A State-of-the-Art Review on the Potential of Waste Cooking Oil as a Sustainable Insulating Liquid for Green Transformers

Abstract

Petroleum-based insulating liquids have traditionally been used in the electrical industry for cooling and insulation. However, their environmental drawbacks, such as non-biodegradability and ecological risks, have led to increasing regulatory restrictions. As a sustainable alternative, vegetable-based insulating liquids have gained attention due to their biodegradability, non-toxicity to aquatic and terrestrial ecosystems, and lower carbon emissions. Adopting vegetable-based insulating liquids also aligns with United Nations Sustainable Development Goals (SDGs) 7 and 13, which focus on cleaner energy sources and reducing carbon emissions. Despite these benefits, most commercially available vegetable-based insulating liquids are derived from edible seed oils, raising concerns about food security and the environmental footprint of large-scale agricultural production, which contributes to greenhouse gas emissions. In recent years, waste cooking oils (WCOs) have emerged as a promising resource for industrial applications through waste-to-value conversion processes. However, their potential as transformer insulating liquids remains largely unexplored due to limited research and available data. This review explores the feasibility of utilizing waste cooking oils as green transformer insulating liquids. It examines the conversion and purification processes required to enhance their suitability for insulation applications, evaluates their dielectric and thermal performance, and assesses their potential implementation in transformers based on existing literature. The objective is to provide a comprehensive assessment of waste cooking oil as an alternative insulating liquid, highlight key challenges associated with its adoption, and outline future research directions to optimize its properties for high-voltage transformer applications.

1. Introduction

The operational reliability of a transformer depends on effective electrical insulation and cooling, as direct coil-to-coil contact can lead to sparking, resulting in thermal and electrical breakdown. According to the literature, dielectric insulation failures account for approximately 75% of high-voltage transformer failures [1]. These unforeseen and premature failures, primarily caused by insulation breakdown, impose significant financial burdens due to costly repairs or replacements, revenue losses, and compromised power delivery efficiency [2]. The insulation system in power transformers consists of passive components, which are broadly classified into solid, liquid, and gaseous insulators [3]. The selection of these materials depends on their specific dielectric properties and the intended application within the transformer. Solid insulators include ceramics, epoxy resin, pressboard, Kraft paper, and thermally upgraded Kraft paper. Liquid insulators comprise mineral oil, synthetic esters, silicone oils, and vegetable oils. Gaseous insulators include compressed air and SF₆ gas. Power transformer insulation majorly depends on two fundamental insulating materials, which are solid insulators like Kraft papers and liquid insulators like mineral oils. The primary source of the Kraft paper insulators is majorly from wood pulps, which contain 40–55% cellulose, 15–35% lignin, and 25–40% hemicellulose [4]. This Kraft paper demonstrates exceptional electrical insulation properties, offering mechanical support and insulation between various conductive parts to prevent electrical discharges. Kraft paper also exhibits good thermal stability, which is vital for maintaining performance at high temperatures [5]. Transformer oils, typically composed of mineral oil or synthetic ester, serve multiple functions as a liquid dielectric, providing electrical insulation between energized components to prevent arcing and short circuits [3]. It facilitates cooling by transferring heat away from the windings and core, ensuring optimal operating temperatures. Additionally, the oil serves as a barrier against moisture and air, protecting the insulating materials from degradation and extending the transformer’s lifespan [6]. Mineral oil insulating liquid, derived from refined crude oil hydrocarbons, has been utilized in the electrical industry for over 130 years. It possesses excellent dielectric and cooling properties, along with remarkable oxidation stability, allowing it to function effectively in both hermetically sealed and free-breathing transformers. Its low pour point is an added advantage, enabling optimal performance in subpolar regions. However, mineral oil has a low ignition point, poor heat resistance, and is non-renewable, posing environmental risks in the event of spills [7]. These drawbacks conflict with the United Nations Sustainable Development Goals (SDGs) 7 and 13, which promote affordable and clean energy as well as climate action. As a result, researchers have shifted their focus to green alternatives, such as vegetable-based insulating liquids [8,9,10,11,12,13]. Vegetable-based insulating liquids, commonly known as natural esters, emerged in the late 1990s as an alternative to conventional insulating liquids. Their application has seen significant success, with commercially available examples including FR3, derived from soybean oil, and Midel 1204, based on rapeseed oil [7,14,15]. Natural esters exhibit excellent insulating properties while having minimal environmental impact. They offer strong fire resistance, high breakdown voltages, and improved electric field distribution [16]. Additionally, their high moisture tolerance helps delay the aging of cellulose paper insulation, enhancing the overall longevity of transformer systems [17]. Despite the remarkable properties of natural esters, their widespread adoption remains a challenge due to several factors. These include poor oxidation stability, which limits their use in free-breathing transformers; inadequate fluidity in subpolar regions; high viscosity; and lower breakdown voltage under lightning impulse at long oil gaps compared to mineral oil [18]. Additionally, the high cost of natural esters, from cultivation to the final product, poses a significant barrier to their commercial viability. Over the years, various enhancement techniques have been explored to improve the performance of natural esters, including chemical modifications [19,20] and the use of additives such as pour point depressants [21,22,23], antioxidants [24,25,26,27], viscosity modifiers [9,28,29], and nanoparticles [30,31,32,33,34,35,36,37]. However, despite these advancements, the global application of natural esters in transformers remains limited. A key concern is that natural esters are primarily derived from cultivated plants, and agricultural practices are known to significantly contribute to anthropogenic greenhouse gas emissions. Reports indicate that agriculture accounts for approximately 84% of global nitrous oxide emissions and 52% of methane emissions. Moreover, to meet the growing demand for food and livestock feed, global crop production is projected to increase by 100% by 2050. To mitigate these environmental impacts, there is a growing interest in utilizing waste-derived vegetable oils, such as waste cooking oil (WCO), as a sustainable alternative to fresh vegetable oils. This approach not only reduces food competition but also minimizes the environmental footprint of agricultural production. Recent advancements have demonstrated the successful conversion of waste cooking oil into biodiesel and diesel blends for fueling diesel engines [38,39,40,41]. Extending this concept to transformer insulation, researchers have begun investigating the potential of waste cooking oils as alternative insulating liquids. Studies have shown that properly treated waste cooking oil can meet standard requirements for breakdown voltage and moisture content, although challenges remain in terms of acidity control [42]. The study presented in Ref. [43] demonstrates that appropriate modification and treatment of waste cooking oil (WCO) can effectively reduce its acidity to below 0.06 mgKOH/g, meeting the standard requirement set by ASTM [44]. Furthermore, research findings in Refs. [45,46] indicate that treated waste cooking oil exhibits a breakdown voltage and thermal conductivity that are 48% and 33% higher, respectively, compared to mineral oil, highlighting its potential as a viable alternative insulating liquid. These findings indicate that waste cooking oil has the potential to become a viable alternative insulating liquid in the near future. Given the limited knowledge available on the potential use of waste cooking oil (WCO) as a transformer insulating liquid, this review explores its feasibility as a sustainable alternative. It examines the necessary purification processes, including physical and chemical treatments, to enhance its suitability for insulation applications. Additionally, the study evaluates the cooling and dielectric properties of purified WCO based on existing literature. A life cycle assessment is also discussed to assess its environmental impact and sustainability. While WCO has not yet been adopted for transformer applications, this review aims to provide a comprehensive assessment of its potential, highlight key challenges associated with its implementation, and outline future research directions to optimize its properties for high-voltage transformer insulation.

2. Overview of Plant-Based Oils and Waste Cooking Oils

2.1. Plant-Based Oils

Vegetable oils are primarily obtained from plant seeds and can be categorized as either edible or non-edible. The extraction process varies depending on the plant source and intended application, with different seeds exhibiting distinct oil yield rates. Examples of high-yield oil-producing plants are soybean seed, safflower seed, neem seed, and canola seed. Common extraction methods include solvent extraction, ultrasound-assisted extraction, and mechanical pressing techniques such as hot-press and cold-press. Among these, the hot-press method has been identified as particularly effective, especially for canola oil extraction, as it enhances yield and efficiency during production [9,47]. Figure 1 presents a schematic illustration of the mechanical press extraction method [48]. To ensure the quality of oil is not compromised during the hot-press extraction process, parameter optimization was investigated in Ref. [48], focusing on pressing time and pressure. It was reported that the hydraulic cold-pressed extraction (HCPE) technique achieves a high oil yield of up to 86.31% within a short extraction time of 30 min. Although ultrasound-assisted extraction has been shown to produce a higher oil yield, it requires more time, is less user-friendly, and necessitates an additional solvent extraction step post-processing [48]. Since this work does not primarily focus on oil extraction methods, further details on optimal extraction techniques that preserve both the physical and chemical properties of the oil can be found in Refs. [47,48,49,50,51,52,53,54,55,56,57]. Vegetable oils have been widely utilized in various industries, including the production of fragrances, perfumes, and luxury soaps. Their application in the power sector as an alternative insulating liquid to mineral oil has gained prominence due to their biodegradability and reduced environmental impact in the event of transformer oil spills.

