Numerical Simulation on Thermal Analysis of a Single Borehole U-Tube

Abstract

Ground source heat pumps (GSHPs) have emerged as a promising solution for effectively harnessing renewable energy sources. However, their efficiency can be compromised by the accumulation of heat in the soil, posing a challenge to their optimal utilization. To mitigate temperature swings, innovative borehole ground heat exchangers (BGHE) incorporating shape-stabilized phase change material (SSPCM) backfill have gained attention. Despite a lack of experimental research, a comprehensive investigation combining meticulous research studies and cutting-edge computer modeling aimed to address the crucial question: How does the choice of backfill material affect soil temperature? Remarkably, sand-SSPCM mixtures demonstrated the superiority of SSPCM over traditional sand backfill, reducing temperature swings by an impressive 10°C near the heater wall. This finding is of paramount significance as maintaining a consistent temperature during the cooling process substantially enhances GSHP system efficiency. The study further explored heat transfer rates, recognizing their pivotal role in optimizing energy utilization within GSHP systems. Numerical modeling showcased a remarkable 16.5% reduction in the heat influence radius after just one operational cycle with the innovative SSPCM backfill. Even after a prolonged 14-hour shutdown, SSPCM consistently maintained a higher temperature compared to its conventional counterpart, highlighting its resilience and capacity to retain thermal properties over an extended period. However, cost-effectiveness must be considered, as the study revealed a decline in cost-effectiveness with increased mass ratios of SSPCM. Designers and engineers face the challenge of weighing the advantages of SSPCM against implementation expenses while striving for the optimal balance. In the domains of heat transfer rate and borehole design, numerous hurdles exist for designers of GSHP thermal equipment. To address these challenges comprehensively, a novel mathematical and numerical methodology was devised. This approach enabled the evaluation of thermal efficiency and performance in U-tube heat exchangers utilizing two distinct backfill materials: grout and SSPCM. Numerical simulations using COMSOL Multiphysics software analyzed the intricate heat transfer processes within a cylindrical borehole configuration, where a U-tube circulating water acted as the working 2 fluid for heat exchange with the surrounding ground. The simulations revealed a 14.6% increase in the total mean resistance of the single U-tube borehole with SSPCM backfill, indicating the enhanced effectiveness of this innovative approach. The research conclusively demonstrated that SSPCM backfill significantly augments the effectiveness and functionality of GSHP systems, emphasizing the need for meticulous design and optimization to strike the delicate balance between benefits and costs. The continued advancement and widespread adoption of GSHP technology are vital to meet escalating energy demands while mitigating adverse environmental effects. As the world increasingly turns to renewable power sources, the significance of this progress cannot be overstated. By leveraging the untapped potential of ground source heat pumps and capitalizing on innovative approaches like SSPCM backfill, a sustainable future becomes an achievable reality.