Compared with conventional second-loop ground-coupled heat pump, direct-expansion ground-coupled heat pump can achieve higher energy efficiency. To maximize the heat exchange rate of direct-expansion heat exchanger, a novel direct-expansion downhole heat exchanger is proposed in this study, using supercritical pressure CO₂ as the refrigerant. The heat exchanger is placed inside a closed water well so that heat transfer can be enhanced by natural convection. A high accuracy three-dimensional transient thermal resistance and capacity model is developed and validated by a field experiment. The measured average effective unit heat flux is significantly improved from a previous study by 168.8%. According to the simulation result, due to the drastic change of pseudocritical CO₂ properties, an improper inlet pressure or U-pipe diameter can lead to a 34.8% reduction in the heat exchange rate. Moreover, the natural convection will aggravate the thermal short-circuit effect between U-pipe branches; however, its positive effect of enhancing heat transfer is more significant. Adding insulation to the upper part of the upward pipe can balance a weaker thermal short-circuit effect with a smaller total heat transfer resistance. With this design, the unit heat flux can be increased by 49.0% from that of conventional direct-expansion heat exchanger.

与传统的二回路地面耦合热泵相比，直接膨胀式地面耦合热泵可实现更高的能源效率。为了使直接膨胀式换热器的换热率最大化，本研究提出了一种以超临界压力CO ₂为制冷剂的新型直接膨胀式井下换热器。热交换器放置在封闭的水井内，因此自然对流可增强热传递。建立了高精度的三维瞬态热阻和容量模型，并通过现场实验进行了验证。与以前的研究相比，测得的平均有效单位热通量显着提高了168.8％。根据模拟结果，由于伪临界CO ₂的急剧变化特性，不合适的入口压力或U形管直径会导致热交换率降低34.8％。而且，自然对流会加剧U型管分支之间的热短路效应。但是，其增强传热的积极作用更为显着。在向上管的上部增加隔热层可以平衡较弱的热短路效应和较小的总传热阻力。通过这种设计，单位热通量可以比传统的直接膨胀式换热器增加49.0％。