## 4 Conclusions  
The doping of electro-insulating liquids (mineral oil, and synthetic and natural esters) with analyzed nanoparticles (C\({}_{60}\), TiO\({}_{2}\)) did not in any way affect the heat exchange capacity of the obtained nanofluids. This is a very important conclusion in the context of doping electro-insulating liquids with nanoparticles to improve their other properties, such as electro-insulating properties (breakdown \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Kind of Liquid**} & \multicolumn{4}{c}{**Liquid Temperature**} \\ \cline{2-5}  & **25 \({}^{\circ}\)**C** & **40 \({}^{\circ}\)**C** & **60 \({}^{\circ}\)**C** & **80 \({}^{\circ}\)**C** \\ \hline MO & 33.9 & 29.4 & 25.2 & 22.4 \\ MO + Span® 20 & 33.4 & 29.2 & 25.5 & 22.3 \\ MO + Span® 20 + TiO\({}_{2}\) & 33.3 & 29.2 & 25.7 & 22.5 \\ \hline SE & 40.4 & 34.1 & 28.6 & 24.9 \\ SE + Span® 20 & 40.4 & 34.0 & 28.9 & 24.8 \\ SE + Span® 20 + TiO\({}_{2}\) & 39.7 & 33.3 & 28.2 & 24.5 \\ \hline NE & 37.8 & 33.0 & 28.4 & 25.1 \\ NE + Span® 20 & 38.1 & 33.1 & 28.3 & 25.2 \\ NE + Span® 20 + TiO\({}_{2}\) & 37.9 & 32.8 & 28.4 & 25.1 \\ \hline \hline \end{tabular}
\end{table} 
Table 8: Calculation results of temperature decrease in pure electro-insulating liquids and nanofluids based on surfactant Span® 20 and titanium dioxide (TiO\({}_{2}\)). voltage, electrical resistivity, electrical permittivity) or aging (dielectric loss factor, the ability to absorb water, release of sludge and sediments). The modification of electro-insulating liquids with fullerene C\({}_{60}\) did not cause changes in thermal conductivity, density, and thermal expansion. The addition of fullerene C\({}_{60}\) caused an increase in kinematic viscosity, which may adversely affect the heat exchange capacity of the obtained nanoliquids. The increase in viscosity was compensated by the increase in specific heat. As a consequence, the heat exchange capacity of insulating liquids did not change due to the addition of nanoparticles to them. The modification of electro-insulating liquids with titanium dioxide TiO\({}_{2}\) nanoparticles (along with the surfactant) did not cause changes in density and thermal expansion. The addition of TiO\({}_{2}\), on the other hand, resulted in an increase in the kinematic viscosity of all liquids and a decrease in specific heat (with the exception of synthetic ester), which may have had a negative impact on the heat exchange capacity of the obtained nanofluids. The increase in viscosity and the decrease in specific heat have been balanced out by the increase in thermal conductivity. In conclusion, the heat exchange capacity of liquids did not change due to the doping with nanoparticles. We recommend that future research concerning the use of electro-insulating nanofluids in electrical power equipment should be conducted in two directions. The first of these is the analysis of the impact of other nanoparticles, used sporadically to improve dielectric properties of electro-insulating liquids on their thermal properties. Such nanoparticles include nanodiamonds (NDs), which improve thermal conductivity [57], and boron nitride nanoparticles [58]. The second direction is the use of well-recognized and widely used nanoparticles in electro-insulating liquids, which significantly improve the thermal properties of other liquids. As it is commonly known, Al\({}_{2}\)O\({}_{3}\) or CuO nanoparticles improve the thermal conductivity of water [52]. In turn, in [53], the beneficial effect of Cu and SiO\({}_{2}\) nanoparticles on the thermal conductivity of other liquids has been proved. It should therefore be assumed that these nanoparticles also improve the thermal properties of insulating liquids, such as mineral oil or synthetic and natural esters. Therefore, it would be necessary to check how the mentioned nanoparticles affect the dielectric properties of the resulting nanoliquids. Thus, this is the opposite approach in relation to that described in this paper.