## 4 Properties of Treated Liquids from Waste Cooking Oil as an Alternative Insulating Liquid
### Influence of WCO Treatment on Thermal Properties
#### 4.1.3 Cold Region Application of WCO-FAME  
In Nordic or subpolar regions where ambient temperatures can drop well below 0 \({}^{\circ}\)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 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 \(\leq\)10 \({}^{\circ}\)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 \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Reference**} & **Pour Point of** & **Pour Point After** & \multirow{2}{*}{**Catalyst**} & **Percentage** \\  & **WCO (\({}^{\circ}\)C)** & **Modification (\({}^{\circ}\)C)** & & **Decrease** \\ \hline
[82] & 9 & \(-3\) & NaOH & 133.33 \\
[76] & 0 & \(-6\) & CaO & Absolute decrease \\  & & & & of 6 units \\  & & & & No observable \\
[46] & \(-28\) & \(-28\) & KOH & difference was reported \\
[140] & \(-15\) & \(-21\) & CaO & 40\% \\
[100] & \(-30\) & \(-39\) & Coal ash & 30\% \\
[142] & 10 & \(-6.7\) & KOH & 167 \\ \hline \hline \end{tabular}
\end{table} 
Table 10: Pour point temperature of treated waste cooking oils.