The insulating properties of vegetable-based oils (natural esters) are summarized in Table 1, where they are compared with mineral oil. Notably, natural esters exhibit high fire and flash points, making them a safer choice for use in sensitive environments. Natural esters differ from mineral oils in terms of molecular structure. While natural esters are composed of triglyceride ester molecules, mineral oils consist of aromatic hydrocarbon molecules, as illustrated in Figure 2 [58]. Due to the molecular structure of natural esters, which contain unsaturated fatty acids, natural esters are more prone to oxidation compared to mineral oils. This susceptibility to oxidation remains a key challenge in their application as transformer insulating liquids [26]. It is important to state that while the oxidation mechanisms of mineral oil and natural esters are similar, they differ at the terminal reaction stage. In natural esters, free electrons cause molecules to bond with each other rather than forming non-soluble polar byproducts, as observed in mineral oils [59]. Consequently, oxidation in natural esters primarily results in increased viscosity. Notably, in free-breathing transformers, the viscosity should not exceed a 10% increase over a 10-year operational period [60].

The practical application of natural esters in transformer insulation has been in use for several decades across various regions, including both subpolar and hot climates. However, mineral oil remains the dominant insulating liquid [3,61]. The slow adoption of natural esters can be attributed to factors such as transformer design, regional climate conditions, compatibility issues, and, most notably, cost [62,63]. The high cost of natural esters, despite their environmental benefits, has driven researchers to explore the potential of waste cooking oils as a more affordable and sustainable alternative, bypassing the planting, harvesting, and extraction stages to reduce costs.

Table 1. Comparison of properties of natural esters with mineral oil [8,63,64,65].

Property	Natural Ester	Mineral Oil
Nature	Renewable and biodegradable	Non-renewable and petroleum-derived
Emission profile	Acceptable	Unacceptable
Cost	High	Prices may increase in the long term
Flash point (°C)	310–343	154
Fire point (°C)	300–365	110–185
Viscosity (cSt) at 40 °C	37.6	13
Density (kg/m3) at 20 °C	0.87–0.62	0.83–0.89
Breakdown voltage (kV)	60	45
Dissipation factor (%) 25 °C	0.08	0.02
Dielectric constant 25 °C	3.3	2.4
Pour point (°C)	−22	−40

Table 1. Comparison of properties of natural esters with mineral oil [8,63,64,65].

Property	Natural Ester	Mineral Oil
Nature	Renewable and biodegradable	Non-renewable and petroleum-derived
Emission profile	Acceptable	Unacceptable
Cost	High	Prices may increase in the long term
Flash point (°C)	310–343	154
Fire point (°C)	300–365	110–185
Viscosity (cSt) at 40 °C	37.6	13
Density (kg/m3) at 20 °C	0.87–0.62	0.83–0.89
Breakdown voltage (kV)	60	45
Dissipation factor (%) 25 °C	0.08	0.02
Dielectric constant 25 °C	3.3	2.4
Pour point (°C)	−22	−40

2.2. Waste Cooking Oils

Waste or used cooking oils are edible oils that have been repeatedly utilized for frying, commonly sourced from restaurants, hotels, households, fast food chains, and other food establishments. Figure 3 presents an example of waste cooking oil collected from a local restaurant. These oils are widely available in large quantities across the globe, with European countries alone generating over 10 million tons annually [43]. Figure 4 provides a graphical representation of waste cooking oil production in various countries, with the United States of America being the largest producer of used cooking oils [66,67].

WCO exists in two distinct forms, yellow grease and brown grease, classified based on their free fatty acid (FFA) content. Yellow grease refers to waste cooking oil with FFA levels below 15%, while brown grease contains FFA levels exceeding 15% [67]. During food frying, particularly at temperatures between 170 and 200 °C, several chemical reactions occur in the oil, altering its composition and quality. The first reaction is hydrolysis, where moisture from the food breaks down triglycerides into glycerol, free fatty acids, monoglycerides, and diglycerides. This process affects the oil’s stability and can lead to undesirable changes in taste and texture. As frying continues, primary oxidation takes place, forming unstable hydroperoxides that eventually degrade into secondary oxidation products such as aldehydes, ketones, and short-chain fatty acids, contributing to rancidity and unpleasant odors. With prolonged reuse, these secondary products can further react to form polymeric compounds, increasing the oil’s viscosity and deteriorating its frying efficiency. At elevated temperatures, the oil also undergoes thermal polymerization, resulting in the formation of high molecular weight cyclic fatty acid monomers, dimers, and oligomers, which further compromise the quality and safety of the oil [68]. Continuous reuse of cooking oil, often driven by economic factors, poses significant health risks due to the accumulation of toxic compounds and oxidized materials such as polyglycerols, acylglycerols, dimeric acids, and polymeric acids [69]. The presence of these harmful substances makes reused oil unsuitable for consumption, as it has been linked to an increased risk of cardiovascular diseases, carcinogenicity, mutagenicity, genotoxicity, and cancer [68]. Given these serious health concerns, the reuse of cooking oil in households and the food industry is strongly discouraged. Figure 5 illustrates the relationship between waste cooking oil consumption and cardiovascular disease [68].

Concerns regarding the reuse of cooking oils have led to their widespread disposal, often inappropriately. Studies have shown that a significant portion of WCO is discarded through drainage systems, leading to water treatment issues such as sewer diameter reduction and blockages in infrastructure [70]. Additionally, more than 16.5 million tons of WCO are generated annually, with much of it being conventionally disposed of into sewage systems or discarded alongside municipal solid waste in landfills [71,72,73]. The spillage of WCO in landfills can contribute to water pollution due to its low solubility, which affects aquatic ecosystems by reducing dissolved oxygen levels and limiting sunlight penetration. Its immiscibility with water can also create a barrier that isolates soil from essential air and moisture, leading to the death of beneficial soil organisms such as earthworms and bacteria. Furthermore, the presence of WCO in the environment can attract rats and vermin, promoting their proliferation and posing further risks to human health [73]. Due to the challenges associated with waste cooking oil disposal, several innovative valorization strategies have emerged to convert WCO into green energy, thereby mitigating its environmental impact and reducing the carbon footprint. This approach aligns with multiple United Nations Sustainable Development Goals (SDGs), including SDG 3 (Good Health and Well-being), SDG 7 (Affordable and Clean Energy), SDG 11 (Sustainable Cities and Communities), SDG 13 (Climate Action), SDG 14 (Life Below Water), and SDG 15 (Life on Land) [69]. Historically, WCO was used as livestock feed in Europe, but this practice was banned approximately 25 years ago due to its association with bovine spongiform encephalopathy (BSE) [74]. While WCO has been utilized in soap production, its application raises environmental concerns as wastewater disposal can introduce polycyclic aromatic hydrocarbons (PAHs) into the surroundings [75]. Biodiesel has been an alternative green transportation fuel over time; however, the recent conflict between Russia and Ukraine has led to a shortage of raw materials for biodiesel production, prompting several European countries to turn to waste cooking oils as a source for green transportation fuel [76]. Waste cooking oil has been a source of biofuel, contributing to approximately 5% of the global market, and is projected to reach a market size of USD 8.9 billion by 2028 [76]. While WCO-derived biodiesel is considered a sustainable fuel alternative, its use in diesel engines contributes to nitrogen oxide (NO_x) emissions, which are known to drive global warming and air pollution [77,78]. In contrast, when considering the use of chemically treated WCO in transformer applications, it can be seen as environmentally friendly since it would be used in a sealed tank with no combustion involved. Unlike biodiesel, which emits nitrogen oxides (NO_x) and contributes to air pollution through combustion, treated WCO in transformers does not release harmful emissions into the environment. The sealed system ensures that the oil remains contained, minimizing the risk of leakage or contamination and preventing pollution. Therefore, a promising and environmentally responsible application of treated WCO is its use in transformer insulation systems, providing a sustainable solution with minimal impact on pollution, as it remains a non-toxic, biodegradable liquid that does not compromise the surrounding environment.

3. Chemical Properties of Waste Cooking Oils and Purification Process

In this section, the properties of waste cooking oil are examined, along with the purification methods required to meet the standard criteria for use as insulating liquids. The chemical composition of WCO is shown in

Table 2. Fatty acids composition of waste cooking oil [43,66].

Fatty Acid Type	Fatty Acid Name	Percentage of Waste Cooking Oil
Saturated Fatty Acids	Palmitic acid	38.35
	Stearic acid	4.33
	Myristic acid	1.03
	Lauric acid	0.34
	Heneicosanoic acid	0.07
Sub-Total		44.12
Monounsaturated Fatty Acids	Oleic acid	43.67
	Cis-11-Eicosenoic acid	0.16
Sub-Total		43.83
Polyunsaturated Fatty Acids	Linoleic acid	11.39
	γ-Linolenic acid	0.37
	Linolenic	0.29
Sub-Total		12.05
Total	-	100

[66], with oleic acid being the predominant component. Several studies have confirmed that the properties of used cooking oils differ significantly from those of unused oils, particularly in their physicochemical characteristics.

Table 3. Physicochemical properties of used and unused vegetable oils [43,44,66,67,82,83].

