Analytical And Experimental Study Of Phase Change Materials Used As Backfill Material For Borehole Heat Exchangers

Ground Source Heat Pumps (GSHP) are widely used for heating and cooling buildings, backfill material is essential in improving the thermal performance of Borehole Heat Exchangers (BHE). In this thesis, an analytical study of the thermal performance of BHE with both phase change materials (PCM) and soil backfill material is compared to and verified against an experimental study.Thermal performance of BHE's made of a U tube and a coaxial pipe where waste animal oil and a mixture of capric and lauric acid were used as PCM backfill material based on differential scanning calorimetry (DSC) were studied analytically. Heat transfer in the PCM and soil backfill was analyzed, and the temperature distribution in both solid and liquid phases was calculated.The results indicate that the larger the Stefan number is, the faster the phase interface moves, and the smaller the total solidification (melting) time of the PCM backfill is.When the mass flow is 180 L/h, the total solidification time is 7h44min with a Stefan number of 0.06, the thermal disturbance distances due to the fluid temperature away from the borehole central line are 0.23 m and 0.66 m with PCM and soil backfill materials, respectively. 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.A BHE thermal performance test system was also set up, and an in situ experiment for double U tube carried out, heat output of single borehole, heat conductivity of soil and the thermal disturbance distance under different heating load conditions was measured.A real-time monitoring system of soil temperature for GSHP which monitors the daily, monthly and annual temperature distribution was developed at different depths of the borehole at New Campus of Tianjin Technology University.A concentric annular pipe borehole heat exchanger laboratory setup was made, mimicking industry geometry and kinetic similarities.A combination of different backfills, fluid mass flows and inlet temperatures of fluid was tested, with fluid inflow into the outer pipe and coming out from the inner pipe were studied. The results show that when the mass flow is 250 L/h, during the complete PCM solidification (melting) time, soil temperature at a distance of 0.1 m from pipe central line decreased 0.7 oC ( heat extraction) or increased 3.5 oC ( heat injection), but soil temperature at a distance of 0.3 m from pipe central line did not change. When the backfill materials are soil or PCM respectively, temperature difference of fluid are 0.16 oC /0.10 oC for heat extraction, and 0.24 oC /0.08 oC for heat injection.Feasibility of practical application of PCM as backfill material was examined, based on an analytical experimental study. The results indicate that using PCM could shorten the distance between boreholes. If the borehole can be sealed well, and low price PCM are readily available, the practical application of PCM as backfill material can have a bright future in conjunction with borehole heat exchanger systems.