Preliminary conditioning of transformer oil using solar heat based on flat-plate and evacuated tube collectors

Abstract

Preheating transformer oil before vacuum dehydration and multistage filtration reduces its viscosity, facilitates circulation, and provides favorable thermal conditions for reconditioning. This study evaluates, using a calculation model, the applicability of flat-plate and evacuated tube solar collectors for a 1 L mobile prototype. With an initial oil temperature of 25 °C, operating temperature of 80 °C, density of 0.89 kg/L, and specific heat capacity of 1.9 kJ/(kg·K), the design heat demand for both configurations was set at 0.040 kWh. At a solar irradiance of 800 W/m², the useful thermal power was 61.2 W for the flat-plate collector and 67.0 W for the evacuated tube collector, with heating times of 39.2 and 35.8 min, respectively. The minimum calculated irradiance for a one-hour cycle was 523 and 478 W/m². An integrated J indicator was proposed to jointly evaluate thermal adequacy, the conditional cumulative separation degree during repeated filtration, and circulation-pump energy consumption. For three cycles, J was 0.760 for the flat-plate and 0.865 for the evacuated tube collector, corresponding to a 13.8% difference. The results indicate that, under the considered prototype conditions, the evacuated tube collector provides higher useful thermal power and shorter heating time. Further experimental validation using a physical prototype and laboratory analysis of transformer oil quality is proposed