexist.The invention is illustrated in the following by reference to batteries, but the applicability of theinvention is wider. For example, heat transfer fluids are used in transformers.“Lithium plating” is a problem that can arise during the charging of lithium ion batteries.Lithium plating is the deposition of lithium on the anode surface, rather than the desiredintercalation of lithium into the anode material. This occurs when lithium is deposited morerapidly than it can be intercalated into the anode, and so is dependent both on charging rate andthe kinetics of intercalation of lithium into the anode material.Lithium plating is exacerbated by operating batteries at low temperatures, as this results inreduced ion diffusion and electrolyte conductivity, increasing resistance, and slower kinetics ofintercalation of lithium into the anode material. Lithium plating can lead to reduced batterycapacity and, in the extreme, short circuiting.Due to the increased internal resistance of cells at sub-zero temperatures and a slowing of theelectro-chemical reactions it is therefore necessary to limit input current during batterycharging. This in turn can lead to very long charging times for battery systems where the celltemperature is below 0°C.To counteract this, it is beneficial to pre-heat battery cells during periods of cold ambienttemperature to reach a more optimal temperature and to achieve a suitable fast charge rate.Using an immersion cooled system provides an excellent thermal transfer between the pre-heating equipment and the battery cells. However the viscosity of some traditional thermaltransfer liquids at sub-zero temperatures impedes the heat transfer due to excessive pressuredrops in the system and a slowing of the liquid flow compared with higher temperatures. Extraenergy is also required from pumping systems to circulate liquid at the higher viscosityexperienced at low temperatures, and the pumps may need to be oversized to compensate.A variety of heat transfer fluids are known and include for example hydrocarbons, fluorinatedhydrocarbons, esters, and water / glycol mixtures.Water / glycol mixtures need to be kept separate from operating electrical components, adding athermal barrier between components and heat transfer fluid. Such water glycol mixtures mayachieve a kinematic viscosity in the region of 100mm2 / s at -40°C (see https: / / detector-cooling.web.cern.ch / data / Table%208-3-1.htm).Hydrocarbons and fluorinated hydrocarbons do not have a good reputation forbiodegradability.Esters are known to be biodegradable to varying degrees and to provide good dielectricproperties. The esters that have been suggested to date for heat transfer fluids include diesters,but their viscosity at e.g. -40°C (~233°K) is too high to provide comparable performance towater / glycol mixtures or fluorinated hydrocarbons.US2012 / 283162 A1 proposed the use of oleyl esters having 23 or more of a total number of aterminal methyl group, a methylene group and an ether group in a main chain, as a base oil forcooling electric motors.US2022 / 0131205 A1 proposes the use of esters in a cooling composition for cooling, amongother things, the battery and / or power electronics of an electric or hybrid vehicle. The onlymonoester exemplified is the highly branched 3,5,5-trimethylhexyl 3,5,5-trimethylhexanoatehaving 18 carbons and disclosed as having a viscosity at -25°C of 55.8 mm2 / s.WO2010 / 116234 discloses a liquid heat exchange medium comprising 90 or more volumepercent of 2-ethylhexyl caprylate (2-ethylhexyl octanoate) and permits presence of other estersof C6-C8 fatty acids with 2 ethyl hexanol.The heat transfer fluids used in the present invention provide heat transfer propertiescomparable to fluorinated hydrocarbons, viscosity much lower than diesters and commonlyused hydrocarbons, and are biodegradable. Further the source materials for the estersdescribed herein are preferably obtainable from biological sources providing environmentalbenefits to their manufacture and use.Accordingly, the present invention provides electrical apparatus comprising at least oneelectrical component in thermal contact with a heat transfer fluid, wherein the heat transferfluid comprises