Phase change material backfill for underground thermal energy storage in direct-expansion CO2 ground-coupled heat pumps

To enhance the short-term underground thermal energy storage capability of direct-expansion ground-coupled heat pump (DX-GCHP) systems, this paper proposes a dynamic thermal resistance and capacity model (TRCM) that incorporates the quasi-static phase-change characteristics of phase change material (PCM). By introducing “equivalent thermal resistance” and a hybrid node network, the model efficiently resolves the complex coupling among CO 2 transcritical/two-phase flow, PCM phase change, and soil heat conduction. Experimental validation against the water-backfilled condition demonstrates that the average absolute error of the wall temperature of the model in cooling and heating modes is only 0.89 °C and 1.14 °C, respectively. Confirming its accuracy in describing CO 2 flow, sensible heat transfer in the backfill, and soil conduction. Based on this validated framework, the model is then extrapolated to predict the PCM-backfilled performance. The results reveal that within a 60-hour operation cycle, the average specific heat exchange rate (SHER) can be improved by up to 23% in cooling mode and by approximately 33% after optimization in heating mode. The parametric study indicates that, under the investigated conditions, the PCM phase-change temperature is the most critical parameter governing storage performance, with optimal values of approximately 30 °C for cooling and 12 °C for heating; both PCM thermal conductivity and casing outer diameter exhibit optimal ranges. These optimal values should be regarded as model-based design references and should be site-specifically adjusted in practical engineering applications. This study provides a reliable analytical tool and essential design guidelines for underground thermal energy storage systems using PCM backfill in DX CO 2 GCHPs.