Dynamic heat transfer analysis of a direct-expansion CO2 downhole heat exchanger

Abstract 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.