Properties	Waste Cooking Oil	Unused Vegetable Oil	Natural Ester Insulating Oils	Standard
Density (g/cm3)	0.94	0.89 [66]	$\le 0.92$	ASTM [84]
Viscosity (mm2/s) @ 40 °C	44.1	39.99 [66]	$\le 50$	IEEE [85]
Moisture (ppm)	1130	105.70–125.8 [67]	$\le 200$	ASTM [86]
Acid value (mgKOH/g)	4.03	0.3 [66]	Max. 0.6	IEC [87]
Flash point (°C)	269	$>220$ [43]	$\ge 275$	ASTM [86]
Pour point (°C)	9	−18 [82]	$\le -10$	IEC [87], ASTM [86]
Breakdown voltage (kV)	27	40 [82]	$\ge 35$	ASTM [88]
Physical color rating	L5.0	Clear and free from suspended particles [43]	Max. 1.0	IEEE C57.147 [85]

compares the physicochemical properties of both used and unused cooking oils with those of standard natural ester insulating liquids. The results indicate that WCO possesses inferior values in key parameters such as moisture content, acidity, flash point, pour point, appearance, and breakdown voltage, rendering it unsuitable for transformer insulation in its raw form. Therefore, proper purification and treatment are essential to upgrade the quality of waste cooking oil before it can be considered for insulation applications in power transformers. It is to be mentioned that there are several modes of recycling waste cooking oils, which are BET (Biodiesel Enterprise Takeback) and TPT (Third-party Takeback), with the United States of America using the TPT and countries like Brazil using the BET [79]. The different approaches used by different countries in the collection of these materials have been extensively documented in Refs. [79,80,81]. During the collection of waste cooking oil, it is crucial to maintain the properties of the oil and avoid contamination, which could interfere with the properties of the oil and increase the cost of purification. To ensure the quality of WCO during transportation for insulation or other applications, proper storage and handling measures must be adopted. The use of sealed, leak-proof, and corrosion-resistant containers, such as food-grade drums or tanks, prevents contamination from external impurities, moisture, or microbial growth. Additionally, strict handling protocols, including proper labeling, temperature control, and hygiene standards, help maintain the oil’s integrity, ensuring its suitability for reuse or further processing [82].

3.1. Purification of Waste Cooking Oil

Purification/filtration is a critical step in the valorization of waste cooking oil, as it removes impurities such as food particles, polymers, carbon residues, and other solids that can interfere with downstream processes like biodiesel production. Several filtration techniques are used depending on the scale and quality requirements. The purification of waste cooking oils can be achieved through various methods, depending on the intended application. Common techniques employed in the purification process include filtration, drying, and acid reduction, either through additives or esterification, as shown in

Table 4. Summary of some pretreatment processes of waste cooking oil.

S/n	Method of Purification	Free Fatty Acids Reduction	Reference
1	Preheated the oil at 50 °C, 1 atm, followed by filtration using cotton cloth.		[93]
2	The oil samples were preheated at 100 °C and filtered.	Metal-catalyzed glycerolysis using zinc dust, ferric chloride, manganese chloride, and stannous chloride, along with supplementation of glycerol and acid treatment using conc. H2SO4, HCl and H3PO4.	[98]
3	The waste cooking oil was preheated and filtered using filter paper (110 mm). Oil degumming at 60 °C, for 30 min/800 rpm using concentrated H3PO4 (2% v/v).	Acid catalysis esterification using 2% (v/v) conc. H2SO4 and methanol at 60 °C with a stirring speed of 1000 rpm for 1 h.	[99]
4	The oil was heated at 60 °C and filtered under vacuum using a cellulose filter paper.	A total of 200 g of oil was heated to 65 °C with stirring, followed by the addition of methanol and catalyst (6:1 oil-to-methanol molar ratio, 5% v/v catalyst).	[92]
5	Vacuum filtration of oil followed by dehumidification at 105 °C.	Free fatty acid treatment using an adsorbent (coal ash). Used cooking oil with 2 g of activated coal and stirred for 60 min at 130 °C.	[76]
6	Waste cooking oil filtration was performed using a vacuum filtration system having a mesh size of 1–5 microns, followed by dehumidification at 120 °C.	The free fatty acids were removed using activated coal ash at a 1:10 ratio at 130 °C for 1 h.	[100]

and also Figure 6. The initial and most critical step is filtration, which can be conducted using vacuum filtration or standard filter paper. For applications such as insulating liquids, where high purity is essential, it is advisable to use low-porosity filter paper to ensure the effective removal of fine particulates. Another effective method for purifying waste cooking oil involves the use of commercial products like Magnesol^® filter media, which provides an efficient and easy filtration process. This method is advantageous as it effectively removes off-flavors, colors, odors, free fatty acids, and total polar materials, significantly improving the quality of the reclaimed oil [89,90,91]. During the dehumidification process, maintaining oil quality is crucial; hence, drying should be performed at moderate temperatures to avoid thermal degradation. This process is often carried out under vacuum conditions, as demonstrated in studies by Refs. [92,93]. The final stage in the purification process, before transesterification, involves the reduction in free fatty acids (FFAs) to prevent saponification reactions during base-catalyzed transesterification. Among the various methods available, the most commonly employed approach for FFA reduction is acid-catalyzed esterification, typically using concentrated sulfuric acid (H₂SO₄) and methanol, as illustrated in Figure 7 [94]. The free fatty acids react with the alcohol in the presence of a catalyst to produce fatty acid methyl esters (FAMEs) and water. It is important to reduce the FFA content to a value less than or equal to 1% to ensure excellent phase separation and to minimize soap formation during the subsequent base-catalyzed transesterification reaction [21,95]. Other catalysts reported in the literature for FFA reduction include hydrochloric acid (HCl), phosphoric acid (H₃PO₄), and coal ash [96,97,98]. Following the pretreatment process, the treated oils undergo transesterification to reduce viscosity by removing glycerol, resulting in a more suitable liquid for intended applications.

The purification of waste cooking oils can effectively utilize conventional vegetable or fat oil refining methods, which include degumming, neutralization, and bleaching. Among these, the total degumming process, commonly referred to as the Dijkstra and Opstal method, has been widely adopted by several researchers [21,31,32,101,102,103,104,105]. In this method, a citric acid solution is added to preheated oil (at approximately 70 °C) and stirred for a few minutes (15–30 min) to facilitate the removal of phospholipids and other impurities [8]. Citric acid acts as a chelating agent by binding to metal ions such as magnesium, calcium, and iron in the oil, which destabilizes and removes the non-hydratable phospholipids (NHPs). This process converts the NHPs into hydratable phospholipids (HPs), which are easier to remove, leading to the formation of a gum phase. The gum can then be separated through centrifugation or water washing. The second step is the neutralization of the degummed oil, achieved by adding an NaOH solution (typically 8–9.5%) at around 70 °C and stirring for 30 min. The soap formed during this reaction is then separated from the oil through centrifugation. To remove any residual soap, the oil is further washed with demineralized water. Lastly, the neutralized oil is bleached using activated clay by adding 4% (w/w) of the clay into the neutralized oil and stirring for approximately 10–20 min at 70 °C. Common examples of bleaching clay in the literature are fuller’s earth, Tonsil Supreme 110 FF, and Tonsil Standard 310 FF [19,21,31,106]. The activated clay adsorbs impurities such as pigments, trace metals, and other contaminants. After this process, the activated clay and the impurities are removed through filtration or centrifugation, leaving behind a cleaner, lighter-colored oil that is more suitable for further processing, such as transesterification.

3.2. Transesterification of Waste Cooking Oil

The transesterification reaction is the reaction between the treated oil and alcohol, mostly methanol due to availability and cost, in the presence of a catalyst, as presented in Figure 8. In addition, the reaction using methanol is often considered faster than ethanol, and also, separation is more difficult when ethanol is used for the transesterification reaction [107,108]. The types of catalysts used during this process could be homogenous, heterogenous, or enzymatic [109]. The efficiency of a homogeneous catalyst is high with low reaction time, low temperature requirement, and high yield, as presented in Table 5, but due to the homogeneous nature, it requires several rinses to remove traces of soap in the oil. This may introduce moisture and impurities in the oil, consequently affecting the quality of the oil as an insulating liquid and possibly corroding the component of the transformer. This may be the reason for the high conductivity of fatty acid methyl ester reported by Refs. [101,106]. The common examples of homogeneous catalysts are NaOH and KOH. However, the heterogeneous catalyst does not dissociate in the oil, therefore providing easy separation and no possibility of corrosion on equipment components. Due to the possibility of reuse, the heterogeneous catalyst has cost benefits over the homogeneous catalyst [109,110]. The significant disadvantage of heterogeneous catalysts is the high reaction temperature and duration. Examples of heterogeneous catalysts are CaO, MgO, BaO, SrO, titanium oxide, and zinc oxide. It is to be mentioned that base heterogeneous catalysts have gained high applicability due to their high activity and stability; however, they require high reaction temperatures, which may hinder their application for industrial purposes. The metal-doped catalysts are now finding application in the replacement of base heterogeneous catalysts (TiO₂–MgO, SO₂²⁻/ZrO₂/SiO₂) [95] due to their low reaction time and excellent yield, as presented in Table 5. The enzyme-based catalyst is also used in the transesterification reaction. The advantage of this catalyst is that it requires no difficult separation of products, it can be reused, and it wastes no water, consequently reducing the cost and environmental pollution [111,112]. In addition, this reaction is not affected by the high free fatty acid content of the base oil. However, these catalysts are expensive, which may eventually affect the production cost during transesterification. Due to strenuous activity in the adoption of catalysts in the transesterification process (washing of biodiesel, separation of by-products, etc.), the application of the supercritical approach (supercritical alcohols) has surfaced as an alternative, as it requires no application of catalysts, is not affected by FFA values, and rapidly transforms oil into FAME. In addition, due to low reaction time, it requires low energy consumption [95,113]. The comparative overview of transesterification methods during fatty acid methyl ester production is presented in Table 6.

