Next Article in Journal
Stochastic Optimization Scheduling Method for Mine Electricity–Heat Energy Systems Considering Power-to-Gas and Conditional Value-at-Risk
Previous Article in Journal
Assessing Lithium-Ion Battery Safety Under Extreme Transport Conditions: A Comparative Study of Measured and Standardised Parameters
Previous Article in Special Issue
Operational Energy Consumption Map for Urban Electric Buses: Case Study for Warsaw
 
 
Font Type:
Arial Georgia Verdana
Font Size:
Aa Aa Aa
Line Spacing:
Column Width:
Background:
Review

Potential of Natural Esters as Immersion Coolant in Electric Vehicles †

by
Raj Shah
1,
Cindy Huang
1,
Gobinda Karmakar
2,
Sevim Z. Erhan
3,
Majher I. Sarker
3 and
Brajendra K. Sharma
3,*
1
Koehler Instrument Company, 85 Corporate Drive, Holtsville, NY 11742, USA
2
Department of Chemistry, Sri Narasingha Vidyapith, Darjeeling 734011, West Bengal, India
3
US Department of Agriculture, Agricultural Research Service, Eastern Regional Research Center, Sustainable Biofuels and Coproducts Research, Wyndmoor, PA 19038, USA
*
Author to whom correspondence should be addressed.
Mention of trade names or commercial products in this article is solely for the purpose of providing specific information and does not imply recommendation or endorsement by the U.S. Department of Agriculture. USDA is an equal opportunity provider and employer.
Energies 2025, 18(15), 4145; https://doi.org/10.3390/en18154145
Submission received: 23 April 2025 / Revised: 11 July 2025 / Accepted: 31 July 2025 / Published: 5 August 2025

Abstract

As the popularity of electric vehicles (EVs) continues to increase, the need for effective and efficient driveline lubricants and dielectric coolants has become crucial. Commercially used mineral oils or synthetic ester-based coolants, despite performing satisfactorily, are not environmentally friendly. The fatty esters of vegetable oils, after overcoming their shortcomings (like poor oxidative stability, higher viscosity, and pour point) through chemical modification, have recently been used as potential dielectric coolants in transformers. The benefits of natural esters, including a higher flash point, breakdown voltage, dielectric character, thermal conductivity, and most importantly, readily biodegradable nature, have made them a suitable and sustainable substitute for traditional coolants in electric transformers. Based on their excellent performance in transformers, research on their application as dielectric immersion coolants in modern EVs has been emerging in recent years. This review primarily highlights the beneficial aspects of natural esters performing dual functions—cooling as well as lubricating, which is necessary for “wet” e-motors in EVs—through a comparative study with the commercially used mineral and synthetic coolants. The adoption of natural fatty esters of vegetable oils as an immersion cooling fluid is a significant sustainable step for the battery thermal management system (BTMS) of modern EVs considering environmental safety protocols. Continued research and development are necessary to overcome the ongoing challenges and optimize esters for widespread use in the rapidly expanding electric vehicle market.

1. Introduction

Electric vehicles (EVs) have attracted significant attention in the past few years. The types of EVs include battery electric vehicles (BEVs), plug-in hybrid electric vehicles (PHEVs), hybrid electric vehicles (HEVs), range-extended electric vehicles (REEVs), and fuel cell electric vehicles (FCEVs) [1,2,3]. All EVs typically consist of a lithium-ion battery and an electric motor (e-motor) [1]. In BEVs, these parts are a substitute of the lead–acid batteries in internal combustion engines (ICEs) of gasoline vehicles [1]. In hybrid EVs, the electric components are used simultaneously with gasoline components [1]. The batteries in BEVs, PHEVs, and REEVs are charged through an external charging station. REEVs are similar to PHEVs except that in REEVs, energy from combusted gasoline charges the depleted batteries and power is not provided directly to the wheels [3]. Fuel cell EVs are unique, as the battery is charged solely through hydrogen fuel cells [2]. This review primarily discusses EVs in the context of BEVs for continuity purposes.
EVs contain fewer moving parts compared to gasoline engine vehicles, and thus require less lubricant and maintenance [4,5]. In gasoline vehicles, the engine oil performs a dual function, both cooling and lubricating the various moving parts. Normally, in EVs, battery coolant and driveline lubricants are separate fluids, with battery coolants primarily being air or water–glycol mixtures. As the design of EVs is evolving, there is a need to develop novel fluids that can address both high temperatures and the high rpm of power sources [5]. EVs still contain gears and bearings that require lubrication [4,6]. In “dry” e-motors, the lubricants and coolants are separate fluids. But in new “wet” e-motors, the motor is encased in the transmission gearbox and differential, which requires fluid that both cools the motor and lubricates the gears [6]. Companies are developing similar fluids for EVs that can function as efficient coolants as well as lubricants for e-motors and batteries [6]. The “wet” e-motor has the advantage of being more compact and efficient by reducing its weight using single immersion fluid, unlike “dry’’ e-motors, which have two separate fluids [6]. However, it also puts a much greater responsibility on the fluid to lubricate and cool while also being chemically compatible with copper components [6].
The main concern with combining the coolant and lubricant into a single fluid is not the lubrication, but the cooling aspect. The excessive heat generated by EVs must be quickly removed from the motors for the safety and greater efficiency of the engine, especially in extended-range vehicles [7,8,9]. Modern EV motors run at an average speed of 20,000 rpm with expectations to go beyond 30,000 rpm, generating extreme heat as well as wear; therefore, novel coolants with special liquid cooling plates filled with a porous medium with excellent thermal management capacity are essential for combating the enormous amount of heat formed [5,10,11]. In addition, lithium-ion batteries, known for their high energy density and low self-discharge rate, have the drawback of producing enormous amounts of heat while charging and discharging [8,12,13]. With the development of ultra-fast charging, an efficient BTMS has become greatly important for the longevity and safety of motors and other components. Extremely high temperatures can degrade the battery, reduce the driving range, or trigger a thermal runaway chain reaction [12,14]. The optimal temperature range varies from each source, ranging anywhere from 15 to 40 °C [12,14,15]. Although sources may disagree on the optimal temperature range, it is known that temperature uniformity is also especially important [12]. The prevention of hotspots has become difficult due to the diversity of battery cell types. With the use of a single coolant and lubrication fluid, the BTMS must use direct immersion. Immersion cooling is more easily adaptable to a wide variety of cell shapes and sizes, unlike indirect cooling, which relies on a careful network of tubes and pipes [16]. There is no clear consensus in the industry on the ideal type of system or fluid to be used. As EVs become more common, there is a clear need to converge on a system and fluid for ease of production and supplier risks [17]. The immersion fluids for modern EVs must have sufficient dielectric characteristics, wear resistance capability, low viscosity, and thermal conductivity, while also being environmentally friendly and suitable in colder weather [18]. Developing fluids that meet all these requirements becomes increasingly challenging. The commercially used synthetic or mineral fluids exhibit satisfactory performance, but are not environmentally safe. Thus, researchers are continuously trying to develop fluids for new EVs, considering all the required specifications and sustainability goals.
The natural fatty esters from vegetable oils have already attracted considerable interest as a potential alternative to the petroleum-based mineral or synthetic oils commercially used in gasoline engines, due to their excellent lubricity, higher viscosity, and biodegradability [19,20,21]. Moreover, due to their higher flash points, dielectric character, enhanced thermal management capacity, and improved safety, these esters have been used as green alternatives to synthetic dielectric coolants and insulating fluids in electric transformers very recently [22,23,24]. Based on these performances, the esters from vegetable oils have an exciting opportunity to be used as immersion fluids that perform as coolants, as well as lubricants of high-performance EVs, without compromising on sustainability aspects. But there are a few pitfalls like poor thermo-oxidative stability, higher viscosity, poor low-temperature fluidity, higher cost, etc., that need to be addressed by suitable chemical transformations and or blending additives before their application in EVs or transformers. The research in this area has great importance considering the environmental safety and renewability of the resource materials.
Nugroho et al. have discussed the efficiency of TiO2 and ZnO nanoparticle-enriched palm oil-based biolubricants to improve the tribological performances in moving parts of EVs [25]. Dekkers et al. screened the thermophysical properties, specifically density, specific heat capacity, viscosity, and thermal conductivity of fluids, in mixtures of non-edible oils, essential oils, and synthetic esters in several experiments and disclosed that formulations consisting of 65% jojoba or cottonseed oil (non-edible oils), 15% pine or tea tree oil, and 20% synthetic ester (MIDEL 7131, a biodegradable synthetic ester developed by M&I Materials Ltd. (Manchester, UK) in 1970) achieved the best performance compared to mineral oil coolants [26]. In another study, biodegradable pentaerythritol ester proved to be a better option as a battery immersion coolant compared to mineral oils [13]. However, reports of the application of modified natural esters as immersion coolants in EVs are limited. This review discusses the potential of various esters derived from vegetable oils as single multifunctional fluids performing as dielectric coolants, as well as lubricants for e-motors in recent EV engines, following their excellent performance in transformers. However, this paper does not discuss the material compatibility or lubricating factors of esters in detail. The advantages of natural esters in terms of properties that are essential for their application in EV engines, adapted from their use in electric transformers, are discussed below in comparison with commercial fluids.

