Nanofluidic two-phase closed thermosyphon-assisted thermoelectric generator for heat recovery from coal spontaneous combustion

The current prevention and control countermeasures for underground coal fire principally rely on various cooling and inhibiting methods. However, because tremendous thermal energy is locked in underground coalfield fires, thermal energy extraction and utilisation is a sustainable and environmentally friendly approach. In this study, a waste heat recovery system based on a two-phase closed thermosyphon-assisted thermoelectric generator technique was established for exploiting the thermal reservoir in underground coalfield fires. To enhance thermal extraction, nanofluid working fluid was used. The optimal concentration of the copper oxide (CuO) nanoparticle in the two-phase closed thermosyphons at three heat source temperatures (100, 200, and 300 °C) was found to be 5.0, 5.0, and 10.0 mass%, respectively. Proper addition of CuO enhanced the thermal transfer in the two-phase closed thermosyphon by 27.90%. Furthermore, the increase in the heat source temperature was positively related to the thermal extraction effect of the two-phase closed thermosyphon. With an optimised two-phase closed thermosyphon, the thermoelectric generator system, combined with forced air cooling, provided an output power of 0.1432–3.1360 W with a thermoelectric conversion efficiency of 0.54%–1.83% for heat sources ranging from 100 to 300 °C.