Based on the data presented in Table 5, it is evident that the percentage yield varies irrespective of the reaction type. This indicates that, apart from the reaction type, the reaction conditions, including the process variables, also play a crucial role. Therefore, optimizing these reaction parameters is essential for achieving a high percentage yield during fatty acid methyl ester production. A summary of the key factors that can affect the percentage yield during FAME production is outlined in Table 7 [20,95]. Among the crucial parameters that affect the yield of FAME during the transesterification process is the reaction temperature, as it significantly influences the reaction kinetics [114,115,116]. This can be attributed to the reduction in oil viscosity at elevated temperatures, which consequently enhances the reaction rate. The investigation reported in Ref. [117] demonstrated that the percentage yield of FAME increases with temperature, reaching an optimum at 60 °C. In addition to temperature, the molar ratio is another critical parameter that significantly influences the overall yield of FAME. A higher alcohol-to-oil ratio shifts the reaction equilibrium toward ester formation, thereby enhancing the conversion efficiency [118]. However, excessively high molar ratios can lead to difficulties in product separation and increased production costs. It has been reported that the optimal molar ratio for achieving maximum yield typically lies within the range of 6:1 to 9:1 [95]. Moreover, the type and concentration of catalyst employed in the process play a pivotal role in determining the reaction rate and final yield. Alkaline catalysts, such as sodium hydroxide and potassium hydroxide, are widely used due to their high catalytic activity and low cost. However, their application is limited when the feedstock contains a high percentage of free fatty acids (FFAs), as this leads to soap formation and a consequent reduction in yield. In such cases, acid catalysts or enzyme-based catalysts may be more suitable alternatives [94].

Other operational parameters, including mixing intensity, reaction time, and reaction mode (batch or continuous), also influence the efficiency of the transesterification process. Adequate mixing enhances mass transfer between immiscible reactants, while sufficient reaction time ensures complete conversion. Furthermore, continuous processes are generally more efficient, cost-effective, and suitable for large-scale production compared to batch operations [20,95].

Table 5. Summary of some transesterification reaction processes using homogenous, heterogenous, enzymatic, and supercritical methods with their percentage yield.

S/n	Transesterification Process	Reaction Temperature and Time	Percentage Yield (%)	Reference
	Homogenous catalysis
1.	The reaction was performed using an alkali catalyst (NaOH, NaOCH3, and KOH) and an enzyme (free and immobilized lipase produced from Bacillus subtilus) with methanol	50 °C/1.5 h for alkaline catalyst and 40 °C/72 h for enzyme	1% KOH yields 97.01% for alkaline catalyst, while 4% w/w enzyme yielded a maximum of 91.61%	[98]
2.	Potassium hydroxide was used as a catalyst, considering molar ratios of 8:1, 10:1, and 12:1	60 °C/1 h at 1000 rpm	The optimal percentage yield of 92.4% was obtained when 0.5 wt.% of KOH and 90 min reaction time were considered	[99]
	Heterogenous catalysis
3	2.5 wt.% catalyst (NaOH/CaO, AlCl3, and NaOH) in methanol (oil-to-methanol ratio 6:1). Excess methanol removed via rotary evaporation	Stirred at 600 rpm for 2 h at 60–65 °C	i. CaO-supported catalyst (from carbide slurry waste): 92.2 ± 0.31 ii. AlCl3: 90.2 ± 0.57 iii. NaOH: 89.7 ± 0.16	[92]
4	Transesterification reaction using CaO and considering the molar ratio of 6:1	Stirred at a speed of 300–400 rpm and 65 °C for 3 h	-	[76]
5	Transesterification reaction using a heterogeneous catalyst, calcium diglyceroxide, with a methanol to WCO molar ratio of 7.46:1	62 °C in the presence of a microwave	Catalyst loading of 1.03% (w/w of WCO) yields biodiesel of 94.86%	[119]
6	7% of CaO (w/w) for the transesterification of sunflower oil with a methanol-to-oil ratio of 6:1	65 °C and 60 min of reaction time, stirred at 500 rpm	Almost 100% yield was reported	[120]
7	10% wt.% of TiO2-MgO with a 50:1 methanol-to-oil ratio	Stirred at 1500 rpm at 160 °C for 6 h	92.3	[121]
8	6wt.% of SO22−/SnO2/SiO2	Stirred at 350 rpm at 150 °C for 1 h	81.4	[122]
	Enzymatic catalysis
9	Transesterification of waste baked duck oil with methanol using Novozym 435 in tert-butyl medium as a catalyst.	45 °C	85.4	[112]
10	50 wt.% of Novozym 435 in waste fish oil with an ethanol-to-oil ratio of 35.45:1	35 °C for 8 h	82.91	[123]
11	40 wt.% of Novozym 435® in WCO with a 6:1 methanol-to-oil ratio.	50 °C for 14 h	72.0	[124]
12	7.5 wt.% of lipase from porcine pancreas in WCO at a methanol-to-oil ratio of 9:1	40 °C for 10 h	92.33	[125]
	Supercritical reaction
13	Canola oil at a 40:1 ethanol-to-oil ratio	350 °C for 30 min. and 200 bar	93.7	[126]
14	WCO at 41:1 methanol-to-oil ratio	287 °C for 30 min	99.6	[127]
15	Rapeseed oil at a 42:1 methanol-to-oil ratio	350 °C for 15 min and 120 bar	93.0	[128]

Table 5. Summary of some transesterification reaction processes using homogenous, heterogenous, enzymatic, and supercritical methods with their percentage yield.

S/n	Transesterification Process	Reaction Temperature and Time	Percentage Yield (%)	Reference
	Homogenous catalysis
1.	The reaction was performed using an alkali catalyst (NaOH, NaOCH3, and KOH) and an enzyme (free and immobilized lipase produced from Bacillus subtilus) with methanol	50 °C/1.5 h for alkaline catalyst and 40 °C/72 h for enzyme	1% KOH yields 97.01% for alkaline catalyst, while 4% w/w enzyme yielded a maximum of 91.61%	[98]
2.	Potassium hydroxide was used as a catalyst, considering molar ratios of 8:1, 10:1, and 12:1	60 °C/1 h at 1000 rpm	The optimal percentage yield of 92.4% was obtained when 0.5 wt.% of KOH and 90 min reaction time were considered	[99]
	Heterogenous catalysis
3	2.5 wt.% catalyst (NaOH/CaO, AlCl3, and NaOH) in methanol (oil-to-methanol ratio 6:1). Excess methanol removed via rotary evaporation	Stirred at 600 rpm for 2 h at 60–65 °C	i. CaO-supported catalyst (from carbide slurry waste): 92.2 ± 0.31 ii. AlCl3: 90.2 ± 0.57 iii. NaOH: 89.7 ± 0.16	[92]
4	Transesterification reaction using CaO and considering the molar ratio of 6:1	Stirred at a speed of 300–400 rpm and 65 °C for 3 h	-	[76]
5	Transesterification reaction using a heterogeneous catalyst, calcium diglyceroxide, with a methanol to WCO molar ratio of 7.46:1	62 °C in the presence of a microwave	Catalyst loading of 1.03% (w/w of WCO) yields biodiesel of 94.86%	[119]
6	7% of CaO (w/w) for the transesterification of sunflower oil with a methanol-to-oil ratio of 6:1	65 °C and 60 min of reaction time, stirred at 500 rpm	Almost 100% yield was reported	[120]
7	10% wt.% of TiO2-MgO with a 50:1 methanol-to-oil ratio	Stirred at 1500 rpm at 160 °C for 6 h	92.3	[121]
8	6wt.% of SO22−/SnO2/SiO2	Stirred at 350 rpm at 150 °C for 1 h	81.4	[122]
	Enzymatic catalysis
9	Transesterification of waste baked duck oil with methanol using Novozym 435 in tert-butyl medium as a catalyst.	45 °C	85.4	[112]
10	50 wt.% of Novozym 435 in waste fish oil with an ethanol-to-oil ratio of 35.45:1	35 °C for 8 h	82.91	[123]
11	40 wt.% of Novozym 435® in WCO with a 6:1 methanol-to-oil ratio.	50 °C for 14 h	72.0	[124]
12	7.5 wt.% of lipase from porcine pancreas in WCO at a methanol-to-oil ratio of 9:1	40 °C for 10 h	92.33	[125]
	Supercritical reaction
13	Canola oil at a 40:1 ethanol-to-oil ratio	350 °C for 30 min. and 200 bar	93.7	[126]
14	WCO at 41:1 methanol-to-oil ratio	287 °C for 30 min	99.6	[127]
15	Rapeseed oil at a 42:1 methanol-to-oil ratio	350 °C for 15 min and 120 bar	93.0	[128]

Table 6. Summary and comparison of reaction types used in FAME production [81,94,95,129].