2. Comparison of Performances of Different Coolants Used in Transformers and EV Engines

2.1. Cooling Systems and Coolants

Air is the most common and economical option for a battery cooling system, as shown in Figure 1, but its limitations have already been seen [12,27]. Particularly in hotter environments, air is not capable of managing the heat from lithium-ion batteries due to air’s low specific heat capacity and poor heat distribution [12,13,27]. The poor distribution of heat can lead to a 5 °C temperature difference in the electrical core [13]. It is estimated that air cooling is possible only if the amount of heat rejected from the battery is less than 4 kW [14]. Currently, air-based BTMS [Figure 1] is used in vehicles such as the Nissan e-NV, Toyota Prius, and Honda Insight [14].
A liquid cooling system is superior to one using air, as liquids have a higher specific heat capacity as well as better thermal conductivity. Liquid-based coolants can be separated into two categories based on indirect and direct cooling methods. As previously stated, indirect cooling relies heavily on the design of the system [13,16]. In indirect cooling (Figure 2b), heat is released from the cells to plates or tubes which contain a circulating fluid that removes the heat [8,15]. The fluid is typically an aqueous solution of water and glycol, as water-based fluids have excellent heat transfer properties [12,18,27]. It is an increasingly popular method of EV cooling, as it is possible to achieve ideal thermal temperatures and uniformity [13,14]. Based on the cooling channels’ position relative to the batteries, an indirect cooling system can be divided into two categories: bottom cooling and side cooling. The Tesla Model 3, for instance, uses the side cooling method with serpentine tubing attached to cylindrical cells [28]. The bottom cooling method, where the cooling plates are installed underneath the cell, is used in vehicles such as the Audi e-Tron, Porsche Taycan, and Mercedes-Benz EQC [12,14].
The downsides of an indirect system are the complexity and costs of developing such a system [13,14]. Maintaining good thermal contact with the cells, additional weight and components, coolant leakage, and safety are some concerns that may be difficult to address when using a complex system [14]. Water and glycol’s high electrical conductivity makes this mixture unsuitable for direct contact with any electrical components [18]. Over time, exposure to high temperatures can cause acidification and corrosion in metals such as carbon steel and copper [13]. Additionally, there is a delay in heat transfer in an indirect system, as the heat must pass through several other materials, all with different specific heat capacities [12,15]. Without proper thermal management in an indirect system, fires can easily start and spread to other cells, causing an explosion [8]. Despite the risks, aqueous coolants are technologically sufficient for standard electric cars without high thermal outputs [17]. However, for ultra-fast charging over 300 kW, current cooling systems struggle to handle the heat generated [14,29].
Direct cooling, also called immersion cooling, is superior compared to indirect cooling due to its thermal contact and low complexity, but also presents some challenges (Figure 2a). Immersion cooling submerges the battery in non-conductive dielectric fluids, which have superior cooling performance and safety compared to aqueous fluids [8,17]. Immersion cooling can be easily adapted for any battery shape, especially cylindrical, because of its high surface-to-volume ratio, allowing greater thermal exchange [16]. Furthermore, in immersion cooling systems, the suppression of thermal runaway is usually observed, as some of the dielectric fluids are also flame-retardant, enhancing the safety of the battery pack. In a test of immersion technology performed by Total Energies, a Volvo XC90 plug-in hybrid improved its charging potential from 3.7 kW to 22 kW, exhibiting seven times more effective cooling capability, and reduced its mass down to 96% of the original weight without having to modify the design of the battery or car [29]. Hemavathi et al., through a numerical simulation analyzing the electrochemical and thermal characteristics of lithium-ion batteries, have proved that the use of the dielectric fluid immersion cooling technique is efficient in BTMS and improved the lifespan of the batteries [30]. Sundin and Sponholtz tested the efficiency of immersion cooling compared to air cooling and found that even with the discharge time halved to 30 min, immersion cooling achieved better results (approximately 26 °C) than an air-cooled battery with a discharge time of one hour (approximately 31.5 °C) [31]. Choi et al. found that immersion cooling reduced the max temperature by 6.7 °C and the temperature difference by 3 °C when comparing an indirect system to an immersion system running under the same conditions [13]. Through simulations, Chen et al. showed that direct liquid coolant can achieve similar cooling results to indirect cooling with a much slower flow speed. For example, in direct cooling, a flow rate of 0.009 m/s is needed compared to the 0.05 m/s needed for indirect cooling to maintain average temperatures just below 31 °C [32]. Mineral oils have been used as immersion cooling fluids in transformers and EVs for a long time due to their reliability, performance, and stability, particularly in electrical applications incompatible with aqueous coolants. However, they are non-renewable and non-biodegradable. The disposal of hazardous byproducts and used oils in the ecosystem can pose significant health risks to living organisms and exacerbate environmental concerns [33,34,35]. Synthetic esters have improved thermal management, dielectric properties, and environmental benefits compared to mineral oils when used as immersion coolants. Challenges are often encountered with synthetic esters due to material compatibility, non-renewability, the cost of fluid, the weight of some fluids, and energy from pumping loss for high-viscosity fluids [8,14]. Most single-phase immersion systems use forced flow with a pump, and there are limited studies testing the cooling abilities without one [16]. Bio-derived esters have much higher thermal conductivity and specific heat capacity compared to mineral oils and therefore are preferred over mineral-based coolants [35]. Moreover, due to their readily biodegradable nature, their use as a potential alternative to synthetic coolants has been increasing recently [36].
The thermal management capacity of different cooling methods for EVs is mentioned in Table 1.