by weight percent of the fluid >95% at least one ester of a C5-C9monocarboxylic acid and a C5-C9 monoalcohol, the at least one ester having a carbon number ofless than 17 the heat transfer fluid not comprising more than 80% by weight 2-ethylhexyloctanoate.Optionally the at least one ester may comprise less than 60% by weight 2-ethylhexyl octanoate,less than 40% by weight 2-ethylhexyl octanoate, less than 20% by weight 2-ethylhexyloctanoate, less than 10% by weight 2-ethylhexyl octanoate, or less than 1% by weight 2-ethylhexyl octanoate. The at least one ester may be essentially free of 2-ethylhexyl octanoate.The C5-C9 monocarboxylic acid is optionally an acyclic monocarboxylic acid, for example alinear monocarboxylic acid or a branched monocarboxylic acid.C5-C9 monocarboxylic acid may, as non-limitative examples, include any of pentanoic acid,hexanoic acid, heptanoic acid, octanoic acid, nonanoic acid, 2-ethylhexanoic acid, or3,5,5-trimethylhexanoic acid.The C5-C9 monoalcohol is optionally an acyclic monoalcohol, for example a linear monoalcoholor a branched monoalcohol and may be a primary, secondary, or tertiary alcohol. The C5-C9monoalcohol when branched may comprise only methyl branches.C5-C9 monoalcohol may, as non-limitative examples, include any of 2-pentanol, 3-pentanol, 2-hexanol, 3-hexanol, 2-heptanol, 3-heptanol, 4-heptanol, 2-octanol, 3-octanol, 4-octanol, 2-nonanol, 3-nonanol, 4-nonanol, 5-nonanol, isononanol, 7-methyloctan-1-ol, 2-ethylhexanol, or3,5,5-Trimethyl-1-hexanol.The electrical apparatus may comprise a heat source in thermal contact with the heat transferfluid to permit heat to be transferred by the heat transfer fluid from the heat source to the atleast one electrical component.The electrical apparatus may comprise a heat sink in thermal contact with the heat transferfluid to permit heat to be transferred by the heat transfer fluid from the at least one electricalcomponent to the heat sink.The at least one electrical component may be immersed in the heat transfer fluid.The apparatus may be configured to supply heat to the at least one electrical component whenthe at least one electrical component is below a first temperature, and to remove heat from theat least one electrical component when the at least one electrical component is above a secondtemperature.The at least one electrical component may comprise a battery, which may be a lithium ionbattery, a lithium metal battery, or any other battery.The heat transfer fluid in the apparatus may comprise by weight percent of the fluid >95% oneester of a C5-C9 monocarboxylic acid and a C5-C9 monoalcohol, and that one ester may be theonly ester present. Some of or each at least one ester in the heat transfer fluid optionally has acarbon number of 16 or less or 15 or less. Some of or each at least one ester in the heat transferfluid optionally has a carbon number of 11 or more, or 12 or more, or 13 or more or 14 or more.The heat transfer fluid in the apparatus may comprise one or more components selected fromthe group anti-oxidants, metal deactivators, friction modifiers, corrosion inhibitors, antifoamadditives, detergents, extreme pressure additives, anti-wear additives, and thermallyconductive particles.A method of operating the electrical apparatus comprises transferring heat from the heattransfer fluid to the at least one electrical component, and operating the electrical componentonce a threshold temperature is exceeded.Specific heat transfer fluids are claimed comprising by weight percent:->95% at least one ester of a C5-C9 monocarboxylic acid and a C5-C9 monoalcohol, the atleast one ester having a carbon number of less than 17and at least one functional additive selected from the group anti-oxidants, metal deactivators,friction modifiers, corrosion inhibitors, antifoam additives, detergents, extreme pressureadditives, and anti-wear additives.The functional additives may comprise by weight percent of the fluid:- ≥0.1% anti-oxidant ≥0.001% metal deactivatorand optionally the functional additives may comprise by weight percent of the fluid:-0.1-1.0% anti-oxidant0.001-0.05% metal deactivatorFurther features of the invention will be apparent from the appended claims and the