Properties	Homogeneous Base Catalysis (e.g., NaOH, KOH)	Heterogeneous Base Catalysis (e.g., CaO, MgO, SrO, TiO2)	Enzyme Catalysis (e.g., Lipase, Novozym 435)	Supercritical Method (e.g., Supercritical Methanol)
Catalyst Solubility	Soluble	Insoluble	Immobilized or soluble (enzyme)	No catalyst required
Reaction rate	Very fast	Moderate–fast	Slow–moderate	Very fast (under high temp/pressure)
Operating Conditions	Mild temperature (60–70 °C), low pressure	Higher temperature (>80 °C)	Mild temperature (25–45 °C), atmospheric pressure	High temp (200–350 °C), high pressure (100–200 bar)
FFA Tolerance	Poor tolerance (<2% FFA)	Poor to moderate (<2% FFA ideal)	Excellent tolerance (high FFA tolerated)	Excellent (not affected by FFA or water)
Soap Formation	High if FFA >2%	Possible at high FFA	None	None
Product Purity	Affected by soap, impurities, and moisture	Higher purity	Very high purity	Very high purity
Catalyst Recovery	Not reusable, requires extensive washing	Easily separable and reusable	Reusable after immobilization	Not applicable
Environmental Impact	Generates soap, wastewater	Low (less effluent generation)	Eco-friendly and clean process	Eco-friendly, no catalyst waste
Corrosion Risk	High (residual alkali, moisture)	Negligible	None	None
Cost Implication	Low catalyst cost, high purification cost	Higher catalyst cost, lower operational cost	High enzyme cost, requires immobilization	High energy and equipment cost
Reusability	Not reusable	Reusable for several cycles	Requires immobilization for reusability	Not applicable
Separation of Products	Difficult (due to soap, water)	Easy	Very easy (no side products)	Easy due to one-step conversion
Industrial Viability	Widely used, but purification is a challenge	Promising, but the temperature requirements are high	Limited by cost, still under development	Technically efficient but expensive
Advancements	Limited	Metal-doped heterogeneous catalysts (e.g., TiO2–MgO, SO42−/ZrO2/SiO2)	Immobilized enzymes, membrane reactor, ultrasound/microwave-assisted	Use of ultrasound, reactive distillation, and membrane reactors

Table 6. Summary and comparison of reaction types used in FAME production [81,94,95,129].

Properties	Homogeneous Base Catalysis (e.g., NaOH, KOH)	Heterogeneous Base Catalysis (e.g., CaO, MgO, SrO, TiO2)	Enzyme Catalysis (e.g., Lipase, Novozym 435)	Supercritical Method (e.g., Supercritical Methanol)
Catalyst Solubility	Soluble	Insoluble	Immobilized or soluble (enzyme)	No catalyst required
Reaction rate	Very fast	Moderate–fast	Slow–moderate	Very fast (under high temp/pressure)
Operating Conditions	Mild temperature (60–70 °C), low pressure	Higher temperature (>80 °C)	Mild temperature (25–45 °C), atmospheric pressure	High temp (200–350 °C), high pressure (100–200 bar)
FFA Tolerance	Poor tolerance (<2% FFA)	Poor to moderate (<2% FFA ideal)	Excellent tolerance (high FFA tolerated)	Excellent (not affected by FFA or water)
Soap Formation	High if FFA >2%	Possible at high FFA	None	None
Product Purity	Affected by soap, impurities, and moisture	Higher purity	Very high purity	Very high purity
Catalyst Recovery	Not reusable, requires extensive washing	Easily separable and reusable	Reusable after immobilization	Not applicable
Environmental Impact	Generates soap, wastewater	Low (less effluent generation)	Eco-friendly and clean process	Eco-friendly, no catalyst waste
Corrosion Risk	High (residual alkali, moisture)	Negligible	None	None
Cost Implication	Low catalyst cost, high purification cost	Higher catalyst cost, lower operational cost	High enzyme cost, requires immobilization	High energy and equipment cost
Reusability	Not reusable	Reusable for several cycles	Requires immobilization for reusability	Not applicable
Separation of Products	Difficult (due to soap, water)	Easy	Very easy (no side products)	Easy due to one-step conversion
Industrial Viability	Widely used, but purification is a challenge	Promising, but the temperature requirements are high	Limited by cost, still under development	Technically efficient but expensive
Advancements	Limited	Metal-doped heterogeneous catalysts (e.g., TiO2–MgO, SO42−/ZrO2/SiO2)	Immobilized enzymes, membrane reactor, ultrasound/microwave-assisted	Use of ultrasound, reactive distillation, and membrane reactors

Table 7. Possible process parameters for optimization of FAME yield.

Possible Variable	Sub-Categories/Conditions	Remarks/Effects	Reference
Temperature	Ambient (20 to 25 °C)	Temperature range affects reaction rate
	Moderate (60–100 °C)		[130,131,132]
	High (>100 but <200)
Pressure	Ambient (101.3 kPa)	Higher pressure may improve reaction yield	[20]
	High (10 MPa)
	Supercritical (>100 MPa)
Mixing rate	Low, Medium, High, Static, Ultrasonic	Affects mass transfer and uniformity	[118]
Reaction time	Total, Continuous, and Batch	Influences the extent of conversion	[133,134]
Molar ratio	Alcohol-to-Glycerol, Alcohol-to-Oil, Alcohol-to-Reagent	Affects the ester yield and cost	[118,135]
Catalyst type	Acid, Alkali, and Enzyme	Different catalysts influence conversion	[136]

Table 7. Possible process parameters for optimization of FAME yield.

Possible Variable	Sub-Categories/Conditions	Remarks/Effects	Reference
Temperature	Ambient (20 to 25 °C)	Temperature range affects reaction rate
	Moderate (60–100 °C)		[130,131,132]
	High (>100 but <200)
Pressure	Ambient (101.3 kPa)	Higher pressure may improve reaction yield	[20]
	High (10 MPa)
	Supercritical (>100 MPa)
Mixing rate	Low, Medium, High, Static, Ultrasonic	Affects mass transfer and uniformity	[118]
Reaction time	Total, Continuous, and Batch	Influences the extent of conversion	[133,134]
Molar ratio	Alcohol-to-Glycerol, Alcohol-to-Oil, Alcohol-to-Reagent	Affects the ester yield and cost	[118,135]
Catalyst type	Acid, Alkali, and Enzyme	Different catalysts influence conversion	[136]

4. Properties of Treated Liquids from Waste Cooking Oil as an Alternative Insulating Liquid

Dielectric insulating liquids are essential in transformers, serving two primary functions: they provide electrical insulation by preventing arcing and short circuits between components, and they facilitate heat dissipation by circulating and transferring heat away from the windings and core. Understanding the properties of chemically treated waste cooking oil is therefore crucial in evaluating its suitability as both an insulating and cooling medium. This section reviews the thermal and electrical characteristics of treated WCO as reported in the literature, focusing on its ability to maintain dielectric strength and effectively manage thermal loads within transformer systems.

4.1. Influence of WCO Treatment on Thermal Properties

In the consideration of WCO as an alternative insulating liquid, it is essential to take factors like viscosity, flash, and the fire point of the treated liquids into consideration. These are crucial as they inform about the thermal stability in the transformer system.

4.1.1. Cooling Efficiency

As presented in Table 3, waste cooking oil exhibits a relatively higher viscosity compared to commercially available natural ester liquids such as Midel 1204 and Midel 1215. Although its viscosity remains within the acceptable range for transformer insulating liquids, the elevated value limits its potential and reduces its effectiveness as a high-performance insulation fluid. The higher viscosity in waste cooking oil is primarily attributed to the presence of long-chain triglycerides, polymerized compounds formed during repeated heating, and residual impurities such as food remnants, free fatty acids, and oxidation by-products [137]. These components increase the internal friction within the fluid, thereby reducing its flowability. In transformer applications, high viscosity negatively impacts the cooling performance by limiting the oil’s ability to circulate and transfer heat efficiently [11]. Consequently, high-viscosity oil may compromise transformer thermal management and increase the risk of overheating due to high pumping power [138]. Therefore, viscosity reduction becomes essential. Transesterification is a general method for reducing the viscosity of waste cooking oil as it breaks down triglycerides into smaller methyl esters, significantly reducing the fluid’s viscosity and enhancing its cooling efficiency.

Table 8. Viscosity of treated and transesterified waste cooking oils.

References	Viscosity of Waste WCO (cSt) (40 °C)	Viscosity After Modification (cSt) (40 °C)	Catalyst	Enhancement (%)
[82]	70	27	NaOH	61.43
[76]	42.8	6	CaO	85.98
[46]	40.84	14.19	KOH	65.25
[140]	41.85	15.23	CaO	63.61
[100]	42.41	4.52	Coal ash	89.34
[141]	24.3	14.76	-	39.26
[142]	32.85	4.2	KOH	87.21

presents findings from the literature on the viscosity of transesterified waste cooking oils for transformer application. It was observed that transesterification effectively reduces the viscosity of waste cooking oil, bringing the modified oils within the range of cooling liquids specified by ASTM D445 at 40 °C [83,139]. This reduction in viscosity can be attributed to the removal of glycerol, which forms the backbone of the triglyceride structure. By eliminating glycerol, the molecular structure becomes less complex, reducing intermolecular friction and thereby enhancing fluidity. The transesterified oils exhibit viscosities comparable to or lower than those of mineral insulating liquids and traditional natural esters, suggesting enhanced cooling performance in transformer applications. The choice of catalyst significantly influences viscosity reduction, with heterogeneous catalysts such as coal ash and CaO achieving the most substantial decreases, while homogeneous catalysts show moderate effectiveness. Although viscosity reduction is affected by factors such as reaction conditions, catalyst loading, and oil composition, further studies could provide deeper insights into optimizing these parameters. Given their reduced viscosity, transesterified waste cooking oils show strong potential for high-performance cooling applications, potentially improving thermal management and reducing operational stress in electrical equipment.

