Experimental investigation on thermal-electric characteristics of curved thermosyphon-thermoelectric generator in waste heat recovery of coal spontaneous combustion

Abstract

The two-phase closed thermosyphon-thermoelectric generator (TPCT-TEG) is promising in waste heat recovery for coal spontaneous combustion (CSC). The thermal resistance is primarily on the solid coal side, but relevant investigation is rare in terms of heat transfer enhancement through optimized pipe design when applying the TPCT-TEG system in CSC. In this work, aiming at improving the heat extraction efficiency and reduce the control thermal resistance at coal side, the thermosyphon with an L-shape curved pipe structure instead of conventional vertical-straight pipe for TPCT is proposed to achieve an enlarged heat exchange area and reduce thermal resistance under a certain burial height in the coal seam. To examine the influence of the curving angle and heat exchange area, a heat transfer performance test for seven TPCTs with various pipe structures is conducted with the conventional vertical-straight TPCT as a benchmark for comparison. The optimal geometrical structure of TPCT is determined accordingly. Then the curved TPCT-TEG unit is integrated with six pieces of Bi2Te3 thermoelectric modules. The experimental system of heat recovery with TPCT-TEG unit for coal spontaneous combustion is established to examine the thermal-electric characteristics of the optimal curved TPCT-TEG unit when applied in the heated coal. The bottom of the coal storage box is electrically heated to simulate the spontaneous combustion of coal containing heat energy. The heat energy extraction and thermoelectric energy conversion performance are investigated under different heating source temperatures from 100 to 300 °C. The results show that the TPCT with a 30° curving angle under an equivalent burial depth as the conventional vertical-straight TPCT has lower thermal resistance (0.08 °C/W) and higher heat transfer efficiency (81.84 %), achieving a 62.1 % reduction of the thermal resistance than the vertical one. The curving angle and the heat exchange area have a synergistic influence on the thermal performance of the TPCT. Meanwhile, the thermal performance of the TPCT is more sensitive to the change of heat exchange area under the test conditions in this work. When applying the curved TPCT-TEG unit to coal, the coal temperature at the bottom and middle locations decreases significantly, the maximum temperature reduction and temperature reduction rate are 203.6 °C and 68.5 %, respectively. When the heating source temperature is 300 °C, the average thermoelectric electromotive force of the TPCT-TEG is 4.36 V. The cumulative power generation of the system is 11.25 kJ under matched electrical load of 33 Ω within 30 h of operation period.