Development of a Phase Change Backfill Material with High Heat Capacity and High Thermal Conductivity Used for Ground Source Heat Pumps

To solve the problems of phase change materials (PCM) including low thermal conductivity and easy washout and loss after liquefaction in current studies on borehole backfill materials, capric acid-lauric acid/expanded graphite (CA-LA/EG) PCM admixture with high thermal conductivity and high heat capacity was developed by using expanded graphite as the carrier. The PCM admixture is mixed with original sands (drilling debris) to make phase change backfill materials (PCBM). When the PCM backfilling is applied, the key issue is to dissipate the large amount of heat stored as latent heat in boreholes to the surrounding formation in time; otherwise, serious heat accumulation would be induced. Numerical simulation of application of the PCBM in multiple intermittent operation cycles was conducted when the admixture percentages and thermal conductivities are different. The result shows that compared with conventional low-conductivity PCM admixtures and ordinary backfill sands such as drilling debris, the CA-LA/EG admixture can mitigate the temperature fluctuation in borehole and improve its heat exchange rate. The heat exchange rate was increased by 11.6% when 30% of CA-LA/EG admixture was added. However, the improvement declined as the number of operation cycle increased, so the intermittent operation mode is suggested to match the PCBM application since the heat in borehole should be dissipated in time to reserve storage capacity in the next operation cycle and continuously utilize the high heat capacity of the PCM.