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

it has been widely proved that phase change materials(PCMs)used as backfill material in ground heat exchangers(GHEs)play a role in enhancing heat transfer and reducing the surrounding temperature fluctuation.However,the feasibility of phase change backfill materials is still under investigation because of the low conductivity and leakage of most phase change materials.In this research,a capric-lauric acid/expanded graphite PCM was prepared and tested.Plus,the heat transfer and flow field were simulated in a ground heat exchanger system using this PCM as backfill material.Fatty acid with different mass ratios between capric acid and lauric acid and PCM composite with different mass ratios of expanded graphite(EG)were prepared and tested via DSC(differential scanning calorimetry),leakage and microstructure analysis.The results showed that the eutectic point was reached when the mass ratio of capric acid was 65% in fatty acid,and the optimal mass ratio between EG and fatty acid was 10:100.Heat transfer analysis in GHE system was investigated via an established and validated single U-tube GHE numerical model with PCM grouting or ordinary grouting.Thermal behaviors such as heat transfer and phase and temperature recovery of backfilling without PCM composite and with PCM composite in the volume fraction of 30%-70% was analyzed.The results showed that the optimal volume fraction of PCM should be 50%.In addition,Thermal performances of GHE with different Reynold numbers(15000,20000,25000,30000)of working fluid were simulated.It was found that a shorter running time should be adopted in GHE system with PCM backfilling as Reynold number increased.In order to investigate the optimal operation mode for GHE with PCM backfilling,intermittent running modes with on/off time ratios of 8:16,10:14 and 12:12,and modes with different interval frequencies but the same operation time were also analysed.It was shown that the heat transfer amount decreased more significantly as the on/off time ratio increased.Plus,it was not suitable for GHE with high percentage of PCM in backfilling to be operated under high on/off ratios in the long term.A higher intermittent frequency(4 h of shutdown after every 4 h of operation)can be adopted in GHE without PCM backfilling because of the improvement in heat exchange,while it is not suitable for GHE with PCM backfilling due to the significant poor recovery behavior compared to a lower intermittent frequency(12 h of shutdown after every 12 h of operation).