A ground source heat pump (GSHP) system utilizes a relatively stable underground temperature to achieve energy-saving for heating and cooling in buildings. Among various types of GSHP systems, Vertical Closed Loop Ground Source Heat Pump (VGSHP) has received increasing attention due to a variety of advantages such as the potential to be installed in a relatively small space and improved energy efficiency. In this research, in order to improve the system performance of a VGSHP, a VGSHP system coupled with heat storage tank was evaluated. For this purpose, the detailed analysis on the operational behavior of heat storage tank and the variations of heat pump energy performance due to the connection with heat storage tank was performed, and characteristics of heat pump part load ratios (PLRs) and the corresponding energy requirements, compared to chiller and boiler based conventional system, were analyzed in detail, using dynamic energy simulation after comprehensive validation process of heat pump performance curves. The results of this study showed that the VGSHP system coupled with heat storage tank showed an energy saving effect of about 20% for cooling and about 77% for heating compared to the conventional air conditioning system, and an energy saving effect of about 2% for cooling and about 15% for heating compared to the VGSHP system without heat storage tank, mainly due to the fact that the operational pattern of heat pump such as PLR and COP characteristics was changed by the operation of heat storage tank. (C) 2018 Elsevier B.V. All rights reserved.