EP4573165A1

EP4573165A1 - Heat transfer fluids

Info

Publication number: EP4573165A1
Application number: EP23758298.6A
Authority: EP
Inventors: Francine TOWNSEND; Christopher Inman; Tom Galvin; Mark LASHBROOK
Original assignee: Midel & Mivolt Fluids Ltd
Current assignee: Midel & Mivolt Fluids Ltd
Priority date: 2022-08-19
Filing date: 2023-08-16
Publication date: 2025-06-25
Also published as: JP2025527546A; WO2024038121A1; KR20250052373A; CN119731287A

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

^{Heat transfer fluids} ^{This invention relates to heat transfer fluids for use in transferring heat from one location to} _{another, to electrical apparatus comprising such heat fluids, and to methods of using such heat} _{transfer fluids in electrical apparatus.} ^{The invention is particularly, although not exclusively, applicable to heat transfer fluids for use} _{in environments where exposure to electricity may occur, and where low temperatures may} _exist. ^{The invention is illustrated in the following by reference to batteries, but the applicability of the} _{invention 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 desired} _{intercalation of lithium into the anode material. This occurs when lithium is deposited more} _{rapidly than it can be intercalated into the anode, and so is dependent both on charging rate and} _{the kinetics of intercalation of lithium into the anode material.} ^{Lithium plating is exacerbated by operating batteries at low temperatures, as this results in} ^{reduced ion diffusion and electrolyte conductivity, increasing resistance, and slower kinetics of} _{intercalation of lithium into the anode material. Lithium plating can lead to reduced battery} _{capacity and, in the extreme, short circuiting.} ^{Due to the increased internal resistance of cells at sub-zero temperatures and a slowing of the} ^{electro-chemical reactions it is therefore necessary to limit input current during battery} _{charging. This in turn can lead to very long charging times for battery systems where the cell} _{temperature is below 0°C.} ^{To counteract this, it is beneficial to pre-heat battery cells during periods of cold ambient} _{temperature 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 thermal} _{transfer liquids at sub-zero temperatures impedes the heat transfer due to excessive pressure} _{drops in the system and a slowing of the liquid flow compared with higher temperatures. Extra} ^{energy is also required from pumping systems to circulate liquid at the higher viscosity} _{experienced 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, fluorinated} ^{hydrocarbons, esters, and water/glycol mixtures.} ^{Water/glycol mixtures need to be kept separate from operating electrical components, adding a} ^{thermal barrier between components and heat transfer fluid. Such water glycol mixtures may} ^{achieve 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 for} _{biodegradability.} _{Esters are known to be biodegradable to varying degrees and to provide good dielectric} _{properties. 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 to} _{water/glycol mixtures or fluorinated hydrocarbons.} ^{US2012/283162 A1 proposed the use of oleyl esters having 23 or more of a total number of a} _{terminal methyl group, a methylene group and an ether group in a main chain, as a base oil for} _{cooling electric motors.} ^{US2022/0131205 A1 proposes the use of esters in a cooling composition for cooling, among} ^{other things, the battery and/or power electronics of an electric or hybrid vehicle. The only} ^{monoester exemplified is the highly branched 3,5,5-trimethylhexyl 3,5,5-trimethylhexanoate} _{having 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 volume} _{percent of 2-ethylhexyl caprylate (2-ethylhexyl octanoate) and permits presence of other esters} _{of C6-C8 fatty acids with 2 ethyl hexanol.} ^{The heat transfer fluids used in the present invention provide heat transfer properties} ^{comparable to fluorinated hydrocarbons, viscosity much lower than diesters and commonly} _{used hydrocarbons, and are biodegradable. Further the source materials for the esters} _{described herein are preferably obtainable from biological sources providing environmental} _{benefits to their manufacture and use.} ^{Accordingly, the present invention provides electrical apparatus comprising at least one} _{electrical component in thermal contact with a heat transfer fluid, wherein the heat transfer} ^{fluid comprises by weight percent of the fluid >95% at least one ester of a C5-C9} ^{monocarboxylic acid and a C5-C9 monoalcohol, the at least one ester having a carbon number of} ^{less than 17 the heat transfer fluid not comprising more than 80% by weight 2-ethylhexyl} ^octanoate. ^{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-ethylhexyl} _{octanoate, 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 a} _{linear 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, or} _{3,5,5-trimethylhexanoic acid.