followingdescription exemplifying, but not limiting, the scope of the invention claimed. Reference is madeto Fig. 1 which shows schematically apparatus showing aspects of the invention as claimed.In Fig. 1 apparatus 1 houses several electrical components 2 (batteries for example) that areimmersed in and so in thermal contact with heat transfer fluid 3 that fills the apparatus. Theapparatus comprises heat source 4 and heat sink 5.Heat source 4 can provide heat to the heat transfer fluid 3, and thereby to the electricalcomponents 2. The heat source can be any convenient source, for example an electrical heater,or a heat exchanger with a heating circuit carrying the same or different heat transfer fluid.Heat sink 5 may absorb heat from the heat transfer fluid 3 to remove the heat from theapparatus. Heat may be removed in any convenient way, for example by radiation to ambient,heat exchange with a cooling circuit carrying the same or different heat transfer fluid.The apparatus 1 is shown as a closed unit, with movement of the heat transfer fluid being byconvection, but normally a pumped system will be required.The heat transfer fluid is thus capable of transferring heat both to and from a part of theelectrical apparatus.The apparatus may be configured to transfer heat to the electrical components when thetemperature of said part is below a threshold temperature. For example, a temperature sensormay be used to detect temperature in the apparatus and heat supplied as required to elevate theelectrical components to the threshold temperature.For batteries, having a dielectric liquid with a very low viscosity at -40°C allows more effectivetransfer of heat from the pre-heating equipment to the battery cells, lower energy demand frompumps and the capability to specify smaller, lighter pumping equipment. This will speed pre-heating of the battery system and allow fast charging current to be applied sooner, as well asmaking the battery charging more efficient overall. This in turn has the potential to significantlyreduce fast charging times under low ambient temperature conditions which brings significantadvantages to consumers.Potential applications include, but are not limited to:- ^ batteries in vehicles (including without limitation land, air, and marine vehicles); ^stationary battery storage, e.g. batteries for storing renewable energy. Storage units areusually charged with surplus energy at night when it is colder and therefore thebatteries may require preheating;^non-battery applications requiring a low viscosity, dielectric heat transfer fluid.Performance as a heat transfer fluid depends upon a number of factors, and the Mouromtseffnumber (Mo) can give an indication of the heat transfer capabilities of fluids. Where ρ is the density, k is the thermal conductivity, Cp is the specific heat and µ is the dynamicviscosity of the heat transfer fluid.The exponents a, b, d and e are system dependent and will differ according to whether there isturbulent flow or laminar flow; but for a defined system provide a means of comparisonbetween heat transfer fluids. The higher the Mo number the better the heat transfer capabilitiesof the fluid in that system.Of note, the denominator is a function of dynamic viscosity which (over a short range oftemperatures and absent any phase changes) can be expected to vary more with temperaturethan the other factors.Table 1 below shows properties for a range of fluids including:- ^ the monoesters of the present invention (shown in Part 1 of the Table); ^ monoesters not in accordance with the present invention; ^ diesters, not in accordance with the present invention; and ^ known non-ester heat transfer fluids. The properties shown [indicating units and methods used] are: ^ Carbon number (for esters) ^ Density at 20°C [kg / dm3- ISO 3675] ^ Specific Heat at 20°C [J / kg K - ASTM D2766] ^ Thermal Conductivity at 40°C [W / m.K - ASTM D7896] ^ Kinematic Viscosity at 40°C [mm2 / s - ISO 3104] ^ Kinematic Viscosity at -30°C [mm2 / s - ISO 3104] ^ Kinematic Viscosity at -40°C [mm2 / s - ISO 3104] ^ Pour Point [°C - ISO 3016] ^ Flash Point [°C - ISO 2719]Where values are shown with an asterisk * values are estimated or from commercial productdata.

