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Based on a quasi 3-D steady model inside and a 2-D unsteady model outside of the borehole of ground heat exchanger(GHE), the relationship between the heat exchange rate per depth of borehole and main parameters (center-to-center distance of two branch pipes, radius and depth of the borehole) has been analyzed. The rational values of those design parameters with considering the heat exchange rate per depth of borehole are obtained. The dynamic simulations of underground temperature field (UTF) of single-pipe and pipe-group of U-tube GHE have been conducted by ANSYS. The change rules of UTF are obtained when ground source heat pump(GSHP) has experienced a complete cooling and heating cycle during one year. The simulation results of a single-pipe show that the action radius of borehole is 1.9 m, i.e., it is reasonable that the borehole center-to-center distance is 3.8 m. The simulation results of pipe-group indicate that temperature of the soil center encircled by four boreholes at the beginning of heating in winter is higher than the initial temperature of soil, but it is in opposite for cooling in summer, which is beneficial to the efficient operation of GSHP. This work can provide guidance for the rational design and operation of GSHP. • Relation of heat exchange rate per depth of borehole(HERPDB) with design parameters. • Rational values of design parameters with considering HERPDB. • Dynamic simulation of underground temperature field (UTF) of a single U-tube GHE. • Dynamic simulation of UTF of pipe-group of U-tube GHE round one year. • Efficient operation of GSHP with borehole center-to-center distance of 3.8 m [ABSTRACT FROM AUTHOR]

• A new ice-storage experimental system with closed and open ice-storage tanks is developed. • The cold release rate of the CIST is more stable than that of the OIST under the same conditions. • The cumulative discharging capacity of CIST with 0 ∘C–3 ∘Cis a certain larger than that of OIST. • The problem of "millennium ice" is solved by combination of the external and internal ice melting. A new ice-storage experimental system with a closed (CIST) and an open (OIST) ice-storage tank has been exploited, with which the external ice melting processes are studied experimentally. Furthermore, the researches on combinations of external and internal ice melting have been carried out. The experiments indicate that the cold release rate of CIST is more stable than that of OIST under the same conditions, especially, the cold release rate of OIST decreases significantly at the later stage of the discharging process. The outlet temperature of the discharge water flowing from the bottom-to-top (Mode 1) is lower than that from the top-to-bottom of the ice-storage tank (Mode 2). With the relatively lower volumetric-flow rate, the outlet temperatures of the discharge water rise slowly, meanwhile, the cold release rate is also relatively lower. To solve the problem of "millennial ice" in the external system without an air-agitation device, two types of the combination of external and internal ice melting have been adopted, one is to adopt internal ice melting at first, and then external ice melting, the other is to adopt simultaneously the internal and external ice melting. The experiments show that the cumulative discharging capacity of the latter has been significantly improved. [ABSTRACT FROM AUTHOR]