} ^{The C5-C9 monoalcohol is optionally an acyclic monoalcohol, for example a linear monoalcohol} _{or a branched monoalcohol and may be a primary, secondary, or tertiary alcohol. The C5-C9} _{monoalcohol 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, or} _{3,5,5-Trimethyl-1-hexanol.} ^{The electrical apparatus may comprise a heat source in thermal contact with the heat transfer} _{fluid to permit heat to be transferred by the heat transfer fluid from the heat source to the at} _{least one electrical component.} ^{The electrical apparatus may comprise a heat sink in thermal contact with the heat transfer} _{fluid to permit heat to be transferred by the heat transfer fluid from the at least one electrical} _{component 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 when} _{the at least one electrical component is below a first temperature, and to remove heat from the} ^{at least one electrical component when the at least one electrical component is above a second} _temperature. ^{The at least one electrical component may comprise a battery, which may be a lithium ion} _{battery, a lithium metal battery, or any other battery.} ^{The heat transfer fluid in the apparatus may comprise by weight percent of the fluid >95% one} ^{ester of a C5-C9 monocarboxylic acid and a C5-C9 monoalcohol, and that one ester may be the} ^{only ester present. Some of or each at least one ester in the heat transfer fluid optionally has a} _{carbon number of 16 or less or 15 or less. Some of or each at least one ester in the heat transfer} _{fluid 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 from} ^{the group anti-oxidants, metal deactivators, friction modifiers, corrosion inhibitors, antifoam} _{additives, detergents, extreme pressure additives, anti-wear additives, and thermally} _{conductive particles.} ^{A method of operating the electrical apparatus comprises transferring heat from the heat} _{transfer fluid to the at least one electrical component, and operating the electrical component} _{once 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 at} ^{least one ester having a carbon number of less than 17} _{and at least one functional additive selected from the group anti-oxidants, metal deactivators,} _{friction modifiers, corrosion inhibitors, antifoam additives, detergents, extreme pressure} _{additives, and anti-wear additives.} The functional additives may comprise by weight percent of the fluid:- ≥0.1% anti-oxidant ≥0.001% metal deactivator ^{and optionally the functional additives may comprise by weight percent of the fluid:-} 0_{.1-1.0% anti-oxidant} _{0.001-0.05% metal deactivator} ^{Further features of the invention will be apparent from the appended claims and the following} _{description exemplifying, but not limiting, the scope of the invention claimed. Reference is made} _{to 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 are} _{immersed in and so in thermal contact with heat transfer fluid 3 that fills the apparatus. The} _{apparatus comprises heat source 4 and heat sink 5.} ^{Heat source 4 can provide heat to the heat transfer fluid 3, and thereby to the electrical} ^{components 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 the} apparatus. 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 by} _{convection, 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 the} _{electrical apparatus.} ^{The apparatus may be configured to transfer heat to the electrical components when the} ^{temperature of said part is below a threshold temperature. For example, a temperature sensor} _{may be used to detect temperature in the apparatus and heat supplied as required to elevate the} _{electrical components to the threshold temperature.} ^{For batteries, having a dielectric liquid with a very low viscosity at -40°C allows more effective} ^{transfer of heat from the pre-heating equipment to the battery cells, lower energy demand from} ^{pumps 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 as} _{making the battery charging more efficient overall. This in turn has the potential to significantly} _{reduce fast charging times under low ambient temperature conditions which brings significant} _{advantages 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 are} u_{sually charged with surplus energy at night when it is colder and therefore the} _{batteries 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 Mouromtseff _{number (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 dynamic} _{viscosity of the heat transfer fluid.} ^{The exponents a, b, d and e are system dependent and will differ according to whether there is} ^{turbulent flow or laminar flow; but for a defined system provide a means of comparison} _{between heat transfer fluids. The higher the Mo number the better the heat transfer capabilities} _{of the fluid in that system.} ^{Of note, the denominator is a function of dynamic viscosity which (over a short range of} _{temperatures and absent any phase changes) can be expected to vary more with temperature} _{than 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 product} _data.