[0002] †isononanol from Evonik – 7-methyloctanol major isomer3,5,5 trimethylhexanoic acid major isomer

[0003] It can be seen that the C5-C9 / C5-C9 monoesters with carbon number less than 17 of the presentinvention show much lower viscosities at -40°C than the diesters, poly-alphaolefins, or C17 andabove monoesters.Although having lower or comparable viscosity to a water / glycol mixture (100mm2 / s), theesters claimed are dielectric materials permitting direct contact with electrical components.Further having lower or comparable viscosity to a water / glycol mixture enables use of pumpswith similar pumping power rather than requiring higher rated pumps.Although having viscosities higher than the exemplified fluorinated material, the esters claimedhave higher thermal conductivity and specific heat (beneficial for heat transfer as can be seenfrom the Mouromtseff number equation given above); are environmentally safer, beingbiodegradable; and further the alcohols and acids from which the esters are made areobtainable from renewable sources.Below carbon number 14 the flash point drops and so carbon numbers of 14 or more may bepreferable in some applications.The above shows the utility of esters as claimed as heat transfer fluids.Advantageously the ester has the formula R1COOR2 wherein R1 and R2 are each hydrocarbonmoieties which may be the same or different. Aliphatic moieties are preferred over aromaticmoieties and acyclic moieties are preferred over cyclic moieties. Unsubstituted aliphaticmoieties are preferred over substituted aliphatic moieties. Aliphatic moieties may be saturatedor unsaturated.The environment in which the heat transfer fluid is used may require the provision of additivesto protect the fluid or components in which it is in contact.Additives may include any of antioxidants, metal deactivators, friction modifiers, corrosioninhibitors, antifoam additives, thermally conductive particles or combinations thereof..Antioxidants limit degradation of the ester. Antioxidants may include, but are not limited to:^ Phenol antioxidants, for example, 2,6-di-tert-butyl-4-methylphenol; 2,6-di-tert-butylphenol; 4,4’-Methylenebis (2,6-di-tertbutylphenol); pentaerythritol tetrakis(3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate) and butylated hydroxyanisole^Aromatic amines, for example phenyl alpha naphthylamines and alkylateddiphenylaminesMetal deactivators limit degradation of the ester or attack on components. Metal deactivatorsmay include but are not limited to triazole-based deactivators, for example Irgamet® 30,Irgamet® 39, Irgamet® BTZ and Irgamet® TTZ (commercially available from BASF).Friction modifiers limit surface effects with surfaces in contact with the heat transfer fluid.Friction modifiers may include but are not limited to: high hydroxyl esters, , boron derivatives,cyclic and acyclic amides.Corrosion inhibitors limit corrosion of surfaces in contact with the heat transfer fluid. Corrosioninhibitors may include but are not limited to: dimercaptothiazoles, mercaptobenzothiazole,triazoles, imidazoles, alkyl amines, amine phosphates, and sulphonatesAntifoam additives limit foaming of the heat transfer fluid. Antifoam additives may include butare not limited to: polyacrylates, and alcohols.Detergents may limit separation of components in the heat transfer fluid or assist in thesuspension of any particulate matter. Detergents may include but are not limited to: phosphateesters, sulphonates, phenates and salicylatesIn some applications extreme pressure additives may be required. Extreme pressure additivesmay include but are not limited to: graphite, carbon-based nanomaterials, molybdenumdisulphide, olefin sulphides, and dithiocarbamates.In some applications anti-wear additives may be required to prevent mechanical damage tosurfaces that the heat transfer fluid is in contact with.. Anti-wear additives include but are notlimited to: metal alkylthiophosphates, ashless dithiophosphates, ashless phosphorothioates,ashless thiophosphates, amine phosphates, triarylphosphates, high hydroxyl esters, ,sulphurised esters, cyclic and acyclic amides, dimer acids and boron derivatives.In some systems heat transfer may be improved by including thermally conductive particles inthe heat transfer fluid. Thermally conductive particles may include, but are not limited to:graphite, carbon-based nanomaterials, and boron nitrideEsters are commonly known compounds and can be manufactured by any suitable process fitfor producing esters.On a laboratory scale, alcohol and carboxylic acid (1 equivalent) may be added to a roundbottom flask fitted with a Dean-Stark trap and a condenser. The reaction mixture may beheated up to 240 °C under nitrogen, held there for 4 hours and water collected in the Dean-Starktrap. Any excess alcohol or carboxylic acid can then be removed using reduced pressurefractional distillation.Modifications and variants to the above disclosure will be evident to the person skilled in theart, yet still remain within the appended claims.