2.2. Dielectric Properties

The dielectric characteristics of fluids are essential, acting as electric insulators to prevent discharges. For EVs that run entirely electrically, the prevention of electrical shorts and corrosion is especially important [7,8,37]. Low electrical conductivity is highly important to prevent electrical shorts, but at the same time, coolants cannot function as insulators alone, otherwise, static charge can build up and cause damage [38]. All flowing liquid generates static electricity. In conductive and semi-conductive liquids, static electricity can be easily dissipated. However, in non-conductive and low-conductive liquids, displacing static charge becomes challenging [39]. This can be solved through anti-static additives [39].
A few important qualities examined in potential dielectric coolants are breakdown voltage, flash point, fire point, pour point, specific heat capacity, viscosity, density, and oxidative stability [40]. Breakdown voltage, also referred to as dielectric breakdown, is the minimum voltage applied for an insulator to become conductive [41]. A minimum of 2.5 kV is required for 800 V batteries [40]. The flash point is the minimum temperature needed for a liquid to give off vapor at concentrations significant enough to be ignitable. There is currently no industry-wide consensus on the requirements or limits for flash points in EV coolants [40]. The fire point is the lowest temperature at which the vapor of a liquid will burn for at least five seconds. Higher values for the qualities listed above are more desirable. The pour point is the minimum temperature at which a liquid will flow through gravity alone. Viscosity plays a major role in the work required by the pump, with lower-viscosity fluids being more efficient in reducing pump energy loss and increasing the heat transfer rate [40]. Viscosity is also important, determining how easily the pump will operate at lower temperatures [40]. Oxidation of oil products can increase conductivity and introduce impurities, affecting other qualities such as breakdown voltage and viscosity [40]. In addition to fluid properties, sustainability and availability in large quantities for mass-produced vehicles are important qualities for this application [13,14].
Some examples of such fluids include fluorinated hydrocarbons, mineral oils, silicone oils, and synthetic or natural esters [14,16]. The physical properties of these dielectric coolants are mentioned in Table 2. Fluorinated hydrocarbons were one of the first fluids used for immersion cooling of power electronics, and were recently introduced into the EV market [13]. The Novec series by 3 M is a line of fluorinated fluids widely used for electrical equipment, in part due to the fluids’ non-flammability [13]. Van Gils et al. showed promising results after submerging a lithium-ion battery at a discharge rate of 5 °C into Novec [13]. The results showed that the temperature rise was limited to below 3 °C and the maximum temperature never rose above the boiling point of the fluid [13]. The downsides are the high costs, volatility, poor biodegradability, environmental issues (PFAS), and density 40% greater than that of water–glycol [13,14,42].
Mineral oils are highly popular as transformer dielectric fluids due to their low cost and low viscosity [43]. Mineral oil has been shown to keep the temperature low and maintain its uniformity throughout the battery cells [13,42]. Research performed by Trimbake et al. on a lithium-ion battery showed that at a charge rate of 2 °C and a discharge of 3 °C, mineral oil maintained temperature uniformity of less than 1 °C difference [42]. Some downsides to mineral oil are its relatively low resistance to fire, lower breakdown voltage, poor oxidative stability, copper corrosion from sulfur impurities, and poor biodegradability [13,14,43,44]. These drawbacks make mineral oil an unsuitable dielectric coolant for EVs.
Table 2. Various physical properties of four dielectric coolants [14,45].
Table 2. Various physical properties of four dielectric coolants [14,45].
PropertiesMineral OilsSilicone OilsSynthetic EstersVegetable Oils
Dielectric Breakdown (KV)30–3536–6045–7082–97
Kinematic Viscosity (cSt)
   at 0 °C<7681–9226–5077–143
   at 40 °C3–1635–4014–2916–37
   at 100 °C2–2.515–174–64–8
Pour Point (°C)−30 to −60 −50 to −60 −40 to −50 −19 to −33
Flash Point (°C)100–170 300–310 250–270 315–328
Fire Point (°C)110–185 340–350 300–310 350–360
Thermal Conductivity (Wm1K1)0.11–0.160.150.13–0.150.16–0.19
Specific heat capacity (Jkg1K1)1600–20001370–15001800–23001500–2100
Dielectric Constant2.12.753.326>3
Silicone oils were introduced as an alternative to mineral oil with good electrical insulating properties, better thermo-oxidative stability, and higher fire points [45]. Research showed that a 48-cell lithium battery submerged in silicone oil only resulted in a 2.5 °C increase in temperature compared to 5.3 °C in an air-cooled battery under the same load [14]. The downsides of silicone oils are their poor biodegradability and high costs.
Esters are oils that have been mainly used as lubricants and insulating oils for power electronic applications, making them highly favorable as an alternative to a single coolant and lubricant fluid [13]. Esters can be divided into two categories: synthetic and natural. Natural esters, also known colloquially as vegetable oils, due to their higher breakdown voltages, flash points, fire points, excellent dielectric properties, and biodegradability, are used as a potential replacement for non-renewable mineral oil-based dielectric fluids in transformers [22,46]. Due to their higher lubricity, their application as biolubricants in different automotive sectors has already been highlighted [19]. Currently, there is little research on the application of esters in lithium-ion batteries. The only ester-based immersion cooling dielectric on the market for EV batteries is M&I Material’s MIVOLT [14]. Therefore, the application of natural esters as dielectric immersion fluids in EVs is an emerging area of research.
Synthetic esters (Pentaerythritol esters, Midel 7131, PriolubeTM EF 3221 (Cargill, Inc., Minneapolis, MN, USA), etc.) are similar to natural esters and have some advantages as dielectric fluids compared to mineral oils [47]. Their synthetic origins allow for more specific properties to be formulated into the fluid [12]. As a result, synthetic esters have lower viscosities and pour points, higher oxidative stability, and more environmental safety on par with mineral oil [12,45,47,48,49], making them a better choice than mineral oils. But they have a lower breakdown voltage, fire point, and lower biodegradability compared to natural esters [14,45,46]. The high cost of synthetic esters may also be an obstacle preventing their widespread adoption in EVs [43]. Currently, synthetic esters are limited to specialty applications in transformers where fire safety is a concern [22,46].

2.3. Comparison with Respect to Moisture Absorption, Hydrolysis Characteristics, and Oxidative Stability

Transformers are electrical components that change voltage levels. Their insulation system is composed of a dielectric coolant that impregnates a solid cellulose composite insulation to eliminate air and increase resistance to electrical breakdown [22,43,50,51]. Transformers place heavy emphasis on cooling efficiency. It is estimated that over 32% of all failures are due to overheating and thermal stress [43,45]. The cellulose paper is considered to be a weak point due to erosion, leading to failures occurring at the oil/solid interface [43]. Over time, the heat causes the production of water and acid in the dielectric fluid, which can be absorbed by the cellulose paper [52]. In a lithium battery, there is no cellulose paper, but there are other sensitive electrical components. Therefore, it is important to examine the effect of moisture content on dielectric fluids and its effect on other components and properties of the fluid, such as dielectric strength.
Natural esters have the ability to prolong the life of cellulose paper due to its moisture absorption and hydrolysis characteristics [53]. Natural esters, in comparison to mineral oil, prolong the lifespan of cellulose paper by a factor of 5 to 12 times before it degrades beyond usability [53,54]. While synthetic ester can have a greater capacity to absorb moisture, the lack of hydrolytic properties eliminates any moisture prevention for both cellulose paper and electrical components when used in lithium-ion batteries [53]. Hydrolysis eliminates the moisture chemically by breaking down bonds, preventing absorption or damage to other components [53]. The estimated end of life for cellulose paper is when a 25% decrease in tensile strength occurs, but this is expensive and impractical to test [50]. The alternative is to test the dielectric properties and dissipation factor of the cellulose paper [51]. While the dissipation factor is not an important factor for battery coolants, it is still used as a sensitive indicator to test the quality of the fluid [40]. Findings in the dissipation factor can still be used as potential predictors for the degradation of electrical components in lithium-ion batteries.
Vegetable oils have been shown to have lower oxidative stability than mineral oils. Although natural esters fill over three million transformers globally, their use is limited to sealed, non-breathable systems [49,53]. Upon exposure to oxygen, triglycerides undergo a reaction with acids and water, resulting in gel formation [55,56]. Acid and water are still good indicators showing whether oxidation occurred, although moisture damage does not appear to be a concern due to the hydrolysis capabilities of natural esters. Studies on transformers also show cellulose degradation and dielectric strength to be relatively unchanged in accelerated oxidation tests (Figure 3 and Figure 4) [57,58]. In Figure 3, the degree of polymerization (DP) of the cellulose papers after aging in different insulating oils is shown. The higher DP of the papers aged with natural esters compared to mineral and synthetic oils indicates higher mechanical strength that increases their lifetimes. However, gelling is a concern. Unlike mineral oils, which form insoluble precipitates, esters polymerize and increase in viscosity, which weakens their cooling properties [23,56,57,59]. The breakdown voltage (kV) of the aged oils in comparison to the new oil at 20% moisture content is shown in Figure 4, in which significant information regarding the dielectric strength of different insulating oils is provided.
Oxidation stability is also a concern that needs to be researched much more extensively, as accelerated aging methods have been known to be unreliable for in-service performance [55]. Accelerated oxidation tests do not differentiate between long-chain and short-chain acids, which significantly counteract cellulose degradation [55]. Only the short-chain acids produced by oxidation harm the cellulose insulation, whereas long-chain acids produced by hydrolysis do not [58]. More studies need to investigate the effects of both long- and short-chain acids on electrical components within lithium-ion batteries. However, the most significant aspect of this shortfall in accelerated aging is the misconception that vegetable oils are not suitable due to their short oxidative stability [55]. More hydrolysis reactions can happen in vegetable oils compared to mineral oil [54]. Vegetable oils could remain serviceable according to industry requirements for just as many years as mineral oil, even if they have poor oxidative stability [55]. Additionally, it has been suggested that poor oxidation stability results for vegetable oils are due to the use of standard ASTM test methods D2112 and D2440, which were developed to evaluate mineral oils and not esters [58]. The concerns for oxidation products in esters are not the same as in mineral oils [59].
Cases of accidental free-breathing transformers have shown very favorable results. A case study of a transformer that had air leakage allowing partial breathing for seven years revealed the transformer was in perfect working order [60]. The breakdown voltage was well over the minimum 30 kV limit for safe operation in a transformer [60]. At the end of another eight-year study in which a purposeful air leak was added, the fluid and internal components were found to be like new [59]. Oxidation resulted in only an 8.6% increase in viscosity, and the transformer passed all IEEE C57.100 electrical stress tests (methods to investigate the effects of operating temperature on the life expectancy of liquid-immersed transformer insulation systems) [59]. Despite these promising results, esters are still not recommended for use in free-breathing systems, as the long-term effects of ester oxidation are not well known and there is still a significant decline in dielectric breakdown, along with an increase in viscosity in an already highly viscous fluid [53,60]. Envirotemp FR3 oil, a natural ester transformer oil, has been shown to be effective in the long term (for up to 10 years when sealed) [55]. The breakdown voltage remained over 40–50 kV compared to the initial value of 80 kV [58]. In accelerated aging tests, FR3 also showed no change in dielectric strength [58].