4.1.2. Fire Safety

The flash point of insulating liquids is a critical safety parameter, particularly for power equipment operating near residential areas and essential facilities. Transformer explosions can lead to catastrophic consequences, including loss of life and extensive property damage. As shown in Figure 9, a tragic transformer explosion emphasizes the importance of using insulating liquids with high thermal and fire safety margins [143]. A higher flash point reduces the risk of fire and explosion, enhancing the overall safety and reliability of transformers [144]. This not only protects surrounding environments but also improves the economic viability of utilities by reducing maintenance costs, minimizing fire-related risks, and extending equipment lifespan. Additionally, insulating liquids with higher flash points contribute to better thermal stability, further ensuring the safe and efficient operation of transformers under varying load conditions. Natural ester insulating liquids are classified as Class K insulating materials due to their high fire points exceeding 300 °C, providing a significant safety advantage over mineral insulating liquids [145]. The flash point temperatures of treated waste cooking oil fatty acid methyl esters are presented in Table 9. A reduction in flash points is generally observed after transesterification, primarily due to the removal of glycerol, the central backbone of triglycerides, which increases the volatility of the resulting esters compared to the original waste cooking oil [101]. For instance, the flash point dropped from 290 °C to 220 °C with NaOH as the catalyst [82], representing a 24.14% decrease. A more pronounced reduction was observed in Ref. [76], where the flash point fell from 230 °C to 140 °C using CaO, equivalent to a 39.13% decrease. The highest reduction occurred in Ref. [142], with a 59.42% decrease from 308.03 °C to 125 °C when KOH was used. Conversely, an anomalous increase in flash point, from 129.6 °C to 235.7 °C, was reported in Ref. [141], suggesting the influence of other process variables such as alcohol recovery, feedstock purity, and transesterification efficiency. Among the various factors contributing to flash point variation, the chemical nature of the base oil and the type of catalyst used are important. However, the most critical determinant is the presence of residual short-chain alcohols (e.g., methanol or ethanol) in the final product [146]. Incomplete purification following the transesterification reaction can leave behind traces of these alcohols, which are highly volatile and can significantly depress the flash point of the resulting esterified insulating fluid [147]. This makes the liquid more prone to vaporization and ignition, thus compromising fire safety. Therefore, implementing rigorous post-reaction purification, such as vacuum distillation, washing, or adsorption, is crucial to ensure that bio-based insulating liquids derived from waste oils meet the safety standards required for use in high-voltage equipment.

Although the flash points presented in Table 9 exceed the minimum requirements for mineral insulating oils as specified by ASTM D3487-09 [148], indicating that the modified oils could serve as viable alternatives to mineral oils, they fall short of the threshold set for natural esters (≥275 °C) according to ASTM D6871-03 [149]. This suggests that while the transesterified waste cooking oils demonstrate improved safety compared to mineral oils, further optimization may be necessary to meet the fire safety standards required for natural ester-based insulating liquids.

Figure 9. Fire ignition in the transformer due to insulation breakdown [150]. This figure was reproduced with permission from Elsevier, Copyright 2025.

Figure 9. Fire ignition in the transformer due to insulation breakdown [150]. This figure was reproduced with permission from Elsevier, Copyright 2025.

Table 9. Flash point temperature of treated waste cooking oil fatty acid methyl esters.

References	Flash Point of Waste WCO (°C)	Flash Point After Modification (°C)	Catalyst	Enhancement (%)
[82]	290	220	NaoH	24.14
[76]	230	140	CaO	39.13
[46]	269	184	KOH	31.59
[140]	265	194	CaO	26.79
[100]	225	210	Coal ash	6.67
[141]	129.6	235.7	-	81.87 increase
[142]	308.03	125	KOH	59.42

Table 9. Flash point temperature of treated waste cooking oil fatty acid methyl esters.

References	Flash Point of Waste WCO (°C)	Flash Point After Modification (°C)	Catalyst	Enhancement (%)
[82]	290	220	NaoH	24.14
[76]	230	140	CaO	39.13
[46]	269	184	KOH	31.59
[140]	265	194	CaO	26.79
[100]	225	210	Coal ash	6.67
[141]	129.6	235.7	-	81.87 increase
[142]	308.03	125	KOH	59.42

4.1.3. Cold Region Application of WCO-FAME

In Nordic or subpolar regions where ambient temperatures can drop well below 0 °C, the application of natural ester insulating liquids is challenged by their relatively high pour point. This limitation often leads to the crystallization of the ester-based fluids at sub-zero temperatures, which impairs both their dielectric and thermal performance [151]. When the temperature falls below the pour point, the insulating liquid begins to solidify, forming a three-dimensional crystalline structure that restricts fluid mobility and disrupts heat dissipation and insulation properties. This behavior is primarily attributed to the molecular structure and composition of natural esters, particularly the high content of saturated and unsaturated fatty acids that influence pour point characteristics [22]. Moreover, during crystallization, the liquid’s affinity for moisture significantly diminishes, resulting in the formation of wax-like residues. While a case study in Ref. [152] reported a successful cold start of a 240 MVA generator step-up transformer filled with natural ester, demonstrating similar dielectric performance to mineral oil during energization and startup, concerns remain. The inherently higher viscosity of natural esters at low temperatures may hinder the operation of mechanical components such as oil pumps and on-load tap changers, potentially affecting overall transformer reliability in cold climates.

Table 10. Pour point temperature of treated waste cooking oils.

Reference	Pour Point of WCO (°C)	Pour Point After Modification (°C)	Catalyst	Percentage Decrease
[82]	9	−3	NaOH	133.33
[76]	0	−6	CaO	Absolute decrease of 6 units
[46]	−28	−28	KOH	No observable difference was reported
[140]	−15	−21	CaO	40%
[100]	−30	−39	Coal ash	30%
[142]	10	−6.7	KOH	167

presents the pour point temperatures of transesterified waste cooking oils as reported by various researchers. The values reported in Refs. [46,100,140] fall below the standard pour point threshold of ≤10 °C, as stipulated by ASTM, IEEE, and IEC guidelines, indicating their potential suitability for application in cold climates [44]. These findings show the promise of WCO-FAMEs as viable alternatives to conventional insulating liquids in low-temperature environments. However, some of the reported values in Table 10 exceed the required pour point limit, raising concerns about the consistency of cold flow performance across different samples. This variability is a call for further investigation into the factors influencing pour point behavior in WCO-FAMEs, such as feedstock composition, processing conditions, and post-treatment, to ensure their reliable application in transformer insulation within subpolar regions.

The cold flow properties of waste cooking oil fatty acid methyl esters can be significantly enhanced through the addition of pour point depressants. At low temperatures, it tends to undergo crystallization, where the saturated molecules form wax crystals that interlock into a rigid, gel-like network, thereby inhibiting flow (Figure 10a). However, the incorporation of PPDs disrupts this crystalline structure by modifying the size, shape, and growth of wax crystals. As illustrated in Figure 10b, the presence of PPDs prevents the development of a continuous gel network, thus maintaining the fluidity of the oil and allowing it to remain pourable even under sub-zero conditions. Several reports have been made on enhancing the pour point temperature of fatty acid methyl esters from waste cooking oils [153,154,155,156,157]; however, these investigations have primarily been directed toward applications in diesel engines, which require distinct physicochemical properties compared to those needed for insulating liquids. Pour point depressants have been employed to enhance the low-temperature performance of insulating liquids. In Ref. [23], the optimization of Viscoplex 10–171 and Viscoplex 10–312 was reported for improving the pour point of natural ester-based insulating liquids. The study revealed that both depressants had no significant adverse effects on the physicochemical and dielectric properties of the oil. A related investigation in Ref. [21] demonstrated that these PPDs effectively reduced the crystallization temperature across all concentrations tested, with optimum performance observed at 0.7 wt.% for both depressants, without compromising the essential properties of the oil. Additionally, Ref. [22] explored the impact of various PPDs, such as poly alpha olefin, polymethacrylate, polyacrylate, and hexyl naphthalene, on natural ester insulating liquids. Among these, poly alpha olefin, hexyl naphthalene, and polymethacrylate exhibited strong pour point depression effects. However, poly alpha olefin and hexyl naphthalene also led to property deterioration, such as increased acid value and higher dielectric dissipation factor, due to their inherent characteristics. The combination of crystallizing fractionation and polymethacrylate yielded the best results, reducing the pour point temperature from −13.5 °C to −27.3 °C. Adopting similar anti-crystallizing agents for waste cooking oil methyl ester could substantially improve its cold flow properties and enhance its viability as a sustainable insulating liquid for transformers operating in subpolar regions.

4.1.4. Acidity of Treated WCO

The acidity of insulating liquids is quantified as the amount of potassium hydroxide (KOH), in milligrams, required to neutralize the acids present in one gram of the liquid, commonly referred to as the total acid number (TAN). A high acid value in insulating liquids can degrade the dielectric strength of the oil and accelerate the deterioration of solid insulation materials, such as Kraft paper. Furthermore, the synergistic effect of acids and moisture within the transformer can lead to corrosion of metallic components and the hydrolytic degradation of cellulose-based insulation [158,159,160]. Although elevated acidity in natural esters is often considered less aggressive toward electrical components due to their inherent chemical structure, excessive acid content can promote oxidative degradation, increase electrical conductivity, and ultimately impair the dielectric performance of the insulating liquid.

The comparative summary of waste cooking oil’s acid values before and after transesterification using various catalysts is presented in

Table 11. Acid value of transesterified waste cooking oil.