Claims

Claims 1. Electrical apparatus comprising at least one electrical component in thermal contactwith a heat transfer fluid, wherein the heat transfer fluid comprises by weight percent ofthe fluid >95% at least one ester of a C5-C9 monocarboxylic acid and a C5-C9monoalcohol, the at least one ester having a carbon number of less than 17 the heattransfer fluid not comprising more than 80% by weight 2-ethylhexyl octanoate.

2. Electrical apparatus, as claimed in Claim 1, further comprising a heat source in thermalcontact with the heat transfer fluid to permit heat to be transferred by the heat transferfluid from the heat source to the at least one electrical component.

3. Electrical apparatus, as claimed in Claim 1 or Claim 2, further comprising a heat sink inthermal contact with the heat transfer fluid to permit heat to be transferred by the heattransfer fluid from the at least one electrical component to the heat sink.

4. Electrical apparatus, as claimed in any of Claims 1 to 3, wherein the at least oneelectrical component is immersed in the heat transfer fluid.

5. Electrical apparatus, as claimed in any of Claims 1 to 4, wherein the apparatus isconfigured to supply heat to the at least one electrical component when the at least oneelectrical component is below a first temperature, and to remove heat from the at leastone electrical component when the at least one electrical component is above a secondtemperature.

6. Electrical apparatus, as claimed in any of Claims 1 to 5, wherein the at least oneelectrical component comprises a battery.

7. Electrical apparatus, as claimed in Claim 6, wherein the battery is a lithium ion battery.

8. Electrical apparatus, as claimed in any of Claims 1 to 7, wherein the heat transfer fluidcomprises by weight percent of the fluid >95% one ester of a C5-C9 monocarboxylic acidand a C5-C9 monoalcohol.

9. Electrical apparatus, as claimed in Claim 8, wherein the one ester of a C5-C9monocarboxylic acid and a C5-C9 monoalcohol is the only ester of a C5-C9monocarboxylic acid and a C5-C9 monoalcohol present in the heat transfer fluid.

10. Electrical apparatus, as claimed in any of Claims 1 to 9, wherein the at least one ester ofa C5-C9 monocarboxylic acid and a C5-C9 monoalcohol comprises one or more of thefollowing features:^ the C5-C9 monocarboxylic acid is an acyclic monocarboxylic acid ^ the C5-C9 monocarboxylic acid is a linear monocarboxylic acid ^ the C5-C9 monocarboxylic acid is a branched monocarboxylic acid ^ the C5-C9 monoalcohol is an acyclic monoalcohol ^ the C5-C9 monoalcohol is a linear monoalcohol ^ the C5-C9 monoalcohol is a branched monoalcohol ^ the C5-C9 monoalcohol is a primary monoalcohol ^ the C5-C9 monoalcohol is a secondary monoalcohol ^the C5-C9 monoalcohol is a tertiary monoalcohol.

11. Electrical apparatus, as claimed in any of Claims 1 to 10, wherein the ester comprises:2-octyl pentanoate, 2-octyl hexanoate, 2-octyl heptanoate, 2-hexyl nonanoate, isononylhexanoate, 7-methyloctanyl hexanoate, 2-ethylhexyl heptanoate, 2-octyl octanoate, 2-ethylhexyl 2-ethylhexanoate, or mixtures thereof.

12. Electrical apparatus, as claimed in any of Claims 1 to 11, wherein the carbon number is13 or more, optionally 14 or more.