3. Modification of Natural Esters

Although the application of bio-derived esters as dielectric coolants for modern EVs and transformers has already been highlighted in recent years, their widespread application is overlooked due to several pitfalls like their poor oxidative stability, high pour point, and viscosity [46,61]. But recently, researchers have mitigated the shortcomings of natural oils through several chemical modifications [19,21].
Natural esters encompass a large group of seed oils that have slightly differing dielectric properties due to various fatty acid compositions [54]. Esters with longer fatty acid chains tend to have a higher breakdown voltage, but longer fatty acid chains are synonymous with higher viscosity [54]. Esters with a higher number of unsaturated fatty acids tend to be more reactive and have lower oxidative stability, but have higher viscosities and pour points [54,62]. In order to achieve suitable oxidative stability and viscosity, there needs to be a careful balance of saturated and unsaturated bonds. Current efforts to modify natural esters lean towards improving the oxidative stability of unsaturated esters along with cold flow properties [62]. At 20 °C, natural esters are three times more viscous than mineral oils, and at 60 °C it was four times, but at temperatures over 100 °C, the viscosity of esters was similar to that of mineral oils [49]. Potential chemical methods that have been applied to modify the esters to improve their oxidative and thermal stability and lower their viscosity and pour points are transesterification, epoxidation, partial hydrogenation, formation of estolides, oligomerization, etc., [19,21] along with the addition of different performance-enhancing additives and diluents.

3.1. Chemical Transformation of Fatty Esters

The chemical transformations performed on vegetable esters can be of two types: reactions to the olefinic functionalities of the fatty acid chains and reactions to the carboxyl groups of the triglyceride esters. Transesterification is a method where the natural long-chain fatty esters are reacted with short-chain alcohols like methanol, ethanol, etc., to form the fatty acid alkyl esters of the respective alcohol. Here, glycerol is removed as a byproduct from the triglyceride backbone [19,23,62]. Once glycerol is removed, the pour point is greatly reduced, along with viscosity [23,62]. However, this lowers the flash point significantly due to a lowering in molecular weight [23]. The viscosity of canola oil was reduced from 33.4 mm2/s to 3.9 mm2/s after transesterification, but the flash point was also reduced from 280 to 167 °C (Table 3) [23]. The addition of side chains onto the individual fatty acids results in a slight increase, by a maximum of 7.0 cSt, in viscosity, but may help to improve oxidative stability [62]. Epoxidation, partial hydrogenation, and oligomerization are the methods in which the reaction is performed in olefinic functional groups [19]. These modifications improve the thermo-oxidative stability and low-temperature properties of the natural esters. Epoxidation is a potential process where reactive carbon–carbon double bonds of the fatty acid chains are modified into epoxy rings by reacting with peracids [63]. The epoxy rings can also be opened by suitable nucleophilic reagents to produce various bio-based products from vegetable oils as per the application requirements. Erhan et al. synthesized a series of derivatives from soybean oil [64,65,66]. They produced lubricant-base stocks from epoxidized soybean oil using acid anhydrides of various chain lengths with improved oxidative stability and low-temperature properties. The fluid was formulated from epoxidized soybean oil by an acid-catalyzed ring-opening followed by esterification of the dihydroxy derivatives with acid anhydrides to attach a side-branched alkyl group. Abdelmalik carried out epoxidation of transesterified palm kernel oil followed by ring-opening reactions using acid anhydrides in the presence of boron trifluoride etherate as a catalyst to produce ester derivatives. The synthesized esters have considerably lower melting points, with a viscosity about four times lower than that of mineral insulating oil, and serve as effective dielectric coolants [62]. Figure 5 shows various ways to chemically transform vegetable oil-based natural ester.

3.2. Improving the Performance of Esters Using Additives/Diluents

The overall performance of the natural esters, however, can be improved significantly by the addition of suitable additives, mineral oils (MOs), or other esters in proper ratios. Viscosity can also be lowered through the use of additives. Benzyl benzoate, when hydrolyzed, generates benzyl alcohol, which can be used to form a dilution with ester [54]. The resulting mixture has a lower viscosity and greater dielectric breakdown [54]. Other additives include mineral oil and other esters, but these are likely to influence the flash point, fire safety, and biodegradability [54]. Esters with a concentration of mineral oil greater than 10% have a flash point lower than 200 °C, which is below the limits for nonflammable classification [67]. Madavan et al. depicted that a decrease in viscosity for both olive oil and coconut oil natural esters has occurred as a result of an increase in the proportion of mineral oil [68]. A similar decrease in viscosity is observed when mineral oil is blended with rapeseed oil [69]. Another approach involves mixing two or more natural esters with different viscosity values to achieve a low-viscosity liquid. Blending rapeseed oil with palm fatty acid ester made it possible to achieve a 51.3% reduction in kinematic viscosity [67]. For fire safety and dielectric properties, an optimum blend of 32.33% methyl ester palm kernel oil and 67.78% refined palm kernel oil was discovered [54]. Blending of mineral oil in natural esters also affects the breakdown voltage of natural esters. Concentrations of mineral oil above 20% resulted in insufficient breakdown voltage (Figure 6) [70]. Breakdown voltage decreases as the percentage of mineral oil increases and reaches a minimum at 85% mineral oil. A further increase in mineral oil increases the breakdown voltage. The pure natural ester (0% mineral oil) showed maximum breakdown voltage (Figure 6).
Attempts to improve the cold-flow properties using pour point depressants have emerged as a significant approach in natural esters. Pour point depressants work by inhibiting the crystal formation of waxes in oils [54], but also affect the acidity and dielectric loss of base oil. Polyacrylate did not affect the acidity of the base liquid but did increase dielectric loss [54]. Hexylnapthalene and polyalphaolefin both showed a slight increase in acidity and dielectric loss [54]. Polymethyl methacrylate (PMMA) was the only depressant that did not have a significant effect on acidity or dielectric loss [23,54,67]. It has been shown to lower the pour point by 10 °C when less than 1% by weight is added [23]. Ethyl acetate was shown to be capable of depressing the pour point by 10 °C when 10% of PMMA by volume is added, but the downside is the large decrease in dielectric breakdown from 60 kV to 20 kV [71]. Different antioxidant additives like propyl gallate, citric acid, tert-butylhydroquinone (TBHQ), butylated hydroxytoluene (BHT), etc., are known to enhance the oxidation stability of natural esters [72,73].
Recently, it has been found that blending nanoparticles with natural esters improved tribological properties significantly along with thermal management. Nugroho et al. disclosed that palm oil-based fluids enriched with nanoparticles present a promising eco-friendly alternative to mineral oils that meet the thermal and tribological demands of EVs. Integrating nanoparticles such as graphene, titanium dioxide, and aluminum oxide into palm oil-based lubricants resulted in a reduction in the coefficient of friction by 26–34% and wear by 13–30% [25]. Their study demonstrated that nanoparticle-enriched biolubricants offer a viable, eco-friendly alternative to conventional lubricants, lowering the environmental impact by reducing CO2 emissions, energy consumption, and thermal management of modern EVs. The investigation carried out by Oparanti et al. demonstrated promising results in terms of physicochemical and dielectric properties enhancement by incorporating different nanoparticles into the base liquid of natural esters. The addition of oleic acid-coated Fe2O3 nanoparticles enhanced the dielectric property and breakdown voltage (19.8%) of rapeseed oil [23].

