Intermittent Mode Analysis Of A Borehole Ground Heat Exchanger With Novel Phase Change Backfill Materials

A study was conducted to investigate heat transfer in a ground heat exchanger (GHE) system with a novel capric acid-lauric acid eutectics/expanded graphite (EG) composite PCM. The composite PCM was prepared and analyzed. The optimal mass ratio of fatty acid to EG was 10:100 with a melting heat of 134.13 kJ/kg. The thermal performances of GHE system with different fraction ratios of the composite PCM mixed in the backfill material were predicted. The heat exchange amount was increased by 17.71% when the PCM fraction ratio was increased from 0% to 70%. However, the average temperature in the borehole after the recovery period increased from 16.94 degrees C to 18.32 degrees C with increasing PCM ratio from 0% to 70% due to heat release during solidification. Moreover, the impacts of different running modes on the heat transfer, PCM utilization, and temperature recovery were investigated. The most rapid reduction of heat exchange amount over time was observed under the on/off ratio of 12:12 and at the PCM ratio of 70%. In addition, the average temperature in the borehole increased by 1.5 degrees C-2.7 degrees C with increasing on/off ratio from 8:16 to 12:12 in all the cases with PCM. Heat exchange amount in the running mode of 4 h interval after every 4 h operation was at least 10.3% higher than that in the mode of 12 h interval after every 12 h operation. By contrast, the temperature and phase recovery significantly deteriorated in the 4 h intermittent mode in the cases with PCM backfilling.