Modeling and techno-economic comparison of two types of coaxial with double U-tube ground heat exchangers

Abstract

Among different types of ground heat exchangers (GHEs), multi-external chamber coaxial GHE is recently a promising design due to its geometrical characteristics of concentric tube-in-tube configuration and maximal heat exchange performance. Nevertheless, the characterization and commercialization of multi-external chamber coaxial GHEs need a deep understanding of the effect of different opt-geometrical and operational parameters on their thermo-economic performance. Therefore, accurate numerical modeling of the transient heat transfer proficiencies and the techno-economic feasibility of the multi-external chamber coaxial GHE and coaxial GHE compared to conventional double U-tube GHE is crucial to promise an optimal design of ground-coupled heat pump systems. This study introduces a comprehensive systematic and techno-economic analysis of the transient heat transfer characteristics of coaxial and multi-external-chamber coaxial GHEs compared to conventional double U-tube GHE. A three-dimensional numerical model was adopted and validated by field experiments. Moreover, a sensitivity analysis was conducted to investigate the impacts of several key operating and design parameters on heat transfer performance under heating and cooling modes conditions. Furthermore, various case studies, along with a comprehensive analysis of multi-objective optimization for minimization of the GHE number and energy consumption, were also carried out. In addition, a techno-economic analysis was also carried out using the total annual cost method to assess the economic viability of the proposed GHE designs. The results reveal that, compared to double U-tube GHE, coaxial GHE has the best thermal performance with maximum and average heat transfer rates of 127.54% and 17.67%, while the multi-external-chamber coaxial GHE is estimated to be 62.38% and 11.72%, respectively. Also, the corresponding total thermal resistance of coaxial GHE is the lowest by 14.86%, and for multi-external-chamber coaxial, it is 10.30%; also coaxial GHE has the lowest pressure drop by 76.45%, followed by multi-external-chamber coaxial GHE with 57.18%. In the case studies, replacing double U-tube GHE with coaxial or multi-external-chamber coaxial GHEs, the GHE number can be decreased by 13.3% for both the designs, or the energy consumption of the water pump by 33.91% and 30.97%, and the energy consumption of the entire system by 17.21% and 10.54%, respectively. The techno-economic analysis results indicate that replacing double U-tube GHE with coaxial GHE is the most feasible alternative, which can decrease the total annual cost by 13.58%, followed by multi-external-chamber coaxial GHE by 6.77%. Further discussion for the deep applications was performed, which concluded the optimal GHE design. The findings of this paper can be used as a guide for future research on GHEs for potential applications of ground-coupled heat pump systems.