4. Promising Esters

Canola oil is one of many esters with a strong potential to replace mineral oil [74]. Canola oil outperformed other oils such as rice bran, palm, corn, and sunflower oil in terms of breakdown voltage, flash point, pour point, and specific heat [74]. Canola oil’s low pour point compared to other esters makes it suitable for use in most cold regions as defined by standards IEC 62770, IEEE 57.147, and ASTM 6871. However, there are regions of Canada and Russia where the temperature reaches below −30 °C, and therefore, canola oil may be unsuitable [74]. Although canola oil has a high kinematic viscosity at 35.14 mm2/s in comparison to mineral oil at 12 mm2/s at 40 °C, it is still well below the IEEE limit of 50 mm2/s [74]. Canola oil also has shown oxidative stability potential, comprising >60% oleic acid, with some genetic variants containing up to 75% oleic acid [74,75]. Oleic acids are monounsaturated fatty acids, which are known to have greater oxidative stability [76]. However, canola oil also contains 27% polyunsaturated fatty acids, which counteract any resistivity susceptibility to oxidation but are still more stable compared to oils like soybean oil with high polyunsaturated fatty acids [54].
Other oils with promising oxidation stability are high oleic sunflower and soybean oil. The first commercial vegetable oil-based coolant for transformers, BIOTEMP®, contains a blend of oils, including sunflower oil with over 80% oleic content [77]. Another transformer oil mentioned previously, Envirotemp FR3, is also made from oleic base oils [77]. Sunflower and soybean oils are typically composed of high concentrations of polyunsaturated fatty acids, but genetic modifications have led to high oleic variants [75,77]. True high oleic sunflower oils with high monounsaturated and low polyunsaturated content were developed in the late 1980s with over 80% oleic content [75,78]. High oleic soybean oil has also been developed in the last few decades and is expected to become the largest high oleic oil product in the next few years [78,79]. Currently, most research on high oleic oils is conducted in the food industry. Jatropha oil is a potential oil with a lower viscosity at 10.45 mm2/s. It has a great flash point, dielectric properties, and a lower pour point [23,54]. However, its low specific heat makes it an unlikely coolant [54].

5. Conclusions

Research on natural esters in transformers provides valuable insight into their potential application in EVs. The ability of the esters to prolong the life of cellulose paper in transformers suggests similar protection for electrical components in EV batteries, motors, and transmission. The effects of the oxidative stability of natural esters have been shown in transformers to be comparable to mineral oil, but there remains a concern about the need for more reliable aging tests and modifications to prevent viscosity changes. Modifications such as transesterification, epoxidation, etc., along with the blending of different additives, can enhance viscosity, pour point, tribological performance, and fire point. The adoption of these modified bio-derived esters in direct immersion cooling systems presents a viable solution for the thermal management of EVs. The benefits of natural esters, including high dielectric strength, fire safety, and environmental friendliness, have made them highly attractive compared to traditional coolants. Continued research and development are necessary to overcome current challenges and optimize esters for widespread use in the rapidly expanding EV market.

Author Contributions

Conceptualization, R.S. and B.K.S.; Resources, S.Z.E., M.I.S. and B.K.S.; Data curation, G.K. and B.K.S.; Writing—original draft, R.S., C.H., G.K. and B.K.S.; Writing—review and editing, R.S., G.K., S.Z.E., M.I.S. and B.K.S.; Supervision, R.S., S.Z.E. and B.K.S.; Funding acquisition, S.Z.E., M.I.S. and B.K.S. All authors have read and agreed to the published version of the manuscript.

Funding

This research work was funded by the United States Department of Agriculture–Agriculture Research Service (USDA-ARS). All opinions expressed in this paper are the authors’ and do not necessarily reflect the policies and views of USDA-ARS.

Conflicts of Interest

Authors Raj Shah and Cindy Huang were employed by the Koehler Instrument Company. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

