Geothermal heat pump systems (GHPSs) are economically efficient and renewable environmentally friendly energy production systems in which the ground acts as a heat source in winter and a heat sink in summer. New methods have been developed to increase the economic efficiency of GHPSs, including using pile foundations as dual-purpose structures in energy production and load transfer from building to ground. The performance of such energy pile foundations in cold climate regions was assessed numerically in this study by considering groundwater flow effects and short-term imbalanced seasonal thermal loadings. The structural behaviour of frictional pile foundations was also analysed using soil elasto-plastic behaviour and assuming non-linear sliding contact at the pile-soil interface. The results indicated that using energy pile foundations under medium groundwater flow (around 1.65E - 8 m/s), the productivity of system is improved by around 20% compared with a saturated conditions with no groundwater flow. They also indicated that no sliding occurred between the frictional pile shaft and the surrounding soil. However, the stresses in the frictional pile shaft decreased significantly in comparison with the end-bearing conditions. Moreover, there was a significant increase in the mobilised shaft friction at the pile-soil interface, particularly in summer mode. (C) 2015 Elsevier B.V. All rights reserved.