Reference	Acidity of Waste WCO	Acidity After Modification	Catalyst	Percentage Decrease
[82]	0.13	0.06	NaOH	53.84
[76]	2.5	0.4	CaO	84
[42]	2.7972	0.2578	NaOH	90.78
[46]	2.7972	0.2561	KOH	90.84
[140]	2.793	0.144	CaO	94.84
[100]	2.82	0.03	Coal ash	98.94
[161]	1.82	0.13	NaOH	92.86
[141]	0.1	0.06	-
[142]	1.122	0.16	KOH	85.74

. A significant reduction in acid value was observed across all studies, confirming the effectiveness of transesterification in enhancing the chemical stability of WCOs for transformer applications. The improvement is primarily attributed to the conversion of free fatty acids and glycerides into methyl esters. Heterogeneous catalysts such as CaO and coal ash achieved the highest reductions, with coal ash exhibiting a remarkable 98.94% decrease. Homogeneous catalysts like NaOH and KOH also demonstrated substantial improvements, though generally slightly lower in comparison. It is to be mentioned that samples with initially low acid values, like in Refs. [82,141], show modest reductions, indicating that transesterification benefits both normal and highly degraded oils. However, when benchmarked against the IEC 62770 standard for natural ester-based insulating liquids [87], which stipulates an acid value of 0.06 mg KOH/g, only the modified oils from Refs. [82,100,141] meet the standard. While the other samples exhibit promising reductions, further purification or process optimization may be necessary to bring their acid values within the acceptable range for safe and reliable application in power transformers.

4.1.5. Breakdown Voltage (BDV) of WCO

The breakdown voltage of an insulating liquid is a critical parameter that defines its ability to withstand electrical stress without failure. It reflects the dielectric strength of the fluid and serves as a key indicator of its suitability for transformer insulation. Among the standard diagnostic tests conducted on insulating liquids, the BDV test is one of the most fundamental and widely used due to its simplicity, rapidity, and ability to provide insightful information about the quality and purity of the liquid. A high BDV generally indicates low moisture and contaminant content, while a low BDV suggests the presence of degradation byproducts, water, or particulates that compromise the insulating performance. For modified waste cooking oil, ensuring a high BDV is essential to validate its potential as an alternative dielectric fluid in power transformers.

Table 12. Breakdown voltage of treated waste cooking oils.

Reference	BDV of WCO (kV)	BDV After Modification (kV)	Catalyst	Percentage Increase
[82]	27	52	NaOH	92.59
[76]	35	40	CaO	14.29
[42]	7	33.4	NaOH	377.14
[46]	7	30	KOH	328.57
[140]	18	37	Cao	10.55
[100]	25.4	45.5	Coal ash	79.13
[161]	2.5	6	NaOH	140
[141]	10.5	38.3	-	264.76

presents the breakdown voltage values of waste cooking oils from different authors. A marked enhancement in BDV is evident across all studies, showing the efficacy of the transesterification process in improving the dielectric strength of WCOs for transformer insulation applications. The oils treated with NaOH exhibited the most significant increases, with BDV rising from 7 kV to 33.4 kV, a 377.14% enhancement [42], while another sample increased from 2.5 kV to 6 kV [161], further affirming NaOH’s effectiveness. Similarly, KOH-catalyzed samples recorded a BDV improvement of 328.57% [46], indicating the superior performance of homogeneous alkali catalysts (NaOH and KOH) in enhancing the electrical insulating properties. In contrast, heterogeneous catalysts such as CaO and coal ash showed more modest improvements, with BDV increases of 10.55% [140] and 79.13% [100], respectively. This disparity emphasizes the influence of catalyst type on BDV enhancement, with homogeneous catalysts generally delivering higher dielectric performance improvements.

From a practical standpoint, industry standards typically require a minimum BDV of 35 kV for transformer insulating oils according to ASTM 1816 and IEC 60156 [162,163]. Several modified WCO samples in Table 12 meet or exceed this benchmark, confirming their potential for use as viable alternatives to conventional transformer oils. However, some modified oils fall below this threshold, suggesting the need for further process optimization, including post-treatment and purification, to fully comply with application-specific requirements.

4.1.6. Oxidation Stability of WCO

In transformer insulation systems, the oxidation stability of insulating liquids is a critical parameter that governs the long-term performance and reliability of the equipment. Liquids with high oxidation stability help maintain the operational integrity of transformers by preserving both cooling and insulating functions over extended service durations. Conversely, insulating liquids that are prone to oxidation degrade over time, producing by-products that negatively affect their dielectric strength and cooling capacity, as well as the mechanical and dielectric properties of the associated solid insulation (typically cellulose-based paper).

Among the common oxidation by-products are aldehydes, ketones, carboxylic acids, and moisture, which accelerate the aging of both the liquid and paper insulation [164]. It is noteworthy that plant-based insulating liquids, such as waste cooking oil (WCO), are inherently more susceptible to oxidation. This susceptibility stems from their molecular composition, typically consisting of fatty acid chains with carbon lengths ranging from 8 to 24 atoms and containing varying degrees of unsaturation (from saturated to mono- and polyunsaturated structures) [62]. The oxidation stability of these oils is directly linked to the structure and quantity of double bonds in the fatty acid chains. A carbon–carbon double bond consists of a sigma (σ) bond and a pi (π) bond, which arise from the sp² hybridization of the carbon atoms [165]. The overlapping of sp² orbitals between two carbon atoms forms a sigma bond (σ), which houses two electrons. The remaining two electrons form the pi bond (π), which is typically more reactive and thus determines the degree of unsaturation and oxidation susceptibility of the oil. Oils with higher levels of unsaturation (i.e., more π bonds) tend to oxidize more rapidly, making them less stable under prolonged thermal and oxidative stress [101].

From Table 2, waste cooking oil exhibits a high proportion of unsaturated fatty acids, which contributes to its increased susceptibility to oxidation. The oxidation stability of methyl esters derived from WCO has been comparatively evaluated against those produced from castor oil, as reported in Ref. [134], using TGA analysis under an oxygen atmosphere. The results demonstrate that castor oil methyl ester possesses significantly higher oxidative onset temperatures (218–230 °C) compared to waste cooking oil methyl ester (187–200 °C), indicating greater resistance to initial thermal oxidation. Furthermore, castor oil methyl ester required higher temperatures to reach 10%, 50%, and 90% mass loss than waste cooking oil methyl ester, confirming its superior thermal stability under oxidative conditions. These differences in thermal and oxidative behavior are closely related to the fatty acid profiles of the two esters [134]. Due to the high percentage of unsaturated fatty acids in the methyl ester from WCO, the oxidation stability seems challenging and may need the inclusion of antioxidants and chemical modification, as reported by Refs. [140,166], respectively. When antioxidants are added to the base samples, they impede the degree of degradation in the oil by preventing the radicals from attacking the oil molecules. In some cases, the antioxidant could be a donor type like butylated hydroxytoluene (BHT), which donates hydrogen, and could also be an acceptor type like superoxide dismutase (SOD), which traps the free radicals, changing them to a stable compound [167]. The study reported in Ref. [168] investigated the effectiveness of three antioxidants, butylated hydroxytoluene (BHT), tert-butylhydroquinone (TBHQ), and pyrogallol (PY), in improving the oxidative stability of waste cooking oil fatty acid methyl esters. Among them, TBHQ demonstrated the superior performance, as confirmed through comprehensive analyses using FTIR and DSC techniques. The oxidative stability of waste cooking oil can also be enhanced through chemical modifications such as epoxidation. This process involves the reaction of the carbon–carbon double bonds in unsaturated fatty acids with an oxidizing agent, typically forming epoxide groups (oxirane rings). By reducing the number of double bonds, which are highly reactive sites for oxidation, epoxidation effectively improves the oxidative resistance of the oil. Studies have shown that epoxidized vegetable oils exhibit higher thermal and oxidative stability, making them more suitable for long-term applications in transformer insulation systems [166,169,170,171,172,173,174]. It is therefore important to consider the optimization of antioxidants and chemical modification parameters on the enhancement of WCO methyl esters for efficient application in transformer application.

5. Compatibility of WCO with Solid Insulators (Papers)

In transformer insulation systems, both liquid and solid insulators are critical to the overall reliability and longevity of the equipment. While the liquid provides insulation and cooling, the paper insulators offer thermal, electrical, and mechanical protection for transformer windings. Numerous studies have shown that the life expectancy of a transformer largely depends on the condition of the solid insulators, primarily cellulose-based papers; unlike insulating liquids, damaged solid insulation cannot be replaced [175,176]. These solid insulators are directly impregnated by the insulating liquid, making compatibility between the two materials essential [177]. Therefore, any alternative insulating liquid, such as transesterified waste cooking oil, must demonstrate strong compatibility with solid insulation to ensure long-term transformer performance and safety. Although there are limited reports on the compatibility of WCO-methyl esters in the literature, the report made by Ref. [100] gives substantial information concerning the long-term compatibility of papers and used cooking oil methyl ester (UCO/ME). The accelerated thermal aging of impregnated thermally enhanced Kraft paper (TEKI) and enhanced Nomex 910 (EN910I) insulation was investigated at 0 h, 48 h, 96 h, and 144 h in two different oils: transformer mineral oil (TO) and used cooking oil methyl ester. The visual aging behavior of the insulating papers immersed in different oil samples over various durations can be found in Ref. [100]. A progressive darkening of the paper samples was observed with increased aging time, indicating thermal and oxidative degradation. In the case of TEKI-TO and TEKI-UCO/ME, the darkening was more pronounced, especially after 144 h, suggesting a higher generation of acidic and oxidative byproducts that accelerate the degradation of cellulose in the paper. Interestingly, for thermally upgraded Kraft paper immersed in TEKI-TO and TEKI-UCO/ME, the papers exhibited similar visual appearances throughout the aging period, with only a slight difference at 96 h. This similarity implies that both mineral oil and methyl ester provided comparable protection to the paper during thermal aging, indicating a similar interaction or stabilization effect from the oils. In contrast, the EN910I papers aged in TO and UCO/ME maintained a more stable coloration even after 144 h, suggesting better oxidative stability and compatibility of these oils with solid insulation. Furthermore, the subsequent micrograph image of these samples presented in Ref. [100] indicates that EN910I paper aged in transformer oil demonstrates the best compatibility, maintaining relatively well-preserved fiber structures even after prolonged thermal exposure. This suggests that transformer oil provides a more stable aging environment, effectively limiting the extent of fiber breakdown and surface deterioration. In contrast, EN910I aged in used cooking oil methyl ester exhibits significant fiber damage, including pronounced distortion, surface cracking, and delamination, resulting in a highly disordered structure with broken and entangled fibrils. These severe structural changes highlight the compromised mechanical integrity of the paper, showcasing the aggressive oxidative environment and acidic by-products associated with the ester, which can accelerate cellulose degradation. This indicates that, despite the potential cooling and dielectric advantages of used cooking oil methyl ester, its long-term compatibility with paper insulation is significantly lower than that of conventional transformer oil. Due to the limited availability of data in the existing literature on the compatibility between waste cooking oil methyl ester and insulation paper, further research is essential to gain a comprehensive understanding of the degradation behavior of paper in the presence of methyl esters. This will help establish the long-term viability and reliability of such alternative insulating fluids in transformer applications.