References

  1. Faraz, A.; Ambikapathy, A.; Thangavel, S.; Logavani, K.; Arun Prasad, G. Battery electric vehicles (BEVs). In Electric Vehicles: Green Energy and Technology; Springer: Singapore, 2021; pp. 137–160. [Google Scholar]
  2. Sanguesa, J.A.; Torres-Sanz, V.; Garrido, P.; Martinez, F.J.; Marquez-Barja, J.M. A review on electric vehicles: Technologies and challenges. Smart Cities 2021, 4, 372–404. [Google Scholar] [CrossRef]
  3. Taghizad-Tavana, K.; Alizadeh, A.A.; Ghanbari-Ghalehjoughi, M.; Nojavan, S. A comprehensive review of electric vehicles in energy systems: Integration with renewable energy sources, charging levels, different types, and standards. Energies 2023, 16, 630. [Google Scholar] [CrossRef]
  4. Butcher, R.; Bradley, N.; Jamieson, M.; Chambers, T. Aspects of Engine Lubricant Operating Conditions and Fuel Economy Differentiation: In-Vehicle Comparisons of Standard Internal Combustion Engine with Two Types of Hybrid Electric; SAE Technical Paper 2024-01-2824; SAE International: Warrendale, PA, USA, 2024. [Google Scholar] [CrossRef]
  5. Arole, K.; Green, M.J.; Liang, H. Thermal and electrical properties of electric vehicle fluids. In Electric Vehicle Tribology, 1st ed.; Cabrera, L.I.F., Erdemir, A., Eds.; Elsevier: Amsterdam, The Netherlands, 2024; pp. 193–206. [Google Scholar]
  6. Sosa, Y. Design requirements and challenges for single electric vehicle fluids. Tribol. Lubr. Technol. 2023, 79, 22–26. [Google Scholar]
  7. McGuire, N.; Writer, S.F. Bubble trouble in electric vehicle fluids. Tribol. Lubr. Technol. 2024, 80, 62–72. [Google Scholar]
  8. Liu, J.; Huang, S.; Chen, H. Recent progress and prospects in oil-immersed battery thermal management system based on single-phase insulating oil: A review. J. Therm. Anal. Calorim. 2024, 149, 4263–4286. [Google Scholar] [CrossRef]
  9. Gangopadhyay, A.; Jost, N.; Mutyala, K.C. Lubrication regimes in battery electric vehicle power unit. In Electric Vehicle Tribology, 1st ed.; Cabrera, L.I.F., Erdemir, A., Eds.; Elsevier: Amsterdam, The Netherlands, 2024; pp. 183–191. [Google Scholar]
  10. Fu, Z.; Zuo, W.; Li, Q.; Zhou, K.; Huang, Y.; Li, Y. Multi-objective optimization of liquid cooling plate partially filled with porous medium for thermal management of lithium-ion battery pack by RSM, NSGA-II and TOPSIS. Energy 2025, 318, 134853. [Google Scholar] [CrossRef]
  11. Fu, Z.; Zuo, W.; Li, Q.; Zhou, K.; Huang, Y.; Li, Y. Performance enhancement studies on the liquid cooling plate fully filled with porous medium for thermal management of lithium-ion battery pack. J. Energy Storage 2025, 116, 116072. [Google Scholar] [CrossRef]
  12. He, L.; Gu, Z.; Zhang, Y.; Jing, H.; Li, P. Review on thermal management of lithium-ion batteries for electric vehicles: Advances, challenges, and outlook. Energ. Fuel 2023, 37, 4835–4857. [Google Scholar] [CrossRef]
  13. Huang, C.; Zhu, H.; Ma, Y.; E, J. Evaluation of lithium battery immersion thermal management using a novel pentaerythritol ester coolant. Energy 2023, 284, 129250. [Google Scholar] [CrossRef]
  14. Roe, C.; Feng, X.; White, G.; Li, R.; Wang, H.; Rui, X.; Li, C.; Zhang, F.; Null, V.; Parkes, M.; et al. Immersion cooling for lithium-ion batteries—A review. J. Power Sources 2022, 525, 231094. [Google Scholar] [CrossRef]
  15. Youssef, R.; Kalogiannis, T.; Behi, H.; Pirooz, A.; Van Mierlo, J.; Berecibar, M. A comprehensive review of novel cooling techniques and heat transfer coolant mediums investigated for battery thermal management systems in electric vehicles. Energy Rep. 2023, 10, 1041–1068. [Google Scholar] [CrossRef]
  16. Liu, Y.H.; Aldan, G.; Huang, X.; Hao, M. Single-phase static immersion cooling for cylindrical lithium-ion battery module. Appl. Therm. Eng. 2023, 233, 121184. [Google Scholar] [CrossRef]
  17. Togun, H.; Aljibori, H.S.S.; Biswas, N.; Mohammed, H.I.; Sadeq, A.M.; Rashid, F.L.; Abdulrazzaq, T.; Zearah, S.A. A critical review on the efficient cooling strategy of batteries of electric vehicles: Advances, challenges, future perspectives. Renew. Sustain. Energy Rev. 2024, 203, 114732. [Google Scholar] [CrossRef]
  18. Claeys, S.G.; Lievens, S.S.; Van De Ven, P.; Zhou, Z.; Miller, S.J.; Schexnaydre, R.J.; Elomari, S. Ester Based Heat Transfer Fluid Useful as a Coolant for Electric Vehicles. U.S. Patent US20120164506A1, 7 November 2011. [Google Scholar]
  19. Karmakar, G.; Ghosh, P.; Sharma, B.K. Chemically modifying vegetable oils to prepare green lubricants. Lubricants 2017, 5, 44. [Google Scholar] [CrossRef]
  20. Mahapatra, A.; Vats, P.; Bambam, A.K.; Kumar, A.; Gajrani, K.K. Sustainable green lubricants. In Performance Characterization of Lubricants, 1st ed.; Kumar, A., Kumar, A., Kumar, A., Eds.; CRC Press: Boca Raton, FL, USA, 2024; pp. 33–47. [Google Scholar]
  21. Sharma, B.K.; Karmakar, G.; Shah, R.; Ghosh, P.; Sarker, M.I.; Erhan, S.Z. Sustainable Lubricant Formulations from Natural Oils: A Short Review. In Green Chemistry and Green Materials from Plant Oils and Natural Acids; Liu, Z., Kraus, G., Eds.; Royal Society of Chemistry: London, UK, 2023; Volume 83, Chapter 10; pp. 170–193. [Google Scholar]
  22. Rafiq, M.; Shafique, M.; Ateeq, M.; Zink, M.; Targitay, D. Natural esters as sustainable alternating dielectric liquids for transformer insulation system: Analyzing the state of the art. Clean Technol. Environ. Policy 2024, 26, 623–659. [Google Scholar] [CrossRef]
  23. Oparanti, S.O.; Rao, U.M.; Fofana, I. Natural esters for green transformers: Challenges and keys for improved serviceability. Energies 2022, 16, 61. [Google Scholar] [CrossRef]
  24. Obebe, E.O.; Hadjadj, Y.; Oparanti, S.O.; Fofana, I. Enhancing the Performance of Natural Ester Insulating Liquids in Power Transformers: A Comprehensive Review on Antioxidant Additives for Improved Oxidation Stability. Energies 2025, 18, 1690. [Google Scholar] [CrossRef]
  25. Nugroho, A.; Kozin, M.; Mamat, R.; Bo, Z.; Ghazali, M.F.; Kamil, M.P.; Puranto, P.; Fitriani, D.A.; Azahra, S.A.; Suwondo, K.P.; et al. Enhancing tribological performance of electric vehicle lubricants: Nanoparticle-enriched palm oil biolubricants for wear resistance. Heliyon 2024, 10, e39742. [Google Scholar] [CrossRef]
  26. Dekkers, M.; Ebrahimiazar, M.; Kazemi, A.; Zargartalebi, M.; Sinton, D. Screening biodegradable alternatives to mineral oil coolants. Energy Convers. Manag. 2025, 338, 119848. [Google Scholar] [CrossRef]
  27. Tormos, B.; Alvis-Sanchez, J. 14-Fluid characteristic and requirements for battery thermal management in battery electric vehicles. In Electric Vehicle Tribology, 1st ed.; Cabrera, L.I.F., Erdemir, A., Eds.; Elsevier: Amsterdam, The Netherlands, 2024; pp. 249–264. [Google Scholar]
  28. Zhao, C.; Cao, W.; Dong, T.; Jiang, F. Thermal behavior study of discharging/charging cylindrical lithium-ion battery module cooled by channeled liquid flow. Int. J. Heat Mass Tran. 2018, 120, 751–762. [Google Scholar] [CrossRef]
  29. TotalEnergies Trials Immersion-Cooled Battery in EV Field Test. Available online: https://www.fuelsandlubes.com/totalenergies-trials-immersion-cooled-battery-in-ev-field-test/ (accessed on 19 July 2022).
  30. Kumaran, A.T.; Hemavathi, S. Optimization of Lithium-ion battery thermal performance using dielectric fluid immersion cooling technique. Process Saf. Environ. Prot. 2024, 189, 768–781. [Google Scholar] [CrossRef]
  31. Sundin, D.W.; Sponholtz, S. Thermal Management of Li-Ion Batteries with Single-Phase Liquid Immersion Cooling. IEEE Open J. Veh. Technol. 2020, 1, 82–92. [Google Scholar] [CrossRef]
  32. Chen, D.; Jiang, J.; Kim, G.; Yang, C.; Pesaran, A. Comparison of different cooling methods for lithium ion battery cells. Appl. Therm. Eng. 2016, 94, 846–854. [Google Scholar] [CrossRef]
  33. Liu, J.; Chen, H.; Yang, M.; Huang, S.; Wang, K. Comparative study of natural ester oil and mineral oil on the applicability of the immersion cooling for a battery module. Renew. Energy 2024, 224, 120187. [Google Scholar] [CrossRef]
  34. Nogueira, T.; Carvalho, J.; Magano, J. Eco-friendly ester fluid for power transformers versus mineral oil: Design considerations. Energies 2022, 15, 5418. [Google Scholar] [CrossRef]
  35. Jacob, J.; Preetha, P.; Thiruthi Krishnan, S. Review on natural ester and nanofluids as an environmental friendly alternative to transformer mineral oil. IET Nanodielectr. 2020, 3, 33–43. [Google Scholar] [CrossRef]
  36. Deshpande, Y.V.; Raut, S.; Madankar, T.A.; Andhare, A. Influence of biodegradable coolants on machinability improvement—A review. Adv. Mater. Process. Technol. 2024, 10, 3229–3264. [Google Scholar] [CrossRef]
  37. Rallabandi, S.; Issac Selvaraj, R.V. Advancements in battery cooling techniques for enhanced performance and safety in electric vehicles: A comprehensive review. Energy Technol. 2024, 12, 2301404. [Google Scholar] [CrossRef]
  38. Rodríguez, E.; Rivera, N.; Fernández-González, A.; Pérez, T.; González, R.; Hernández Battez, A. Electrical compatibility of transmission fluids in electric vehicles. Tribol. Int. 2022, 171, 107544. [Google Scholar] [CrossRef]
  39. Zhan, J.; Deng, Y.; Ren, J.; Gao, Y.; Liu, Y.; Rao, S.; Li, W.; Gao, Z. Cell design for improving low-temperature performance of lithium-ion batteries for electric vehicles. Batteries 2023, 9, 373. [Google Scholar] [CrossRef]
  40. Daccord, R.; Kientz, T.; Bouillot, A. Aging of a dielectric fluid used for direct contact immersion cooling of batteries. Front. Mech. Eng. 2023, 9, 1212730. [Google Scholar] [CrossRef]
  41. Chavan, S.; Venkateswarlu, B.; Prabakaran, R.; Salman, M.; Joo, S.W.; Choi, G.S.; Kim, S.C. Thermal runaway and mitigation strategies for electric vehicle lithium-ion batteries using battery cooling approach: A review of the current status and challenges. J. Energy Storage 2023, 72, 108569. [Google Scholar] [CrossRef]
  42. Trimbake, A.; Singh, C.P.; Krishnan, S. Mineral Oil Immersion Cooling of Lithium-Ion Batteries: An Experimental Investigation. J. Electrochem. Energy Convers. Storage 2021, 19, 021007. [Google Scholar] [CrossRef]
  43. Jiosseu, J.L.; Mengournou, G.M.; Nkoutcha, E.T.; Imano, A.M. Effect of ageing of monoesters and mineral oil on the propagation of creeping discharges. J. Electrostat. 2023, 123, 103810. [Google Scholar] [CrossRef]
  44. Tiwari, R.; Agrawal, P.S.; Belkhode, P.N.; Ruatpuia, J.V.L.; Rokhum, S.L. Hazardous effects of waste transformer oil and its prevention: A review. Next Sustain. 2024, 3, 100026. [Google Scholar] [CrossRef]
  45. Amin, D.; Walvekar, R.; Khalid, M.; Vaka, M.; Mubarak, N.M.; Gupta, T.C.S.M. Recent Progress and Challenges in Transformer Oil Nanofluid Development: A Review on Thermal and Electrical Properties. IEEE Access. 2019, 7, 151422–151438. [Google Scholar] [CrossRef]
  46. McShane, C.P. Vegetable-oil-based dielectric coolants. IEEE Ind. Appl. Mag. 2002, 8, 34–41. [Google Scholar] [CrossRef]
  47. Rozga, P.; Beroual, A.; Przybylek, P.; Jaroszewski, M.; Strzelecki, K. A review on synthetic ester liquids for transformer applications. Energies 2020, 13, 6429. [Google Scholar] [CrossRef]
  48. Lyutikova, M.N.; Ridel, A.V.; Konovalov, A.A. Dielectric Liquids: Past, Present, Future. Power Technol. Eng. 2023, 57, 615–622. [Google Scholar] [CrossRef]