13. Electrical apparatus, as claimed in any of Claims 1 to 12, wherein the heat transfer fluidcomprises at least one functional additive selected from the group anti-oxidants, metaldeactivators, friction modifiers, corrosion inhibitors, antifoam additives, detergents,extreme pressure additives, anti-wear additives, and thermally conductive particles.

14. A method of operating electrical apparatus as claimed in any preceding claim,comprising transferring heat from the heat transfer fluid to the at least one electricalcomponent, and operating the at least one electrical component once a thresholdtemperature is exceeded.

15. A heat transfer fluid comprising by weight percent:->95% at least one ester of a C5-C9 monocarboxylic acid and a C5-C9 secondarymonoalcohol, the at least one ester having a carbon number of less than 17 theheat transfer fluid not comprising more than 80% by weight 2-ethylhexyloctanoateandat least one functional additive selected from the group anti-oxidants, metaldeactivators, friction modifiers, corrosion inhibitors, antifoam additives,detergents, extreme pressure additives, and anti-wear additives.

16. A heat transfer fluid, as claimed in Claim 15 in which the functional additives compriseby weight percent of the fluid:-≥0.1% anti-oxidant ≥0.001% metal deactivator optionally, in which the functional additives comprise by weight percent of the fluid:-0.1-1.0% anti-oxidant0.001-0.05% metal deactivator.

17. A heat transfer fluid, as claimed in any of Claims 15 to 16 wherein the heat transfer fluidcomprises >95% one ester of a C5-C9 linear monocarboxylic acid and a C5-C9 linearsecondary monoalcohol.

18. A heat transfer fluid, as claimed in Claim 17 wherein the one ester of a C5-C9 linearmonocarboxylic acid and a C5-C9 linear secondary monoalcohol is the only ester of a C5-C9 linear monocarboxylic acid and a C5-C9 linear secondary monoalcohol present.

19. A heat transfer fluid, as claimed in any of Claims 15 to 18, the at least one ester of a C5-C9 monocarboxylic acid and a C5-C9 monoalcohol comprises one or more of thefollowing features:^ the C5-C9 monocarboxylic acid is an acyclic monocarboxylic acid ^ the C5-C9 monocarboxylic acid is a linear monocarboxylic acid ^ the C5-C9 monocarboxylic acid is a branched monocarboxylic acid ^ the C5-C9 monoalcohol is an acyclic monoalcohol ^ the C5-C9 monoalcohol is a linear monoalcohol ^ the C5-C9 monoalcohol is a branched monoalcohol ^ the C5-C9 monoalcohol is a primary monoalcohol ^ the C5-C9 monoalcohol is a secondary monoalcohol ^ the C5-C9 monoalcohol is a tertiary monoalcohol.

20. A heat transfer fluid, as claimed in any of Claims 15 to 19, wherein the ester comprises:2-octyl pentanoate, 2-octyl hexanoate, 2-octyl heptanoate, 2-hexyl nonanoate, isononylhexanoate, 7-methyloctanyl hexanoate, 2-ethylhexyl heptanoate, 2-octyl octanoate, 2-ethylhexyl 2-ethylhexanoate, or mixtures thereof.

21. A heat transfer fluid, as claimed in any of Claims 15 to 20, wherein the carbon number is13 or more, optionally 14 or more.

22. A heat transfer fluid, as claimed in any of Claims 15 to 21, wherein the C6-C9 linearmonocarboxylic acid is heptanoic acid and the C6-C9 linear secondary monoalcohol is 2-octanol.

23. Use of a heat transfer fluid comprising by weight percent of the fluid >95% at least oneester of a C5-C9 monocarboxylic acid and a C5-C9 monoalcohol and having a carbonnumber of less than 17, the heat transfer fluid not comprising more than 80% by weight2-ethylhexyl octanoate, to deliver heat to at least one electrical component at atemperature between 0°C and -40°C , optionally between -30°C and -40°C.

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