6. Sustainability and Economic Viability of Waste Cooking Oil Valorization

The increasing global emphasis on sustainability and environmental responsibility has spurred a growing demand for bio-based alternatives to conventional petroleum-derived products. As a result, the market prices of environmentally friendly materials such as biolubricants, biosurfactants, bioasphalts, and 3D printing resins have risen significantly [178]. A key factor contributing to this price surge is the high cost of feedstocks, which can constitute more than 50% of the total production expenses. These feedstocks are predominantly sourced from virgin plant-based oils, which require resource- and energy-intensive agricultural operations, including land preparation, planting, irrigation, fertilization, harvesting, and oil extraction. While these activities elevate production costs, they also exacerbate environmental degradation through greenhouse gas (GHG) emissions and increased global warming potential (GWP) [9].

In contrast, waste cooking oil offers a sustainable and cost-effective alternative feedstock. Typically considered a low-value or even waste material, WCO is readily available from residential, commercial, and industrial sources with minimal or no purchase cost. Its valorization into high-value bioproducts, such as insulating liquids for transformers, can significantly reduce dependency on virgin feedstocks, enhance the economic feasibility of bio-based production chains, and align with circular economy principles. By diverting WCO from improper disposal channels (e.g., drainage systems and landfills), this approach also mitigates environmental hazards such as waterway pollution, soil contamination, and sewage blockages.

Furthermore, the conversion of WCO into value-added products reduces the carbon footprint associated with traditional fossil-derived materials. Reusing WCO prevents emissions from both the waste management process and virgin oil production, contributing to lower net GHG emissions. The integration of WCO into industrial applications represents a practical and scalable approach to supporting green chemistry principles, promoting resource efficiency, and advancing the United Nations Sustainable Development Goals (SDGs), particularly those related to responsible consumption, climate action, and clean industry [179].

Challenges Associated with Waste Cooking Oil Valorization and Future Direction

Despite the clear environmental and economic advantages of waste cooking oil valorization, it still faces some challenges hindering its widespread implementation and industrial scalability, which are stated as follows:

i.

Supply chain constraints.

One of the most critical issues is the inconsistency in WCO supply. The lack of a well-established and organized collection infrastructure often results in significant quantities of WCO being improperly discarded. In many regions, over half of the used cooking oil is released into sewage systems, leading not only to environmental pollution but also to the loss of valuable raw materials that could otherwise be repurposed. Establishing robust collection networks involving households, restaurants, and food processing industries is essential for ensuring a steady and reliable feedstock supply for industrial applications.

ii.

Variability in oil properties.

WCO quality varies depending on the source (households, restaurants, industries), frying duration, and type of food cooked. WCO is therefore highly heterogeneous, with its physicochemical properties varying widely depending on the type of oil used, the cooking duration, the temperature, and the nature of the food materials fried. Contaminants include water, food particles, polymers (from repeated heating), and heavy metals, which affect processing and catalyst performance. This variability can significantly influence the efficiency and outcome of conversion processes such as transesterification. Therefore, preliminary laboratory assessments of each collected WCO batch are necessary to determine key parameters such as acidity, moisture content, and fatty acid composition.

iii.

Cost of Collection, Transportation, and Processing

The economic viability of WCO valorization depends not only on the low cost of the raw material but also on the logistics associated with its collection, storage, and transport. In decentralized collection systems, the cost per liter of collected WCO can increase substantially, especially in remote areas. Thus, techno-economic analysis is critical to identify cost bottlenecks and optimize the overall process, particularly when aiming for applications in high-performance sectors such as transformer insulation.

iv.

Carbon Emissions During the Conversion Process

While WCO valorization is generally considered environmentally beneficial, the conversion process, especially transesterification, can be a significant source of carbon emissions. Studies have shown that this stage alone accounts for a substantial proportion of the total carbon footprint of the entire process. According to Refs. [179,180], the transesterification stage can be responsible for approximately 68% to 75% of the total carbon emissions during WCO processing. This indicates the need for greener processing technologies. One promising alternative is the supercritical methanol process, which has been reported to produce fewer GHG emissions compared to conventional alkali-catalyzed methods [181,182]. Although this method requires higher energy inputs, it eliminates the need for catalysts and simplifies post-reaction purification, which can offset its energy demands and reduce overall environmental impacts.

v.

Research and Development Needs

Ongoing research is essential to improve the environmental performance and scalability of WCO valorization technologies. This includes developing low-energy, low-emission processing methods, refining supply chain logistics, and exploring catalytic systems that enhance conversion efficiency while minimizing waste and emissions. Life cycle assessment (LCA) and techno-economic analysis (TEA) should be applied in tandem to identify optimization opportunities across the value chain and guide policy interventions that promote the use of WCO as a mainstream industrial feedstock.

vi.

Future directions

Due to the various challenges associated with modified waste cooking oils, such as oxidation instability, variability in feedstock composition, and suboptimal dielectric properties, it is imperative to conduct more in-depth research into advanced chemical modification techniques. These modifications should aim to improve thermal stability, oxidative resistance, and compatibility with solid insulation materials, making waste cooking oil a more viable candidate for transformer applications. Furthermore, the development of a thermal resistance model for chemically treated waste cooking oils would provide important information concerning their behavior under real-world operating conditions. Such a model would support application-oriented validation of the oil and also guide formulation and performance optimization, enabling the use of WCO-based insulating liquids in next-generation, environmentally friendly transformers.

7. Conclusions

The valorization of waste cooking oil (WCO) represents a promising pathway toward achieving circular economy goals, reducing environmental pollution, and mitigating dependency on fossil-based resources. Despite its abundance and potential environmental benefits, the valorization of WCO is hindered by several challenges, including feedstock variability, contamination, logistical constraints, and technical limitations in processing. Traditionally utilized in biodiesel production for combustion engines, WCO-based fuels, while renewable, still contribute to carbon dioxide and nitrogen oxide emissions during combustion, thus presenting limited environmental advantages. In contrast, the application of WCO in non-combustion sectors, particularly in transformer insulation, offers a safer, cleaner, and more sustainable alternative. This approach not only extends the lifecycle of WCO but also aligns with the global shift toward eco-friendly materials in energy infrastructure. Recent studies have shown that treated WCO possesses desirable characteristics for transformer applications, including good cooling performance and inherent fire safety. However, despite these advantages, WCO-based insulating liquids typically do not meet the strict K-class fire resistance standards, which require a fire point above 300 °C. Additionally, several physicochemical properties, such as acidity, oxidation stability, and cold flow behavior, exhibit significant variability due to differences in feedstock origin, frying conditions, and treatment methods. These inconsistencies raise concerns about the long-term stability and reliability of WCO in electrical insulation systems and warrant deeper scientific inquiry. One particularly critical but underexplored area is the compatibility between fatty acid methyl esters derived from WCO and other transformer components such as cellulose-based insulation paper. The solid–liquid interactions within the oil–paper insulation system are vital to the dielectric performance and aging behavior of transformers. However, the current literature offers limited insight into these interactions, particularly under accelerated aging or thermal stress conditions. Furthermore, the conversion process itself raises environmental concerns, particularly regarding energy consumption, chemical usage during pretreatment (e.g., transesterification, bleaching), and byproduct management. Therefore, integrating cleaner, low-energy technologies, such as supercritical or enzymatic processes, alongside life cycle and techno-economic analyses will be essential to fully assess the sustainability of WCO-derived insulating liquids. In addition, developing efficient and standardized WCO collection infrastructures is crucial for reliable feedstock availability. Social and cultural barriers, informal markets, and inadequate regulatory oversight continue to hinder efficient WCO recovery in many regions. Promoting public awareness, incentivizing waste oil collection, and enforcing legislation against unsafe reuse are necessary policy steps to support a sustainable WCO supply chain.

In summary, WCO offers a compelling opportunity for resource recovery and environmental preservation when applied in non-fuel domains such as transformer insulation. However, a successful transition requires a multidisciplinary research approach to address its physicochemical variability, material compatibility, environmental footprint, and regulatory integration. Future work should focus on advanced treatment protocols, optimizing performance additives, and evaluating aging behavior in real-life operating conditions. Furthermore, successful large-scale implementation depends not only on technological innovation but also on supportive regulatory frameworks, robust collection systems, and increased public awareness. Moving forward, an integrated approach combining economic incentives, cleaner technologies, and circular economy principles will be essential to fully realize the value of WCO as a valuable and sustainable asset for the electrical power industry.