  49. Park, T.W.; Han, S.H. Numerical analysis of local hot-spot temperatures in transformer windings by using alternative dielectric fluids. Electr Eng. 2015, 97, 261–268. [Google Scholar] [CrossRef]
  50. Li, S.; Zhao, X.; Liao, R.; Yang, L.; Guo, P. Study on ageing characteristics of insulating pressboard impregnated by mineral-vegetable oil. In Proceedings of the IEEE Conference on Electrical Insulation and Dielectric Phenomena, Toronto, ON, Canada, 16–19 October 2016; IEEE: New York, NY, USA, 2016. [Google Scholar]
  51. Fofana, I.; Wasserberg, V.; Borsi, H.; Gockenbach, E. Challenge of mixed insulating liquids for use in high-voltage transformers. II. Investigations of mixed liquid impregnated paper insulation. IEEE Electr. Insul. Mag. 2002, 18, 5–16. [Google Scholar] [CrossRef]
  52. Qin, J.; Peng, X.; Qiu, Q.; Tang, C. A new type of nano APTES-hBN modified palm oil as natural ester insulating oil with upgraded thermal aging characteristics. Renew. Energy 2022, 200, 743–750. [Google Scholar] [CrossRef]
  53. Gutiérrez, C.M.; Alonso, C.O.; Diego, C.F.; Diego, I.F.; Salas, C.O.; Köseoğlu, A.K.; Altay, R.; Fernández, A.O. Compatibility of Esters with Cellulosic Insulation Materials. In Alternative Liquid Dielectrics for High Voltage Transformer Insulation Systems: Performance Analysis and Applications, 1st ed.; Rao, U.M., Fofana, I., Sarathi, R., Eds.; Wiley: Hoboken, NJ, USA, 2021; pp. 43–84. [Google Scholar]
  54. Oparanti, S.O.; Fofana, I.; Jafari, R.; Zarrougui, R.; Abdelmalik, A.A. Canola oil: A renewable and sustainable green dielectric liquid for transformer insulation. Ind. Crops Prod. 2024, 215, 118674. [Google Scholar] [CrossRef]
  55. Martin, D.; Saha, T.; Mcpherson, L. Condition monitoring of vegetable oil insulation in in-service power transformers: Some data spanning 10 years. IEEE Electr. Insul. Mag. 2017, 33, 44–51. [Google Scholar] [CrossRef]
  56. Rao, U.M.; Fofana, I.; Rozga, P.; Picher, P.; Sarkar, D.K.; Karthikeyan, R. Influence of Gelling in Natural Esters Under Open Beaker Accelerated Thermal Aging. IEEE Trans. Dielectr. Electr. Insul. 2023, 30, 413–420. [Google Scholar] [CrossRef]
  57. Lu, W.; Liu, Q.; Wang, Z.D. Gelling behaviour of natural ester transformer liquid under thermal ageing. In Proceedings of the 2012 International Conference on High Voltage Engineering and Application, Shanghai, China, 17–20 September 2012; IEEE: New York, NY, USA, 2012. [Google Scholar]
  58. Tenbohlen, S.; Koch, M. Aging Performance and Moisture Solubility of Vegetable Oils for Power Transformers. IEEE Trans. Power Syst. 2010, 25, 825–830. [Google Scholar] [CrossRef]
  59. McShane, C.P.; Corkran, J.L.; Rapp, K.J.; Luksich, J. Natural Ester Dielectric Fluid Development. In Proceedings of the 2005/2006 IEEE/PES Transmission and Distribution Conference and Exhibition, Dallas, TX, USA, 21–24 May 2006; IEEE: New York, NY, USA, 2006. [Google Scholar]
  60. Shinde, R. Global Footprint of Free Breathing Transformer with Natural Ester. In Proceedings of the 2019 IEEE 20th International Conference on Dielectric Liquids (ICDL), Rome, Italy, 23–27 June 2019; IEEE: New York, NY, USA, 2019. [Google Scholar]
  61. Ortiz, A.; Delgado, F.; Ortiz, F.; Fernandez, I.; Santisteban, A. The aging impact on the cooling capacity of a natural ester used in power transformers. Appl. Therm. Eng. 2018, 144, 797–803. [Google Scholar] [CrossRef]
  62. Abdelmalik, A.A. Chemically modified palm kernel oil ester: A possible sustainable alternative insulating fluid. Sustain. Mater. Technol. 2014, 1–2, 42–51. [Google Scholar] [CrossRef]
  63. Goud, V.V.; Patwardhan, A.V.; Pradhan, N.C. Kinetics of in situ epoxidation of natural unsaturated triglycerides catalysed by acidic ion exchange resin. Ind. Eng. Chem. Res. 2007, 46, 3078–3085. [Google Scholar] [CrossRef]
  64. Hwang, H.S.; Erhan, S.Z. Modification of epoxidized soybean oil for lubricant formulation with improved oxidative stability and low pour point. J. Am. Oil Chem. Soc. 2001, 78, 1179–1184. [Google Scholar] [CrossRef]
  65. Sharma, B.K.; Adhvaryu, A.; Liu, Z.; Erhan, S. Chemical modification of vegetable oils for lubricant applications. J. Am. Oil Chem. Soc. 2006, 83, 129–136. [Google Scholar] [CrossRef]
  66. Moser, B.R.; Sharma, B.K.; Doll, K.M.; Erhan, S.Z. Diester from oleic acid: Synthesis, low temperature properties, and oxidation stability. J. Am. Oil Chem. Soc. 2007, 84, 675–680. [Google Scholar] [CrossRef]
  67. Yang, T.; Wang, F.; Yao, D.; Li, J.; Zheng, H.; Yao, W.; Lv, Z.; Huang, Z. Low-Temperature Property Improvement on Green and Low-Carbon Natural Ester Insulating Oil. IEEE Trans. Dielectr. Electr. Insul. 2022, 29, 1459–1464. [Google Scholar] [CrossRef]
  68. Madavan, R.; Saroja, S.; Karthick, A.; Murugesan, S.; Mohanavel, V.; Velmurugan, P.; Al Obaid, S.; Alfarraj, S.; Sivakumar, S. Performance analysis of mixed vegetable oil as an alternative for transformer insulation oil. Biomass Convers. Biorefin. 2022, 15, 1939–1944. [Google Scholar] [CrossRef]
  69. Radha, R.; Iruthayarajan, M.W.; Bakrutheen, M. Performance of natural high oleic ester based blended oil insulation for transformer. In Proceedings of the 10th IEEE International Conference on Intelligent Systems and Control (ISCO), Coimbatore, India, 7–8 January 2016; pp. 1–5. [Google Scholar] [CrossRef]
  70. Beldjilali, A.; Idir, O.; Saidi-Amroun, N.; Saidi, M.; Moulai, H. Electrical and physicochemical properties and transient charging currents in mineral and vegetable oils mixture. IEEE Trans. Dielectr. Electr. Insul. 2018, 25, 1739–1748. [Google Scholar] [CrossRef]
  71. Abeysundara, D.C.; Weerakoon, C.; Lucas, J.R.; Gunatunga, K.A.I.; Obadage, K.C. Coconut oil as an alternative to transformer oil. In Proceedings of the ERU Symposium, Moratuwa, Sri Lanka; 2001. [Google Scholar]
  72. Pagger, E.P.; Pattanadech, N.; Uhlig, F.; Muhr, M. Dielectric Insulating Liquids. In Biological Insulating Liquids; Springer: Cham, Switzerland, 2023. [Google Scholar] [CrossRef]
  73. Deepa, S.N.; Srinivasan, A.D.; Anusha, M.; Veeramanju, K.T.; Chaitra, M.R. Review on the Addition of Antioxidants and Nanoparticles to Natural Ester as an Alternative to Transformer Oil. In Proceedings of the International Conference on Advances in Renewable Energy and Electric Vehicles, Nitte, India, 22–23 December 2022; Springer Nature: Singapore, 2022; pp. 217–258. [Google Scholar]
  74. Hamid, M.H.A.; Ishak, M.T.; Din, M.F.M.; Suhaimi, N.S.; Katim, N.I.A. Dielectric properties of natural ester oils used for transformer application under temperature variation. In Proceedings of the IEEE International Conference on Power and Energy (PECon), Melaka, Malysia, 28–29 November 2016; IEEE: New York, NY, USA, 2017. [Google Scholar]
  75. Boss, P.; Oommen, T.V. New insulating fluids for transformers based on biodegradable high oleic vegetable oil and ester fluid. In Proceedings of the IEEE Colloquium on Insulating Liquids, Leatherhead, UK, 27 May 1999; IET: London, UK, 2002. [Google Scholar]
  76. Hosier, I.L.; Guushaa, A.; Westenbrink, E.W.; Rogers, C.; Vaughan, A.S.; Swingler, S.G. Aging of biodegradable oils and assessment of their suitability for high voltage applications. IEEE Trans. Dielectr. Electr. Insul. 2011, 18, 728–738. [Google Scholar] [CrossRef]
  77. Rafiq, M.; Lv, Y.Z.; Zhou, Y.; Ma, K.B. Use of vegetable oils as transformer oils—A review. Renew. Sustain. Energy Rev. 2015, 52, 308–324. [Google Scholar] [CrossRef]
  78. Oommen, T.V.; Claiborne, C. Biodegradable Insulating Fluid from High Oleic Vegetable Oils. In Proceedings of the CIGRE Symposium, Paris, France, 15–17 September 1998; CIGRE: Paris, France, 1998. [Google Scholar]
  79. Brentin, R. Soybean Oil: Lubricating Performance. In Proceedings of the 2019 STLE Annual Meeting, Nashville, TN, USA, 19–23 May 2019. [Google Scholar]
Figure 1. Diagram of an air-cooled battery. The arrows dictate direction of flow of air.
Figure 1. Diagram of an air-cooled battery. The arrows dictate direction of flow of air.
Energies 18 04145 g001
Figure 2. Diagram of (a) direct and (b) indirect liquid cooling systems for EV batteries. The arrows dictate direction of flow of coolants.
Figure 2. Diagram of (a) direct and (b) indirect liquid cooling systems for EV batteries. The arrows dictate direction of flow of coolants.
Energies 18 04145 g002
Figure 3. Degree of polymerization of paper after accelerated aging using various oils. Nynas is a mineral oil, Midel 7131 is synthetic ester (pentaerythritol-tetra ester), and the rest are various natural esters [58].
Figure 3. Degree of polymerization of paper after accelerated aging using various oils. Nynas is a mineral oil, Midel 7131 is synthetic ester (pentaerythritol-tetra ester), and the rest are various natural esters [58].
Energies 18 04145 g003
Figure 4. Breakdown voltage at 20% moisture saturation after accelerated aging [58].
Figure 4. Breakdown voltage at 20% moisture saturation after accelerated aging [58].
Energies 18 04145 g004
Figure 5. Different chemical routes for modifying the triglyceride ester of vegetable oil [21].
Figure 5. Different chemical routes for modifying the triglyceride ester of vegetable oil [21].
Energies 18 04145 g005
Figure 6. Changes in breakdown voltage with varying mineral oil and natural ester ratios [70]. (The figures in the x-axis indicate the % of mineral oil in the mixture of ester and mineral oil. 1:MO 100%; 2:95%; 3:90%; 4:85%; 5:50%; 6:25%; 7:15%; 8:10%; 9:5%; 10:0%).
Figure 6. Changes in breakdown voltage with varying mineral oil and natural ester ratios [70]. (The figures in the x-axis indicate the % of mineral oil in the mixture of ester and mineral oil. 1:MO 100%; 2:95%; 3:90%; 4:85%; 5:50%; 6:25%; 7:15%; 8:10%; 9:5%; 10:0%).
Energies 18 04145 g006
Table 1. Comparison of various thermal management systems for EVs [14].
Table 1. Comparison of various thermal management systems for EVs [14].
BTMSThermal Conductivity (Wm−1K−1)Specific Heat Capacity (Jkg−1K−1)Convective Heat-Transfer Coefficient (Wm−2K−1)
Air cooling0.0242100610–100
Indirect Cooling (water–glycol)0.3892332310,000
Immersion cooling single-phase0.129–0.151370–22412000
Immersion cooling double-phase0.075130020,800
Table 3. Physical properties of vegetable oils before and after transesterification [23].
Table 3. Physical properties of vegetable oils before and after transesterification [23].
Vegetable Oils (Normal/
Transesterified)
Kinematic Viscosity at 40 °C (cSt)Pour Point (°C)Flash Point (°C)
Rapeseed oil36.14−8297
Transesterified Rapeseed oil4.615−18175
Jatropha oil32.443≥240
Transesterified Jatropha oil10.450191
Palm Kernel oil44.4926239
Transesterified Palm Kernel oil4.57−6148
Neem oil43.757209
Transestrified Neem oil5.533175
Canola oil33.4−24280
Transestrified Canola oil3.9−25167
Disclaimer/Publisher’s Note: The statements, opinions and data contained in all publications are solely those of the individual author(s) and contributor(s) and not of MDPI and/or the editor(s). MDPI and/or the editor(s) disclaim responsibility for any injury to people or property resulting from any ideas, methods, instructions or products referred to in the content.

