Latent versus sensible thermal storage for load-shifting energy management in medium-deep U-type geothermal heat pump systems: A comparative performance analysis

Thermal energy storage can enhance load shifting and extend heat-pump shutdown periods in medium-deep U-type borehole heat exchanger (MDUBHE) heating systems. However, latent and sensible thermal energy storage are commonly evaluated separately, and their differences in heat-supply capability, operating performance, and storage-volume requirements under unified conditions remain unclear. In this study, PCM-based latent storage and water-tank-based sensible storage were integrated with an MDUBHE heat-pump system and compared using consistent boundary conditions and control strategies. Seven PCM materials were first evaluated, followed by analyses of storage volume, dynamic charging and discharging, seasonal heat supply, building-load regulation, volumetric utilization, and marginal benefits. A system without thermal storage was also considered as the baseline. PCM47 achieved a seasonal energy recovery ratio of 99.68%, a heat-supply contribution of 17.50%, and an independent heating duration of 778.65 h, showing the best combined performance among the candidate materials. As the PCM volume increased from 10 to 50 m 3 , its seasonal contribution and independent heating duration increased from 4.65% to 20.46% and from 186.20 to 899.62 h, respectively. At the same volume of 50 m 3 , the PCM tank supplied 605.29 MWh of heat and provided 899.62 h of independent heating, compared with 175.68 MWh and 241.08 h for the water tank. The average heat-pump COP values of the latent-storage, sensible-storage, and no-storage systems were 5.48, 5.53, and 5.62, respectively, while the corresponding seasonal performance factors (SPFs) were 4.36, 4.52, and 4.78. Although latent storage caused stronger ground-side thermal disturbances and a moderate efficiency penalty, its heat supply per unit volume was 3.45–3.92 times that of sensible storage. Under equivalent heat-supply performance, PCM storage reduced the required volume by 72.34–74.78%. These results quantify the trade-off between load-shifting capability, operating efficiency, and storage compactness, providing guidance for storage-technology selection and capacity sizing in MDUBHE heating systems.