†^{isononanol from Evonik – 7-methyloctanol major isomer} 3_{,5,5 trimethylhexanoic acid major isomer}

^{It can be seen that the C5-C9/C5-C9 monoesters with carbon number less than 17 of the present} _{invention show much lower viscosities at -40°C than the diesters, poly-alphaolefins, or C17 and} _{above monoesters.} ^{Although having lower or comparable viscosity to a water/glycol mixture (100mm2/s), the} ^{esters claimed are dielectric materials permitting direct contact with electrical components.} _{Further having lower or comparable viscosity to a water/glycol mixture enables use of pumps} _{with similar pumping power rather than requiring higher rated pumps.} ^{Although having viscosities higher than the exemplified fluorinated material, the esters claimed} ^{have higher thermal conductivity and specific heat (beneficial for heat transfer as can be seen} _{from the Mouromtseff number equation given above); are environmentally safer, being} _{biodegradable; and further the alcohols and acids from which the esters are made are} _{obtainable from renewable sources.} ^{Below carbon number 14 the flash point drops and so carbon numbers of 14 or more may be} _{preferable 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 hydrocarbon} ^{moieties which may be the same or different. Aliphatic moieties are preferred over aromatic} _{moieties and acyclic moieties are preferred over cyclic moieties. Unsubstituted aliphatic} _{moieties are preferred over substituted aliphatic moieties. Aliphatic moieties may be saturated} _{or unsaturated.} ^{The environment in which the heat transfer fluid is used may require the provision of additives} _{to protect the fluid or components in which it is in contact.} ^{Additives may include any of antioxidants, metal deactivators, friction modifiers, corrosion} _{inhibitors, 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-} b_{utylphenol; 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 alkylated} d_{iphenylamines} ^{Metal deactivators limit degradation of the ester or attack on components. Metal deactivators} _{may 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. Corrosion} _{inhibitors may include but are not limited to: dimercaptothiazoles, mercaptobenzothiazole,} _{triazoles, imidazoles, alkyl amines, amine phosphates, and sulphonates} ^{Antifoam additives limit foaming of the heat transfer fluid. Antifoam additives may include but} _{are not limited to: polyacrylates, and alcohols.} ^{Detergents may limit separation of components in the heat transfer fluid or assist in the} ^{suspension of any particulate matter. Detergents may include but are not limited to: phosphate} ^{esters, sulphonates, phenates and salicylates} _{In some applications extreme pressure additives may be required. Extreme pressure additives} _{may include but are not limited to: graphite, carbon-based nanomaterials, molybdenum} _{disulphide, olefin sulphides, and dithiocarbamates.} ^{In some applications anti-wear additives may be required to prevent mechanical damage to} ^{surfaces that the heat transfer fluid is in contact with.. Anti-wear additives include but are not} _{limited 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 in} _{the heat transfer fluid. Thermally conductive particles may include, but are not limited to:} _{graphite, carbon-based nanomaterials, and boron nitride} ^{Esters are commonly known compounds and can be manufactured by any suitable process fit} _{for producing esters.} ^{On a laboratory scale, alcohol and carboxylic acid (1 equivalent) may be added to a round} ^{bottom flask fitted with a Dean-Stark trap and a condenser. The reaction mixture may be} _{heated up to 240 °C under nitrogen, held there for 4 hours and water collected in the Dean-Stark} _{trap. Any excess alcohol or carboxylic acid can then be removed using reduced pressure} _{fractional distillation.} ^{Modifications and variants to the above disclosure will be evident to the person skilled in the} _{art, yet still remain within the appended claims.}