Share and Cite

MDPI and ACS Style

Shah, R.; Huang, C.; Karmakar, G.; Erhan, S.Z.; Sarker, M.I.; Sharma, B.K. Potential of Natural Esters as Immersion Coolant in Electric Vehicles. Energies 2025, 18, 4145. https://doi.org/10.3390/en18154145

AMA Style

Shah R, Huang C, Karmakar G, Erhan SZ, Sarker MI, Sharma BK. Potential of Natural Esters as Immersion Coolant in Electric Vehicles. Energies. 2025; 18(15):4145. https://doi.org/10.3390/en18154145

Chicago/Turabian Style

Shah, Raj, Cindy Huang, Gobinda Karmakar, Sevim Z. Erhan, Majher I. Sarker, and Brajendra K. Sharma. 2025. "Potential of Natural Esters as Immersion Coolant in Electric Vehicles" Energies 18, no. 15: 4145. https://doi.org/10.3390/en18154145

APA Style

Shah, R., Huang, C., Karmakar, G., Erhan, S. Z., Sarker, M. I., & Sharma, B. K. (2025). Potential of Natural Esters as Immersion Coolant in Electric Vehicles. Energies, 18(15), 4145. https://doi.org/10.3390/en18154145

Note that from the first issue of 2016, this journal uses article numbers instead of page numbers. See further details here.

Article Metrics

Article metric data becomes available approximately 24 hours after publication online.
Back to TopTop