Three kinds of ground source energy system used as heat sink directly for cooling the buildings in summer are presented, which are underground water system, ground heat exchange system and underground sink system, respectively. Correspondingly, three numerical models are established. Based on a rural housing in Jinghai County, Tianjin, the simulated operating results of three systems are analyzed and compared. The applied size parameters for direct cooling system (DCS) were coincident with traditional compression refrigeration system. The results indicate that the outlet water temperature of the double-wells circulating system remains stable and the system operation effect is satisfied. Hence, it is recommended as a prior way for DCS to give long-term, safe and stable operation. However, the outlet water temperature of the U-shaped tube system easily exceeds 23 degrees C after 10 days of running, which is the maximum temperature limit for DCS due to heat transfer effect. This is not recommended. The feasibility of using an underground sink for DCS depends on reasonable analysis of the adaptability between the building cooling load and the cooling capability of the sink. Because of economical and ecological reasons, it is still recommended as an auxiliary cooling system. (C) 2011 Elsevier B.V. All rights reserved.