Study of Thermal Performance of Coaxial Type Borehole Heat Exchanger with Different Backfill Materials

Abstract

This paper presents thermal performance of coaxial type borehole heat exchanger (CBHE) with fluid inflow into the outer pipe and coming out from the inner pipe. Fluid temperature distribution was calculated, the effect of backfill materials and fluid flow rate on fluid temperature were conducted. When PCMs (a mixture of capric and lauric acid) were used 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 mixed acid, respectively, the corresponding fluid temperature difference are 0.24?C/0.08?C, and which decreases as mass flow increases. 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 0.15, and the corresponding distances are 0.18 m and 0.50 m, respectively, indicating using PCM could shorten the distance between boreholes.