Claims

Claims 1^{. Electrical apparatus comprising at least one electrical component in thermal contact} ^{with a heat transfer fluid, wherein the heat transfer fluid comprises by weight percent of} ^{the fluid >95% at least one ester of a C5-C9 monocarboxylic acid and a C5-C9} ^{monoalcohol, the at least one ester having a carbon number of less than 17 the heat} ^{transfer 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 thermal} ^{contact with the heat transfer fluid to permit heat to be transferred by the heat transfer} ^{fluid 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 in} ^{thermal contact with the heat transfer fluid to permit heat to be transferred by the heat} ^{transfer 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 one} ^{electrical component is immersed in the heat transfer fluid.} ^{5. Electrical apparatus, as claimed in any of Claims 1 to 4, wherein the apparatus is} c_{onfigured to supply heat to the at least one electrical component when the at least one} _{electrical component is below a first temperature, and to remove heat from the at least} _{one electrical component when the at least one electrical component is above a second} _temperature. _{6. Electrical apparatus, as claimed in any of Claims 1 to 5, wherein the at least one} _{electrical 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 fluid} _{comprises by weight percent of the fluid >95% one ester of a C5-C9 monocarboxylic acid} _{and a C5-C9 monoalcohol.} _{9. Electrical apparatus, as claimed in Claim 8, wherein the one ester of a C5-C9} _{monocarboxylic acid and a C5-C9 monoalcohol is the only ester of a C5-C9} _{monocarboxylic 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 of} _{a C5-C9 monocarboxylic acid and a C5-C9 monoalcohol comprises one or more of the} _{following 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, isononyl} ^{hexanoate, 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 is} ^{13 or more, optionally 14 or more.} ^{13. Electrical apparatus, as claimed in any of Claims 1 to 12, wherein the heat transfer fluid} ^{comprises at least one functional additive selected from the group anti-oxidants, metal} d_{eactivators, 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 electrical} _{component, and operating the at least one electrical component once a threshold} _{temperature 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 secondary} _{monoalcohol, the at least one ester having a carbon number of less than 17 the} _{heat transfer fluid not comprising more than 80% by weight 2-ethylhexyl} _octanoate _and ^{at least one functional additive selected from the group anti-oxidants, metal} d_{eactivators, 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 comprise} b_{y weight percent of the fluid:-} ≥0.1% anti-oxidant ≥0.001% metal deactivator o^{ptionally, in which the functional additives comprise by weight percent of the fluid:-} 0_{.1-1.0% anti-oxidant} _{0.001-0.05% metal deactivator.} ^{17. A heat transfer fluid, as claimed in any of Claims 15 to 16 wherein the heat transfer fluid} ^{comprises >95% one ester of a C5-C9 linear monocarboxylic acid and a C5-C9 linear} ^{secondary monoalcohol.} ^{18. A heat transfer fluid, as claimed in Claim 17 wherein the one ester of a C5-C9 linear} m_{onocarboxylic 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 the} _{following 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, isononyl} ^{hexanoate, 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 is} ^{13 or more, optionally 14 or more.} ^{22. A heat transfer fluid, as claimed in any of Claims 15 to 21, wherein the C6-C9 linear} m_{onocarboxylic 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 one} _{ester of a C5-C9 monocarboxylic acid and a C5-C9 monoalcohol and having a carbon} _{number of less than 17, the heat transfer fluid not comprising more than 80% by weight} _{2-ethylhexyl octanoate, to deliver heat to at least one electrical component at a} _{temperature between 0°C and -40°C , optionally between -30°C and -40°C.}