Heat Transfer Analysis of Centric Borehole Heat Exchanger with Different Backfill Materials

Abstract

Thermal performance of BHE\'s made of a coaxial pipe with fluid inflow into the outer pipe and coming out from the inner pipe were studied. Fluid temperature distribution was calculated, the impact of backfill materials and fluid flow rate on fluid temperature were conducted. When a mixture of capric and lauric acid were used as phase change materials (PCM) for backfilling, the temperature distribution in both solid and liquid phases was calculated for heat injection. The results show that when the backfill materials are soil or PCM respectively, temperature difference of fluid are 0.24 oC/0.08oC, and which decreases as mass flow increases, this is because although both flow rate and transferred heat increase over flow velocity, the flow rate increased faster than the heat, thus, the corresponding temperature difference is smaller. The results also indicate that the larger the Stefan number is, the faster the phase interface moves, and the smaller the total melting time of the PCM backfill is; When the mass flow is 180 L/h, the total melting time is 4h02min with a Stefan number of 0.12, and the corresponding distances are 0.18 m and 0.50 m, respectively, indicating using PCM could shorten the distance between boreholes.